RESUMO
Synaptic plasticity is believed to be the cellular basis for experience-dependent learning and memory. Although long-term depression (LTD), a form of synaptic plasticity, is caused by the activity-dependent reduction of cell surface α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors (AMPA receptors) at postsynaptic sites, its regulation by neuronal activity is not completely understood. In this study, we showed that the inhibition of toll-like receptor-9 (TLR9), an innate immune receptor, suppresses N-methyl-d-aspartate (NMDA)-induced reduction of cell surface AMPA receptors in cultured hippocampal neurons. We found that inhibition of TLR9 also blocked NMDA-induced activation of caspase-3, which plays an essential role in the induction of LTD. siRNA-based knockdown of TLR9 also suppressed the NMDA-induced reduction of cell surface AMPA receptors, although the scrambled RNA had no effect on the NMDA-induced trafficking of AMPA receptors. Overexpression of the siRNA-resistant form of TLR9 rescued the AMPA receptor trafficking abolished by siRNA. Furthermore, NMDA stimulation induced rapid mitochondrial morphological changes, mitophagy, and the binding of mitochondrial DNA (mtDNA) to TLR9. Treatment with dideoxycytidine and mitochondrial division inhibitor-1, which block mtDNA replication and mitophagy, respectively, inhibited NMDA-dependent AMPA receptor internalization. These results suggest that mitophagy induced by NMDA receptor activation releases mtDNA and activates TLR9, which plays an essential role in the trafficking of AMPA receptors during the induction of LTD.
Assuntos
DNA Mitocondrial , Hipocampo , Depressão Sináptica de Longo Prazo , Receptor Toll-Like 9 , DNA Mitocondrial/genética , DNA Mitocondrial/metabolismo , Hipocampo/metabolismo , Imunidade Inata , N-Metilaspartato/farmacologia , N-Metilaspartato/metabolismo , Neurônios/metabolismo , Receptores de AMPA/genética , Receptores de AMPA/metabolismo , Receptores de N-Metil-D-Aspartato/genética , Receptores de N-Metil-D-Aspartato/metabolismo , RNA Interferente Pequeno/metabolismo , Receptor Toll-Like 9/genética , Receptor Toll-Like 9/metabolismo , Células HEK293RESUMO
Long-term depression (LTD) is a form of synaptic plasticity thought to be the cellular basis of experience-dependent learning and memory. LTD is caused by an activity-dependent decrease in cell surface α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPA receptors) at the postsynaptic sites. However, the mechanism through which AMPA receptors are removed from the cell surface via neuronal activity is not fully understood. In this study, we showed that small interfering RNA (siRNA)-mediated knockdown of sterile alpha and toll/interleukin receptor motif containing 1 (SARM1) in cultured hippocampal neurons prevented the N-methyl-d-aspartate (NMDA)-induced reduction in cell surface AMPA receptors. However, the control RNA did not affect NMDA-mediated AMPA receptor trafficking. Overexpression of the siRNA-resistant form of SARM1 in SARM1-knocked-down neurons restored AMPA receptor trafficking. However, overexpression of SARM1, which lacks the mitochondrial transport signal, in the SARM1-knocked-down neurons did not restore NMDA-dependent AMPA receptor endocytosis. Moreover, the inhibition of the NADase activity of SARM1 blocked the NMDA-induced reduction of cell surface AMPA receptors. These results suggest that both the mitochondrial localization and NADase activity of SARM1 are essential for NMDA receptor-dependent AMPA receptor internalization in the hippocampal neurons.