Takunori Satoh

A novel subtype of G-protein-coupled receptor kinase, GRK7, in teleost cone photoreceptors

Two kinds of retinal cDNA fragments (OlGRK‐R and ‐C) encoding the putative G‐protein‐coupled receptor kinases (GRKs) were isolated from medaka, Oryzias latipes. OlGRK‐R appears to be closely related to the rhodopsin kinase (RK) found in the outer segments of mammalian photoreceptors, but the deduced amino acid sequence of OlGRK‐C shows less than 50% identity to those of GRKs known to date, suggesting that OlGRK‐C is a novel GRK subtype (GRK7). The mRNA of OlGRK‐R is detectable in rods, and that of OlGRK‐C is found in all four types of cone photoreceptor. The C‐terminal of OlGRK‐R has a consensus sequence for farnesylation, whereas, surprisingly, OlGRK‐C has a consensus sequence for geranylgeranylation. Our result are consistent with the concept that lower vertebrates have rod‐ and cone‐specific opsin kinases.

Molecular cloning and characterization of the putative ultraviolet-sensitive visual pigment of goldfish

A cDNA full length encoding a putative ultraviolet (UV)-sensitive visual pigment of goldfish was isolated. The deduced amino acid sequence shows 64% identity to those of human blue and chicken violet, and less identity (40-49%) to those of other vertebrate visual pigment. The mRNA is localized in the miniature short single cone cells, which are known to have a sensitivity maximum in the near UV-region.

The primary structure and distribution of killifish visual pigments

Five cDNA fragments (KFH-R, -V, -G, -B and -Rh) encoding the putative visual pigments of killifish were isolated and sequenced. Judging from the deduced amino acid sequences, each cDNA falls into a different group of the five major families of vertebrate visual pigment genes. In situ hybridization localized the mRNA of KFH-R and -G to the principle and accessory members, respectively, of double cones. Visual pigment genes KFH-Rh, -B and -V were expressed in the rods, and the long and short single cones, respectively. It is suggested that the relationships between the cell types and their respective visual pigment gene groups may be a common pattern among teleost fishes.

Primary structure and characterization of a bullfrog visual pigment contained in small single cones

A cDNA fragment encoding a putative visual pigment (FCV pigment) was isolated from the bullfrog, Rana catesbeiana. Its deduced amino acid sequence shows high similarities to those of short wavelength-sensitive pigments such as human blue-, chicken violet- and goldfish ultraviolet-sensitive pigments. An antiserum against its C-terminal amino acid sequence recognized the outer segments of small cone photoreceptor cells without oil droplets. It is suggested that the FCV pigment is a short wavelength-sensitive pigment contained in small single cones which have not been characterized previously.

Arrestins expressed in killifish photoreceptor cells

Two kinds of cDNA fragments (KfhArr‐R and KfhArr‐C) encoding the putative arrestins of killifish, Oryzias latipes, were isolated. The distributions of these transcripts were investigated by in situ hybridization, and it was demonstrated that KfhArr‐R and KfhArr‐C are expressed in, respectively, rod and all four types of cone cells. The deduced amino acid sequences of KfhArr‐R and KfhArr‐C are closely related to human S‐antigen (rod arrestin) and X‐arrestin (cone arrestin), respectively. Phylogenetic analysis of arrestin sequences suggests that vertebrate visual arrestins form a single cluster distinct from other arrestins and diverged to form rod and cone subtypes before the divergence between teleosts and tetrapods. It is speculated that the divergence pattern of vertebrate visual arrestins may prove to be reflected in the divergence of the proteins participating in the respective phototransduction cascades.

Identification of rod- and cone-specific phosducins in teleost retinas.

Phosducin (PD) is a regulatory protein of vertebrate phototransduction cascades. In mammalian retina, it has been thought that only one kind of PD commonly exists in both rods and cones. However, we have found two kinds of PD (OlPD‐R and OlPD‐C) in the retina of a teleost, medaka (Oryzias latipes). In situ hybridization and immunohistochemical analysis demonstrated that OlPD‐R and ‐C are selectively expressed in rods and cones, respectively. The antiserum against medaka PDs recognized two kinds of proteins in bluegill (Lepomis macrochirus) retina. These results suggest that rod‐ and cone‐specific PDs exist in teleost retinas, probably creating differences in light adaptation between rods and cones.

A third photoreceptor-specific GRK found in the retina of Oryzias latipes (Japanese killifish).

Abstract We previously reported that the teleost fish medaka (Oryzias latipes, Japanese killifish), possesses two kinds of G protein-coupled receptor kinases (GRKs) in the retina with different localizations: GRK7 (OlGRK-C) in cones and GRK1 (OlGRK-R1) in rods. To further clarify the diversity of teleost photoreceptor GRKs, we sought other medaka GRKs. We found an additional cDNA that encodes a second retina-specific GRK1 (OlGRK-R2). In situ hybridization experiments demonstrated that OlGRK-R2 mRNA is selectively expressed in rods. Sequence analysis of the Fugu rubripes genomic database unveiled a larger diversity of GRKs than previously expected. We also describe the light-dependent regulation of GRK1, a phenomenon that has not been found in other species. Immunocytochemical analysis indicated that OlGRK-R2 is localized in rod outer segments, independent of light condition. OlGRK-R1 is localized in the rod inner segments and synaptic termini of dark-adapted eyes, and moves to rod outer segments after light adaptation. Our studies suggest that the two medaka GRKs are not functionally redundant, and demonstrate a complicated light-dependent regulation of GRK1 in vivo.