Preferential inactivation of Scn1a in parvalbumin interneurons increases seizure susceptibility

Abstract

Voltage-gated sodium channels (VGSCs) are essential for the generation and propagation of action potentials in electrically excitable cells. Dominant mutations in SCN1A, which encodes the Nav1.1 VGSC α-subunit, underlie several forms of epilepsy, including Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFS+). Electrophysiological analyses of DS and GEFS+ mouse models have led to the hypothesis that SCN1A mutations reduce the excitability of inhibitory cortical and hippocampal interneurons. To more directly examine the relative contribution of inhibitory interneurons and excitatory pyramidal cells to SCN1A-derived epilepsy, we first compared the expression of Nav1.1 in inhibitory parvalbumin (PV) interneurons and excitatory neurons from P22 mice using fluorescent immunohistochemistry. In the hippocampus and neocortex, 69% of Nav1.1 immunoreactive neurons were also positive for PV. In contrast, 13% and 5% of Nav1.1 positive cells in the hippocampus and neocortex, respectively, were found to co-localize with excitatory cells identified by CaMK2α immunoreactivity. Next, we reduced the expression of Scn1a in either a subset of interneurons (mainly PV interneurons) or excitatory cells by crossing mice heterozygous for a floxed Scn1a allele to either the Ppp1r2-Cre or EMX1-Cre transgenic lines, respectively. The inactivation of one Scn1a allele in interneurons of the neocortex and hippocampus was sufficient to reduce thresholds to flurothyl- and hyperthermia-induced seizures, whereas thresholds were unaltered following inactivation in excitatory cells. Reduced interneuron Scn1a expression also resulted in the generation of spontaneous seizures. These findings provide direct evidence for an important role of PV interneurons in the pathogenesis of Scn1a-derived epilepsies.

Dissecting the phenotypes of Dravet syndrome by gene deletion

“…Selective deletion of Na v 1.1 channels in forebrain GABAergic interneurons fully reproduces the complex epilepsy and behavioural phenotypes of Dravet syndrome in these mice Ogiwara et al, 2013). Moreover, selective deletion of Na v 1.1 channels in excitatory neurons does not cause the disease traits of Dravet syndrome and can actually oppose the lethal effects of the disease by reducing the frequency of premature death (Dutton et al, 2012;Ogiwara et al, 2013). Thus, in mouse genetic models of Scn1a deletion, which reproduce all of the complex facets of Dravet syndrome, no evidence has emerged that alteration of excitability of pyramidal neurons contributes to causing disease phenotypes.…”

“…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.…”

“…Deletion of both Na v 1.1 alleles in PV-expressing neurons, a more severe genetic ablation than Dravet syndrome, caused seizures and premature death (Ogiwara et al, 2013). In addition, heterozygous deletion of Na v 1.1 in PPP1R2-expressing interneurons, which include forebrain PVexpressing interneurons but also interneurons positive for reelin (RELN) and neuropeptide Y (NPY) (Belforte et al, 2010), also increased sensitivity to chemically-induced seizures (Dutton et al, 2012).…”

Dysfunction of hippocampal interneurons in epilepsy

“…In fact, Kv3.2 knock-out mice show an increased susceptibility to epileptic seizures and disturbed cortical rhythmic activity [34] . In a study of the relative contribution of inhibitory interneurons and excitatory pyramidal cells to SCN1A-derived epilepsy, Dutton found that the inactivation of one Scn1a allele in PV interneurons reduces the seizure threshold, whereas the threshold is unaltered following its inactivation in excitatory cells [36] . These results demonstrated that specific ion channels of interneurons play a signifi cant role in epileptogenesis.…”