doi:10.1093/brain/awv142

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Dissecting the phenotypes of Dravet syndrome by gene deletion Moran Rubinstein, Sung Han, Chao Tai, Ruth E. Westenbroek, Avery Hunker, Todd Scheuer and William A. Catterall *These authors contributed equally to this work. Neurological and psychiatric syndromes often have multiple disease traits, yet it is unknown how such multi-faceted deficits arise from single mutations. Haploinsufficiency of the voltage-gated sodium channel Nav1.1 causes Dravet syndrome, an intractable childhood-onset epilepsy with hyperactivity, cognitive deficit, autistic-like behaviours, and premature death. Deletion of Nav1.1 channels selectively impairs excitability of GABAergic interneurons. We studied mice having selective deletion of Nav1.1 in parvalbumin- or somatostatin-expressing interneurons. In brain slices, these deletions cause increased threshold for action potential generation, impaired action potential firing in trains, and reduced amplification of postsynaptic potentials in those interneurons. Selective deletion of Nav1.1 in parvalbumin- or somatostatin-expressing interneurons increases susceptibility to thermally-induced seizures, which are strikingly prolonged when Nav1.1 is deleted in both interneuron types. Mice with global haploinsufficiency of Nav1.1 display autistic-like behaviours, hyperactivity and cognitive impairment. Haploinsufficiency of Nav1.1 in parvalbuminexpressing interneurons causes autistic-like behaviours, but not hyperactivity, whereas haploinsufficiency in somatostatin-expressing interneurons causes hyperactivity without autistic-like behaviours. Heterozygous deletion in both interneuron types is required to impair long-term spatial memory in context-dependent fear conditioning, without affecting short-term spatial learning or memory. Thus, the multi-faceted phenotypes of Dravet syndrome can be genetically dissected, revealing synergy in causing epilepsy, premature death and deficits in long-term spatial memory, but interneuron-specific effects on hyperactivity and autistic-like behaviours. These results show that multiple disease traits can arise from similar functional deficits in specific interneuron types.

Department of Pharmacology, University of Washington, Seattle, WA 98195-7280 Correspondence to: William A. Catterall, Department of Pharmacology, Box 357280 University of Washington Seattle, WA 98195-7280, USA E-mail: [email protected]

Keywords: Dravet syndrome; sodium channel; Nav1.1; interneurons; parvalbumin; somatostatin Abbreviations: EPSC/P = excitatory postsynaptic current/potential; IPSC = inhibitory postsynaptic currents; PV = parvalbumin; SST = somatostatin

Introduction Voltage-gated sodium (Nav) channels regulate neuronal excitability by initiation and propagation of action potentials

and amplification of Heterozygous truncation of-function mutations in the pore-forming subunit

subthreshold synaptic events. mutations and other severe lossthe SCN1A gene, which encodes of Nav1.1 sodium channels, cause

Received November 4, 2014. Revised March 17, 2015. Accepted April 4, 2015. Advance Access publication May 27, 2015 ß The Author (2015). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email: [email protected]

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Dravet syndrome (also called severe myoclonic epilepsy of infancy), a rare genetically dominant, intractable convulsive disorder. Dravet syndrome symptoms begin during the first year of life, with seizures often associated with fever, and progress to prolonged refractory seizures and frequent episodes of status epilepticus. During and after the second year of life, patients develop psychomotor delay, ataxia, cognitive impairment, and social interaction deficits (Genton et al., 2011; Li et al., 2011). 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., 2007, 2013; Cheah et al., 2012; Dutton et al., 2012; Kalume et al., 2013; Rubinstein et al., 2014; Tai et al., 2014). Moreover, specific heterozygous deletion of Nav1.1 in forebrain GABAergic interneurons leads to seizures, premature death (Cheah et al., 2012; 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. Parvalbumin (PV, encoded by PVALB)-expressing and somatostatin (SST)-expressing interneurons account for a large fraction of total interneurons. For instance, in layer V of the cerebral cortex these two classes include 480% of total interneurons (Rudy et al., 2011). Previous studies suggested that dysfunction of PV interneurons might contribute to seizures and premature death in Dravet syndrome. Deletion of both Nav1.1 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 Nav1.1 in PPP1R2expressing 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). Here we investigated the specific contributions of PV- and SST-expressing interneurons to the physiological and behavioural consequences of Dravet syndrome using mice with selective heterozygous and homozygous deletion of Nav1.1 in PV-expressing interneurons, SST-expressing interneurons, or both. Our results demonstrate that haploinsufficiency of Nav1.1 in either PV-expressing or SST-expressing interneurons results in reduced excitability of these interneuron types, spontaneous epileptic cortical discharge patterns without convulsions, consistent with absence or partial seizures, and increased sensitivity to thermally-induced seizures. Moreover, deletion in both of these interneuron types increased the susceptibility and severity of thermally-induced seizures and the frequency of premature death synergistically. In contrast, contributions to behavioural deficits in Dravet syndrome segregated by interneuron type. Reduced excitability of PV-expressing neurons caused autistic-like social interaction deficits, whereas impaired excitability of SST-expressing neurons resulted in hyperactivity. Deletion in both PV and SST interneurons impaired long-term spatial

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memory in context-dependent fear conditioning, but was not sufficient to recapitulate the full extent of the cognitive impairment observed in Dravet syndrome mice. Our results dissect the differential contributions of interneuron subtypes to the complex phenotypes of Dravet syndrome. These data reveal that impairment of different classes of interneurons is responsible for distinct aspects of the Dravet syndrome phenotype, and suggest that cellular deficits in different classes of interneurons may underlie multi-faceted disease traits in complex genetic syndromes.

Materials and methods For thermal induction of seizures and electrophysiology, we used male and female mice aged 21–24 days (postnatal Day 21 age group) or 32–37 days (postnatal Day 35 age group). Behavioural tests were performed on males aged 2–6 months. For EEG recording, fine silver wire EEG electrodes were implanted under ketamine/xylazine (130/8.8 mg/kg) anaesthesia (Kalume et al., 2013). EEG recordings were collected in conscious mice using a low impedance head-stage connected to RZ2D bioamp processor (Tucker Davis Technologies) sampled at 6 kHz. 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, 2013). The open field test, the three chamber social interaction test, Barnes circular maze, novel object recognition and Y-maze spontaneous alteration test were performed as described (Han et al., 2012). Our experimental techniques are described in detail in the Supplementary material.

Results Impairment of excitability of PV- and SST-expressing cortical interneurons by selective deletion of Nav1.1 channels Layer V pyramidal cells are the principal output neurons of the cerebral cortex, and their activity is tightly controlled by layer V PV-expressing, fast-spiking interneurons and SSTexpressing Martinotti cells (Rudy et al., 2011). Electrophysiological recordings in cortical slices from mice with global deletion of Nav1.1 demonstrated reduced excitability of both PV-expressing and SST-expressing interneurons (Tai et al., 2014). Similar recordings were made here to assess the degree of functional haploinsufficiency of Nav1.1 upon specific Cre-dependent deletion. Cre expression in Scn1a + / + (wild-type) and Scn1afl/ + (Dravet syndrome, DS) was monitored by the tdTomato reporter, and recordings were made from the most brightly labelled cells. Haploinsufficiency of Nav1.1 in PV-expressing interneurons (PVCre, Scn1afl/ + , termed PV-DS) or SST-expressing interneurons (SSTCre, Scn1afl/ + , termed SST-DS) resulted

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in increased threshold for action potential generation (Fig. 1A and B) and higher rheobase (Fig. 1C). In addition, both the number and frequency of action potentials fired in response to incremental steps of depolarizing current

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injection were reduced by deletion of Nav1.1 in either cell type (Fig. 1D–G and Supplementary Fig. 1). Layer V interneurons in PV-DS and SST-DS mice also exhibited increased failure of action potential firing in response to

Figure 1 Deletion of Nav1.1 results in reduced excitability in layer V cortical interneurons. (A) Examples of action potentials recorded in PV- and SST-expressing layer V cortical interneurons. Scn1a + / + are depicted in black. (B and C) Threshold (B) and rheobase for action potentials (C) in cortical PV (blue) and SST (yellow) neurons. Action potentials were evoked by 10 ms depolarizing current injection. (D– G) Firing of cortical neurons with selective deletion of Nav1.1 in response to 1-s long depolarizing current injection at the indicated intensities. (D) Sample traces of whole-cell current-clamp recordings of PV-expressing neurons in response to injection of 240 pA current. (E) Average number of action potentials in response to 1 s depolarizing current injection at the indicated intensity (n = 15–19). (F) Sample traces of whole-cell current-clamp recordings of SST-expressing neurons in response to injection of 160 pA current (n = 15–17). (G) Average number of action potentials in response to 1 s depolarizing current injection at the indicated intensity. (H–K) Failure rates for action potential generation in cortical neurons with selective deletion of Nav1.1. Per cent failure of action potentials in PV (H and I) and SST (J and K) interneurons. A train of 100 pulses, each 10 ms long at the minimal current required to trigger action potentials, was given at the indicated frequencies. Sample traces for 100 pulses at 10 Hz are shown (H and J) n = 15–17. Statistical analysis was done using Student’s t-test. *P 5 0.05, **P 5 0.01, ***P 5 0.001.

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short repetitive stimulation at frequencies ranging from 5 to 50 Hz (Fig. 1H–K). These results reveal a common deficit in electrical excitability of these two classes of interneurons, as reflected in their increased threshold and rheobase for action potential firing and failures of action potential firing during trains. The functional impairments observed here are similar or identical in magnitude to the deficits previously recorded in mice with Nav1.1 channels deleted globally (Tai et al., 2014), demonstrating that the Cre-Lox method has been fully effective in creating functional haploinsufficiency of Nav1.1 channels in both of these cell types. In addition, the similar impairment indicates that network changes caused by loss of Nav1.1 channels in excitatory neurons and other classes of interneurons in our global knockout

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mice do not substantially alter the functional cellular deficits beyond those caused by cell-autonomous heterozygous deletion of Nav1.1 in these PV- and SST-expressing interneurons.

Spontaneous synaptic activity of cortical interneurons As a measure of the impact of deletion of Nav1.1 channels on the function of neural circuits, we examined the effects of PV-DS and SST-DS mutations on spontaneous, action potential-driven, excitatory and inhibitory synaptic activity recorded from layer V pyramidal neurons. Deletion of Nav1.1 in PV-expressing neurons led to reduction in spontaneous inhibitory postsynaptic current (IPSC) frequency

Figure 2 Deletion of Nav1.1 in PV neurons, but not in SST neurons, alters the excitation/inhibition balance. Spontaneous IPSC and spontaneous EPSC were recorded in layer V pyramidal neurons from mice with specific deletion of Nav1.1 in PV or SST mice. (A–D) PV-DS mice. Example traces of spontaneous IPSC (A) and cumulative histogram of inter-event intervals (B, P = 0.05, Kolmogorov-Smirnov test) with mean values  SEM (inset) of spontaneous IPSC frequency. Spontaneous IPSC amplitude was 35.9  3.1 pA in PVCre Scn1a + / + and 36  3.4 pA in PVCre Scn1afl/ + , P = 0.49. n = 21–24. Example traces of spontaneous EPSC (C) and cumulative histogram of interevent intervals (D, P = 0.038, Kolmogorov-Smirnov test) with means  SEM (inset) of sEPSC frequency. Spontaneous EPSC amplitude was 25.3  1 pA in PVCre Scn1a + / + and 26.7  2.2 pA in PVCre Scn1afl/ + , P = 0.28, n = 19–20. (E–H) SST-DS mice. Example traces of spontaneous IPSC (E) and cumulative histogram of interevent intervals (F) with means  SEM (inset) of spontaneous IPSC frequency. Spontaneous IPSC amplitude was 39.5  5.3 pA in SSTCre Scn1a + / + and 39.4  3 pA in SSTCre Scn1afl/ + , P = 0.48. n = 25. Example traces of spontaneous EPSC (G) and cumulative histogram of interevent intervals (H) with means  SEM (inset) of spontaneous EPSC frequency. Spontaneous EPSC amplitude was 28  1.4 pA in SSTCre Scn1a + / + and 27.9  1.3 pA in and SSTCre Scn1afl/ + , P = 0.37, n = 19–21. Statistical analysis was done using Student’s t-test. *P 5 0.05, **P 5 0.01.

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with no alteration in the spontaneous IPSC amplitudes (Fig. 2A and B). Associated with this reduction in inhibitory synaptic neurotransmission, we observed a significant enhancement in spontaneous excitatory postsynaptic current (EPSC) frequency with no change in amplitude (Fig. 2C and D). These results indicate that the cortical network in layer V is hyperexcitable and hyperactive in PV-DS mice. In contrast to the results with PV-DS mice, the frequencies of spontaneous IPSCs and spontaneous EPSCs were not changed in neurons from SST-DS mice (Fig. 2E–H), probably because they make synapses on the distal dendrites far from our recordings in cell bodies (see ‘Discussion’ section).

Deficits of excitability and synaptic amplification in hippocampal interneurons Interneurons in the hippocampus also express PV and SST, and were shown to be involved in seizure generation in Dravet syndrome (Liautard et al., 2013). As a complement to our studies of cortical interneurons, we tested the functional consequence of selective deletion of Nav1.1 in CA1 horizontal stratum oriens lacunosum-moleculare (O-LM) cells. 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 Nav1.1 (Rubinstein et al., 2014). Similar recordings were made here in mice expressing Cre recombinase in both PV-expressing and SST-expressing interneurons (PV&SST). Recordings were made from the brightly labelled horizontal tdTomato-expressing cells, and all of the cells displayed the characteristic sag and firing pattern of O-LM cells (Supplementary Fig. 2A) (Maccaferri and McBain, 1996; Zemankovics et al., 2010). Similar to cortical interneurons, haploinsufficiency of Nav1.1 (PV&SST-DS) resulted in increased threshold and rheobase for action potential firing (Fig. 3A–C) and reduced number of action potentials in prolonged depolarizations (Supplementary Fig. 2B), demonstrating the importance of Nav1.1 in excitability of OL-M cells. To test the effects of selective deletion of Nav1.1 on evoked synaptic activity, a stimulating electrode was placed in the alveus to activate the axons of CA1 pyramidal cells antidromically (Maccaferri and McBain, 1995; Pouille and Scanziani, 2004), and the stimulation intensity was set to produce a firing probability of 50% during trains of stimuli (i.e. 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

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potentials in PV&SST-DS interneurons was 7 mV more depolarized (Fig. 3D and E), and accordingly the nearthreshold evoked excitatory postsynaptic potential (EPSP) amplitude was increased (Fig. 3G). Sodium currents combine to enhance and amplify the response to subthreshold synaptic inputs (Carter et al., 2012), and deletion of Nav1.1 resulted in a marked shortening of the EPSP half-width (Fig. 3F and H), which was accompanied by reduction in the time integral of depolarization during the EPSP (Fig. 3I). Associated with the larger and shorter EPSPs, the latency to spike generation and the spike jitter were significantly reduced (Fig. 3D and Supplementary Fig. 2C and D). Together, these data demonstrate that selective deletion of Nav1.1 hampers the excitability of multiple types of interneurons in both the cerebral cortex and the hippocampus. This reduced excitability in both brain regions recapitulates the cellular effects observed previously in Dravet syndrome mice with global deletion of Nav1.1 (Rubinstein et al., 2014; Tai et al., 2014), confirming effective reduction of Nav1.1 currents in these classes of interneurons upon Cre expression.

Seizures in mice with selective deletion of Nav1.1 channels in PV-expressing interneurons Loss-of-function mutations in SCN1A are associated with a range of epilepsies, all involving infantile onset of febrile seizures that proceed beyond childhood. With global Dravet syndrome mice, seizures can be induced by mild elevation of core body temperature late in the third week of life (Oakley et al., 2009), providing a natural pathophysiological stimulus for measurement of seizure susceptibility. Therefore, we tested mice with selective deletion of Nav1.1 for their susceptibility to thermally-induced seizures at two different ages, postnatal Day 21, the age at which Dravet syndrome mice with global Nav1.1 deletion first experience spontaneous seizures (Yu et al., 2006; Oakley et al., 2009), and postnatal Day 35 following the period of most intense spontaneous seizures (Kalume et al., 2013). Body core temperature was increased up to 42 C, and none of the Scn1a + / + mice tested experienced seizures in this temperature range. PV-DS mice did not have thermallyinduced seizures at postnatal Day 21 (Fig. 4A); however, seizures could be evoked in 50% of PV-DS mice at postnatal Day 35 (Fig. 4B). Previous studies report that adequate Cre expression in this mouse line is restricted to neurons with the strongest PV expression (Madisen et al., 2010; Ogiwara et al., 2013). Therefore, we also tested mice that were heterozygous for deletion of Scn1a but homozygous for expression of PV-Cre (PVCre/Cre, Scn1afl/ + ). In these mice, the expression of Cre protein is potentially 2-fold higher, and therefore Scn1a deletion should be achieved in PV neurons with lower activation of the PV promoter. Indeed, homozygous expression of PV-Cre increased the number of Cre-expressing cells by 25%

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(Supplementary Fig. 3). PV-DS mice with homozygous Cre expression were already sensitive to thermal induction of seizures at postnatal Day 21 (Fig. 4D, blue). Moreover, at postnatal Day 35, all the tested PVCre/Cre, Scn1afl/ + mice

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had thermally-induced seizures (Fig. 4E, blue), and their seizures were longer compared to PVCre, Scn1afl/ + mice (Fig. 4C, F, blue; 19.3  3 s and 135.2  18.7 s in PVCre, Scn1afl/ + and PVCre/Cre, Scn1afl/ + , respectively, P 5 0.01).

Figure 3 Deletion of Nav1.1 results in reduced excitability of CA1 stratum oriens hippocampal interneurons. (A–C) Action potentials evoked by direct depolarization of cell body via the patch pipette. (A) Examples of action potentials. (B and C) Threshold (B) and rheobase (C) of action potentials, n = 7. (D–G) Properties of synaptically evoked action potentials and near-threshold EPSPs in stratum oriens neurons. Stimulating electrode was placed in the alveus and the stimulation strength was set to produce firing probability of 50% during trains of 10 stimuli at 1 Hz. Representative action potentials (D) and mean values  SEM for threshold for action potential generation (E). (F–I) Parameters for near-threshold EPSPs. Examples of EPSPs (F). Mean values  SEM for EPSP amplitude (G), duration (H) and time integral (I), n = 7–9. To control for changes in properties of voltage-dependent currents activated by EPSPs of different amplitude, we examined the duration of EPSPs with similar amplitude. These were also of shorter duration in PV&SST-DS (Supplementary Fig. 2E and F), indicating that loss of Nav1.1 currents, and not voltage-dependent changes in the kinetics of the EPSP due to its higher amplitude, cause reduced amplification. Statistical analysis was done using Student’s t-test. **P 5 0.01, ***P 5 0.001.

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Figure 4 Epileptic phenotype of mice with deletion of Nav1.1 in PV- and SST-expressing interneurons. (A–C) Epileptic phenotypes of mice with heterozygous expression of Cre. (A–B) Percentage of mice remaining free of behavioural seizures (SZ) at the indicated body core temperatures at postnatal Day 21 (P21, A) and postnatal Day 35 (P35, B). Selective deletion of Nav1.1 in PV-expressing neuron (blue, n = 10), SSTexpressing neurons (yellow, n = 8), or combined deletion of Nav1.1 in both PV and SST expressing neurons (green, n = 17). (C) Mean values  SEM for seizure duration at postnatal Day 35. (D–F) Epileptic phenotypes of mice with homozygous expression of PV Cre. Mean values  SEM for the percentage of mice remaining free of behavioural seizures (SZ) at the indicated body core temperatures at postnatal Day 21 (D) and postnatal Day 35 (E). (F) Mean values  SEM for seizure duration at postnatal Day 35. Statistical analysis was done using one-way ANOVA, **P 5 0.01. PVCre/Cre-DS, n = 9, SSTCre/Cre-DS, n = 5, PVCre/Cre&SST-DS, n = 8. The epileptic phenotype SSTCre, Scn1afl/ + was indistinguishable from that of SSTCre/Cre, Scn1afl/ + (P 4 0.05). Similarly the phenotype of PVCre/Cre, SSTCre, Scn1afl/ + was indistinguishable from that of PVCre/Cre, SSTCre/Cre, Scn1afl/ + (n = 4 for each) and these data were pooled. Scn1a + / + mice of either genotype do not have seizures at the tested temperatures (n = 3–7 for each genotype). (G–I) Examples of EEG recordings from cortical lead depicting a spontaneous electrographic seizure in PVCre/Cre-DS (G), SSTCre-DS (H) and PVCre/Cre&SST-DS (I).

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Thus, we observed a gene-dosage effect in the epileptic phenotypes of PV-DS mice. Heterozygous deletion of Nav1.1 in some PV interneurons is sufficient to cause thermally-induced seizures, and increasing the number of affected PV interneurons by homozygous expression of PV-Cre leads to earlier onset of temperature induced seizures and longer duration of seizures. We did not observe any spontaneous convulsive seizures in PVCre/Cre-DS mice during routine handling or continuous home cage video recordings between postnatal Days 22–26 (n = 5), the period of most intense and frequent spontaneous seizures in mice with global deletion of Nav1.1 (Kalume et al., 2013). In contrast, video-EEG recordings of PVCre/Cre, Scn1afl/ + revealed brief (5 s) bilateral abnormal cortical discharge pattern that was associated with a short pause of behaviour without convulsions, consistent with absence or partial seizures (Fig. 4G). Together, these results demonstrate that deletion of Nav1.1 only in PV neurons is sufficient to cause mild spontaneous seizures and thermally-induced generalized tonic-clonic seizures, but not premature death.

Seizures in mice with selective deletion of Nav1.1 channels in SST-expressing interneurons Loss of function of Nav1.1 in SST neurons was also sufficient to induce thermal seizures at postnatal Day 35 but not at postnatal Day 21 (Fig. 4A–C). Cre expression under the SST promoter is highly efficient (Taniguchi et al., 2011), and mice with homozygous expression of Cre (SSTCre/Cre, Scn1afl/ + ) were indistinguishable from mice expressing only one allele of SST-Cre (SSTCre, Scn1afl/ + ) (Fig. 4A–F). Video-EEG recordings revealed brief episodes of abnormal cortical discharge patterns that were not associated with convulsions (Fig. 4H). No premature death or convulsive seizures were observed during routine handling or continuous home cage video recordings from postnatal Days 22–26 (n = 3). Together, these results demonstrate that deletion of Nav1.1 only in SST neurons is sufficient to cause susceptibility to thermally-induced generalized tonic-clonic seizures as well as mild spontaneous seizures, but not premature death.

Synergistic seizure susceptibility and severity with combined deletion of Nav1.1 in PV and SST interneurons As PV-expressing and SST-expressing neurons both contributed to epilepsy in Dravet syndrome, we wondered whether deletion of Nav1.1 in both types of interneurons would have synergistic effects. Although PV-DS and SSTDS mice were not susceptible to thermally-induced seizures at postnatal Day 21, 88% of PV&SST-DS mice (15/17) had thermally-induced seizures (Fig. 4A). Moreover, at postnatal Day 35, seizure duration in PV&SST-DS mice was

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6-fold longer than in PV-DS mice and 3-fold longer than in SST-DS mice (Fig. 4C). In addition, while deletion of Nav1.1 in PV or SST alone did not cause increased mortality, we did observe occasional premature deaths in PV&SST-DS mice (93% survival at 3 months). As observed in PV-DS mice, homozygous expression of PV-Cre in PVCre/ Cre &SST-DS mice increased the sensitivity to thermallyinduced seizures (Fig. 4D and E) as well as seizure duration (Fig. 4F). Thus, heterozygous deletion of Nav1.1 simultaneously in both types of interneurons results in earlier onset of seizure susceptibility and longer duration of thermallyinduced seizures, and those effects are greater than additive in several respects. Nevertheless, similar to PV-DS and SST-DS, no spontaneous convulsive seizures were observed in PVCre/Cre&SST-DS mice during routine handling or continuous home cage video recordings between postnatal Days 22 and 26 (n = 4); however, EEG recordings of PVCre/Cre&SST-DS revealed abnormal cortical discharge patterns that were not associated with observable convulsions (Fig. 4I).

Epileptic phenotype of mice with homozygous deletion of Scn1a Mice with homozygous deletion of Scn1a have severe ataxia, frequent seizures and all die by postnatal Day 14 (Yu et al., 2006; Kalume et al., 2007; Ogiwara et al., 2007). Homozygous deletion of Scn1a in PV-expressing neurons (PVCre, Scn1afl/fl) resulted in severe ataxia, spontaneous convulsive seizures, high sensitivity to thermallyinduced seizures, and frequent premature death, with only 40% survival at 3 months of age (Fig. 5A–C). Thus, the degree of loss of Nav1.1 function in PV-expressing neurons has dramatic effects on the epileptic phenotype. In contrast, the epileptic phenotype of mice with complete deletion of Scn1a in SST-expressing neurons (SSTCre, Scn1afl/fl) was similar to SST-DS mice. These mice had seizures at high temperature (Fig. 5A and B) and no premature death was observed (Fig. 5C). Mice with complete deletion of Nav1.1 in both PV and SST interneurons (PVCre, SSTCre, Scn1afl/fl) exhibited ataxia, frequent spontaneous convulsive seizures, and thermally induced seizures at low temperatures (Fig. 5A and B). Moreover, spontaneous death was frequent (Fig. 5C). These results with homozygous deletion of Nav1.1 in PV&SST-DS mice support the conclusion that deletion in both PV- and SST-expressing neurons has synergistic effects on epilepsy and premature death.

Specific effect of deletion of Nav1.1 in SST-expressing interneurons on hyperactivity Our previous results show that Dravet syndrome mice with global deletion of Scn1a display behavioural traits that are found in patients with Dravet syndrome, including

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Figure 6 Selective deletion of Nav1.1 in SST-expressing neurons causes hyperactivity. Distance moved in the open field test for PV (A, n = 8), SST (B, n = 6), and PV&SST mice (C, n = 10). The behaviour of SSTCre and SSTCre/Cre mice was indistinguishable in all the tests performed and the data were pooled together. Statistical analysis was done using Student’s t-test. *P 5 0.05.

Specific effect of deletion of Nav1.1 in PV-expressing interneurons on autistic-like behaviours Figure 5 Epileptic phenotype of mice with selective deletion of both alleles encoding Nav1.1. (A–C) Epileptic phenotypes of Scn1afl/fl mice. (A) Percentage of mice remaining free of behavioural seizures (SZ) at the indicated body core temperatures at postnatal Day 21. (B) Means values  SEM for the temperature of seizure induction. (C) Survival plots for PVCre, Scn1afl/fl mice (n = 22, blue), SSTCre, Scn1afl/fl mice (n = 15, yellow) and PVCre, SSTCre, Scn1afl/fl mice (n = 8, green). Statistical analysis was done using one-way ANOVA. *P 5 0.05.

hyperactivity, autistic-like social interaction deficits, and cognitive impairments (Han et al., 2012). Moreover, while floxed Scn1a + / + mice were indistinguishable from wild-type mice, Scn1afl/ + mice with specific heterozygous deletion of Nav1.1 in forebrain interneurons displayed all Dravet syndrome-associated behavioural deficits (Han et al., 2012). However, which types of interneurons are responsible for these behavioural phenotypes is not known. To answer this question, we performed behavioural tests with PV-DS, SST-DS, and PV&SST-DS mice, focusing on behaviours that we have previously shown to be altered in Dravet syndrome mice. As a first step, we measured the total distance moved during 10 min in the open field test to examine general locomotor activity in a novel environment. PV-DS mice displayed normal locomotor activity (Fig. 6A). However, SST-DS mice (Fig. 6B) and PV&SST-DS mice (Fig. 6C) travelled significantly longer distances. These data indicate that deficit of excitability in SST-expressing interneurons contributes to hyperactivity in Dravet syndrome mice, whereas impairment of excitability in PVexpressing interneurons does not.

We tested the social behaviour of these mice with the threechamber test of social interaction. In this test, mice are placed in the central compartment of a three-chambered plexiglass box and allowed to interact with an object (a small empty cage) in one side chamber or with a stranger mouse (in an identical small cage) in the other side chamber. Scn1a + / + mice spent 2-fold more time interacting with a stranger mouse than with the empty cage (Fig. 7A and B). In contrast, PV-DS mice displayed a social interaction deficit and exhibited no preference for interaction with a stranger mouse compared to the empty cage (Fig. 7A and B). No social deficit was apparent for SST-DS mice, which displayed a strong preference for interaction with the stranger mouse (Fig. 7C and D). The double Cre-expressing PV&SST-DS mice recapitulated the autistic-like social deficits of the PVDS mice (Fig. 7E and F). These data provide evidence that reduced electrical excitability in PV-expressing interneurons contributes specifically to social interaction deficits in Dravet syndrome, but impairment of excitability of SST-expressing interneurons does not.

Effect of deletion of Nav1.1 in PV- and SST-expressing interneurons on spatial learning and memory Cognitive impairment is characteristic in patients with Dravet syndrome. Similarly, Dravet syndrome mice with global or specific heterozygous deletion of Scn1a in forebrain interneurons, have deficits in short-term and longterm context-dependent fear conditioning and in spatial memory (Han et al., 2012). In contrast, PV-DS,

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improve their performance, and their latency to reach the target hole decreased with training similar to littermate controls (Fig. 8E). During the probe trial on the fifth day, both groups of mice remembered the spatial location of the target hole (Fig. 8F). Thus, deletion of Nav1.1 in PV and SST neurons does not recapitulate the spatial learning deficit observed in Dravet syndrome mice (Han et al., 2012). To further examine short-term memory and working memory, we tested PVCre/Cre&SST-DS in novel object recognition and spontaneous alteration in the Y-maze. PVCre/ Cre &SST-DS performance was similar to that of littermate controls in both tests (Fig. 8G–I), indicating that loss function of Nav1.1 in PV and SST neurons does not impair short-term working memory. Altogether, while simultaneous deletion of Nav1.1 in PV and SST expressing neurons can affect long-term fear memory, other class(es) of interneurons are involved in causing more severe short-term memory impairment and spatial learning deficits observed in mice with global deletion of Nav1.1.

Dissecting behavioural phenotypes in Dravet syndrome

Figure 7 Selective deletion of Nav1.1 in PV-expressing neurons causes social deficit. Three-chamber test of social interaction for PV (A and B, n = 8–9), SST (C and D, n = 11–14) and PV&SST (E and F, n = 6–7) mice. Time in each chamber (A, C and E) and total interaction time (B, D and F): O = object (an empty small mouse cage); C = centre; M = mouse (a stranger mouse in an identical small cage). Statistical analysis was done using Student’s ttest., *P 5 0.05, **P 5 0.01, ***P 5 0.001.

PVCre/Cre-DS, SST-DS and PV&SST-DS mice did not demonstrate deficits in context-dependent fear-conditioning. All of these mice displayed robust freezing behaviour when they returned to the fearful spatial context of a foot shock 30 min and 24 h later (Fig. 8A–C). In contrast, PVCre/Cre&SST-DS mice demonstrated normal freezing behaviour immediately following a foot shock and 30 min later, but their freezing behaviour was significantly reduced after 24 h. These results indicate that simultaneous deletion of Nav1.1 in both PVand SST-expressing neurons causes impairment of long-term context-dependent fear memory (Fig. 8D). We further assessed spatial learning in the absence of a fear stimulus using the Barnes circular maze. PVCre/ Cre &SST-DS and littermate controls were trained three times per day for four consecutive days to find a specific target hole. In contrast to mice with global deletion Scn1a (Han et al., 2012), PVCre/Cre&SST-DS mice were able to

Taken together, our behavioural data demonstrate that impairment of excitability in different subtypes of interneurons is differentially involved in the distinct behavioural deficits that we have observed in Dravet syndrome mice. These results reveal how a single gene mutation can result in multi-faceted neurophysiological and behavioural traits in a complex neuropsychiatric syndrome. Hyperactivity, impaired social interactions, and cognitive impairment are all characteristics of children with Dravet syndrome (Genton et al., 2011; Li et al., 2011). Therefore, it is conceivable that these traits in patients with Dravet syndrome may also involve impaired excitability of different classes of interneurons. Dissection of these phenotypes of Dravet syndrome by specific gene deletion in mice is considered further below.

Discussion Loss-of-function mutations in Nav1.1 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). The complementary and coordinated functions of these two types of interneurons provide versatile and powerful control of the activity in neuronal circuits. However, it is not known how

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Figure 8 Cognitive phenotype of mice with selective deletion of Nav1.1. Context-dependent fear conditioning test in PVCre (A, n = 1619 for Scn1a + / + and Scn1afl/ + , respectively), SST (B, n = 10), PV&SST mice (C, n = 10) and PVCre/Cre&SST mice (D, n = 7–9). Freezing behaviour of PVCre (n = 6–9) were indistinguishable from that of PVCre/Cre (n = 10) and these data were pooled in A. Basal = freezing time (%) during exploration of a cage containing an electrical grid as a floor and markings for identification; training = freezing time (%) during presentation of a 0.6 mA, 2 s long foot shock; 30 min = freezing time (%) upon return to the same cage after 30 min but without shock; 24 h = freezing time (%) upon return to the same cage after 24 h but without shock. (E) In the Barnes circular maze, both PVCre/Cre&SST-DS mice and littermate controls decreased their latency to the target hole similarly over the 4-day repeated training trials. (F) Latency to the target hole during the probe trial on the fifth day (n = 6 for each genotype). (G) In the novel object recognition test PVCre/Cre&SST-DS mice and littermate controls had normal recognition memory for a preconditioned object (F = familiar), which was presented 2 min before the test, and spent more time with the novel object (N = novel). (H) The normalized ratio of time spent with the familiar object divided by time spent with the novel object (n = 10–11 for each genotype). (I) Y-maze task: spontaneous alternation behaviour during a 10-min session (n = 5 for each genotype). Statistical analysis was done using Student’s t-test. *P 5 0.05, **P 5 0.01.

impaired electrical excitability in these two classes of interneurons would contribute to the multi-faceted neurological and behavioural phenotypes in Dravet syndrome. Here we show that Nav1.1 is important for normal synaptic boosting and action potential firing of both PV- and SSTexpressing interneurons and that reduced excitability of these major classes of interneurons contributes synergistically to the epilepsy and differentially to the behavioural deficits associated with Dravet syndrome.

Deletion of Nav1.1 in pyramidal neurons does not cause Dravet syndrome phenotypes Previous studies have shown that global haploinsufficiency of Nav1.1 channels has no detectable effect on sodium

currents or action potential firing in pyramidal neurons in hippocampus or cerebral cortex (Yu et al., 2006; Ogiwara et al., 2007; Rubinstein et al., 2014; Tai et al., 2014). Selective deletion of Nav1.1 channels in forebrain GABAergic interneurons fully reproduces the complex epilepsy and behavioural phenotypes of Dravet syndrome in these mice (Cheah et al., 2012; Ogiwara et al., 2013). Moreover, selective deletion of Nav1.1 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.

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Deletion of Nav1.1 in PV-expressing neurons results in reduced excitability, seizures and autistic-like behaviour In contrast to the lack of effect in pyramidal neurons, deletion of Nav1.1 in PV-expressing interneurons in cortical layer V resulted in reduced excitability due to increased threshold and rheobase, reduced number and frequency of action potentials, and inability to sustain firing during trains of depolarizations (Fig. 1). These deficits are similar to the loss of excitability observed in mice with global deletion of Nav1.1 (Tai et al., 2014). Moreover, deletion of Nav1.1 selectively in PV neurons was sufficient to increase the excitation/inhibition ratio (Fig. 2), as observed previously in Dravet syndrome mice (Han et al., 2012). Together, our physiological recordings confirmed that Cre expression in PV interneurons results in reduced function of Nav1.1, and the reduced excitability of PV interneurons in PV-DS mice is comparable to the functional deficits observed in Dravet syndrome mice. These deficits in excitability at the cellular level were sufficient to cause thermally-induced seizures in PVCre, Scn1afl/ + mice (heterozygous expression of Cre and Scn1a) (Fig. 4). Homozygous expression of Cre in PVCre/Cre, Scn1afl/ + mice resulted in brief spontaneous non-convulsive seizures, and increased the sensitivity to thermally-induced generalized tonic-clonic seizures. Generalized tonic-clonic seizures could be evoked as early as postnatal Day 21, and their duration was longer at postnatal Day 35 (Fig. 4). These results demonstrate the contribution of PV interneurons to epilepsy in Dravet syndrome (Fig. 4). Although heterozygous deletion of Nav1.1 is the relevant genetic ablation in patients with Dravet syndrome, and mice with homozygous global deletion of Nav1.1 do not survive beyond postnatal Day 14 (Yu et al., 2006; Ogiwara et al., 2007), we also tested the epileptic phenotype in mice with cell-specific homozygous deletion of Scn1a. Similar to previous findings (Ogiwara et al., 2013), PVCre, Scn1afl/fl mice had increased mortality and experienced seizures at or slightly above normal body temperature (Fig. 5). Thus, selective deletion of Nav1.1 in PV neurons demonstrated gene dosage effects that exacerbated the epileptic phenotype in two ways: by increasing the number of cells in which Scn1a gene is deleted via homozygous expression of Cre in PVCre/Cre mice and by increasing the loss of excitability via deletion of both alleles in Scn1afl/fl mice. Unlike other generalized epilepsy disorders, patients with Dravet syndrome have autism-spectrum behaviours (Li et al., 2011). Reduction in the number of PV neurons was observed in multiple mouse models of autism spectrum disorders (Gogolla et al., 2009). Strikingly here, PV-DS mice showed reduced social interaction in the threechamber test (Fig. 7), similar to globally deleted Dravet syndrome mice (Han et al., 2012). PV-DS mice were not hyperactive (Fig. 6) and performed similarly to control

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animals in context-dependent fear conditioning, indicating normal spatial learning and memory in this experimental paradigm (Fig. 8). Altogether, our experiments provide a clear dissection of the complex phenotypes of Dravet syndrome and show that dysfunction of PV-expressing interneurons due to loss of Nav1.1 is sufficient to cause epilepsy and social interaction deficit, but not hyperactivity or loss of context-dependent fear memory.

Deletion of Nav1.1 in SST-expressing neurons results in seizures and hyperactivity We tested two types of SST-positive neurons: cortical layer V Martinotti cells, which express SST (Silberberg and Markram, 2007), and hippocampal horizontal O-LM cells, which express both SST and PV (Maccaferri et al., 2000; Ferraguti et al., 2004). Deletion of Nav1.1 in either type of SST-expressing neuron resulted in increased threshold and rheobase for action potential firing (Figs 1 and 3). Moreover, SST-DS neurons failed to sustain firing during prolonged depolarizations (Fig. 1F and G, and Supplementary Fig. 2B) and trains of action potentials (Fig. 1J and K). In the hippocampus, SST-DS interneurons had higher threshold for synaptically evoked action potentials and shorter duration of near threshold EPSPs (Fig. 3). Together, these data demonstrate the importance of Nav1.1 in setting the threshold and initiation of action potentials in multiple types of interneurons (Rubinstein et al., 2014; Tai et al., 2014), and confirm loss of function of Nav1.1 on SST-Cre expression. Like PV-DS mice, SST-DS mice had mild spontaneous seizures and were also susceptible to thermal induction of generalized tonic-clonic seizures (Fig. 4). However, thermally-induced seizures occurred at high temperatures with both heterozygous and homozygous deletion of Nav1.1, and no premature death was observed (Fig. 5). Thus, heterozygous deletion of Nav1.1 channels only in SST-expressing neurons produced a mild epileptic phenotype. In layer V of the cerebral cortex, SST-expressing Martinotti cells are increasingly recruited by sustained high-frequency stimulation and provide frequency-dependent disynaptic inhibition to pyramidal cells (Silberberg and Markram, 2007). This local inhibitory circuit is disrupted in Dravet syndrome mice (Tai et al., 2014). However, this feedback circuit would not be expected to be active under basal conditions of electrical activity because an intense burst of action potentials is required to produce inhibition (Silberberg and Markram, 2007). Moreover, SST-expressing neurons also target PV interneurons (Letzkus et al., 2011; Pfeffer et al., 2013), and reduction in their excitability might have opposing effects in small circuits. In agreement with these characteristics, we found that the excitation/inhibition balance of spontaneous excitatory and inhibitory synaptic activity recorded at the cell body of pyramidal neurons was unchanged in SST-DS cortical

Dissecting Dravet syndrome phenotypes

slices (Fig. 2E–H). It may be that stronger stimuli, such as higher temperatures, are needed to reveal the effects of reduced synaptic inhibition in SST-DS mice. Hyperactivity is a well-documented behavioural characteristic in children with Dravet syndrome (Li et al., 2011) and in Dravet syndrome mice (Han et al., 2012; Ito et al., 2012). We found that SST-DS mice were hyperactive in an open field test (Fig. 6). However, SST-DS mice did not show any deficit in social interaction in the three-chamber test (Fig. 7) or in the context-dependent fear-conditioning test (Fig. 8). Taken together, our results with SST-DS mice further dissect the phenotypes of Dravet syndrome by showing that Nav1.1 is important for normal function of SST neurons and that specific loss of excitability of these neurons contributes to epilepsy and hyperactivity but not to autistic-like behaviours or deficit in spatial learning and memory as assessed in the context-dependent fear conditioning test.

Synergistic effects of deletion of Nav1.1 in PV- and SST-expressing neurons on epilepsy and premature death Activity in neuronal circuits is tightly regulated by different subtypes of inhibitory interneurons (Klausberger and Somogyi, 2008; Wolff et al., 2014). PV-expressing, fastspiking interneurons and SST-expressing Martinotti cells have complementary roles in providing inhibitory synaptic input at the cell bodies and basal dendrites versus the distal dendrites of layer V pyramidal cells, respectively. Our results show that reduced excitability of both PV and SST interneurons contributes to Dravet syndrome, and further that deletion of Nav1.1 in both types of interneurons simultaneously results in more severe epilepsy. PV&SST-DS mice were sensitive to thermally-induced seizures at postnatal Day 21, when PV-DS or SST-DS mice were seizurefree (Fig. 4A). At postnatal Day 35, when all the genotypes had thermally-induced seizures, the seizures in PV&SST-DS mice were much longer in duration (Fig. 4C and F). Moreover, some PV&SST-DS mice died prematurely, whereas we did not observe any premature death in PVDS mice with heterozygous or homozygous expression of PV-Cre or in SST-DS mice. Comparison of homozygous Scn1afl/fl mice also demonstrated synergistic effects. Thermally-induced seizures of PVCre, SSTCre, Scn1afl/fl mice occurred at lower temperatures compared to either PVCre, Scn1afl/fl or SSTCre, Scn1afl/fl (Fig. 5B), and all PVCre, SSTCre, Scn1afl/fl mice died, whereas 40% of PVCre, Scn1afl/fl survived at 3 months of age, and no mortality was observed in SSTCre, Scn1afl/fl mice (Fig. 5C). The synergy observed for deletion of Nav1.1 channels in PV- and SSTexpressing interneurons in epilepsy and premature death may reflect the synergistic effects of impairing their two complementary forms of inhibition in the cell body/

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proximal dendritic versus distal dendritic compartments of cortical pyramidal neurons.

Differential effects of deletion in PV- and SST-expressing interneurons on behaviour Dysfunction of interneurons, and the resulting excitation/ inhibition imbalance are involved in the aetiology of many neurological and psychiatric syndromes such as epilepsy, autism and schizophrenia. However, it is unclear how specific interneuron deficits contribute to pathophysiology of the complex behavioural phenotypes (Marin, 2012). Our experiments provide a clear dissection of the roles of PVexpressing versus SST-expressing interneurons in causing hyperexcitability and autistic-like phenotypes in Dravet syndrome mice. SST-DS mice are hyperactive, whereas PV-DS mice are not. PV-DS mice show autistic-like behaviours, whereas SST-DS mice do not. Behavioural tests with PV&SST-DS mice showed reduced social interaction that was similar to PV-DS and hyperactivity that was similar to SST-DS (Figs 6 and 7). These results suggest that the effects of deletion of Nav1.1 channels in PV-DS mice are truly primary in causing autistic-like phenotypes, and further deletion in SST-expressing interneurons has no additional effect on that phenotype. Similarly, it seems that deletion of Nav1.1 channels in SST-DS mice is truly primary in causing hyperactivity and further deletion in PVexpressing interneurons has no additional effect on that phenotype, even in the sensitized background of SST-DS mice. In contrast, while PV- and SST-DS mice had normal freezing behaviour in the context-dependent fear conditioning test, PVCre/Cre&SST-DS mice had reduced freezing after 24 h, demonstrating synergistic effects of PV and SST in impairment of long-term fear-related spatial memory (Fig. 8D). Interestingly, it was recently shown that memory retrieval circuits change with time (Do-Monte et al., 2015), so it is possible that normal firing of other classes of interneurons is sufficient for retrieval of fearful memory early after the event, while function of PV and SST interneurons is essential for retrieval at later time points. In this context, it was surprising that PVCre/Cre&SST-Dravet syndrome mice did not have a deficit in spatial learning in the Barnes maze, as indicated by their ability to improve their performance each day and to respond normally in the probe trial (Fig. 8E and F). It is conceivable that repeated training each day for 4 days is sufficient to overcome the deficit in long-term memory.

Contribution of other interneurons to Dravet syndrome The data presented here clearly dissect the roles of PV- and SST-expressing interneurons in Dravet syndrome and its associated comorbidities. However, mice with deletion of

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Nav1.1 in all forebrain interneurons recapitulated all the epileptic symptoms (Yu et al., 2006; Oakley et al., 2009, 2013; Kalume et al., 2013), hyperactivity, reduced sociability and cognitive deficits (Cheah et al., 2012; Han et al., 2012). 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 Nav1.1 (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. 8), short-term fear memory and spatial learning in the Barnes maze were preserved, indicating less severe cognitive impairment compared to Dravet syndrome mice. These results show that the severe deficit in spatial learning and memory in Dravet syndrome mice requires additional deletion of Nav1.1 channels in interneurons other than those that express PV and SST. Future studies of additional classes of interneurons by specific deletion of Nav1.1 channels using the Cre-Lox method may give further insights into the complete set of interneurons in which Nav1.1 channels must be deleted to fully replicate Dravet syndrome.

Complex behavioural deficits in neuropsychiatric disease Beyond their direct implications for Dravet syndrome, our results provide the first example, to our knowledge, of dissection of complex epileptic and behavioural phenotypes by gene deletion in single classes of interneurons in any neuropsychiatric disease. Because complex combinations of phenotypes are common in neuropsychiatric syndromes (Marin, 2012), this study may provide a precedent for other, more widespread disorders by showing that a single genetic mutation, which causes a common deficit at the cellular level in most or all interneurons, can nevertheless cause complex disease outcomes through the differential physiological and behavioural impacts of deficits in specific classes of interneurons. Such ‘interneuronopathies’ may be a common source of complexity in both genetic and idiopathic forms of neuropsychiatric disease.

Funding Research reported in this publication was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under award number R01 NS25704 and by a research grant from the Simons Foundation Autism Research Initiative. The content is solely the responsibility of the authors and does not

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necessarily represent the official views of the National Institutes of Health or the Simons Foundation.

Supplementary material Supplementary material is available at Brain online.

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