The observation that astrocytes could amplify the neurotoxic effects initiated by microglia (Fig. 2J) suggested that pro-inflammatory cytokines secreted by activated microglia such as TNFα and IL1β could activate the astrocytes and induce the transcription of inflammatory neurotoxic mediators (Fig. 2D–F and Fig. S6A–B). Consistent with this possibility, the receptors for TNFα and IL1β are highly expressed in primary mouse and human astrocytes, but not microglia (Fig. 5A–B, Fig. S11A–B). Nurr1 protein was expressed in resting human and mouse astrocytes (Fig. S1E and F) and Nurr1 mRNA was induced by IL1β or TNFα (Fig. 5C, D and Fig. S11C,D). To test the possibility that Nurr1 also participates in a signal-dependent negative feedback mechanism in astrocytes, primary mouse and human astrocytes were infected with shCtrl- and shNurr1-lentivirues. Activated astrocytes can up-regulate many pro- inflammatory genes, including the iNOS and Ncf1 genes upon IL1β and TNFα stimulation, which are essential enzymes for NO and reactive oxygen species (ROS) production, respectively. Knockdown of Nurr1 in astrocytes drastically increased mRNA expression of both iNOS and Ncf1 in response to IL1β and TNFα and up-regulated
