[Show abstract][Hide abstract] ABSTRACT: The visual cycle system in a primitive chordate, ascidian Ciona intestinalis, was studied by whole-mount in situ hybridization and by whole-mount immunohistochemistry. Three visual cycle proteins, Ciona homologue of RGR (Ci-opsin3), CRALBP (Ci-CRALBP), and BCO/RPE65 (Ci-BCO/RPE65) were widely distributed in the brain vesicle and visceral ganglion. To identify the visual cycle system in a primitive chordate, we compared the localization of photoreceptor-specific proteins (visual pigment and arrestin) and visual cycle proteins (Ci-opsin3 and Ci-CRALBP). The ascidian visual cycle is composed of two cellular compartments, the photoreceptors and the brain vesicle, but some photoreceptor cells also contain visual cycle proteins.
[Show abstract][Hide abstract] ABSTRACT: The absorption of light by rhodopsin leads to the cis-to-trans isomerization of the chromophore to generate all-trans-retinal. In the visual cycle, the resultant all-trans-retinal is converted back into the 11-cis-retinal. In the mammalian eye, the retinal pigment epithelium (RPE) plays an essential role in the visual cycle. We have identified cDNA clones encoding three putative visual cycle proteins, homologs of mammalian retinal G-protein-coupled receptor (RGR), cellular retinaldehyde-binding protein (CRALBP) and beta-carotene 15,15'-monooxygenase (BCO)/RPE65 in a primitive chordate, ascidian Ciona intestinalis. The mRNAs for these proteins are specifically expressed in the central nervous system during embryonic development. In the larva, the transcripts were widely distributed in the brain vesicle and visceral ganglion. Since visual pigment, Ci-opsin1, is solely expressed in photoreceptor cells, the visual cycle in this primitive chordate may take place in two compartments, which are coupled into a cycle by the direct flow of retinoids though the intercellular matrix. The Ci-opsin3, an ascidian homolog of mammalian RGR, was expressed in HEK 293S cells and purified after binding of retinal. The chromophore of Ci-opsin3 is in an all-trans-retinal and it is isomerized to an 11-cis-form upon absorption of light. Mammalian CRALBP and BCO/RPE65 are believed to play critical roles in the process of reisomerization of all-trans-retinoid to 11-cis-retinoid in RPE. The present data suggest that isomerization of all-trans-retinoid to 11-cis-retinoid occurs in the brain vesicle and visceral ganglion of a primitive chordate.
Full-text · Article · May 2003 · The Journal of Comparative Neurology