Characterizing the role of sodium channels in mouse models of Dravet Syndrome

Abstract

Voltage-gated sodium channels (Nav) are responsible for the initiation and propagation of action potentials in excitable cells. The channels isoforms Nav1.1, Nav1.2, Nav1.3 and Nav1.6 are dynamically expressed in the developing central nervous system and are essential for proper network function. Heterozygous loss-of-function mutations in SCN1A, the gene encoding Nav1.1, leads to Dravet Syndrome (DS), a pharmacoresistant infantile-onset epilepsy syndrome with co-morbidities of cognitive impairment and premature death. Previous studies using a mouse model of DS heterozygous for a global deletion of Scn1a (Scn1a+/-), revealed reduced sodium currents and impaired excitability in GABAergic interneurons. We generated a floxed Scn1a mouse line and used the Cre-Lox method driven by an enhancer from the DLX1,2 locus to conditionally delete one or both copies of Scn1a in forebrain GABAergic neurons. Mice with this specific deletion had selective loss of NaV1.1 channels in GABAergic interneurons of the cerebral cortex and hippocampus, died prematurely following generalized tonic-clonic seizures, were equally susceptible to thermal induction of seizures as global Scn1a+/- mice, and demonstrated impaired cognitive function. Evidently, loss of NaV1.1 channels in forebrain GABAergic neurons is sufficient to cause epilepsy, premature death, and cognitive impairments in DS. Initial characterization of the DS mouse revealed a striking strain difference with respect to survival and seizure susceptibility. Studies also found an interneuron-specific increase in Nav1.3 with complete loss of Nav1.1, suggesting Nav1.3 as a possible precipitating factor or genetic modifier in DS. To evaluate the role of Nav1.3 in DS progression, we measured channel expression in non-epileptic mouse and human cortical tissue. We found Nav1.3 was expressed at high levels in embryonic life and declined after birth coincident with increased NaV1.1 channel expression in both species. The onset of seizures in mouse and human follows the developmental decrease in Nav1.3 to less than half of its maximal level suggesting that its loss, coupled with failure of normal expression of NaV1.1 channels, may contribute to the time of onset of seizures in DS. We tested whether genetic deletion of NaV1.3 channels would exacerbate the early phase of DS in mice and found that heterozygous loss of Nav1.3 does not lead to impaired survival or increased sensitivity to thermally induced seizures in DS mice. Our results support the hypothesis that declining expression of NaV1.3 channels to below 50% of maximum, in the face of heterozygous loss-of-function mutation of the NaV1.1 channel, may be one of the precipitating factors contributing to the time of onset of DS.