Translation regulation of ATF4 by the termination complex Hbs1-Pelo is required for visual system development and function

Inherited retinal diseases are a class of genetically heterogenous disorders characterized by mutations in genes required for retinal function, resulting in progressive loss of vision in human patients. One such deletion mutation in the translation termination factor, HBS1L, results in a suite of developmental anomalies in human patients, including progressive vision loss defects. HBS1L, and its interaction partner Pelota (Pelo), are required for recycling stalled ribosomes on mRNAs, but the specific mRNA target causative of vision defects seen with HBS1L deletion remains unknown. Further, the specific cell types in the visual system that require HBS1L for proper development and function are also unknown. Here, we discover that loss of the Drosophila HBS1L homolog, Hbs1, results in reduced expression of the stress responsive Activating Transcription Factor 4 (ATF4) which is encoded by an mRNA containing multiple upstream open reading frames (uORFs) in its 5 leader. We corroborate these results in cultured human cells, where we find that HBS1L and Pelo promote translation reinitiation at the ATF4 ORF by facilitating proper translation termination at the preceding uORFs. Like human HBS1L deficiency patients, loss of function Drosophila mutants for Hbs1, pelo, and ATF4 show vision defects as measured by electroretinograms (ERG). Depleting Hbs1 in lamina neurons replicated the ERG defects seen in Hbs1 mutants, suggesting that Hbs1-Pelo is required for proper lamina neuron function. Further confocal analysis of Hbs1 mutants revealed vacuolization defects in the lamina layer, which are indicative of defective synapse formation between lamina neurons and photoreceptors. Strikingly, restoring ATF4 expression in the lamina partially rescued ERG defects in Hbs1 mutants, indicating that ATF4 is likely a relevant mRNA target regulated by Hbs1-Pelo in these cells. Together, our data support a model wherein Hbs1-Pelo mediated translation regulation of ATF4 in lamina neurons underlies the inherited retinal disease caused by HBS1L deletion.