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

Adipose triglyceride lipase affects triacylglycerol metabolism at brain barriers

Currently, little is known about the role of intracellular triacylglycerol (TAG) lipases in the brain. Adipose triglyceride lipase (ATGL) is encoded by the PNPLA2 gene and catalyzes the rate-limiting step of lipolysis. In this study, we investigated the effects of ATGL deficiency on brain lipid metabolism in vivo using an established knock-out mouse model (ATGL-ko). A moderate decrease in TAG hydrolase activity detected in ATGL-ko versus wild-type brain tissue was accompanied by a 14-fold increase in TAG levels and an altered composition of TAG-associated fatty acids in ATGL-ko brains. Oil Red O staining revealed a severe accumulation of neutral lipids associated to cerebrovascular cells and in distinct brain regions namely the ependymal cell layer and the choroid plexus along the ventricular system. In situ hybridization histochemistry identified ATGL mRNA expression in ependymal cells, the choroid plexus, pyramidal cells of the hippocampus, and the dentate gyrus. Our findings imply that ATGL is involved in brain fatty acid metabolism, particularly in regions mediating transport and exchange processes: the brain-CSF interface, the blood-CSF barrier, and the blood-brain barrier.

Temporal and spatial changes of free cholesterol and neutral lipids in rat brain after transient middle cerebral artery occlusion

In order to examine lipid metabolism in relation to neural process following brain ischemia, we investigated temporal and spatial changes of free cholesterol (FC) and neutral lipids (NLs) after 90 min of transient middle cerebral artery occlusion (MCAO). Filipin and Nile Red stainings were performed to detect mainly FC and NLs, respectively. Double stainings for Nile Red plus ED1, MAP2, or GFAP were performed in order to identify cell type of positive stainings. Filipin stanining decreased during 1-7 day and lost at 21 day after transient MCAO in the ischemic core, but did not change in the penumbra. Nile Red positive droplets reached the maximum at 7 day after transient MCAO and gradually decreased in the core, while the peak time delayed in the penumbra. MAP2 immunoreactivity lost at 7 day in the core, and increased in the penumbra during 7-56 day. Most Nile Red positive droplets were double positive for ED1 in the core, and were localized within GFAP positive cells in the penumbra. These results suggest that changes of FC and NLs are different temporally and spatially between the core and penumbra in relation to degenerative and regenerative neural processes following brain ischemia.

Pathways of incorporation of fatty acid into glycerolipids of the murine peripheral nervous system in vivo: alterations in the dysmyelinating mutant trembler mouse

In vivo glycerolipid metabolism was studied in sciatic nerves of normal and Trembler mice. The results showed that two kinetically independent pathways were implicated in the labeling of diacylglycerophospholipids from [3H]palmitate: the Kennedy pathway and a 'direct acylation' pathway. In normal nerves, 45% of the glycerophospholipids were labeled, with a rate constant k3 = 3.9 x 10(-3) min-1, from phosphatidic acid and diacylglycerol intermediates, themselves formed with a rate constant of k1 = 0.24 min-1 from a free 3H-fatty acid pool, FFA1, that represents 45% of the total injected label. The remaining 55% of the glycerophospholipids were labeled from a kinetically distinct free 3H-fatty acid pool, FFA2, with a rate constant of k4 = 9.8 x 10(-2) min-1, via a process that does not implicate a detectably labeled metabolic intermediate ('direct acylation'). Glycerophospholipid labeling via the Kennedy pathway in the Trembler mouse sciatic nerves was reduced to 75% of the normal level, while labeling via the 'direct acylation' pathway was increased 1.4-fold. The values of the rate constants for free 3H-fatty acid utilisation (k1 and k4) were both increased about 2.5-fold, while that of glycerophospholipid formation from diacylglycerol (k3) was close to normal.

1-Acyl-2-lysophosphatidylcholine transport across the blood-retina and blood-brain barrier

The transport of lysophospholipids through the rat blood-retina and blood-brain barrier was determined by using radioactive 1-palmitoyl-2-lysophosphatidylcholine (Pam-lysoPtdCho) and by measuring the uptake of this labeled compound into the retina and various brain regions after short in situ carotid perfusion. The transport was not affected by probenecid (0.25 mM), but it was inhibited, in a dose-dependent manner, by circulating albumin which is able to bind tightly to lysophosphatidylcholine and lowered the availability of the latter for tissue extraction. Radiotracer transfer in the retina was higher than in brain regions. The permeability-surface area products (PS) changed with the inclusion of unlabeled Pam-lysoPtdCho, showing that transport across retinal and brain microvessels is mainly saturable. The data provided an estimate of transport constants (Vmax, Km and non-saturable constant Kd). However, we could not distinguish whether this saturable process represents the saturation of a transport carrier or simple passive diffusion followed by the saturation of enzymatic reactions. In brain tissue lipid extract, 20 s after carotid injection, radiolabel was associated by 45% to unmetabolized Pam-lysoPtdCho. Partial acylation to phosphatidylcholine, as well as hydrolysis and redistribution of the fatty acyl moiety into main phospholipid classes also occurred. The present results, compared to our previous data, indicate that PamlysoPtdCho is transported faster and/or in greater amounts than unesterified fatty acids.

Turnover of palmitic and arachidonic acids in the phospholipids from different brain areas of adult and aged rats

[3H]Palmitic acid and [14C]arachidonic acid were injected together into the cerebral ventricle of 4-month and 24-month-old rats. At different time intervals from the injection, the distribution of these fatty acids in the lipids from different brain areas was examined. The fatty acids were rapidly incorporated into the lipids through different mechanisms. The time-specific activity relationship indicate that the utilization of the fatty acid differs according to the different areas and aging decreases the utilization of both the fatty acids. The decline of arachidonic acid incorporation into phospholipids is particularly evident, indicating that aging affects mainly the utilization of polyunsaturated fatty acids.

Metabolic turnover of fatty acids and acylglycerols in rat sciatic nerve

To explain the discrepancy between the low level and high metabolic activity of endoneurial free fatty acids (FFAs) and triacylglycerol (TG), levels of de novo synthesized FFA and acylglycerols were measured in rat sciatic endoneurium at various intervals after endoneurial microinjection of [14C]acetate. Soon after injection (less than 10 min), the [14C]acetate was metabolized to FFA and incorporated into diacylglycerol (DG), TG, sterols, ceramides, and various phospholipids. The proportions of 14C-labeled FFA, DG, TG, and ceramides to total 14C-labeled lipids decreased, whereas those of phospholipids and cerebrosides increased with time after injection. These findings suggest that rapid turnover of FFA and TG may contribute to their low level in sciatic endoneurium. The de novo synthesized fatty acids were largely incorporated into phosphatidylcholine (approximately 50% of total 14C-labeled phospholipids), probably via the cytidine nucleotide pathway using 1,2-DG as a metabolic intermediate. Hydrolysis of [14C]phosphatidylcholine revealed that fatty acids were labeled at both the C-1 (approximately 43%) and C-2 (approximately 57%) positions. On the other hand, a temporal association between decreased amounts of 14C-label in ceramides and increased amounts of 14C-label in sphingomyelin and galactocerebrosides supports the hypothesis that peripheral nerve galactocerebroside is derived, in vivo, from ceramide via acylation of sphingosine. This exclusive labeling of endoneurial lipids by endoneurial microinjection of labeled precursor provides a unique model for studying synthesis and metabolic turnover of membrane lipids in experimental neuropathies.

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)

Biosynthesis of triglyceride and other fatty acyl esters by developing rat brain

The biosynthesis of triglyceride from 1,2-diglyceride and long-chain acyl coenzyme A (CoA) was studied in developing rat brain. Diglyceride acyltransferase activity was highest in a microsomal fraction, had a neutral pH optimum, and was stimulated by MgCl2. Palmitoyl CoA and oleoyl CoA served equally well as acyl donors. The enzyme catalyzed the acylation of both endogenous diglyceride and several naturally occurring and synthetic exogenous diglycerides. In addition, short-chain primary and secondary alcohols were found to be acylated under these conditions. A second acylation system, active at low pH, was found to catalyze esterification of ethanol and cholesterol, but not diglyceride, with free fatty acid. These results demonstrate that brain has the capacity to acylate a wide variety of physiological and nonphysiological hydroxyl compounds.

Effects of postdecapitation ischemia on the metabolism of [14C]arachidonic acid and [14C]palmitic acid in the mouse brain

The effect of postdecapitation ischemia on the labeling of the free fatty acid pool and their incorporation in lipids was examined during the first 10 min after decapitation in mouse brain that had been injected intracerebrally with either [1-14C]arachidonic acid or [1-14C]palmitic acid. One min after decapitation, animals injected with labeled arachidonic acid exhibited a greatly reduced incorporation of label in brain phospholipids, diglycerides, and triglycerides. When radioactive palmitic acid was used, brain lipids exhibited considerably less inhibition of label. However, a similar degree of inhibition was observed 10 min after decapitation with both fatty acids. At this time, free arachidonic acid had decreased 84% as compared to the 24% decrease observed in the controls, and about 77% of the free palmitic acid remained in the free fatty acid fraction as compared with 30% in the controls. This decreased labeling may reflect ATP shortage that affects the fatty acid activation-reacylation reactions or the enzymes involved. Alternatively, the enhanced endogenous free arachidonic acid may compete with the radiolabeled arachidonic acid resulting in an inhibition of lipid labeling. Inhibition of label may have been greater in radiolabeled arachidonic acid than palmitic because of the larger accumulation of the former endogenous fatty acid during early ischemia.