Acylation of lysophosphatidylcholine by brain membranes

Docosahexaenoic acid (DHA): An essential nutrient and a nutraceutical for brain health and diseases

Docosahexaenoic acid (DHA), a polyunsaturated fatty acid (PUFA) enriched in phospholipids in the brain and retina, is known to play multi-functional roles in brain health and diseases. While arachidonic acid (AA) is released from membrane phospholipids by cytosolic phospholipase A2 (cPLA2), DHA is linked to action of the Ca2+-independent iPLA2. DHA undergoes enzymatic conversion by 15-lipoxygenase (Alox 15) to form oxylipins including resolvins and neuroprotectins, which are powerful lipid mediators. DHA can also undergo non-enzymatic conversion by reacting with oxygen free radicals (ROS), which cause the production of 4-hydoxyhexenal (4-HHE), an aldehyde derivative which can form adducts with DNA, proteins and lipids. In studies with both animal models and humans, there is evidence that inadequate intake of maternal n-3 PUFA may lead to aberrant development and function of the central nervous system (CNS). What is less certain is whether consumption of n-3 PUFA is important in maintaining brain health throughout one's life span. Evidence mostly from non-human studies suggests that DHA intake above normal nutritional requirements might modify the risk/course of a number of diseases of the brain. This concept has fueled much of the present interest in DHA research, in particular, in attempts to delineate mechanisms whereby DHA may serve as a nutraceutical and confer neuroprotective effects. Current studies have revealed ability for the oxylipins to regulation of cell redox homeostasis through the Nuclear factor (erythroid-derived 2)-like 2/Antioxidant response element (Nrf2/ARE) anti-oxidant pathway, and impact signaling pathways associated with neurotransmitters, and modulation of neuronal functions involving brain-derived neurotropic factor (BDNF). This review is aimed at describing recent studies elaborating these mechanisms with special regard to aging and Alzheimer's disease, autism spectrum disorder, schizophrenia, traumatic brain injury, and stroke.

Differential disposition of lysophosphatidylcholine in diabetes compared with raised glucose: implications for prostaglandin production in the diabetic kidney glomerulus in vivo

An early increased formation of renal prostaglandins in diabetes which follows the hydrolysis of cellular phospholipids by cytosolic phospholipase A2 is of considerable importance in determining subsequent cellular function. As the disposition of concomitantly formed lysophosphatidylcholine may also affect cellular function, we investigated the cellular fate of exogenous lysophosphatidylcholine in mesangial cell-enriched glomerular cores and showed that in cells taken from diabetic rats there is an increased net reformation of phosphatidylcholine. Positional distribution of labelled palmitate from sn-1 position palmitate-labelled lysophosphatidylcholine showed distribution to both sn-1 and sn-2 position of the phosphatidylcholine formed with a significantly increased sn-2 position labelling in diabetes. Although both a coenzyme A-dependent acyltransferase activity and a coenzyme A-independent transacylase activity could be shown in these cells, the increased phosphatidylcholine formation in cells taken from diabetic animals was due to an increase in coenzyme A-independent transacylase activity. By contrast, an increase in coenzyme-A independent transacylase activity could not be demonstrated in cultured mesangial cells maintained with prolonged raised glucose concentrations. Cell homogenates possess the ability to transfer fatty acid from lysophosphatidylcholine to lysophosphatidylcholine and lysophosphatidylethanolamine with subsequent formation of phosphatidylcholine and phosphatidylethanolamine, respectively. In preparations from diabetic animals phosphatidylethanolamine formed in this manner was increased in the presence of an inhibitor of cytosolic phospholipase A2, indicating that it may provide a substrate for phospholipase A2 activity; an effect not seen in cultured cells maintained at raised glucose concentrations. It is concluded that one effect of an altered disposition of lysophosphatidylcholine in cells from diabetic animals would be to spare fatty acids released following phospholipase A2 hydrolysis of phospholipid, possibly providing the substrate for prostaglandin production, an effect not seen with raised glucose alone.

Subcellular fractions of bovine brain degrade phosphatidylcholine by sequential deacylation of the sn-1 and sn-2 positions

Phosphatidylcholine (PC) metabolism was investigated using cytosol (fraction I) and particulate fractions of bovine brain that were enriched with microsomes (fraction II), plasma membranes (fraction III) or mitochondria (fraction IV). Fractions I-III incubated with 1-palmitoyl-2-[14C]arachidonoyl-sn-glycero-3-phosphocholine yielded [14C]arachidonic acid at near equal rates, whereas only fraction I accumulated significant amounts of 2-[14C]arachidonoyl-sn-glycero-3-phosphocholine. Much slower rates of arachidonic acid release were observed using an ether PC (1-O-hexadecyl-2-[3H]arachidonoyl-sn-glycero-3-phosphocholine). Moreover, arachidonic acid yield from the diacyl, but not ether PC was slowed by pretreating fractions I-III, but not IV, with phenylmethylsulfonyl fluoride (PMSF). Coincident with this decreased arachidonic acid, 2-[14C]arachidonoyl-sn-glycero-3-phosphocholine was increased, indicating high PLA1 activity. Taken together these data suggest that arachidonic release was largely dependent on initial deacylation of position sn-1. Incubating each untreated fraction with 2-[3-H]arachidonoyl-sn-glycero-3-phosphocholine yielded [3H]arachidonic acid (lysophospholipase A2 activity) at rate that was substantially greater than that using the comparable PMSF-treated fraction. Thus, the large effect of PMSF on arachidonic acid release can be accounted for if much of the fatty acid formation arose from the sequential sn-1 and sn-2 deacylation of diacyl-PC by phospholipase A1 and lysophospholipase A2. When PMSF-treated fractions were incubated with 2-[3H]arachidonoyl-sn-glycero-3-phosphocholine, [3H]PC accumulated at low rates that were enhanced by adding coenzyme A or stearoyl-coenzyme A. Thus, the lysophospholipid was also reacylated to form PC, but this reaction was negligible in the absence of PMSF and added cofactors. In summary, we conclude that, in brain subcellular fractions, deacylation of the sn-1 position of diacyl-PC proceeded more rapidly than sn-2 hydrolysis. There was substantial further metabolism of 2-acyl lysophospholipids due to the combined activities of a PMSF-sensitive and -insensitive lysophospholipase. Finally, the sequential deacylation of diacyl-PC by phospholipase A1 and lysophospholipase A2 probably accounted for the major portion of arachidonic acid produced.

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.