The Role of Angiotensin II/AT1 Receptor Signaling in Regulating Retinal Microglial Activation

PURPOSE. This study explored whether the proangiogenic factor Angiotensin II (AngII) had a direct effect on the activation state of microglia via the Angiotensin type 1 receptor (AT1-R).

METHODS. Microglial dynamic activity was investigated in live retinal flatmounts from adult Cx3Cr1^+GFP mice under control, AngII (5 lM) or AngII (5 lM) þ candesartan (0.227 lM) conditions. The effects of intravitreal administration of AngII (10 mM) were also investigated at 24 hours, with retinae processed for immunocytochemistry, flow cytometry, or inflammatory quantitative PCR arrays.

RESULTS. We found FACS isolated retinal microglia expressed AT1-R. In retinal flatmounts, microglia showed characteristic movement of processes under control conditions. Perfusion of AngII induced an immediate change in process length (42%, P < 0.05) and activation state of microglia that was ameliorated by AT1-R blockade, suggesting a direct effect of AngII on microglia via the AT1-R. Intravitreal injection of AngII induced microglial activation after 24 hours, which was characterized by increased soma size (23%, P < 0.001) and decreased process length (20%, P < 0.05). Further analysis indicated a significant decrease in the number of microglial contacts with retinal neurons (saline 15.6 6 2.31 versus AngII 7.8 6 1.06, P < 0.05). Retinal cytokine and chemokine expression was modulated, indicative of an inflammatory retinal phenotype.

CONCLUSIONS. We show that retinal microglia express AT1-R and their activation state is significantly altered by the angiogenic factor, AngII. Specifically, AngII may directly activate AT1-Rs on microglia and contribute to retinal inflammation. This may have implications for diseases like diabetic retinopathy where increases in AngII and inflammation have been shown to play an important role.