Light Threshold–Controlled Cone α-Transducin Translocation

Abstract

Light-induced translocation of rod alpha-transducin (rTalpha, GNAT1) has been recognized as one of the mechanisms for light adaptation in rods. However, cone alpha-transducin (cTalpha, GNAT2) has not been shown to have such light-dependent redistribution. To investigate potential reasons for the restriction of cTalpha to the cone outer segment, the authors established a transient transgenic strategy to express cone Talpha within rod photoreceptor cells, and the location of the cone Talpha within rods and cones was examined under different light conditions. Vector DNA that expresses cTalpha and green fluorescent protein (GFP) bicistronically under control of the cytomegalovirus (CMV) promoter was injected subretinally into the eyes of neonatal rats, and this was followed by electroporation. The localization of cTalpha in rods and cones under different light conditions was determined by immunofluorescent techniques. Injection of the cDNA constructs resulted in the successful transient transfection of retinal cells. When cTalpha was exogenously expressed in rods, its localization paralleled that of endogenous rTalpha under light and dark conditions. Further experiments, with higher intensity light (7000 lux), demonstrated that endogenous cTalpha can also translocate in cone photoreceptor cells to the same extent it does in rods under 600 lux light. The authors successfully established an in vivo transient retinal transfection model. The demonstration of cTalpha translocation in rods indicates cTalpha is not inherently prevented from translocating. The novel observation of cTalpha translocation under high-intensity light suggests a light threshold regulates the redistribution of cTalpha possibly as a protective response against very bright light.