Abstracts

Rationale: Neurons found in some epileptic patients display disruption of the cytoskeleton. Whether these alterations in cytoskeletal proteins constitute a cause or consequence of epilepsy remains unknown. Despite the cytoskeleton's key role in shaping neuronal networks during brain development, it is unknown whether postnatal modification of cytoskeletal structure 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 one month after birth. This is the first animal model with loss of function for Ndel1 in the postnatal brain at a developmental stage equivalent to a ~2.5 year-old human. We tested the hypothesis that postnatal dysregulation of the cytoskeleton can lead to defects in hippocampal structure and neuronal excitability. Methods: Knockout mice for Ndel1 (Ndel1 CKO) were generated by breeding CaMKIIα-Cre transgenic mice with Ndel1-LoxP mice. Immunohistochemical, confocal/electron microscope imaging, behavioral (video and hippocampal depth electrode) and cellular electrophysiological techniques were used to assess morpho-functional properties of hippocampal CA1 subfield in Ndel1 -/- and +/+ littermates. Results: Ndel1 CKO mice experience spontaneous recurrent seizures (SRS), exhibit frequent interictal spikes and die unexpectedly at an early age (average CKO mice lifespan, ~11 weeks). Anatomically, CKO mice display postnatal hippocampal lamination defects with CA1 pyramidal cells misaligned in two layers. The CA1 dysplasia also involves primitive pyramidal cell dendritic arbors (~40% decrease in total dendritic length), ~55% reduction in the number of pyramidal cell dendritic microtubules, atrophied spines and ~48% reduction in the number of asymmetric synapses (P<0.01). These morphological abnormalities were paralleled by an increased excitability of CA1 pyramidal cells: ~26% increase in input resistance, ~35% decrease in threshold current and ~51% enhanced firing (P<0.05). Interneuron abnormalities in CKO mice included: ~54% reduction in the number of CA1 symmetric synapses, as well as ~40% decrease in the frequency of miniature inhibitory post-synaptic currents recorded in pyramidal cells (P<0.01). Conclusions: We identified several postnatal mechanisms underlying SRS in Ndel1 CKO, i.e., CA1 dendritic/synaptic pathologies, postnatal dispersion and hyperexcitability of pyramidal cells and an interneuronopathy. These findings suggest that dispersion of CA1 pyramidal neurons, combined with reduced inhibitory drive from interneurons on these principal cells, may contribute to seizure activity in our mouse model. Overall, our study advances the notion that the postnatal disruption of the cytoskeleton may be an important determinant of the epileptic state.