Functional identification of an opsin kinase underlying inactivation of the pineal bistable opsin parapinopsin in zebrafish

Abstract

In the pineal organ of zebrafish larvae, the bistable opsin parapinopsin alone generates color opponency between UV and visible light. Our previous study suggested that dark inactivation of the parapinopsin photoproduct, which activates G-proteins, is important for the regulation of the amount of the photoproduct. In turn, the photoproduct is responsible for visible light sensitivity in color opponency. Here, we found that an opsin kinase or a G-protein-coupled receptor kinase (GRK) is involved in inactivation of the active photoproduct of parapinopsin in the pineal photoreceptor cells of zebrafish larvae. We investigated inactivation of the photoproduct in the parapinopsin cells of various knockdown larvae by measuring the light responses of the cells using calcium imaging. We found that GRK7a knockdown slowed recovery of the response of parapinopsin photoreceptor cells, whereas GRK1b knockdown or GRK7b knockdown did not have a remarkable effect, suggesting that GRK7a, a cone-type GRK, is mainly responsible for inactivation of the parapinopsin photoproduct in zebrafish larvae. We also observed a similar knockdown effect on the response of the parapinopsin photoreceptor cells of mutant larvae expressing the opsin SWS1, a UV-sensitive cone opsin, instead of parapinopsin, suggesting that the parapinopsin photoproduct was inactivated in a way similar to that described for cone opsins. We confirmed the immunohistochemical distribution of GRK7a in parapinopsin photoreceptor cells by comparing the immunoreactivity to GRK7 in GRK7a-knockdown and control larvae. These findings suggest that in pineal photoreceptor cells, the cone opsin kinase GRK7a contributes greatly to the inactivation of parapinopsin, which underlies pineal color opponency.

Phototoxicity avoidance is a potential therapeutic approach for retinal dystrophy caused by EYS dysfunction

“…Unlike GRK1, the amount of research on GRK7 is still limited because of its lack of rodent models, as is true of EYS. Although there are several reports describing grk7-knockdown (50,51) or -KO ( 52) zebrafish, all of them focused on their impaired response recovery without long-term observation and did not investigate light-induced damage or retinal degeneration. In general, cone cells are not saturated as easily as rod cells and continue to respond even under bright illumination (53).…”

Optogenetic manipulation of Gq- and Gi/o-coupled receptor signaling in neurons and heart muscle cells

“…Given that these bistable rhodopsins are sensitive to stimulating light of diverse wavelengths, they may be useful for manipulating various cell and tissue functions in vivo using light of different wavelengths. Animal bistable rhodopsins are endogenously expressed in various regions of the brain including photoreceptive tissues such as pineal and parapineal organs ( Kawano-Yamashita et al, 2011 ; Kawano-Yamashita et al, 2020 ; Kawano-Yamashita et al, 2015 ; Kawano-Yamashita et al, 2007 ; Koyanagi et al, 2004 ; Koyanagi et al, 2015 ; Shen et al, 2021 ; Wada et al, 2012 ; Wada et al, 2021 ; Wada et al, 2018 ). If a wide area of the brain of Tg zebrafish is irradiated with white light, it may also activate endogenous bistable rhodopsins in addition to transgene-expressed rhodopsins and affect the functions of neurons or other tissues.…”

Optogenetic manipulation of Gq- and Gi/o-coupled receptor signaling in neurons and heart muscle cells

“…Given that these bistable rhodopsins are sensitive to stimulating light of diverse wavelengths, they may be useful for manipulating various cell and tissue functions in vivo using light of different wavelengths. Animal bistable rhodopsins are endogenously expressed in various regions of the brain including photoreceptive tissues such as pineal and parapineal organs ( Kawano-Yamashita et al, 2011, Kawano-Yamashita et al, 2020, Kawano-Yamashita et al, 2015, Kawano-Yamashita et al, 2007, Koyanagi et al, 2004, Koyanagi et al, 2015, Shen et al, 2021, Wada et al, 2012, Wada et al, 2021, Wada et al, 2018 ). If a wide area of the brain of Tg fish is irradiated with white light, it may also activate endogenous bistable rhodopsins in addition to transgene-expressed rhodopsins and affect the functions of neurons or other tissues.…”