title: Endoplasmic reticulum stress-associated cone photoreceptor degeneration in cyclic nucleotide-gated channel deficiency.
abstract: Cyclic nucleotide-gated (CNG) channels play a pivotal role in phototransduction. Mutations in the cone CNG channel subunits CNGA3 and CNGB3 account for >70% of all known cases of achromatopsia. Cones degenerate in achromatopsia patients and in CNGA3(-/-) and CNGB3(-/-) mice. This work investigates the molecular basis of cone degeneration in CNG channel deficiency. As cones comprise only 2-3% of the total photoreceptor population in the wild-type mouse retina, we generated mouse lines with CNG channel deficiency on a cone-dominant background, i.e. CNGA3(-/-)/Nrl(-/-) and CNGB3(-/-)/Nrl(-/-) mice. The retinal phenotype and potential cell death pathways were examined by functional, biochemical, and immunohistochemical approaches. CNGA3(-/-)/Nrl(-/-) and CNGB3(-/-)/Nrl(-/-) mice showed impaired cone function, opsin mislocalization, and cone degeneration similar to that in the single knock-out mice. The endoplasmic reticulum stress marker proteins, including Grp78/Bip, phospho-eIF2α, phospho-IP(3)R, and CCAAT/enhancer-binding protein homologous protein, were elevated significantly in CNGA3(-/-)/Nrl(-/-) and CNGB3(-/-)/Nrl(-/-) retinas, compared with the age-matched (postnatal 30 days) Nrl(-/-) controls. Along with these, up-regulation of the cysteine protease calpains and cleavage of caspase-12 and caspase-7 were found in the channel-deficient retinas, suggesting an endoplasmic reticulum stress-associated apoptosis. In addition, we observed a nuclear translocation of apoptosis-inducing factor (AIF) and endonuclease G in CNGA3(-/-)/Nrl(-/-) and CNGB3(-/-)/Nrl(-/-) retinas, implying a mitochondrial insult in the endoplasmic reticulum stress-activated cell death process. Taken together, our findings suggest a crucial role of endoplasmic reticulum stress in cone degeneration associated with CNG channel deficiency.
authors.affiliation: Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73104, USA.
authors.forename: Arjun
authors.initials: A
authors.lastname: Thapa
authors.forename: Lynsie
authors.initials: L
authors.lastname: Morris
authors.forename: Jianhua
authors.initials: J
authors.lastname: Xu
authors.forename: Hongwei
authors.initials: H
authors.lastname: Ma
authors.forename: Stylianos
authors.initials: S
authors.lastname: Michalakis
authors.forename: Martin
authors.initials: M
authors.lastname: Biel
authors.forename: Xi-Qin
authors.initials: XQ
authors.lastname: Ding
mesh_terms: D000818:Animals; D016923:Cell Death; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000235:genetics*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D000091342:Endoplasmic Reticulum Chaperone BiP; D020287:In Situ Nick-End Labeling; D015640:Ion Channel Gating; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D017949:Retinal Cone Photoreceptor Cells / Q000378:metabolism*
chemical_list: C532622:CNGB3 protein, mouse; C518237:Cnga3 protein, mouse; D054815:Cyclic Nucleotide-Gated Cation Channels; D000091342:Endoplasmic Reticulum Chaperone BiP; C000717248:Hspa5 protein, mouse