Subneurotoxic copper(II)-induced NF-κB-dependent microglial activation is associated with mitochondrial ROS

Dual role of superoxide dismutase 2 induced in activated microglia: oxidative stress tolerance and convergence of inflammatory responses

Microglia are activated quickly in response to external pathogens or cell debris and clear these substances via the inflammatory response. However, excessive activation of microglia can be harmful to host cells due to the increased production of reactive oxygen species and proinflammatory cytokines. Superoxide dismutase 2 (SOD2) is reportedly induced under various inflammatory conditions in the central nervous system. We herein demonstrated that activated microglia strongly express SOD2 and examined the role of SOD2, focusing on regulation of the microglial activity and the susceptibility of microglia to oxidative stress. When rat primary microglia were treated with LPS, poly(I:C), peptidoglycan, or CpG oligodeoxynucleotide, respectively, the mRNA and protein levels of SOD2 largely increased. However, an increased expression of SOD2 was not detected in the primary neurons or astrocytes, indicating that SOD2 is specifically induced in microglia under inflammatory conditions. The activated microglia showed high tolerance to oxidative stress, whereas SOD2 knockdown conferred vulnerability to oxidative stress. Interestingly, the production of proinflammatory cytokines was increased in the activated microglia treated with SOD2 siRNA compared with that observed in the control siRNA-treated cells. Pretreatment with NADPH oxidase inhibitors, diphenylene iodonium and apocynin, decreased in not only reactive oxygen species generation but also the proinflammatory cytokine expression. Notably, SOD2 knockdown largely potentiated the nuclear factor κB activity in the activated microglia. Taken together, increased SOD2 conferred tolerance to oxidative stress in the microglia and decreased proinflammatory cytokine production by attenuating the nuclear factor κB activity. Therefore, SOD2 might regulate neuroinflammation by controlling the microglial activities.

Cu(II) enhances the effect of Alzheimer's amyloid-β peptide on microglial activation

Background

Aggregated forms of amyloid-β (Aβ) peptides are important triggers for microglial activation, which is an important pathological component in the brains of Alzheimer’s patients. Cu(II) ions are reported to be coordinated to monomeric Aβ, drive Aβ aggregation, and potentiate Aβ neurotoxicity. Here we investigated whether Cu(II) binding modulates the effect of Aβ on microglial activation and the subsequent neurotoxicity.

Methods

Aβ peptides were incubated with Cu(II) at an equimolar ratio to obtain the Cu(II)-Aβ complex. Primary and BV-2 microglial cells were treated with Cu(II)-Aβ, Aβ, or Cu(II). The tumor necrosis factor-α (TNF-α) and nitric oxide levels in the media were determined. Extracellular hydrogen peroxide was quantified by a fluorometric assay with Amplex Red. Mitochondrial superoxide was detected by MitoSOX oxidation.

Results

Incubation of Cu(II) with Aβ confers different chemical properties on the resulting complex. At the subneurotoxic concentrations, Cu(II)-Aβ (but not Aβ or Cu(II) alone) treatment induced an activating morphological phenotype of microglia and induced the microglial release of TNF-α and nitric oxide as well as microglia-mediated neuronal damage. Cu(II)-Aβ-triggered microglial activation was blocked by nuclear factor (NF)-κB inhibitors and was accompanied with NF-κB activation. Moreover, Cu(II)-Aβ induced hydrogen peroxide release, which was not affected by NADPH oxidase inhibitors. Mitochondrial superoxide production was increased after Cu(II)-Aβ stimulation. N-acetyl-cysteine, a scavenger of reactive oxygen species (ROS), inhibited Cu(II)-Aβ-elicited microglial release of TNF-α and nitric oxide as well as the microglia-mediated neurotoxic effect.

Conclusion

Our observations suggest that Cu(II) enhances the effect of Aβ on microglial activation and the subsequent neurotoxicity. The Cu(II)-Aβ-triggered microglial activation involves NF-κB activation and mitochondrial ROS production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0343-3) contains supplementary material, which is available to authorized users.