The changing phenotype of microglia from homeostasis to disease

Luo, Xiao-Guang; Chen, Sheng-Di

After considerable time and research, we have recognized the "double-edged sword" nature of microglial cells. On one hand, significant evidence from in vitro and in vivo studies has associated neuronal injury with microglial activation [138] [139] [140] [141] . This evidence results from an inflammatory phenotype of microglia releasing neurotoxic factors, mediators and reactive oxygen species [138] [139] [140] [141] . On the other hand, several other studies have highlighted the beneficial and important roles of microglia in neuronal regeneration, repair and neurogenesis [142] [143] [144] [145] [146] . These seemingly paradoxical results cannot be directly compared, because they come from different experimental sets that vary in terms of the stimulus, timing of microglial activation and age of animals. Thus, whether microglia have positive or negative effects on neuronal survival is contextdependent.