RasV12 Expression in Microglia Promotes Retinal Inflammatory Circuits and Impairment of Photoreceptor

Abstract Background Microglial activation is commonly observed in neurodegenerative diseases. However, its role in the complex inflammatory network remains unclear. In this study, we generated a microglial activation mouse model by inducing the expression of a constitutively active form of Ras in microglia.Methods The double transgenic lines CAG-LSL-RasV12-IRES-EGFP; Cx3cr1CreER (Cx3cr1-RasV12 mice) and CAG-LSL-EGFP;Cx3cr1CreER (control mice) were generated. The expression of RasV12 was induced in microglia by tamoxifen administration. Effects of the expression of RasV12 in the retina were examined by immunohistochemistry of frozen sections, RT-qPCR, and live imaging.Results RasV12 expression in retinal microglial cells promoted cell proliferation, cytokine expression, and phagocytosis. RasV12-expressing microglia migrated towards the apical and basal regions of the retina. Examination of GFAP expression revealed activation of Müller glia in the retina. We also observed loss of the photoreceptors in the outer nuclear layer (ONL) in close proximity to microglial cells. However, no neurodegeneration was observed in the inner nuclear layer (INL) or ganglion cell layer (GCL). Furthermore, we found that RasV12-expressing microglia in the ONL were smaller in size and engulfed photoreceptors. In contrast, the morphology of RasV12-expressing microglia in the GCL and INL resembled resting microglia. In conclusion, RasV12-induced microglial activation impaired rod photoreceptor survival in the ONL, but not in other regions of the retina.Conclusion The expression of RasV12 is sufficient to activate microglia, and our results suggested that microenvironment cues may modulate the microglial phenotypic features and effects of microglial activation.