Abstracts

Rationale: The epileptic brain is characterized by a number of pathological changes of the hippocampal dentate gyrus, including the appearance of granule cells with deformed dendritic structures. These deformities include compressed dendritic trees, abnormal dendritic orientations and recurrent basal dendrites, and are present in epileptic tissue from both animals and humans. Moreover, dendritic abnormalities certainly alter the pattern and processing of afferent inputs, perhaps contributing to epileptogenesis and/or impaired mnemonic function. The cellular mechanisms by which these changes come about, however, remain mysterious. In particular, the role of immature vs. mature granule cells is unresolved. Previous studies have demonstrated that disruption of immature granule cells accounts almost exclusively for the development of another pathological change; the formation of hilar basal dendrites (Walter et al., J. Neurosci., 2007). Here, we examined mature granule cells using real-time confocal imaging to assess their potential role in dentate pathogenesis. Methods: Organotypic hippocampal explant cultures were made from neonatal Thy1-YFP expressing mice. In these mice, yellow fluorescent protein (YFP) is expressed in a subset of granule cells, revealing their complete morphology. Select neurons in these explants were imaged at defined intervals over a period of ten days using a Leica SP5 confocal microscope equipped with a humidified, temperature controlled chamber perfused with 95% air/5% CO2. Explant preparation - likely in response to deafferentation and trauma - is known to lead to changes reminiscent of the epileptic brain, such as mossy fiber sprouting. Whether explant preparation also disrupts the dendritic structure of mature granule cells was examined here. Results: Surprisingly, these studies revealed that mature granule cells with initially normal morphologies could develop recurrent basal dendrites by movement of existing apical dendrites to the basal pole of the cell. This appeared to result as a consequence of the soma translocating into an adjacent apical dendrite. As the soma moved up this dendrite, neighboring dendrites were shifted to the basal pole of the cell. Movement of the soma also led to the absorption of dendritic branch points, increasing the number of primary dendrites possessed by the cell. The somatic movement producing these dendritic changes likely reflects granule cell dispersion, which also occurred in these explants. Conclusions: Together, these findings suggest that granule cell dispersion, in addition to disrupting the normally compact granule cell body layer, also disrupts granule cell dendritic structure. Moreover, these finding demonstrate in principal, that recurrent basal dendrites can form by movement of pre-existing apical dendrites. Combined with previous studies examining immature granule cells, the current study examining mature granule cells suggests that both young and old cells contribute to dentate pathogenesis in epilepsy. This work was supported by the Cincinnati Children’s Hospital Research Foundation.