Dietary vitamins A and E influence retinyl ester composition and content of the retinal pigment epithelium

Metabolic Pathway Coupled with Fermentation Process Optimization for High-Level Production of Retinol in Yarrowia lipolytica

Retinol is a lipid-soluble form of vitamin A that is crucial for human visual and immune functions. The production of retinol through microbial fermentation has been the focus of recent exploration. However, the obtained titer remains limited and the product is often a mixture of retinal, retinol, and retinoic acid, necessitating purification. To achieve efficient biosynthesis of retinol in Yarrowia lipolytica, we improved the metabolic flux of β-carotene to provide sufficient precursors for retinol in this study. Coupled with the optimization of the expression level of β-carotene 15,15'-dioxygenase, de novo production of retinol was achieved. Furthermore, Tween 80 was used as an extractant and butylated hydroxytoluene as an antioxidant to extract intracellular retinol and prevent retinol oxidation, respectively. This strategy significantly increased the level of retinol production. By optimizing the enzymes converting retinal to retinol, the proportion of extracellular retinol in the produced retinoids reached 100%, totaling 1042.3 mg/L. Finally, total retinol production reached 5.4 g/L through fed-batch fermentation in a 5 L bioreactor, comprising 4.2 g/L extracellular retinol and 1.2 g/L intracellular retinol. This achievement represents the highest reported titer so far and advances the industrial production of retinol.

Environmentally Driven Color Variation in the Pearl Oyster Pinctada margaritifera var. cumingii (Linnaeus, 1758) Is Associated With Differential Methylation of CpGs in Pigment- and Biomineralization-Related Genes

Today, it is common knowledge that environmental factors can change the color of many animals. Studies have shown that the molecular mechanisms underlying such modifications could involve epigenetic factors. Since 2013, the pearl oyster Pinctada margaritifera var. cumingii has become a biological model for questions on color expression and variation in Mollusca. A previous study reported color plasticity in response to water depth variation, specifically a general darkening of the nacre color at greater depth. However, the molecular mechanisms behind this plasticity are still unknown. In this paper, we investigate the possible implication of epigenetic factors controlling shell color variation through a depth variation experiment associated with a DNA methylation study performed at the whole genome level with a constant genetic background. Our results revealed six genes presenting differentially methylated CpGs in response to the environmental change, among which four are linked to pigmentation processes or regulations (GART, ABCC1, MAPKAP1, and GRL101), especially those leading to darker phenotypes. Interestingly, the genes perlucin and MGAT1, both involved in the biomineralization process (deposition of aragonite and calcite crystals), also showed differential methylation, suggesting that a possible difference in the physical/spatial organization of the crystals could cause darkening (iridescence or transparency modification of the biomineral). These findings are of great interest for the pearl production industry, since wholly black pearls and their opposite, the palest pearls, command a higher value on several markets. They also open the route of epigenetic improvement as a new means for pearl production improvement.

Peropsin modulates transit of vitamin A from retina to retinal pigment epithelium

Peropsin is a non-visual opsin in both vertebrate and invertebrate species. In mammals, peropsin is present in the apical microvilli of retinal pigment epithelial (RPE) cells. These structures interdigitate with the outer segments of rod and cone photoreceptor cells. RPE cells play critical roles in the maintenance of photoreceptors, including the recycling of visual chromophore for the opsin visual pigments. Here, we sought to identify the function of peropsin in the mouse eye. To this end, we generated mice with a null mutation in the peropsin gene (Rrh). These mice exhibited normal retinal histology, normal morphology of outer segments and RPE cells, and no evidence of photoreceptor degeneration. Biochemically, Rrh^-/- mice had ∼2-fold higher vitamin A (all-trans-retinol (all-trans-ROL)) in the neural retina following a photobleach and 5-fold lower retinyl esters in the RPE. This phenotype was similar to those reported in mice that lack interphotoreceptor retinoid-binding protein (IRBP) or cellular retinol-binding protein, suggesting that peropsin plays a role in the movement of all-trans-ROL from photoreceptors to the RPE. We compared the phenotypes in mice lacking both peropsin and IRBP with those of mice lacking peropsin or IRBP alone and found that the retinoid phenotype was similarly severe in each of these knock-out mice. We conclude that peropsin controls all-trans-ROL movement from the retina to the RPE or may regulate all-trans-ROL storage within the RPE. We propose that peropsin affects light-dependent regulation of all-trans-ROL uptake from photoreceptors into RPE cells through an as yet undefined mechanism.

Retinal vascular changes induced by the oxidative stress of alpha-tocopherol deficiency contrasted with diabetic microangiopathy

It has been proposed that oxidative tissue damage is involved in the development of diabetic angiopathies. To evaluate this hypothesis, experiments were conducted to identify the retinal vessel changes induced by the oxidative stress related to alpha-tocopherol deficiency and examine possible similarities with the lesions characteristic of diabetic retinopathy. Twenty-one-day-old male Fisher 344 albino rats were divided randomly to receive a basal, chemically defined diet either with (adequate group) or without (deficient group) alpha-tocopherol. After 6 and 8 months, some rats (n = 3 per group) were killed and the eyes removed. In order to evaluate cell integrity and localization of lipofuscin-specific autofluorescence by light and fluorescence microscopy, some of the retinas were prepared for cryostat-sections while others were digested by elastase to isolate intact retinal vasculatures. After 8 and 14 months, the central retina of one eye per rat (n = 6 to 8 per group) was examined by electron microscopy for retinal capillary basement membrane (RCBM) thickening and other ultrastructural changes. At 6 and 8 months, the deficient rats exhibited extensive shortening and disarray of rod outer segments (ROS), marked loss of photoreceptor cells, and pronounced increases in the numbers of granules with lipofuscin-specific autofluorescence in the retinal pigment epithelium (RPE) and retinal vessels. At 14 months, the ultrastructure revealed that the damage to ROS involved disruption of membranes and that the capillary lipofuscin was contained mainly within the endothelial cells. Membrane remnants were found in the lipofuscin granules of both the RPE and retinal vessels. In addition, there was an increase in RCBM thickness (98.7 +/- 2.6 nm vs. 86.9 +/- 2.9 nm). RCBM thickening was the only finding common with diabetic retinopathy, and the thickening was 13.6%, significantly less than that reported in diabetic rat models with 8 and 14 months durations (34% and 53.1%, respectively). Capillary lipofuscin accumulation, which was prominent in the deficient rats, is not notable in diabetes. Both the moderate RCBM thickening and marked lipofuscin accumulations seen in alpha-tocopherol-deficient rats were similar to changes occurring in the aging process, though more pronounced. The spectrum of microangiopathies characteristic of diabetic retinopathy did not develop in alpha-tocopherol-deficient rats. These findings suggest that oxidative damage, though probably involved, is unlikely to play a predominant role in the development of diabetic retinal microangiopathies.

Long-term variations in cyclic light intensity and dietary vitamin A intake modulate lipofuscin content of the retinal pigment epithelium

Experiments were conducted to determine whether the intensity of cyclic light exposure to the retina over a long period of time affects retinoid-dependent accumulation of lipofuscin in the retinal pigment epithelium (RPE). Albino rats were maintained from weaning on diets either containing (+A) or lacking (-A) retinyl palmitate, which can be metabolized to the retinoids involved in the visual cycle. Animals in each dietary group were divided between bright (L) and dim (D) cyclic light treatments. Thus, the experiments employed the following four treatment groups: +A/D, +A/L, -A/D, and -A/L. After 6, 12, and 15 months from the start of the treatments, animals in each group were killed for quantitative determination of: 1) retinal photoreceptor densities; 2) RPE lipofuscin content; and 3) RPE lipofuscin fluorescence intensity. Animals in the L groups had a lower volume of RPE lipofuscin than those in the D groups fed the same diet. Among the -A rats, this reduced lipofuscin volume could be attributed to a light-enhanced depletion of vitamin A from the retina and an accompanying loss of photoreceptor cells. In the +A animals, however, there were no differences in photoreceptor densities between the D and L groups. In the -A rats, the volume of RPE lipofuscin decreased between 6 and 15 months of age, whereas it increased in the +A animals. In contrast, lipofuscin fluorescence intensity increased between 6 and 15 months of age in all four treatment groups. However, in the +A rats, the fluorescence intensity was lower in the L than in the D group at all three ages. In the -A groups, light level had no effect on lipofuscin fluorescence intensity. At all three ages, fluorescence intensity was lower in the -A animals than in +A rats. Thus, at light intensities below those that induce acute retinal degeneration, long-term exposure to higher intensity light inhibits the age-related increase in RPE lipofuscin volume. A decrease in the volume of RPE lipofuscin after the retina is depleted of vitamin A suggests that lipofuscin is turned over, and that RPE lipofuscin content is determined by a balance between the rates at which lipofuscin is formed and at which it is eliminated from the RPE. An age-related increase in lipofuscin-specific fluorescence intensity after vitamin A depletion from the retina suggests that lipofuscin fluorophores may continue to form slowly from retinoids that have been modified such that they can no longer enter the visual cycle.

Visual sensitivity and interphotoreceptor retinoid binding protein in the mouse: regulation by vitamin A

Interphotoreceptor retinoid binding protein (IRBP) is a retinoid and fatty acid binding glycoprotein secreted by rod and cone photoreceptors in all vertebrates. IRBP is believed to serve as a carrier for retinoids in the bleaching and regeneration cycle of rhodopsin. IRBP protein has been found to be decreased in vitamin A-deprived rats; it is rapidly recovered after retinol repletion. To understand the mechanism for this recovery, we determined whether vitamin A affects transcription and translation of the IRBP gene. Wild-type and transgenic mice harboring the IRBP promoter-CAT reporter fusion gene were maintained on a retinol-deficient diet supplemented with retinoic acid (-A) or on a control diet (+A) for up to 60 wk postweaning. Some of the -A mice were given retinol repletion for 7 days (-A+A). Electroretinography analysis revealed alterations in waveform and a 2 log unit decrease in b-wave sensitivity in the -A mice over a broad range of stimulus wavelengths. Retinol repletion effected a full recovery. Immunochemistry showed a significant decrease in the immunogold-labeled IRBP between the retinal pigment epithelium and the outer segments of the -A mice compared with +A and -A+A mice. Northern blots showed no differences in the amounts of IRBP or CAT mRNA between these three treatment groups. These results suggest that the regulation of IRBP by retinol is not transcriptional.

Vitamin A incorporation into lipofuscin-like inclusions in the retinal pigment epithelium

Intravitreal injection of the protease inhibitor leupeptin causes a rapid accumulation of lipofuscin-like autofluorescent inclusions in the retinal pigment epithelium (RPE) of the eye. In vitamin A-deprived animals, similar inclusions form in response to leupeptin treatment, but they do not become autofluorescent. Because vitamin A is necessary to the development of fluorescence, it appears likely that retinoids are directly incorporated into the inclusions. Experiments were conducted to determine whether this is the case. Rats were reared on a diet containing retinoic acid as the only retinoid. Retinoic acid cannot be utilized in visual transduction by the retina. When the eyes had been over 90% depleted of visual cycle retinoids, the animals were given a single intramuscular injection of 3H-all-trans retinol. After 7 days, when visual cycle retinoids had returned to an average of almost 70% of normal, the animals were given an intravitreal injection of leupeptin in each eye. At either 1 day or 7 days after the leupeptin treatment, some of the animals were dark-adapted for at least 12 h. The eyes were enucleated and fixed under dim red light. A region of each retina just superior to the optic nerve head was examined with electron microscopic autoradiography. At both one day and 7 days after the leupeptin treatment, the radiolabel in the RPE was primarily associated with the leupeptin induced inclusion bodies. Label was also present in the photoreceptor outer segments. The localization of vitamin A to the leupeptin-induced inclusions in the RPE strongly suggests that vitamin A is covalently bound to outer segment proteins that have been phagocytosed by the RPE but remain undegraded due to protease inhibition. This bound vitamin A is probably responsible for the autofluorescence of the leupeptin-induced inclusions. Vitamin A is not likely to be bound through a Schiff base linkage, since retinal-Schiff base compounds do not exhibit lipofuscin-like fluorescence.

Influence of dietary vitamin A on autofluorescence of leupeptin-induced inclusions in the retinal pigment epithelium

Substantial evidence indicates that the major precursors for retinal pigment epithelial (RPE) lipofuscin are molecular components of the photoreceptor outer segments. Previous experiments have demonstrated that the retinoids required for transduction by the photoreceptors promote RPE lipofuscin fluorophore formation. Animals deficient in these retinoids accumulate lipofuscin in the RPE at greatly reduced rates. The protein components of the photoreceptor outer segments also appear to be involved in the formation of RPE lipofuscin fluorophores. When degradation of phagocytosed photoreceptor outer segment proteins by the RPE is blocked, this tissue rapidly becomes engorged with phagosome-derived inclusions that have fluorescence properties similar to lipofuscin. Experiments were conducted to determine whether the development of lipofuscin-like fluorescence in these inclusions was dependent on the availability of retinoids. Rats were fed diets containing vitamin A either in the form of retinyl palmitate (+A), which can be metabolically converted into the retinoids involved in vision, or retinoic acid (-A), which does not support visual function. After the retinas of the -A animals had been depleted of retinoids involved in vision, animals from both groups were given intraocular injections of the protease inhibitor leupeptin. Two days later, the amounts of lipofuscin-like autofluorescence from the RPEs of rats in each group were determined. Leupeptin treatment produced an increase in this fluorescence in RPEs of the +A animals, but not in the RPEs of the rats fed the -A diet, despite the fact that phagosome-like inclusions accumulated in the RPE in both dietary groups. This finding suggests that retinoids involved in the visual process are probably directly involved in RPE lipofuscin fluorophore formation.

Age-related alterations in vitamin A metabolism in the rat retina

Vitamin A plays a central role in visual transduction and in maintaining the structural integrity of the retina. It is possible that age-related alterations in vitamin A metabolism in the eye could contribute to the impairment of visual function that occurs during senescence. Therefore, investigations were conducted to determine whether the metabolism of this vitamin in the rat retina was altered during aging. Pigmented rats aged 12-, 22-, and 32 months were dark-adapted, and one eye from each animal was enucleated under dim red light. The neural retinas were separated from the retinal pigment epithelium (RPE)-choroid-scleral complexes, and the amounts and forms of vitamin A in both tissues were determined. The animals were then fully light-adapted, and the same measurements were performed on the tissues from the remaining eye of each rat. A number of age-related alterations in the vitamin A composition and content of the retina and RPE were observed. The most pronounced of these changes were significant increases in the ratios of retinyl palmitate to retinyl stearate with advancing age in both the neural retina and RPE. The total vitamin A ester contents of the RPEs increased during senescence in the dark-adapted state, but not in the light-adapted state. Retinyl ester levels in the neural retinas, on the other hand, did not differ significantly between 12- and 32-month-old animals in either the light-adapted or dark-adapted states. The amounts of all-trans retinol in the neural retinas decreased during aging, mainly in the dark-adapted state, whereas aging had no influence on RPE all-trans retinol content. The age-related alterations in metabolism of vitamin A that these observations reflect may be related to certain changes in visual function that occur during senescence.

Light and aging effects on vitamin E in the retina and retinal pigment epithelium

Experiments were performed to determine the effects of senescence and light adaptation on vitamin E levels in the neural retina and RPE-choroid-sclera of pigmented rats. Aging resulted in significant increases in alpha-tocopherol levels in both tissues, the effect being most pronounced in the RPE-choroid-sclera. The state of light adaptation had no influence on alpha-tocopherol levels in the neural retina at any of the ages examined, whereas in the RPE-choroid-sclera, alpha-tocopherol levels were substantially higher in light-adapted than in dark-adapted animals at all three ages (12, 22, and 32 months) at which they were measured. The effect of light adaptation on RPE-choroid-scleral alpha-tocopherol levels was most pronounced in the oldest age group.