Abstract TMP30: Activation of Neuronal NMDA Receptors After Ischemia as Signal to Trigger Neuronal Il-4 Production

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Abstract

After cerebral ischemia, the loss of blood supply triggers a series of pathological changes known as ischemic cascades. One of the key components of this cascade is excitotoxicity, triggered by the excessive extracellular glutamate level, overactivation of N-methyl-D-aspartate (NMDA)-glutamate subtype receptor, calcium overload and neuronal injury. Another component of this cascade is microglia (MΦ) over-activation with harmful pro-inflammatory responses. Recent studies from our group suggest that the viable neurons in the ischemic penumbra produce and release anti-inflammatory IL-4. This neuron-derived IL-4 could act as a signal to local MΦ to prevent their over-activation and to enhance their reparative phenotype. In this study, we investigated the mechanisms underlying IL-4 production and release by neurons under the sublethal ischemia and how this process may affect MΦ. By using an in vitro ischemic injury model (oxygen glucose deprivation, OGD) in rat primary cortical neurons, we showed that only a brief transient OGD that causes minimal injury (LDH release assay), rapidly induced neuronal IL-4 (mRNA and protein) production lasting for 6-24h after reperfusion. To elucidate what aspect of OGD triggers IL-4 production/release, we exposed neurons to different insults and found that 50μM glutamate (dose causing no to mild injury), via NMDA receptor (effect reversed by NMDA, but no Kainate or AMPA antagonists), is optimal for induction of IL-4 production and release. NMDA at a low 50μM dose reproduced this effect. D-serine, an agonist to the glycine binding site on the NMDA receptor, enhanced the IL-4 release, but not mRNA expression. A role of downstream calcineurin and NFAT in transcriptional regulation of IL-4 synthesis is investigated. Finally, conditioned media from OGD- or sublethal dose of NMDA-treated neurons was capable of polarizing MΦ toward an anti-inflammatory” and phagocytic phenotype, without inducing MΦ proliferation. For the first time, this study demonstrate that ischemia-induced excitatory transmission may have important task in regulating microglia phenotype through IL-4/IL-4R signaling. This cross-talk between neurons and microglia through IL-4 could be a potential therapeutic target for ischemic stroke.

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