Purified rat brain myelin contains measurable acyl-CoA:lysophospholipid acyltransferase(s) but little, if any, glycerol-3-phosphate acyltransferase

Emerging Role of Phospholipids and Lysophospholipids for Improving Brain Docosahexaenoic Acid as Potential Preventive and Therapeutic Strategies for Neurological Diseases

Docosahexaenoic acid (DHA, 22:6n-3) is an omega-3 polyunsaturated fatty acid (PUFA) essential for neural development, learning, and vision. Although DHA can be provided to humans through nutrition and synthesized in vivo from its precursor alpha-linolenic acid (ALA, 18:3n-3), deficiencies in cerebral DHA level were associated with neurodegenerative diseases including Parkinson’s and Alzheimer’s diseases. The aim of this review was to develop a complete understanding of previous and current approaches and suggest future approaches to target the brain with DHA in different lipids’ forms for potential prevention and treatment of neurodegenerative diseases. Since glycerophospholipids (GPs) play a crucial role in DHA transport to the brain, we explored their biosynthesis and remodeling pathways with a focus on cerebral PUFA remodeling. Following this, we discussed the brain content and biological properties of phospholipids (PLs) and Lyso-PLs with omega-3 PUFA focusing on DHA’s beneficial effects in healthy conditions and brain disorders. We emphasized the cerebral accretion of DHA when esterified at sn-2 position of PLs and Lyso-PLs. Finally, we highlighted the importance of DHA-rich Lyso-PLs’ development for pharmaceutical applications since most commercially available DHA formulations are in the form of PLs or triglycerides, which are not the preferred transporter of DHA to the brain.

Phosphatidylserine in the brain: metabolism and function

Phosphatidylserine (PS) is the major anionic phospholipid class particularly enriched in the inner leaflet of the plasma membrane in neural tissues. PS is synthesized from phosphatidylcholine or phosphatidylethanolamine by exchanging the base head group with serine, and this reaction is catalyzed by phosphatidylserine synthase 1 and phosphatidylserine synthase 2 located in the endoplasmic reticulum. Activation of Akt, Raf-1 and protein kinase C signaling, which supports neuronal survival and differentiation, requires interaction of these proteins with PS localized in the cytoplasmic leaflet of the plasma membrane. Furthermore, neurotransmitter release by exocytosis and a number of synaptic receptors and proteins are modulated by PS present in the neuronal membranes. Brain is highly enriched with docosahexaenoic acid (DHA), and brain PS has a high DHA content. By promoting PS synthesis, DHA can uniquely expand the PS pool in neuronal membranes and thereby influence PS-dependent signaling and protein function. Ethanol decreases DHA-promoted PS synthesis and accumulation in neurons, which may contribute to the deleterious effects of ethanol intake. Improvement of some memory functions has been observed in cognitively impaired subjects as a result of PS supplementation, but the mechanism is unclear.

Fatty acid synthesizing enzymes intrinsic to myelin

A recent study showing incorporation of acetyl groups from neuronal N-acetylaspartate into myelin lipids suggested the presence of fatty acid synthesizing enzymes in myelin that utilize the acetyl groups liberated by myelin-associated aspartoacylase [J. Neurochem. 78 (2001) 736]. We report here detection of the fatty acid synthase (FAS) complex and acetyl-CoA carboxylase (ACC) in purified myelin. The activity of myelin FAS was approximately half that of cytosolic FAS and, unlike the latter, required detergent for activation. Intrinsic association of FAS with myelin was indicated by failure to remove the activity with NaCl or Na-taurocholate. Myelin-associated ACC was approximately 10% of cytosolic ACC in myelin isolated by gradient centrifugation, and this was reduced by half following osmotic shock; this suggested bimodal distribution of myelin ACC, some being loosely associated within inter-lamellar cytoplasmic spaces and the remainder more firmly associated in a manner that resists NaCl/Na-taurocholate treatments. These results, in combination with earlier findings, provide a possible mechanism for the observed incorporation of neuronal NAA acetyl groups into myelin lipids.

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.

Peripheral nerve sphingomyelin and cerebroside are both formed via two metabolically and kinetically distinct pathways in vivo

We have studied the labeling kinetics of peripheral nerve sphingolipids in vivo. The kinetic analysis of the labeling profiles observed for the various sphingolipids demonstrated that 90% of cerebrosides, but only 30% of sphingomyelin, were synthesized via a de novo synthesized ceramide intermediate following the injection of 1-4 pmol [3H]palmitate into mouse sciatic nerves. The remaining sphingolipid labeling (30% of the total) was due to direct acylation events, using free fatty acids originating from a pool different from those implicated in the de novo ceramide pathway. Direct acylation events ceased within 1 h following substrate administration, while labeling via the ceramide pathway continued through 5 h. The results provide the first in vivo demonstration that the formation of cerebrosides and sphingomyelin in peripheral nerves in situ can be simultaneously assured via two metabolically and kinetically distinct pathways that employ different fatty acid pools.

Molecular species of choline and ethanolamine glycerophospholipids in rat brain myelin during development

The composition of the molecular species of various phospholipid subclasses was examined in myelin isolated from brain of 15-, 21- and 90-day-old rats. The molecular species of diacylglycerophosphocholine (PtdCho), diacylglycerophosphoethanolamine (PtdEtn) and plasmenyl-ethanolamine (PlsEtn) were quantified by high-performance liquid chromatography (HPLC) after phospholipase C treatment and dinitrobenzoyl derivatization. In rat brain myelin, each phospholipid subclass showed a specific pattern of molecular species that changed during development. PtdCho contained large amounts of saturated/monounsaturated and disaturated species and low amounts of saturated/polyunsaturated species. During brain development, the levels of saturated/monounsaturated molecular species increased whereas those of the disaturated and saturated/polyunsaturated species decreased. PtdEtn were characterized by their low levels of disaturated species and a high content of saturated/monounsaturated and saturated/polyunsaturated species, of which those containing fatty acids of the n-3 series decreased, whereas those containing fatty acids of the n-6 series did not change during brain development. The levels of saturated/monounsaturated species increased in PtdEtn. No disaturated molecular species could be detected in PlsEtn. This alkenylacyl subclass contained large amounts of saturated/polyunsaturated, saturated/monounsaturated and dimonounsaturated molecular species. During development, the levels of saturated/polyunsaturated molecular species decreased while those of the two others increased. The data indicated that myelin sheaths undergo phospholipid changes during brain development and maturation.

Axon-myelin transfer of phospholipids and phospholipid precursors. Labeling of myelin phosphoinositides through axonal transport

Previous studies have provided evidence for axon-to-myelin transfer of intact lipids and lipid precursors for reutilization by myelin enzymes. Several of the lipid constituents of myelin showed significant contralateral/ipsilateral ratios of incorporated radioactivity, indicative of axonal origin, whereas proteins and certain other lipids did not participate in this transfer-reutilization process. The present study will examine the labeling of myelin phosphoinositides by this pathway. Both 32PO4 and [3H]inositol were injected monocularly into 7-9-wk-old rabbits and myelin was isolated 7 or 21 days later from pooled optic tracts and superior colliculi. In total lipids 32P counts of the isolated myelin samples showed significant contralateral/ipsilateral ratios as well as increasing magnitude of contralateral-ipsilateral differences during the time interval. Thin-layer chromatographic isolation of the myelin phosphoinositides revealed significant 32P-labeling of these species, with PIP and PIP2 showing time-related increases. This resembled the labeling pattern of the major phospholipids from rabbit optic system myelin in a previous study and suggested incorporation of axon-derived phosphate by myelin-associated enzymes. The 32P label in PI, on the other hand, remained constant between 7 and 21 days, suggesting transfer of intact lipid. This was supported by the labeling pattern with [3H]inositol, which also showed no increase over time for PI. These results suggest axon-myelin transfer of intact PI followed by myelin-localized incorporation of axon-derived phosphate groups into PIP and PIP2. The general topic of axon-myelin transfer of phospholipids and phospholipid precursors is reviewed.

Phospholipid acylation by mouse sciatic nerve microsomes

The partition of 0.3 nmol of [1-14C]oleoyl-CoA in the microsomes (10 micrograms proteins) from mouse sciatic nerves is unaffected by the presence of lysophospholipids and is about 45% of the total oleoyl-CoA (77% of the acylglycerophosphocholine partition in the membrane). The concentration of both oleoyl-CoA and acylglycerophosphocholine is over 1 mM in the membrane. There is a selective acyl transfer from acyl-CoA to lysolipid acceptors (oleoyl greater than myristoyl, palmitoyl, stearoyl much greater than eicosanoyl greater than docosanoyl, tetracosanoyl). The exogenous acyl acceptors are acylglycerophosphocholine and acylglycerophosphoinositol and to a lesser extent acylglycerophosphoethanolamine, but not acylglycerophosphoserine. A PC formation from acylGPC in the absence of exogenous acyl donors or from oleoyl-CoA in the absence of exogenous acyl acceptor was also observed.

Phosphatidate phosphohydrolase in purified rat brain myelin

Highly purified myelin from rat brain stem has been shown to contain phosphatidate phosphohydrolase, an enzyme which converts phosphatidate to diacylglycerol. The high levels relative to cytosol and microsomes (17% and 22%, respectively) tended to preclude contamination by these fractions as the source of activity. Additional evidence came from study of repeated purification, mixing experiments, and washing of the myelin with salt and detergent. We conclude that this enzyme, in addition to being widely distributed in other subcellular fractions, is intrinsic to the myelin membrane. Through its activity it generates a key substrate for the cytidine (Kennedy) pathway which was previously shown to occur in this membrane.