Preservation of endoplasmic reticulum (ER) Ca 2+ stores by deletion of inositol‐1,4,5‐trisphosphate receptor type 1 promotes ER retrotranslocation, proteostasis, and protein outer segment localization in cyclic nucleotide‐gated channel‐deficient cone photoreceptors

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Endoplasmic reticulum (ER) Ca 2+ homeostasis relies on an appropriate balance between efflux‐ and influx‐channel activity responding to dynamic changes of intracellular Ca 2+ levels. Dysregulation of this complex signaling network has been shown to contribute to neuronal and photoreceptor death in neuro‐ and retinal degenerative diseases, respectively. In mice with cone cyclic nucleotide‐gated (CNG) channel deficiency, a model of achromatopsia/cone dystrophy, cones display early‐onset ER stress‐associated apoptosis and protein mislocalization. Cones in these mice also show reduced cytosolic Ca 2+ level and subsequent elevation in the ER Ca 2+ ‐efflux‐channel activity, specifically the inositol‐1,4,5‐trisphosphate receptor type 1 (IP 3 R1), and deletion of IP 3 R1 results in preservation of cones. This work investigated how preservation of ER Ca 2+ stores leads to cone protection. We examined the effects of cone specific deletion of IP 3 R1 on ER stress responses/cone death, protein localization, and ER proteostasis/ER‐associated degradation. We demonstrated that deletion of IP 3 R1 improves trafficking of cone‐specific proteins M‐/S‐opsin and phosphodiesterase 6C to cone outer segments and reduces localization to cone inner segments. Consistent with the improved protein localization, deletion of IP 3 R1 results in increased ER retrotranslocation protein expression, reduced proteasome subunit expression, reduced ER stress/cone death, and reduced retinal remodeling. We also observed the enhanced ER retrotranslocation in mice that have been treated with a chemical chaperone, supporting the connection between improved ER retrotranslocation/proteostasis and alleviation of ER stress. Findings from this work demonstrate the importance of ER Ca 2+ stores in ER proteostasis and protein trafficking/localization in photoreceptors, strengthen the link between dysregulation of ER Ca 2+ homeostasis and ER stress/cone degeneration, and support an involvement of improved ER proteostasis in ER Ca 2+ preservation‐induced cone protection; thereby identifying IP 3 R1 as a critical mediator of ER stress and protein mislocalization and as a potential target to preserve cones in CNG channel deficiency.