Contribution of activated glial cells in epileptogenesis after status epilepticus

[Speaker] Fumikazu Sano:1,2
[Co-author] Eiji Shigetomi:1, Schuichi Koizumi:1, Hideaki Kanemura:2, Katsuhiko Mikoshiba:3, Masao Aihara:2
1:Department of Neuropharmacology, Interdisciplinary Graduate School of Medicine, University of Yamanashi, Japan, 2:Department of Pediatrics, Faculty of Medicine, University of Yamanashi, Japan, 3:Laboratory for Developmental Neurobiology, RIKEN Brain Science Institute, Japan

Background: Epileptogenesis, i.e., the process leading to epilepsy, is a common sequel of brain insults such as status epilepticus (SE). Although extensive activation of glial cells has been well described during a latent period in various animal models of epilepsy, it remains unknown whether such a glial activation has a causality to form epileptogenesis and what a molecular mechanism for such glia-mediated epileptogenesis is. Here we show that a sequence of glial activation, i.e. initial microglial activation, followed by astrocytic activation, is a cause of epileptogenesis.
Methods: Pilocarpine was administrated to induce SE in 8-week-old C57BL/6J male mice. Microglial activation and astrogliosis were investigated with immunohistochemistry and qRT-PCR. For functional analysis of astrocytes, Ca2+ imaging were performed from hippocampal slices 4 weeks after SE, using either GCaMP3 or Fluo-4AM. To inhibit acute seizure-induced microglia activation, mice were administrated minocycline after the first SE.
Results: We detected a significant activation of microglia 1-3 days after SE, which was followed by the increase in activation of astrocytes in CA1 from 7-28 days after SE. We screened several molecules, and found that mRNAs of Tnf and Il1b were significantly upregulated in the isolated hippocampal microglia at only 1 day after SE. Inhibition of microglial activation by minocycline resulted in decrease in these cytokines and subsequent activation of astrocytes, suggesting that initial activation of microglia and microglia-derived proinflammatory cytokines should be responsible subsequent astrocytic activation. To characterize the activated astrocytes by SE, we performed Ca2+ imaging in astrocytes in situ. Activated astrocytes displayed significantly larger and more frequent Ca2+ signals 4 weeks after SE. Both pharmacological depletion of intracellular Ca2+ stores and genetic deletion of IP3 receptor type2 (IP3R2) significantly reduced these extraordinary Ca2+ signals. A temporal pattern of epileptogenesis (susceptibility to the pilocarpine-induced SE) was well associated with that of astrocytic Ca2+ excitability. In IP3R2 mice, SE induced neither the increase in Ca2+ excitation nor epileptogenesis, suggesting that enhanced Ca2+ signals in astrocytes should be responsible to form epileptogenesis.
Conclusion: A sequential activation of glial cells, i.e., the initial activation of microglia, followed by astrocytic activation, is essential for in glia-mediated epileptogenesis.
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