To define the cell types responsible for LPS-mediated inflammation in the SN, we evaluated the responses of human and mouse microglia, astrocytes and neurons to LPS. These experiments demonstrated that microglia are orders of magnitude more responsive than astrocytes or neurons, exemplified by the pattern of TNFα induction in primary mouse and human microglia and astrocytes and the neuronal Neuro 2A (mouse neuroblastoma) cell line (Fig. 2A, Fig. S4F). These results are consistent with the expression patterns of TLR4, co-receptors and down-stream signaling molecules in neurons and glial cells (Fig. S4A–E). Although, TLR4 expression was virtually absent from the neuronal cell lines examined, we tested whether LPS could directly induce the death of these cells. Three different neuronal cell lines, Neuro2A, SK-N-SH and PC12, were incubated with LPS for 24h. No significant cell death was observed by TUNEL assay or caspase-3 cleavage, in contrast to the effects of TNFα plus cyclohexamide (CHX) treatment (Fig. 2B and Fig. S4G). In addition, knockdown of Nurr1 in Neuro2A cells did not increase the sensitivity to LPS or death signaling (TNFα plus CHX) as determined by TUNEL assay (Fig. 2C).
