Metabolism of hexacosatetraenoic acid (C26:4,n-6) in immature rat brain

Influence of very-long-chain polyunsaturated fatty acids on membrane structure and dynamics

The unique attributes of very-long-chain polyunsaturated fatty acids (VLC-PUFAs), their long carbon chains (n > 24) and high degree of unsaturation, impart unique chemical and physical properties to this class of fatty acids. The changes imparted by VLC-PUFA 32:6 n-3 on lipid packing and the compression moduli of model membranes were evaluated from π-A isotherms of VLC-PUFA in 1,2-distearoyl-sn-3-glycero-phosphocholine (DSPC) lipid monolayers. To compare the attractive or repulsive forces between VLC-PUFA and DSPC lipid monolayers, the measured mean molecular areas (MMAs) were compared with the calculated MMAs of an ideal mixture of VLC-PUFA and DSPC. The presence of 0.1, 1, and 10 mol % VLC-PUFA shifted the π-A isotherm to higher MMAs of the lipids comprising the membrane and the observed positive deviations from ideal behavior of the mixed VLC-PUFA:DSPC monolayers correspond to repulsive forces between VLC-PUFAs and DSPC. The MMA of the VLC-PUFA component was estimated using the measured MMAs of DSPC of 47.1 ± 0.7 Å²/molecule, to be 15,000, 1100, and 91 Å²/molecule at 0.1, 1, and 10 mol % VLC-PUFA:DSPC mixtures, respectively. The large MMAs of VLC-PUFA suggest that the docosahexaenoic acid tail reinserts into the membrane and adopts a nonlinear structure in the membrane, which is most pronounced at 0.1 mol % VLC-PUFA. The presence of 0.1 mol % VLC-PUFA:DSPC also significantly increased the compression modulus of the membrane by 28 mN/m compared with a pure DSPC membrane. The influence of VLC-PUFA on lipid "flip-flop" was investigated by sum-frequency vibrational spectroscopy. The incorporation of 0.1 mol % VLC-PUFA increased the DSPC flip-flop rate fourfold. The fact that VLC-PUFA promotes lipid translocation is noteworthy as retinal membranes require a high influx of retinoids which may be facilitated by lipid flip-flop.

Regional changes in CNS and retinal glycerophospholipid profiles with age: a molecular blueprint

We present here a quantitative molecular blueprint of the three major glycerophospholipid (GPL) classes, phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylethanolamine (PE), in retina and six regions of the brain in C57Bl6 mice at 2, 10, and 26 months of age. We found an age-related increase in molecular species containing saturated and monoenoic FAs and an overall decrease in the longer-chain PUFA molecular species across brain regions, with loss of DHA-containing molecular species as the most consistent and dramatic finding. Although we found very-long-chain PUFAs (VLC-PUFAs) (C28) in PC in the retina, no detectable levels were found in any brain region at any of the ages examined. All brain regions (except hippocampus and retina) showed a significant increase with age in PE plasmalogens. All three retina GPLs had di-PUFA molecular species (predominantly 44:12), which were most abundant in PS (∼30%). In contrast, low levels of di-PUFA GPL (1-2%) were found in all regions of the brain. This study provides a regional and age-related assessment of the brain's lipidome with a level of detail, inclusion, and quantification that has not heretofore been published.

alpha-Synuclein abnormalities in mouse models of peroxisome biogenesis disorders

alpha-Synuclein (alphaS) is a presynaptic protein implicated in Parkinson's disease (PD). Growing evidence implicates mitochondrial dysfunction, oxidative stress, and alphaS-lipid interactions in the gradual accumulation of alphaS in pathogenic forms and its deposition in Lewy bodies, the pathological hallmark of PD and related synucleinopathies. The peroxisomal biogenesis disorders (PBD), with Zellweger syndrome serving as the prototype of this group, are characterized by malformed and functionally impaired peroxisomes. Here we utilized the PBD mouse models Pex2-/-, Pex5-/-, and Pex13-/- to study the potential effects of peroxisomal dysfunction on alphaS-related pathogenesis. We found increased alphaS oligomerization and phosphorylation and its increased deposition in cytoplasmic inclusions in these PBD mouse models. Furthermore, we show that alphaS abnormalities correlate with the altered lipid metabolism and, specifically, with accumulation of long chain, n-6 polyunsaturated fatty acids that occurs in the PBD models.

Retinal very long-chain PUFAs: new insights from studies on ELOVL4 protein

Compared with other mammalian tissues, retina is highly enriched in PUFA. Long-chain PUFA (LC-PUFA; C18-C24) are essential FAs that are enriched in the retina and are necessary for maintenance of normal retinal development and function. The retina, brain, and sperm also contain very LC-PUFA (VLC-PUFA; >C24). Although VLC-PUFA were discovered more than two decades ago, very little is known about their biosynthesis and functional roles in the retina. This is due mainly to intrinsic difficulties associated with working on these unusually long polyunsaturated hydrocarbon chains and their existence in small amounts. Recent studies on the FA elongase elongation of very long chain fatty acids-4 (ELOVL4) protein, however, suggest that VLC-PUFA probably play some uniquely important roles in the retina as well as the other tissues. Mutations in the ELOVL4 gene are found in patients with autosomal dominant Stargardt disease. Here, we review the recent literature on VLC-PUFA with special emphasis on the elongases responsible for their synthesis. We focus on a novel elongase, ELOVL4, involved in the synthesis of VLC-PUFA, and the importance of these FAs in maintaining the structural and functional integrity of retinal photoreceptors.

20:5n-3 but not 22:6n-3 is a preferred substrate for synthesis of n-3 very-long- chain fatty acids (C24-C36) in retina

The objective of this study was to determine if 20:5n-3 or 22:6n-3 is the primary precursor of very-long-chain fatty acids (VLCFAs; C24-C36) synthesized in retina. Rats were fed semisynthetic, nutritionally complete diet containing 20% (w/w) fat with 3% (w/w) of 22:6n-3. After 6 weeks feeding, the vitreal fluid of each eye was injected with [3H]20:5n-3 or [3H]22:6n-3. Rats were then maintained under constant light (330 lux) or dark conditions for 48 hr. After 48 hr in vivo metabolism, the amount of label present in individual fatty acids was determined in major phospholipids in retina. For [3H]22:6n-3, 90% of total incorporation remained in 22:6n-3, whereas for [3H]20:5n-3 the label was actively incorporated into pentaenoic and hexaenoic VLCFAs up to 34 carbon chain length. 22:5n-3 derived from [3H]20:5n-3 was among the most highly labeled fatty acids. These observations suggest that 22:6n-3 is incorporated directly into retinal phospholipids without further metabolism, whereas 20:5n-3 and 22:5n-3 are metabolically active precursors for synthesis of VLCFAs.

Dietary 20:4n-6 and 22:6n-3 modulates the profile of long- and very-long-chain fatty acids, rhodopsin content, and kinetics in developing photoreceptor cells

The objective of this study was to determine whether addition of dietary 20:4n-6 and 22:6n-3 to a conventional infant formula fat blend influences membrane long-chain and very-long-chain fatty acid composition, rhodopsin content, and rhodopsin kinetics in developing rat photoreceptor cells. The dietary fats were formulated based on the fat composition of a conventional infant formula providing an 18:2n-6/18:3n-3 ratio of 7:1 (SMA, Wyeth Nutritionals), which served as the control fat blend. This dietary fat blend was modified to contain 20:4n-6 [arachidonic acid (AA)], 22:6n-3 [docosahexaenoic acid (DHA)], AA + DHA, or an 18:2n-6/18:3n-3 ratio of 4:1 (alpha-linolenic acid). Dams were fed diets from birth, and rat pups were fed the same diet after weaning. Retinas and rod outer segments were prepared in the dark from pups at 2, 3, and 6 wk of age for fatty acid analysis of individual phospholipids, rhodopsin content, and rhodopsin disappearance kinetics after light exposure. Feeding AA + DHA in the diet increased 22:6n-3 levels in phosphatidylcholine and phosphatidylethanolamine. In phosphatidylcholine, total n-6 tetraenoic very-long-chain fatty acids and total n-3 pentaenoic and n-3 hexaenoic very-long-chain fatty acids increased after feeding AA and DHA, respectively. Developmental changes were characterized by a decrease in 20:4n-6 in the major phospholipids, whereas 22:6n-3 increased with age in rod outer segments. The highest rhodopsin content occurred in the retina of rats fed diets containing AA and/or DHA. The kinetics of rhodopsin disappearance after light exposure was highest in rats fed DHA at 6 wk of age. This study demonstrates that small manipulations of the dietary level of 20:4n-6 and 22:6n-3 are important determinants of fatty acid composition of membrane lipid and visual pigment content and kinetics in the developing photoreceptor cell.

Very long chain fatty acids in higher animals--a review

Fatty acids with greater than 22 carbon atoms (very long chain fatty acids, VLCFA) are present in small amounts in most animal tissues. Saturated and monoenoic VLCFA are major components of brain, while the polyenoic VLCFA occur in significant amounts in certain specialized animal tissues such as retina and spermatozoa. Biosynthesis of VLCFA occurs by carbon chain elongation of shorter chain fatty acid precursors while beta-oxidation takes place almost exclusively in peroxisomes. Mitochondria are unable to oxidize VLCFA because they lack a specific VLCFA coenzyme A synthetase, the first enzyme in the beta-oxidation pathway. VLCFA accumulate in the tissues of patients with inherited abnormalities in peroxisomal assembly, and also in individuals with defects in enzymes catalyzing individual reactions along the beta-oxidation pathway. It is believed that the accumulation of VLCFA in patient tissues contributes to the severe pathological changes which are a feature of these conditions. However, little is known of the role of VLCFA in normal cellular processes, and of the molecular basis for their contribution to the disease process. The present review provides an outline of the current knowledge of VLCFA including their biosynthesis, degradation, possible function and involvement in human disease.

Differences in the metabolism of eicosatetraenoic (20:4(n - 6)), tetracosatetraenoic (24:4(n - 6)) and triacontatetraenoic (30:4(n - 6)) acids in human neutrophils

The metabolism of [1-14C]eicosatetraenoic (arachidonic, 20:4(n - 6)), [1-14C]tetracosatetraenoic (24:4(n - 6)) and [1-14C]triacontatetraenoic (30:4(n - 6)) acids was studied in intact human neutrophils. [1-14C]20:4(n - 6) and [1-14C]24:4(n - 6) were efficiently taken up by the neutrophils, esterified into neutral lipids and phospholipids, and elongated by up to four carbon units. In contrast, [1-14C]30:4(n - 6) was poorly incorporated into the cells and remained predominantly in the original unesterified form. The [1-14C]tetraenoic fatty acids were mainly esterified into triacylglycerol, suggesting that this lipid class is important in the intracellular trafficking of polyunsaturated fatty acids. The leukocytes demonstrated a low capacity to beta-oxidize and desaturate the fatty acid substrates. In the presence of calcium ionophore A23187 the neutrophils converted [1-14C]20:4(n - 6) to a variety of radiolabelled oxygenated fatty acid derivatives including prostaglandins, thromboxanes, mono- and dihydroxylated fatty acids and leukotrienes. The major eicosanoid products were 5-monohydroxy-20:4(n - 6) and leukotriene B4. In contrast, [1-14C]24:4(n - 6) was metabolized to radiolabelled monohydroxylated fatty acids (predominantly the 9-hydroxy positional isomer) but not to other lipoxygenase or cyclooxygenase products by the calcium ionophore-stimulated cells. Negligible oxygenated fatty acid compounds were formed from [1-14C]30:4(n - 6), indicating that it is a poor substrate for the neutrophil cyclooxygenase and lipoxygenase enzymes.

Very long-chain fatty acids in peroxisomal disease

Fatty acids with from 24 to 28 carbon atoms (very long-chain fatty acids, VLCFA) are present in small amounts in all mammalian tissues. Even longer chain fatty acids with from 30 to 38 carbon atoms (ultra-long-chain fatty acids, ULCFA) are found in certain specialized tissues including retina, brain, and spermatozoa. In patients with inherited defects in peroxisomal structure and/or function, there is an accumulation of VLCFA in most tissues, while VLCFA and ULCFA levels are increased in brain. The most pronounced changes occur in those patients who have defects in peroxisomal assembly (Zellweger syndrome, infantile Refsum's disease, and neonatal adrenoleukodystrophy). In the brain of these individuals, ULCFA are distributed largely in molecular species of phosphatidylcholine with penta-, hexa-, and heptaenoic acids. In contrast, patients with X-linked adrenoleukodystrophy have increased levels of phosphatidylcholine with monoenoic rather than polyenoic ULCFA. A defect in a peroxisomal VLCFA CoA synthetase or ligase has been reported for these patients, but assembly of their peroxisomes is apparently normal. We speculate that ULCFA are normal products of carbon chain elongation. We have confirmed this by demonstrating the elongation of [1-14C]hexacosatetraenoic acid (26:4n-6) by rat brain in vivo to a series of longer chain tetraenoic acids with carbon chain lengths up to 34. Elongation to ULCFA can occur as well in non-neural tissues as shown by detection of labeled saturated and monoenoic fatty acids with up to 32 carbon atoms after incubation of normal and Zellweger syndrome fibroblasts with [2-14C] acetate. Increased labeling of VLCFA and ULCFA is observed in cells from patients with peroxisomal disorders. Our data suggest that ULCFA with up to at least 32 carbon atoms are formed normally, as a part of the elongation process in most mammalian tissues, and that control of carbon chain elongation is a major function of peroxisomes. Impairment of this function as occurs in peroxisomal disease results in the accumulation of VLCFA and ULCFA. The relative enrichment in normal tissues of ULCFA such as 32:6n-3 in ram and bull spermatozoa and 36:4n-6 in human and rat brain suggests a probable physiological role for this class of fatty acids in these tissues.