The dominant cold-sensitive Out-cold mutants of Drosophila melanogaster have novel missense mutations in the voltage-gated sodium channel gene paralytic

Investigating Developmental and Epileptic Encephalopathy Using Drosophila melanogaster

Developmental and epileptic encephalopathies (DEEs) are the spectrum of severe epilepsies characterized by early-onset, refractory seizures occurring in the context of developmental regression or plateauing. Early infantile epileptic encephalopathy (EIEE) is one of the earliest forms of DEE, manifesting as frequent epileptic spasms and characteristic electroencephalogram findings in early infancy. In recent years, next-generation sequencing approaches have identified a number of monogenic determinants underlying DEE. In the case of EIEE, 85 genes have been registered in Online Mendelian Inheritance in Man as causative genes. Model organisms are indispensable tools for understanding the in vivo roles of the newly identified causative genes. In this review, we first present an overview of epilepsy and its genetic etiology, especially focusing on EIEE and then briefly summarize epilepsy research using animal and patient-derived induced pluripotent stem cell (iPSC) models. The Drosophila model, which is characterized by easy gene manipulation, a short generation time, low cost and fewer ethical restrictions when designing experiments, is optimal for understanding the genetics of DEE. We therefore highlight studies with Drosophila models for EIEE and discuss the future development of their practical use.

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

Voltage-gated sodium (Na_v) channels play a central role in the generation and propagation of action potentials in excitable cells such as neurons and muscles. To determine how the phenotypes of Na_v-channel mutants are affected by other genes, we performed a forward genetic screen for dominant modifiers of the seizure-prone, gain-of-function Drosophila melanogaster Na_v-channel mutant, para^Shu. Our analyses using chromosome deficiencies, gene-specific RNA interference, and single-gene mutants revealed that a null allele of glutathione S-transferase S1 (GstS1) dominantly suppresses para^Shu phenotypes. Reduced GstS1 function also suppressed phenotypes of other seizure-prone Na_v-channel mutants, para^GEFS+ and para^bss. Notably, para^Shu mutants expressed 50% less GstS1 than wild-type flies, further supporting the notion that para^Shu and GstS1 interact functionally. Introduction of a loss-of-function GstS1 mutation into a para^Shu background led to up- and down-regulation of various genes, with those encoding cytochrome P450 (CYP) enzymes most significantly over-represented in this group. Because GstS1 is a fly ortholog of mammalian hematopoietic prostaglandin D synthase, and in mammals CYPs are involved in the oxygenation of polyunsaturated fatty acids including prostaglandins, our results raise the intriguing possibility that bioactive lipids play a role in GstS1-mediated suppression of para^Shu phenotypes.

Milk-whey diet substantially suppresses seizure-like phenotypes of paraShu, a Drosophila voltage-gated sodium channel mutant

The Drosophila mutant para^Shu harbors a dominant, gain-of-function allele of the voltage-gated sodium channel gene, paralytic (para). The mutant flies display severe seizure-like phenotypes, including neuronal hyperexcitability, spontaneous spasms, ether-induced leg shaking, and heat-induced convulsions. We unexpectedly found that two distinct food recipes used routinely in the Drosophila research community result in a striking difference in severity of the para^Shu phenotypes. Namely, when para^Shu mutants were raised on the diet originally formulated by Edward Lewis in 1960, they showed severe neurological defects as previously reported. In contrast, when they were raised on the diet developed by Frankel and Brousseau in 1968, these phenotypes were substantially suppressed. Comparison of the effects of these two well-established food recipes revealed that the diet-dependent phenotypic suppression is accounted for by milk whey, which is present only in the latter. Inclusion of milk whey in the diet during larval stages was critical for suppression of the adult para^Shu phenotypes, suggesting that this dietary modification affects development of the nervous system. We also found that milk whey has selective effects on other neurological mutants. Among the behavioral phenotypes of different para mutant alleles, those of para^GEFS+ and para^bss were suppressed by milk whey, while those of para^DS and para^ts1 were not significantly affected. Overall, our study demonstrates that different diets routinely used in Drosophila labs could have considerably different effects on neurological phenotypes of Drosophila mutants. This finding provides a solid foundation for further investigation into how dietary modifications affect development and function of the nervous system and, ultimately, how they influence behavior.

Preclinical Animal Models for Dravet Syndrome: Seizure Phenotypes, Comorbidities and Drug Screening

Epilepsy is a common chronic neurological disease affecting almost 3 million people in the United States and 50 million people worldwide. Despite availability of more than two dozen FDA-approved anti-epileptic drugs (AEDs), one-third of patients fail to receive adequate seizure control. Specifically, pediatric genetic epilepsies are often the most severe, debilitating and pharmaco-resistant forms of epilepsy. Epileptic syndromes share a common symptom of unprovoked seizures. While some epilepsies/forms of epilepsy are the result of acquired insults such as head trauma, febrile seizure, or viral infection, others have a genetic basis. The discovery of epilepsy associated genes suggests varied underlying pathologies and opens the door for development of new "personalized" treatment options for each genetic epilepsy. Among these, Dravet syndrome (DS) has received substantial attention for both the pre-clinical and early clinical development of novel therapeutics. Despite these advances, there is no FDA-approved treatment for DS. Over 80% of patients diagnosed with DS carry a de novo mutation within the voltage-gated sodium channel gene SCN1A and these patients suffer with drug resistant and life-threatening seizures. Here we will review the preclinical animal models for DS featuring inactivation of SCN1A (including zebrafish and mice) with an emphasis on seizure phenotypes and behavioral comorbidities. Because many drugs fail somewhere between initial preclinical discovery and clinical trials, it is equally important that we understand how these models respond to known AEDs. As such, we will also review the available literature and recent drug screening efforts using these models with a focus on assay protocols and predictive pharmacological profiles. Validation of these preclinical models is a critical step in our efforts to efficiently discover new therapies for these patients. The behavioral and electrophysiological drug screening assays in zebrafish will be discussed in detail including specific examples from our laboratory using a zebrafish scn1 mutant and a summary of the nearly 3000 drugs screened to date. As the discovery and development phase rapidly moves from the lab-to-the-clinic for DS, it is hoped that this preclinical strategy offers a platform for how to approach any genetic epilepsy.

Insight into Notch Signaling Steps That Involve pecanex from Dominant-Modifier Screens in Drosophila

Notch signaling plays crucial roles in intercellular communications. In Drosophila, the pecanex (pcx) gene, which encodes an evolutionarily conserved multi-pass transmembrane protein, appears to be required to activate Notch signaling in some contexts, especially during neuroblast segregation in the neuroectoderm. Although Pcx has been suggested to contribute to endoplasmic reticulum homeostasis, its functions remain unknown. Here, to elucidate these roles, we performed genetic modifier screens of pcx We found that pcx heterozygotes lacking its maternal contribution exhibit cold-sensitive lethality, which is attributed to a reduction in Notch signaling at decreased temperatures. Using sets of deletions that uncover most of the second and third chromosomes, we identified four enhancers and two suppressors of the pcx cold-sensitive lethality. Among these, five genes encode known Notch-signaling components: big brain, Delta (Dl), neuralized (neur), Brother of Bearded A (BobA), a member of the Bearded (Brd) family, and N-ethylmaleimide-sensitive factor 2 (Nsf2). We showed that BobA suppresses Dl endocytosis during neuroblast segregation in the neuroectoderm, as Brd family genes reportedly do in the mesoderm for mesectoderm specification. Analyses of Nsf2, a key regulator of vesicular fusion, suggested a novel role in neuroblast segregation, which is distinct from Nsf2's previously reported role in imaginal tissues. Finally, jim lovell, which encodes a potential transcription factor, may play a role in Notch signaling during neuroblast segregation. These results reveal new research avenues for Pcx functions and Notch signaling.

Insights into DDT Resistance from the Drosophila melanogaster Genetic Reference Panel

Insecticide resistance is considered a classic model of microevolution, where a strong selective agent is applied to a large natural population, resulting in a change in frequency of alleles that confer resistance. While many insecticide resistance variants have been characterized at the gene level, they are typically single genes of large effect identified in highly resistant pest species. In contrast, multiple variants have been implicated in DDT resistance in Drosophila melanogaster; however, only the Cyp6g1 locus has previously been shown to be relevant to field populations. Here we use genome-wide association studies (GWAS) to identify DDT-associated polygenes and use selective sweep analyses to assess their adaptive significance. We identify and verify two candidate DDT resistance loci. A largely uncharacterized gene, CG10737, has a function in muscles that ameliorates the effects of DDT, while a putative detoxifying P450, Cyp6w1, shows compelling evidence of positive selection.

Preparing for Winter: The Transcriptomic Response Associated with Different Day Lengths in Drosophila montana

At northern latitudes, the most robust cue for assessing the onset of winter is the shortening of day lengths. Many species use day length as a cue to increase their cold tolerance and/or enter into diapause, but little is known about changes in gene expression that occur under different day lengths. We investigate the gene expression changes associated with differences in light/dark cycles in Drosophila montana, a northerly distributed species with a strong adult photoperiodic reproductive diapause. To examine gene expression changes induced by light both prior to and during diapause, we used both nondiapausing and diapausing flies. We found that the majority of genes that are differentially expressed between different day lengths in nondiapausing and diapausing flies differ. However, the biological processes involved were broadly similar. These included neuron development and metabolism, which are largely consistent with an increase in cold tolerance previously observed to occur in these flies. We also found that many genes associated with reproduction change in expression level between different day lengths, suggesting that D. montana use changes in day length to cue changes in reproduction both before and after entering into diapause. Finally, we also identified several interesting candidate genes for light-induced changes including Lsp2, para, and Ih.

Model systems for studying cellular mechanisms of SCN1A-related epilepsy

Mutations in SCN1A, the gene encoding voltage-gated sodium channel NaV1.1, cause a spectrum of epilepsy disorders that range from genetic epilepsy with febrile seizures plus to catastrophic disorders such as Dravet syndrome. To date, more than 1,250 mutations in SCN1A have been linked to epilepsy. Distinct effects of individual SCN1A mutations on neuronal function are likely to contribute to variation in disease severity and response to treatment in patients. Several model systems have been used to explore seizure genesis in SCN1A epilepsies. In this article we review what has been learned about cellular mechanisms and potential new therapies from these model systems, with a particular emphasis on the novel model system of knock in Drosophila and a look toward the future with expanded use of patient-specific induced pluripotent stem cell-derived neurons.

Disruption of Endocytosis with the Dynamin Mutant shibirets1 Suppresses Seizures in Drosophila

One challenge in modern medicine is to control epilepsies that do not respond to currently available medications. Since seizures consist of coordinated and high-frequency neural activity, our goal was to disrupt neurotransmission with a synaptic transmission mutant and evaluate its ability to suppress seizures. We found that the mutant shibire, encoding dynamin, suppresses seizure-like activity in multiple seizure-sensitive Drosophila genotypes, one of which resembles human intractable epilepsy in several aspects. Because of the requirement of dynamin in endocytosis, increased temperature in the shi(ts1) mutant causes impairment of synaptic vesicle recycling and is associated with suppression of the seizure-like activity. Additionally, we identified the giant fiber neuron as critical in the seizure circuit and sufficient to suppress seizures. Overall, our results implicate mutant dynamin as an effective seizure suppressor, suggesting that targeting or limiting the availability of synaptic vesicles could be an effective and general method of controlling epilepsy disorders.

Molecular biology of insect sodium channels and pyrethroid resistance

Voltage-gated sodium channels are essential for the initiation and propagation of the action potential in neurons and other excitable cells. Because of their critical roles in electrical signaling, sodium channels are targets of a variety of naturally occurring and synthetic neurotoxins, including several classes of insecticides. This review is intended to provide an update on the molecular biology of insect sodium channels and the molecular mechanism of pyrethroid resistance. Although mammalian and insect sodium channels share fundamental topological and functional properties, most insect species carry only one sodium channel gene, compared to multiple sodium channel genes found in each mammalian species. Recent studies showed that two posttranscriptional mechanisms, alternative splicing and RNA editing, are involved in generating functional diversity of sodium channels in insects. More than 50 sodium channel mutations have been identified to be responsible for or associated with knockdown resistance (kdr) to pyrethroids in various arthropod pests and disease vectors. Elucidation of molecular mechanism of kdr led to the identification of dual receptor sites of pyrethroids on insect sodium channels. Many of the kdr mutations appear to be located within or close to the two receptor sites. The accumulating knowledge of insect sodium channels and their interactions with insecticides provides a foundation for understanding the neurophysiology of sodium channels in vivo and the development of new and safer insecticides for effective control of arthropod pests and human disease vectors.

Mutation in the RmβAOR gene is associated with amitraz resistance in the cattle tick Rhipicephalus microplus

We aimed to describe the evolution of resistance to amitraz in Rhipicephalus microplus in the field and to test the association between amitraz resistance and the frequency of a mutation in the β-adrenergic octopamine receptor gene (RmβAOR). We established six populations of Rhipicephalus microplus ticks in similar paddocks by the admixture of ticks from strains known to be susceptible and resistant to amitraz and synthetic pyrethroids. Each population was managed using one of three acaricide treatment regimes: always amitraz, always spinosad, or rotation between amitraz and spinosad. We used microsatellites to elucidate population structure over time, an SNP in the para-sodium channel gene previously demonstrated to confer resistance to synthetic pyrethroids to quantify changes in resistance to synthetic pyrethroids over time, and a nonsynonymous SNP in the RmβAOR, a gene that we proposed to confer resistance to amitraz, to determine whether selection with amitraz increased the frequency of this mutation. The study showed panmixia of the two strains and that selection of ticks with amitraz increased the frequency of the RmβAOR mutation while increasing the prevalence of amitraz-resistance. We conclude that polymorphisms in the RmβAOR gene are likely to confer resistance to amitraz.

A knock-in model of human epilepsy in Drosophila reveals a novel cellular mechanism associated with heat-induced seizure

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.

Footprints of positive selection associated with a mutation (N1575Y) in the voltage-gated sodium channel of Anopheles gambiae

Insecticide resistance is an ideal model to study the emergence and spread of adaptative variants. In the African malaria mosquito, Anopheles gambiae, this is complemented by a strong public health rationale. In this insect, resistance to pyrethroid and DDT insecticides is strongly associated with the mutations L1014F and L1014S within the para voltage-gated sodium channel (VGSC). Across much of West Africa, 1014F frequency approaches fixation. Here, we document the emergence of a mutation, N1575Y, within the linker between domains III-IV of the VGSC. In data extending over 40 kbp of the VGSC 1575Y occurs on only a single long-range haplotype, also bearing 1014F. The 1014F-1575Y haplotype was found in both M and S molecular forms of An. gambiae in West/Central African sample sites separated by up to 2,000 km. In Burkina Faso M form, 1575Y allele frequency rose significantly from 0.053 to 0.172 between 2008 and 2010. Extended haplotype homozygosity analysis of the wild-type 1575N allele showed rapid decay of linkage disequilibrium (LD), in sharp contrast to the extended LD exhibited by 1575Y. A haplotype with long-range LD and high/increasing frequency is a classical sign of strong positive selection acting on a recent mutant. 1575Y occurs ubiquitously on a 1014F haplotypic background, suggesting that the N1575Y mutation compensates for deleterious fitness effects of 1014F and/or confers additional resistance to insecticides. Haplotypic tests of association suggest the latter: The 1014F-1575Y haplotype confers a significant additive benefit above 1014F-1575N for survival to DDT (M form P = 0.03) and permethrin (S form P = 0.003).

Effect of sodium channel abundance on Drosophila development, reproductive capacity and aging

The voltage-gated Na (+) channels (VGSC) are complex membrane proteins responsible for generation and propagation of the electrical signals through the brain, the skeletal muscle and the heart. The levels of sodium channels affect behavior and physical activity. This is illustrated by the maleless mutant allele (mle (napts)) in Drosophila, where the decreased levels of voltage-gated Na(+) channels cause temperature-sensitive paralysis. Here, we report that mle (napts) mutant flies exhibit developmental lethality, decreased fecundity and increased neurodegeneration. The negative effect of decreased levels of Na(+) channels on development and ts-paralysis was more pronounced at 18 and 29°C than at 25°C, suggesting particular sensitivity of the mle (napts) flies to temperatures above and below normal environmental conditions. Similarly, longevity of mle (napts) flies was unexpectedly short at 18 and 29°C compared with flies heterozygous for the mle (napts) mutation. Developmental lethality and neurodegeneration of mle (napts) flies was partially rescued by increasing the dosage of para, confirming a vital role of Na(+) channels in development, longevity and neurodegeneration of flies and their adaptation to temperatures.

Evolution of acaricide resistance: phenotypic and genotypic changes in field populations of Rhipicephalus (Boophilus) microplus in response to pyrethroid selection pressure

There have been few, if any, studies of arthropod field populations quantifying the kinetics of evolution of phenotypic and genotypic resistance to chemicals in response to the presence or absence of selection pressure. A prospective intervention study was undertaken over 2 years in Mexico to measure changes in resistance phenotype and genotype in the presence or absence of pyrethroid selection pressure on field populations of Rhipicephalus (Boophilus) microplus ticks on 11 farms with varying degrees of pyrethroid resistance. The resistance phenotype was evaluated by bioassay in a larval packet test expressed as the resistance factor (RF) derived from probit analysis of dose mortality regressions, and resistance genotype by an allele-specific PCR (AS-PCR) to determine the frequency of a sodium channel mutation (F1550I) associated with pyrethroid resistance. To validate the AS-PCR, a Pyrosequencing™ method was developed to detect the F1550I mutation. There was good concordance with the genotypes identified by both Pyrosequencing™ and AS-PCR (Kappa: 0.85). On five farms cypermethrin (CY) was exclusively used at intervals and on six farms amitraz was used. On two of the five CY-treated farms, the experiment was prematurely terminated due to unacceptably high levels of tick resistance. For all five farms, after 8-24 months of continued selection pressure with CY, the RF had increased 2-125-fold. The frequency of the resistance allele increased on all five farms from a starting range of 5-46% to a range of 66-95% after 8-24 months. On six farms treated with amitraz neither the RF nor the frequency of the resistance allele changed. A clear correlation between the phenotype and genotype was found in three of four treated farms confirming that the F1550I mutation is a major cause of synthetic pyrethroid resistance in Mexico. These results show that the pyrethroid resistance trait is stable (> 2 years) and that resistance is acquired much faster than it is lost. Hence, alternation of pyrethroid acaricide with other chemicals is likely to lead to the stepwise acquisition of synthetic pyrethroid resistance but not additional prolongation of its efficacious lifespan.

Mechanisms underlying insect chill-coma

At their critical thermal minimum (CT(min)) insects enter chill-coma, a reversible state where neuromuscular transmission and movement cease. The physiological mechanisms responsible for the insect CT(min) remain poorly understood despite the regular use of chill-coma onset and recovery as a means to assess evolved or acquired variation in low temperature tolerance. In this review, we summarize the use of chill-coma as a metric of thermal tolerance to date, and synthesise current knowledge on the nature and plasticity of lower thermal limits to present probable physiological mechanisms of cold-induced failure. Chill-coma is likely to be driven by an inability to maintain ionic homeostasis through the effects of temperature on ion-motive ATPases, ion channel gating mechanisms, and/or the lipid membrane, leading to a loss of nerve and muscle excitability.

Old gene duplication facilitates origin and diversification of an innovative communication system--twice

The genetic basis of parallel innovation remains poorly understood due to the rarity of independent origins of the same complex trait among model organisms. We focus on two groups of teleost fishes that independently gained myogenic electric organs underlying electrical communication. Earlier work suggested that a voltage-gated sodium channel gene (Scn4aa), which arose by whole-genome duplication, was neofunctionalized for expression in electric organ and subsequently experienced strong positive selection. However, it was not possible to determine if these changes were temporally linked to the independent origins of myogenic electric organs in both lineages. Here, we test predictions of such a relationship. We show that Scn4aa co-option and rapid sequence evolution were tightly coupled to the two origins of electric organ, providing strong evidence that Scn4aa contributed to parallel innovations underlying the evolutionary diversification of each electric fish group. Independent evolution of electric organs and Scn4aa co-option occurred more than 100 million years following the origin of Scn4aa by duplication. During subsequent diversification of the electrical communication channels, amino acid substitutions in both groups occurred in the same regions of the sodium channel that likely contribute to electric signal variation. Thus, the phenotypic similarities between independent electric fish groups are also associated with striking parallelism at genetic and molecular levels. Our results show that gene duplication can contribute to remarkably similar innovations in repeatable ways even after long waiting periods between gene duplication and the origins of novelty.