Abstract
Purpose :
During the visual (retinoid) cycle, clearance of all-trans-retinaldehyde (atRAL) released from photoactivated rhodopsin involves reisomerization of it back to 11-cis-retinaldehyde, the chromophore binding with opsin to regenerate rhodopsin, in retinal pigment epithelial (RPE) cells. Given that disrupted atRAL clearance will cause severe RPE/photoreceptor dystrophy as well as the production of toxic atRAL condensation byproducts that constitute RPE lipofuscin, free atRAL and RPE lipofuscin are both considered to implicate the etiology of age-related macular degeneration (AMD). As the precursor of bisretinoid constituents of RPE lipofuscin, free atRAL exhibits higher cytotoxicity to RPE cells than bisretinoids A2E and all-trans-retinaldehyde dimer (atRAL-dimer), thereby revealing that the conversion of free atRAL to bisretinoid fluorophores is likely an antidotal process at the start. Only when accumulated beyond a critical level, A2E and atRAL-dimer have the ability to trigger the death of RPE cells. In the present study, an alternative metabolic pathway of the visual chromophore atRAL in RPE cells is elucidated.
Methods :
Primary porcine RPE cells laden with atRAL, and irradiated atRAL or atRAL-dimer in RPE cells and cell-free systems were analyzed by high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). Cellular toxicity was evaluated by MTS assays.
Results :
By using primary porcine RPE cells, we found that, after treatment with atRAL, atRAL-dimer readily formed and accumulated in a concentration- and time-dependent manners. Moreover, atRAL-dimer was highly vulnerable to light, and HPLC analysis revealed that irradiation of atRAL-dimer by light produced a series of hydrophilic products that might be relatively easy to be metabolically eliminated. More importantly, exposure of primary porcine RPE cells fed with atRAL-dimer to light showed no cytotoxic effects.
Conclusions :
We proposed an alternative metabolic pathway of atRAL in RPE cells: atRAL that evades the visual (retinoid) cycle enters into RPE cells, and undergoes the condensation reaction to generate atRAL-dimer. Light will facilitate the removal of atRAL-dimer without causing RPE cell death. This is the first report on a new concept regarding how atRAL is metabolized in RPE cells.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.