Activation of microglia by secreted amyloid precursor protein evokes release of glutamate by cystine exchange and attenuates synaptic function.
Microglial activation as part of a chronic inflammatory response is a prominent component of Alzheimer's disease. Secreted forms of the beta-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 beta-peptide. This sAPP-evoked glutamate release was completely blocked by inhibition of the cystine-glutamate antiporter by alpha-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 (N(G)-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.
Barger
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SW
Department of Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas, USA. BargerStevenW@exchange.uams.edu
Basile
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AS
D015074:2-Aminoadipic Acid / Q000494:pharmacology; D016564:Amyloid beta-Protein Precursor / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D018920:Coculture Techniques; D017077:Culture Media, Conditioned / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000378:metabolism*; D006624:Hippocampus / Q000166:cytology / Q000378:metabolism; D017628:Microglia / Q000187:drug effects / Q000502:physiology*; D009474:Neurons / Q000378:metabolism; D009498:Neurotoxins / Q000378:metabolism / Q000494:pharmacology*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D052248:Nitric Oxide Synthase Type I; D052247:Nitric Oxide Synthase Type II; D009994:Osmolar Concentration; D051381:Rats; D013569:Synapses / Q000187:drug effects / Q000502:physiology*
D016564:Amyloid beta-Protein Precursor; D002352:Carrier Proteins; D017077:Culture Media, Conditioned; D004791:Enzyme Inhibitors; D018691:Excitatory Amino Acid Antagonists; D009498:Neurotoxins; D015074:2-Aminoadipic Acid; D018698:Glutamic Acid; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; D052247:Nitric Oxide Synthase Type II; C496322:Nos1 protein, rat; C496318:Nos2 protein, rat; D002118:Calcium