Regular ArticleSeizures induce proliferation and dispersion of doublecortin-positive hippocampal progenitor cells
Introduction
Epileptic activity affecting the adult hippocampus leads to a variety of structural changes that eventually cause long-term deficits of hippocampal function in humans (Sutula et al., 1989, Houser, 1990, Dalby and Mody, 2001, Helmstaedter, 2002). Among other pathological findings an abnormal broadening of the dentate gyrus GCL in epileptic hippocampi is often observable (Houser, 1990) but to date it remains unclear if this morphological change represents a consequence or a possible cause of epilepsy (Blumcke et al., 2002). Not only mature neurons are subject to alterations in the epileptic hippocampus as seizure-activity also strongly induces cell proliferation and net neurogenesis in rodent models of temporal lobe epilepsy (Bengzon et al., 1997, Parent et al., 1997, Gray and Sundstrom, 1998). The majority of seizure-induced neurons shows no obvious differences compared to physiologically generated nerve cells and most of them fully differentiate into mature granule cells. However, it is unclear if enhanced neurogenesis represents an attempt of the injured brain to replace lost cells or if seizure-generated neurons contribute to epilepsy-related hippocampal dysfunction. Therefore, we tried to answer the following questions in this study: (i) which cells respond to seizure-activity? (ii) do newborn granule cells contribute to long-term structural changes within the GCL of seizure-induced animals?
It has long been tacitly assumed that there was only one type of precursor cell in the adult dentate gyrus and that thereby the measurement of cell proliferation independent of different cell types was sufficient to describe the response. There is now ample evidence that this simplistic view is not sufficient (Kempermann et al., 2004). Within the neuronal lineage we distinguish radial glia-like type-1 precursor cells (Yamaguchi et al., 2000, Filippov et al., 2003, Fukuda et al., 2003, Kronenberg et al., 2003), which divide rarely and do not respond to physiologic neurogenic stimuli. In contrast, the transiently amplifying progenitor cells (type-2a and b) are induced by physiologic stimuli such as physical activity (Kronenberg et al., 2003). Type-3 cells are blast-like late progenitor cells, which do not show this induction, but can exhibit first morphological signs of neuronal maturation (Brandt et al., 2003, Brown et al., 2003).
We hypothesized that those precursor cells that are in a migratory state could be particularly sensitive to the effects of seizure activity. In contrast to the effects of physiologic neurogenic stimuli, the pathologic stimulus of seizures might stimulate the division of hippocampal progenitor cells “too late” and at a time when their primary goal is to migrate and initiate terminal differentiation. In other words, our hypothesis was that seizures might preferentially affect type-3 progenitor cells. Therefore, we closely analyzed precursor cell division and the subsequent migration pattern of newborn granule cells within the adult dentate gyrus. We here show that seizure-induced neurogenesis is primarily mediated by type-3 progenitor cells. The pathological activation of type-3 cells may be responsible for the observed dispersion of immature neurons within the adult GCL. The presented results offer a first step towards an explanation for the development of structural alterations within the adult GCL mediated by seizure-activated progenitors.
Section snippets
Animals
Twelve 8-week-old female mice (19–24 g) expressing enhanced GFP under the control of nestin gene regulatory regions (Yamaguchi et al., 2000, Sawamoto et al., 2001) were randomly assigned to two experimental groups (control and seizure animals; set-1). Animals of the seizure group received 35 mg/kg KA (Kainic acid, Sigma) in PBS, control animals received an identical volume of sterile PBS (V = approximately 200 μL). Approximately 20–40 min after KA-injection, all KA-treated mice displayed status
KA-induced seizures induce cell proliferation in the adult dentate gyrus and result in a stable increase in net neurogenesis
Several studies indicated that generalized seizure-activity increases the proliferation of dividing cells in the adult rodent hippocampus (Bengzon et al., 1997, Parent et al., 1997, Scott et al., 1998). Nine days after the injection of KA the number of BrdU-positive cells in the dentate gyrus was dramatically increased compared to control animals (Figs. 1A, B, E). Control animals showed 764 ± 30 BrdU-positive cells per GCL whereas seizure animals had 4623 ± 659 BrdU-positive cells (P < 0.001).
Discussion
Human temporal lobe epilepsy (TLE) is associated with a variety of neuropathological changes such as hippocampal cell loss and widening of the dentate gyrus granule cell layer that eventually lead to impaired hippocampal function (Houser, 1990, Helmstaedter, 2002, Pitkanen and Sutula, 2002). The finding that seizure activity also strongly increases cell proliferation in rodent models of TLE raised the question if and how newborn cells may contribute to epileptogenesis (Parent and Lowenstein,
Acknowledgments
We would like to thank Irene Thun and Ruth Zarmstorff for excellent technical support. The nestin-GFP transgenic mice were provided by Masahiro Yamaguchi, University of Tokyo. This study was supported by the VolkswagenStiftung and Deutsche Forschungsgemeinschaft (JE297/1-1 to S.J. and GRK-238 to B.R.).
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Contributed equally.