We have previously shown that the docosahexaenoate (22:6n-3) status in membrane phospholipids influences the biosynthesis and accumulation of phosphatidylserine (PS) in brain microsomes and C6 glioma cells. In the present study, we investigated whether the observed effect of membrane docosahexaenoic acid status on PS accumulation is universal or occurs specifically in neuronal tissues. We observed that rat brain cortex, brain mitochondria, and olfactory bulb, where 22:6n-3 is highly concentrated, contain significantly higher levels of PS in comparison to liver and adrenal, where 22:6n-3 is a rather minor component. Phospholipid molecular species analysis revealed that in brain cortex, mitochondria, and olfactory bulb 18:0,22:6n-3 was the most abundant species representing 45-65% of total PS. In nonneuronal tissues such as liver and adrenal, 18:0,20:4n-6 was the major PS species. Dietary depletion of n-3 fatty acids during prenatal and postnatal developmental periods decreased the brain 22:6n-3 content by more than 80%, with a concomitant increase in 22:5n-6 in all tissues. Under these conditions, an approximately 30-35% reduction in total PS in rat brain cortex, brain mitochondria, and olfactory bulb was observed, while PS levels in liver and adrenal were unchanged. The observed reduction of PS content in neuronal membranes appears to be due to a dramatic reduction of 18:0,22:6n-3-PS without complete replacement by 18:0,22:5n-6-PS. These results establish that variations in membrane 22:6n-3 fatty acid composition have a profound influence on PS accumulation in neuronal tissues where 22:6n-3 is abundant. These data have implications in neuronal signaling events where PS is believed to play an important role.

Phospholipids and their acyl group composition are important in providing the proper membrane environment for membrane protein structure and function. In particular, the highly unsaturated phospholipids in synaptic plasma membranes in the CNS are known to play an important role in modulating receptor function and neurotransmitter release processes. Apolipoprotein E (apoE) is a major apolipoprotein in the CNS, mediating the transport of cholesterol, phospholipids and their fatty acids, particularly in reparative mechanisms during neuronal injury. This study was performed to determine whether deficiency in the apoE gene contributes to an alteration of the phospholipids in synaptic plasma membranes. Phospholipid molecular species were identified and quantitated by HPLC/electrospray ionization-mass spectrometry. Analysis of the different phospholipid classes in membranes of apoE-deficient and C57BL/6 J mice indicated no obvious differences in the distribution of different phospholipid classes but substantial differences in composition of phospholipid molecular species. Of special interest was the prevalence of phospholipids (phosphatidylcholine, diacyl-phosphatidylethanolamine, and phosphatidylserine) with 22:6n-3 in both the sn-1 and sn-2 positions of SPM and these phospholipid species were significantly higher in apoE-deficient mice as compared to control mice. Since polyunsaturated fatty acids in neurons are mainly supplied by astrocytes, these results revealed a new role for apoE in regulating polyunsaturated phospholipid molecular species in neuronal membranes.

Neuronal membranes are highly enriched with docosahexaenoic acid (22:6n-3), and its content can be altered by ethanol consumption. We have previously reported that the 22:6n-3 status in membrane affects the biosynthesis of phosphatidylserine (PS), a phospholipid class which contains an exceptionally high proportion of 22:6n-3. The aim of the present study is to investigate the effect of chronic ethanol exposure on PS accumulation in relation to the 22:6n-3 status. C-6 glioma cells were enriched with 25 microM 22:6n-3 for 48 h and the PS accumulation was first evaluated in comparison to nonenriched cells as well as cells enriched with arachidonic acid (20:4n-6). Electrospray liquid chromatography-mass spectrometry analysis revealed that cells treated with 22:6n-3 showed significantly higher accumulation of PS in comparison to nonenriched or 20:4n-6-enriched cells, primarily due to an increase of 1-stearoyl-2-docosahexaenoyl-glycerophosphoserine (18:0,22:6-PS). Chronic ethanol exposure selectively affected the accumulation of PS in 22:6n-3-enriched cells. After cells were exposed to 20 or 50 mM ethanol for 4 wk, accumulation of 18:0,22:6-PS upon 22:6n-3 supplementation was significantly lower, resulting in a drastic reduction of total PS. Concomitantly, ethanol-treated cells showed lower incorporation of serine in comparison to control cells. From these data, it was concluded that supplementation of cells with 22:6n-3 promotes the accumulation of PS and chronic ethanol treatment diminishes this effect at least in part through impaired serine incorporation processes. Attenuated accumulation of 22:6n-3 in PS and the reduction of PS thus may have significant implications in pathophysiological effects of ethanol, especially in tissues with abundant 22:6n-3.