All-trans retinal levels and formation of lipofuscin precursors after bleaching in rod photoreceptors from wild type and Abca4 mice

Abstract

The accumulation of lipofuscin in the cells of the retinal pigment epithelium (RPE) is thought to play an important role in the development and progression of degenerative diseases of the retina. The bulk of RPE lipofuscin originates in reactions of the rhodopsin chromophore, retinal, with components of the photoreceptor outer segment. The 11-cis retinal isomer is generated in the RPE and supplied to rod photoreceptor outer segments where it is incorporated as the chromophore of rhodopsin. It is photoisomerized during light detection to all-trans and subsequently released by photoactivated rhodopsin as all-trans retinal, which is removed through reduction to all-trans retinol in a reaction requiring metabolic input in the form of NADPH. Both 11-cis and all-trans retinal can form lipofuscin precursor fluorophores in rod photoreceptor outer segments. Increased accumulation of lipofuscin has been suggested to result from excess formation of lipofuscin precursors due to buildup of all-trans retinal released by light exposure. In connection with this suggestion, the Abca4 transporter protein, an outer segment protein defects in which result in recessive Stargardt disease, has been proposed to promote the removal of all-trans retinal by facilitating its availability for reduction. To examine this possibility, we have measured the outer segment levels of all-trans retinal, all-trans retinol, and of lipofuscin precursors after bleaching by imaging the fluorescence of single rod photoreceptors isolated from wild type and Abca4−/− mice. We found that all-trans retinol and all-trans retinal levels increased after bleaching in both wild type and Abca4−/− rods. At all times after bleaching, there was no significant difference in all-trans retinal levels between the two strains. All-trans retinol levels were not significantly different between the two strains at early times, but were lower in Abca4−/− rods at times longer than 20 min after bleaching. Bleaching in the presence of lower metabolic substrate concentrations resulted in higher all-trans retinal levels and increased formation of lipofuscin precursors in both wild type and Abca4−/− rods. The results show that conditions that result in buildup of all-trans retinal levels result in increased generation of lipofuscin precursors in both wild type and Abca4−/− rods. The results are consistent with the proposal that Abca4 facilitates the reduction of all-trans retinal to retinol; absence of Abca4 however does not appear to be associated with higher all-trans retinal levels compared to wild type.

Interphotoreceptor retinoid–binding protein removes all-trans-retinol and retinal from rod outer segments, preventing lipofuscin precursor formation

“…8). This makes it unlikely that LFPs originate from free-dwelling all-trans-retinal, and it is consistent with the suggestion that they form from all-trans-retinal trapped in the form of a Schiff base with phosphatidylethanolamine (10). Another possibility would be that a small amount of chromophore remains trapped in aggregated opsin within which the Schiff base is retained.…”

“…Accumulation of all-trans-retinal due to metabolic limitations has been shown to result in excess formation of LFPs in rod photoreceptor outer segments (10). All-trans-retinal also accumulates in Rdh8 Ϫ/Ϫ rod outer segments, which lack the enzyme needed for the reduction of retinal to retinol (3,48,50).…”

“…In intact rods with a functioning Rdh8 enzyme, even in the absence of IRBP, it has not been possible to resolve any LFP formation from all-trans-retinal released by photoactivated rhodopsin (Fig. 10C) (10,35). In the intact retina, rods will be in the presence of IRBP, which strongly promotes the clearance of all-trans-retinal ( Fig.…”

Oxidized Lipids Decrease Phagocytic Activity of ARPE‐19 Cells In Vitro

“…Results of our recent study suggested that moderate photoreactivity of BRex not subjected to oxidation, could be attributed hydrophobic endogenous sensitizers present in Folch’ extracts of neural retinas. One of these sensitizers is all‐trans retinal (atRAL), a product of photobleaching of the visual pigment rhodopsin, which may temporarily accumulate in POS . It has previously been shown that atRAL generates 1 O 2 with quantum yield about 0.3 .…”

“…One of these sensitizers is all-trans retinal (atRAL), a product of photobleaching of the visual pigment rhodopsin, which may temporarily accumulate in POS. [11,38] It has previously been shown that atRAL generates 1 O 2 with quantum yield about 0.3. [39] Not oxidized BRex may contain small amounts of atRAL, which could contribute to generation of singlet oxygen by the photoexcited LUV prepared from not oxidized BRex (0 h).…”