Lithium-Responsive Seizure-Like Hyperexcitability Is Caused by a Mutation in theDrosophilaVoltage-Gated Sodium Channel Geneparalytic

Abstract

Shudderer (Shu) is an X-linked dominant mutation in Drosophila melanogaster identified more than 40 years ago. A previous study showed that Shu caused spontaneous tremors and defects in reactive climbing behavior, and that these phenotypes were significantly suppressed when mutants were fed food containing lithium, a mood stabilizer used in the treatment of bipolar disorder (Williamson, 1982). This unique observation suggested that the Shu mutation affects genes involved in lithium-responsive neurobiological processes. In the present study, we identified Shu as a novel mutant allele of the voltage-gated sodium (Nav) channel gene paralytic (para). Given that hypomorphic para alleles and RNA interference–mediated para knockdown reduced the severity of Shu phenotypes, Shu was classified as a para hypermorphic allele. We also demonstrated that lithium could improve the behavioral abnormalities displayed by other Nav mutants, including a fly model of the human generalized epilepsy with febrile seizures plus. Our electrophysiological analysis of Shu showed that lithium treatment did not acutely suppress Nav channel activity, indicating that the rescue effect of lithium resulted from chronic physiological adjustments to this drug. Microarray analysis revealed that lithium significantly alters the expression of various genes in Shu, including those involved in innate immune responses, amino acid metabolism, and oxidation-reduction processes, raising the interesting possibility that lithium-induced modulation of these biological pathways may contribute to such adjustments. Overall, our findings demonstrate that Nav channel mutants in Drosophila are valuable genetic tools for elucidating the effects of lithium on the nervous system in the context of neurophysiology and behavior.

Reduced Function of the Glutathione S-Transferase S1 Suppresses Behavioral Hyperexcitability inDrosophilaExpressing Mutant Voltage-Gated Sodium Channels

“…The effects of the deficiency on para Shu were evaluated by examining the F1 female progeny trans-heterozygous for para Shu and the deficiency (e.g., para Shu /+ ; Df/+) for their reactive climbing behavior (see Materials and Methods). As reported previously, para Shu heterozygous females have a severe defect in climbing behavior due to spontaneous tremors and uncoordinated movements (KAAS et al 2016). Our initial screen identified several chromosomal deficiencies that significantly improved the climbing behavior of para Shu females (Supplemental Table 1; deficiencies that resulted in CI>0.4 are shaded).…”

“…Reactive climbing: The reactive climbing assay was performed as previously described (KAAS et al 2016), using a countercurrent apparatus originally invented by Seymour Benzer (BENZER 1967). Five to seven-day-old females (~20) were placed into one tube (tube #0), tapped to the bottom, and allowed 15 sec to climb, at which point those that had climbed were transferred to the next tube.…”

“…A number of para mutant alleles have been identified in Drosophila. They display a variety of physiological and behavioral phenotypes: lethality, olfactory defects, spontaneous tremors, neuronal hyperexcitability, resistance to insecticides, and paralysis or seizure in response to heat, cold, or mechanical shock (SUZUKI et al 1971;GANETZKY AND WU 1982;LILLY et al 1994;MARTIN et al 2000;LINDSAY et al 2008;PARKER et al 2011;SUN et al 2012;SCHUTTE et al 2014;KAAS et al 2016). One of these more recently characterized Na v -channel gene mutants, para Shu , is a dominant gain-of-function allele formerly referred to as Shudderer due to the "shuddering" or spontaneous tremors it causes (WILLIAMSON 1971;WILLIAMSON 1982).…”

“…One of these more recently characterized Na v -channel gene mutants, para Shu , is a dominant gain-of-function allele formerly referred to as Shudderer due to the "shuddering" or spontaneous tremors it causes (WILLIAMSON 1971;WILLIAMSON 1982). This allele contains a missense mutation that results in the replacement of an evolutionarily conserved methionine residue in Na v -channel homology domain III (Kaas et al 2016). Adult para Shu mutants exhibit various dominant phenotypes in addition to shuddering, such as defective climbing behavior, increased susceptibility to electroconvulsive and heatinduced seizures, and short lifespan.…”

“…Adult para Shu mutants exhibit various dominant phenotypes in addition to shuddering, such as defective climbing behavior, increased susceptibility to electroconvulsive and heatinduced seizures, and short lifespan. They also have an abnormal down-turned wing posture and an indented thorax, both of which are thought to be caused by neuronal hyperexcitability (WILLIAMSON 1982;KAAS et al 2016;KASUYA et al 2019). In the current study, we carried out a forward genetic screen for dominant modifiers of para Shu and found that the phenotypes are significantly suppressed by loss-of-function mutations in the glutathione S-transferase S1 (GstS1) gene.…”