Gene expression in activated brain microglia: Identification of a proteinase inhibitor that increases microglial cell number

Microglia, the intrinsic immune cells of the central nervous system, are activated in a variety of inflammatory brain diseases in which they play a pathogenetic role. However, mechanisms underlying activation are largely unknown. To begin elucidating molecular mechanisms associated with activation, we characterized the pattern of gene expression in virtually pure dissociated microglial cultures, using RT-PCR differential display. Microglia were activated with bacterial lipopolysaccharide (LPS), a traditional stimulant, and the profile of gene expression was compared to that in basal, control cultures. Activation resulted in altered expression of six genes. The cDNAs were isolated, sequenced and characterized. Homology searches identified three novel genes, and two that exhibited very high sequence similarity to the gene encoding squamous cell carcinoma antigen (SCCA). SCCA (1 and 2) are tandemly arranged genes that encode two serine proteinase inhibitors (serpins). SCCA has been detected exclusively in cancer cells, and is a plasma marker for squamous cell carcinoma. Immunoblot analysis indicated that gene expression was accompanied by a 5-fold increase in the synthesis of SCCA protein in LPS-activated microglia. To assess potential biological actions of the SCCA serpins, SCCA1 protein was added to cultures. SCCA1 altered microglial morphology, and elicited a dramatic, 5-fold increase in cell number within 72 h. The effects appeared to be cell-specific, since the protein had no effect on other cell types: cortical astrocytes and neurons from cortex or basal forebrain were unaffected. We tentatively conclude that SCCA1 may play a cell-specific role in increasing cell number, a critical early step in microglial activation and brain inflammation. More generally, differential display of genes in the microglial model system may help define patterns of expression associated with CNS disease, thereby identifying pathogenetic mechanisms and new therapeutic targets.