The incorporation of radioactive fatty acids into the phospholipids of nerve-cell-body membranes in vivo

Metabolic pathways for the degradation of phosphatidic acid in isolated nuclei from cerebellar cells

The aim of the present research was to analyse the pathways for phosphatidic acid metabolism in purified nuclei from cerebellar cells. Lipid phosphate phosphatase and diacylglyceride lipase activities were detected in nuclei from cerebellar cells. It was observed that DAGL activity makes up 50% of LPP activity and that PtdOH can also be metabolised to lysophosphatidic acid. With a nuclear protein content of approximately 40 μg, the production of diacylglycerol and monoacylglycerol was linear for 30 min and 5 min, respectively, whereas it increased with PtdOH concentrations of up to 250 μM. LysoPtdOH, sphingosine 1-phosphate and ceramide 1-phosphate, which are alternative substrates for LPP, significantly reduced DAG production from PA. DAG and MAG production increased in the presence of Triton X-100 (1 mM) whereas no modifications were observed in the presence of ionic detergent sodium deoxycholate. Ca²+ and Mg²+ stimulated MAG production without affecting DAG formation whereas fluoride and vanadate inhibited the generation of both products. Specific PtdOH-phospholipase A1 and PtdOH-phospholipase A2 were also detected in nuclei. Our findings constitute the first reported evidence of active PtdOH metabolism involving LPP, DAGL and PtdOH-selective PLA activities in purified nuclei prepared from cerebellar cells.

Functional role of phospholipids in the nuclear events

This review presents the structural and functional role of phospholipids in chromatin and nuclear matrix as well as the difference in composition and turnover compared to those present in the nuclear membrane. Nuclei have a very active lipid metabolism which seems to play an important role in the transduction of the signals to the genome in response to agonists acting at the plasma membrane level. The evidence on the presence of phospholipid-calcium-dependent protein kinase C (PKC) in nuclei and enzymes of phospholipids turnover is given. Protein kinase C interacts with nuclear phosphoinositol and sphingomyelin cycles products. This fact evidences about possibility that signal transduction events could also occur at the nuclear level during induction of cell proliferation, differentiation and apoptosis.

Phospholipid signalling in the nucleus. Een DAG uit het leven van de inositide signalering in de nucleus

Diverse methodologies, ranging from activity measurements in various nuclear subfractions to electron microscopy, have been used to demonstrate and establish that many of the key lipids and enzymes responsible for the metabolism of inositol lipids are resident in nuclei. PtdIns(4)P, PtdIns(4,5)P2 and PtdOH are all present in nuclei, as well as the corresponding enzyme activities required to synthesise and metabolise these compounds. In addition other non-inositol containing phospholipids such as phosphatidylcholine constitute a significant percentage of the total nuclear phospholipid content. We feel that it is pertinent to include this lipid in our discussion as it provides an alternative source of 1, 2-diacylglycerol (DAG) in addition to the hydrolysis of PtdIns(4, 5)P2. We discuss at length data related to the sources and possible consequences of nuclear DAG production as this lipid appears to be increasingly central to a number of general physiological functions. Data relating to the existence of alternative pathways of inositol phospholipid synthesis, the role of 3-phosphorylated inositol lipids and lipid compartmentalisation and transport are reviewed. The field has also expanded to a point where we can now also begin to address what role these lipids play in cellular proliferation and differentiation and hopefully provide avenues for further research.

The potential for platelet-activating factor synthesis in brain: properties of cholinephosphotransferase and 1-alkyl-sn-glycero-3-phosphate acetyltransferase in microsomal fractions of immature rabbit cerebral cortex

The synthesis of platelet-activating factor (PAF) was studied in microsomal fractions of cerebral cortices of 15-day-old rabbits. These included: a total microsomal fraction P3, rough and smooth microsomes, R and S, and microsomal fraction P derived from isolated nerve cell bodies. Cholinephosphotransferase (CPT) generating PAF from alkylacetylglycerol had the highest specific activities in fractions R and P (24 and 6 times the homogenate values, based on membrane phospholipid content). This CPT activity differed from that which synthesized phosphatidylcholine as the latter was sensitive to dithiothreitol inhibition and was more readily inhibited by Triton X-100. As the CPT activity for PAF synthesis relies on the production of alkylacetylglycerol we studied the acetyltransferase which forms 1-alkyl-2-acetyl-sn-glycero-3-phosphate (AAGP). This enzyme had the highest specific activity in fraction R, followed by fractions P3 and P. There was evidence that the acetyltransferase was more active in a phosphorylated form. NaF maximized the recovery of AAGP products in the assays. The pH optimum for acetylation was in a range of 8.0-9.0. Lyso PAF did not inhibit the formation of AAGP and the rates of formation of PAF by acetylation were less than 5% of values for AAGP synthesis. During AAGP formation there was no evidence for subsequent alkylacetylglycerol formation in the absence of NaF, but a small formation of radioactive PAF could be demonstrated from AAGP under the CPT assay conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipids in the nucleus--metabolism and possible functions

Most of the phospholipids in the nuclear envelope are contained in the double nuclear membrane, and this has an active lipid metabolism consistent with its origins as a component of the endoplasmic reticular system. However, even after removal of the nuclear membrane with detergents, some phospholipids, mostly of unknown location and function, remain. Amongst these are all of the components of what appears to be a nuclear polyphosphoinositide signalling system, distinct from the well-established inositide pathway found in the plasma membrane. The consequences for nuclear function of the activation of these two inositide pathways are discussed, with a detailed consideration of proposed intranuclear functions for protein kinase C, and the maintenance of nuclear Ca2+ homoeostasis.

Phosphatidylinositol synthetase activities in neuronal nuclei and microsomal fractions isolated from immature rabbit cerebral cortex

The synthesis of phosphatidylinositol was studied using a nuclear fraction N1, a microsomal fraction P3, rough (R) and smooth (S) microsomal fractions and a microsomal fraction P derived from isolated nerve cell bodies. Each fraction was prepared using cerebral cortices of 15-day-old rabbits. In assays using CDP-diacylglycerol (prepared from egg phosphatidylcholine) and myo[3H]inositol at pH 7.4, fraction N1 had the highest maximal specific rates of phosphatidylinositol synthetase (EC 2.7.8.11) (expressed per mumol phospholipid in the fraction). However the three microsomal fractions achieved maximal specific activities at liponucleotide concentrations close to 50 microM, while fraction N1 required 200 microM concentrations. In certain cases (25-120 microM CDP-diacylglycerol, and at higher pH values) fraction R had specific activities which equalled or surpassed those of N1. However, with respect to inositol, fraction N1 had a distinctly lower Km than was shown for fractions R or P3. Each of the microsomal fractions and N1 required Mg2+ for the reaction, but for N1, maximal rates could be sustained at 0.1 mM, while for the microsomal fractions the optimal Mg2+ concentration was 1 mM. For each fraction Mn2+ could not replace Mg2+ in the reaction and Mn2+ was inhibitory. The optimal pH for the reaction was between 8.0 and 9.0. Phosphatidylinositol synthetase could also be shown using fraction N1 enriched in endogenous CDP-diacylglycerol. The relatively high specific activities of fraction N1, and the differences found between N1 and the microsomal fractions, for optimal CDP-diacylglycerol and Mg2+ concentrations and for Km values for inositol, support the existence of a neuronal nuclear phosphatidylinositol synthetase.

Sources for brain arachidonic acid uptake and turnover in glycerophospholipids

Brain arachidonic acid comes from linoleic acid and arachidonic acid in the blood. Part of the brain arachidonic acid is elongated to adrenic acid, 22:4 (n = 6), especially in higher animals. With labeled arachidonic acid injected into cerebral ventricles of mice, the highest specific radioactivity was in triacylglycerols. The highest labeling in PtdCho and PtdIns was found at 15 to 60 minutes. Labeling of PtdEtn was much less. The molecular species with 16:0 and 18:1 were labeled better than those with 18:0. Adrenic acid was preferred by alkylacyl-GroPEtn. The highest level of labeling by arachidonic acid was found in the choline plasmalogens and the alkylacyl-GroPCho at 24 hours after injection. The PtdCho arachidonic acid turned over several times within 24 hours. Part of this turnover probably represents the transfer of labeled arachidonic acid to unlabeled ether-linked choline glycerophospholipids, including 1-alkyl-2-lyso-GroPCho, also known as lyso platelet activating factor. The energy-independent transfer of arachidonic acid from PtdCho to ether-linked choline glycerophospholipids may follow removal of their arachidonic acid by phospholipase A2 due to receptor activation. The lack of pulse labeling of ether-linked choline glycerophospholipids complicates the study of their function.

The formation of phosphatidic acid de novo: a comparison of activities in neuronal nuclei and microsomes isolated from immature rabbit cerebral cortex

The formation of phosphatidic acid from sn-glycerol 3-phosphate was studied in neuronal nuclear fraction N1 and a microsomal fraction P3, isolated from cerebral cortices of 15-day-old rabbits. Two assays were used, employing dithiothreitol, MgCl2, NaF and (A) sn-glycerol 3-phosphate, [14C]oleate, ATP and CoA or (B) sn-[3H]glycerol 3-phosphate and oleoyl-CoA. In both assays fraction N1 had specific rates of phosphatidic acid labelling (expressed per mumol phospholipid in the fraction) which were 5- to 6-times the corresponding values for P3. In contrast to N1, the formation of phosphatidic acid by fraction P3 was more sensitive to inhibition at high concentrations of oleoyl-CoA and was greatly dependent upon the presence of NaF. In the absence of this salt, P3 showed decreased phosphatidate formation and increased levels of radioactive monoacylglycerols. Using cerebral cortex, rough (R) and smooth (S) microsomal fractions were prepared, as was a microsomal fraction P from isolated nerve cell bodies. P had specific rates of phosphatidic acid labelling which were 2-3 times the values for P3, but were about 50% of the N1 values. This indicates a concentration of phosphatidate synthesis in the nucleus within the nerve cell. Specific rates for fraction R were higher and were similar to those of N1. In S, P3 and R the specific rates of phosphatidic acid synthesis paralleled specific RNA contents and indicated a location for phosphatidic acid synthesis within the rough endoplasmic reticulum.

Utilization of plasma fatty acid in rat brain: distribution of [14C]palmitate between oxidative and synthetic pathways

Radioactivity within individual brain compartments was determined from 5 min to 44 h after intravenous injection of [14C]palmitate into awake Fischer-344 rats, aged 21 days or 3 months. Total radioactivity peaked broadly between 15 min and 1 h after injection, declined rapidly between 1 and 2 h, and then more slowly. In 3-month-old rats, the lipid and protein brain fractions were maximally labeled within 15 min after [14C]palmitate injection, then retained approximately constant label for up to 2 days. Radioactivity in the aqueous brain fraction comprised mainly radioactive glutamate and glutamine, and peaked at 45 min, when it comprised 48% of total brain radioactivity, then decreased to 27% of the total at 4 h, 15% at 20 h, and 10% at 44 h. Percent distribution of radioactivity within the different brain compartments, 4 h after intravenous injection of [14C]palmitate, was similar in 21-day-old and 3-month-old rats, despite higher net brain uptake in the younger animals. The results indicate that about 50% of plasma [14C]palmitate that enters the brain of adult rats is incorporated rapidly into stable protein and lipid compartments. The remaining [14C]palmitate enters the aqueous fraction after beta-oxidation, and is slowly lost. At 4 h after injection, 73% of brain radioactivity is within the stable brain compartments; this fraction increases to 86% by 20 h.

The CMP-stimulated production of diacylglycerol and CDPdiacylglycerol in neuronal nuclei labelled with radioactive arachidonate

A neuronal nuclear fraction (N1), isolated from immature rabbit cerebral cortex, was preincubated with [3H]arachidonate, ATP, CoA, Mg2+ and 1-acyl-sn-glycero-3-phosphocholine or 1-acyl-sn-glycero-3-phosphoinositol. Using the former lysophospholipid, a sizeable incorporation of radioactivity was seen in N1 phosphatidylcholine. In subsequent incubations in the presence of CMP and EGTA, there was a generation of radioactive diacylglycerol in N1 and a corresponding decline in phosphatidylcholine radioactivity. Both these changes could be blocked by the addition of CDPcholine. In incubations using N1 phosphatidylinositol or phosphatidylethanolamine prelabelled with [3H]arachidonate, no evidence was found to support a direct generation of diacylglycerol from these phospholipids. The back reaction of cholinephosphotransferase in N1 is likely the principal source of diacylglycerols bearing arachidonate. Using either lysophospholipid in the preincubations described in the opening sentence, more than half of the incorporated radioactivity derived from [3H]arachidonate was found in N1 phosphatidylinositol. In subsequent incubations with EGTA and CMP there was a production of radioactive CDPdiacylglycerol and a decline in radioactive phosphatidylinositol. Both events could be blocked by the presence of myo-inositol. Radioactive CDPdiacylglycerol, produced in N1 in the presence of CMP and EGTA, was converted back into phosphatidylinositol by the addition of myo-inositol. The production of CDPdiacylglycerol is likely the result of the back reaction of CDPdiacylglycerol:inositol phosphatidate transferase in N1.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Acylation of lysoglycerophospholipids by adrenal membranes

1. Substantial differences were found in the acyl donor and lyso-acceptor specificities among subcellular membranes and with respect to different regions of the adrenal gland. 2. In the presence of Mg2+-ATP and CoASH, adrenal microsomes were actively transferring arachidonate to lysophospholipids with acyl acceptor specificity in the order: 1-acyl-GPI greater than 1-acyl-GPC greater than 1-acyl-GP. However, when oleoyl-CoA was used, acyl acceptor specificity for the microsomal transferases was in the order: 1-acyl-GPC greater than 1-acyl-GP greater than 1-acyl-GPI. 3. Mitochondrial membranes had very low acyl transfer activity and they preferred 1-acyl-GPC over other lyso-acceptors. 4. The chromaffin granules were apparently lacking this type of activity.

Incorporation of [1-14C[palmitic acid into neutral lipids and phospholipids of rat cerebral cortex in vitro

Incorporation of [1-14C]palmitic acid into neutral lipids and phospholipids of rat cerebral cortex was examined in vitro in normal Krebs--Ringer bicarbonate buffer containing 3% (wt/vol) albumin and 0.75 mM palmitic acid. Under standard assay conditions, radioactivity in the triacylglycerol fraction increased rapidly during the first 30 min, and then decreased after 60 min, with corresponding increase in radioactivity in phosphatidyl choline, phosphatidyl ethanolamine, and a fraction of phosphatidyl inositol plus phosphatidyl serine. Diacylglycerol was shown to be an intermediate metabolite. Radioactivity increased in triacylglycerol, and decreased in phosphatidyl choline and phosphatidyl ethanolamine throughout incubation under N2 gas. In the fraction of phosphatidyl inositol plus phosphatidyl serine, radioactivity decreased after 30 min during incubation under N2 gas. A possible acylation--deacylation cycle, in which triacylglycerol could be a source of free fatty acids for phospholipids, is discussed.

A comparison of lysophosphatidylcholine acyltransferase activities in neuronal nuclei and microsomes isolated from immature rabbit cerebral cortex

Using neuronal nuclei (N1) and microsomes (P3) isolated from cerebral cortices of 15-day-old rabbits, the activity of lysophosphatidylcholine acyltransferase (acyl CoA: 1-acyl-sn-glycerol-3-phosphorylcholine acyltransferase) was studied using palmitoyl-, oleoyl- and arachidonoyl-CoA and a pool of lysophosphatidylcholine labelled with [3H]palmitate, [3H]stearate or [3H]oleate. Generally, in the acylation of the three radioactive lysophosphatidylcholines with arachidonoyl-CoA, the N1-specific acylation activities were two to seven times those of P3. For oleoyl-coA smaller N1 : P3 specific activity ratios were found, differing significantly from unity for only the 1-palmitoyl and 1-stearoyl lysophosphatidylcholines. The N1 : P3 ratios for the two unsaturated acyl-CoA thioesters were usually found to increase as the lysophosphatidylcholine concentration was lowered from 100 to 25 microM. Thus, nuclear acylation rates, particularly with arachidonoyl-CoA, were less affected by lowering the acceptor concentration than were microsomal activities. At the high lysophosphatidylcholine concentration (100 microM), arachidonoyl-CoA was a superior substrate to oleoyl-CoA in the nuclear acylations of the 1-palmitoyl or 1-stearoyl acceptors. Such a preference was never seen for the microsomal fraction. Using oleoyl- and arachidonoyl-CoA, the nuclear enzymes also showed a greater preference for the 1-palmitoyl homologue over the 1-oleoyl homologue than did the microsomal enzymes. These results support the existence of neuronal nuclear lysophosphatidylcholine acyltransferases with different substrate preferences than shown by the microsomal fraction.

Growth of adult superior cervical ganglion explants in serum-free media

Neurite outgrowth from explants of superior cervical ganglion from adult rats can be achieved in a serum-free medium. Extensive neurite outgrowth occurred from ganglion explants maintained in Eagle's minimum essential medium supplemented with either 10% (V/V) fetal calf serum or 1% (W/V) bovine serum albumin and nerve growth factor. After one week in culture, the ATP content of explants maintained in the serum-free medium was slightly higher than that noted in explants cultured in the presence of fetal calf serum and amounts of phosphocreatine were significantly lower. Despite these differences in high energy phosphate content, the abundance and morphology of neuritic outgrowth were essentially the same from explants cultured in the two types of media. Comparable activities of a number of NADP+-dependent dehydrogenases were noted in explants maintained in the two types of media. Increases in the activities of the oxidative enzymes of the pentose pathway, which occur in axotomized ganglia in vivo, were observed in the cultured ganglion explants. NADP+-dependent isocitrate dehydrogenase activity remained constant in ganglion explants in vitro, and measurements of this activity were employed in a new method to quantitate neurite outgrowth. The activity of isocitrate dehydrogenase in lyophilized neurite processes that had grown out onto a Millipore filter substrate correlated well with visual estimates of neuritic outgrowth. Substitution of delipidated for normal bovine serum albumin in the growth medium resulted in a significant decrease in neuritic outgrowth from ganglion explants from both adult and weanling rats. Addition of fatty acids to media containing delipidated bovine serum albumin enhanced neuritic outgrowth in explants of weanling rats. Thus, lipophilic substances bound to bovine serum albumin including fatty acids appear necessary for optimal growth of neurites from explants of the rat superior cervical ganglion.