An Scn1a epilepsy mutation in Scn8a alters seizure susceptibility and behavior

Abstract

Understanding the role of SCN8A in epilepsy and behavior is critical in light of recently identified human SCN8A epilepsy mutations. We have previously demonstrated that Scn8amed and Scn8amed-jo mice carrying mutations in the Scn8a gene display increased resistance to flurothyl and kainic acid-induced seizures; however, they also exhibit spontaneous absence seizures. To further investigate the relationship between altered SCN8A function and epilepsy, we introduced the SCN1A-R1648H mutation, identified in a family with generalized epilepsy with febrile seizures plus (GEFS+), into the corresponding position (R1627H) of the mouse Scn8a gene. Heterozygous R1627H mice exhibited increased resistance to some forms of pharmacologically and electrically induced seizures and the mutant Scn8a allele ameliorated the phenotype of Scn1a-R1648H mutants. Hippocampal slices from heterozygous R1627H mice displayed decreased bursting behavior compared to wild-type littermates. Paradoxically, at the homozygous level, R1627H mice did not display increased seizure resistance and were susceptible to audiogenic seizures. We furthermore observed increased hippocampal pyramidal cell excitability in heterozygous and homozygous Scn8a-R1627H mutants, and decreased interneuron excitability in heterozygous Scn8a-R1627H mutants. These results expand the phenotypes associated with disruption of the Scn8a gene and demonstrate that an Scn8a mutation can both confer seizure protection and increase seizure susceptibility.

Regulation of Thalamic and Cortical Network Synchrony by Scn8a

“…We therefore performed immunohistochemistry for Scn8a and the inhibitory marker GAD65/67 in mouse brain slices and counted the percentage of double-labeled Scn8a processes. Consistent with previous findings, we found evidence for the expression of Scn8a in axons in cortical inhibitory cells (5.6% of Scn8a segments colocalized with GAD65/67, n = 3 animals, Figures 3A and 3B) (Lorincz and Nusser, 2008; Makinson et al, 2016). GAD65/67-negative Scn8a processes that predominate in the cortex likely represent axon initial segments (AISs) of excitatory projection neurons as previously described (Lorincz and Nusser, 2008).…”

“…The Scn8a manipulations reported here are designed to reduce channel expression (Figure S1). Consistent with this general principle, a mutation in Scn8a with both GOF and loss-of-function (LOF) properties exhibits a complex seizure phenotype displaying both pro-and anticonvulsant phenotypes (Makinson et al, 2016). …”

“…The susceptibility of mice with cell-type-specific deletion of Scn8a to chemically induced seizures was assessed by exposure to flurothyl (Makinson et al, 2016; Martin et al, 2007; Papale et al, 2009). Scn8a fl/+ FoxG1 animals (purple box) exhibited increased latencies to the generalized tonic clonic seizure (GTCS) but not to the initial myoclonic jerk (MJ), whereas Scn8a fl/+ mice expressing either of the excitatory cell-specific Cre transgenes, Emx1 or Camk2a Cre (blue box), exhibited increased latencies to both seizure behaviors (Figure 1; see Table S1 for statistics).…”

“…To evaluate Scn8a -associated seizure protection directly in a model of epilepsy, we generated mice carrying the Scn1a -R1648H genetic epilepsy with febrile seizures plus (GEFS+) mutation (Makinson et al, 2016; Martin et al, 2010) and excitatory cell-specific deletion of Scn8a as outlined in Figure S4E. Deletion was found both to increase seizure latencies and to normalize survival (Figure S4F; Seizure latencies, GTCS, Scn8a fl/+ no-Cre , 443 ± 29 s, Scn8a fl/+ no-Cre Scn1a RH / + , 332 ± 15 s, Scn8a fl/+ Emx1 , 714 ± 50 s, Scn1a RH / + Scn8a fl/+ Emx1 , 612 ± 43 s, One-way ANOVA, Dunnett’s post hoc; Survival, Scn8a fl/+ no- Cre , 9/9, 100%, Scn8a fl/+ Emx1 , 7/7, 100%, Scn8a fl/+ no- Cre Scn1a RH/+ , 8/12, 66.7%, Scn1a RH / + Scn8a fl / + Emx1 , 8/8, 100%, Mantel-Cox test).…”

“…We previously reported that mutations that reduce the expression or activity of Scn8a increase seizure resistance and ameliorate seizure phenotypes in Scn1a epilepsy models (Hawkins et al, 2011; Makinson et al, 2014, 2016; Martin et al, 2007). Here we show that selective deletion of Scn8a in excitatory, but not inhibitory, neurons is sufficient to protect against induced seizures and hippocampal epileptiform bursts, reduce cortical pyramidal cell excitability, and increase seizure resistance in an Scn1a epileptic mutant (Figures 1, S3, and S4).…”