Role and Mechanism of Microglial Activation in Iron-Induced Selective and Progressive Dopaminergic Neurodegeneration

Parkinson's disease (PD) patients have excessive iron depositions in substantia nigra (SN). Neuroinflammation characterized by microglial activation is pivotal for dopaminergic neurodegeneration in PD. However, the role and mechanism of microglial activation in iron-induced dopaminergic neurodegeneration in SN remain unclear yet. This study aimed to investigate the role and mechanism of microglial beta-nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2) activation in iron-induced selective and progressive dopaminergic neurodegeneration. Multiple primary midbrain cultures from rat, NOX2(+/+) and NOX2(-/-) mice were used. Dopaminergic neurons, total neurons, and microglia were visualized by immunostainings. Cell viability was measured by MTT assay. Superoxide (O-2(center dot-)) and intracellular reactive oxygen species (iROS) were determined by measuring SOD-inhibitable reduction of tetrazolium salt WST-1 and DCFH-DA assay. mRNA and protein were detected by real-time PCR and Western blot. Iron induces selective and progressive dopaminergic neurotoxicity in rat neuron-microglia-astroglia cultures and microglial activation potentiates the neurotoxicity. Activated microglia produce a magnitude of O-2(center dot-) and iROS, and display morphological alteration. NOX2 inhibitor diphenylene iodonium protects against iron-elicited dopaminergic neurotoxicity through decreasing microglial O-2(center dot-) generation, and NOX2(-/-) mice are resistant to the neurotoxicity by reducing microglial O-2(center dot-) production, indicating that iron-elicited dopaminergic neurotoxicity is dependent of NOX2, a O-2(center dot-)-generating enzyme. NOX2 activation is indicated by the increased mRNA and protein levels of subunits P47 and gp91. Molecules relevant to NOX2 activation include PKC-sigma, P38, ERK1/2, JNK, and NF-KBP65 as their mRNA and protein levels are enhanced by NOX2 activation. Iron causes selective and progressive dopaminergic neurodegeneration, and microglial NOX2 activation potentiates the neurotoxicity. PKC-sigma, P38, ERK1/2, JNK, and NF-KBP65 are the potential molecules relevant to microglial NOX2 activation.