Microglia maintain retinal redox homeostasis following ablation of rod photoreceptors in zebrafish

Microglia rapidly respond to injury, stress, and perturbations to neurons in the brain and retina and perform phagocytosis to clear dying cells and debris. Oxidative stress is a frequent feature of neurodegeneration, and while glia are crucial for managing such stress, microglia may also be dysfunctional in diseased tissue. Here we examine the role of microglia in management of oxidative stress upon death of rod photoreceptors in the larval zebrafish retina. Using rho:nfsb-eGFP transgenic zebrafish and treatment with the pro-drug metronidazole (MTZ), we coupled the generation of reactive oxygen species (ROS) in dying rods to their ablation. Microglia efficiently engulfed and cleared the ROS-laden rods, effectively undertaking the oxidative load. Despite abundant ROS upon MTZ-mediated cell death, oxidative stress overall was minimal in retinal tissue when microglia were present, indicating that they rapidly and efficiently performed redox functions. In irf8-/- mutants, which are deficient in microglia, retinas with MTZ-induced rod ablation showed widespread ROS that localized, at least in part, to Muller glia. Further, there was evidence of increased oxidative stress, and increased numbers of "off-target" inner retinal neurons that stained positive for the cell death marker TUNEL. Supplementation with the antioxidant Glutathione (GSH) reduced the number of off-target TUNEL+ cells detected in microglia-deficient retinas following rod ablation. Our results indicate that microglial redox functions are important in restoring homeostasis following acute retinal damage.