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Hongwei Ma, Arjun Thapa, Lynsie Morris, Mirja Koch, Martin Biel, Stylianos Michalakis, Xi-Qin Ding; Inhibition of Protein Kinase G Activity Improves Cone Survival in Cyclic Nucleotide-gated Channel Deficiency. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1259.
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Cone phototransduction mediated by cyclic nucleotide-gated (CNG) channels is essential for central and color vision and for visual acuity. Naturally occurring mutations in the cone CNG channel subunit CNGA3 are associated with achromatopsia and cone dystrophy. We have shown that cyclic guanosine monophosphate (cGMP) is remarkably elevated in CNG channel-deficient retina. We also observed an increased activity and expression of the cGMP-dependent protein kinase G (PKG) in CNG channel deficiency. This work examined whether the elevated PKG function plays a role in cone cell death.
Wild type (C57BL/6), and three CNG channel-deficient mouse lines, Cnga3-/-, Cnga3-/-/Nrl-/- (Cnga3 deficiency on a cone-dominant background), and Cnga3-/-/Pkg1-/- (deficiency of Cnga3 and Pkg1, which encodes mouse PKG1), were used in this study. Inhibition of PKG activity was achieved by treating mice with the PKG inhibitors KT5823 and Rp-8-Br-cGMPS. Retinal cGMP level and PKG activity were analyzed by ELISA. Cone death was evaluated by TUNEL on retinal cross sections, and cone density and expression levels of cone specific proteins were evaluated by immunohistochemical labeling and western blotting.
Treatment with PKG inhibitors effectively reduced PKG activity to the control levels. The TUNEL positive cells were significantly reduced in Cnga3-/-/Nrl-/- mice treated with PKG inhibitors. Cone density and expression levels of M-opsin and S-opsin were greatly increased in PKG inhibitor-treated mice, compared with vehicle-treated controls. In addition, the expression levels of the endoplasmic reticulum stress marker p-eIF2α and cleavage of the apoptotic protein caspase-7 were significantly reduced in Cnga3-/-/Nrl-/- mice treated with PKG inhibitors. Moreover, cone density was significantly increased in Cnga3-/-/Pkg1-/- mice compared with age-matched Cnga3-/- controls.
This work demonstrates that PKG plays a role in cone death. Thus, suppressing PKG activity may represent a new approach to reduce cone cell death associated with cGMP accumulation, including cone death caused by CNG channel deficiency and by photoreceptor phosphodiesterase deficiency.
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