The objective is to investigate the metabolism of cyclic nucleotides in normal and diseased visual cells with a special emphasis in the less biochemically characterized cones. In addition, we wish to investigate the mechanism by which elevated cyclic nucleotide levels affect the viability of photoreceptor cells in some retinal degenerations. Our work has suggested that in contrast to the function that cGMP plays in rods, cAMP may be more important than cGMP in cone-dominant retinas since cAMP concentration is higher, cAMP levels are decreased by light, and elevated levels of cAMP seem to be more toxic and cause the specific degeneration of cones. However, to establish that cones and rods utilize different cyclic nucleotides as intracellular regulators it is necessary to study isolated cone photoreceptors because the rest of the retina interferes with its own cAMP metabolism. Using isolated cone cells we will study their cyclic nucleotide content, activities, kinetics and regulation of cyclic nucleotide synthetic and degradative enzymes, and investigate if cyclic nucleotide action is modulated by the phosphorylation of specific proteins. We will determine whether light reduces cyclic nucleotide levels and whether activities of the metabolic enzymes are coupled to bleaching of the visual pigment. The same enzyme systems will also be studied in developing cone-like photoreceptors grown in culture, obtained from dissociated retinas of chick embryos. In addition, we will continue to collect baseline data on cyclic nucleotide metabolism in normal human retina to be used for comparison with results from retinas affected with Retinitis Pigmentosa, macular degeneration or other retinal diseases, whenever these retinas become available. To gain insight into how elevated cyclic nucleotide levels participate in visual cell degeneration, we will study first degeneration of rods in the rd mouse, since we have several probes on hand and clues about what components need to be analyzed. In the future, when more is known about cones and probes are developed, we will focus our attention on cone degeneration. We will use antibodies and molecular biology methods to measure concentration and synthesis of cGMP-phosphodiesterase (PDE), the activity of which is abnormal in rd retina, and mRNAs that code for G-protein and rhodopsin, which are needed for PDE activation. We will also clone cDNAs for PDE and rhodopsin from control and rd retinas. Analyses of the cDNA sequences may reveal anomalies associated with the cause of the rd mouse disease and further our understanding of photoreceptor degeneration.
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