Genetic background modulates impaired excitability of inhibitory neurons in a mouse model of Dravet syndrome

Abstract

Dominant loss-of-function mutations in voltage-gated sodium channel NaV1.1 cause Dravet Syndrome, an intractable childhood-onset epilepsy. NaV1.1+/− Dravet Syndrome mice in C57BL/6 genetic background exhibit severe seizures, cognitive and social impairments, and premature death. Here we show that Dravet Syndrome mice in pure 129/SvJ genetic background have many fewer seizures and much less premature death than in pure C57BL/6 background. These mice also have a higher threshold for thermally induced seizures, fewer myoclonic seizures, and no cognitive impairment, similar to patients with Genetic Epilepsy with Febrile Seizures Plus. Consistent with this mild phenotype, mutation of NaV1.1 channels has much less physiological effect on neuronal excitability in 129/SvJ mice. In hippocampal slices, the excitability of CA1 Stratum Oriens interneurons is selectively impaired, while the excitability of CA1 pyramidal cells is unaffected. NaV1.1 haploinsufficiency results in increased rheobase and threshold for action potential firing and impaired ability to sustain high-frequency firing. Moreover, deletion of NaV1.1 markedly reduces the amplification and integration of synaptic events, further contributing to reduced excitability of interneurons. Excitability is less impaired in inhibitory neurons of Dravet Syndrome mice in 129/SvJ genetic background. Because specific deletion of NaV1.1 in forebrain GABAergic interneuons is sufficient to cause the symptoms of Dravet Syndrome in mice, our results support the conclusion that the milder phenotype in 129/SvJ mice is caused by lesser impairment of sodium channel function and electrical excitability in their forebrain interneurons. This mild impairment of excitability of interneurons leads to a milder disease phenotype in 129/SvJ mice, similar to Genetic Epilepsy with Febrile Seizures Plus in humans.

Dissecting the phenotypes of Dravet syndrome by gene deletion

“…This reduced excitability in both brain regions recapitulates the cellular effects observed previously in Dravet syndrome mice with global deletion of Na v 1.1 (Rubinstein et al, 2014;Tai et al, 2014), confirming effective reduction of Na v 1.1 currents in these classes of interneurons upon Cre expression.…”

“…EEG signals were processed off-line with a 0-70 Hz lowpass filter. Brain slices were prepared and electrophysiological studies were carried out as described (Rubinstein et al, 2014;Tai et al, 2014). Thermal induction of seizures was performed as described (Oakley et al, 2009.…”

“…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.…”

“…O-LM cells are critical for regulation of action potential firing of CA1 pyramidal cells and thereby exert important inhibitory control of circuit function and prevent epilepsy (Maccaferri and McBain, 1995;Pouille and Scanziani, 2004;Coulter et al, 2011). These cells represent a distinct subtype of interneurons that express both SST and PV (Maccaferri et al, 2000;Ferraguti et al, 2004), and their excitability was shown to be reduced in Dravet syndrome mice with global deletion of Na v 1.1 (Rubinstein et al, 2014). Similar recordings were made here in mice expressing Cre recombinase in both PV-expressing and SST-expressing interneurons (PV&SST).…”

“…five action potentials out of 10 stimuli delivered to the alveus at 1 Hz). This protocol effectively normalized for changes in intrinsic excitability (Rubinstein et al, 2014). The threshold for synaptically evoked action potentials in PV&SST-DS interneurons was $7 mV more depolarized ( Fig.…”