title: Origin of the vertebrate visual cycle.
abstract: In vertebrates, the absorption of light by rhodopsin leads to the isomerization of 11-cis-retinal chromophore to its all-trans form. In the visual cycle, all-trans retinal is converted back to 11-cis retinal. Mammalian visual cycle takes place in photoreceptor cells and retinal pigment epithelial (RPE) cells, while that of cephalopods is completed within a photoreceptor cell. To identify visual cycle system in the primitive chordate ascidians, we studied the localization of the ascidian visual cycle genes and proteins by in situ hybridization and whole-mount immunohistochemistry, respectively. We identified four genes encoding putative visual cycle proteins, homologs of retinal G protein-coupled receptor (Ci-opsin3), cellular retinaldehyde-binding protein (Ci-CRALBP), beta-carotene 15,15'monooxygenase (Ci-BCO) and RPE-specific 65 kDa protein (Ci-RPE65) in the ascidian, Ciona intestinalis. In contrast to Ci-BCO, which is predominantly localized in ocellus photoreceptor cells of the larva, Ci-RPE65 is not significantly expressed in the ocellus and brain vesicle of the larva. Ci-RPE65 is expressed in the neural complex, a photoreceptor organ of the adult ascidian, at a level comparable with that of Ci-opsin3 and Ci-CRALBP. Proteins of Ci-opsin3, Ci-CRALBP and Ci-BCO are localized in photoreceptor cells. These results suggest that the larval visual cycle uses Ci-opsin3 as a photoisomerase, while the visual cycle of the adult photoreceptors is RPE65-dependent. The colocalization of visual cycle proteins in the photoreceptor cells suggest that ascidian visual cycle takes place in a photoreceptor cell as seen in the cephalopod visual cycle.
authors.affiliation: Department of Life Science, Graduate School of Life Science, University of Hyogo, Hyogo, Japan.
authors.forename: Noriko
authors.initials: N
authors.lastname: Takimoto
authors.forename: Takehiro
authors.initials: T
authors.lastname: Kusakabe
authors.forename: Motoyuki
authors.initials: M
authors.lastname: Tsuda
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001921:Brain / Q000502:physiology; D002938:Ciona intestinalis / Q000254:growth & development / Q000502:physiology; D005136:Eye Proteins / Q000502:physiology; D018462:Photobiology; D017956:Photoreceptor Cells, Invertebrate / Q000502:physiology*; D020419:Photoreceptor Cells, Vertebrate / Q000502:physiology; D012168:Retinal Pigments / Q000502:physiology*; D014714:Vertebrates
chemical_list: D005136:Eye Proteins; D012168:Retinal Pigments