Biosynthesis of molecular species of inositol, choline, serine, and ethanolamine glycerophospholipids in the bovine retina

Overview of how N32 and N34 elovanoids sustain sight by protecting retinal pigment epithelial cells and photoreceptors

The essential fatty acid DHA (22:6, omega-3 or n-3) is enriched in and required for the membrane biogenesis and function of photoreceptor cells (PRCs), synapses, mitochondria, etc. of the CNS. PRC DHA becomes an acyl chain at the sn-2 of phosphatidylcholine, amounting to more than 50% of the PRC outer segment phospholipids, where phototransduction takes place. Very long chain PUFAs (n-3, ≥ 28 carbons) are at the sn-1 of this phosphatidylcholine molecular species and interact with rhodopsin. PRC shed their tips (DHA-rich membrane disks) daily, which in turn are phagocytized by the retinal pigment epithelium (RPE), where DHA is recycled back to PRC inner segments to be used for the biogenesis of new photoreceptor membranes. Here, we review the structures and stereochemistry of novel elovanoid (ELV)-N32 and ELV-N34 to be ELV-N32: (14Z,17Z,20R,21E,23E,25Z,27S,29Z)-20,27-dihydroxydo-triaconta-14,17,21,23,25,29-hexaenoic acid; ELV-N34: (16Z,19Z,22R,23E,25E,27Z,29S,31Z)-22,29-dihydroxytetra-triaconta-16,19,23,25,27,31-hexaenoic acid. ELVs are low-abundance, high-potency, protective mediators. Their bioactivity includes enhancing of antiapoptotic and prosurvival protein expression with concomitant downregulation of proapoptotic proteins when RPE is confronted with uncompensated oxidative stress. ELVs also target PRC/RPE senescence gene programming, the senescence secretory phenotype in the interphotoreceptor matrix, as well as inflammaging (chronic, sterile, low-grade inflammation). An important lesson on neuroprotection is highlighted by the ELV mediators that target the terminally differentiated PRC and RPE, sustaining a beautifully synchronized renewal process. The role of ELVs in PRC and RPE viability and function uncovers insights on disease mechanisms and the development of therapeutics for age-related macular degeneration, Alzheimer's disease, and other pathologies.

In-plate toxicometabolomics of single zebrafish embryos

Toxicometabolomic studies involving zebrafish embryos have become increasingly popular for linking apical endpoints to biochemical perturbations as part of adverse outcome pathway determination. These experiments involve pooling embryos to generate sufficient biomass for metabolomic measurement, which adds both time and cost. To address this limitation, we developed a high-throughput toxicometabolomic assay involving single zebrafish embryos. Incubation, microscopy, embryo extraction, and instrumental metabolomic analysis were all performed in the same 96-well plate, following acquisition of conventional toxicological endpoints. The total time for the assay (including testing of 6 doses/n = 12 embryos per dose plus positive and negative controls, assessing conventional endpoints, instrumental analysis, data processing and multivariate statistics) is <14 days. Metabolomic perturbations at low dose were linked statistically to those observed at high dose and in the presence of an adverse effect, thereby contextualizing omic data amongst apical endpoints. Overall, this assay enables collection of high resolution metabolomic data in a high throughput manner, suitable for mode of action hypothesis generation in the context of pharmaceutical or toxicological screening.

Docosahexaenoic acid signalolipidomics in nutrition: significance in aging, neuroinflammation, macular degeneration, Alzheimer's, and other neurodegenerative diseases

Essential polyunsaturated fatty acids (PUFAs) are critical nutritional lipids that must be obtained from the diet to sustain homeostasis. Omega-3 and -6 PUFAs are key components of biomembranes and play important roles in cell integrity, development, maintenance, and function. The essential omega-3 fatty acid family member docosahexaenoic acid (DHA) is avidly retained and uniquely concentrated in the nervous system, particularly in photoreceptors and synaptic membranes. DHA plays a key role in vision, neuroprotection, successful aging, memory, and other functions. In addition, DHA displays anti-inflammatory and inflammatory resolving properties in contrast to the proinflammatory actions of several members of the omega-6 PUFAs family. This review discusses DHA signalolipidomics, comprising the cellular/tissue organization of DHA uptake, its distribution among cellular compartments, the organization and function of membrane domains rich in DHA-containing phospholipids, and the cellular and molecular events revealed by the uncovering of signaling pathways regulated by DHA and docosanoids, the DHA-derived bioactive lipids, which include neuroprotectin D1 (NPD1), a novel DHA-derived stereoselective mediator. NPD1 synthesis agonists include neurotrophins and oxidative stress; NPD1 elicits potent anti-inflammatory actions and prohomeostatic bioactivity, is anti-angiogenic, promotes corneal nerve regeneration, and induces cell survival. In the context of DHA signalolipidomics, this review highlights aging and the evolving studies on the significance of DHA in Alzheimer's disease, macular degeneration, Parkinson's disease, and other brain disorders. DHA signalolipidomics in the nervous system offers emerging targets for pharmaceutical intervention and clinical translation.

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.

Relationship between fatty acid accretion, membrane composition, and biologic functions

Dietary fat affects metabolic pathways for phospholipid biosynthesis in tissues in a coordinated fashion. This may be important to aspects of development that concern phosphatidylcholine metabolism or regulatory processes that depend on signals from a changing milieu in the microenvironment of the membrane. Dietary fat influences the phosphatidylethanolamine (PE) composition in many membranes of the brain and retina and may by altered by small changes in the content of 20:4(6) and 22:6(3). Membrane PE fatty acids that contain one, four, or six double bonds and the ratio of 22:5(6) to 22:6(3) in PE that contains four to six double bonds are also affected. An increase in the omega 6 fatty acid content of membranes is associated with increased PE methyltransferase activity and decreased phosphocholine transferase activity, thus indicating a mechanism by which change in an exogenous factor (e.g., dietary fat intake) may alter neural phospholipid biosynthesis. Small changes in the composition of dietary fat intake change the composition of brain membranes during development. It is provocative to ponder whether diet could be used to induce formation of membrane structures that are more resistant to specific insults that cause degeneration of brain structural material, to ensure optimal functional compositions, or to reverse degenerative changes that occur in neural membrane structure and function.

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.

Impact of dietary fatty acid balance on membrane structure and function of neuronal tissues

Neural tissue has generally been viewed as resistant to structural changes induced by exogenous factors. Research has shown that the brain responds to changes in diet by altering neurotransmitter synthesis, and by shifting neuroendocrine controls over a variety of physiological events. Animal model research also indicates that fatty acid constituents and synthesis of brain structural lipid in membranes undergoing turnover can be altered by changing the composition of dietary fat. In growing animals, the balance between dietary omega 6 and omega 3 fatty acids influences brain phospholipid fatty acid composition, phosphatidylethanolamine methyltransferase activity, and rate of phosphatidylcholine biosynthesis via the CDP-choline pathway. It is concluded that biosynthetic control mechanisms regulating synthesis of brain structural lipid, in particular phosphatidylcholine, respond to exogenous factors and represent a normal physiological response by the brain. This response may provide a mechanism for therapeutic treatment of disorders involving degeneration of brain structural lipid.

Metabolic labeling of normal canine rod outer segment phospholipids in vivo and in vitro

Twenty-four hours after the intravitreal injection of [3H]palmitate and [14C]docosahexaenoate in dogs, the rod outer segment phospholipids are highly labeled. Palmitate is found predominantly in phosphatidylcholine (PC), with lesser amounts in phosphatidylethanolamine (PE) and very little in either phosphatidylserine (PS) or phosphatidylinositol (PI). Docosahexaenoate most heavily labeled PE followed by PC, with lesser amounts in PS and very little in PI. Two-hour incubations of 3 mm trephine buttons removed from dog retinas produced very similar patterns of labeling with palmitate and docosahexaenoate. In vitro incubation of retina buttons with [3H]arachidonate produced heavy labeling of PI, with much less in PC and very little in either PS or PE. [3H]Glycerol labeled in PC, PI and PE in descending order but PS almost not at all. [3H]Serine labeled PS predominantly, but small amounts were found in PC, PE and PI. The trephine retina buttons can be utilized for multiple-precursor incubations and studies of differential metabolism in retinal regions, particularly when studying scarce tissue from mutant animals or humans with inherited retinal degenerations.

Labeling of phosphatidylcholines of retina subcellular fractions by [1-14C]eicosatetraenoate (20:4(n-6)), docosapentaenoate (22:5(n-3)) and docosahexaenoate (22:6(n-3))

The labeling of molecular species of phosphatidylcholine (PC) has been studied in bovine retinas incubated for 2 h with (1-14C)-labeled (n-6) eicosatetraenoate (n-3) docosapentaenoate and (n-3) docosahexaenoate (20:4, 22:5 and 22:6, respectively) and in four subcellular fractions isolated after such incubations. Of the total radioactivity incorporated in PC, the following percentages of the above fatty acids, respectively, are found in its dipolyunsaturated species: 58, 56 and 53% in rod outer segments; 29, 41 and 49% in mitochondria; 24, 28 and 39% in microsomes; 12, 14 and 16% in postmicrosomal supernatants; 28, 36 and 58% in entire retinas. The remainder percentages are in tetra-, penta- and hexaenoic species of PC, respectively. The levels of pentaenoic species in the PCs of all fractions are similar, while tetraenes are lowest and hexaenes highest in photoreceptor membranes. Dipolyunsaturated species are highly concentrated in photoreceptor membranes, but are minor components of mitochondrial, microsomal and cytosolic PC. The specific radioactivities of tetraenoic, pentaenoic and hexaenoic PCs are decreasingly lower in the following order: postmicrosomal supernatants, microsomes, mitochondria, photoreceptor membranes. In contrast, the specific radioactivities of dipolyunsaturated PCs are higher in mitochondria and microsomes than in the other fractions, especially with 22:5 and 22:6. It is suggested that mitochondria as well as the endoplasmic reticulum could play a role in the synthesis and further modifications of dipolyunsaturated PCs before being supplied to photoreceptor membranes.

Labeling of lipids of retina subcellular fractions by [1-14C]eicosatetraenoate (20:4(n-6)) docosapentaenoate (22:5(n-3)) and docosahexaenoate (22:6(n-3))

(1-14C)-labeled (n-6) eicosatetraenoate, (n-3) docosapentaenoate and (n-3) docosahexaenoate (20:4, 22:5 and 22:6, respectively) are efficiently taken up and actively esterified into the lipids of bovine retina after 2 h incubation. Photoreceptor membranes, mitochondria, microsomes and postmicrosomal supernatants, which display significant differences in phospholipid and fatty acid compositions, are isolated after such incubations to study the labeling of lipids. The lipid classes preferentially labeled with the acids (1) largely differ among and within subcellular fractions, while (2) some common features in the treatment of the three polyenes are observed in each fraction. In all of them, the three acids are actively incorporated in phosphatidylcholine; ethanolamine glycerophospholipid, phosphatidylserine (PS) and phosphatidylinositol (PI) are highly labeled with 22:6, 22:5 and 20:4 respectively; within ethanolamine glycerophospholipid, the three label phosphatidylethanolamine in preference to plasmenylethanolamine. Most of the 14C esterified in mitochondria is in phospholipids. The endoplasmic reticulum produces in addition highly labeled triacylglycerols, also found in cytosol. High levels of 14C-labeled diacylglycerols are observed exclusively in photoreceptor membranes, where the specific radioactivity of PI is very high. The total amounts of 14C incorporated (1) are in general similar within a given fraction for the three polyenes, but (2) largely differ among fractions. The labeling of the highly unsaturated phospholipids of photoreceptor membranes is the lowest, while the postmicrosomal supernatant (whose lipids are relatively the poorest in polyenoic fatty acids) contains most of the labeled lipids isolated from retinas under these conditions. The results indicate that polyunsaturated species of retina phospholipids undergo an active synthesis and turnover, as well as an intense intracellular traffic among membranes.

Effects of aging on the composition and metabolism of docosahexaenoate-containing lipids of retina

The amount of docosahexaenoate (22:6n-3)-containing phospholipid species decreases with aging in the rat retina. Most lipids, but especially choline and serine glycerophospholipids, show a significant fall in 22:6n-3, which is not compensated by increases in other polyenoic fatty acids. The decrease not only affects 22:6 but also various very long chain n-3 hexaenoic fatty acids which, in phosphatidylcholine, have up to 36 carbon atoms, and which are probably synthesized by successive elongations of 22:6n-3. The in vitro incorporation of [2-3H]glycerol into retinal lipids indicates that the de novo biosynthetic pathways are not impaired by aging. The incorporation of [1-14C]docosahexaenoate is significantly stimulated into all lipids of aged retinas, but to the largest extent in those showing the largest decreases in 22:6, especially in choline glycerophospholipids. The results indicate that the decreased levels of 22:6 with aging are due not to an impaired activity of the enzymes involved in the synthesis and turnover of phospholipids but to a decreased availability of this polyene in the retina. It is suggested that this may stem from a defect in some of the enzymatic steps that lead to the synthesis of 22:6n-3, probably that catalyzed by delta 4 desaturase, the effect on longer hexaenes being secondary to the decreased synthesis of 22:6.

Cationic amphiphilic drugs perturb the metabolism of inosititides and phosphatidic acid in photoreceptor membranes

Incubation of purified bovine photoreceptor rod outer segments with [gamma-32P]ATP resulted in the labeling of phosphatidylinositol 4-phosphate (PIP) and phosphatidic acid (PA) with little labeling of phosphatidylinositol 4,5-bisphosphate (PIP2). Propranolol inhibited in a dose-dependent manner the labeling of PA and enhanced that of PIP. Various cationic amphiphilic drugs also were tested for these effects. Propranolol had the same effects on high-speed rat brain particulate material. While this particular preparation displayed more labeling of PIP2, propranolol was ineffective, as it was on retinal PIP-kinase. Ca2+-activated polyphosphoinositide phosphodiesterase activity in nerve-ending membranes also was inhibited by propranolol. It is concluded that cationic amphiphilic drugs can inhibit diacylglycerol kinase and the polyphosphoinositide phosphodiesterase and stimulate the phosphatidylinositol-kinase (but not PIP-kinase).

De novo biosynthesis of docosahexaenoyl-phosphatidic acid in bovine retinal microsomes

Lysophosphatidic acid stimulated several-fold the formation of docosahexaenoyl-phosphatidic acid from 14C-labeled docosahexaenoic acid (22:6 (n-3] in the bovine retina. 1-Palmitoyl- and 1-oleoyl-sn-glycerol 3-phosphate were the preferred acceptors. Most of the activity was localized in the 105 000 X g microsomal fraction. Despite the very high content of 22:6 in the phospholipids of photoreceptor membranes, only about 1% of the microsomal activity was found in discs isolated from rod outer segments. The newly synthesized docosahexaenoyl-phosphatidic acid was further metabolized to diacylglycerols, triacylglycerols, phosphatidylcholine and phosphatidylserine. The de novo synthesis of docosahexaenoyl-phosphatidylcholine was stimulated by 1 mM CDPcholine. Lysophosphatidic acid and lysophosphatidylcholine up to 50 microM do not compete with each other for 22:6 in the formation of their respective diacylated lipids. This suggests that this fatty acid is introduced into phosphatidic acid and phosphatidylcholine via different acylation systems. We conclude that, in addition to the deacylation-acylation cycle, there is also an active pathway for the acylation of 22:6 into glycerolipids during the de novo biosynthesis of phosphatidic acid.