Exogenous reelin prevents granule cell dispersion in experimental epilepsy

Temporal lobe epilepsy (TLE) is often accompanied by granule cell dispersion (GCD), a migration defect of granule cells in the dentate gyrus. We have previously shown that a decrease in the expression of reelin, an extracellular matrix protein important for neuronal positioning, is associated with the development of GCD in TLE patients. Here, we used unilateral intrahippocampal injection of kainate (KA) in adult mice which is also associated with GCD formation and a decrease of reelin expression. In this mouse epilepsy model we aimed to prevent GCD development by the application of exogenous reelin. As a prerequisite we analyzed whether the reelin signaling transduction cascade was preserved in the KA-injected hippocampus. Using in situ hybridization and Western blot analysis we found that the expression of the reelin signaling components, apolipoprotein E receptor 2, the very-low-density lipoprotein receptor and the intracellular adaptor protein disabled 1, was maintained in dentate granule cells after KA injection. Next, recombinant reelin was infused into the KA-injected hippocampus by osmotic minipumps over a period of 2 weeks. Quantitative analysis of granule cell layer width revealed a significant reduction of GCD in reelin-treated, but not in saline-infused animals when compared to KA injection alone. Our findings highlight the crucial role of reelin for the maintenance of granule cell lamination in the dentate gyrus of adult mice and show that a reelin deficiency is causally involved in GCD development.