Ectopic expression of cone-specific G-protein-coupled receptor kinase GRK7 in zebrafish rods leads to lower photosensitivity and altered responses

Non-technical summary When rod and cone photoreceptors in the eye respond to light, they need to recover, and the first step in recovery involves a protein called G-protein receptor kinase (GRK). Rods, which underlie night vision, employ a variant called GRK1, whereas cones, which mediate day vision, typically employ a variant called GRK7. We have engineered rod cells in the zebrafish retina that additionally express the cone variant, GRK7. By recording electrically from these modified rods, we have found that they are less sensitive to light than normal rods, in that regard mimicking cones. We have also found evidence to suggest that the size of the cell's response to a single photon (the smallest particle of light) is normal when recovery is mediated by GRK1, but is small (and hence somewhat cone-like) when mediated by GRK7. These results help us understand the differences between rod and cone photoreceptors.To investigate the roles of G-protein receptor kinases (GRKs) in the light responses of vertebrate photoreceptors, we generated transgenic zebrafish lines, the rods of which express either cone GRK (GRK7) or rod GRK (GRK1) in addition to the endogenous GRK1, and we then measured the electrophysiological characteristics of single-cell responses and the behavioural responses of intact animals. Our study establishes the zebrafish expression system as a convenient platform for the investigation of specific components of the phototransduction cascade. The addition of GRK1 led to minor changes in rod responses. However, exogenous GRK7 in GRK7-tg animals led to lowered rod sensitivity, as occurs in cones, but surprisingly to slower response kinetics. Examination of responses to long series of very dim flashes suggested the possibility that the GRK7-tg rods generated two classes of single-photon response, perhaps corresponding to the interaction of activated rhodopsin with GRK1 (giving a standard response) or with GRK7 (giving a very small response). Behavioural measurement of optokinetic responses (OKR) in intact GRK7-tg zebrafish larvae showed that the overall rod visual pathway was less sensitive, in accord with the lowered sensitivity of the rods. These results help provide an understanding for the molecular basis of the electrophysiological differences between cones and rods.