Guanylate cyclase-activating protein (GCAP) 1 rescues cone recovery kinetics in GCAP1/GCAP2 knockout mice

Abstract

Mediated by guanylate cyclase-activating proteins (GCAPs), cytoplasmic Ca 2؉ levels regulate the activity of photoreceptor guanylate cyclase (GC) and the synthesis of cGMP, the internal transmitter of phototransduction. When GCAP1 is expressed in transgenic mice on a GCAP null background, it restores the wild-type flash responses in rod photoreceptors. In this communication, we explored the role of GCAP1 in cone photoreceptors by using electroretinograms (ERGs). Under cone isolation conditions, ERGs recorded from mice lacking both GCAP1 and GCAP2 had normal amplitudes of the saturated a-wave and b-wave. However, recordings from these mice demonstrated a widened b-wave and increased sensitivity of both M-and UV-cone systems. Paired-flash ERGs revealed a delayed recovery of both the cone driven b-wave and a-wave and suggest that the delay originated from the photoreceptors. To test whether GCAP1 could restore normal cone response recovery, mice that expressed only transgenic GCAP1 in the absence of wild-type GCAP expression were tested. Immunohistochemical analysis demonstrated that cones of these mice expressed high levels of GCAP1. Paired-flash ERGs showed that the recovery of the cone-driven a-wave was restored to normal, whereas recovery of the cone-driven b-wave was slightly faster than that observed in wild-type mice. These studies reveal that, similar to rods, deletion of GCAP1 and GCAP2 delays the recovery of light responses in cones and GCAP1 restores the recovery of cone responses in the absence of GCAP2.

A Calcium-Relay Mechanism in Vertebrate Phototransduction

“…ERG recordings on GCAPs −/− mice indicated that deletion of GCAPs delayed the recovery of cone light responses, which were partially restored in mice expressing transgenic GCAP1 on a GCAP null background. 16,17 In a more recent study, single-cell recordings from cones of GCAPs −/− mice gave a detailed picture of the physiological function of GCAPs in cone photoreceptors. 47 Characteristic features of light responses were a 2-fold longer time-to-peak and a 3-fold longer integration time, indicating that GCAPs are required for an efficient shutoff of the response on a short time scale, which is typical for cone response kinetics.…”

The Diversity of Calcium Sensor Proteins in the Regulation of Neuronal Function

“…GCAPs are the only known activators of retinal guanylyl cyclases (GCs) and are known to be physiological regulators of light adaptation (Mendez et al 2001;Burns et al 2002;Howes et al 2002;Pennesi et al 2003). They are unusual in that they activate GCs when in their Ca 2þ -free form but become inhibitors of GCs at higher Ca 2þ concentrations (Dizhoor and Hurley 1996).…”

The Function of Guanylate Cyclase 1 and Guanylate Cyclase 2 in Rod and Cone Photoreceptors

“…Recovery from Test Flashes as Determined by Paired Flash Analysis-A paired flash protocol was used to evaluate the ability of GC deletion mutant retinas to recover responsiveness after light stimulation (23,30,35). In this method, short test flashes followed by varying interstimulus intervals, and a probe flash were used to test for a-wave recovery (36,37).…”