Microglial activation and cell death induced by the mitochondrial toxin 3-nitropropionic acid: in vitro and in vivo studies

Metabolic impairment of neurons has been implicated in several neurological disorders, but it is not at present known whether such metabolic impairment has deleterious effects on microglia, the phagocytic cells of the central nervous system (CNS). In the present study, we examined whether metabolic impairment induced by 3-nitropropionic acid (3-NP), an irreversible inhibitor of succinate dehydrogenase, affects the function and viability of microglia in vitro and in vivo. Treatment of HMO6 human microglia cell line with 3-NP induced the elevation of intracellular Ca2+ concentration ([Ca2+](i)) and activation of microglia with production of reactive oxygen species (ROS). Exposure of HMO6 cells to 3-NP also induced cell death as indicated by nuclear fragmentation in a dose- and time-dependent manner. Trolox, an antioxidant agent, was effective in reduction in ROS production and cell death caused by 3-NP. Consistent with in vitro findings, intrastriatal injection of 3-NP in adult rats resulted in an increase in ROS production in microglia in vivo, as evidenced by the oxidation of the reduced MitoTracker probe. ROS production induced by 3-NP was inhibited when trolox was coinjected with 3-NP. Caspase-3 immunoreactivity was demonstrated in OX-42+ microglia in the core and penumbra area of the 3-NP-injected striatum. Apoptotic cell death of microglia was also demonstrated by terminal deoxynucleotidyl- transferase-mediated biotin-dUTP nick end labeling reaction in the 3-NP-induced lesion area. The present results indicate that metabolic impairment in the CNS could involve both activation and cell death of microglia and contribute to pathology in neurodegenerative diseases. (C) 2003 Elsevier Science (USA). All rights reserved.