Visual pigments consist of an apoprotein opsin and a chromophore, 11-cis retinal. Light-induced isomerization of 11-cis retinal to all-trans retinal initiates vision. Efficient regeneration of 11-cis retinal is essential for normal vision. The retinoid visual cycle refers to a multi-step process to regenerate 11-cis retinal. A key step in the visual cycle is the conversion of all-trans retinyl ester into 11-cis retinol, catalyzed by retinoid isomerase in the RPE. Previously, we identified RPE65 as the retinoid isomerase. Multiple recessive RPE65 mutations are known to cause retinal dystrophies. Recent genetic studies identified the first dominant point mutation of RPE65, D477G, associating with retinitis pigmentosa (RP) in human patients. Unlike the recessive mutations, the patients carrying a single copy of D477G develop progressive vision loss, suggesting a novel and yet to be understood mechanism for retinal dystrophy. This project will elucidate the pathogenic mechanism by which D477G impairs vision. We have recently generated a heterozygous D477G knock-in (KI) mouse. D477G KI mice displayed delayed dark-adaptation and decreased 11-cis retinal regeneration, suggesting that D477G may function as a dominant negative mutant that interferes with the isomerase activity of WT RPE65 and impairs the visual cycle. We will investigate the impacts of D477G KI on the visual cycle and retinal structure and function. We will also elucidate the mechanism by which D477G disturbs the visual cycle. We will investigate if D477G protein may physically interact with WT RPE65 and affect its conformation, leading to decreased enzymatic activity or stability of WT RPE65. These studies will not only reveal a new pathogenic mechanism for inherited retinal dystrophies, but also contribute to the understanding of structure and function of RPE65 and the visual cycle. Diabetic retinopathy (DR) is a major blinding disorder. The implication of the visual cycle in DR has not been investigated. Our preliminary studies demonstrated that generation of 11-cis retinal and visual pigments is deficient in diabetic rats, while opsin levels are unchanged. The consequent increase of free opsin may contribute to photoreceptor degeneration and visual impairment in DR. Further, we found that a visual cycle protein, interphotoreceptor retinoid-binding protein (IRBP) is down-regulated in diabetic retinas. An objective of this project is to investigate the causative role of the visual cycle dysfunction in visual deficits in early DR. We will determine if administration of a chromophore to diabetic mice can ameliorate vison loss in early DR. We will also induce diabetes in mice with ablation or over-expression of IRBP to evaluate the impacts of IRBP down-regulation on the impaired rhodopsin regeneration and visual deficits induced by diabetes. This study will explore an undocumented association of a disturbed visual cycle with DR and has potential to identify a new therapeutic strategy for DR.
Vitamin A is essential for normal vision. The retinoid visual cycle is a multi-step pathway to recycle 11-cis retinal, the active form of vitamin A for forming visual pigments, an essential process for normal vision. This project is to study how visual cycle dysfunctions lead to vision loss in inherited retinal diseases and diabetic retinopathy, a common complication of diabetes leading to blindness. These studies will contribute to our understanding of some retinal degenerative diseases and to the development of new treatment strategies for these blinding diseases.
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