Oligomeric amyloid β facilitates microglial excitotoxicity by upregulating tumor necrosis factor-α and downregulating excitatory amino acid transporter 2 in astrocytes

Abstract

Objectives Soluble oligomeric amyloid β (oAβ) directly causes synaptic dysfunction and neuronal death, which are involved in the pathogenesis of Alzheimer's disease. In contrast, several lines of evidence have shown that glia-mediated excitotoxicity is also involved in the disease progression of Alzheimer's disease. However, it is still unclear how oAβ induces glia-mediated excitotoxicity. Therefore, we tried to clarify the mechanism of oAβ-induced glia-mediated excitotoxicity using mouse primary cultures. Methods Glial glutamate production was assessed using a colorimetric assay kit. Glial production of tumor necrosis factor-α (TNF-α) was evaluated using a specific enzyme-linked immunosorbent assay kit. Microglial survival was examined using the MTS assay. mRNA and protein expression levels of excitatory amino acid transporter 2 were determined using quantitative polymerase chain reaction and western blotting, respectively. Results We showed that oAβ-stimulation induces glutamate release from astrocyte/microglia co-cultures, but not from single culture of astrocytes or microglia. oAβ increased TNF-α release from astrocytes, but not from microglia, and the astrocyte-derived TNF-α enhanced glutamate release from microglia. TNF-α elongated microglial survival and triggered sustained microglial glutamate release in a positive feedback mechanism through TNF-α receptor 1. Furthermore, treatment of oAβ decreased astrocytic expression of excitatory amino acid transporter 2, which plays a pivotal role in homeostasis of the extracellular glutamate level. Conclusions The present findings suggest that oAβ contributes to glia-mediated excitotoxicity through the two-hit mechanism by suppressing astrocytic glutamate uptake through excitatory amino acid transporter 2 and enhancing microglial glutamate release, and triggers chronic glial neuroinflammation through the TNF-α/TNF-α receptor 1 positive feedback loop in Alzheimer's disease.