Expression of two temporally distinct microglia-related gene clusters after spinal cord injury

The dual role of microglia in cytotoxicity and neuroprotection is believed to depend on the specific, temporal expression of microglial-related genes. To better clarify this issue, we used high-density oligonucleotide microarrays to examine microglial gene expression after spinal cord injury (SCI) in rats. We compared expression changes at the lesion site, as well as in rostral and caudal regions after mild, moderate, or severe SCI. Using microglial-associated anchor genes, we identified two clusters with different temporal profiles. The first, induced by 4 h postinjury to peak between 4 and 24 h, included interleukin-1β, interleukin-6, osteopontin, and calgranulin, among others. The second was induced 24 h after SCI, and peaked between 72 h and 7 days; it included C1qB, Galectin-3, and p22^phox. These two clusters showed similar expression profiles regardless of injury severity, albeit with slight decreases in expression in mild or severe injury vs. moderate injury. Expression was also decreased rostral and caudal to the lesion site. We validated the expression of selected cluster members at the mRNA and protein levels. In addition, we demonstrated that stimulation of purified microglia in culture induces expression of C1qB, Galectin-3, and p22^phox. Finally, inhibition of p22^phox activity within microglial cultures significantly suppressed proliferation in response to stimulation, confirming that this gene is involved in microglial activation. Because microglial-related factors have been implicated both in secondary injury and recovery, identification of temporally distinct clusters of genes related to microglial activation may suggest distinct roles for these groups of factors.