From bench to drug: human seizure modeling using Drosophila

Abstract

Studies of human seizure disorders have revealed that susceptibility to seizures is greatly influenced by genetic factors. In addition to causing epilepsy, genetic factors can suppress seizures and epileptogenesis. Examination of seizure-suppressor genes is challenging in humans. However, such genes are readily identified and analyzed in a Drosophila animal model of epilepsy. In this article, the epilepsy phenotype of Drosophila seizure-sensitive mutants is reviewed. A novel class of genes called seizure-suppressors is described. Mutations defining suppressors revert the "epilepsy" phenotype of neurological mutants. We conclude this review with particular discussion of a seizure-suppressor gene encoding DNA topoisomerase I (top1). Mutations of top1 are especially effective at reverting the seizure-sensitive phenotype of Drosophila epilepsy mutants. In addition, an unexpected class of anti-epileptic drugs has been identified. These are DNA topoisomerase I inhibitors such as camptothecin and its derivatives; several candidates are comparable or perhaps better than traditional anti-epileptic drugs such as valproate at reducing seizures in Drosophila drug-feeding experiments.

Figure 1. Behavioral and electrophysiological responses of BS mutants to mechanical and electrical stimulation

BS flies exhibit a unique behavioral repertoire following a mechanical stimulation (“bang”), such as a tap on the bench-top and vortex on the vortex mixer: they undergo seizure-like behavioral activity (hyperactivity) and subsequent paralysis. The seizure-like behavior is manifested as intense abnormal contraction, wing-flapping, proboscis extension, and leg-shaking; the paralysis is the cessation of physical activity. Upon recovery from paralysis, flies undergo additional bouts of spontaneous seizure-like behavioral activity that vary in intensity depending on the genotype of the fly. The entire cycle from the initial seizure-like behavior to the restoration of normal behavior is termed as the recovery time. This complex behavioral response can be mimicked on an electrophysiological level by administration of a high-frequency ECS (electroconvulsive shock) to the brain of the fly, which results in seizure-like activity followed by synaptic failure. As the fly recovers, a second bout of seizure-like activity is typically seen. Following ECS-induced seizure-like activity, flies exhibit transiently elevated seizure thresholds, corresponding to the behavioral refractory period.

Figure 2. Seizure-like activity in intact sda and wild type CS flies

The BS mutant sda fly is more susceptible to seizure-like activity than the wild type CS fly and therefore has a much lower seizure threshold. (A) Seizure-like activity is elicited in a sda fly by a high frequency stimulus of low strength (8 V) and displayed at a high sweep speed. The HF stimulus (labeled HF) is a short wavetrain (0.5 ms pulses at 200 Hz for 300 ms) of electrical stimuli delivered to the brain. Recording is from a muscle fiber (DLM, dorsal longitudinal muscle) and reflects the activity of the single DLM motoneuron that innervates it. The seizure-like activity is widespread as similar firing can be found in recordings from seven different muscle groups in the fly following HF stimulation (Kuebler and Tanouye, 2000). (B) A low-voltage HF stimulus of 8 V fails to elicit seizure-like activity in a wild type CS fly because the stimulus is below the seizure threshold. Following the HF stimulus artifact, there is no seizure-like activity observed in this recording displayed at a high sweep speed. Note also that there is no period of synaptic failure and single-pulse stimulation of the GF (0.5 Hz) continues to evoke DLM potentials. Two such effective single-pulse stimuli are depicted in this trace; each was effective in evoking a DLM potential. (C) Seizure-like activity is elicited in a wild type CS fly by a high voltage HF stimulus (32 V) which is above the threshold for seizure. Vertical calibration bar is 20 mV, horizontal calibration bar is 300 ms (figure modified from Kuebler and Tanouye, 2002, figure 1).

Figure 3. The comparison between genetic interaction and drug feeding

Drug treatment by CPT phenocopies the genetic interaction between bss and top1^JS in reduction of the recovery time. The average recovery time of bss is around 120s (3dp), it is reduced to around 70 s in the double mutant top1^JS bss, and 60 s in the CPT-treated bss mutant.