Germline Disruption of Retinal Pigment Epithelium‐Expressed Zebrafish <i>rlbp1b</i> ^−/− Results in Selective Dim Light Visual Behavior Deficits and Provides a Screening Platform for Evaluating the Pathogenicity of Human <scp> <i>RLBP1</i>

Cellular retinaldehyde binding protein (CRALBP) plays a crucial role in the visual cycle by chaperoning 11-cis-retinoids. Mutations in its encoding gene RLBP1 lead to inherited retinal diseases with the common feature of poor night vision. Zebrafish possess two RLBP1 paralogs, rlbp1a and rlbp1b, with distinct retinal expression profiles, providing a bespoke opportunity to dissect cell-specific functions of CRALBP. Here, we first resolved conflicting reports on paralog expression by interrogating zebrafish single-cell RNA-sequencing datasets, which revealed predominant rlbp1a expression in Müller glia and rlbp1b expression in the RPE. Using CRISPR-generated zebrafish knockouts, we demonstrated that loss of RPE-expressed rlbp1b selectively impaired optokinetic responses (OKR) with a ~50% reduction in saccade frequency relative to wildtype. This impaired OKR response is only seen when under dim light conditions with no defect seen in standard or bright light rearing conditions. This recapitulates the night blindness presentation in patients with RLBP1 mutations. Retinoid profiling of rlbp1b knockout larvae showed significant decreases in 11-cis-retinal (62% reduced) and all-trans-retinal (69% reduced) levels. To explore mechanistic changes following rlbp1b loss, unbiased proteomic profiling was carried out on rlbp1b knockout adult zebrafish eyes. This confirmed the knockout of Cralbpb and revealed significant disruption of proteins involved in vitamin A metabolism, lipid metabolism/storage and ferroptosis. To explore the utility of zebrafish for in vivo pathogenicity assessment of RLBP1, we established a complementation assay using transgenic zebrafish. Although expression of wildtype EGFP-tagged human RLBP1 did not rescue the visual deficit, expression of zebrafish Cralbp to the RPE restored dim light vision, whereas zebrafish Cralbp harboring the human pathogenic p.R151Q mutation failed to do so. Together, these findings underscore the predominant role of RPE-expressed CRALBP in sustaining visual function under low-light conditions and establish a zebrafish platform for functional evaluation of RLBP1 variants.