Activation of microglia by secreted amyloid precursor protein evokes release of glutamate by cystine exchange and attenuates synaptic function

• Published in: Journal of neurochemistry 2001-02, Vol.76 (3), p.846-854
• Main Authors: Barger, Steven W., Basile, Anthony S.
• Format: Article
• Language: English
• Subjects: 2-Aminoadipic Acid - pharmacology; Alzheimer's disease; Amyloid beta-Protein Precursor - metabolism; Amyloid beta-Protein Precursor - pharmacology; Animals; Biological and medical sciences; Calcium - metabolism; Carrier Proteins - antagonists & inhibitors; Carrier Proteins - physiology; Coculture Techniques; Culture Media, Conditioned - pharmacology; cystine; Enzyme Inhibitors - pharmacology; Excitatory Amino Acid Antagonists - pharmacology; Fundamental and applied biological sciences. Psychology; glutamate; Glutamic Acid - metabolism; Hippocampus - cytology; Hippocampus - metabolism; Isolated neuron and nerve. Neuroglia; microglia; Microglia - drug effects; Microglia - physiology; Neurons - metabolism; Neurotoxins - metabolism; Neurotoxins - pharmacology; nitric oxide; Nitric Oxide Synthase - antagonists & inhibitors; Nitric Oxide Synthase Type I; Nitric Oxide Synthase Type II; Osmolar Concentration; Rats; synapse; Synapses - drug effects; Synapses - physiology; Vertebrates: nervous system and sense organs; xc exchange
• ISSN: 0022-3042; 1471-4159
• DOI: 10.1046/j.1471-4159.2001.00075.x

Microglial activation as part of a chronic inflammatory response is a prominent component of Alzheimer's disease. Secreted forms of the β‐amyloid precursor protein (sAPP) previously were found to activate microglia, elevating their neurotoxic potential. To explore neurotoxic mechanisms, we analyzed microglia‐conditioned medium for agents that could activate glutamate receptors. Conditioned medium from primary rat microglia activated by sAPP caused a calcium elevation in hippocampal neurons, whereas medium from untreated microglia did not. This response was sensitive to the NMDA receptor antagonist, aminophosphonovaleric acid. Analysis of microglia‐conditioned by HPLC revealed dramatically higher concentrations of glutamate in cultures exposed to sAPP. Indeed, the glutamate levels in sAPP‐treated cultures were substantially higher than those in cultures treated with amyloid β‐peptide. This sAPP‐evoked glutamate release was completely blocked by inhibition of the cystine–glutamate antiporter by α‐aminoadipate or use of cystine‐free medium. Furthermore, a sublethal concentration of sAPP compromised synaptic density in microglia–neuron cocultures, as evidenced by neuronal connectivity assay. Finally, the neurotoxicity evoked by sAPP in microglia‐neuron cocultures was attenuated by inhibitors of either the neuronal nitric oxide synthase (NG‐propyl‐l‐arginine) or inducible nitric oxide synthase (1400 W). Together, these data indicate a scenario by which microglia activated by sAPP release excitotoxic levels of glutamate, probably as a consequence of autoprotective antioxidant glutathione production within the microglia, ultimately causing synaptic degeneration and neuronal death.