Anticonvulsant valproate reduces seizure-susceptibility in mutant Drosophila

Effects of valproate on seizure-like activity in Drosophila melanogaster with a knockdown of Ube3a in different neuronal populations as a model of Angelman Syndrome

Angelman Syndrome is a rare, genetically induced neurodevelopmental disorder. This disorder stems from a mutation or deletion of the maternal UBE3A gene. Characteristics of this disease include developmental delay, recurring seizures, and severe intellectual disabilities. We studied seizure activity in male Drosophila melanogaster with a knockdown of Ube3a in different neuronal populations (GABAergic, glutamatergic, mushroom body, and all neurons) and investigated the effects of the antiseizure medication (ASM) on seizure-like activity. Epileptiform activity was monitored in individual fruit flies using imaging chambers and mechanically induced seizures using a vortex assay. A positive control was also used: eas (easily shocked seizure phenotype). Seizure activity was analyzed for sums of seizure durations, number of seizures, and total time to return to normal activity. Ube3a knockdowns in GABAergic neurons elicited more seizure-like episodes than knockdowns in glutamatergic neurons and were on par with the positive control group and those with knockdowns in the mushroom bodies. We have established a method whereby valproate could be administered through food rather than through injections to effectively treat epileptiform activity. We demonstrated that if Ube3a is not knocked down pan-neuronally, Angelman Syndrome seizure-like activity can be studied using Drosophila melanogaster and therefore allows for high-throughput drug discovery.

Drosophila melanogaster as a versatile model organism to study genetic epilepsies: An overview

Epilepsy is one of the most prevalent neurological disorders, affecting more than 45 million people worldwide. Recent advances in genetic techniques, such as next-generation sequencing, have driven genetic discovery and increased our understanding of the molecular and cellular mechanisms behind many epilepsy syndromes. These insights prompt the development of personalized therapies tailored to the genetic characteristics of an individual patient. However, the surging number of novel genetic variants renders the interpretation of pathogenetic consequences and of potential therapeutic implications ever more challenging. Model organisms can help explore these aspects in vivo. In the last decades, rodent models have significantly contributed to our understanding of genetic epilepsies but their establishment is laborious, expensive, and time-consuming. Additional model organisms to investigate disease variants on a large scale would be desirable. The fruit fly Drosophila melanogaster has been used as a model organism in epilepsy research since the discovery of "bang-sensitive" mutants more than half a century ago. These flies respond to mechanical stimulation, such as a brief vortex, with stereotypic seizures and paralysis. Furthermore, the identification of seizure-suppressor mutations allows to pinpoint novel therapeutic targets. Gene editing techniques, such as CRISPR/Cas9, are a convenient way to generate flies carrying disease-associated variants. These flies can be screened for phenotypic and behavioral abnormalities, shifting of seizure thresholds, and response to anti-seizure medications and other substances. Moreover, modification of neuronal activity and seizure induction can be achieved using optogenetic tools. In combination with calcium and fluorescent imaging, functional alterations caused by mutations in epilepsy genes can be traced. Here, we review Drosophila as a versatile model organism to study genetic epilepsies, especially as 81% of human epilepsy genes have an orthologous gene in Drosophila. Furthermore, we discuss newly established analysis techniques that might be used to further unravel the pathophysiological aspects of genetic epilepsies.

Fly seizure EEG: field potential activity in the Drosophila brain

Hypersynchronous neural activity is a characteristic feature of seizures. Although many Drosophila mutants of epilepsy-related genes display clear behavioral spasms and motor unit hyperexcitability, field potential measurements of aberrant hypersynchronous activity across brain regions during seizures have yet to be described. Here, we report a straightforward method to observe local field potentials (LFPs) from the Drosophila brain to monitor ensemble neural activity during seizures in behaving tethered flies. High frequency stimulation across the brain reliably triggers a stereotypic sequence of electroconvulsive seizure (ECS) spike discharges readily detectable in the dorsal longitudinal muscle (DLM) and coupled with behavioral spasms. During seizure episodes, the LFP signal displayed characteristic large-amplitude oscillations with a stereotypic temporal correlation to DLM flight muscle spiking. ECS-related LFP events were clearly distinct from rest- and flight-associated LFP patterns. We further characterized the LFP activity during different types of seizures originating from genetic and pharmacological manipulations. In the 'bang-sensitive' sodium channel mutant bangsenseless (bss), the LFP pattern was prolonged, and the temporal correlation between LFP oscillations and DLM discharges was altered. Following administration of the pro-convulsant GABA_A blocker picrotoxin, we uncovered a qualitatively different LFP activity pattern, which consisted of a slow (1-Hz), repetitive, waveform, closely coupled with DLM bursting and behavioral spasms. Our approach to record brain LFPs presents an initial framework for electrophysiological analysis of the complex brain-wide activity patterns in the large collection of Drosophila excitability mutants.

North Sea Progressive Myoclonus Epilepsy is Exacerbated by Heat, A Phenotype Primarily Associated with Affected Glia

Progressive myoclonic epilepsies (PMEs) comprise a group of rare disorders of different genetic aetiologies, leading to childhood-onset myoclonus, myoclonic seizures and subsequent neurological decline. One of the genetic causes for PME, a mutation in the gene coding for Golgi SNAP receptor 2 (GOSR2), gives rise to a PME-subtype prevalent in Northern Europe and hence referred to as North Sea Progressive Myoclonic Epilepsy (NS-PME). Treatment for NS-PME, as for all PME subtypes, is symptomatic; the pathophysiology of NS-PME is currently unknown, precluding targeted therapy. Here, we investigated the pathophysiology of NS-PME. By means of chart review in combination with interviews with patients (n = 14), we found heat to be an exacerbating factor for a majority of NS-PME patients (86%). To substantiate these findings, we designed a NS-PME Drosophila melanogaster model. Downregulation of the Drosophila GOSR2-orthologue Membrin leads to heat-induced seizure-like behaviour. Specific downregulation of GOSR2/Membrin in glia but not in neuronal cells resulted in a similar phenotype, which was progressive as the flies aged and was partially responsive to treatment with sodium barbital. Our data suggest a role for GOSR2 in glia in the pathophysiology of NS-PME.

De Novo Variants in WDR37 Are Associated with Epilepsy, Colobomas, Dysmorphism, Developmental Delay, Intellectual Disability, and Cerebellar Hypoplasia

WD40 repeat-containing proteins form a large family of proteins present in all eukaryotes. Here, we identified five pediatric probands with de novo variants in WDR37, which encodes a member of the WD40 repeat protein family. Two probands shared one variant and the others have variants in nearby amino acids outside the WD40 repeats. The probands exhibited shared phenotypes of epilepsy, colobomas, facial dysmorphology reminiscent of CHARGE syndrome, developmental delay and intellectual disability, and cerebellar hypoplasia. The WDR37 protein is highly conserved in vertebrate and invertebrate model organisms and is currently not associated with a human disease. We generated a null allele of the single Drosophila ortholog to gain functional insights and replaced the coding region of the fly gene CG12333/wdr37 with GAL4. These flies are homozygous viable but display severe bang sensitivity, a phenotype associated with seizures in flies. Additionally, the mutant flies fall when climbing the walls of the vials, suggesting a defect in grip strength, and repeat the cycle of climbing and falling. Similar to wall clinging defect, mutant males often lose grip of the female abdomen during copulation. These phenotypes are rescued by using the GAL4 in the CG12333/wdr37 locus to drive the UAS-human reference WDR37 cDNA. The two variants found in three human subjects failed to rescue these phenotypes, suggesting that these alleles severely affect the function of this protein. Taken together, our data suggest that variants in WDR37 underlie a novel syndromic neurological disorder.

Glial Ca2+signaling links endocytosis to K+ buffering around neuronal somas to regulate excitability

Glial-neuronal signaling at synapses is widely studied, but how glia interact with neuronal somas to regulate their activity is unclear. Drosophila cortex glia are restricted to brain regions devoid of synapses, providing an opportunity to characterize interactions with neuronal somas. Mutations in the cortex glial NCKX^zydeco elevate basal Ca²⁺, predisposing animals to seizure-like behavior. To determine how cortex glial Ca²⁺ signaling controls neuronal excitability, we performed an in vivo modifier screen of the NCKX^zydeco seizure phenotype. We show that elevation of glial Ca²⁺ causes hyperactivation of calcineurin-dependent endocytosis and accumulation of early endosomes. Knockdown of sandman, a K_2P channel, recapitulates NCKX^zydeco seizures. Indeed, sandman expression on cortex glial membranes is substantially reduced in NCKX^zydeco mutants, indicating enhanced internalization of sandman predisposes animals to seizures. These data provide an unexpected link between glial Ca²⁺ signaling and the well-known role of glia in K⁺ buffering as a key mechanism for regulating neuronal excitability.

Shortened Lifespan and Other Age-Related Defects in Bang Sensitive Mutants of Drosophila melanogaster

Mitochondrial diseases are complex disorders that exhibit their primary effects in energetically active tissues. Damage generated by mitochondria is also thought to be a key component of aging and age-related disease. An important model for mitochondrial dysfunction is the bang sensitive (bs) mutants in Drosophila melanogaster Although these mutants all show a striking seizure phenotype, several bs mutants have gene products that are involved with mitochondrial function, while others affect excitability another way. All of the bs mutants (parabss , eas, jus, ses B, tko are examined here) paralyze and seize upon challenge with a sensory stimulus, most notably mechanical stimulation. These and other excitability mutants have been linked to neurodegeneration with age. In addition to these phenotypes, we have found age-related defects for several of the bs strains. The mutants eas, ses B, and tko display shortened lifespan, an increased mean recovery time from seizure with age, and decreased climbing ability over lifespan as compared to isogenic CS or w1118 lines. Other mutants show a subset of these defects. The age-related phenotypes can be rescued by feeding melatonin, an antioxidant, in all the mutants except ses B The age-related defects do not appear to be correlated with the seizure phenotype. Inducing seizures on a daily basis did not exacerbate the phenotypes and treatment with antiepileptic drugs did not increase lifespan. The results suggest that the excitability phenotypes and the age-related phenotypes may be somewhat independent and that these phenotypes mutants may arise from impacts on different pathways.

The imbalance of serotonergic circuitry impairing the crop supercontractile muscle activity and the mitochondrial morphology of PD PINK1B9Drosophila melanogaster are rescued by Mucuna pruriens

Despite its great potentiality, little attention has been paid to modelling gastrointestinal symptoms of Parkinson's disease (PD) in Drosophila melanogaster (Dm). Our previous studies on standardized Mucuna pruriens extract (Mpe) have shown usefulness in the Drosophila model of PD. In this communication, we provide new information on the effect of Mpe on basal and serotonin treated contractions in the crop (i.e., an important and essential part of the gut) in Drosophila PD mutant for PTEN-induced putative kinase 1 (PINK1^B9) gene. The effect of Mpe on PINK1^B9 supplied with standard diet to larvae and/or adults, were assayed on 10-15 days old flies. Conversely from what we observed in the wild type flies, recordings demonstrated that exogenous applications of serotonin on crop muscles of untreated PINK1^B9 affect neither the frequency nor the amplitude of the crop contraction, while the same muscle parameters are enhanced following brain injections of serotonin, thus suggesting that PINK1^B9 mutants may likely have an impairment in the serotonergic pathways. Also, the mitochondrial morphology in the crop muscles is strongly compromised, as demonstrated by the transmission electron microscopy analysis. The Mpe treatment rescued the crop muscle parameters and also the mitochondrial morphology when supplied to both larvae and adults. Overall, this study strengthens the relevance of using PINK1^B9 Dm as a translational model to study the gastrointestinal symptoms in PD and also confirms the useful employment of M. pruriens for PD treatment.

Rogdi Defines GABAergic Control of a Wake-promoting Dopaminergic Pathway to Sustain Sleep in Drosophila

Kohlschutter-Tönz syndrome (KTS) is a rare genetic disorder with neurological dysfunctions including seizure and intellectual impairment. Mutations at the Rogdi locus have been linked to development of KTS, yet the underlying mechanisms remain elusive. Here we demonstrate that a Drosophila homolog of Rogdi acts as a novel sleep-promoting factor by supporting a specific subset of gamma-aminobutyric acid (GABA) transmission. Rogdi mutant flies displayed insomnia-like behaviors accompanied by sleep fragmentation and delay in sleep initiation. The sleep suppression phenotypes were rescued by sustaining GABAergic transmission primarily via metabotropic GABA receptors or by blocking wake-promoting dopaminergic pathways. Transgenic rescue further mapped GABAergic neurons as a cell-autonomous locus important for Rogdi-dependent sleep, implying metabotropic GABA transmission upstream of the dopaminergic inhibition of sleep. Consistently, an agonist specific to metabotropic but not ionotropic GABA receptors titrated the wake-promoting effects of dopaminergic neuron excitation. Taken together, these data provide the first genetic evidence that implicates Rogdi in sleep regulation via GABAergic control of dopaminergic signaling. Given the strong relevance of GABA to epilepsy, we propose that similar mechanisms might underlie the neural pathogenesis of Rogdi-associated KTS.

Opposite effects of 5-HT/AKH and octopamine on the crop contractions in adult Drosophila melanogaster: Evidence of a double brain-gut serotonergic circuitry

This study showed that in adult Drosophila melanogaster, the type of sugar-either present within the crop lumen or in the bathing solution of the crop-had no effect on crop muscle contraction. What is important, however, is the volume within the crop lumen. Electrophysiological recordings demonstrated that exogenous applications of serotonin on crop muscles increases both the amplitude and the frequency of crop contraction rate, while adipokinetic hormone mainly enhances the crop contraction frequency. Conversely, octopamine virtually silenced the overall crop activity. The present study reports for the first time an analysis of serotonin effects along the gut-brain axis in adult D. melanogaster. Injection of serotonin into the brain between the interocellar area shows that brain applications of serotonin decrease the frequency of crop activity. Based on our results, we propose that there are two different, opposite pathways for crop motility control governed by serotonin: excitatory when added in the abdomen (i.e., directly bathing the crop) and inhibitory when supplied within the brain (i.e., by injection). Finally, our results point to a double brain-gut serotonergic circuitry suggesting that not only the brain can affect gut functions, but the gut can also affect the central nervous system. On the basis of our results, and data in the literature, a possible mechanism for these two discrete serotonergic functions is suggested.

Mutations of the Calcium Channel Gene cacophony Suppress Seizures in Drosophila

Bang sensitive (BS) Drosophila mutants display characteristic seizure-like phenotypes resembling, in some aspects, those of human seizure disorders such as epilepsy. The BS mutant para^bss1, caused by a gain-of-function mutation of the voltage-gated Na⁺ channel gene, is extremely seizure-sensitive with phenotypes that have proven difficult to ameliorate by anti-epileptic drug feeding or by seizure-suppressor mutation. It has been presented as a model for intractable human epilepsy. Here we show that cacophony (cac^TS2), a mutation of the Drosophila presynaptic Ca⁺⁺ channel α₁ subunit gene, is a particularly potent seizure-suppressor mutation, reverting seizure-like phenotypes for para^bss1 and other BS mutants. Seizure-like phenotypes for para^bss1 may be suppressed by as much as 90% in double mutant combinations with cac^TS2. Unexpectedly, we find that para^bss1 also reciprocally suppresses cac^TS2 seizure-like phenotypes. The cac^TS2 mutant displays these seizure-like behaviors and spontaneous high-frequency action potential firing transiently after exposure to high temperature. We find that this seizure-like behavior in cac^TS2 is ameliorated by 85% in double mutant combinations with para^bss1.

Seizure disorders, such as epilepsy, are a serious health concern because of the large number of patients affected and a limited availability of treatment options. About 10% of the population will have at least one seizure during their lifetime, and 1% will experience persistent, recurrent epileptic seizures. Moreover, for about one-third of patients, epilepsy is intractable with seizures that are not controlled with any available drugs. Genetic seizure suppressors are modifier mutations that are capable of reverting seizure susceptibility to wild type levels when combined with seizure-prone mutants in double mutant individuals. Suppressors are valuable in providing an experimental approach that can provide insight into mechanisms underlying seizure susceptibility. Also, they identify novel gene products that may be targets for therapeutic drug development. In the present study we show that a severe seizure phenotype of the Drosophila paralytic^bss1(para^bss1) mutant is 90% suppressed by the N-type calcium channel mutation cacophony^TS2(cac^TS2). The effect of suppression is not restricted to para^bss1, but cac^TS2 can also revert seizure-like phenotypes of other Drosophila mutants like easily-shocked (eas) and slamdance (sda). Thus, cac^TS2 acts as a highly potent, general seizure suppressor mutation. A surprising finding in this study is co-suppression: para^bss1 also suppresses a seizure phenotype in cac^TS2 mutants induced by elevated temperature. A current view of complex diseases such as epilepsy, is that multiple genes and environmental factors can each contribute small, additive effects that can summate to produce a disease state when some threshold value is exceeded. Our findings indicate that different pathogenic ion channel mutations can sometimes form therapeutic combinations with effects that mask one another.

Drosophila sodium channel mutations: Contributions to seizure-susceptibility

This paper reviews Drosophila voltage-gated Na(+) channel mutations encoded by the para (paralytic) gene and their contributions to seizure disorders in the fly. Numerous mutations cause seizure-sensitivity, for example, para(bss1), with phenotypes that resemble human intractable epilepsy in some aspects. Seizure phenotypes are also seen with human GEFS+ spectrum mutations that have been knocked into the Drosophila para gene, para(GEFS+) and para(DS) alleles. Other para mutations, para(ST76) and para(JS) act as seizure-suppressor mutations reverting seizure phenotypes in other mutants. Seizure-like phenotypes are observed from mutations and other conditions that cause a persistent Na(+) current through either changes in mRNA splicing or protein structure.

A new model to study sleep deprivation-induced seizure

BACKGROUND AND STUDY OBJECTIVES

A relationship between sleep and seizures is well-described in both humans and rodent animal models; however, the mechanism underlying this relationship is unknown. Using Drosophila melanogaster mutants with seizure phenotypes, we demonstrate that seizure activity can be modified by sleep deprivation.

DESIGN

Seizure activity was evaluated in an adult bang-sensitive seizure mutant, stress sensitive B (sesB(9ed4)), and in an adult temperature sensitive seizure mutant seizure (sei(ts1)) under baseline and following 12 h of sleep deprivation. The long-term effect of sleep deprivation on young, immature sesB(9ed4) flies was also assessed.

SETTING

Laboratory.

PARTICIPANTS

Drosophila melanogaster.

INTERVENTIONS

Sleep deprivation.

MEASUREMENTS AND RESULTS

Sleep deprivation increased seizure susceptibility in adult sesB(9ed4)/+ and sei(ts1) mutant flies. Sleep deprivation also increased seizure susceptibility when sesB was disrupted using RNAi. The effect of sleep deprivation on seizure activity was reduced when sesB(9ed4)/+ flies were given the anti-seizure drug, valproic acid. In contrast to adult flies, sleep deprivation during early fly development resulted in chronic seizure susceptibility when sesB(9ed4)/+ became adults.

CONCLUSIONS

These findings show that Drosophila is a model organism for investigating the relationship between sleep and seizure activity.

Activity-dependent regulation of astrocyte GAT levels during synaptogenesis

Astrocytic uptake of GABA through GABA transporters (GATs) is an important mechanism regulating excitatory/inhibitory balance in the nervous system, however mechanisms by which astrocytes regulate GAT levels are undefined. Here we show at mid-pupal stages the Drosophila CNS neuropil is devoid of astrocyte membranes and synapses. Astrocyte membranes subsequently infiltrate the neuropil coordinate with synaptogenesis and a strocyte ablation reduces synapse numbers by half, indicating that Drosophila astrocytes are pro-synaptogenic. Shortly after synapses form in earnest, the GABA transporter, GAT, is up-regulated in astrocytes. Ablation or silencing of GABAergic neurons or disruption of metabotropic GABA receptor (GABA_BR1/2) signaling in astrocytes leads to decreased astrocytic GAT levels. Interestingly, developmental depletion of astrocytic GABA_BR1/2 signaling suppresses mechanosensory-induced seizure activity in mutants with hyperexcitable neurons. These data reveal astrocytes actively modulate GAT expression via metabotropic GABA receptor signaling, and highlight the importance of precise regulation of astrocytic GAT in modulation of seizure activity.

Regulation of membrane excitability: a convergence on voltage-gated sodium conductance

The voltage-gated sodium channel (Nav) plays a key role in regulation of neuronal excitability. Aberrant regulation of Nav expression and/or function can result in an imbalance in neuronal activity which can progress to epilepsy. Regulation of Nav activity is achieved by coordination of a multitude of mechanisms including RNA alternative splicing and translational repression. Understanding of these regulatory mechanisms is complicated by extensive genetic redundancy: the mammalian genome encodes ten Navs. By contrast, the genome of the fruitfly, Drosophila melanogaster, contains just one Nav homologue, encoded by paralytic (DmNa v ). Analysis of splicing in DmNa v shows variants exhibit distinct gating properties including varying magnitudes of persistent sodium current (INaP). Splicing by Pasilla, an identified RNA splicing factor, alters INaP magnitude as part of an activity-dependent mechanism. Enhanced INaP promotes membrane hyperexcitability that is associated with seizure-like behaviour in Drosophila. Nova-2, a mammalian Pasilla homologue, has also been linked to splicing of Navs and, moreover, mouse gene knockouts display seizure-like behaviour.Expression level of Navs is also regulated through a mechanism of translational repression in both flies and mammals. The translational repressor Pumilio (Pum) can bind to Na v transcripts and repress the normal process of translation, thus regulating sodium current (INa) density in neurons. Pum2-deficient mice exhibit spontaneous EEG abnormalities. Taken together, aberrant regulation of Nav function and/or expression is often epileptogenic. As such, a better understanding of regulation of membrane excitability through RNA alternative splicing and translational repression of Navs should provide new leads to treat epilepsy.

Seizure-sensitivity in Drosophila is ameliorated by dorsal vessel injection of the antiepileptic drug valproate

Drosophila is a powerful model organism that can be used for the development of new drugs directed against human disease. A limitation is the ability to deliver drugs for testing. We report on a novel delivery system for treating Drosophila neurological mutants, direct injection into the circulatory system. Using this method, we show that injection of the antiepileptic drug valproate can ameliorate seizure-sensitive phenotypes in several mutant genotypes in the bang-sensitive (BS) paralytic mutant class, sda, eas, and para(bss1). This drug-injection method is superior to drug-feeding methods that we have employed previously, presumably because it bypasses potent detoxification systems present in the fly. In addition, we find that utilizing blood-brain barrier mutations in the background may further improve the injection results under certain circumstances. We propose that this method of drug delivery is especially effective when using Drosophila to model human pathologies, especially neurological syndromes.

A novel Drosophila SOD2 mutant demonstrates a role for mitochondrial ROS in neurodevelopment and disease

Reactive oxygen species (ROS) play essential roles in cell signaling, survival, and homeostasis. Aberrant ROS lead to disease and contribute to the aging process. Numerous enzymes and vigilant antioxidant pathways are required to regulate ROS for normal cellular health. Mitochondria are a major source of ROS, and mechanisms to prevent elevated ROS during oxidative phosphorylation require super oxide dismutase (SOD) activity. SOD2, also known as MnSOD, is targeted to mitochondria and is instrumental in regulating ROS by conversion of superoxides to hydrogen peroxide, which is further broken down into H(2)O and oxygen. Here, we describe the identification of a novel mutation within the mitochondrial SOD2 enzyme in Drosophila that results in adults with an extremely shortened life span, sensitivity to hyperoxia, and neuropathology. Additional studies demonstrate that this novel mutant, SOD2(bewildered), exhibits abnormal brain morphology, suggesting a critical role for this protein in neurodevelopment. We investigated the basis of this neurodevelopmental defect and discovered an increase in aberrant axonal that could underlie the aberrant neurodevelopment and brain morphology defects. This novel allele, SOD2(bewildered), provides a unique opportunity to study the effects of increased mitochondrial ROS on neural development, axonal targeting, and neural cell degeneration in vivo.

Increased persistent Na+ current contributes to seizure in the slamdance bang-sensitive Drosophila mutant

There is clinical need to extend the understanding of epilepsy and to find novel approaches to treat this condition. Bang-sensitive (bs) Drosophila mutants, which exhibit reduced thresholds for seizure, offer an attractive possibility to combine tractable genetics, electrophysiology, and high-throughput screening. However, despite these advantages, the precise electrophysiological aberrations that contribute to seizure have not been identified in any bs mutant. Because of this, the applicability of Drosophila as a preclinical model has not yet been established. In this study, we show that electroshock of bs slamdance (sda) larvae was sufficient to induce extended seizure-like episodes. Whole cell voltage-clamp recordings from identified motoneurons (termed aCC and RP2) showed synaptic currents that were greatly increased in both amplitude and duration. Current-clamp recordings indicated that these inputs produced longer-lived plateau depolarizations and increased action potential firing in these cells. An analysis of voltage-gated currents in these motoneurons, in both first and third instar larvae, revealed a consistently increased persistent Na(+) current (I(Nap)) and a reduced Ca(2+) current in first instar larvae, which appeared normal in older third instar larvae. That increased I(Nap) may contribute to seizure-like activity is indicated by the observation that feeding sda larvae the antiepileptic drug phenytoin, which was sufficient to reduce I(Nap), rescued both seizure-like episode duration and synaptic excitation of motoneurons. In contrast, feeding of either anemone toxin, a drug that preferentially increases I(Nap), or phenytoin to wild-type larvae was sufficient to induce a bs behavioral phenotype. Finally, we show that feeding of phenytoin to gravid sda females was sufficient to both reduce I(Nap) and synaptic currents and rescue the bs phenotype in their larval progeny, indicating that a heightened predisposition to seizure may arise as a consequence of abnormal embryonic neural development.

Mutations in prickle orthologs cause seizures in flies, mice, and humans

Epilepsy is heritable, yet few causative gene mutations have been identified, and thus far no human epilepsy gene mutations have been found to produce seizures in invertebrates. Here we show that mutations in prickle genes are associated with seizures in humans, mice, and flies. We identified human epilepsy patients with heterozygous mutations in either PRICKLE1 or PRICKLE2. In overexpression assays in zebrafish, prickle mutations resulted in aberrant prickle function. A seizure phenotype was present in the Prickle1-null mutant mouse, two Prickle1 point mutant (missense and nonsense) mice, and a Prickle2-null mutant mouse. Drosophila with prickle mutations displayed seizures that were responsive to anti-epileptic medication, and homozygous mutant embryos showed neuronal defects. These results suggest that prickle mutations have caused seizures throughout evolution.

Seizure and epilepsy: studies of seizure disorders in Drosophila

Despite the frequency of seizure disorders in the human population, the genetic and physiological basis for these defects has been difficult to resolve. Although many genetic contributions to seizure susceptibility have been identified, these involve disparate biological processes, many of which are not neural specific. The large number and heterogeneous nature of the genes involved makes it difficult to understand the complex factors underlying the etiology of seizure disorders. Examining the effect known genetic mutations have on seizure susceptibility is one approach that may prove fruitful. This approach may be helpful in both understanding how different physiological processes affect seizure susceptibility and identifying novel therapeutic treatments. We review here factors contributing to seizure susceptibility in Drosophila, a genetically tractable system that provides a model for human seizure disorders. Seizure-like neuronal activities and behaviors in the fruit fly are described, as well as a set of mutations that exhibit features resembling some human epilepsies and render the fly sensitive to seizures. Especially interesting are descriptions of a novel class of mutations that are second-site mutations that act as seizure suppressors. These mutations revert epilepsy phenotypes back to the wild-type range of seizure susceptibility. The genes responsible for seizure suppression are cloned with the goal of identifying targets for lead compounds that may be developed into new antiepileptic drugs.

A Drosophila systems model of pentylenetetrazole induced locomotor plasticity responsive to antiepileptic drugs

Background

Rodent kindling induced by PTZ is a widely used model of epileptogenesis and AED testing. Overlapping pathophysiological mechanisms may underlie epileptogenesis and other neuropsychiatric conditions. Besides epilepsy, AEDs are widely used in treating various neuropsychiatric disorders. Mechanisms of AEDs' long term action in these disorders are poorly understood. We describe here a Drosophila systems model of PTZ induced locomotor plasticity that is responsive to AEDs.

Results

We empirically determined a regime in which seven days of PTZ treatment and seven days of subsequent PTZ discontinuation respectively cause a decrease and an increase in climbing speed of Drosophila adults. Concomitant treatment with NaVP and LEV, not ETH, GBP and VGB, suppressed the development of locomotor deficit at the end of chronic PTZ phase. Concomitant LEV also ameliorated locomotor alteration that develops after PTZ withdrawal. Time series of microarray expression profiles of heads of flies treated with PTZ for 12 hrs (beginning phase), two days (latent phase) and seven days (behaviorally expressive phase) showed only down-, not up-, regulation of genes; expression of 23, 2439 and 265 genes were downregulated, in that order. GO biological process enrichment analysis showed downregulation of transcription, neuron morphogenesis during differentiation, synaptic transmission, regulation of neurotransmitter levels, neurogenesis, axonogenesis, protein modification, axon guidance, actin filament organization etc. in the latent phase and of glutamate metabolism, cell communication etc. in the expressive phase. Proteomic interactome based analysis provided further directionality to these events. Pathway overrepresentation analysis showed enrichment of Wnt signaling and other associated pathways in genes downregulated by PTZ. Mining of available transcriptomic and proteomic data pertaining to established rodent models of epilepsy and human epileptic patients showed overrepresentation of epilepsy associated genes in our PTZ regulated set.

Conclusion

Systems biology ultimately aims at delineating and comprehending the functioning of complex biological systems in such details that predictive models of human diseases could be developed. Due to immense complexity of higher organisms, systems biology approaches are however currently focused on simpler organisms. Amenable to modeling, our model offers a unique opportunity to further dissect epileptogenesis-like plasticity and to unravel mechanisms of long-term action of AEDs relevant in neuropsychiatric disorders.

Invertebrate and fungal model organisms: emerging platforms for drug discovery

Early-stage translational research programs have increasingly exploited yeast, worms and flies to model human disease. These genetically tractable organisms represent flexible platforms for small molecule and drug target discovery. This review highlights recent examples of how model organisms are integrated into chemical genomic approaches to drug discovery with an emphasis on fungal yeast, nematode Caenorhabditis elegans and fruit fly Drosophila melanogaster. The roles of these organisms are expanding as novel models of human disease are developed and novel high-throughput screening technologies are created and adapted for drug discovery.

DNA topoisomerase I inhibitors ameliorate seizure-like behaviors and paralysis in a Drosophila model of epilepsy

The Drosophila DNA topoisomerase type I mutant allele, top1JS is an effective general seizure-suppressor mutation, reverting seizure-sensitive phenotypes of several mutant strains in a genetic model of epilepsy. Seizure-suppression is caused by reduced transcription of the top1 (topoisomerase I gene) gene [Song J, Hu J, Tanouye MA. (2007) Seizure suppression by top1 mutations in Drosophila. J Neurosci 27(11):2927-2937]. Here, we examine the possibility that pharmaceutical inhibition of Top1 (topoisomerase I protein) enzymatic activity may also be effective at reducing seizure phenotypes. We investigate the effect of vertebrate Top1 inhibitor camptothecin (CPT) along with two related compounds, apigenin and kaempferol, when fed to seizure-sensitive mutant Drosophila. All three Top1 inhibitors were found to suppress phenotypes in these mutants. In particular, for drug treatments, the recovery time from seizure and paralysis is greatly reduced compared with untreated animals. Intriguingly we find that chronic drug treatments result in a small reduction in seizure sensitivity. Taken together, the results suggest that Top1 inhibitors may have the potential to be developed into effective anti-epileptic drugs, especially for brain tumor patients presenting with epilepsy.

Neuropathology in Drosophila mutants with increased seizure susceptibility

Genetic factors are known to contribute to seizure susceptibility, although the long-term effects of these predisposing factors on neuronal viability remain unclear. To examine the consequences of genetic factors conferring increased seizure susceptibility, we surveyed a class of Drosophila mutants that exhibit seizures and paralysis following mechanical stimulation. These bang-sensitive seizure mutants exhibit shortened life spans and age-dependent neurodegeneration. Because the increased seizure susceptibility in these mutants likely results from altered metabolism and since the Na(+)/K(+) ATPase consumes the majority of ATP in neurons, we examined the effect of ATPalpha mutations in combination with bang-sensitive mutations. We found that double mutants exhibit strikingly reduced life spans and age-dependent uncoordination and inactivity. These results emphasize the importance of proper cellular metabolism in maintaining both the activity and viability of neurons.

From bench to drug: human seizure modeling using Drosophila

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.

Seizure suppression by top1 mutations in Drosophila

DNA topoisomerase I is an essential nuclear enzyme involved in resolving the torsional stress associated with DNA replication, transcription, and chromatin condensation. Here we report the discovery of a seizure-suppressor mutant, top1(JS), which suppresses seizures in a Drosophila model of human epilepsy. A P-element mutagenesis screen using easily shocked seizure-sensitive mutant as a genetic background identified top1(JS), which plays a novel role in regulating nervous system excitability. Plasmid rescue, excision, complementation, and sequencing analyses verified that top1(JS) results from a P-element insertion in the 5' untranslated region. Quantitative reverse transcription analysis on wild-type and mutant fly heads showed that the top1(JS) mutation causes reduced transcription level in the CNS, suggesting a partial loss-of-function mutation. Electrophysiological experiments revealed normal seizure thresholds in top1(JS) mutants, which are different from other seizure suppressors identified previously, suggesting a novel mechanism underlying seizure suppression by top1(JS). The pharmacological camptothecin feeding experiment and cell death analysis suggested that the seizure suppression by top1(JS) may occur via increased neuronal apoptosis. Furthermore, overexpression of the DIAP1 (Drosophila inhibitor of apoptosis 1) gene rescues top1(JS) suppression, providing additional support for a neural apoptosis suppression mechanism. The top1(JS) mutation is the first viable partial loss-of-function mutation identified in higher eukaryotes, and the results presented here point to a novel function for topo I in construction and/or maintenance of circuits required for seizure propagation in vivo.

Development of aDrosophilaseizure model forin vivohigh-throughput drug screening

An important application of model organisms in neurological research has been to identify and characterise therapeutic approaches for epilepsy, a recurrent seizure disorder that affects > 1% of the human population. Proconvulsant-treated rodent models have been widely used for antiepileptic drug discovery and development, but are not suitable for high-throughput screening. To generate a genetically tractable model that would be suitable for large-scale, high-throughput screening for antiepileptic drug candidates, we characterized a Drosophila chemical treatment model using the GABA(A) receptor antagonist picrotoxin. This proconvulsant, delivered to Drosophila larvae via simple feeding methods suitable for automated screening, generated robust generalised seizures with lethality occurring at doses between 0.3 and 0.5 mg/mL. Electrophysiological analysis of CNS motor neuron output in picrotoxin-treated larvae revealed generalised seizures within minutes of drug exposure. At subthreshold doses for seizure induction, picrotoxin produced an increased frequency of motor neuron action potential bursting, indicating that CNS GABAergic transmission regulates patterned activity. Mutants in the Drosophila Rdl GABA(A) receptor are resistant to picrotoxin, confirming that seizure induction occurs via a conserved GABA(A) receptor pathway. To validate the usefulness of this model for in vivo drug screening, we identified several classes of neuroactive antiepileptic compounds in a pilot screen, including phenytoin and nifedipine, which can rescue the seizures and lethal neurotoxicity induced by picrotoxin. The well-defined actions of picrotoxin in Drosophila and the ease with which compounds can be assayed for antiseizure activity makes this genetically tractable model attractive for high-throughput in vivo screens to identify novel anticonvulsants and seizure susceptibility loci.

Glutamatergic synapses of Drosophila neuromuscular junctions: a high-resolution model for the analysis of experience-dependent potentiation

The glutamatergic synapses of developing neuromuscular junctions (NMJ) of Drosophila larvae are readily accessible, morphologically simple, and physiologically well-characterized. They therefore have a long and highly successful tradition as a model system for the discovery of genetic and molecular mechanisms of target recognition, synaptogenesis, NMJ development, and synaptic plasticity. However, since the development and the activity-dependent refinement of NMJs are concurrent processes, they cannot easily be separated by the widely applied genetic manipulations that mostly have chronic effects. Recent studies have therefore begun systematically to incorporate larval foraging behavior into the physiological and genetic analysis of NMJ function in order to analyze potential experience-dependent changes of glutamatergic transmission. These studies have revealed that recent crawling experience is a potent modulator of glutamatergic transmission at NMJs, because high crawling activities result after an initial lag-phase in several subsequent phases of experience-dependent synaptic potentiation. Depending on the time window of occurrence, four distinct phases of experience-dependent potentiation have been defined. These phases of potentiation can be followed from their initial induction (phase-I) up to the morphological consolidation (phase-III/IV) of previously established functional changes (phase-II). This therefore establishes, for the first time, a temporal hierarchy of mechanisms involved in the use-dependent modification of glutamatergic synapses.

Metabolic disruption in Drosophila bang-sensitive seizure mutants

We examined a number of Drosophila mutants with increased susceptibility to seizures following mechanical or electrical stimulation to better understand the underlying factors that predispose neurons to aberrant activity. Several mutations in this class have been molecularly identified and suggest metabolic disruption as a possible source for increased seizure susceptibility. We mapped the bang-sensitive seizure mutation knockdown (kdn) to cytological position 5F3 and identified citrate synthase as the affected gene. These results further support a role for mitochondrial metabolism in controlling neuronal activity and seizure susceptibility. Biochemical analysis in bang-sensitive mutants revealed reductions in ATP levels consistent with disruption of mitochondrial energy production in these mutants. Electrophysiological analysis of mutants affecting mitochondrial proteins revealed an increased likelihood for a specific pattern of seizure activity. Our data implicate cellular metabolism in regulating seizure susceptibility and suggest that differential sensitivity of neuronal subtypes to metabolic changes underlies distinct types of seizure activity.

Seizure suppression by gain-of-function escargot mutations

Suppressor mutations provide potentially powerful tools for examining mechanisms underlying neurological disorders and identifying novel targets for pharmacological intervention. Here we describe mutations that suppress seizures in a Drosophila model of human epilepsy. A screen utilizing the Drosophila easily shocked (eas) "epilepsy" mutant identified dominant suppressors of seizure sensitivity. Among several mutations identified, neuronal escargot (esg) reduced eas seizures almost 90%. The esg gene encodes a member of the snail family of transcription factors. Whereas esg is normally expressed in a limited number of neurons during a defined period of nervous system development, here normal esg was expressed in all neurons and throughout development. This greatly ameliorated both the electrophysiological and the behavioral epilepsy phenotypes of eas. Neuronal esg appears to act as a general seizure suppressor in the Drosophila epilepsy model as it reduces the susceptibility of several seizure-prone mutants. We observed that esg must be ectopically expressed during nervous system development to reduce seizure susceptibility in adults. Furthermore, induction of esg in a small subset of neurons (interneurons) will reduce seizure susceptibility. A combination of microarray and computational analyses revealed 100 genes that represent possible targets of neuronal esg. We anticipate that some of these genes may ultimately serve as targets for novel antiepileptic drugs.

Modeling age-related diseases in Drosophila: can this fly?

Human neurodegenerative diseases are characterized by progressive neuronal cell loss often resulting in memory and cognitive decline, motor dysfunction, and ultimately premature death. Despite the prevalence of these diseases, there are no effective cures. Insight into many of these syndromes has come from the identification of single gene mutations that are associated with inherited forms of the disease. This has led to the development of animal models in which the pathogenesis caused by these genes can be rigorously examined. Due to their short life span and powerful genetic potential, several attempts have been made to model neurodegenerative diseases in the fruit fly Drosophila melanogaster. This review will describe how these models were generated and how faithfully they recapitulate human disease. In addition, how fly models can be used to identify genetic modifiers of known disease genes and what these have revealed about the biochemical pathways underlying disease pathogenesis is discussed. Finally, the review will describe how fly models can be used to identify new therapeutic targets and test the effectiveness of new drugs.

Potassium bromide, an anticonvulsant, is effective at alleviating seizures in the Drosophila bang-sensitive mutant bang senseless

Human seizure disorders are a major health concern due to the large number of affected individuals, the potentially devastating consequences of untreated seizure occurrences, and the lack of an effective treatment for all patients. Although anticonvulsants have proven very helpful in treating seizures and remain the best option available for treatment, not all afflicted individuals respond to medication and many only do so in unique drug combinations or at the cost of adverse side-effects. Therefore, new and more effective anticonvulsants are continually sought after to combat this illness. In this study, we present results which offer the possibility of using Drosophila bang-sensitive (BS) mutants as a tool to screen anticonvulsants. By feeding the BS mutants a known anticonvulsant, potassium bromide, we have demonstrated that the drug dramatically reduces the seizures of bang senseless, the most severe of the BS mutants. This methodology suggests that the Drosophila system can potentially be a powerful instrument for assaying and testing new compounds with anticonvulsant properties.