Epilepsy in Dcx knockout mice associated with discrete lamination defects and enhanced excitability in the hippocampus

Mice lacking doublecortin and doublecortin-like kinase 2 display altered hippocampal neuronal maturation and spontaneous seizures

Mutations in doublecortin (DCX) are associated with intractable epilepsy in humans, due to a severe disorganization of the neocortex and hippocampus known as classical lissencephaly. However, the basis of the epilepsy in lissencephaly remains unclear. To address potential functional redundancy with murin Dcx, we targeted one of the closest homologues, doublecortin-like kinase 2 (Dclk2). Here, we report that Dcx; Dclk2-null mice display frequent spontaneous seizures that originate in the hippocampus, with most animals dying in the first few months of life. Elevated hippocampal expression of c-fos and loss of somatostatin-positive interneurons were identified, both known to correlate with epilepsy. Dcx and Dclk2 are coexpressed in developing hippocampus, and, in their absence, there is dosage-dependent disrupted hippocampal lamination associated with a cell-autonomous simplification of pyramidal dendritic arborizations leading to reduced inhibitory synaptic tone. These data suggest that hippocampal dysmaturation and insufficient receptive field for inhibitory input may underlie the epilepsy in lissencephaly, and suggest potential therapeutic strategies for controlling epilepsy in these patients.

Polarity regulation in migrating neurons in the cortex

The formation of the cerebral cortex requires migration of billions of cells from their birth position to their final destination. A motile cell must have internal polarity in order to move in a specified direction. Locomotory polarity requires the coordinated polymerization of cytoskeletal elements such as microtubules and actin combined with regulated activities of the associated molecular motors. This review is focused on migrating neurons in the developing cerebral cortex, which need to attain internal polarity in order to reach their proper target. The position and dynamics of the centrosome plays an important function in this directed motility. We highlight recent interesting findings connecting polarity proteins with neuronal migration events regulated by the microtubule-associated molecular motor, cytoplasmic dynein.

Abnormal network activity in a targeted genetic model of human double cortex

In human patients, cortical dysplasia produced by Doublecortin (DCX) mutations lead to mental retardation and intractable infantile epilepsies, but the underlying mechanisms are not known. DCX(-/-) mice have been generated to investigate this issue. However, they display no neocortical abnormality, lessening their impact on the field. In contrast, in utero knockdown of DCX RNA produces a morphologically relevant cortical band heterotopia in rodents. On this preparation we have now compared the neuronal and network properties of ectopic, overlying, and control neurons in an effort to identify how ectopic neurons generate adverse patterns that will impact cortical activity. We combined dynamic calcium imaging and anatomical and electrophysiological techniques and report now that DCX(-/-)EGFP(+)-labeled ectopic neurons that fail to migrate develop extensive axonal subcortical projections and retain immature properties, and most of them display a delayed maturation of GABA-mediated signaling. Cortical neurons overlying the heterotopia, in contrast, exhibit a massive increase of ongoing glutamatergic synaptic currents reflecting a strong reactive plasticity. Neurons in both experimental fields are more frequently coactive in coherent synchronized oscillations than control cortical neurons. In addition, both fields displayed network-driven oscillations during evoked epileptiform burst. These results show that migration disorders produce major alterations not only in neurons that fail to migrate but also in their programmed target areas. We suggest that this duality play a major role in cortical dysfunction of DCX brains.

Dcx reexpression reduces subcortical band heterotopia and seizure threshold in an animal model of neuronal migration disorder

Disorders of neuronal migration can lead to malformations of the cerebral neocortex that greatly increase the risk of seizures. It remains untested whether malformations caused by disorders in neuronal migration can be reduced by reactivating cellular migration, and whether such repair can decrease seizure risk. Here we show, in a rat model of subcortical band heterotopia (SBH) generated by in utero RNAi of Dcx, that aberrantly positioned neurons can be stimulated to migrate by re-expressing Dcx after birth. Re-starting migration in this way both reduces neocortical malformations and restores neuronal patterning. We find further that the capacity to reduce SBH has a critical period in early postnatal development. Moreover, intervention after birth reduces convulsant-induced seizure threshold to levels similar to that of malformation-free controls. These results suggest that disorders of neuronal migration may be eventually treatable by re-engaging developmental programs both to reduce the size of cortical malformations and to reduce seizure risk.