Abstracts

Rationale: Despite the cytoskeleton's key role in shaping neuronal networks during brain development, it is unknown whether postnatal modification of cytoskeletal structure and/or function can lead to epilepsy. Here, we generated an animal model of epilepsy caused by postnatal knockout of the microtubule-associated/signaling protein Ndel1 from forebrain excitatory neurons, and using this model, tested the hypothesis that postnatal dysregulation of the cytoskeleton can lead to aberrant hippocampal structure and neuronal hyperexcitability. Methods: Ndel1 CKO mice were generated by breeding CaMKIIalpha-Cre transgenic mice with Ndel1-LoxP mice. Following deletion of LoxP-Ndel1 alleles at P15, Ndel1 expression reaches a minimum at P32. Confocal/electron microscopy, video-EEG, molecular, cellular and behavioral techniques were used to study CA1 hippocampus in Ndel1 CKO and WT mice. Genome-wide transcriptome profiling of hippocampi derived from both CKO and control mice was performed using RNA sequencing. Results: CKO mice display postnatal hippocampal lamination defects (i.e., misalignment of CA1 pyramidal [pyr] cells). This CA1 dysplasia also involves primitive pyr cell dendritic arbors, atrophied spines, ~48% reduction in the number of asymmetric synapses and ~55% reduction in the number of pyr cell dendritic microtubules (P < 0.01). These structural abnormalities are paralleled by increased excitability of CA1 pyr cells: ~26% increase in input resistance, ~35% decrease in threshold current and ~51% enhanced firing (P < 0.05). Further, CKO have interneuron defects, including: ~54% reduction in the number of CA1 symmetric synapses, ~40% decrease in the frequency of mIPSCs recorded in pyr cells (P < 0.01), decreased Reelin-positive cell density in stratum oriens and reduced immunoreactivity for Reelin in CA1. Ndel1 CKO mice exhibit spontaneous recurrent seizures (SRS), frequent interictal spikes and die prematurely (~10.5 weeks). Finally, genome-wide transcriptome analysis of CKO hippocampi revealed deregulation of genes associated with human epilepsy that can be rescued with Reelin treatment. Reelin supplementation decreased the fragmentation of dendritic MTs, improved dendritic deficits and ameliorated the dispersion of pyr cells by 25% (P < 0.05). Conclusions: We have identified several postnatal changes in hippocampi of Ndel1 CKO mice that likely contribute to SRS, specifically CA1 dendritic/synaptic pathologies, postnatal dispersion and hyperexcitability of CA1 pyramidal cells, and an interneuronopathy. Reelin administration ameliorated ultrastructural, cellular and anatomical defects, contributing to the maintenance of postnatal CA1 integrity and function. The identification of genes linked to epilepsy combined with morphofunctional abnormalities in Ndel1 CKO mice suggest that Ndel1 may represent an important target for the study of epilepsy and advances the notion that the postnatal disruption of the cytoskeleton may be an important determinant of the epileptic state. Funding: Alberta Children's Hospital Research Institute CIHR