Mesio-temporal lobe epilepsy (MTLE) is often accompanied by granule cell dispersion (GCD), a widening of the granule cell layer. The molecular determinants of GCD are poorly understood. Here, we used an animal model to study whether GCD results from an increased dentate neurogenesis associated with an abnormal migration of the newly generated granule cells. Adult mice were given intrahippocampal injections of kainate (KA) known to induce focal epileptic seizures and GCD, comparable to the changes observed in human MTLE. Ipsilateral GCD progressively developed after KA injection and was paralleled by a gradual decrease in the expression of doublecortin, a marker of newly generated granule cells, in the dentate subgranular layer. Staining with Fluoro-Jade B revealed little cell degeneration in the subgranular layer on the KA-injected side. Labeling with bromodeoxyuridine showed an early, transient increase in mitotic activity in the dentate gyrus of the KA-injected hippocampus that gave rise to microglial cells and astrocytes but not to new neurons. Moreover, at later time points, there was a virtually complete cessation of mitotic activity in the injected hippocampus (where GCD continued to develop), but not on the contralateral side (where no GCD was observed). Finally, a significant decrease in reelin mRNA synthesis in the injected hippocampus paralleled the development of GCD, and neutralization of reelin by application of the CR-50 antibody induced GCD. These results show that GCD does not result from increased neurogenesis but reflects a displacement of mature granule cells, most likely caused by a local reelin deficiency.

CASK is a multi-domain scaffolding protein that interacts with the transcription factor TBR1 and regulates expression of genes involved in cortical development such as RELN. Here we describe a previously unreported X-linked brain malformation syndrome caused by mutations of CASK. All five affected individuals with CASK mutations had congenital or postnatal microcephaly, disproportionate brainstem and cerebellar hypoplasia, and severe mental retardation.

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The extracellular matrix protein Reelin is an essential regulator of neuronal migration and lamination in the developing and mature brain. Lack of Reelin causes severe disturbances in cerebral layering, such as the reeler phenotype and granule cell dispersion in temporal lobe epilepsy. Reelin is synthesized and secreted by Cajal-Retzius cells and GABAergic interneurons, and its function depends on proteolytic cleavage after secretion. The mechanisms regulating these processes are largely unknown. Here, we used rat hippocampal slice cultures to investigate the effect of neuronal activation and hyperexcitation on Reelin synthesis, secretion, and proteolytic processing. We show that enhanced neuronal activity does not modulate Reelin synthesis or secretion. Moreover, we found that intracellular Reelin resides predominantly in the endoplasmic reticulum before it is constitutively secreted via the early secretory pathway. Epileptiform activity, however, impairs the proteolytic processing of Reelin and leads to accumulation of Reelin in the extracellular matrix. We found that both conditions, epileptiform activity and impaired proteolytic cleavage of Reelin, cause granule cell dispersion via inhibition of metalloproteinases. Taken together, our results strongly suggest that secretion of Reelin is activity-independent and that proteolytic processing of Reelin is required for the maintenance of granule cell lamination in the dentate gyrus.

ABSTRACT:Early life stress predisposes to the development of psychiatric disorders. In this context the hippocampal formation is of particular interest, because it is affected by stress on the structural and cognitive level. Since little is known how early life stress is translated on the molecular level, we mimicked early life stress in mouse models and analyzed the expression of the glycoprotein Reelin, a master molecule for development and differentiation of the hippocampus. From postnatal day 1 (P1) to P14, mouse pups were subjected to one of the following treatments: nonhandling (NH), handling (H), maternal separation (MS), and early deprivation (ED) followed by immediate (P15) or delayed (P70) real time RT-PCR analysis of reelin mRNA expression. We show that at P15, reelin mRNA levels were significantly increased in male H and ED groups when compared with the NH group. In contrast, no stress-induced alterations of reelin mRNA expression were found in female animals. This sex difference in stress-mediated stimulation of reelin expression was maintained into adulthood, since at P70 intergroup differences were still found in male, but not in female mice. On the cellular level, however, we did not find any significant differences in cell densities of Reelin-immunolabeled neurons between treatment groups or sexes, but an overall reduction of Reelin-expressing neurons in the adult hippocampus when compared to P15. To address the question whether corticosterone mediates the stress-induced up-regulation of reelin gene expression, we used age-matched hippocampal slice cultures derived from male and female mouse pups. Quantitative determination of mRNA levels revealed that corticosterone treatment significantly up-regulated reelin mRNA expression in male, but not in female hippocampi. Taken together, these results show a sex-specific regulation of reelin gene expression by early life experience, most likely mediated by corticosterone. V V C 2010 Wiley-Liss, Inc.