Activity-Dependent Alternative Splicing Increases Persistent Sodium Current and Promotes Seizure

Activity of voltage-gated Na channels (Na-v) is modified by alternative splicing. However, whether altered splicing of human Na(v)s contributes to epilepsy remains to be conclusively shown. We show here that altered splicing of the Drosophila Nav (paralytic, DmNa(v)) contributes to seizure-like behavior in identified seizure mutants. We focus attention on a pair of mutually exclusive alternate exons (termed K and L), which form part of the voltage sensor (S4) in domain III of the expressed channel. The presence of exon L results in a large, non-inactivating, persistent INap. Many forms of human epilepsy are associated with an increase in this current. In wild-type (WT) Drosophila larvae, similar to 70-80% ofDmNav transcripts contain exon L, and the remainder contain exonK. Splicing ofDmNav to include exon L is increased to similar to 100% in both the slamdance and easily-shocked seizure mutants. This change to splicing is prevented by reducing synaptic activity levels through exposure to the antiepileptic phenytoin or the inhibitory transmitter GABA. Conversely, enhancing synaptic activity in WT, by feeding of picrotoxin is sufficient to increase INap and promote seizure through increased inclusion of exon L to 100%. We also show that the underlying activity-dependent mechanism requires the presence of Pasilla, an RNA-binding protein. Finally, we use computational modeling to show that increasing INap is sufficient to potentiate membrane excitability consistent with a seizure phenotype. Thus, increased synaptic excitation favors inclusion of exon L, which, in turn, further increases neuronal excitability. Thus, at least in Drosophila, this self-reinforcing cycle may promote the incidence of seizure.