highroad is induced by retinoids and clears mutant Rhodopsin-1 in Drosophila Retinitis Pigmentosa models.
The light detecting protein, Rhodopsin, requires retinoid chromophores for their function. In vertebrates, retinoids also serve as signaling molecules, but whether these molecules similarly regulate gene expression in Drosophila remains unclear. Here, we report the identification of a retinoid-inducible gene in Drosophila, highroad, which is required for photoreceptors to clear folding-defective mutant Rhodopsin-1 proteins. Specifically, we identified highroad through an in vivo RNAi based genetic interaction screen with one such folding defective Rhodopsin-1 mutant, ninaEG69D. CRISPR-Cas9-mediated deletion of highroad results in the stabilization of folding-defective mutant Rhodopsin-1 proteins, and acceleration of the age-related retinal degeneration phenotype of ninaEG69D mutants. Elevated highroad transcript levels are detected ninaEG69D flies, and interestingly, deprivation of retinoids in the fly diet blocks this effect. Consistently, mutations in the retinoid transporter santa maria impairs the induction of highroad in ninaEG69D flies. In cultured S2 cells, highroad expression is induced by retinoic acid treatment. These results indicate that cellular quality control mechanism against misfolded Rhodopsin-1 involves regulation of gene expression by retinoids.