The dynamics of the LPS triggered inflammatory response of murine microglia under different culture and in vivo conditions

Overall, the inflammatory potential of lipopolysaccharide (LPS) in vitro and in vivo was investigated using different omics technologies. We investigatedthehippocampalresponsetointracerebroventricular(i.c.v)LPSinvivo,atboththetranscriptionalandproteinlevel.Here,atimecourse analysis of interleukin-6 (IL-6) and monocyte chemotactic protein-1 (MCP-1) showed a sharp peak at 4 h and a return to baseline at 16 h. The expressionofinflammatorymediatorswasnottemporallycorrelatedwithexpressionofthemicrogliamarkerF4/80,whichdidnotpeakuntil2days after LPS injection. Of 480 inflammation-related genes present on a microarray, 29 transcripts were robustly up-regulated and 90% of them were alsodetectedinLPSstimulatedprimarymicroglia(PM)cultures.Furtherinvitrotoinvivocomparisonshowedthatthecounterregulationresponse observed in vivo was less evident in vitro, as transcript levels in PM decreased relatively little over 16 h. This apparent deficiency of homeostatic controlof the innateimmune response incultures may alsoexplainwhya group of genes comprisingtnf receptorassociatedfactor-1,endothelin-1 and schlafen-1 were regulated stronglyin vitro, but not in vivo. When the overall LPS-induced transcriptional response of PM was examined on a large Affymetrix chip, chemokines and cytokines constituted the most strongly regulated and largest groups. Interesting new microglia markers included interferon-induced protein with tetratricopeptide repeat (ifit), immune responsive gene-1 (irg-1) and thymidylate kinase family LPSinducible member (tyki). The regulation of the former two was confirmed on the protein level in a proteomics study. Furthermore, conspicuous regulation of severalgene clusters wasidentified,for instancethatof genes pertaining tothe extra-cellularmatrixand enzymatic regulationthereof. Although most inflammatory genes induced in vitro were transferable to our in vivo model, the observed discrepancy for some genes potentially represents regulatory factors present in the central nervous system (CNS) but not in vitro.