ap20187 and Retinitis-Pigmentosa

We previously reported a transgenic Xenopus laevis model of retinitis pigmentosa in which tadpoles express the bovine form of P23H rhodopsin (bP23H) in rod photoreceptors. In this model, retinal degeneration was dependent on light exposure. Here, we investigated ultrastructural changes that occurred in the rod photoreceptors of these retinas when exposed to light.. Tadpoles expressing bP23H in rods were transferred from constant darkness to a 12-hour light:12-hour dark (12L:12D) regimen. For comparison, transgenic tadpoles expressing an inducible form of caspase 9 (iCasp9) were reared in a 12L:12D regimen, and retinal degeneration was induced by administration of the drug AP20187. Tadpoles were euthanized at various time points, and eyes were processed for confocal light and transmission electron microscopy.. We observed defects in outer and inner segments of rods expressing bP23H that were aggravated by light exposure. Rod outer segments exhibited vesiculations throughout and were rapidly phagocytosed by the retinal pigment epithelium. In rod inner segments, we observed autophagic compartments adjacent to the endoplasmic reticulum and extensive vesiculation at later time points. These defects were not found in rods expressing iCasp9, which completely degenerated within 36 hours after drug administration.. Our results indicate that ultrastructural defects in outer and inner segment membranes of bP23H expressing rods differ from those observed in drug-induced apoptosis. We suggest that light-induced retinal degeneration caused by P23H rhodopsin occurs via cell death with autophagy, which may represent an attempt to eliminate the mutant rhodopsin and/or damaged cellular compartments from the secretory pathway.

Topics: Animals; Animals, Genetically Modified; Autophagy; Caspase 9; Disease Models, Animal; Light; Photoperiod; Radiation Injuries, Experimental; Retinal Photoreceptor Cell Inner Segment; Retinal Rod Photoreceptor Cells; Retinitis Pigmentosa; Rhodopsin; Rod Cell Outer Segment; Tacrolimus; Xenopus laevis

Because of their high cone/rod ratio, Xenopus laevis may be a useful system for examining rod-cone interactions during retinal degeneration and mechanisms that underlie secondary cone degeneration. The authors developed an inducible model of retinitis pigmentosa (RP) in X. laevis to investigate these issues.. The authors generated transgenic X. laevis that express a modified caspase-9 (iCasp9) under the control of the X. laevis rod opsin promoter. iCasp9 is activated by the compound AP20187, resulting in an apoptotic cascade. Confocal microscopy, Western blot analysis, and electroretinography (ERG) were used to determine the effects of AP20187 on transgenic retinas.. AP20187 induced rod cell apoptosis in transgenic tadpoles and postmetamorphic frogs. Longitudinal results indicate rod cell death led to cone cell dysfunction within 3 months; however, cone function was reinstated after 6 months. Returning cone function may be associated with increased numbers of morphologically normal cone cells and thickening of the inner nuclear layer.. These studies indicate that X. laevis may be a useful system for examining cone dysfunction associated with rod death in RP and longer term regeneration of cone responses. This inducible model of RP is unique in that rod death proceeds through a well-understood mechanism, rod death can be carefully controlled to occur at any stage of development, and the stimulus for rod death can be removed at any time.

Topics: Animals; Animals, Genetically Modified; Apoptosis; Blotting, Western; Caspase 9; Disease Models, Animal; Electroretinography; Enzyme Activation; Gene Expression Regulation, Enzymologic; Green Fluorescent Proteins; Microscopy, Confocal; Opsins; Promoter Regions, Genetic; Retina; Retinal Cone Photoreceptor Cells; Retinal Rod Photoreceptor Cells; Retinitis Pigmentosa; Tacrolimus; Xenopus laevis