Abstracts

Rationale: Many animal models of temporal lobe epilepsy (TLE) exhibit altered neurogenesis arising from progenitors within the dentate gyrus subgranular zone (SGZ), the neural stem cell niche in the adult hippocampus. Aberrant integration of new neurons into the existing circuit is thought to contribute to epileptogenesis. In particular, adult-born neurons that exhibit ectopic migration and hilar basal dendrites are suggested to be pro-epileptogenic. Loss of Reelin may contribute to these morphological changes in patients with epilepsy. Reelin signaling activates the adaptor protein Disabled1 (Dab1), which can mediate cell migration, differentiation, and dendritic growth. We previously demonstrated that conditional deletion of Dab1 from postnatal mouse SGZ progenitors (using nestin-driven tamoxifen-inducible Cre mice, nestin-CreER^T2, crossed with Dab1^fl/fl mice) generated dentate granule cells (DGCs) with abnormal dendritic development and ectopic placement, similar to that observed in human and experimental TLE. Methods: To test the hypothesis that loss of Reelin signaling in SGZ progenitors is epileptogenic, we treated nestin-CreER^T2/R26R-YFP (a recombination reporter)/Dab1^fl/fl mice with tamoxifen on postnatal days 7 and 8 and performed chronic video-EEG recordings for 10 days, 8-10 weeks later. We then measured latency to, and severity of pilocarpine-induced status epilepticus (SE). Phenotypes of Dab1 deficient cells were determined using immunohistochemistry for YFP and doublecortin, glial fibrillary acidic protein, NeuN, Prox1, or S100β. We quantified abnormal morphological phenotypes including ectopic cell placement and atypical dendritic arbors. Results: Mice did not exhibit spontaneous seizures, but showed a significantly reduced latency to seizure onset in Dab1^fl/fl mice. Over 90% of Dab1^fl/fl mice developed generalized motor convulsions with tonic-clonic movements, rearing, and falling. This was in contrast to <20% of mice with wild-type Dab1. There was also a significant inverse correlation between recombination efficiency, measured by YFP expression and loss of Dab1 immunoreactivity, and time to the first sustained seizure. Prox1 immunohistochemistry showed that Dab1 deletion altered DGC progenitor migration, including a non-cell autonomous influence on adjacent DGCs that did not undergo recombination. We also noted a dispersion of the CA1 pyramidal layer, even in the absence of Cre directed deletion of Dab1, likely due to hypomorphic effects in the Dab1^fl/fl mice. This abnormality did not correlate with seizure susceptibility. Conclusions: Our results indicate that postnatal loss of Dab1 generates newborn DGCs with abnormal phenotypes that contribute to seizure susceptibility. These findings suggest that the misplacement of adult-born granule cells contributes to circuit instability. Video/EEG evaluation at later time points in the future may reveal the development of spontaneous seizures with the accumulation of Dab1-deficient neurons. NIH NS058585 (JMP) and Epilepsy Foundation (MJK)