Specific deletion of Na _V 1.1 sodium channels in inhibitory interneurons causes seizures and premature death in a mouse model of Dravet syndrome

Abstract

Heterozygous loss-of-function mutations in the brain sodium channel Na V 1.1 cause Dravet syndrome (DS), a pharmacoresistant infantile-onset epilepsy syndrome with comorbidities of cognitive impairment and premature death. Previous studies using a mouse model of DS revealed reduced sodium currents and impaired excitability in GABAergic interneurons in the hippocampus, leading to the hypothesis that impaired excitability of GABAergic inhibitory neurons is the cause of epilepsy and premature death in DS. However, other classes of GABAergic interneurons are less impaired, so the direct cause of hyperexcitability, epilepsy, and premature death has remained unresolved. We generated a floxed Scn1a mouse line and used the Cre-Lox method driven by an enhancer from the Dlx1,2 locus for conditional deletion of Scn1a in forebrain GABAergic neurons. Immunocytochemical studies demonstrated selective loss of Na V 1.1 channels in GABAergic interneurons in cerebral cortex and hippocampus. Mice with this deletion died prematurely following generalized tonic-clonic seizures, and they were equally susceptible to thermal induction of seizures as mice with global deletion of Scn1a. Evidently, loss of Na V 1.1 channels in forebrain GABAergic neurons is both necessary and sufficient to cause epilepsy and premature death in DS.V oltage gated sodium (Na V ) channels are composed of a 260-kDa pore-forming α subunit and one or more smaller auxiliary β subunits (1, 2). The Na V 1.1, Na V 1.2, Na V 1.3, and Na V 1.6 isoforms are highly expressed in the brain, where they initiate and propagate action potentials in neurons. Na V 1.1 and Na V 1.3 are prominently expressed in the cell soma and axon initial segment where they integrate incoming information from the dendrites, whereas Na V 1.2 channels are found in unmyelinated axons and dendrites, and Na V

Dissecting the phenotypes of Dravet syndrome by gene deletion

“…Loss-of-function mutations in Na v 1.1 cause Dravet syndrome in mice by selective impairment of excitability in GABAergic interneurons without detectable effects on excitatory neurons (Yu et al, 2006;Ogiwara et al, 2007;Cheah et al, 2012;Dutton et al, 2012;Tai et al, 2014). PV interneurons preferentially form synapses at the perisomatic region of their target cells, thereby controlling neuronal activity and spike output, whereas SST-expressing interneurons preferentially contact the distal dendrites, which allow them to efficiently control the impact of synaptic inputs near these sites (Klausberger and Somogyi, 2008;Harris and Mrsic-Flogel, 2013).…”

“…Studies of mouse genetic models demonstrated that Dravet syndrome is caused by disinhibition due to reduced excitability of GABAergic inhibitory interneurons without a corresponding change in the activity of excitatory neurons (Yu et al, 2006;Ogiwara et al, 2007Ogiwara et al, , 2013Cheah et al, 2012;Dutton et al, 2012;Kalume et al, 2013;Rubinstein et al, 2014;Tai et al, 2014). Moreover, specific heterozygous deletion of Na v 1.1 in forebrain GABAergic interneurons leads to seizures, premature death Kalume et al, 2013;Ogiwara et al, 2013), cognitive deficits, and autistic features (Han et al, 2012), similar to those in patients with Dravet syndrome.…”

“…In contrast, not all of these symptoms were observed in comparable severity in PV&SST-DS mice. The epileptic phenotype of PV&SST-DS mice is mild compared to global or interneuron-specific deletion of Na v 1.1 (Yu et al, 2006;Ogiwara et al, 2007;Oakley et al, 2009;Cheah et al, 2012;Kalume et al, 2013) because they did not have spontaneous generalized tonic-clonic seizures, their thermallyinduced generalized tonic-clonic seizures occurred at higher temperatures, and premature death was infrequent. Moreover, while long-term memory in the context-dependent fear conditioning test was impaired (Fig.…”

Severe peri-ictal respiratory dysfunction is common in Dravet syndrome

“…Mice with an Scn1a R1407X/+ mutation have cardiac myocyte dysfunction and die spontaneously at a young age after severe bradycardia, which has been interpreted as indicating that these mice die from intrinsic cardiac dysfunction (7). However, selective knockout of Scn1a only in brain interneurons results in seizures and spontaneous death (43). In mice with global knockout of Scn1a, heat-induced seizures are associated with progressively worsening bradycardia followed by terminal asystole (40).…”

Sudden unexpected death in a mouse model of Dravet syndrome

“…Our original mouse model, generated by global heterozygous KO of Scn1a, develops several key phenotypic features of DS, including epilepsy with early onset (beginning at P21), high susceptibility to hyperthermia-induced seizures, ataxia, spontaneous seizures with age-dependent severity, sleep and circadian abnormalities, and premature deaths (19)(20)(21)(22)(23)(24)(25). We performed simultaneous video-EEG-ECG recordings on these mice and discovered interictal, periictal, and ictal abnormalities of neural regulation of the heart associated with their Scn1a mutation and with SUDEP susceptibility.…”