Conversion of all-trans-retinal into all-trans-retinal dimer reflects an alternative metabolic/antidotal pathway of all-trans-retinal in the retina

Free all-trans-retinal (atRAL) and retinal pigment epithelium (RPE) lipofuscin are both considered to play etiological roles in Stargardt disease and age-related macular degeneration. A2E and all-trans-retinal dimer (atRAL-dimer) are two well characterized bisretinoid constituents of RPE lipofuscin. In this study, we found that, after treatment of primary porcine RPE (pRPE) cells with atRAL, atRAL-dimer readily formed and accumulated in a concentration- and time-dependent manner, but A2E was barely detected. Cell-based assays revealed that atRAL, the precursor of atRAL-dimer, significantly altered the morphology of primary pRPE cells and decreased cell viability at a concentration of 80 m regardless of light exposure. By contrast, atRAL-dimer was not cytotoxic and phototoxic to primary pRPE cells. Compared with atRAL and A2E, atRAL-dimer was more vulnerable to light, followed by the generation of its photocleaved products. Moreover, we observed the presence of atRAL-dimer in reaction mixtures of atRAL with porcine rod outer segments (ROS), RPE/choroid, or neural retina. Taken together, we here proposed an alternative metabolic/antidotal pathway of atRAL in the retina: atRAL that evades participation of the visual (retinoid) cycle undergoes a condensation reaction to yield atRAL-dimer in both ROS and RPE. Translocation of atRAL, all-trans N-retinylidene-phosphatidylethanolamine (NR-PE), atRAL-dimer, and photocleavage products of atRAL-dimer from ROS into RPE is accomplished by phagocytosing shed ROS on a daily basis. Without causing damage to RPE cells, light breaks up total atRAL-dimer within RPE cells to release low-molecular-weight photocleavage fragments. The latter, together with ROS-atRAL-dimer photocleavage products, may easily move across membranes and thereby be metabolically eliminated.