Abstracts

Rationale: In the adult hippocampus neural stem cells proliferate and differentiate in support of learning and memory throughout the life mammals. Pathological changes that occur to this population of cells, including ectopic migration of newborn neurons, increased proliferation of the neural stem cell pool, mossy fiber sprouting, and aberrant synaptic connectivity has been shown to contribute to the formation of spontaneous reoccurring seizures (SRS). (Cho et al., Nat Comm. 2015 Mar 26;6:660) In order to eventually target pro-epileptic adult neurogenesis in the clinical setting, it will be important to identify molecular players involved in the control of aberrant neurogenesis after seizures. Previous work from our lab has shown that the bHLH transcription factor NeuroD1, which has been shown to play an essential role in the differentiation and maturation of adult-generated neurons in the hippocampus, is up-regulated after seizure, and is found widely in the hilar ectopic granule cell (EGC) population. This indicates that by modulating levels of NeuroD1 we may be able to affect SRS. Methods: To determine the role of NeuroD1 in pro-aberrant adult neurogenesis we chose to use Nestin-CreET2;Rosa YFP transgenic mice bred to NeuroD1 conditional knockout (cKO). We administered tamoxifen to these mice for 5 days prior to inducing status epilepticus (SE) to remove NeuroD1 from the nestin expressing stem/progenitors in the hippocampus. After waiting a period of 5 weeks we implanted these same mice with wireless EEG transmitters to record seizure activity for a total of 2 consecutive weeks. After the recording time period was finished, animals were euthanized to look at histology of the hippocampus. Results: Mice treated with pilocarpine to induce status SE showed a significant up-regulation of NeuroD1+ immature neuroblasts located in both the granule cell layer (GCL) and cells ectopically localized to the hilar region of the hippocampus (p=0.0013 n=6 wt, n=6 cKO for GCL, p=0.0016 n=6 wt n=6 for Hilus). Removal of NeuroD1 in nestin-expressing stem/progenitors and their progeny resulted in recombined cells in the hippocampal GCL and hilus as quantified by yellow fluorescent protein (YFP) that lacked NeuroD expression. As expected, cKO mice showed significantly fewer total number of YFP+/ND+ adult-generated neurons after pilocarpine treatment compared to WT (n=1 wt n=6 cKO). Surprisingly, there was no change in SRS in NeuroD1 cKO mice (p=0.8826 n=11 wt, n=10 cKO). Consistent with this finding, the total number of adult-generated neurons in the pilocarpine model, especially the number of mature ectopic granule cells was unchanged after NeuroD cKO (p=0.1342 n=9 wt n=10 cKO). Conclusions: NeuroD cKO does not reduce aberrant neurogenesis below a threshold to impact SRS suggesting strategies to reduce SRS will need to achieve a greater removal of aberrant adult-generated neurons. Funding: This work was supported by grants from National Institute of Health (NIH) (R01NS081203, R01NS090926, R01NS089770, and R01NS093992), Department of Defense W81XWH-15-1-0399, American Heart Association 15GRNT25750034, a grant from the Texas Institute for Brain Injury and Repair and an NIH pre-doctoral training grant (5T32GM083831-05).