The Role of Endoplasmic Reticulum Calcium Channels in Cone Degeneration Resulting from CNG Channel Deficiency

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ABSTRACT The cone photoreceptor cyclic nucleotide-gated (CNG) channel is essential for cone phototransduction. Mutations in the CNGA3 and CNGB3 genes encoding the cone channel subunits account for about 80% of all cases of achromatopsia, and are associated with progressive cone dystrophies. Cones in patients and in mouse models of CNG channel deficiency degenerate over time. Using CNG channel-deficient (CCD) mouse models, we found that CNG channel deficiency leads to endoplasmic reticulum (ER) stress-associated cone death. We also observed that CCD retinas display increased activity and expression levels of the ER calcium- releasing channels inositol 1,4,5-trisphosphate receptor (IP3R) and ryanodine receptor (RyR), and treatment with inhibitors of IP3R and RyR reduces ER stress and cone death. The objective of this study is to understand the mechanisms of ER calcium channel-associated cone death in CNG channel deficiency. We will determine whether the loss of functional CNG channels leads to impaired ER calcium homeostasis/ER calcium depletion, impaired protein processing, and ER stress, and whether suppressing ER calcium channels will reduce ER stress/cone death. Three specific aims will address our questions. Aim 1 is to determine the role of ER calcium channels in CCD ER stress and cone death. We will evaluate the effects of IP3R and RyR inhibition on ER stress and cone death. Conditional knockout and adeno-associated viral (AAV)-mediated CRISPR/Cas9 genome editing approaches will be used to deplete ER calcium channels. Aim 2 is to determine the role of ER calcium channels in CCD cone opsin mistrafficking. CCD mice display cone opsin mistrafficking, and ER chaperons and inhibitors for IP3R improve cone opsin trafficking. We will evaluate the effects of ER calcium channel inhibition/depletion on the cellular localization of cone opsin and other cone outer segment proteins in CCD mice. We will also investigate whether promoting ER protein processing/ER-associated protein degradation improves cone protein trafficking. Aim 3 is to determine how cGMP/PKG (cGMP-dependent protein kinase) signaling induces CCD ER stress and cone death, and whether ER calcium channels are the significant targets. CCD retinas show elevated cGMP/PKG signaling, and suppressing cGMP/PKG signaling reduces ER stress and cone death. We will examine the effects of cGMP/PKG signaling on the expression and activity of the ER calcium channels. We will also evaluate the effects of cGMP/PKG signaling on other unfolded protein response/ER stress components. Upon completion of the proposed study, we will better understand the mechanisms of cone degeneration in CNG channel deficiency. Specifically, we will know whether ER calcium channels play a role in ER stress and cone death. This information is vital for the development of ER calcium channel-based therapeutic strategies for photoreceptor preservation.