Network dysfunction precedes neurodegeneration in a dox-regulatable TDP-43 mouse model of ALS-FTD.

Neuronal hyperexcitability is a hallmark of amyotrophic lateral sclerosis (ALS) but its relationship with the TDP-43 aggregates that comprise the predominant pathology in over 90% of ALS cases remains unclear. Emerging evidence indicates that TDP-43 pathology induces neuronal hyperexcitability, which may contribute to excitotoxic neuronal death. To characterize TDP-43 mediated network excitability changes in a disease-relevant model, we performed in vivo continuous electroencephalography monitoring and ex vivo acute hippocampal slice electrophysiology in rNLS8 mice (males and females), which express human TDP-43 with a defective nuclear localization signal (hTDP-43ΔNLS). Surprisingly, we identified the presence of seizures in approximately 64% of rNLS8 mice beginning around 2.5 weeks after transgene induction (off-DOX). More broadly, we observed longitudinal changes in cortical EEG patterns and circuit hyperexcitability preceding neurodegeneration of vulnerable hippocampal subfields. Consistent with previous reports, we have observed broad dysregulation of AMPA subunit expression in mice expressing hTDP-43ΔNLS. These changes were most pronounced in the hippocampus, where we hypothesized they promote hyperexcitability and ultimately, excitotoxic cell death. Interestingly, hippocampal injection of AAV encoding inhibitory DREADDs (hM4Di) and daily activation with CNO ligand rescued anxiety deficits on the elevated zero maze but did not reduce neurodegeneration. Moreover, therapeutic doses of the anti-seizure medications, valproic acid and levetiracetam, did not improve behavior or prevent neurodegeneration. These results highlight the complex relationship between TDP-43 -mediated neuronal hyperexcitability and neurodegeneration. Although targeting hyperexcitability may ameliorate some behavioral deficits, our study suggests it may not be sufficient to halt or slow neurodegeneration in TDP-43-related proteinopathies. Cytoplasmic aggregates of TDP-43 are the predominant pathology in over 90% of ALS and 50% of frontotemporal lobar degeneration cases. Understanding how TDP-43 pathology promotes neurodegeneration may lead to therapeutic strategies to slow disease progression in humans. In this study, we identified hippocampal network hyperexcitability and generalized seizures that preceded neurodegeneration in the inducible rNLS8 mouse model. Local suppression of hippocampal hyperexcitability with chemogenetics (hM4Di) improved behavioral function but did not reduce neuron loss. Systemic anti-seizure medications had no beneficial effects. These results highlight the complexity of TDP-43 induced excitability changes but ultimately suggest that directly targeting hyperexcitability may not be therapeutically effective.