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Stylianos Michalakis, Mirja Koch, Hongwei Ma, Xi-Qin Ding, Martin Biel; cGMP-cGKII signalling contributes to cone photoreceptor degeneration in achromatopsia. Invest. Ophthalmol. Vis. Sci. 2016;57(12):165.
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© 2017 Association for Research in Vision and Ophthalmology.
Achromatopsia (ACHM) is a genetically and clinically well-defined inherited retinal disorder. Other than originally assumed cone photoreceptors do degenerate over time. Our knowledge about the mechanisms underlying the progressive degeneration of cones in ACHM is still limited. One important observation is that degenerating cones show a marked accumulation of the second messenger cyclic guanosine monophosphate (cGMP). It is still unclear whether this cGMP accumulation is directly linked to cell death or results as a side effect of the degenerative disease.
We evaluated the progression of cone photoreceptor degeneration in the Cnga3 knockout (KO) mouse model of ACHM using in vivo fundus fluorescence imaging and ex vivo morphological/histological analysis after labeling of cone photoreceptors with specific markers (eGFP, peanut agglutinin or glycogen phosphorylase). The contribution of the cGMP producing enzyme guanylyl cyclase E (GC-E) and the cGMP effectors cGMP-dependent kinase (cGK) type I and II was analyzed using gene KO mouse models and/or specific shRNAs delivered to the retina using AAV8Y733F-pseudotyped adeno-associated virus 2 (AAV) vectors. To probe the effect of cGK overactivation on photoreceptor viability we expressed a constitutively active cGK variant using AAV vectors and monitored the retinal morphology.
shRNA-mediated knock down (KD) of the retinal GC-E significantly reduced the rate of cone degeneration in Cnga3 KO mice, confirming a causal link between cGMP and cone degeneration. In addition, overexpression of a constitutively active cGK variant in wildtype photoreceptors induced degeneration, resulting in significant loss of photoreceptors within 5 weeks. Both cGKs were expressed in cone photoreceptors. However, genetic inactivation of cGKI in Cnga3 KO mice had no obvious effect on cone viability. By contrast, deletion of cGKII significantly reduced the number of TUNEL-positive cells and delayed degeneration of Cnga3 KO cones, resulting in up to 3-fold higher numbers of surviving cones in Cnga3/cGKII double KO mice compared to Cnga3 KO mice. Finally, KD of cGKII using specific AAV-shRNA also significantly reduced the rate of cone degeneration in Cnga3 KO mice.
These findings establish a causal link between cGMP/cGKII signaling and cone photoreceptor degeneration and highlight cGKII as a novel target for neuroprotection in ACHM and cone dystrophies.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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