Duplication and Divergence of Zebrafish Cralbp Genes Uncovers a Novel Role for Rpe- and Mueller-Cralbp in Cone Vision

Purpose. During vertebrate phototransduction 11-cis-retinal is isomerised to all-trans-retinal. Light sensitivity is restored by recombination of apo-opsin with 11-cis-retinal to regenerate visual pigments. The conversion of all-trans retinal back to 11-cis-retinal is known as the visual cycle. Within the retina, cellular retinal-binding protein (CRALBP) is abundantly expressed in the retinal pigment epithelium (RPE) and Mueller glia. CRALBP expressed in the RPE is known to facilitate the rate of the rod visual cycle. Recent evidence suggests a role for Mueller glia in an alternate cone visual cycle. Here we characterise the role of RPE- and Mueller- CRALBP in cone vision. Methods. CRALBP orthologues rlbp1a and rlbp1b were identified in zebrafish by bioinformatic METHODS: The spatial and developmental expression of rlbp1a and rlbp1b was determined by in situ hybridisation and immunohistochemistry. Depletion of the expression of the corresponding Cralbp a and Cralbp b proteins was achieved by microinjection of antisense morpholinos. Visual function was analysed in 5 day post fertilisation (dpf) larvae using the optokinetic response assay. Results. The zebrafish genome contains two CRALBP ohnologues, rlbp1a and rlbp1b. These genes have functionally diverged, exhibiting differential expression at 5 dpf in RPE and Mueller glia, respectively. Depletion of CRALBP in the RPE or Mueller glia results in abnormal cone visual behaviour. Conclusions. This study suggests that cone photoreceptors incorporate 11-cis-retinoids derived from the rod and cone visual cycles into their visual pigments, and that Mueller-CRALBP participates in the cone visual cycle.