Vision in civilized man is mediated largely by cone photoreceptors. Despite the importance of cones, much more is known about the biology of rods. The first step in visual perception is absorption of a photon by a visual pigment molecule, which induces 11-cis to all-trans isomerization of the retinaldehyde chromophore. Before light-sensitivity can be restored, all-trans-retinaldehyde must be re-isomerized to 11-cis-retinaldehyde. This involves a multi-step process called the visual cycle, which has been worked out mainly in rod-dominant species. Several lines of published evidence suggest that cone photoreceptors regenerate visual pigment by an alternate pathway. Nothing is known, however, about the biochemistry of this alternate pathway. We have recently identified three new catalytic activities in cone-dominant ground squirrel and chicken retinas that mediate regeneration of 11-cis-retinaldehyde from all-trans-retinol.
The first aim of this application will be to purify and clone the mRNA for: (1) a new 11-cis-retinol dehydrogenase distinct from the 11-cis-retinol dehydrogenase type-5 in retinal pigment epithelial cells; (2) an all-trans-retinol isomerase that catalyzes the direct conversion of all-trans-retinol to 11-cis-retinol utilizing fatty-acyl-CoAs as an energy source; and (3) an 11-cis-retinyl-ester synthase that acts by a mechanism clearly distinct from that of lecithin-retinol acyl transferase (LRAT). We plan to characterize these proteins functionally, using in vitro biochemical, cell culture expression, and mouse transgenic/knockout systems. Recessive Stargardt's disease is an inherited form of macular degeneration caused by mutations in the ABCR gene. During the previous funding period, we generated mice with a knockout mutation in abcr. The phenotype in these animals is strikingly similar to the clinical phenotype in patients with recessive Stargardt's disease, including accumulation of lipofuscin in the retinal pigment epithelium. Lipofuscin accumulation appears to be a critical event in the development of retinal pathology.
The second aim of this application is to test a promising strategy for inhibiting lipofuscin deposition and photoreceptor degeneration in abcr-/- mice. If successful, this strategy should lead to the initiation of clinical trials on patients with active Stargardt's disease.
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