Retinoid processing in cone and Müller cell lines

To determine whether cones and Müller cells in the rod dominated retina cooperate to regenerate the 11-cis retinal chromophore via the retinoid cycle, two cell lines from the rod dominated retinas of Murine were used for this study: 661W, a mouse cell line derived from cones, and rMC-1, a rat Müller cell line. Retinoid cycle enzymes were analyzed by RT–PCR, and their catalytic activity was detected by incubation with retinoids and analyzed by HPLC. We found that 661W cells are capable of reducing all-trans retinal to all-trans retinol due to the presence of multiple dehydrogenases and to generate minor amounts of retinyl-ester. The rMC-1 cells take up all-trans retinol and oxidize it to all-trans retinal or esterify it to retinyl-ester, but are incapable of isomerizing all-trans retinoids to 11-cis retinoids. This could be a reflection of lack of necessary activities in Müller cells in vivo, which suggests that Müller cells do not contribute to retinoid cycling by regenerating 11-cis retinoids. Alternatively, this could be due to the potential that rMC-1, as a transformed cell line, has stopped expressing the proteins needed for the regeneration of 11-cis retinoids.