InteractiveFly: GeneBrief

seizure: Biological Overview | References


Gene name - seizure

Synonyms - erg

Cytological map position - 60B5-60B6

Function - transmembrane protein

Keywords - voltage-gated potassium channel, essential for protecting flies from acute heat-induced seizures, acts in excitatory neurons, the octopaminergic system and neuropile ensheathing and perineurial glia, mutations in sei cause extensive structural remodeling of the myofibrillar organization, expressed widely in the CNS - neuromuscular junction, heart

Symbol - sei

FlyBase ID: FBgn0003353

Genetic map position - chr2R:24,047,411-24,053,035

NCBI classification - Ion transport protein: This family is 6 transmembrane helices in which the last two helices flank a loop which determines ion selectivity

Cellular location - surface transmembrane



NCBI links: EntrezGene, Nucleotide, Protein
BIOLOGICAL OVERVIEW

Neuronal physiology is particularly sensitive to acute stressors that affect excitability, many of which can trigger seizures and epilepsies. Although intrinsic neuronal homeostasis plays an important role in maintaining overall nervous system robustness and its resistance to stressors, the specific genetic and molecular mechanisms that underlie these processes are not well understood. This study used a reverse genetic approach in Drosophila to test the hypothesis that specific voltage-gated ion channels contribute to neuronal homeostasis, robustness, and stress resistance. The activity of the voltage-gated potassium channel encoded by seizure (sei), an ortholog of the mammalian EAG-related gene (ERG) channel family, is essential for protecting flies from acute heat-induced seizures. Although sei is broadly expressed in the nervous system, the data indicate that its impact on the organismal robustness to acute environmental stress is primarily mediated via its action in excitatory neurons, the octopaminergic system, as well as neuropile ensheathing and perineurial glia. Furthermore, these studies suggest that human mutations in the human ERG channel (hERG), which have been primarily implicated in the cardiac Long QT Syndrome (LQTS), may also contribute to the high incidence of seizures in LQTS patients via a cardiovascular-independent neurogenic pathway (Hill, 2019).

Neuronal homeostatic responses to acute and long-term environmental stressors are essential for maintaining robust behavioral outputs and overall organismal fitness. Many environmental stressors, such as changes in temperature or oxygen availability, impact various aspects of neuronal system function. Nervous systems must therefore compensate, in a homeostatic manner, in order to continue functioning in the presence of these stressors. At the neuronal level, the homeostatic response to stress depends on both synaptic and cell-intrinsic physiological processes that enable neurons to stably maintain optimal activity patterns. The synaptic processes include both presynaptic mechanisms related to neurotransmitter release and postsynaptic mechanisms controlling neurotransmitter receptor localization, turnover, and control of downstream signaling pathways. Previous theoretical and empirical studies in both invertebrate and mammalian species have suggested that neuronal intrinsic robustness depends on the expression and activity of specific combinations of ion channels and transporters, which can vary across neuronal cell types and individuals. While some of the transcriptional and physiological processes that enable neurons to adjust their intrinsic activity levels in response to long-term stressors have been identified, primarily via the altered conductance of voltage-gated ion channels, most of the genetic and molecular mechanisms that mediate susceptibility to acute, environmentally-induced seizures, such as fever-induced febrile seizures, remain unknown (Hill, 2019).

In humans, seizures result from a diverse set of mechanisms that lead to an abnormal increase in electrical activity of the nervous system. A wide range of stressors have been associated with triggering seizures, including fevers, flickering lights, sleep deprivation, and emotional stress. A handful of genetic mutations have been linked to febrile and photosensitive seizures, yet these only account for a small percentage of individuals experiencing seizures in response to these and other stressors (Hill, 2019).

Because of its small size, large surface-to-volume ratio, and its inability to internally regulate body temperature, the fruit fly Drosophila melanogaster, represents an excellent model for studying mechanisms underlying the neuronal response to acute heat stress. To date, forward genetic screens in Drosophila have identified several mutations that lead to heat-induced seizures and paralysis. These mutations seem to primarily affect the function of genes that encode voltage-gated sodium and potassium channels, and proteins associated with their neuronal function. This study tested the hypothesis that the knockdown of genes that are specifically important for the intrinsic neurophysiological homeostatic response to acute heat stress, would have little impact on fly behavior at permissive temperatures, but would lead to rapid paralysis under acute heat-stress conditions (Hill, 2019).

To test this hypothesis, a reverse genetic approach was applied to identify candidate genes specifically involved in the neuronal homeostatic response to acute heat stress. By using a tissue-specific RNAi knockdown screen of voltage-gated potassium channels, seizure (sei), the fly ortholog of the mammalian hERG channel (KCNH2) [Jackson, 1985; Jackson, 1984; Titus, 1997; Wang, 1997; Zheng, 2014], was identified as an essential element in the neuronal homeostatic response to acute heat stress. The sei gene was originally identified in a Drosophila forward mutagenesis screen for temperature-sensitive (ts) structural alleles of essential genes associated with neuronal excitability. Although the original screen was designed to identify point-mutations that would lead to proteins that are functional at permissive temperature but would inactivate at non-permissive high temperature due to misfolding, it was recently shown that null alleles of sei and neuronal RNAi knockdown lead to high temperature-induced seizures, as in the original 'ts' alleles (Zheng, 2014). These data indicate that the original sei alleles isolated were not true ts alleles. Instead, these data suggest that sei is not required for baseline neural excitability, but does play a role in the ability of neurons to maintain adaptive firing rates when exposed to acute heat stress. Furthermore, previous work has shown that temporal downregulation of sei expression specifically in neurons of the adult fly, or pharmacologically blocking SEI channel activity only in the adult stage, is sufficient to increase susceptibility to acute heat-induced seizures. These data suggest that the impact of sei mutations on stress-induced seizures is primarily a consequence of physiological rather than developmental processes (Hill, 2019).

Previous studies have indicated that individuals who carry some of the dominant hERG mutations that cause the cardiac Long QT Syndrome (LQTS), often also suffer from high prevalence of generalized seizures. Yet, it is currently assumed that seizures in these patients represent a derived secondary outcome of the primary LQTS cardiac pathology. However, the data presented in this study, as well as previous studies that showed that ERG channels are expressed in mammalian neuronal tissues, and contribute to intrinsic spike frequency adaptation in cultured mouse neuroblastoma cells and cerebellar Purkinje neurons, suggest that ERG channels also have a specific function within the nervous system (Hill, 2019).

By utilizing existing and novel genetic tools, this study shows that the ERG channel sei is indeed essential for maintaining neuronal robustness under acute heat stress conditions in Drosophila. Specifically, an intersectional approach, combining the UAS-GAL4 and LexAOp-LexA binary transgene expression systems, was used to show that sei is broadly expressed in the nervous system, in both neurons and glia. Yet, using RNAi to downregulate sei expression in specific cell types, this study demonstrates that the contribution of sei to organismal behavioral resistance to acute heat stress is primarily mediated via its specific action in excitatory cholinergic and glutamatergic neurons, the octopaminergic system, as well as non-neuronal glia. Furthermore, by generating a CRISPR/cas9-derived GFP-tagged allele of the native sei locus, it was also shown that at the subcellular level, sei exerts its action primarily in axons and associated glia. Together, these studies indicate that mutations in hERG-like potassium channels may contribute directly to the etiology of stress-induced seizures in susceptible individuals by limiting the intrinsic neuronal homeostatic response to acute environmental stressors, possibly via homeostatic axonal spike frequency adaptation (Hill, 2019).

Previous theoretical and empirical studies of neural circuit adaptability, and by extension, the ability of animals to maintain robust and adaptive behavioral outputs in unstable environments, depends on both the intrinsic homeostatic capacity of neurons to maintain an optimal activity pattern, and the ability of neural circuits to maintain stable outputs via the homeostatic regulation of neuronal connectivity and synaptic activity. Yet, despite its high incidence, the majority of genetic and molecular factors that regulate neuronal homeostasis, and increase susceptibility to seizures, remain mostly unknown. This study shows that the Drosophila voltage-gated potassium channels sei, Shab, and Shal impact the neuronal homeostatic response to acute heat stress. Furthermore, this study shows that the organismal capacity to buffer the effects of acute heat stress depends on the independent activity of sei in both neurons and glia. Although sei is broadly expressed in the nervous system, its contribution to the overall organismal resistance to acute heat stress seems to be specifically driven by its action in cholinergic and glutamatergic excitatory neurons, and neuropile-ensheathing glia, as well as to a lesser extent in the modulatory octopaminergic system and perineurial glia. In addition, this study developed genetic tools to show that SEI is expressed in the axons of neurons and in glia. Together, these data highlight the important role of sei in the organismal homeostatic response to acute environmental stress, by providing robustness to both the intrinsic activity of specific neuronal populations, and the neural circuits that harbor them. It is expected that future work on other voltage gated potassium channel genes, and well as other ion channels and transporters, in both neurons and glia, will continue to shed light on the genetic programs that control robust and homeostatic processes within the nervous system (Hill, 2019).

How ERG-type potassium channels might contribute to neuronal intrinsic homeostasis during bouts of acute stress is not well understood. Nonetheless, in vivo and in vitro studies in Drosophila and mammalian models have suggested that ERG channels have little effect on baseline neuronal firing rate, but can prevent rapid firing in response to environmental or electrophysiological stimuli that induce hyperexcitability [Zheng, 2014; Chiesa, 1997; Sacco, 2003]. Previous work has shown that in Drosophila motor neurons, basal neuronal firing patterns are unaffected by the sei mutation at optimal 25°C, but become hyperexcitable in response to a rapid temperature increase (Zheng, 2014). Similarly, in electrophysiological studies of mammalian brain slices, in vitro cultured neurons, and heterologously-expressed mammalian ERG channels, pharmacological blockers of hERG channels have little effect on firing rates in response to small current injections, but greatly diminish spike frequency adaptation in response to large current injections, resulting in rapid firing rates. The presence of SEI channels specifically in axons suggests that they do not affect the propagation of dendritic potentials, but rather limit the rate of action potential generation and propagation, which is sufficient to prevent rapid firing rates. Together, these data suggest a model whereby ERG-like potassium channels play a crucial role in mediating the neuronal homeostatic response to acute stress by protecting neurons from rapid increase in firing rates, and therefore, support neuronal robustness when exposed to extreme environmental fluctuations (Hill, 2019).

At the neuronal network level, seizures are thought to result from an imbalance between excitatory and inhibitory neural signaling pathways. This study found that knocking down sei specifically in all cholinergic neurons, the primary excitatory pathway in the fly central nervous system, is sufficient to phenocopy the effects of sei null mutations on the organismal resistance to heat-induced seizures. These results are similar to previous studies, which showed that increasing activity of the cholinergic system in flies, via genetic manipulations of voltage gated sodium channels and optogenetic neural activation, is sufficient to increase seizure-related and paralytic behavior. The simplest interpretation of these data together is that the lack of sei in cholinergic excitatory neurons makes them hypersensitive to heat-induced hyperexcitability, which subsequently surpasses the buffering capacity of the inhibitory neurotransmission pathways, and therefore leads to the rapid development of generalized seizures and paralysis (Hill, 2019).

Additionally, a large increase was observed in seizure susceptibility when sei is knocked down in glutamatergic neurons. Although it has been previously demonstrated that the activity of motor neurons, which in insects are primarily glutamatergic, is increased in seizure-susceptible mutant flies, this finding suggests that the decreased intrinsic ability of motor neurons to resist acute stress is sufficient for inducing organismal seizure-like phenotype. Nevertheless, it cannot be excluded that the observed effect of knocking down sei expression in glutamatergic neurons on organismal sensitivity to heat stress is mediated via the action of a small number of modulatory glutamatergic neurons within the central nervous system (Hill, 2019).

An impairment was observed in the organismal homeostatic response to acute heat stress when sei is specifically knocked-down in the modulatory octopaminergic system. Previous studies of the octopaminergic system in Drosophila and other insects have indicated that octopamine and related biogenic amines have broad impact on diverse neuronal processes at the developmental and physiological timescales. Because sei mutant flies seem to have normal behaviors when housed under constant optimal conditions, it is likely that the effects of knocking down sei in octopaminergic neurons on heat-induced seizures are physiological, not developmental. Although it is not currently known which specific elements of the octopaminergic system play a role in the organismal response to acute heat stress, previous work has shown that exogenous application of octopamine in Drosophila increases contraction force of muscles and their response to synaptically driven contractions. Therefore, one possible mechanism by which sei knockdown in octopaminergic neurons might affect observed heat-induced seizures is via the direct modulation of the neuromuscular junction. Octopamine has also been shown to play important roles in the central nervous system, including modulation of behaviors related to motivation, sleep, aggression, social behaviors and learning and memory. Therefore, sei knockdown in octopaminergic neurons may result in a broader shift in synaptic processes associated with the homeostatic maintenance of the balance between excitatory and inhibitory pathways under acute heat stress conditions (Hill, 2019).

The important role of sei activity in regulating the capacity of the nervous system to buffer acute environmental stress is further supported by the discovery that its knockdown in glia also increased susceptibility to acute heat-induced seizures. These data are in agreement with previously published studies, which demonstrated that the knockdown of genes associated with ionic homeostasis in glia can increase seizure susceptibility in Drosophila. Previous studies have suggested that some glia are important for maintaining synaptic activity and homeostasis, and that disrupting glia functions could contribute to the etiology of seizures because of their role in modulating extracellular potassium concentration, adenosine levels, the size of the extracellular space, and uptake of neurotransmitters. Of all the glia, the data indicate that sei is specifically important in neuropile-ensheathing glia. A complete picture of the specific functions of neuropile ensheathing glia has yet to emerge, yet studies manipulating genes in this cell type have implicated roles in phagocytosis of injured neurons, organization of neural circuits, and glutamate metabolism. However, how the action of voltage gated ion channels such as sei in glia might affect these specific processes remains mostly unknown. Nevertheless, glial expression of another voltage gated potassium channel that is associated with human epilepsy, KCNJ10, has been shown to lead to epileptic activity in a mouse model, possibly via its role in buffering extracellular potassium and glutamate. Whether hERG-like channels play a similar role in glia remains to be explored (Hill, 2019).

Together, these provide important insights into the possible role of hERG channels in regulating neuronal robustness and susceptibility to stress-induced seizures. From a clinical perspective, the data suggest that the high incidence of generalized seizures that has been reported in LQTS patients that carry mutations in the hERG genes might not be a secondary cardiogenic comorbidity, as is currently often assumed. Instead, because about 40% of patients that carry LQTS-related hERG mutations have reported a personal history of seizures, as compared to less than 20% in LQTS patients with similar cardiac pathologies that are due to mutations in other genes, it is hypothesized that seizure etiology in many LQTS patients is likely due to the direct impact of mutations in hERG on nervous system functions, independent of their cardiovascular condition. Therefore, it is predicted that it is possible that some unidentified mutations in hERG might be causally related to epilepsies, independent of the presentation of any LQTS-related pathologies, and may represent novel genetic risk factors for seizures (Hill, 2019).

These studies provide compelling evidence that hERG channels play an essential role in protecting the nervous system from acute environmental stressors, such as heat, which could potentially lead to hyperexcitability and seizures. Furthermore, this study shows that in Drosophila, the activity of the ERG channel sei contributes to neuronal and behavioral robustness via its action in independent cell types in the nervous system. These important insights should help better understand how the nervous system responds to acute environmental stressors, and possibly provide important mechanistic insights into some of the known pathologies associated with hERG mutations in human patients (Hill, 2019).

Age-dependent electrical and morphological remodeling of the Drosophila heart caused by hERG/seizure mutations

Understanding the cellular-molecular substrates of heart disease is key to the development of cardiac specific therapies and to the prevention of off-target effects by non-cardiac targeted drugs. One of the primary targets for therapeutic intervention has been the human ether a go-go (hERG) K+ channel that, together with the KCNQ channel, controls the rate and efficiency of repolarization in human myocardial cells. Neither of these channels plays a major role in adult mouse heart function; however, this study shows that the hERG homolog seizure (sei), along with KCNQ, both contribute significantly to adult heart function as they do in humans. In Drosophila, mutations in or cardiac knockdown of sei channels cause arrhythmias that become progressively more severe with age. Intracellular recordings of semi-intact heart preparations revealed that these perturbations also cause electrical remodeling that is reminiscent of the early afterdepolarizations seen in human myocardial cells defective in these channels. In contrast to KCNQ, however, mutations in sei also cause extensive structural remodeling of the myofibrillar organization, which suggests that hERG channel function has a novel link to sarcomeric and myofibrillar integrity. It is concluded that deficiency of ion channels with similar electrical functions in cardiomyocytes can lead to different types or extents of electrical and/or structural remodeling impacting cardiac output (Ocorr, 2017).

Channel dysfunction, or channelopathies, underlie a number of cardiac disorders such as long QT syndrome (LQTS) and are thought to contribute to sudden cardiac arrest, infant sudden death syndrome and increased risk of cardiac arrhythmias. The human ether-a-go-go related K+ channel (hERG) along with the KCNQ K+ channel (IKr, and IKs respectively) are the major contributors to cardiac repolarization in humans. Defects in KCNQ and hERG channels have been shown to cause LQT1 and LQT2, respectively and cardiac arrhythmias in humans. The hERG channel in particular has been a major target for the development of anti-arrhythmia drugs and can be inhibited by a variety of drugs that do not specifically target the heart. Previous work has shown that the KCNQ K+ channel is functional in the adult fly heart and that mutations in this channel contribute to cardiac arrhythmias. In the fly, seizure and KCNQ mutations do not significantly affect cardiac development, although in the mouse some hERG mutations cause looping defects and embryonic lethality. Importantly for heart function studies, survivorship of adult flies is not acutely affected by badly functioning hearts that would quickly result in death in vertebrates, likely because the fly does not rely on the heart for oxygen distribution, which is carried out by a separate system of tracheoles (Ocorr, 2017).

The current data demonstrate that the sei gene, the fly homolog of hERG, along with a number of other genes encoding K+ channels, are also expressed in fly myocardial cells and likely contribute to the repolarization capacity of the heart. This set of K+ channels is reminiscent of what is observed in vertebrate hearts. The primary effect of sei dysfunction in mutants was bradycardia and this phenotype could be replicated by cardiac-specific and adult, cardiac-specific sei KD as well as acute application of selective hERG antagonists. However, the increases in SI seen in both KD experiments are different from the mutant phenotype and possibly reflect some compensatory genetic alterations in the systemic sei mutants. The current data also show that, as in humans, mutations in sei cause arrhythmias and electrical remodeling in the form of AP bursts that are likely triggered by EADs. In addition this study shows that systemic mutants or cardiac-specific KD of sei, but not of KCNQ, reduced heart contractility, and were associated with structural remodeling (Ocorr, 2017).

These effects appear to be specific to alterations in sei as most of the sei cardiac function phenotypes can be rescued by over-expression of the wt channel. In addition, the morphological/functional effects of channel dysfunction appear not to be due to developmental defects but are adult stage- and cardiac-specific (Ocorr, 2017).

The intracellular electrical activity in adult fly cardiomyocytes appears more nodal or atrial-like and is similar to what has previously been observed in larva although more robust in terms of the resting potential and AP amplitude. Frequent AP bursts were observed in both sei and KCNQ mutants and in hearts from old flies compared to young Wt fly hearts reminiscent of the increase in electrical remodeling with disease and age in human myocardial cells. However, there appears to be a significant difference between mutations in KCNQ and sei in terms of their effects on electrical and morphological remodeling, with mutations in KCNQ resulting in significantly more electrical arrhythmia at younger ages compared to sei mutations. In addition, although 100% of APs recorded from both old KCNQ and sei mutants are arrhythmogenic, the severity of the events appears to be worse in KCNQ mutants. Notably, the ability to simultaneously record both intracellular APs while optically monitoring intact heart function demonstrate that the arrhythmogenic APs that were observed correlate directly with unsustained fibrillatory contractions of the heart wall revealed in M-modes (Ocorr, 2017).

Although both sei and KCNQ channel mutations cause electrical remodeling and arrhythmia in the fly heart model they appear to have differing effects on muscle structure. Mutations in sei appear to be linked to an increase in myofibrillar disorganization and this effect on myocardial structure is manifest by reductions in fractional shortening. In addition, the observed reductions in shortening velocities under loaded conditions suggests that the ability to generate tension is significantly lower in hearts from sei mutants than for controls or KCNQ mutants and are consistent with the reduced ability to sustain an isometric contraction in the sei mutant hearts. Together these results suggest differential roles for these channels on structural and electrical integrity of the adult heart (Ocorr, 2017).

The microarray results suggest that the different effects of the two K+ channel mutants are the result of underlying differences in gene expression. The observation that many pathways involved in metabolism are significantly upregulated in sei mutants is consistent with previous observations in rabbits, although in that study both LQT1 and LQT2 models exhibited similar effects. In particular, in the adult fly heart the Wnt signaling pathway appears to be selectively perturbed in the sei mutant hearts, as evidenced by a downregulation of many of its pathway components. This is consistent with previous results demonstrating that mutation or cardiac KD of the TCF co-factor encoded by pygo cause bradycardia and morphological remodeling similar to that observed for sei mutants. Importantly, this study now demonstrates an interaction between sei channel mutations and mutations in pygo. Wnt signaling has previously been shown to play significant roles in cardiac development in flies and vertebrates and in cardiac disease. This study has shown in the fly that pygo likely plays a role in the maintenance of cardiac function and structure in the adult and the current data suggest that Pygo and Sei channel dysfunction are likely linked genetically. Many components of both the canonical and non-canonical pathways exhibit reduced expression, which would suggest an overall reduction in Wnt signaling. However, downregulation was also observed of potential negative effectors of Wnt signaling, such as APC, the core of the destruction complex, and CtBP, which has been shown to mediate both activation and repression of transcription. Both effects could be expected to result in increased stabilization of β-catenin. Thus, the exact role of different Wnt signaling components in maintaining cardiac structure and function remains to be determined (Ocorr, 2017).

Electrical and morphological remodeling have previously been shown to be linked; for example, knockout of the heart development transcription factor Ptx1 was shown to affect both electrical and morphological remodeling in mouse and humans. A number of studies have documented pro-fibrotic and apoptotic effects of atrial fibrillation as well as tachypacing in cardiomyocytes and in response to sustained atrial fibrillation in patients. Further, a genetic variant situated close to the long QT syndrome (LQTS) type 2 gene KCNH2 has been shown to be associated with early onset AF. Thus, cellular/molecular links between channel function, electrical activity and morphological remodeling and ultimately heart failure have been suggested but have yet to be clearly elucidated. An understanding of how hERG channels interact with cellular pathways involved in electrical and structural remodeling and with other repolarizing currents such as IKs, mediated by KCNQ channels, will be important to the development of novel anti-arrhythmia therapies. These data using the fly heart model now provides the first clear genetic and physiological evidence that some channelopathies may be contributing to cardiac remodeling during disease progression via Wnt signaling (Ocorr, 2017).

Natural antisense transcripts regulate the neuronal stress response and excitability

Neurons regulate ionic fluxes across their plasma membrane to maintain their excitable properties under varying environmental conditions. However, the mechanisms that regulate ion channels abundance remain poorly understood. This study shows that pickpocket 29 (ppk29), a gene that encodes a Drosophila degenerin/epithelial sodium channel (DEG/ENaC), regulates neuronal excitability via a protein-independent mechanism. The mRNA 3'UTR of ppk29 affects neuronal firing rates and associated heat-induced seizures by acting as a natural antisense transcript (NAT) that regulates the neuronal mRNA levels of seizure (sei), the Drosophila homolog of the human Ether-a-go-go Related Gene (hERG) potassium channel. The regulatory impact of ppk29 mRNA on sei is independent of the sodium channel it encodes. Thus, these studies reveal a novel mRNA dependent mechanism for the regulation of neuronal excitability that is independent of protein-coding capacity (Zheng, 2014).

The Drosophila erg K+ channel polypeptide is encoded by the seizure locus

The eag family of K+ channels contains three known subtypes: eag, elk, and erg. Genes representing the first two subtypes have been identified in flies and mammals, whereas the third subtype has been defined only by the human HERG gene, which encodes an inwardly rectifying channel that is mutated in some cardiac arrhythmias. To establish the predicted existence of a Drosophila gene in the erg subfamily and to learn more about the structure and biological function of channels within this subfamily, a search was undertaken for the Drosophila counterpart of HERG. This paper report the isolation and characterization of the Drosophila erg gene. It corresponds with the previously identified seizure (sei) locus, mutations of which cause a temperature-sensitive paralytic phenotype associated with hyperactivity in the flight motor pathway. These results yield new insights into the structure and evolution of the eag family of channels, provide a molecular explanation for the sei mutant phenotype, and demonstrate the important physiological roles of erg-type channels from invertebrates to mammals (Titus, 1997).

The seizure locus encodes the Drosophila homolog of the HERG potassium channel

Mutations in the seizure (sei) locus cause temperature-induced hyperactivity, followed by paralysis. Gene cloning studies have established that the seizure gene product is the Drosophila homolog of HERG, a member of the eag family of K+ channels implicated in one form of hereditary long QT syndrome in humans. A series of five null alleles with premature stop codons are all recessive, but viable. A missense mutation in the sei gene, which changes the charge at a conserved glutamate residue near the outer mouth of the pore, has a semidominant phenotype, suggesting that the mutant seizure protein acts as a poison in a multimeric complex. Transformation rescue of a null allele with a cDNA under the control of an inducible promoter demonstrates that induced expression of seizure potassium channels in adults rescues the paralytic phenotype. This rescue decays with a t1/2 of approximately 1-1.5 d after gene induction is discontinued, providing the first estimate of ion channel stability in an intact, multicellular animal (Wang, 1997).

Enhancer of seizure: a new genetic locus in Drosophila melanogaster defined by interactions with temperature-sensitive paralytic mutations

Mutations in the enhancer of seizure [e(sei)] locus have been isolated on the basis of their ability to cause temperature-induced paralysis of alleles at the seizure (sei) locus at temperatures at which these mutations ordinarily do not paralyze. This enhancer is specific to the seizure locus and is without effect on other temperature-sensitive paralytic mutants including para, nap, tip-E and shi. This suggests that the enhancer responds specifically to the mechanism of paralysis mediated by the seizure mutations. The e(sei) is a recessive mutation which maps to 39.0 on the left arm of chromosome 3. Deficiency mapping has placed it at 69A4-B5 on the salivary gland polytene chromosome map. When a new enhancer allele was isolated following P-M hybrid dysgenesis, there was a concomitant P-element insertion at 69B. In the absence of seizure mutations, the enhancer mutation causes non-temperature dependent hyperactivity when agitated and interferes with the climbing response. Electrophysiological studies examined the effects of increasing temperature on electrical activity in the adult giant fiber/flight muscle system. Neuronal hyperactivity was seen in both e(sei) and sei single mutant homozygotes, but not in wild type. The hyperactivity was more severe in the sei;e(sei) double mutants. The correlation between the physiological effects and the mutant behavior suggests that both sei and e(sei) cause membrane excitability defects. Since previous work has shown that seizure mutants affect [3H]saxitoxin binding to the voltage-sensitive sodium channel, e(sei) may code for a gene product which interacts with this channel (Kasbekar, 1987).

Genetic modifications of voltage-sensitive sodium channels in Drosophila: gene dosage studies of the seizure locus

A genetic locus has been identified in Drosophila melanogaster whose product appears to have a structural role in the formation of functional voltage-sensitive sodium channels. This locus, designated seizure, is defined by two temperature-sensitive alleles (seits-1 and seits-2), each of which causes convulsive seizures followed by a rapid but reversible paralysis of adults at restrictive temperatures above 38 degrees C. Previous work had shown that seits-2 extracts display an altered pH dependence and an abnormally high Kd for [3H]-saxitoxin binding at high temperatures, suggesting that sodium channels in seits-2 mutants have an altered structure. These binding studies have now been extended to extracts of seits-1 which have a Kd not significantly different from wild-type at all assay temperatures. However, seits-1 extracts show a reduced number of saxitoxin binding sites (Bmax) relative to wild-type. This reduction is only 5%-18% at 0°C but is 17%-37% at 39°C, suggesting that under certain conditions sodium channels in the seits-1 mutant are more labile than those of wild-type. Cytogenetic studies demonstrate that the seizure locus maps within region 60A to 60B8-10 on the second chromosome. Gene dosage analysis of approximately 99.7% of the genome, including this second chromosome region, failed to detect a wild-type locus whose dose affected saxitoxin-binding activity. Nevertheless, the mutant seits-2 allele has codominant and dose-dependent effects on paralytic behavior and saxitoxin-binding activity. Nevertheless, the mutant seits-2 allele has codominant and dose-dependent effects on paralytic behavior and saxitoxin-binding activity (Jackson, 1995).

Two types of mutants affecting voltage-sensitive sodium channels in Drosophila melanogaster

Voltage-sensitive sodium channels have a key role in the genesis of propagated action potentials. Mutations that affect these channels might be used like specific pharmacological agents in studies of channel structure and regulation. This study found that mutations which cause a reversible, temperature-induced paralysis in the fruit fly Drosophila melanogaster often affect the voltage-sensitive sodium channel. Using 3H-saxitoxin binding to membrane preparations, this study has identified two types of presumptive sodium channel mutants. One mutation, seizurets-2 (seits-2), appears to alter saxitoxin-binding sites structurally. The second, no-action-potential (napts), reduces the number of saxitoxin-binding sites, but appears not to alter the receptor structure (Jackson, 1984).


Functions of Seizure orthologs in other species

Functional roles of an ERG current isolated in cerebellar Purkinje neurons

Transcripts encoding ERG potassium channels are expressed by most neurons of the CNS. By patch-clamp whole cell recording from Purkinje neurons in slices of young (5-9 days old) mouse cerebellum it has been possible to isolate a tail current [IK(ERG)] with the same characteristics as previously described for ERG channels. In zero external Ca2+ and high K+ (40 mM) the V1/2 of activation was -50.7 mV, the V1/2 of inactivation was -70.6 mV, and the deactivation rate was double exponential and voltage dependent. IK(ERG) was 93.0% blocked by WAY-123,398 (1 microM) and 78.2% by haloperidol (2 microM). The role of IK(ERG) on evoked firing was studied in adult mice, where WAY-123,398 application decreased the first spike latency, increased the firing frequency, and suppressed the frequency adaptation. However, the shape of individual action potentials was not affected. Stimulation of presynaptic climbing fibers evoked the Purkinje neuron 'complex spike,' composed of an initial spike and several spikelets. IK(ERG) block caused an increase of the number of spikelets of the 'complex spike.' These data show, for the first time, an IK(ERG) in a neuron of the CNS, the cerebellar Purkinje neuron, and indicate that such a current is involved in the control of membrane excitability, firing frequency adaptation, and in determining the effects of the climbing fiber synapse (Sacco, 2003).

A novel role for HERG K+ channels: spike-frequency adaptation

The regular firing of a Hodgkin-Huxley neurone endowed with fast Na+ and delayed K+ channels can be converted into adapting firing by appending HERG (human eag-related gene) channels. The computer model predictions were verified by studying the firing properties of F-11 DRG neurone x neuroblastoma hybrid cells induced to differentiate by long-term exposure to retinoic acid. These cells, which express HERG currents (IHERG), show clear spike-frequency adaptation of their firing when current clamped with long depolarizations. In agreement with the prediction, the selective blocking of IHERG by class III antiarrhythmic drugs always led to the disappearance of the spike-frequency adaptation, and the conversion of adapting firing to regular firing. 4. It is proposed that, in addition to their role in the repolarization of the heart action potential, HERG channels may sustain a process of spike-frequency adaptation, and hence contribute to the control of burst duration in a way that is similar to that of the K+ currents, IAHP, IC and IM. In addition to the known cardiac arrhythmia syndrome (LQT2), genetic mutations or an altered HERG expression could lead to continuous hyperexcitable states sustained by the inability of nerve or endocrine cells to accommodate to repetitive stimuli. This might help in clarifying the pathogenesis of still undefined idiopathic familial epilepsies (Chiesa, 1997).


REFERENCES

Search PubMed for articles about Drosophila Seizure

Chiesa, N., Rosati, B., Arcangeli, A., Olivotto, M. and Wanke, E. (1997). A novel role for HERG K+ channels: spike-frequency adaptation. J Physiol 501 ( Pt 2): 313-318. PubMed ID: 9192303

Hill, A. S., Jain, P., Folan, N. E. and Ben-Shahar, Y. (2019). The Drosophila ERG channel Seizure plays a role in the neuronal homeostatic stress response. PLoS Genet 15(8): e1008288. PubMed ID: 31393878

Jackson, F. R., Wilson, S. D., Strichartz, G. R. and Hall, L. M. (1984). Two types of mutants affecting voltage-sensitive sodium channels in Drosophila melanogaster. Nature 308(5955): 189-191. PubMed ID: 6322008

Jackson, F. R., Gitschier, J., Strichartz, G. R. and Hall, L. M. (1985). Genetic modifications of voltage-sensitive sodium channels in Drosophila: gene dosage studies of the seizure locus. J Neurosci 5(5): 1144-1151. PubMed ID: 2582101

Kasbekar, D. P., Nelson, J. C. and Hall, L. M. (1987). Enhancer of seizure: a new genetic locus in Drosophila melanogaster defined by interactions with temperature-sensitive paralytic mutations. Genetics 116(3): 423-431. PubMed ID: 2440763

Ocorr, K., Zambon, A., Nudell, Y., Pineda, S., Diop, S., Tang, M., Akasaka, T. and Taylor, E. (2017). Age-dependent electrical and morphological remodeling of the Drosophila heart caused by hERG/seizure mutations. PLoS Genet 13(5): e1006786. PubMed ID: 28542428

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Biological Overview

date revised: 28 September 2019

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