Metabolism of linolenic acid and docosahexaenoic acid in rat retinas and rod outer segments

Understanding the Impact of Polyunsaturated Fatty Acids on Age-Related Macular Degeneration: A Review

Age-related Macular Degeneration (AMD) is a multifactorial ocular pathology that destroys the photoreceptors of the macula. Two forms are distinguished, dry and wet AMD, with different pathophysiological mechanisms. Although treatments were shown to be effective in wet AMD, they remain a heavy burden for patients and caregivers, resulting in a lack of patient compliance. For dry AMD, no real effective treatment is available in Europe. It is, therefore, essential to look for new approaches. Recently, the use of long-chain and very long-chain polyunsaturated fatty acids was identified as an interesting new therapeutic alternative. Indeed, the levels of these fatty acids, core components of photoreceptors, are significantly decreased in AMD patients. To better understand this pathology and to evaluate the efficacy of various molecules, in vitro and in vivo models reproducing the mechanisms of both types of AMD were developed. This article reviews the anatomy and the physiological aging of the retina and summarizes the clinical aspects, pathophysiological mechanisms of AMD and potential treatment strategies. In vitro and in vivo models of AMD are also presented. Finally, this manuscript focuses on the application of omega-3 fatty acids for the prevention and treatment of both types of AMD.

Dysfunctional peroxisomal lipid metabolisms and their ocular manifestations

Retina is rich in lipids and dyslipidemia causes retinal dysfunction and eye diseases. In retina, lipids are not only important membrane component in cells and organelles but also fuel substrates for energy production. However, our current knowledge of lipid processing in the retina are very limited. Peroxisomes play a critical role in lipid homeostasis and genetic disorders with peroxisomal dysfunction have different types of ocular complications. In this review, we focus on the role of peroxisomes in lipid metabolism, including degradation and detoxification of very-long-chain fatty acids, branched-chain fatty acids, dicarboxylic acids, reactive oxygen/nitrogen species, glyoxylate, and amino acids, as well as biosynthesis of docosahexaenoic acid, plasmalogen and bile acids. We also discuss the potential contributions of peroxisomal pathways to eye health and summarize the reported cases of ocular symptoms in patients with peroxisomal disorders, corresponding to each disrupted peroxisomal pathway. We also review the cross-talk between peroxisomes and other organelles such as lysosomes, endoplasmic reticulum and mitochondria.

Fatty acid oxidation and photoreceptor metabolic needs

Photoreceptors have high energy demands and a high density of mitochondria that produce ATP through oxidative phosphorylation (OXPHOS) of fuel substrates. Although glucose is the major fuel for CNS brain neurons, in photoreceptors (also CNS), most glucose is not metabolized through OXPHOS but is instead metabolized into lactate by aerobic glycolysis. The major fuel sources for photoreceptor mitochondria remained unclear for almost six decades. Similar to other tissues (like heart and skeletal muscle) with high metabolic rates, photoreceptors were recently found to metabolize fatty acids (palmitate) through OXPHOS. Disruption of lipid entry into photoreceptors leads to extracellular lipid accumulation, suppressed glucose transporter expression, and a duel lipid/glucose fuel shortage. Modulation of lipid metabolism helps restore photoreceptor function. However, further elucidation of the types of lipids used as retinal energy sources, the metabolic interaction with other fuel pathways, as well as the cross-talk among retinal cells to provide energy to photoreceptors is not fully understood. In this review, we will focus on the current understanding of photoreceptor energy demand and sources, and potential future investigations of photoreceptor metabolism.

Polyunsaturated Fatty Acid Biosynthesis Involving Δ8 Desaturation and Differential DNA Methylation of FADS2 Regulates Proliferation of Human Peripheral Blood Mononuclear Cells

Polyunsaturated fatty acids (PUFAs) are important for immune function. Limited evidence indicates that immune cell activation involves endogenous PUFA synthesis, but this has not been characterised. To address this, we measured metabolism of 18:3n-3 in quiescent and activated peripheral blood mononuclear cells (PBMCs), and in Jurkat T cell leukaemia. PBMCs from men and women (n = 34) were incubated with [1-¹³C]18:3n-3 with or without Concanavalin A (Con. A). 18:3n-3 conversion was undetectable in unstimulated PBMCs, but up-regulated when stimulated. The main products were 20:3n-3 and 20:4n-3, while 18:4n-3 was undetectable, suggesting initial elongation and Δ8 desaturation. PUFA synthesis was 17.4-fold greater in Jurkat cells than PBMCs. The major products of 18:3n-3 conversion in Jurkat cells were 20:4n-3, 20:5n-3, and 22:5n-3. ¹³C Enrichment of 18:4n-3 and 20:3n-3 suggests parallel initial elongation and Δ6 desaturation. The FADS2 inhibitor SC26196 reduced PBMC, but not Jurkat cell, proliferation suggesting PUFA synthesis is involved in regulating mitosis in PBMCs. Con. A stimulation increased FADS2, FADS1, ELOVL5 and ELOVL4 mRNA expression in PBMCs. A single transcript corresponding to the major isoform of FADS2, FADS20001, was detected in PBMCs and Jurkat cells. PBMC activation induced hypermethylation of a 470bp region in the FADS2 5'-regulatory sequence. This region was hypomethylated in Jurkat cells compared to quiescent PBMCs. These findings show that PUFA synthesis involving initial elongation and Δ8 desaturation is involved in regulating PBMC proliferation and is regulated via transcription possibly by altered DNA methylation. These processes were dysregulated in Jurkat cells. This has implications for understanding the regulation of mitosis in normal and transformed lymphocytes.

Synthesis of docosahexaenoic acid from eicosapentaenoic acid in retina neurons protects photoreceptors from oxidative stress

Oxidative stress is involved in activating photoreceptor death in several retinal degenerations. Docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, protects cultured retina photoreceptors from apoptosis induced by oxidative stress and promotes photoreceptor differentiation. Here, we investigated whether eicosapentaenoic acid (EPA), a metabolic precursor to DHA, had similar effects and whether retinal neurons could metabolize EPA to DHA. Adding EPA to rat retina neuronal cultures increased opsin expression and protected photoreceptors from apoptosis induced by the oxidants paraquat and hydrogen peroxide (H2 O2 ). Palmitic, oleic, and arachidonic acids had no protective effect, showing the specificity for DHA. We found that EPA supplementation significantly increased DHA percentage in retinal neurons, but not EPA percentage. Photoreceptors and glial cells expressed Δ6 desaturase (FADS2), which introduces the last double bond in DHA biosynthetic pathway. Pre-treatment of neuronal cultures with CP-24879 hydrochloride, a Δ5/Δ6 desaturase inhibitor, prevented EPA-induced increase in DHA percentage and completely blocked EPA protection and its effect on photoreceptor differentiation. These results suggest that EPA promoted photoreceptor differentiation and rescued photoreceptors from oxidative stress-induced apoptosis through its elongation and desaturation to DHA. Our data show, for the first time, that isolated retinal neurons can synthesize DHA in culture. Docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in retina photoreceptors, and its precursor, eicosapentaenoic acid (EPA) have multiple beneficial effects. Here, we show that retina neurons in vitro express the desaturase FADS2 and can synthesize DHA from EPA. Moreover, addition of EPA to these cultures protects photoreceptors from oxidative stress and promotes their differentiation through its metabolization to DHA.

The ins and outs of cholesterol in the vertebrate retina

The vertebrate retina has multiple demands for utilization of cholesterol and must meet those demands either by synthesizing its own supply of cholesterol or by importing cholesterol from extraretinal sources, or both. Unlike the blood-brain barrier, the blood-retina barrier allows uptake of cholesterol from the circulation via a lipoprotein-based/receptor-mediated mechanism. Under normal conditions, cholesterol homeostasis is tightly regulated; also, cholesterol exists in the neural retina overwhelmingly in unesterified form, and sterol intermediates are present in minimal to negligible quantities. However, under certain pathological conditions, either due to an inborn error in cholesterol biosynthesis or as a consequence of exposure to selective inhibitors of enzymes in the cholesterol pathway, the ratio of sterol intermediates to cholesterol in the retina can rise dramatically and persist, in some cases resulting in progressive degeneration that significantly compromises the structure and function of the retina. Although the relative contributions of de novo synthesis versus extraretinal uptake are not yet known, herein we review what is known about these processes and the dynamics of cholesterol in the vertebrate retina and indicate some future avenues of research in this area.

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.

A cholesterol-enriched diet induces ultrastructural changes in retinal and macroglial rabbit cells

The purpose of this study was to ascertain whether the excess of cholesterol in rabbits induces ultrastructural retinal changes similar to those observed in human age-related macular degeneration (AMD). New Zealand rabbits were divided into two groups: Control (GO; n=10), fed standard diet for 8 months; hypercholesterolemic (G1; n=10), fed with 0.5% cholesterol-enriched diet for 8 months. Eyes were processed for transmission electron microscopy (TEM) and immunohistochemistry (anti-glial fibrillary acidic protein, GPAP). In comparison with GO, G1 exhibited alterations in all the retinal layers that were more intense in areas overlying altered retinal pigment epithelium (RPE). RPE changes showed no preferential location. In G1, Bruch's membrane was thicker as a result particle build-up in the collagen layers; the cytoplasm of RPE showed dense bodies, debris from cell membranes, vacuoles and numerous clumps of lipids; necrosis and apoptosis were detected in different retinal layers; Müller cells and astrocytes were reactive with instances of apoptosis and necrosis; some Müller cells filled up the empty spaces left by degenerated neurons in all retinal layers; some Müller cell nuclei were displaced to the nerve-fiber layer (NFL); epiretinal perivascular astrocytes contained drops of lipids; the NFL had very few astrocytes and the basal membranes of capillaries in the NFL was thicker. Excess cholesterol induces ultrastructural changes in the rabbit retina similar to those in human AMD. Given that lipid intake is most dependent on food composition, dietary regimen could help induce or prevent retinal disease.

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.

The role of omega-3 long-chain polyunsaturated fatty acids in health and disease of the retina

In this work we advance the hypothesis that omega-3 (omega-3) long-chain polyunsaturated fatty acids (LCPUFAs) exhibit cytoprotective and cytotherapeutic actions contributing to a number of anti-angiogenic and neuroprotective mechanisms within the retina. omega-3 LCPUFAs may modulate metabolic processes and attenuate effects of environmental exposures that activate molecules implicated in pathogenesis of vasoproliferative and neurodegenerative retinal diseases. These processes and exposures include ischemia, chronic light exposure, oxidative stress, inflammation, cellular signaling mechanisms, and aging. A number of bioactive molecules within the retina affect, and are effected by such conditions. These molecules operate within complex systems and include compounds classified as eicosanoids, angiogenic factors, matrix metalloproteinases, reactive oxygen species, cyclic nucleotides, neurotransmitters and neuromodulators, pro-inflammatory and immunoregulatory cytokines, and inflammatory phospholipids. We discuss the relationship of LCPUFAs with these bioactivators and bioactive compounds in the context of three blinding retinal diseases of public health significance that exhibit both vascular and neural pathology. How is omega-3 LCPUFA status related to retinal structure and function? Docosahexaenoic acid (DHA), a major dietary omega-3 LCPUFA, is also a major structural lipid of retinal photoreceptor outer segment membranes. Biophysical and biochemical properties of DHA may affect photoreceptor membrane function by altering permeability, fluidity, thickness, and lipid phase properties. Tissue DHA status affects retinal cell signaling mechanisms involved in phototransduction. DHA may operate in signaling cascades to enhance activation of membrane-bound retinal proteins and may also be involved in rhodopsin regeneration. Tissue DHA insufficiency is associated with alterations in retinal function. Visual processing deficits have been ameliorated with DHA supplementation in some cases. What evidence exists to suggest that LCPUFAs modulate factors and processes implicated in diseases of the vascular and neural retina? Tissue status of LCPUFAs is modifiable by and dependent upon dietary intake. Certain LCPUFAs are selectively accreted and efficiently conserved within the neural retina. On the most basic level, omega-3 LCPUFAs influence retinal cell gene expression, cellular differentiation, and cellular survival. DHA activates a number of nuclear hormone receptors that operate as transcription factors for molecules that modulate reduction-oxidation-sensitive and proinflammatory genes; these include the peroxisome proliferator-activated receptor-alpha (PPAR-alpha) and the retinoid X receptor. In the case of PPAR-alpha, this action is thought to prevent endothelial cell dysfunction and vascular remodeling through inhibition of: vascular smooth muscle cell proliferation, inducible nitric oxide synthase production, interleukin-1 induced cyclooxygenase (COX)-2 production, and thrombin-induced endothelin 1 production. Research on model systems demonstrates that omega-3 LCPUFAs also have the capacity to affect production and activation of angiogenic growth factors, arachidonic acid (AA)-based vasoregulatory eicosanoids, and MMPs. Eicosapentaenoic acid (EPA), a substrate for DHA, is the parent fatty acid for a family of eicosanoids that have the potential to affect AA-derived eicosanoids implicated in abnormal retinal neovascularization, vascular permeability, and inflammation. EPA depresses vascular endothelial growth factor (VEGF)-specific tyrosine kinase receptor activation and expression. VEGF plays an essential role in induction of: endothelial cell migration and proliferation, microvascular permeability, endothelial cell release of metalloproteinases and interstitial collagenases, and endothelial cell tube formation. The mechanism of VEGF receptor down-regulation is believed to occur at the tyrosine kinase nuclear factor-kappa B (NFkappaB). NFkappaB is a nuclear transcription factor that up-regulates COX-2 expression, intracellular adhesion molecule, thrombin, and nitric oxide synthase. All four factors are associated with vascular instability. COX-2 drives conversion of AA to a number angiogenic and proinflammatory eicosanoids. Our general conclusion is that there is consistent evidence to suggest that omega-3 LCPUFAs may act in a protective role against ischemia-, light-, oxygen-, inflammatory-, and age-associated pathology of the vascular and neural retina.

Effects of docosahexaenoic acid on retinal development: cellular and molecular aspects

We have recently shown that docosahexaenoic acid (DHA) is necessary for survival and differentiation of rat retinal photoreceptors during development in vitro. In cultures lacking DHA, retinal neurons developed normally for 4 d; then photoreceptors selectively started an apoptotic pathway leading to extensive degeneration of these cells by day 11. DHA protected photoreceptors by delaying the onset of apoptosis; in addition, it advanced photoreceptor differentiation, promoting opsin expression and inducing apical differentiation in these neurons. DHA was the only fatty acid having these effects. Mitochondrial damage accompanied photoreceptor apoptosis and was markedly reduced upon DHA supplementation. This suggests that a possible mechanism of DHA-mediated photoreceptor protection might be the preservation of mitochondrial activity; a critical amount of DHA in mitochondrial phospholipids might be required for proper functioning of these organelles, which in turn might be essential to avoid cell death. Müller cells in culture appeared to be involved in DHA processing: they took up DHA, incorporated it into glial phospholipids, and channeled it to photoreceptors in coculture. Both Müller cells, when cocultured with neuronal cells, and the glial-derived neurotrophic factor (GDNF) protected photoreceptors from cell death. These results suggest that glial cells may play a central role in regulating photoreceptor survival during development through the provision of trophic factors. The multiple effects of DHA on photoreceptors suggest that, in addition to its structural role, DHA might be one of the trophic factors required by these cells.

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.

Phospholipid incorporation and metabolic conversion of n-3 polyunsaturated fatty acids in the Y79 retinoblastoma cell line

The metabolic conversion of n-3 fatty acids was studied in the human Y79 retinoblastoma cell line. Cultured cells were exposed to increasing concentrations of either 18:3n-3, 22:5n-3, or 22:6n-3, and their phospholipid fatty acid composition was analyzed after 72 hr. Cells internalized the supplemental fatty acids and proceeded to their metabolic conversion. Supplemental 22:6n-3 was directly esterified into cell phospholipids, at levels typical for normal neural retinas (41% by weight of phosphatidylethanolamine fatty acids, and 24% of phosphatidylcholine fatty acids). In contrast, 18:3n-3 was mainly converted to 20:5n-3 and 22:5n-3, both of which appeared in cell phospholipids after exposure to low external concentrations of 18:3n-3 (10 microg/ml). Y79 cells can proceed to the metabolic conversion of 18:3n-3 through elongation and Delta6- and Delta5-desaturation. When cells were exposed to high external concentrations of 18:3n-3 (30 microg/ml), the supplemental fatty acid was directly incorporated, and its relative content increased in both phospholipid classes to the detriment of all other n-3 fatty acids. Cells cultured in the presence of 22:5n-3 did not incorporate 22:6n-3 into their phospholipids but did incorporate 20:5n-3 and 22:5n-3. The data suggest that Y79 cells can proceed to the microsomal steps of n-3 metabolism, involving elongation, desaturation, and chain shortening of 22C fatty acids. Although Y79 cells avidly used supplemental 22:6n-3 for phospholipid incorporation at levels typical for normal photoreceptor cells, they failed to match such levels through metabolic conversion of n-3 parent fatty acids. The terminal step of the very long-chain polyunsaturated fatty acid synthesis, consisting in Delta6-desaturation followed by peroxisomal chain shortening of 24C-fatty acids, could be rate-limiting in Y79 cells.

Essential fatty acids in infant nutrition: lessons and limitations from animal studies in relation to studies on infant fatty acid requirements

Animal studies have been of pivotal importance in advancing knowledge of the metabolism and roles of n-6 and n-3 fatty acids and the effects of specific dietary intakes on membrane composition and related functions. Advantages of animal studies include the rigid control of fatty acid and other nutrient intakes and the degree, timing, and duration of deficiency or excess, the absence of confounding environmental and clinical variables, and the tissue analysis and testing procedures that cannot be performed in human studies. However, differences among species in nutrient requirements and metabolism and the severity and duration of the dietary treatment must be considered before extrapolating results to humans. Studies in rodents and nonhuman primates fed diets severely deficient in alpha-linolenic acid (18:3n-3) showed altered visual function and behavioral problems, and played a fundamental role by identifying neural systems that may be sensitive to dietary n-3 fatty acid intakes; this information has assisted researchers in planning clinical studies. However, whereas animal studies have focused mainly on 18:3n-3 deficiency, there is considerable clinical interest in docosahexaenoic acid (22:6n-3) and arachidonic acid (20:4n-6) supplementation. Information from animal studies suggests that brain and retinal concentrations of 22:6n-3 plateau with 18:3n-3 intakes of approximately 0.7% of energy, but this requirement is influenced by dietary 18:2n-6 intake. Blood and tissue concentrations of 22:6n-3 increase as 22:6n-3 intake increases, with adverse effects on growth and function at high intakes. Animal studies can provide important information on the mechanisms of both beneficial and adverse effects and the pathways of brain 22:6n-3 uptake.

Visual acuity and blood lipids in term infants fed human milk or formulae

This multicenter, parallel group study determined plasma phospholipid and red blood cell (RBC) phosphatidylcholine and phosphatidylethanolamine fatty acids, plasma cholesterol, apo A-1 and B, growth and visual acuity (using the acuity card procedure) in term infants fed from birth to 90 d of age with formula containing palm-olein, high oleic sunflower, coconut and soy oil (22.2% 16:0, 36.2% 18:1, 18% 18:2n-6, 1.9% 18:3n-3) (n = 59) or coconut and soy oil (10.3% 16:0 18:6% 18:1, 34.2% 18:2n-6, 4.7% 18:3n-3) (n = 57) or breast-fed (n = 56) with no formula supplementation. Different centers in North America were included to overcome potential bias due to differences in n-6 or n-3 fatty acids at birth or in breast-fed infants that might occur in a single-site study. Plasma and RBC phospholipid docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (AA, 20:4n-6), cholesterol and apo B were significantly lower in the formula- than breast-fed infants. There were no differences in looking acuity or growth among the breast-fed and formula-fed infants. No significant relations were found between DHA and looking acuity, or AA and growth within or among any of the infant groups. This study provides no evidence to suggest the formula provided inadequate n-6 or n-3 fatty acids for growth and looking acuity for the first 3 mon after birth.

Alterations in rabbit retina lipid metabolism induced by detachment. Decreased incorporation of [3H]DHA into phospholipids

BACKGROUND

Docosahexaenoic acid (22:6n-3, DHA) is found in high concentration in phospholipids from retinal membranes, and is essential for their function. This study investigated the effect of in vivo retinal detachment on in vitro lipid metabolism using [3H]DHA.

METHODS

Rabbit retina was detached from the retinal pigment epithelium by injecting physiological saline into the subretinal space of the eye. Retinal samples from control (non-operated) and sham (operated, no detachment) animals, and from attached and detached retinal areas from the same eye, were incubated in vitro with [3H]DHA for 4 hours, and then prepared for biochemical and autoradiographic analysis.

RESULTS

In control and sham retinas, [3H]DHA was preferentially esterified into phospholipids (82%) with low labeling of free fatty acids (FFA) (5%). In samples from detached areas of the retina, a higher proportion of [3H]DHA was recovered in the FFA pool (up to 30%) and its esterification was shunted into triacylglycerol, thereby reducing the formation of [3H]DHA-phospholipids. Changes were sustained through 48 hours of postdetachment. High labeling of inner segments and synaptic terminals was observed autoradiographically in control retinas, while in detached retinas, clusters of labeling were detected in the neural retina, and eventually within the photoreceptor layer.

CONCLUSION

Retinal detachment induces longlasting changes in lipid metabolism which are reflected in lower labeling of [3H]DHA-phospholipids. Metabolic changes, sustained through 48 hours, may lead to inadequate synthesis/turnover of phospholipids, among them, those containing DHA, possibly resulting in defective disc membrane assembly with subsequent deterioration of visual cells.

Abnormal plasma lipids of patients with Retinitis pigmentosa

Retinitis pigmentosa (RP) is a hereditary retinal degeneration of unknown etiology, resulting in progressive night blindness, loss of peripheral vision, abnormal retinal pigmentation and reduced electroretinographic response. Docosahexaenoic acid (22:6 omega 3) is found in high concentration in the rod outer segment membranes of the retina. Previous reports of low 22:6 omega 3 in blood lipids or phospholipids in RP patients prompted us to evaluate the complete fatty acid (FA) profiles of plasma phospholipids (PL), cholesteryl esters, triglycerides (TG) and nonesterified fatty acids (NEFA) in ten patients with RP. In the PL fraction, we found significantly depressed levels of 22:6 omega 3, 22:5 omega 3, total omega 3, 22:5 omega 6, 22:4 omega 6 and total omega 6 polyunsaturated FA (PUFA), and elevated total saturated acids. Plasma TG showed normal levels of PUFA, normal total saturated FA and total monounsaturated FA. The NEFA fraction showed significant elevation in total saturated FA with depressed total omega 6 PUFA. Evidence is accumulating mulating that RP is associated with abnormal PUFA and lipid metabolism.

Docosahexaenoic acid is taken up by the inner segment of frog photoreceptors leading to an active synthesis of docosahexaenoyl-inositol lipids: similarities in metabolism in vivo and in vitro

Retinal uptake and metabolism of docosahexaenoic acid (DHA) was studied in vivo in frogs 1, 2, and 6 hours after dorsal lymph sac injections of [3H]-DHA (50 microCi/g). Light microscope autoradiography and biochemical techniques were used to compare the profiles of cellular uptake and lipid labeling with those obtained from 6 hour [3H]-DHA retinal incubations (final DHA concentration, 0.11 and 25 microM). Light microscope autoradiography demonstrated that rod photoreceptor ellipsoids and synaptic terminals preferentially labeled both in vivo and in vitro conditions. Also, the cytoplasm and oil droplets of retinal pigment epithelial cells became very heavily labeled after 6 hours of in vivo labeling. Phosphatidic acid showed the highest labeling in one hour, while other phospholipids accumulated label throughout the 6 hours. At that time point, most label was recovered in phosphatidyl-ethanolamine (37%), phosphatidylcholine (27%), and phosphatidylinositol (16%), the latter displaying 1.6-fold higher labeling than phosphatidylserine. The profile of labeled lipids was similar to that obtained in vitro when the concentration of DHA was in the nanomolar range. Our results suggest that de novo lipid synthesis is a major route for esterification of [3H]-DHA into retinal lipids, giving rise to an early and rapid labeling of DHA-phosphatidylinositol, both in vivo and in vitro, when DHA is present at low concentrations. Furthermore, the profile of labeled retinal cells under in vivo conditions closely resembles in vitro DHA labeling.

Comparison of uptake and incorporation of docosahexaenoic and arachidonic acids by frog retinas

Vertebrate retinas, especially photoreceptor cellular membranes, contain high levels of docosahexaenoic acid (DHA) and relatively low levels of arachidonic acid (AA). The present study was designed to test the hypothesis that DHA enrichment in the retina is the result of preferential uptake and incorporation relative to other fatty acids. Frog retinas were incubated in vitro with [3H]DHA or [3h]AA for up to 6 h, and the amounts of label in retinal lipids were quantitated. The incorporation of DHA into total lipids, triglycerides, phosphatidylcholine, and phosphatidylethanolamine was similar to that of AA when each was presented as the only substrate, and was linear with fatty acid concentration and incubation time. The addition of excess, unlabeled AA reduced the uptake and incorporation of DHA into retinal lipids. A slightly greater inhibition was noted for the uptake and incorporation of AA in the presence of unlabeled DHA. There was about 2-3 fold greater incorporation of DHA into phosphatidic acid, diglycerides, and phosphatidylinositol compared with AA, whereas the reverse was found for phosphatidylserine. The different levels of DHA and AA in retinal phospholipids cannot be explained by different rates of uptake and incorporation of these fatty acids into lipids, although some slight enrichment of DHA may be possible by this mechanism. The linear incorporation with fatty acid concentration suggests that the difference could be accomplished by controlling the amount and type of fatty acids delivered to the retina by the adjacent pigment epithelium.

Enrichment of polyunsaturated fatty acids from rat retinal pigment epithelium to rod outer segments

Polyunsaturated fatty acids (PUFA), especially docosahexaenoic acid (DHA, 22:6n-3), are enriched in phospholipids of vertebrate rod outer segments (ROS). Retinal ROS can incorporate 22 carbon (C-22) PUFA from the plasma pool where C-20 PUFA are predominant. In this study, we analyzed the fatty acid composition of retinal pigment epithelium (RPE) and ROS from rats fed different fatty acid supplements to determine whether this enrichment is at the photoreceptor-RPE boundary or the RPE-choriocapillaris boundary. Long Evans rats were raised from birth for 13-14 weeks on a diet supplemented with 10% (wt/wt) hydrogenated coconut oil (COC; 0.2% 18:2n-6, no 18:3n-3), safflower oil (SAF; 73.8% 18:2n-6, 0.1% 18:3n-3), or linseed oil (LIN; 16.4% 18:2n-6, 52.2% 18:3n-3). These diets were chosen because they increased plasma levels of 20:3n-9, 20:4n-6, and 20:5n-3, respectively. These three fatty acids served as metabolic markers. Plasma levels of 22:6n-3 were reduced by the COC and SAF diets. The RPE incorporated 20:3n-9, 20:4n-6, and 20:5n-3 from the plasma. However, the levels of 20:3n-9 and 20:5n-3 were very low in ROS and 20:4n-6 was not significantly elevated in the ROS of the SAF diet group. The relative amount of total C-20 PUFA in phospholipids in RPE was similar to that found in plasma and was about 4-16 times (depending on different lipid classes) that in the ROS. In contrast, C-22 PUFA (22:6n-3 and 22:5n-6) showed a step-wise, average 3-5 fold increase in concentration from the plasma to the RPE to the ROS.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipids of frog retinal pigment epithelium: comparison with rod outer segments, retina, plasma and red blood cells

The glycerolipid and fatty acid compositions of frog retinal pigment epithelium (RPE) were determined and compared with rod outer segments (ROS), retina, plasma, and red blood cells (RBC). The glycerolipid class composition of RPE was similar to RBC and ROS or retina, with phosphatidylcholine and phosphatidylethanolamine being the major components. The fatty acid composition of RPE differed substantially from that of plasma or RBC; the former contained much higher levels of C-20 and C-22 polyunsaturated fatty acids (PUFAs), such as 20:4n-6 and 22:6n-3, but less C-18 mono-, dienoic, and trienoic acids. The difference between RPE and ROS or retina with respect to fatty acid profile was also dramatic; RPE had relatively less 22:6n-3, but more 20:4n-6 and 18:2n-6, than ROS or retina. These results suggest that frog RPE cells may selectively take up C-20 and C-22 PUFAs from the circulation, but preferentially deliver 22:6n-3 to the ROS and retina. Fatty acid analyses show that 20:4n-6 and 22:6n-3 were unevenly distributed among RPE glycerolipids; phosphatidic acid, diglyceride, triglyceride, and phosphatidylserine are relatively more enriched in 22:6n-3 compared with 20:4n-6. This information might imply that these two PUFAs are metabolized differently inside the frog RPE cells.