Modeling NaV1.1/SCN1A sodium channel mutations in a microcircuit with realistic ion concentration dynamics suggests differential GABAergic mechanisms leading to hyperexcitability in epilepsy and hemiplegic migraine.

Loss of function mutations of SCN1A, the gene coding for the voltage-gated sodium channel NaV1.1, cause different types of epilepsy, whereas gain of function mutations cause sporadic and familial hemiplegic migraine type 3 (FHM-3). However, it is not clear yet how these opposite effects can induce paroxysmal pathological activities involving neuronal networks' hyperexcitability that are specific of epilepsy (seizures) or migraine (cortical spreading depolarization, CSD). To better understand differential mechanisms leading to the initiation of these pathological activities, we used a two-neuron conductance-based model of interconnected GABAergic and pyramidal glutamatergic neurons, in which we incorporated ionic concentration dynamics in both neurons. We modeled FHM-3 mutations by increasing the persistent sodium current in the interneuron and epileptogenic mutations by decreasing the sodium conductance in the interneuron. Therefore, we studied both FHM-3 and epileptogenic mutations within the same framework, modifying only two parameters. In our model, the key effect of gain of function FHM-3 mutations is ion fluxes modification at each action potential (in particular the larger activation of voltage-gated potassium channels induced by the NaV1.1 gain of function), and the resulting CSD-triggering extracellular potassium accumulation, which is not caused only by modifications of firing frequency. Loss of function epileptogenic mutations, on the other hand, increase GABAergic neurons' susceptibility to depolarization block, without major modifications of firing frequency before it. Our modeling results connect qualitatively to experimental data: potassium accumulation in the case of FHM-3 mutations and facilitated depolarization block of the GABAergic neuron in the case of epileptogenic mutations. Both these effects can lead to pyramidal neuron hyperexcitability, inducing in the migraine condition depolarization block of both the GABAergic and the pyramidal neuron. Overall, our findings suggest different mechanisms of network hyperexcitability for migraine and epileptogenic NaV1.1 mutations, implying that the modifications of firing frequency may not be the only relevant pathological mechanism.

Excitatory and inhibitory neuron defects in a mouse model of Scn1b-linked EIEE52.

OBJECTIVE: Human variants in voltage-gated sodium channel (VGSC) α and ß subunit genes are linked to developmental and epileptic encephalopathies (DEEs). Inherited, biallelic, loss-of-function variants in SCN1B, encoding the ß1/ß1B subunits, are linked to early infantile DEE (EIEE52). De novo, monoallelic variants in SCN1A (Nav1.1), SCN2A (Nav1.2), SCN3A (Nav1.3), and SCN8A (Nav1.6) are also linked to DEEs. While these VGSC-linked DEEs have similar presentations, they have diverse mechanisms of altered neuronal excitability. Mouse models have suggested that Scn2a-, Scn3a-, and Scn8a-linked DEE variants are, in general, gain of function, resulting in increased persistent or resurgent sodium current (INa ) and pyramidal neuron hyperexcitability. In contrast, Scn1a-linked DEE variants, in general, are loss-of-function, resulting in decreased INa and hypoexcitability of fast-spiking interneurons. VGSC ß1 subunits associate with Nav1.1, Nav1.2, Nav1.3, and Nav1.6 and are expressed throughout the brain, raising the possibility that insults to both pyramidal and interneuron excitability may drive EIEE52 pathophysiology. METHODS: We investigated excitability defects in pyramidal and parvalbumin-positive (PV +) interneurons in the Scn1b-/- model of EIEE52. We also used Scn1bFL/FL mice to delete Scn1b in specific neuronal populations. RESULTS: Scn1b-/- cortical PV + interneurons were hypoexcitable, with reduced INa density. Scn1b-/- cortical pyramidal neurons had population-specific changes in excitability and impaired INa density. Scn1b deletion in PV + neurons resulted in 100% lethality, whereas deletion in Emx1 + or Camk2a + neurons did not affect survival. INTERPRETATION: This work suggests that SCN1B-linked DEE variants impact both excitatory and inhibitory neurons, leading to the increased severity of EIEE52 relative to other DEEs.

Analgesic-antitumor peptide inhibits the migration and invasion of HepG2 cells by an upregulated VGSC ß1 subunit.

Analgesic-antitumor peptide (AGAP), one of the scorpion toxin polypeptides, has been shown to have an antitumor activity. Recombinant AGAP (rAGAP) was shown to affect the migration and invasion of HepG2 cells via a voltage-gated sodium channel (VGSC) ß1 subunit. The VGSC ß1 subunit was validated as a cell adhesion molecule (CAM) in human hepatocellular carcinoma (HCC) cell lines. rAGAP suppresses the migration and invasion of HepG2 cells but has no significant effect of human liver HL7702 cells without ß1 subunit expression. rAGAP inhibits the migration and invasion of the cells when the VGSC ß1 subunit is overexpressed in HL7702 cells. To explain these findings, VGSC ß1 subunit messenger RNA (mRNA) and protein levels were measured. The ß1 subunit protein level was upregulated in a dose-dependent manner following treatment with rAGAP while there was no significant change in the mRNA level, so rAGAP might be an active component of the VGSC ß1 subunit.

The effects of eslicarbazepine on persistent Na⁺ current and the role of the Na⁺ channel ß subunits.

Eslicarbazepine is the major active metabolite of eslicarbazepine acetate, a once-daily antiepileptic drug approved in Europe as adjunctive therapy for refractory partial-onset seizures in adults. This study was aimed to determine the effects of eslicarbazepine on persistent Na(+) currents (INaP) and the role of ß subunits in modulating these effects. To study the role of ß subunits of the Na(+) channel we used a mouse line genetically lacking either the ß1 or ß2 subunit, encoded by the SCN1B or SCN2B gene, respectively. Whole cell patch-clamp recordings were performed on CA1 neurons in hippocampal slices under control conditions and application of 300 µM eslicarbazepine. We examined INaP in acutely isolated CA1 neurons and repetitive firing in hippocampal slices of mice lacking ß subunits and corresponding wild-type littermates. We found that eslicarbazepine caused a significant reduction of maximal INaP conductance and an efficient reduction of the firing rate in wild-type mice. We have shown previously a paradoxical increase of conductance of INaP caused by carbamazepine in mice lacking ß1 subunits in the subthreshold range, leading to a failure in affecting neuronal firing (Uebachs et al., 2010). In contrast, eslicarbazepine did not cause this paradoxical effect on INaP in SCN1B null mice. Consequently, the effects of eslicarbazepine on repetitive firing were maintained in these animals. These results indicate that eslicarbazepine exerts effects on INaP similar to those known for carbamazepine. However, in animals lacking the ß1 Na(+) channel subunit these effects are maintained. Therefore, eslicarbazepine potentially overcomes a previously described putative mechanism of resistance to established Na(+) channel acting antiepileptic drugs.

Loss of ß1 accessory Na+ channel subunits causes failure of carbamazepine, but not of lacosamide, in blocking high-frequency firing via differential effects on persistent Na+ currents.

PURPOSE: In chronic epilepsy, a substantial proportion of up to 30% of patients remain refractory to antiepileptic drugs (AEDs). An understanding of the mechanisms of pharmacoresistance requires precise knowledge of how AEDs interact with their targets. Many commonly used AEDs act on the transient and/or the persistent components of the voltage-gated Na(+) current (I(NaT) and I(NaP) , respectively). Lacosamide (LCM) is a novel AED with a unique mode of action in that it selectively enhances slow inactivation of fast transient Na(+) channels. Given that functional loss of accessory Na(+) channel subunits is a feature of a number of neurologic disorders, including epilepsy, we examined the effects of LCM versus carbamazepine (CBZ) on the persistent Na(+) current (I(NaP) ), in the presence and absence of accessory subunits within the channel complex. METHODS: Using patch-clamp recordings in intact hippocampal CA1 neurons of Scn1b null mice, I(NaP) was recorded using slow voltage ramps. Application of 100 µm CBZ or 300 µm LCM reduced the maximal I(NaP) conductance in both wild-type and control mice. KEY FINDINGS: As shown previously by our group in Scn1b null mice, CBZ induced a paradoxical increase of I(NaP) conductance in the subthreshold voltage range, resulting in an ineffective block of repetitive firing in Scn1b null neurons. In contrast, LCM did not exhibit such a paradoxical increase, and accordingly maintained efficacy in blocking repetitive firing in Scn1b null mice. SIGNIFICANCE: These results suggest that the novel anticonvulsant LCM maintains activity in the presence of impaired Na(+) channel ß(1) subunit expression and thus may offer an improved efficacy profile compared with CBZ in diseases associated with an impaired expression of ß sub-units as observed in epilepsy.