G-protein β-Complex Is Crucial for Efficient Signal Amplification in Vision

Abstract

Purpose: : Highly efficient visual signal amplification mediated by heterotrimeric G-protein transducin (Gt) allows retinal rod photoreceptors to detect single photons of light. While much is known about the role of the rod Gtα-subunit in phototransduction, the physiological function of the auxiliary Gtβγ-complex in this process remains controversial. To investigate the role of Gtβγ in the activation of phototransduction, we generated mice lacking the rod-specific Gtγ-subunit (Gtγ^-/-) and characterized the function of their rods.

Methods: : Retina morphology in Gtγ^-/- and WT mice was analyzed by light and electron microscopy and immunohistochemistry. Levels of Gt subunits in purified mouse rod outer segments were quantified by Western blotting. Retina function was assessed by ERG. Scotopic visual contrast sensitivity was determined from behavioral experiments (optometry). Light responses from single mouse rods were recorded with a suction electrode. A mathematical model of phototransduction was applied to extract key parameters of its activation and inactivation.

Results: : Retina morphology was normal in 2 m.o. Gtγ^-/- animals. However, both scotopic ERG a-wave and visual contrast sensitivity were significantly impaired in Gtγ^-/- mice. Rods lacking Gtγ-subunit were 90 times less sensitive to light than WT rods. Notably, their signal amplification was reduced by 33-fold, including a 6-fold decrease caused by a corresponding reduction in the Gtα content in their outer segments. This leaves the lack of Gtβγ-complex as the source for the remaining 5.5-fold decrease in amplification demonstrating its crucial role for the efficient activation of Gt in intact rods. Finally, photoresponse inactivation was significantly accelerated in Gtγ^-/- rods leading to additional 3-fold decline in their sensitivity. Our mathematical model identified the acceleration of rhodopsin shutoff as a cause for the faster response inactivation in Gtγ^-/- rods.

Conclusions: : Our biochemical, behavioral, and physiological results demonstrate that the Gtβγ-complex is critical for the efficient activation of Gt and the high amplification of visual signal in phototransduction in intact mouse rods.

Keywords: electrophysiology: non-clinical • photoreceptors