Duplication and divergence of zebrafish CRALBP genes uncovers novel role for RPE- and Muller-CRALBP in cone vision.

PURPOSE

During vertebrate phototransduction 11-cis-retinal is isomerized 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 Müller glia. CRALBP expressed in the RPE is known to facilitate the rate of the rod visual cycle. Recent evidence suggests a role for Müller glia in an alternate cone visual cycle. In this study, the role of RPE- and Müller-CRALBP in cone vision was characterized.

METHODS

The 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 hybridization 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 analyzed in 5-day post fertilization (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 Müller glia, respectively. Depletion of CRALBP in the RPE or Müller glia results in abnormal cone visual behavior.

CONCLUSIONS

The results suggest that cone photoreceptors incorporate 11-cis-retinoids derived from the rod and cone visual cycles into their visual pigments and that Müller-CRALBP participates in the cone visual cycle.