Variation in rhodopsin kinase expression alters the dim flash response shut off and the light adaptation in rod photoreceptors.

Rod photoreceptors are exquisitely sensitive light detectors that are perfectly suited for function in dim light.1 The detection of light and the conversion of its energy into an electric signal take place at the membrane discs in the outer segments of rod photoreceptors. Phototransduction is initiated by the absorption of a photon by a molecule of visual pigment, rhodopsin.2 In its active state, rhodopsin (R*) binds to a G protein, transducin, triggering the exchange of guanosine-5′-triphosphate (GTP) for guanosine-5′-diphosphate (GDP) on its α-subunit (Tα). In turn, the activated Tα-GTP binds to an effector enzyme, cGMP phosphodiesterate (PDE) which eventually results in closure of cGMP-gated channels in the outer segment. The amplification of the rod signal is produced by two phototransduction components: a single R* activating multiple Tα subunits, and a single Tα/PDE complex hydrolyzing multiple cGMP molecules. Response termination requires the timely inactivation of both R* and Tα/PDE. First, R* is partially inactivated on phosphorylation by rhodopsin kinase (GRK1),3,4 a reaction inhibited in darkness by recoverin.5 Phosphorylated rhodopsin is then completely inactivated on binding to arrestin.6 Transducin is inactivated in a GTP hydrolysis reaction catalyzed by regulator of G protein signalling 9 (RGS9)7 which returns transducin into its inactive GDP-bound state. The molecular mechanisms that rate-limit the inactivation of the transduction cascade and dominate the light response shut off have been an active area of research but are still subjects of debate. While early studies indicated that shut off of the transduction cascade is controlled by the inactivation of R*8, a recent study demonstrated that the inactivation of the Tα/PDE complex is the rate-limiting step in the shut off of the light response in mouse rods.9 The same study stated that overexpression of rhodopsin kinase does not affect the termination of the light response and concluded that the inactivation of R* is very rapid (≤80 ms) and substantially faster than that of Tα/PDE (see also Ref. 10). However, whether the inactivation of R* by rhodopsin kinase is slow enough to modulate the overall response kinetics in rods remains controversial.11 More importantly, it is not known whether rhodopsin phosphorylation affects the function of rods during light adaptation. We recently generated transgenic mice with rods and cones overexpressing GRK1 driven by the full length rhodopsin kinase promoter12 in preparation for studying how GRK1 expression modulates cone function. We performed initial recordings from the rods of these mice to confirm that, as previously suggested, overexpression of GRK1 in mouse rods does not affect the kinetics of their responses.9 Surprisingly, we observed a notable acceleration of rod response shut off in rods overexpressing GRK1. We proceeded to characterize in detail the effect of GRK1 expression level on the function of mouse rods in darkness and during background adaptation. Our results demonstrate that R* inactivation by rhodopsin kinase affects the kinetics of the single-photon response and plays a role in the background adaptation of mammalian rods.