Chemical induction and reversal of photoreceptor dystrophy in retinal organoids.

Activating transcription factor 6 (ATF6) controls a signal transduction pathway of the unfolded protein response (UPR). Loss-of-function ATF6 mutations cause inherited photoreceptor diseases. Ceapin-A7 is a non-toxic, cell-permeable small molecule that selectively inhibits ATF6. We investigated how ATF6 inhibition impacts photoreceptors by treating wild-type human retinal organoids with Ceapin-A7 and evaluating the effects via single-nucleus RNA sequencing (snRNA-seq) and microscopy after 2 months. Ceapin-A7 administration effectively suppressed ATF6-dependent and UPR-related gene expression and led to fewer rod and cone inner/outer segments, malformed/stunted cone segments, and Müller gliosis, while no photoreceptor cell death was observed. When Ceapin-A7 was removed from retinal organoid media after extended exposure, photoreceptor segment quantity increased, indicating that photoreceptors were able to restore segments. These findings demonstrate that ATF6 is selectively required for photoreceptor inner/outer segment integrity. Furthermore, chemically restoring ATF6 activity promotes photoreceptor segment growth, identifying ATF6/UPR signaling as an attractive pathway to treat photoreceptor diseases.