Abstracts

Rationale: Neurogenesis in the adult hippocampus has been shown to be influenced by prolonged seizures. Such hippocampal neurogenesis would be expected to be dependent on a resident or nearby stem cell population. However, studies of the hippocampus have only identified precursors resembling progenitor cells, in that they have limited self-renewal capacity. This raises the questions of whether there is a quiescent latent population of stem cells in the hippocampus and, more importantly, how this population is regulated.Methods: We used 20mM KCl to depolarize cultures of hippocampal cells, freshly isolated from an adult mouse, and then assessed the number of neurospheres generated 10 days later using a previously described in vitro assay. To investigate neurogenesis following prolonged seizures, we used the pilocarpine mouse model.Results: We found there was a 3-fold increase in the total number of neurospheres generated following high extracellular KCl concentrations. Application of high extracellular KCl led to the emergence of a subpopulation (~5%) of very large neurospheres, 250-550μm in diameter. Approximately 36% of the large neurospheres could be expanded for at least 10 passages. After differentiation, small hippocampal neurospheres exclusively gave rise to astrocytes, while those derived from large neurospheres generated a large number of neurons. Thus, neural excitation caused by depolarization activated a latent population with the characteristics of stem cells: self-renewal and multipotentiality. We next investigated whether the latent hippocampal population identified in vitro could also be activated by synaptic excitation in vivo. The increased prolonged synaptic activity found in status epilepticus (SE) results in increased neurogenesis in the dentate gyrus. We found that mice which experienced SE for at least 2 hours and were studied after 2 days had a four fold increase in the number of hippocampal neurospheres compared to control mice, similar to the effect observed in the KCl-stimulated cultures. In contrast, mice that suffered only sporadic seizures showed no such increase, suggesting that only SE stimulates the latent KCl-activatable precursor population. The latent precursor population in mice subjected to SE was almost completely activated, and subsequent KCl induced-depolarization in vitro resulted in no further activation. Prolonged excitation was also a requirement for activation in vitro; at least 24 hours exposure to high levels of KCl was required to produce a significant effect. Thus, it seems that the in vitro and in vivo mechanisms of activation are similar and that both are linked to prolonged neural excitation. Conclusions: We conlcude that the adult hippocampus contains a large number of latent precursors, including a self-renewing stem cell population, which only become activated following hyperpolarization in vitro or after prolonged seizures in vivo. The discovery of this latent population provides the first demonstration of an activatable stem cell in the adult hippocampus.