Abstract
Purpose:
Rhodopsin deactivation requires phosphorylation by rhodopsin kinase (Grk1) and arrestin-1 (Arr1) binding, and mice lacking Grk1 or Arr1 show progressive photoreceptor degeneration when exposed to steady, relatively dim light. In the Arr1-/- retina, light induces the infiltration of activated microglia into the outer nuclear layer (ONL) and engulfment of photoreceptor somata prior to significant ONL thinning. We have investigated whether the light-dependent degeneration in the Grk1-/- mouse is likewise preceded by microglial activation, which we hypothesize accelerates ONL thinning.
Methods:
Mice were born and reared in 24 hour darkness before exposure to ~200 lux continuous white light for 6 to 240 hrs. Fd-OCT imaging assessed the integrity and thickness of the layers of the retina over time before the animals were sacrificed for immunohistochemistry. Vibratome sections of paraformaldehyde-fixed sections were immunostained by standard methods and the number, location, and morphology of microglia were quantified across sections and animals.
Results:
Microglia in the Grk1-/- mouse migrated vertically from the plexiform layers towards the ONL and increased in number throughout the course of light exposure, similar to the microglial changes observed with light exposure in the Arr1-/- retina. Despite this migratory response, however, the displaced microglia did not infiltrate the ONL and engulf photoreceptor somata even after 240 hrs of light. Increased OCT light scattering within the photoreceptor layer, which coincided with ultrastructural disruption in the Arr1-/- retina at 24 hrs, was not observed by 240 hrs of light exposure in the Grk1-/-. Both OCT and histological analysis of Grk1-/- retinas showed a small degree of ONL thinning by 240 hrs but these changes were modest compared to Arr1-/- retinas.
Conclusions:
The same intensity of light exposure that triggers activation of microglia into amoeboid phagocytes in Arr1-/- retinas does not yield phagocytes in the light-exposed Grk1-/- retina, even after a 10-fold longer duration. Although steady light induces microglia of the Grk1-/- retina to migrate towards the ONL and proliferate, the signal to infiltrate and engulf dead or dying nuclei appears to be absent or unsuccessfully transmitted.