Unique molecular species composition of glycerolipids of frog rod outer segments

HTRA1 synergizes with oxidized phospholipids in promoting inflammation and macrophage infiltration essential for ocular VEGF expression

Understanding oxidative stress and HTRA1 locus in abnormal angiogenesis resulting in wet AMD pathology is an important step in developing a novel therapeutic approach. Using subretinal injection of oxLDL into C57BL/6 mice, we observed a lesion resembling the features of choroidal neovascularization (CNV), including macrophage infiltration, increased VEGF expression, and neovascularization. However, incubating ARPE-19 cells with oxLDL–a carrier of oxidized phospholipids–resulted in increased expression of inflammatory cytokines and chemoattractant proteins that recruited monocytes, but no substantial increase in expression of VEGF. Furthermore, incubation of ARPE-19 with oxLDL induced higher expression of HTRA1, which we showed to synergize with oxLDL in elevating the expression of inflammatory cytokines and chemoattractant factors. To investigate the role of macrophage infiltration on these expression changes, we treated cultured J774 macrophages with oxLDL and applied the conditioned medium onto ARPE-19 cells. This treatment was found to greatly enhance the expression of VEGF in ARPE-19, indicating the necessity of macrophage secretory products to induce increased expression of VEGF in retinal pigment epithelium. Gene expression analysis revealed that oxLDL induced the expression of Wnt3A in macrophages, a key activator of canonical Wnt signaling pathways. In addition, western blot analysis showed that the macrophage conditioned media further enhanced the reduction of phosphorylated β-catenin induced by oxLDL. Lastly, we investigated HTRA1 as a potential target for AMD therapeutics. We demonstrated the ability of anti-HTRA1 antibody in vitro to neutralize the protease activity of HTRA1 and reduce the inflammatory and angiogenic response to oxidative stress. Finally, we validated the neutralizing effect of anti-HTRA1 antibody in vivo by evaluating lesion size and protein expression in a laser-photocoagulation murine model of CNV. We found that the combination of oxLDL and HTRA1 enhanced CNV size, which was reversed by the addition of anti-HTRA1 antibody. This study not only provides preliminary evidence that HTRA1 may be a viable target for AMD therapeutics but also elucidates the biochemical mechanisms by which this therapeutic effect may be mediated.

Survival signaling in retinal pigment epithelial cells in response to oxidative stress: significance in retinal degenerations

Photoreceptor survival depends on the integrity of retinal pigment epithelial (RPE) cells. The pathophysiology of several retinal degenerations involves oxidative stress-mediated injury and RPE cell death; in some instances it has been shown that this event is mediated by A2E and its epoxides. Photoreceptor outer segments display the highest DHA content of any cell type. RPE cells are active in DHA uptake, conservation, and delivery. Delivery of DHA to photoreceptor inner segments is mediated by the interphotoreceptor matrix. DHA is necessary for photoreceptor function and at the same time is a target of oxidative stress-mediated lipid peroxidation. It has not been clear whether specific mediators generated from DHA contribute to its biological properties. Using ARPE-19 cells, we demonstrated the synthesis of 10,17S-docosatriene [neuroprotectin Dl (NPDI)]. This synthesis was enhanced by the calcium ionophore A-23187, by IL-1 3P, or by supplying DHA. Added NPD1 (50nM) potently counteracted H2O2/tumor necrosis factor-alpha oxidative stress-triggered apoptotic DNA damage in RPE. NPD1 also up-regulated the anti-apoptotic proteins Bcl-2 and Bcl-xL and decreased pro-apoptotic Bax and Bad expression. Moreover, NPD1 (50nM) inhibited oxidative stress-induced caspase-3 activation. NPD1 also inhibited IL-1beta-stimulated expression of COX-2. Furthermore, A2E-triggered oxidative stress induction of RPE cell apoptosis was also attenuated by NPD1. Overall, NPD1 protected RPE cells from oxidative stress-induced apoptosis. In conclusion, we have demonstrated an additional function of the RPE: its capacity to synthesize NPD1. This new survival signaling is potentially of interest in the understanding of the pathophysiology of retinal degenerations and in exploration of new therapeutic modalities.

Cell survival matters: docosahexaenoic acid signaling, neuroprotection and photoreceptors

Recent data have provided important clues about the molecular mechanisms underlying certain retinal degenerative diseases, including retinitis pigmentosa and age-related macular degeneration. Photoreceptor cell degeneration is a feature common to these diseases, and the death of these cells in many instances seems to involve the closely associated retinal pigment epithelial (RPE) cells. Under normal circumstances, both cell types are subject to potentially damaging stimuli (e.g. sunlight and high oxygen tension). However, the mechanism or mechanisms by which homeostasis is maintained in this part of the eye, which is crucial for sight, are an unsolved riddle. The omega-3 fatty acid family member docosahexaenoic acid (DHA), which is enriched in these cells, is the precursor of neuroprotectin D1 (NPD1). NPD1 inhibits oxidative-stress-mediated proinflammatory gene induction and apoptosis, and consequently promotes RPE cell survival. This enhanced understanding of the molecular basis of endogenous anti-inflammatory and neuroprotective signaling in the RPE presents an opportunity for the development of therapies for retinal degenerative diseases.

Neuroprotectin D1 (NPD1): a DHA-derived mediator that protects brain and retina against cell injury-induced oxidative stress

The biosynthesis of oxygenated arachidonic acid messengers triggered by cerebral ischemia-reperfusion is preceded by an early and rapid phospholipase A2 activation reflected in free arachidonic and docosahexaenoic acid (DHA) accumulation. These fatty acids are released from membrane phospholipids. Both fatty acids are derived from dietary essential fatty acids; however, only DHA, the omega-3 polyunsaturated fatty acyl chain, is concentrated in phospholipids of various cells of brain and retina. Synaptic membranes and photoreceptors share the highest content of DHA of all cell membranes. DHA is involved in memory formation, excitable membrane function, photoreceptor cell biogenesis and function, and neuronal signaling, and has been implicated in neuroprotection. In addition, this fatty acid is required for retinal pigment epithelium cell (RPE) functional integrity. Here we provide an overview of the recent elucidation of a specific mediator generated from DHA that contributes at least in part to its biological significance. In oxidative stress-challenged human RPE cells and rat brain undergoing ischemia-reperfusion, 10,17S-docosatriene (neuroprotectin D1, NPD1) synthesis evolves. In addition, calcium ionophore A23187, IL-1beta, or the supply of DHA enhances NPD1 synthesis. A time-dependent release of endogenous free DHA followed by NPD1 formation occurs, suggesting that a phospholipase A2 releases the mediator's precursor. When NPD1 is infused during ischemia-reperfusion or added to RPE cells during oxidative stress, apoptotic DNA damage is down-regulated. NPD1 also up-regulates the anti-apoptotic Bcl-2 proteins Bcl-2 and BclxL and decreases pro-apoptotic Bax and Bad expression. Moreover, NPD1 inhibits oxidative stress-induced caspase-3 activation. NPD1 also inhibits IL-1beta-stimulated expression of COX-2. Overall, NPD1 protects cells from oxidative stress-induced apoptosis. Because photoreceptors are progressively impaired after RPE cell damage in retinal degenerative diseases, understanding of how these signals contribute to retinal cell survival may lead to the development of new therapeutic strategies. Moreover, NPD1 bioactivity demonstrates that DHA is not only a target of lipid peroxidation, but rather is the precursor to a neuroprotective signaling response to ischemia-reperfusion, thus opening newer avenues of therapeutic exploration in stroke, neurotrauma, spinal cord injury, and neurodegenerative diseases, such as Alzheimer disease, aiming to up-regulate this novel cell-survival signaling.

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.

Phosphoinositides, ezrin/moesin, and rac1 regulate fusion of rhodopsin transport carriers in retinal photoreceptors

The post-Golgi trafficking of rhodopsin in photoreceptor cells is mediated by rhodopsin-bearing transport carriers (RTCs) and regulated by the small GTPase rab8. In this work, we took a combined pharmacological-proteomic approach to uncover new regulators of RTC trafficking toward the specialized light-sensitive organelle, the rod outer segment (ROS). We perturbed phospholipid synthesis by activating phospholipase D with sphingosine 1-phosphate (S1P) or inhibiting phosphatidic acid phosphohydrolase by propranolol (Ppl). S1P stimulated the overall rate of membrane trafficking toward the ROS. Ppl stimulated budding of RTCs, but blocked membrane delivery to the ROS. Ppl caused accumulation of RTCs in the vicinity of the fusion sites, suggesting a defect in tethering, similar to the previously described phenotype of the rab8T22N mutant. Proteomic analysis of RTCs accumulated upon Ppl treatment showed a significant decrease in phosphatidylinositol-4,5-bisphosphate-binding proteins ezrin and/or moesin. Ppl induced redistribution of moesin, actin and the small GTPase rac1 from RTCs into the cytosol. By confocal microscopy, ezrin/moesin and rac1 colocalized with rab8 on RTCs at the sites of their fusion with the plasma membrane; however, this distribution was lost upon Ppl treatment. Our data suggest that in photoreceptors phosphatidylinositol-4,5-bisphosphate, moesin, actin, and rac1 act in concert with rab8 to regulate tethering and fusion of RTCs. Consequentially, they are necessary for rhodopsin-laden membrane delivery to the ROS, thus controlling the critical steps in the biogenesis of the light-detecting organelle.

Docosahexaenoic, arachidonic, palmitic, and oleic acids are differentially esterified into phospholipids of frog retina

Docosahexaenoic acid (22:6n-3) is highly enriched in the retina. To determine if retinal cells take up and metabolize fatty acids in a specific manner, retinas from Rana pipiens were incubated for 3 h with an equimolar mixture of tritiated 22:6n-3, arachidonic acid (20:4n-6), palmitic acid, and oleic acid. The radiolabeling of retinal lipids was determined and compared to the endogenous fatty acid content of the lipids. The results showed that in most, but not all, cases, the relative labeling with the four precursor fatty acids was similar to their relative abundance in each glycerolipid. Thus, during retinal glycerolipid synthesis, either through de novo or acyl exchange reactions, fatty acids are incorporated in proportions reflecting their steady-state mass levels. Since other studies with labeled glycerol have shown greater differences between early labeling patterns and molecular species mass, the final incorporation we report may be due primarily to acyl exchange reactions.

Protection of photoreceptor cells in adult rats from light-induced degeneration by adaptation to bright cyclic light

Light history has been shown to affect the susceptibility of the albino rat retina to the damaging effects of constant light exposure. Retinas of animals raised in relatively bright cyclic light are protected against light-induced degeneration compared with dim-reared animals. These effects were observed in animals raised from birth in bright cyclic light and are part of an adaptive response that protects photoreceptors from stress-induced degeneration. To determine if retinas of adult animals are capable of such adaptive changes or flexibility by switching between different light environments which do not pathologically damage photoreceptor cells, albino rats were maintained in less than 250 lux cyclic light for more than 3 weeks. At 12-13 weeks of age, they were placed into 800 lux cyclic light for 1 week, after which they were exposed to constant illumination of 1500-lux for 1, 3 or 7 days. Retinal function was evaluated by electroretinography and photoreceptor cell death was quantified by measuring outer nuclear layer thickness. After 1 week in bright cyclic light, the retinas were completely protected against 1 day constant light exposure that significantly damaged retinas of animals without 800 lux cyclic light adaptation. Significant protection was also observed in 3 day constant light exposed animals; limited protection occurred after 7 days exposure. These results indicate that the retinas of adult rats adapted to bright cyclic light within certain ranges that did not significantly damage photoreceptor cells are protected from constant light challenge. This phenomenon is a post-developmental response that demonstrates a remarkable plasticity of the retina. The mechanism(s) underlying the ability of this adaptation/flexibility in protecting photoreceptors could involve endogenous molecules that encompass many aspects of retinal cell and molecular biology and physiology. Identification of these molecules may provide insight into the development of therapeutic approaches to treat retinal degeneration.

Strong association of unesterified [3H]docosahexaenoic acid and [3H-docosahexaenoyl]phosphatidate to rhodopsin during in vivo labeling of frog retinal rod outer segments

Docosahexaenoic acid (DHA, 22:6n-3), the most prevalent fatty acid in phospholipids of rod outer segments (ROS), is essential for visual transduction and daily renewal of ROS membranes. We investigated the association of [3H]DHA-lipids to rhodopsin in ROS from frogs (Rana pipiens) after in vitro (4 hrs) and in vivo (1 day and 32 days) labeling. Lipids from lyophilized ROS were sequentially extracted with hexane (neutral lipids), chloroform:methanol (phospholipids) and acidified chloroform:methanol (acidic phospholipids). After in vitro labeling, free [3H]DHA was easily extracted with hexane (66% of total ROS free DHA), implying a weak association with proteins (rhodopsin). In contrast, after in vivo labeling free [3H]DHA was mainly recovered in the acidic solvent extract (89-99%). Of all phospholipids, [3H-DHA]phosphatidic acid (PA) displayed the highest binding to rhodopsin after both in vitro (43% in acidic extract) and in vivo (>70%) labeling suggesting a possible modulatory role of free DHA and DHA-PA in visual transduction.

Rhodopsin trafficking and its role in retinal dystrophies

We review the sorting/targeting steps involved in the delivery of rhodopsin to the outer segment compartment of highly polarized photoreceptor cells. The transport of rhodopsin includes (1) the sorting/budding of rhodopsin-containing vesicles at the trans-Golgi network, (2) the directional translocation of rhodopsin-bearing vesicles through the inner segment, and (3) the delivery of rhodopsin across the connecting cilium to the outer segment. Several independent lines of evidence suggest that the carboxyl-terminal, cytoplasmic tail of rhodopsin is involved in the post-Golgi trafficking of rhodopsin. Inappropriate subcellular targeting of naturally occurring rhodopsin mutants in vivo leads to photoreceptor cell death. Thus, the genes encoding mutations in the cellular components involved in photoreceptor protein transport are likely candidate genes for retinal dystrophies.

The unique lipid composition of gecko (Gekko Gekko) photoreceptor outer segment membranes

This study investigated the lipid and fatty acid composition of gecko photoreceptor outer segment membranes which contain the P521 cone-type pigment. The lipids of gecko photoreceptor outer segment membranes were first extracted and separated by thin layer chromatography (TLC) and then analyzed by gas chromatography (GC). Our results show that gecko photoreceptor outer segment membranes contain less phosphatidylethanolamine (PE) and more phosphatidylcholine (PC) and phosphatidylserine (PS) compared with those of bovine and frog. The content of the polyunsaturated fatty acid, docosahexaenoic acid (DHA), in PC and PS is also the highest yet reported (55 and 63%, respectively). These lipid differences may provide some insight into the specific lipid requirements of cone-type pigments.

Molecular species analysis of phospholipids

The elucidation of phospholipid molecular species composition provides detailed structural information concerning various lipids and thus offers descriptions of crucial determinants of membrane physical and biological properties. Various methods differing in labor intensity, mode of separation and detection, type of calibration, as well as other factors, have been published. Thus precision and accuracy are expected to vary considerably between methods. Qualitative and quantitative aspects of different procedures for molecular species analysis of individual phospholipid classes are discussed in this review. Special emphasis has been given to the characterization of biological tissue samples.

Post-Golgi vesicles cotransport docosahexaenoyl-phospholipids and rhodopsin during frog photoreceptor membrane biogenesis

Post-Golgi vesicles budding from the trans-Golgi network (TGN) are involved in the vectorial transport and delivery of rhodopsin to photoreceptor rod outer segments (ROS). We report here that newly synthesized docosahexaenoyl (DHA) phospholipids are sequestered and cotransported by rhodopsin-bearing post-Golgi vesicles to ROS. Frog retinas were pulse-labeled with [35S]methionine/cysteine and [3H]DHA prior to ROS isolation and subcellular fractionation. After a 1-h pulse, relatively uniform [3H]DHA-lipid labeling (DPM/microg protein) was observed in all fractions enriched in post-Golgi vesicles, TGN, Golgi, and endoplasmic reticulum (ER) membranes. During the subsequent 2-h chase translocation of free [3H]DHA from ROS to the photoreceptor inner segment contributed to an additional overall increase in labeling of lipids. The specific activity (dpm/nmol DHA) in ER-enriched fraction was similar or higher than in other subcellular fractions after both the pulse and the chase, indicating that the bulk of [3H]DHA-lipids was synthesized in the ER. After the chase a 2-fold increase in labeling of lipids in the ER and Golgi and a 2.6-fold in lighter TGN-enriched fractions was observed. The highest labeling was in the post-Golgi vesicle fraction (4-fold increase), with [3H]DHA-phosphatidylcholine and [3H]DHA-phosphatidylethanolamine showing the greatest increase. At the same time, newly synthesized [35S]rhodopsin shifted from the ER and Golgi toward TGN and post-Golgi fractions. Therefore, sequestration and association of [35S]rhodopsin and [3H]DHA-lipids in a TGN membrane domain occurs prior to their exit and subsequent vectorial cotransport on post-Golgi vesicles to ROS. Labeling of ROS lipids was very low, with phosphatidylinositol and diacylglycerols displaying the highest labeling. This indicates that other mechanisms by-passing Golgi, i.e. facilitated by lipid carrier proteins, may also contribute to molecular replacement of disc membrane DHA-phospholipids, particularly phosphatidylinositol.

Review: pharmacological manipulation of docosahexaenoic-phospholipid biosynthesis in photoreceptor cells: implications in retinal degeneration

Docosahexaenoic acid (22:6n-3, DHA) is derived in vertebrate animals from n-3 fatty acids present in the diet (i.e., alpha-linolenic acid, 18:3n-3 and/or other n-3-long chain polyunsaturated fatty acids) and is found in very high concentrations in phospholipids from membranes of the central nervous system. Disk membranes of photoreceptor outer segments and synaptic terminals display a preferential enrichment in DHA-phospholipids that appears to be necessary for normal excitable membrane functions. Because of the relevance of adequate DHA-phospholipid synthesis and sorting toward new assembled disk membranes and synaptic terminals, as well as the pathophysiological implications of abnormal DHA metabolism (including its synthesis, delivery to the retina, and incorporation into lipids by de novo and turnover pathways), we reviewed recent studies of: a) the preferential uptake and retention of DHA by photoreceptors and its metabolism as it is activated to DHA-CoA and incorporated preferentially into phospholipids; b) pharmacological manipulations using amphiphilic cationic drugs (i.e., propranolol) to show an active esterification of DHA into lipids via de novo synthesis; and c) perturbations in DHA metabolism in retinas from dogs with progressive rod-cone degeneration (prcd).

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.

Metabolism of lipid molecular species in rat rod outer segments

We have studied the metabolism of selected diacyl molecular species of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) and diacylglycerol (DG) from rat rod outer segments (ROS). Rats were injected intravitreally with [2-3H]glycerol. At 1, 2, 3, 4 and 6 days post-injection, ROS phospholipids and DG were isolated by two-dimensional thin-layer chromatography (TLC), derivatized, and fractionated into molecular species by high-performance liquid chromatography (HPLC). Selected molecular species were quantitated and counted for radioactivity. We found the following. In PC and PE, the specific activities of 22:6-22:6, 18:1-22:6 and 16:0-22:6 were highest at day 1 and then decreased in a nearly exponential manner. In contrast, the specific activities of 18:0-22:6 and 18:0-20:4 were substantially lower than these three molecular species and changed little over the 6-day period. In PS, the specific activities of 22:6-22:6, 18:0-22:6 and 18:1-22:6 were similar and did not reach their maximum until the 3rd or 4th days. In PC, the specific activities of the five molecular species examined were two to three times higher at day 1 than the same species in PE and PS. In PI and DG, the major molecular species were 16:0-20:4 and 18:0-20:4. The specific activities of these two molecular species at day 1 were about ten times higher than those of 20:4-containing species in PE and PC, and showed the most rapid turnover of any of the molecular species examined in this study.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatty acid and molecular species compositions of phospholipids and diacylglycerols from rat retinal membranes

The relative proportions and compositions of the diacyl molecular species of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidic acid (PA), and diacylglycerol (DG) from rat retinal membranes were determined. Two membrane fractions were derived by discontinuous sucrose gradient floatation: rod outer segment (ROS) and 'rest of the retina' (ROR). ROR is defined as those membranes pelleted as 100,000 g following removal of the ROS. Diacylglycerols were prepared from PC, PE and PS by phospholipase C treatment and were converted into the corresponding 1,2-diacylglycerobenzoates (DGBZ). PI, PA, and DG were converted into 1,2-diacylglyceroacetates (DGAC) by acetolysis. The molecular species of the DGBZ and DGAC were resolved by reverse-phase HPLC and detected by UV absorption at 230 and 210 nm, respectively. Fatty acid methyl esters of PC, PE, PS, PI and DG from ROS and ROR were prepared and analysed by GLC. The fatty acid and molecular species patterns of PC, PE and PS were similar in both membrane fractions, although the levels of docosahexaenoic acid (22:6 omega 3) and 22:6-containing molecular species were lower in ROR than in ROS, PE and PS were enriched in 22:6 omega 3 and 18:0, an evidenced by the high levels of 18:0-22:6 and 22:6-22:6 molecular species. PC contained relatively more saturated and monoene species, such as 16:0-16:0, 16:0-18:0, 16:0-18:0, 16:0-18:1 and 18:0-18:1. The fatty acids and molecular species patterns of DG, PI and PA in ROS and ROR differed from those of PC, PE and PS.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphoinositide metabolism in frog rod outer segments

Previous studies have shown that vertebrate rod outer segments (ROS) have a light activated phospholipase C which hydrolyzes phosphatidylinositol-4,5-bisphosphonate (PIP2). Three different experimental approaches have been used to test the hypothesis that the phosphatidylinositol (PI) biosynthetic cycle is present in ROS and that PIP2 can be regenerated from DG independent of rod inner segments. In the first study, enzyme activities of the PI cycle were assayed simultaneously in the presence of CTP, myo-inositol and [gamma-32P]ATP using endogenous lipids as substrates. Under these conditions, broken (leaky) ROS prepared by continuous sucrose gradient centrifugation showed PI, PIP and DG kinase activities similar to those found in intact ROS and non-ROS membranes, whereas PI synthetase activity was much lower in the leaky ROS than in the other two fractions. The relative distribution of PI synthetase specific activity in the three membrane preparations was similar to that of the microsomal enzyme marker cytochrome c reductase. ROS prepared by discontinuous sucrose gradient centrifugation showed only 2-3% of whole homogenate PI synthetase or phosphatidyl: cytidyl transferase activities, and the distribution of activities was the same as for microsomal and mitochondrial marker enzymes. In the second study, whole retinas were incubated with myo-[2-3H]inositol or [2-3H]glycerol in vitro, and the time course of incorporation of radioactivity into PI and other phospholipids was determined for ROS and three other retinal fractions. Over a 10-hr period, the rate of incorporation of myo-[2-3H]inositol or [2-3H]glycerol into PI in ROS was lowest among the various retinal fractions. In the third study, chemical analysis of the molecular species composition of PI, DG and phosphatidic acid (PA) from ROS shows that PA is substantially different from PI and DG, the latter two being quite similar. These results are consistent with a precursor-product relationship between PI and DG, but not with the conversion of DG to PA or of PA to PI. Taken together, these three studies indicate that ROS do not have PI synthetase or phosphatidyl: cytidyl transferase activities, but do have DG, PI and PIP kinase activities. Thus, the PI in ROS lost through rapid turnover must be replaced with molecules derived from de novo synthesis in the inner segment of the photoreceptor cell.