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.
Highlights
The pineal and related organs of most nonmammalian vertebrates have direct light sensitivity [1,2,3,4,5,6,7]
Previous studies showed that GRK1b, GRK7a, and GRK7b are distributed in the pineal organs in zebrafish [29, 30]
We investigated which of the three G-protein-coupled receptor kinase (GRK) is involved in the inactivation of light-activated parapinopsin 1 (PP1) via a combination of calcium imaging of PP1 cells and GRK1b, GRK7a, or GRK7b knockdown with antisense morpholino oligonucleotides (MOs)
Summary
The pineal and related organs of most nonmammalian vertebrates have direct light sensitivity [1,2,3,4,5,6,7]. We functionally identified a GRK involved in the dark inactivation of light-activated PP1 in zebrafish larvae via a combination of GRK knockdown using morpholino oligonucleotides and calcium imaging of PP1 cells after light activation.
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