Effect of dietary fat and environmental lighting on the phospholipid molecular species of rat photoreceptor membranes

Retinal light damage: mechanisms and protection

By its action on rhodopsin, light triggers the well-known visual transduction cascade, but can also induce cell damage and death through phototoxic mechanisms - a comprehensive understanding of which is still elusive despite more than 40 years of research. Herein, we integrate recent experimental findings to address several hypotheses of retinal light damage, premised in part on the close anatomical and metabolic relationships between the photoreceptors and the retinal pigment epithelium. We begin by reviewing the salient features of light damage, recently joined by evidence for retinal remodeling which has implications for the prognosis of recovery of function in retinal degenerations. We then consider select factors that influence the progression of the damage process and the extent of visual cell loss. Traditional, genetically modified, and emerging animal models are discussed, with particular emphasis on cone visual cells. Exogenous and endogenous retinal protective factors are explored, with implications for light damage mechanisms and some suggested avenues for future research. Synergies are known to exist between our long term light environment and photoreceptor cell death in retinal disease. Understanding the molecular mechanisms of light damage in a variety of animal models can provide valuable insights into the effects of light in clinical disorders and may form the basis of future therapies to prevent or delay visual cell loss.

Selective decrease of bis(monoacylglycero)phosphate content in macrophages by high supplementation with docosahexaenoic acid

Bis(monoacylglycero)phosphate (BMP) is a unique phospholipid (PL) preferentially found in late endosomal membranes, where it forms specialized lipid domains. Recently, using cultured macrophages treated with anti-BMP antibody, we showed that BMP-rich domains are involved in cholesterol homeostasis. We had previously stressed the high propensity of BMP to accumulate docosahexaenoic acid (DHA), compared with other PUFAs. Because phosphatidylglycerol (PG) was reported as a precursor for BMP synthesis in RAW macrophages, we examined the effects of PG supplementation on both FA composition and amount of BMP in this cell line. Supplementation with dioleoyl-PG (18:1/18:1-PG) induced BMP accumulation, together with an increase of oleate proportion. Supplementation with high concentrations of didocosahexaenoyl-PG (22:6/22:6-PG) led to a marked enrichment of DHA in BMP, resulting in the formation of diDHA molecular species. However, the amount of BMP was selectively decreased. Similar effects were observed after supplementation with high concentrations of nonesterified DHA. Addition of vitamin E prevented the decrease of BMP and further increased its DHA content. Supplementation with 22:6/22:6-PG promoted BMP accumulation with an enhanced proportion of 22:6/22:6-BMP. DHA-rich BMP was significantly degraded after cell exposure to oxidant conditions, in contrast to oleic acid-rich BMP, which was not affected. Using a cell-free system, we showed that 22:6/22:6-BMP is highly oxidizable and partially protects cholesterol oxidation, compared with 18:1/18:1-BMP. Our data suggest that high DHA content in BMP led to specific degradation of this PL, possibly through the diDHA molecular species, which is very prone to peroxidation and, as such, a potential antioxidant in its immediate vicinity.

Different mechanisms of saturated versus polyunsaturated FFA-induced apoptosis in human endothelial cells

Apoptosis and underlying mechanisms were evaluated in human umbilical vein endothelial cells (HUVECs), in target tissues of late diabetic vascular complications [human aortic endothelial cells (HAECs) and human retinal endothelial cells (HRECs)], and in endothelial progenitor cells (EPCs) exposed to FFAs, which are elevated in obesity and diabetes. Saturated stearic acid concentration dependently induced apoptosis that could be mediated via reduced membrane fluidity, because both apoptosis and membrane rigidity are counteracted by eicosapentaenoic acid. PUFAs triggered apoptosis at a concentration of 300 micromol/l in HUVECs, HAECs, and EPCs, but not HRECs, and, in contrast to stearic acid, involved caspase-8 activation. PUFA-induced apoptosis, but not stearic acid-induced apoptosis, strictly correlated (P < 0.01) with protein expression of E2F-1 (r = 0.878) and c-myc (r = 0.966). Lack of c-myc expression and activity owing to quiescence or transfection with dominant negative In373-Myc, respectively, renders HUVECs resistant to PUFA-induced apoptosis. Because c-myc is abundant in growing cells only, apoptosis triggered by PUFAs, but not by saturated stearic acid, obviously depends on the growth/proliferation status of the cells. Finally, this study shows that FFA-induced apoptosis depends on the vascular origin and growth/proliferation status of endothelial cells, and that saturated stearic acid-induced apoptosis and PUFA-induced apoptosis are mediated via different mechanisms.

Lipidomic analysis of the retina in a rat model of Smith-Lemli-Opitz syndrome: alterations in docosahexaenoic acid content of phospholipid molecular species

Smith-Lemli-Opitz syndrome (SLOS) is a complex hereditary disease caused by an enzymatic defect in the last step of cholesterol biosynthesis. Progressive retinal degeneration occurs in an AY9944-induced rat model of SLOS, with biochemical and electroretinographic hallmarks comparable with the human disease. We evaluated alterations in the non-sterol lipid components of the retina in this model, compared with age-matched controls, using lipidomic analysis. The levels of 16:0-22:6 and 18:0-22:6 phosphatidylcholine molecular species in retinas were less by > 50% and > 33%, respectively, in rats treated for either 2 or 3 months with AY9944. Relative to controls, AY9944 treatment resulted in > 60% less di-22:6 and > 15% less 18:0-22:6 phosphatidylethanolamine molecular species. The predominant phosphatidylserine (PS) molecular species in control retinas were 18:0-22:6 and di-22:6; notably, AY9944 treatment resulted in > 80% less di-22:6 PS, relative to controls. Remarkably, these changes occurred in the absence of n3 fatty acid deficiency in plasma or liver. Thus, the retinal lipidome is globally altered in the SLOS rat model, relative to control rats, with the most profound changes being less phosphatidylcholine, phosphatidylethanolamine, and PS molecular species containing docosahexaenoic acid (22:6). These findings suggest that SLOS may involve additional metabolic compromise beyond the primary enzymatic defect in the cholesterol pathway.

Environmental light and heredity are associated with adaptive changes in retinal DHA levels that affect retinal function

Retinas of rats and mice react to environmental and genetic stimuli by altering the level of DHA in their rod outer segment membranes. We propose that this adaptation is a neuroprotective response to control the number of photons captured by rhodopsin and the efficiency of visual transduction, under conditions where excessive activation of the transduction cascade could lead to cell death.

How much blue light should an IOL transmit?

Older, and even some modern, intraocular lenses (IOLs) transmit potentially hazardous ultraviolet radiation (UVR) to the retina. In addition, IOLs transmit more blue and green light to the retina for scotopic vision than the crystalline lenses they replace, light that is also potentially hazardous. The severity of UVR-blue type phototoxicity increases with decreasing wavelength, unlike the action spectrum of blue-green type retinal phototoxicity and the luminous efficiency of scotopic vision which both peak in the blue-green part of the optical spectrum around 500 nm. Theoretically, UVR+blue absorbing IOLs provide better retinal protection but worse scotopic sensitivity than UVR-only absorbing IOLs, but further study is needed to test this analysis. UVR is potentially hazardous and not useful for vision, so it is prudent to protect the retina from it with chromophores in IOLs. Determining authoritatively how much blue light an optimal IOL should block requires definitive studies to determine (1) the action spectrum of the retinal phototoxicity potentially involved in human retinal ageing, and (2) the amount of shorter wavelength blue light required for older adults to perform essential activities in dimly lit environments.

Retinal fatty acid binding protein reduce lipid peroxidation stimulated by long-chain fatty acid hydroperoxides on rod outer segments

In the present study we have investigated the effect of partially purified retinal fatty acid binding protein (FABP) against nonenzymatic lipid peroxidation stimulated by hydroperoxides derived from fatty acids on rod outer segment (ROS) membranes. Linoleic acid hydroperoxide (LHP), arachidonic acid hydroperoxide (AHP) and docosahexaenoic acid hydroperoxide (DHP) were prepared from linoleic acid, arachidonic acid and docosahexaenoic acid, respectively, by means of lipoxidase. ROS membranes were peroxidized using an ascorbate-Fe(+2) experimental system. The effect on the peroxidation of ROS containing different amounts of lipid hydroperoxides (LOOH) was studied; ROS deprived of exogenously added LOOH was utilized as control. The degradative process was measured simultaneously by determining chemiluminescence and fatty acid composition of total lipids isolated from ROS. The addition of hydroperoxides to ROS produced a marked increase in light emission. This increase was hydroperoxide concentration-dependent. The highest value of activation was produced by DHP. The decrease percentage of the more polyunsaturated fatty acids (PUFAs) (20:4 n6 and 22:6 n3) was used to evaluate the fatty acid alterations observed during the process. We have compared the fatty acid composition of total lipids isolated from native ROS and peroxidized ROS that were incubated with and without hydroperoxides. The major difference in the fatty acid composition was found in the docosahexaenoic acid content, which decreased by 45.51+/-1.07% in the peroxidized group compared to native ROS; the decrease was even higher, 81.38+/-1.11%, when the lipid peroxidation was stimulated by DHP. Retinal FABP was partially purified from retinal cytosol. Afterwards, we measured its effect on the reaction of lipid peroxidation induced by LOOH. As a result, we observed a decrease of chemiluminescence (inhibition of lipid peroxidation) when adding increasing amounts (0.2 to 0.6 mg) of retinal FABP to ROS. The inhibitory effect reaches its highest value in the presence of DHP (41.81+/-10.18%). Under these conditions, bovine serum albumin (BSA) produces a smaller inhibitory effect (20.2+/-7.06%) than FABP.

Dietary n-3 FA modulate long and very long chain FA content, rhodopsin content, and rhodopsin phosphorylation in rat rod outer segment after light exposure

A previous study has shown that the long and very long chain FA (VLCFA) content of the rat retina responds to changes in dietary n-6/n-3 ratio of the fat fed (1). The present study tested whether similar changes in these FA are associated with alterations in rhodopsin content and rhodopsin phosphorylation after light treatment. Weanling rats were fed diets containing 20% (w/w, 40% energy) fat with either high (4.8%, w/w) or low (1.2%, w/w) n-3 FA. After 6 wk of feeding, half of the animals in each group were exposed to light for 48 h at 350 lx or were kept in complete darkness. In the rod outer segment, the high n-3 diet treatment increased the level of 20:5n-3 and 22:6n-3 and reduced the levels of 20:4n-6 and 24:4n-6 in PC, PE, and PS. After the feeding of a high n-3 FA diet, total n-3 pentaenoic VLCFA from C24 to C34 increased in PC, whereas the n-6 tetra- and pentaenoic VLCFA decreased. No changes occurred in n-3 hexaenoic VLCFA regardless of the level of 22:6n-3 in the diet. After light exposure, animals fed a high n-3 FA diet showed reduction in 22:6n-3 as well as in n-6 and n-3 VLCFA in PC. FFA and TG fractions contained increased levels of both 20:4n-6 and 22:6n-3 after light exposure. Dark-adapted rhodopsin content and rhodopsin phosphorylation in the rod outer segment of rats fed the low n-3 FA diet were higher than in animals fed a high n-3 FA diet. After light exposure, animals fed the low n-3 FA diet lost more rhodopsin compared to animals fed the high n-3 FA diet, resulting in less phosphorylation of rhodopsin. Results indicate that the FA composition, rhodopsin content, and phosphorylation in visual cells is influenced by the dietary n-3 FA fed as well as by light exposure. The results also imply that 22:6n-3 may not be the precursor for synthesis of hexaenoic VLCFA.

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.

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.

A hypothesis to explain the reduced blood levels of docosahexaenoic acid in inherited retinal degenerations caused by mutations in genes encoding retina-specific proteins

Some humans and animals with inherited retinal degenerations (RD) have lower blood levels of docosahexaenoic acid (22:6n-3) than controls. As a result of recent studies, clearly the low blood 22:6n-3 phenotype is found in multiple RD phenotypes and no mutation thus far identified in humans or animals is involved in lipid metabolism. Therefore, it seems reasonable to suggest that the primary defect is not in 22:6n-3 metabolism, but rather in some common convergent pathway that ultimately leads to the reduction of blood and tissue 22:6n-3 levels. One possibility is that the different mutations produce a metabolic stress that provokes structural and biochemical adaptive changes in photoreceptor cells and their rod outer segments. If the stress is oxidant, the retina could downregulate 22:6n-3 and upregulate antioxidant defenses. How such a stress could lead to changes in blood levels of 22:6n-3 is not obvious. However, the consistent finding of the 22:6n-3 phenotype in many different retinal degeneration genotypes suggests that some form of communication exists between the retina and other tissues that serves to reduce blood levels of 22:6n-3.

Neonatal polyunsaturated fatty acid metabolism

The importance of n-6 and n-3 polyunsaturated fatty acids (PUFA) in neonatal development, particularly with respect to the developing brain and retina, is well known. This review combines recent information from basic science and clinical studies to highlight recent advances in knowledge on PUFA metabolism and areas where research is still needed on infant n-6 and n-3 fatty acid requirements. Animal, cell culture, and infant studies are consistent in demonstrating that synthesis of 22:6n-3 involves C24 PUFA and that the amounts of 18:2n-6 and 18:3n-3 influence PUFA metabolism. Studies to show that addition of n-6 fatty acids beyond delta6-desaturase alters n-6 fatty acid metabolism with no marked increase in tissue 20:4n-6 illustrate the limitations of analyses of tissue fatty acid compositions as an approach to study the effects of diet on fatty acid metabolism. New information to show highly selective pathways for n-6 and n-3 fatty acid uptake in brain, and efficient pathways for conservation of 22:6n-3 in retina emphasizes the differences in PUFA metabolism among different tissues and the unique features which allow the brain and retina to accumulate and maintain high concentrations of n-3 fatty acids. Further elucidation of the delta6-desaturases involved in 24:5n-6 and 22:6n-3 synthesis; the regulation of fatty acid movement between the endoplasmic reticulum and peroxisomes; partitioning to acylation, desaturation and oxidation; and the effects of dietary and hormonal factors on these pathways is needed for greater understanding of neonatal PUFA metabolism.

Effect of diet on the fatty acid and molecular species composition of dog retina phospholipids

Dogs were born to mothers fed commercial diets low or enriched in n-3 fatty acids and raised on those diets until they were about 50 d old. Retinas were removed, lipids were extracted, and total phospholipids were analyzed for fatty acid and molecular species composition. Animals from the low n-3 group had significantly lower retinal levels of 22:6n-3 and higher levels of n-6 fatty acids, especially 20:4n-6 and 22:5n-6. There was no difference in the retinal levels of 18:2n-6, and only small differences were found in saturated and monounsaturated fatty acids. The most dramatic differences in molecular species occurred in 22:6n-3-22:6n-3 (4.7 vs. 0.8%) and 18:0-22:6n-3 (27.6 vs. 14.4%); total molecular species containing 22:6n-3 were significantly lower in the low n-3 group (45.5 vs. 24.0%). Molecular species containing 20:4n-6 and 22:5n-6 were greater in the low n-3 animals (13.0 vs. 25.7%), as were molecular species containing only saturated and monounsaturated fatty acids (40.8 vs. 35.4%). These results show that modest differences in the amount of n-3 fatty acids in the diets of dogs can have profound effects on the fatty acid and molecular species composition of their retinas.

Effect of dietary fat on the response of the rat retina to chronic and acute light stress

We designed an experiment to study the role of light history and polyunsaturated fatty acids (PUFA) on the susceptibility of the albino rat retina to light damage. Albino rats were born to dams that had been kept in either 1 lx or 250 lx cyclic light for 0-4 days prior to delivery and fed one of three diets containing either 10% (by weight) hydrogenated coconut oil (COC, no n-3 nor n-6 fatty acids), safflower oil (SAF, high n-6, < 0.1% n-3 fatty acids), or linseed oil (LIN, high n-3, low n-6 fatty acids). After weaning, the rats were maintained in the same light environment and fed the same diets for 9 weeks, at which time some were killed and their retinas processed for morphometric analysis. Animals raised in bright cyclic light had shorter ROS lengths and thinner outer nuclear layers (ONL) than rats raised in dim cyclic light. The LIN animals had a thinner ONL than animals of the SAF or COC groups. Rats from each diet and light rearing groups were exposed to constant illumination of 2000 lx for 24 hr, after which they and non-exposed controls were placed in 1 lx cyclic light for 10 days and analysed for changes in ONL thickness. In the 250 lx (bright; B) groups, there was no effect of acute light stress on ONL thickness, although both control and stressed LIN animals had a thinner ONL than the corresponding COC and SAF groups. However, in the rats raised in 1 lx cyclic light, acute stress resulted in significant retinal damage (i.e. decrease in ONL thickness) in the three diet groups combined. The superior region was damaged the most and the severity was dependent on diet, as evidenced by the LIN group having a greater reduction in ONL thickness than the SAF group after light stress. From these results we conclude that rats on diets high in n-3 fatty acids are more susceptible to photoreceptor cell loss than animals fed n-6 or no polyunsaturated fatty acids when raised in dim cyclic light. These results show that both diet and light history play a role in the susceptibility of the retina to acute and chronic effects of light and suggest a role for lipid peroxidation in retinal light damage.