Purpose: The role of activated retinal microglia in driving retinal degeneration has been implicated in a number of in vivo disease models. Here, we investigated the primary consequences of microglial activation by the specific expression of constitutively active Ras in microglia in a transgenic mouse model before the onset of any degenerative changes in the retina.
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, and the retinas 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 toward the inner and outer layers of the retina. Examination of glial fibrillary acidic protein (GFAP) expression revealed activation of Müller glia in the retina. We also observed loss of the photoreceptors in the outer nuclear layer in close proximity to microglial cells. However, no significant neurodegeneration was detected in the inner nuclear layer (INL) or ganglion cell layer (GCL). The morphology of RasV12-expressing microglia in the GCL and INL retained more ramified features compared with the predominantly-ameboid morphology found in outer retinal microglia.
Conclusions: The expression of RasV12 is sufficient to activate microglia and lead to photoreceptor degeneration. Neurons in the inner side of the retina were not damaged by the RasV12-activated microglia, suggesting that microenvironment cues may modulate the microglial phenotypic features and effects of microglial activation.