Small molecule modulation of ATF6 affects photoreceptor integrity in human retinal organoids

Activating Transcription Factor 6 (ATF6) is a key component of the unfolded protein response, a protein quality control system of the cell. Previous studies found that mutations in the ATF6 gene resulted in inherited vision loss diseases such as achromatopsia (ACHM). Furthermore, retinal organoids (RO) derived from ACHM patients with ATF6 mutations revealed that loss of ATF6 function causes severe defects in cone photoreceptor development and increased ER stress in cone cells. Despite the clear connection between ATF6 function and human photoreceptor integrity, pharmacological efforts to manipulate ATF6 and investigate its effect on the retina remain limited. In particular, small molecule drugs are effective agents that can target certain molecular pathways, but their use has been overlooked in research on ATF6 in human vision. This study aims to use small molecule ATF6 modulators to chemically interrogate the role of ATF6 in the maintenance of photoreceptors in ROs. This study developed ROs from wildtype human embryonic stem cells and treated mature ROs with the ATF6 inhibitor Ceapin-A7 (CA7), to investigate whether ATF6 is essential for maintaining photoreceptor integrity. Fluorescent microscopy cross-sections of ROs revealed that CA7-treated organoids showed shorter and fewer photoreceptor segments (i.e., brush border). RNA-Seq revealed significant disruption of genes involved in photoreceptor structural integrity and protein quality control in CA7-treated ROs. These results suggest that chemical inhibition of ATF6 disrupts the integrity of photoreceptors in ROs. Importantly, no cell death was observed with ATF6 inhibition. Based on this, we propose that ATF6 is essential for the maintenance of photoreceptor segments, and that its loss leads to visual defects in patients carrying these alleles. Knowledge gained from these studies could provide insight to the disease mechanism of ACHM and build the foundation for developing treatment options to prevent or reverse vision loss in patients with ATF6 mutations.