Diminished Cone Sensitivity in cpfl3 Mice Is Caused by Defective Transducin Signaling

Purpose Cone photoreceptor function loss 3 ( Gnat2 cpfl3/cpfl3 or cpfl3 ) is a mouse model commonly used as a functional cones null from a naturally occurring mutation in the α-subunit of cone transducin ( Gnat2 ). We nevertheless detected robust cone-mediated light responses from cpfl3 animals, which we now explore. Methods Recordings were made from whole retina and from identified cones with whole-cell patch clamp in retinal slices. Relative levels of GNAT2 protein and numbers of cones in isolated retinas were compared between cpfl3 , rod transducin knockout ( Gnat1 − / − ), cpfl3 / Gnat1 − / − double mutants, and control C57Bl/6J age-matched mice at 4, 9, and 14 weeks of age. Results Cones from cpfl3 and cpfl3 / Gnat1 − / − mice 2 to 3 months of age displayed normal dark currents but greatly reduced sensitivity and amplification constants. Responses decayed more slowly than in control (C57Bl/6J) mice, indicating an altered mechanism of inactivation. At dim light intensities rod responses could be recorded from cpfl3 cones, indicating intact rod/cone gap junctions. The cpfl3 and cpfl3 / Gnat1 − / − mice express two-fold less GNAT2 protein compared with C57 at 4 weeks, and a four-fold decrease by 14 weeks. This is accompanied by a small decrease in the number of cones. Conclusions Cplf3 cones can respond to light with currents of normal amplitude and cannot be assumed to be a Gnat2 null. The decreased sensitivity and amplification rate of cones is not explained by a reduction in GNAT2 protein level, but instead by abnormal interactions of the mutant transducin with rhodopsin and the effector molecule, cGMP phosphodiesterase.