Evidence for RPE65-independent vision in the cone-dominated zebrafish retina

An enzyme-based cyclic pathway for trans to cis isomerization of the chromophore of visual pigments (11-cis-retinal) is intrinsic to vertebrate cone and rod vision. This process, called the visual cycle, is mostly characterized in rod-dominated retinas and essentially depends on RPE65, an all-transto 11-cis-retinoid isomerase. Here we analyzed the role of RPE65 in zebrafish, a species with a cone-dominated retina. We cloned zebrafish RPE65 and showed that its expression coincided with photoreceptor development. Targeted gene knock-down of RPE65 resulted in morphologically altered rod outer segments and overall reduced 11-cis-retinal levels. Cone vision of RPE65-deficient larvae remained functional as evidenced by behavioral tests and by metabolite profiling for retinoids. Furthermore, all-trans retinylamine, a potent inhibitor for the rod visual cycle, reduced 11-cis-retinal levels of control larvae to a similar extend, but showed no additive effects in RPE65-deficient larvae. Thus, our study of zebrafish provides in vivo evidence for the existence of an RPE65-independent pathway for the regeneration of 11-cis-retinal for cone vision.