Downregulation of cGMP phosphodiesterase induced by expression of GTPase- deficient cone transducin in mouse rod photoreceptors

Purpose. Photoexcitation of vertebrate retinal rod photoreceptors stimulates GTP binding to the transducin α subunit. Like other GTP-binding proteins, transducin restores itself to an inactive form by hydrolyzing its bound GTP. The role of GTP hydrolysis in phototransduction was investigated. Methods. A mutant form of cone transducin α deficient in its ability to hydrolyze bound GTP was expressed in mouse rod photoreceptors. Results. Expression of the mutant cone transducin α at levels threefold to sixfold higher than endogenous rod transducin α led to a specific depletion of the transducin target, cGMP phosphodiesterase, and a decrease in the cGMP level. Suction electrode recordings revealed abnormally prolonged flash responses, decreased maximal response amplitudes, and a shift in the stimulus-response relation to higher flash strengths. Conclusions. Rods expressing high levels of GTPase-deficient cone transducin α have reduced levels of phosphodiesterase catalytic subunits and cGMP. These changes are associated with prolonged flash responses, reduced dark current, and decreased sensitivity to light.