Involvement of proinflammatory factors, apoptosis, caspase-3 activation and Ca2+ disturbance in microglia activation-mediated dopaminergic cell degeneration

Increasing evidences suggest that activated microglia may contribute to neurodegeneration in Parkinson's disease (PD). In the present study, primary ventral mesencephalic (VM) cultures from E14 rats and PC12 cells were utilized as in vitro models to examine the mechanism underlying microglia activation mediated dopaminergic neurodegeneration. Using lipopolysaccharide (LPS) (1-100 ng/ml) as a tool, we observed that microglia activation-mediated a selective dopaminergic neurodegeneration in VM neuron-glia cultures, which was supported by the further study showing that conditioned medium (CM) from microglia-enriched cultures treated with LPS (10-100 ng/ml) decreased PC12 cell viability. The results from antibody neutralization, NO inhibition and superoxide neutralization suggested that the dopaminergic cell death was due to the production of microglia-derived proinflammatory factors (TNF-α, NO and superoxide), among which reactive oxygen species (ROS) might outweigh proinflammatory cytokines. Apoptosis assay on PC12 cells and primary dopaminergic neurons showed that apoptosis was a mechanism for both microglia activation-mediated dopaminergic cell death. Through Western blot and immunocytochemistry, we found that caspase-3 activation was involved in both dopaminergic cell injuries. Finally, the results from laser scanning confocal microscope demonstrated that PC12 cell intracellular free Ca2+ ([Ca2+]i) increased early after CM treatment. [Ca2+]i increase involved influx of calcium from the extracellular milieu and release from intracellular stores and participated in the CM-induced PC12 cell apoptosis. Further investigations indicated that TNF-α, IL-1β, NO and superoxide contributed at different degrees to CM-induced [Ca2+]i increase and apoptosis in PC12 cells. Using primary VM cultures and PC12 cells, our study shows the roles of proinflammatory factors, apoptosis, caspase-3 activation and Ca2+ disturbance in microglia activation-mediated dopaminergic cell degeneration. Understanding the mechanism for microglia activation-mediated dopaminergic neurodegeneration may contribute to the development of new neuroprotective strategies against PD.