An epilepsy-causing human sodium channel mutation reduces activity in inhibitory neurons.

摘要

Epileptic seizures are the result of abnormal synchronized electrical discharges within the Central Nervous System (CNS). One type of epilepsy is Genetic Epilepsy with Febrile Seizures Plus (GEFS+) which is inherited as an autosomal dominant disorder. The R1648H mutation of the SCN1A sodium channel gene, among several others, causes GEFS+. The SCN1A gene encodes the pore forming alpha subunit of the voltage gated Nav1.1 sodium channel in the CNS, which underlies the generation of the action potential upstroke.;Our lab and others have reported gain-of-function defects owing to abnormal inactivation of the R1648H mutant using heterologous expression systems. However, this approach does not give an idea of the effects of the mutant in the native neuronal environment or of the cellular dynamic processes actually involved in epileptogenesis.;Our goal was to determine the effect of the Nav1.1 channel mutation at the molecular, cellular and network levels using a mouse model, to provide insights into the mechanisms of seizures in GEFS+. To accomplish this, we constructed a transgenic mouse expressing R1648H Nav1.1 channels under the native promoter. The transgenic channels are also resistant to saxitoxin, so that the mutant channel properties can be studied in isolation from the toxin sensitive endogenous sodium channels. We studied the changes in sodium channel gating properties of mutant channels in the presence of saxitoxin in dissociated neurons from transgenic mice. We found reduced sodium channel activity in cortical bipolar-shaped neurons from R1648H transgenic mice compared to the wild-type mice.;However, in the transgenic mice, the Nav1.1 channels are over-expressed and thus it is not a good model for the human disease. Therefore, we constructed a knock-in mouse model carrying the same R1648H mutation in Nav1.1 channels. We studied the effect of the mutation on sodium channel properties in cortical neurons and found loss-of-function defects in bipolar-shaped interneurons, which led us to predict reduced firing frequency in these neurons. Current clamp experiments showed that mutant interneurons have significant difficulty in sustaining repetitive firing. This decreased firing and might lead to reduced inhibition and failure to suppress hyperactivity in excitatory neurons, thus resulting to seizures in GEFS+.