Npas4-Homer1a pathway regulates neuronal homeostasis in animal model of epilepsy

[Speaker] Taku Nagai:1
[Co-author] Wei Shan:1, Motoki Tanaka:2, Norimichi Itoh:1, Yoko Furukawa-Hibi:1, Toshitaka Nabeshima:3,4, Masahiro Sokabe:2, Kiyofumi Yamada:1
1:Department of Neuropsychopharmacology and Hospital Pharmacy, Nagoya University Graduate School of Medicine, Japan, 2:Mechanobiology Laboratory, Nagoya University Graduate School of Medicine, Japan, 3:Advanced Diagnostic System Research Laboratory, Graduate School of Health Sciences, Fujita Health University, Japan, 4:Aino University, Japan

Epilepsy is a common and refractory neurological disorder. Previous studies indicated that multiple factors are involved in epileptogenesis because they affect the balance between excitatory and inhibitory neurons, leading to neuronal hyperexcitability and recurrent seizures. Neuronal intrinsic homeostatic scaling-down of excitatory synapse has been implicated in epilepsy pathogenesis to prevent the neuronal circuits from hyper-excitability. Recent findings suggest a role for Npas4, an activity-dependent neuron-specific transcription factor, in epileptogenesis; however, the underlying mechanisms by which Npas4 regulates epilepsy remain unclear. We herein propose that limbic seizure activity up-regulates Npas4-homer1a signaling in the hippocampus, thereby contributing to epileptogenesis in mice. The expression level of Npas4 mRNA was significantly increased after the pentylenetetrazol (PTZ) treatment. When the time course for the development of PTZ-induced kindling was compared between wild-type and Npas4 KO mice, Npas4 KO mice developed kindling more rapidly than their wild-type littermates. The expression of Homer1a mRNA and protein in the hippocampus increased after seizure activity. Npas4 increased Homer1a promotor activity in COS7 cells. The PTZ-stimulated induction of Homer1a mRNA and protein was attenuated in the hippocampus of Npas4 KO mice. The combination of fluorescence in situ hybridization and immunohistochemical analyses revealed that Homer1a mRNA co-localized with the Npas4 protein after the convulsive seizure response. PTZ reduced excitatory synaptic transmission at AC-CA3 synapses through the Npas4-mediated down-regulation of postsynaptic AMPARs in hippocampal CA3 neurons. The AAV-mediated expression of Homer1a resulted in lower AMPA GluA1 subunit levels in the hippocampal plasma membrane fraction than in that from AAV-EGFP-transfected Npas4 KO mice. The development of kindling was more strongly suppressed in AAV-Homer1a-microinjeted Npas4 KO mice than in AAV-EGFP-microinjected Npas4 KO mice. These results indicate that the activation of Npas4 plays a significant role in neuronal homeostatic mechanisms during epileptogenesis through the induction of Homer1a. Based on the results of the present study, we propose that Npas4 functions as a molecular switch to initiate homeostatic scaling and the targeting of Npas4-Homer1a signaling may provide new approaches for the treatment of epilepsy.
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