A Knock-In Model of Human Epilepsy in Drosophila Reveals a Novel Cellular Mechanism Associated with Heat-Induced Seizure

Developmental and Cell Biology, University of California, Irvine, Irvine, California 92697-1280, and Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, Providence, Rhode Island 02912.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 10/2012; 32(41):14145-55. DOI: 10.1523/JNEUROSCI.2932-12.2012
Source: PubMed


Over 40 missense mutations in the human SCN1A sodium channel gene are linked to an epilepsy syndrome termed genetic epilepsy with febrile seizures plus (GEFS+). Inheritance of GEFS+ is dominant, but the underlying cellular mechanisms remain poorly understood. Here we report that knock-in of a GEFS+ SCN1A mutation (K1270T) into the Drosophila sodium channel gene, para, causes a semidominant temperature-induced seizure phenotype. Electrophysiological studies of GABAergic interneurons in the brains of adult GEFS+ flies reveal a novel cellular mechanism underlying heat-induced seizures: the deactivation threshold for persistent sodium currents reversibly shifts to a more negative voltage when the temperature is elevated. This leads to sustained depolarizations in GABAergic neurons and reduced inhibitory activity in the central nervous system. Furthermore, our data indicate a natural temperature-dependent shift in sodium current deactivation (exacerbated by mutation) may contribute to febrile seizures in GEFS+ and perhaps normal individuals.

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    • "High-volume heat-induced seizure assay. In our previous study, seizure assays in knock-in flies were conducted by heating 1 fly/vial in a water bath with continuous monitoring of seizure activity for 120 s (Sun et al. 2012). With this method, the seizure activity could be modeled by the equation t ϭ 120 p, where t is total time spent seizing in seconds and p is the average probability of seizing, calculated by evaluating seizure probability at 1-s intervals (1 Hz). "
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    ABSTRACT: Hundreds of mutations in the SCN1A sodium channel gene confer a wide spectrum of epileptic disorders, requiring efficient model systems to study cellular mechanisms and identify potential therapeutic targets. We have recently demonstrated that Drosophila knock-in flies carrying a GEFS+ causing SCN1A mutation (K1270T) results in a primarily conditional increase in sodium current activity that contributes to a heat-induced seizure phenotype. To determine whether different SCN1A mutations cause distinct phenotypes in Drosophila as they do in humans, this paper focuses on a knock-in line carrying a mutation that causes a more severe seizure disorder, Dravet Syndrome (DS), in humans. Introduction of the DS SCN1A mutation (S1231R) into the Drosophila sodium channel gene, para, results in flies that exhibit spontaneous and heat-induced seizures with distinct characteristics and lower onset temperature than the GEFS+ flies. Electrophysiological studies of GABAergic interneurons in the brains of adult DS flies reveal, for the first time in an in vivo model system, that a missense DS mutation causes a constitutive and conditional reduction in sodium current activity and repetitive firing. In addition, feeding with the serotonin precursor 5-HTP suppresses heat-induced seizures in DS but not GEFS+ flies. The distinct alterations of sodium currents in DS and GEFS+ GABAergic interneurons demonstrate that both loss- and gain-of-function alterations in sodium currents are capable of causing reduced repetitive firing and seizure phenotypes. The mutation-specific effects of 5-HTP on heat-induced seizures suggest the serotonin pathway as a potential therapeutic target for DS.
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