Cell Commun Signal. 2014 Oct 17;12:67. doi: 10.1186/s12964-014-0067-5.
The rod photoreceptor cGMP-gated cation channel, consisting of three α- and one β subunit, controls ion flow into the rod outer segment (ROS). In addition to the β-subunit, the Cngb1 locus encodes an abundant soluble protein, GARP2 that binds stoichiometrically to rod photoreceptor cGMP phosphodiesterase type 6 (PDE6). To examine the in vivo functional role of GARP2 we generated opsin promoter-driven transgenic mice overexpressing GARP2 three-fold specifically in rod photoreceptors.
In the GARP2 overexpressing transgenic mice (tg), the endogenous channel β-subunit, cGMP phosphodiesterase α-subunit, peripherin2/RDS and guanylate cyclase I were present at WT levels and were properly localized within the ROS. While localized properly within ROS, two proteins cGMP phosphodiesterase α-subunit (1.4-fold) and cGMP-gated cation channel α-subunit (1.2-fold) were moderately, but significantly elevated. Normal stratification of all retinal layers was observed, and ROS were stable in numbers but were 19% shorter than WT. Analysis of the photoresponse using electroretinography (ERG) showed that tg mice exhibit no change in sensitivity indicating overall normal rod function, however two parameters of the photoresponse significantly differed from WT responses. Fitting of the rising phase of the ERG a-wave to an accepted model of phototransduction showed a two-fold increase in phototransduction gain in the tg mice. The increase in gain was confirmed in isolated retinal tissue and by suction electrode recordings of individual rod photoreceptor cells. A measure of response recovery, the dominant time constant (τD) was elevated 69% in isolated retina compared to WT, indicating slower shutoff of the photoresponse.
GARP2 may participate in regulating visual signal transduction through a previously unappreciated role in regulating phototransduction gain and recovery.
棒状光感受器 cGMP 门控阳离子通道由三个α亚基和一个β亚基组成，控制离子流入棒状光感受器外段（ROS）。除了β亚基之外，Cngb1 基因座还编码一种丰富的可溶性蛋白 GARP2，它与视蛋白启动子驱动的转基因小鼠中三倍过表达 GARP2，该转基因小鼠仅在棒状光感受器中特异性过表达 GARP2。
在 GARP2 过表达的转基因小鼠（tg）中，内源性通道β亚基、cGMP 磷酸二酯酶α亚基、外周蛋白 2/RDS 和鸟苷酸环化酶 I 的水平与 WT 相同，并在 ROS 中正确定位。虽然在 ROS 中正确定位，但两种蛋白 cGMP 磷酸二酯酶α亚基（增加 1.4 倍）和 cGMP 门控阳离子通道α亚基（增加 1.2 倍）适度但显著升高。所有视网膜层的正常分层都观察到，ROS 的数量稳定，但比 WT 短 19%。使用视网膜电图（ERG）分析光反应表明，tg 小鼠的敏感性没有变化，表明整体正常的杆状功能，但是光反应的两个参数与 WT 反应明显不同。将 ERG a 波的上升相拟合到公认的光转导模型中表明，tg 小鼠的光转导增益增加了两倍。在分离的视网膜组织中和通过单个棒状光感受器细胞的抽吸电极记录中证实了增益的增加。反应恢复的一个度量标准，即主导时间常数（τD）在分离的视网膜中比 WT 升高 69%，表明光反应的关闭较慢。
GARP2 可能通过在调节光转导增益和恢复方面发挥以前未被认识到的作用参与调节视觉信号转导。
