The broad objective of this project is to understand the underlying events that lead to photoreceptor degeneration when problems with retinoid metabolism arise and to develop methods to prevent cones from degenerating. In Leber Congenital Amaurosis type 2 (LCA2), generation of the native chromophore of visual pigments (11-cis retinal) is inhibited. In mouse models for LCA2, cone cells die rapidly. Damage is most severe with short wavelength sensitive (SWS1) cones. This pattern roughly parallels the pathogenesis of LCA2. Studies from other laboratories have demonstrated that gene therapy can restore vision but appears limited by irreversible cone loss prior to the treatment. Early administration of an exogenous source of 11-cis retinal to mouse models improved cone survival, but recent work from this laboratory demonstrated that this was ineffective when mice were subjected to room light. These results suggest that (1) cone opsin/11-cis retinal interactions are important in preventing cone death, and (2) 11-cis retinal will not be the solution to treating LCA2 because normal light conditions negates its benefits. Because cone opsins are constitutively active but deactived with 11-cis retinal, the hypothesis for this project is that the increased levels of active cone opsins lead to cone degeneration in LCA2. Thus, light-insensitive small molecules that deactivate cone opsins will be protective to cone cells when endogenous 11-cis retinal is limited. Preliminary data indicate that beta ionone, a truncated analog of 11-cis retinal, improved survival of middle/long wavelength-sensitive (M/LWS) cones but not SWS1 cones. Consistent with the hypothesis, beta ionone is an inverse agonist to M/LWS cone opsins but an agonist to SWS1 cone opsins. This proposal aims to improve the survival of all cone types in mouse models for LCA2 such that reintroduction of the missing gene later in development can still improve vision;identify new compounds that can deactivate cone opsins; ensure that they will not severely impede vision in wild-type mice;and determine the impact of these compounds on trafficking of and post-translational modifications to cone opsins in cell culture and animal models. Fluorescence microscopy, electroretinography, mass spectrometry, and biochemicial methods will be used to assess the effects of test compounds on cone cell survival, function, and opsin properties. The use of opsin inverse agonists may have broader applicability in improving photoreceptor survival for other visual problems associated with compromised retinoid processing such as Stargardt's, retinitis pigmentosa, and aging.
Cones are used for daily bright light and color vision, but defects in Vitamin A processing in the eye can lead to cone cell degeneration, which is what happens with Leber Congenital Amaurosis type 2. The goals of this project are to understand this degeneration process and to develop therapeutic approaches to keeping cones alive by targeting the protein components of cone visual pigments. This approach may be effective in slowing down rod and/or cone photoreceptor cell death associated with other visual defects such as Stargardt's disease, retinitis pigmentosa, and aging that arise from abnormal Vitamin A metabolism.
