β-Amyloid-Stimulated Microglia Induce Neuron Death via Synergistic Stimulation of Tumor Necrosis Factor α and NMDA Receptors

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Abstract
Although abundant reactive microglia are found associated with β-amyloid (Aβ) plaques in Alzheimer's disease (AD) brains, their contribution to cell loss remains speculative. A variety of studies have documented the ability of Aβ fibrils to directly stimulate microgliain vitroto assume a neurotoxic phenotype characterized by secretion of a plethora of proinflammatory molecules. Collectively, these data suggest that activated microglia play a direct role in contributing to neuron death in AD rather than simply a role in clearance after plaque deposition. Although it is clear the Aβ-stimulated microglia acutely secrete toxic oxidizing species, the identity of longer-lived neurotoxic agents remains less defined. We used Aβ-stimulated conditioned media from primary mouse microglia to identify more stable neurotoxic secretions. The NMDA receptor antagonists memantine and 2-amino-5-phosphopetanoic acid as well as soluble tumor necrosis factor α (TNFα) receptor protect neurons from microglial-conditioned media-dependent death, implicating the excitatory neurotransmitter glutamate and the proinflammatory cytokine TNFα as effectors of microglial-stimulated death. Neuron death occurs in an oxidative damage-dependent manner, requiring activity of inducible nitric oxide synthase. Toxicity results from coincident stimulation of the TNFα and NMDA receptors, because stimulations of either alone are insufficient to initiate cell death. These findings suggest the hypothesis that AD brains provide the appropriate microglial-mediated inflammatory environment for TNFα and glutamate to synergistically stimulate toxic activation of their respective signaling pathways in neurons as a contributing mechanism of cell death.

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