The rd mouse is a model of an inherited retinal degeneration which has a defect in its phototransduction cascade. The beta-subunit of cGMP-PDE is the candidate gene for retina degeneration in this mouse. To study the mechanisms underlying degeneration and the potential for gene repair, we have expressed a normal bovine cGMP-PDE beta-subunit (beta-PDE) in the rd mouse. In these transgenic animals, we were able to restore the beta- PDE enzyme activity and 'rescue' the photoreceptors. While the 'rescue' of the retina in these animals confirms that the beta-PDE is the rd gene, the processes resulting from this defective gene which lead to the loss of all rods and cones remain to be resolved. Clearly, the normal functioning of rod-specific PDE is fundamental to maintaining the integrity of the retina, thus understanding its role in retinal degeneration is critical. The long-term goal of this proposal is to reveal mechanisms linking a gene defect in the rod phototransduction cascade to the loss of all rods as well as cones, as occurs in many human inherited retinal degenerations. A further goal is to develop pharmacologic interventions to inhibit these mechanisms. Specifically, we will test the idea that there is a 'window' of expression which the phototransduction mechanism can tolerate and that beta-PDE expression above or below that level leads to rod degeneration. We will test this concept by examining appropriate transgenic, rd/rd,rd/+ and normal +/+ mice using in situ hybridization and immunocytochemical methods. We will also test the hypothesis that the heterozygote rd/+ is adapted to compensate for reduced beta-PDE levels by regulating the number of cGMP-gated Na+/Ca++ channels. The improper sorting of opsin is a hallmark of all rodent models of retinal degenerations. We will test if the normal sorting of opsin is restored concomitant with the restoration of PDE activity in transgenic rd/rd mice. We will determine the cause of the gradient of degeneration observed in some transgenic mice which has a pattern similar to 'sectoral' RP. Finally, we will develop strategies for therapeutic intervention in rd. We will test our hypothesis that the rd defect opens an excessive number of cGMP-gated ion channels, resulting in a catastrophic ion influx. Several drugs which specifically block Na+/Ca++ channels will be used to control this influx. Secondarily, we will attempt to block destructive cascades which may cause the loss of the cones, specifically the activation of degradative enzymes and the generation of free radicals. We will use specific enzyme inhibitors and free-radical scavengers to intervene in these processes.
