Form and Function in the Scn1b-null Cerebellar Cortex: Implications for Epileptic Encephalopathy
Winters, Jesse
2017
Abstract
Dravet syndrome is a catastrophic, pediatric epileptic encephalopathy which is caused by genetic mutations is voltage-gated Na+ channel (VGSC) subunit genes. In addition to severe seizures, patients present with cognitive impairment and ataxia. VGSCs are composed of a single alpha subunit and two beta subunits, one of which is beta1. Most cases of Dravet syndrome are linked to haploinsufficiency in the alpha subunit gene SCN1A. However a growing number of new cases of SCN1B-linked Dravet syndrome are being discovered. SCN1B encodes the VGSC beta1 subunit as well as the secreted splice variant beta1B. Beta1 subunits are multifunctional, with roles in modulating Na+ current as well as developmental processes, including axon pathfinding and neuronal migration. The Scn1b-null mouse model of Dravet syndrome has severe seizures, an ataxic gait, and micro-organizational defects in the cerebellar cortex. We characterized the Scn1b-null ataxic gait using footprint analysis and found that these mice have a significant widening of paw positioning. We hypothesized that Scn1b may be critical for normal cerebellar cortical function. We determined that the migration and morphology of Purkinje cells are normal in mice lacking Scn1b. In contrast, electrophysiological analyses showed a significant reduction in action potential frequency in Scn1b-null Purkinje cells and molecular layer interneurons in cerebellar slice recordings. We hypothesized that a reduction in parallel fiber, climbing fiber, and/or basket interneuron synapses formed with Purkinje cells in the Scn1b-null cerebellar cortex could be contributing to this reduced excitability. This is plausible because beta1 and beta1B are members of the Ig-superfamily of cell adhesion molecules, which are known for their involvement in axon pathfinding, cell migration, and synaptogenesis. We used specific antibody markers to immunolabel synapses in the wildtype and Scn1b-null cerebellar cortex. We then prepared serial images using confocal microscopy and reconstructed the Purkinje cell surfaces in 3-dimensions. Quantification of these distinct classes of synapses on the Purkinje cell surface showed that they occur at normal densities in the mutant mouse in three separate lobules examined. We conclude that Scn1b is required for normal cerebellar cortical activity but is not required for the formation of cerebellar synapses in vivo. Abnormal function of excitatory parallel fiber synapses or climbing fiber synapses could be responsible for the observed hypoexcitablity in Scn1b-null Purkinje cells. Alternatively, reduced cell surface expression of VGSCs at the Purkinje cell axon initial segment could be leading to the defect. We propose that deficits in cerebellar cortical function may contribute to the pathological features of SCN1B-linked Dravet syndrome.Subjects
Dravet syndrome SCN1B Scn1b-null mouse epileptic encephalopathy cerebellar cortex Purkinje cell
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