Abstracts

Rationale: Several models of epilepsy demonstrate increased neurogenesis following an epileptogenic insult. Evidence suggests that newly generated neurons produced just prior to or after an initial prolonged seizure episode contribute to hyperexcitability upon integrating aberrantly into the existing circuit. In particular, newly generated neurons exhibit ectopic migration and hilar basal dendrites. Loss of Reelin signaling may account, in part, for these morphological changes in patients with epilepsy. A potential effector of Reelin signaling is the Notch pathway. Known for its regulation of stem cell renewal, Notch signaling has been shown to be involved in the maturation of adult-born neurons including their migration and growth of dendrites. Thus, in addition to producing hyperexcitability, changes that result in neural progenitor cell (NPC) dysfunction in chronic epilepsy may cause long term deficits in NPC self-renewal and neuronal maturation resulting in comorbidities such as depression, anxiety, and memory loss. The aim of this set of experiments is to evaluate how early steps in the neurogenic program are altered by an epileptogenic insult, focusing on the Notch pathway.Methods: To examine NPC alterations in an animal model of mTLE, we induced status epilepticus (SE) in mice with pilocarpine and identified changes in dentate gyrus subgranular zone (SGZ) progenitors by immunohistochemistry for glial fibrillary acidic protein (GFAP) and Sox2. We quantified the number of type-1 and type-2 NPCs within the SGZ at 24 hours, 3, 7, 21, 28, and 56 days after a 90-minute SE episode. We also examined expression of the Notch intracellular domain (NICD), a marker of active notch signaling, by immunolabeling. Proliferating cells and immature neurons were identified using antibodies to the cell proliferation marker Ki67 and the neuroblast marker doublecortin (DCX). Results: Immunolabeling for Sox2 and GFAP showed an increase in type-1 progenitors 24 hours after SE, followed by a decrease below baseline 5 weeks after SE. A similar transient increase in expression of NICD was observed in the subgranular zone (SGZ), with levels returning to baseline 7 days post-SE. We found that the NICD protein localized to a select number of GFAP-positive radial fibers of the putatively quiescent stem cell pool in the SGZ. An expected increase in DCX expression was also seen after SE. Conclusions: These data indicate that seizures stimulate type-1 neural progenitor proliferation in the dentate SGZ (as shown previously by others), and that altered Notch signaling may be involved in this process. Moreover, differential NICD expression suggests heterogeneity among the SGZ progenitor population. Future targeting of the quiescent subset of SGZ progenitors may yield a therapeutic strategy to preserve the pool of proliferating cells in chronic epilepsy. NIH NS058585 (JMP) and EF (MJK)