The Role of Microglia and NADPH Oxidase in Traumatic Spinal Cord Injury

Abstract

Spinal cord injury (SCI) results in delayed biochemical changes that contribute to secondary tissue damage and dysfunction. The microglial-induced inflammatory response, long implicated in secondary tissue damage after SCI, includes release of various factors known to be toxic to neurons. However, the response of microglia and their subsequent role remains controversial and poorly understood. The present study evaluated the acute and chronic gene expression profiles in rats after traumatic SCI. Using anchor genes selected from the literature because of their relationship to microglial-induced inflammation, high density microarray expression profiles were examined after contusion. Two temporally correlated clusters were identified: one was expressed immediately after injury, peaking between 4 and 24 hours and declining to baseline or below baseline levels; the second was expressed chronically after injury, and remained elevated up to 6 months after injury. Expression profiles for genes of each group were confirmed using real-time quantitative PCR, immunoblotting, and immunohistochemistry. One of the genes chronically expressed from 24 hours to 6 months postinjury was p22^phox, a component of the NADPH oxidase enzyme. To investigate the role of this chronically expressed gene after SCI and the effect of modification of its activity, the NADPH oxidase inhibitor apocynin was added to purified microglia cultures and microglial activation after LPS stimulation was investigated. Inhibition of p22^phox/NADPH oxidase resulted in significant suppression of microglial proliferation, nitric oxide production, and microglial-induced neurotoxicity. This study demonstrates the potential utility of microarray technology in identifying possible targets for future therapeutics. Additionally, it identifies p22^phox and NADPH as potential components of microglial-induced neurotoxicity, with implications for neuroprotective therapy.