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A.M. Dizhoor, A.B. Savchenko, M.L. Woodruff, I.V. Peshenko, E.V. Olshevskaya, R. Barrett, R.A. Bush, P.A. Sieving, G.L. Fain; Activation of PDE6 Cascade in the Dark Reverses Photoreceptor Degeneration Caused by Y99C Mutation in GCAP1 . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1093.
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© ARVO (1962-2015); The Authors (2016-present)
The Y99C GCAP1 reduces Ca2+–sensitivity of retinal guanylyl cyclase (retGC), elevates the free intracellular cGMP and Ca2+ concentrations, and causes degeneration of photoreceptors in transgenic mice . We hypothesize that activation of PDE6 in the dark can slow retinal degeneration in the Y99C mice by lowering the free cGMP and Ca2+concentrations.
A mutation, G90D, in rhodopsin that activates the PDE6 cascade in the dark , was introduced into a fast degenerating mouse line, L53 , expressing the Y99C GCAP1 (Y99C+); a single allele for wild type rhodopsin (R) remained . The Y99C+;G90D+;R+/– mice were analyzed for ERG, retinal morphology, Ca2+–sensitivity of retGC, and the intracellular free Ca2+concentrations .
Ca2+ sensitivity of retGC in the L53 mice (both Y99C+;R+/+ and Y99C+;R+/–) was shifted toward higher than normal dark–adapted free Ca2+ concentrations. They rapidly lost ERG responses and nearly all photoreceptor nuclei by the age of 6 months. The maximal amplitude of the ERG a–wave in the Y99C+;R+/– mice after 120 days was less than 5% of normal (15 µV ±15 SD, n=27), often with no measurable a–wave at saturating flash strength. In the Y99C+;G90D+;R+/– retinas, retGC also remained active at high free Ca2+ concentrations in vitro, but the dark adapted free intracellular Ca2+ level was closure to the wild type. The Y99C+;G90D+;R+/– mice had clear ERG–responses at > 8 months of age (the oldest age group tested). The light–sensitivity of ERG was reduced, but the maximal a–wave reached half the normal amplitude (227 µV±76 SD, n=58) and half of the rod nuclei remained in their retinas after 6 months of age; the morphology of the photoreceptors was similar to the wild type.
Activation of PDE6 in the dark by G90D rhodopsin can effectively restore near–normal levels of cGMP and Ca2+ in Y99C rods thus rescuing them from degeneration caused by abnormal cGMP synthesis.
. Olshevskaya et al., (2004) J. Neuroscience 24, 6078;  Sieving et al., (2001) J. Neuroscience 21, 5449;  Woodruff et al., (2002) J.Physiol (London) 542, 843.
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