Disturbed retinoid metabolism upon loss of <i>rlbp1a</i> impairs cone function and leads to subretinal lipid deposits and photoreceptor degeneration in the zebrafish retina.

Abstract

The <i>RLBP1</i> gene encodes the 36 kDa cellular retinaldehyde-binding protein, CRALBP, a soluble retinoid carrier, in the visual cycle of the eyes. Mutations in <i>RLBP1</i> are associated with recessively inherited clinical phenotypes, including Bothnia dystrophy, retinitis pigmentosa, retinitis punctata albescens, fundus albipunctatus, and Newfoundland rod-cone dystrophy. However, the etiology of these retinal disorders is not well understood. Here, we generated homologous zebrafish models to bridge this knowledge gap. Duplication of the <i>rlbp1</i> gene in zebrafish and cell-specific expression of the paralogs <i>rlbp1a</i> in the retinal pigment epithelium and <i>rlbp1b</i> in Müller glial cells allowed us to create intrinsically cell type-specific knockout fish lines. Using <i>rlbp1a</i> and <i>rlbp1b</i> single and double mutants, we investigated the pathological effects on visual function. Our analyses revealed that <i>rlbp1a</i> was essential for cone photoreceptor function and chromophore metabolism in the fish eyes. <i>rlbp1a-</i>mutant fish displayed reduced chromophore levels and attenuated cone photoreceptor responses to light stimuli. They accumulated 11-<i>cis</i> and all-<i>trans</i>-retinyl esters which displayed as enlarged lipid droplets in the RPE reminiscent of the subretinal yellow-white lesions in patients with <i>RLBP1</i> mutations. During aging, these fish developed retinal thinning and cone and rod photoreceptor dystrophy. In contrast, <i>rlbp1b</i> mutants did not display impaired vision. The double mutant essentially replicated the phenotype of the <i>rlbp1a</i> single mutant. Together, our study showed that the <i>rlbp1a</i> zebrafish mutant recapitulated many features of human blinding diseases caused by <i>RLBP1</i> mutations and provided novel insights into the pathways for chromophore regeneration of cone photoreceptors.