Altered gene expression in cultured microglia in response to simulated blast overpressure: possible role of pulse duration

Abstract

Blast overpressure has long been known to cause barotrauma to air-filled organs such as lung and middle ear. However, experience in Iraq and Afghanistan is revealing that individuals exposed to explosive munitions can also suffer traumatic brain injury (TBI) even in the absence of obvious external injury. The interaction of a blast shock wave with the brain in the intact cranial vault is extremely complex making it difficult to conclude that a blast wave interacts in a direct manner with the brain to cause injury. In an attempt to "isolate" the shock wave and test its primary effects on cells, we exposed cultured microglia to simulated blast overpressure in a barochamber. Overpressures ranging from 15 to 45 psi did not change microglial Cox-2 levels or TNF-α secretion nor did they cause cell damage. Microarray analysis revealed increases in expression of a number of microglial genes relating to immune function and inflammatory responses to include Saa3, Irg1, Fas and CxCl10. All changes in gene expression were dependent on pulse duration and were independent of pressure. These results indicate that microglia are mildly activated by blast overpressure and uncover a heretofore undocumented role for pulse duration in this process.

Fig. 1

Effects of blast overpressure on cultured microglia. Cells were exposed to simulated blast overpressure of 20 psi and assessed for activation 3, 6 or 24 h after treatment. Microglial activation was assessed by immunoblotting for cellular Cox-2 protein (A). Cox-2 levels on blots (left y-axis) were quantified via scanning and TNF-α (right y-axis) secreted into the media was quantified by Elisa (B). As a positive control, microglia were exposed to LPS and harvested for Cox-2 and TNF-α analysis 24 h after treatment. Data are presented as mean fold-change ± SEM. * p < 0.001, Dunnett's test.

Fig. 2

Summary effects of blast overpressure on microglial activation. The effects of exposure of BV-2 microglial cells to blast overpressure Cox-2 protein levels at 3, 6 or 24 h is shown from 40 independent experiments. Cox-2 immunoblots for each experiment were scanned and normalized versus controls carried out for each separate experiment. Results (as fold-change) are presented as control versus pressure (A) or pulse duration (B). The symbol (X) indicates experiments where the protective parafilm cover on tissue culture dishes was broken during the procedure. These samples were included only when they did not show evidence of contamination.