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

Neuronal excitability modulates developmental time of Drosophila melanogaster

Developmental time is a fundamental life history trait that affects the reproductive success of animals. Developmental time is known to be regulated by many genes and environmental conditions, yet mechanistic understandings of how various cellular processes influence the developmental timing of an organism are lacking. The nervous system is known to control key processes that affect developmental time, including the release of hormones that signal transitions between developmental stages. Here we show that the excitability of neurons plays a crucial role in modulating developmental time. Genetic manipulation of neuronal excitability in Drosophila melanogaster alters developmental time, which is faster in animals with increased neuronal excitability. We find that selectively modulating the excitability of peptidergic neurons is sufficient to alter developmental time, suggesting the intriguing hypothesis that the impact of neuronal excitability on DT may be at least partially mediated by peptidergic regulation of hormone release. This effect of neuronal excitability on developmental time is seen during embryogenesis and later developmental stages. Observed phenotypic plasticity in the effect of genetically increasing neuronal excitability at different temperatures, a condition also known to modulate excitability, suggests there is an optimal level of neuronal excitability, in terms of shortening DT. Together, our data highlight a novel connection between neuronal excitability and developmental time, with broad implications related to organismal physiology and evolution.

SIK3 and Wnk converge on Fray to regulate glial K+ buffering and seizure susceptibility

Glial cells play a critical role in maintaining homeostatic ion concentration gradients. Salt-inducible kinase 3 (SIK3) regulates a gene expression program that controls K+ buffering in glia, and upregulation of this pathway suppresses seizure behavior in the eag, Shaker hyperexcitability mutant. Here we show that boosting the glial SIK3 K+ buffering pathway suppresses seizures in three additional molecularly diverse hyperexcitable mutants, highlighting the therapeutic potential of upregulating glial K+ buffering. We then explore additional mechanisms regulating glial K+ buffering. Fray, a transcriptional target of the SIK3 K+ buffering program, is a kinase that promotes K+ uptake by activating the Na+/K+/Cl- co-transporter, Ncc69. We show that the Wnk kinase phosphorylates Fray in Drosophila glia and that this activity is required to promote K+ buffering. This identifies Fray as a convergence point between the SIK3-dependent transcriptional program and Wnk-dependent post-translational regulation. Bypassing both regulatory mechanisms via overexpression of a constitutively active Fray in glia is sufficient to robustly suppress seizure behavior in multiple Drosophila models of hyperexcitability. Finally, we identify cortex glia as a critical cell type for regulation of seizure susceptibility, as boosting K+ buffering via expression of activated Fray exclusively in these cells is sufficient to suppress seizure behavior. These findings highlight Fray as a key convergence point for distinct K+ buffering regulatory mechanisms and cortex glia as an important locus for control of neuronal excitability.

The environmental toxicant ziram enhances neurotransmitter release and increases neuronal excitability via the EAG family of potassium channels

Environmental toxicants have the potential to contribute to the pathophysiology of multiple complex diseases, but the underlying mechanisms remain obscure. One such toxicant is the widely used fungicide ziram, a dithiocarbamate known to have neurotoxic effects and to increase the risk of Parkinson's disease. We have used Drosophila melanogaster as an unbiased discovery tool to identify novel molecular pathways by which ziram may disrupt neuronal function. Consistent with previous results in mammalian cells, we find that ziram increases the probability of synaptic vesicle release by dysregulation of the ubiquitin signaling system. In addition, we find that ziram increases neuronal excitability. Using a combination of live imaging and electrophysiology, we find that ziram increases excitability in both aminergic and glutamatergic neurons. This increased excitability is phenocopied and occluded by null mutant animals of the ether a-go-go (eag) potassium channel. A pharmacological inhibitor of the temperature sensitive hERG (human ether-a-go-go related gene) phenocopies the excitability effects of ziram but only at elevated temperatures. seizure (sei), a fly ortholog of hERG, is thus another candidate target of ziram. Taken together, the eag family of potassium channels emerges as a candidate for mediating some of the toxic effects of ziram. We propose that ziram may contribute to the risk of complex human diseases by blockade of human eag and sei orthologs, such as hERG.

The Drosophila ERG channel seizure plays a role in the neuronal homeostatic stress response

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. Here we 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. We found that the activity of the voltage-gated potassium channel seizure (sei), an ortholog of the mammalian ERG channel family, is essential for protecting flies from acute heat-induced seizures. Although sei is broadly expressed in the nervous system, our 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, our 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.

ERG3 potassium channel-mediated suppression of neuronal intrinsic excitability and prevention of seizure generation in mice

KEY POINTS

ERG3 channels have a high expression level in the central nervous system. Knockdown of ERG3 channels enhances neuronal intrinsic excitability (caused by decreased fast afterhyperpolarization, shortened delay time to the generation of an action potential and enhanced summation of somatic excitatory postsynaptic potentials) in hippocampal CA1 pyramidal neurons and dentate gyrus granule cells. The expression of ERG3 protein is reduced in human and mouse hippocampal epileptogenic foci. Knockdown of ERG3 channels in hippocampus enhanced seizure susceptibility, while mice treated with the ERG channel activator NS-1643 were less prone to epileptogenesis. The results provide strong evidence that ERG3 channels have a crucial role in the regulation of neuronal intrinsic excitability in hippocampal CA1 pyramidal neurons and dentate gyrus granule cells and are critically involved in the onset and development of epilepsy.

ABSTRACT

The input-output relationship of neuronal networks depends heavily on the intrinsic properties of their neuronal elements. Profound changes in intrinsic properties have been observed in various physiological and pathological processes, such as learning, memory and epilepsy. However, the cellular and molecular mechanisms underlying acquired changes in intrinsic excitability are still not fully understood. Here, we demonstrate that ERG3 channels are critically involved in the regulation of intrinsic excitability in hippocampal CA1 pyramidal neurons and dentate gyrus granule cells. Knock-down of ERG3 channels significantly increases neuronal intrinsic excitability, which is mainly caused by decreased fast afterhyperpolarization, shortened delay time to the generation of an action potential and enhanced summation of somatic excitatory postsynaptic potentials. Interestingly, the expression level of ERG3 protein is significantly reduced in human and mouse brain tissues with temporal lobe epilepsy. Moreover, ERG3 channel knockdown in hippocampus significantly enhanced seizure susceptibility, while mice treated with the ERG channel activator NS-1643 were less prone to epileptogenesis. Taken together, our results suggest ERG3 channels play an important role in determining the excitability of hippocampal neurons and dysregulation of these channels may be involved in the generation of epilepsy. ERG3 channels may thus be a novel therapeutic target for the prevention of epilepsy.

Functional Loss of Bmsei Causes Thermosensitive Epilepsy in Contractile Mutant Silkworm, Bombyx mori

The thermoprotective mechanisms of insects remain largely unknown. We reported the Bombyx mori contractile (cot) behavioral mutant with thermo-sensitive seizures phenotype. At elevated temperatures, the cot mutant exhibit seizures associated with strong contractions, rolling, vomiting, and a temporary lack of movement. We narrowed a region containing cot to ~268 kb by positional cloning and identified the mutant gene as Bmsei which encoded a potassium channel protein. Bmsei was present in both the cell membrane and cytoplasm in wild-type ganglia but faint in cot. Furthermore, Bmsei was markedly decreased upon high temperature treatment in cot mutant. With the RNAi method and injecting potassium channel blockers, the wild type silkworm was induced the cot phenotype. These results demonstrated that Bmsei was responsible for the cot mutant phenotype and played an important role in thermoprotection in silkworm. Meanwhile, comparative proteomic approach was used to investigate the proteomic differences. The results showed that the protein of Hsp-1 and Tn1 were significantly decreased and increased on protein level in cot mutant after thermo-stimulus, respectively. Our data provide insights into the mechanism of thermoprotection in insect. As cot phenotype closely resembles human epilepsy, cot might be a potential model for the mechanism of epilepsy in future.

Inhibition of Schistosoma mansoni ether-a-go-go related gene-encoded potassium channels leads to hypermotility and impaired egg production

The purpose of this work was to investigate the effect of ether-a-go-go related gene (ERG) potassium channel inhibition on Schistosoma mansoni. Use of dofetilide to block the schistosome ERGs resulted in a striking 'corkscrew' effect. The worms were unable to control their motility; they were hypermotile. The treated worms produced abnormal eggs, some of which consisted of little more than a spine. One of the S. mansoni ERGs (SmERGs), Smp_161140, was chosen for further study by RNAi. The transcript was knocked down to 50% compared to the controls. These RNAi-treated worms demonstrated seizure-like movements. In S. mansoni, as in other organisms, ERG channels seem to play a role in regulating muscle excitability. This work shows that egg production can be greatly reduced by effectively targeting muscle coordination in these important parasites.

Comparison of larval and adult Drosophila astrocytes reveals stage-specific gene expression profiles

The analysis of adult astrocyte glial cells has revealed a remarkable heterogeneity with regard to morphology, molecular signature, and physiology. A key question in glial biology is how such heterogeneity arises during brain development. One approach to this question is to identify genes with differential astrocyte expression during development; certain genes expressed later in neural development may contribute to astrocyte differentiation. We have utilized the Drosophila model and Translating Ribosome Affinity Purification (TRAP)-RNA-seq methods to derive the genome-wide expression profile of Drosophila larval astrocyte-like cells (hereafter referred to as astrocytes) for the first time. These studies identified hundreds of larval astrocyte-enriched genes that encode proteins important for metabolism, energy production, and protein synthesis, consistent with the known role of astrocytes in the metabolic support of neurons. Comparison of the larval profile with that observed for adults has identified genes with astrocyte-enriched expression specific to adulthood. These include genes important for metabolism and energy production, translation, chromatin modification, protein glycosylation, neuropeptide signaling, immune responses, vesicle-mediated trafficking or secretion, and the regulation of behavior. Among these functional classes, the expression of genes important for chromatin modification and vesicle-mediated trafficking or secretion is overrepresented in adult astrocytes based on Gene Ontology analysis. Certain genes with selective adult enrichment may mediate functions specific to this stage or may be important for the differentiation or maintenance of adult astrocytes, with the latter perhaps contributing to population heterogeneity.

The role of cAMP in synaptic homeostasis in response to environmental temperature challenges and hyperexcitability mutations

Homeostasis is the ability of physiological systems to regain functional balance following environment or experimental insults and synaptic homeostasis has been demonstrated in various species following genetic or pharmacological disruptions. Among environmental challenges, homeostatic responses to temperature extremes are critical to animal survival under natural conditions. We previously reported that axon terminal arborization in Drosophila larval neuromuscular junctions (NMJs) is enhanced at elevated temperatures; however, the amplitude of excitatory junctional potentials (EJPs) remains unaltered despite the increase in synaptic bouton numbers. Here we determine the cellular basis of this homeostatic adjustment in larvae reared at high temperature (HT, 29°C). We found that synaptic current focally recorded from individual synaptic boutons was unaffected by rearing temperature (<15°C to >30°C). However, HT rearing decreased the quantal size (amplitude of spontaneous miniature EJPs, or mEJPs), which compensates for the increased number of synaptic releasing sites to retain a normal EJP size. The quantal size decrease is accounted for by a decrease in input resistance of the postsynaptic muscle fiber, indicating an increase in membrane area that matches the synaptic growth at HT. Interestingly, a mutation in rutabaga (rut) encoding adenylyl cyclase (AC) exhibited no obvious changes in quantal size or input resistance of postsynaptic muscle cells after HT rearing, suggesting an important role for rut AC in temperature-induced synaptic homeostasis in Drosophila. This extends our previous finding of rut-dependent synaptic homeostasis in hyperexcitable mutants, e.g., slowpoke (slo). In slo larvae, the lack of BK channel function is partially ameliorated by upregulation of presynaptic Shaker (Sh) IA current to limit excessive transmitter release in addition to postsynaptic glutamate receptor recomposition that reduces the quantal size.

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.

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. Here we show that pickpocket 29 (ppk29), a gene that encodes a Drosophila degenerin/epithelial sodium channel (DEG/ENaC), regulates neuronal excitability via a protein-independent mechanism. We demonstrate that 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-à-go-go Related Gene (hERG) potassium channel. We find that the regulatory impact of ppk29 mRNA on sei is independent of the sodium channel it encodes. Thus, our studies reveal a novel mRNA dependent mechanism for the regulation of neuronal excitability that is independent of protein-coding capacity.

DOI:http://dx.doi.org/10.7554/eLife.01849.001

Neurons communicate with one another via electrical signals known as action potentials. These signals are generated when a stimulus causes sodium and potassium ion channels in the cell membrane to open, leading to an influx of sodium ions, followed by an efflux of potassium ions. Changes in temperature affect the rate at which ion channels open and close, and thus affect how easy it is for a stimulus to trigger an action potential. In response to a sudden rise in temperature, neurons must adjust the number of ion channels in their membranes to ensure that they do not become hyperexcitable, which could result in epilepsy.

Now, Zheng et al. have revealed one possible mechanism for how neurons do this. In the fruit fly, Drosophila, a gene for a potassium channel is found on the same chromosomal location as a gene for a sodium channel, and some of the genetic elements that regulate the expression of these two genes even overlap. However, the genes are on opposite strands of the DNA double helix. This means that when the genes are transcribed to produce molecules of messenger RNA (mRNA), which is usually single stranded, some of the mRNA molecules will pair up to form double-stranded mRNA molecules. This is significant because such RNA ‘duplexes’ have been shown to inhibit the translation of conventional single-stranded mRNA molecules into proteins, or to lead to their complete degradation.

Zheng et al. found that flies with mutations in the potassium channel gene display seizures in response to sudden changes in temperature. However, insects with mutations in the sodium channel gene are not affected because, surprisingly, they have a higher than expected number of potassium channels. It turns out that the mutant sodium channel mRNA molecules are unable to form RNA duplexes with potassium channel mRNA molecules: these duplexes would normally limit the number of potassium channels so, in their absence, the number of potassium channels increases, and this protects the flies from seizures.

Zheng et al. also uncovered a novel mechanism by which mRNA molecules can regulate gene expression independent of their role as templates for proteins. Further work is required to determine whether this mechanism is also present in other organisms, including humans.

DOI:http://dx.doi.org/10.7554/eLife.01849.002

Potassium channels in Drosophila: historical breakthroughs, significance, and perspectives

Drosophila has enabled important breakthroughs in K(+) channel research, including identification and fi rst cloning of a voltage-activated K(+) channel, Shaker, a founding member of the K(V)1 family. Drosophila has also helped in discovering other K(+) channels, such as Shab, Shaw, Shal, Eag, Sei, Elk, and also Slo, a Ca(2+) - and voltage-dependent K(+) channel. These findings have contributed significantly to our understanding of ion channels and their role in physiology. Drosophila continues to play an important role in ion channel studies, benefiting from an unparalleled arsenal of genetic tools and availability of tens of thousands of genetically modified strains. These tools allow deletion, expression, or misexpression of almost any gene in question with temporal and spatial control. The combination of these tools and resources with the use of forward genetic approach in Drosophila further enhances its strength as a model system. There are many areas in which Drosophila can further help our understanding of ion channels and their function. These include signaling pathways involved in regulating and modulating ion channels, basic information on channels and currents where very little is currently known, and the role of ion channels in physiology and pathology.

A mutation in Drosophila Aldolase causes temperature-sensitive paralysis, shortened lifespan, and neurodegeneration

We describe the characterization of m4, an autosomal recessive, temperature-sensitive paralytic mutant in Drosophila that is associated with shortened lifespan and neurodegeneration. Deletion mapping places the mutation in the gene encoding the glycolytic enzyme, Aldolase. The mutant enzyme contains a single amino acid substitution, which results in decreased steady-state levels of Aldolase with a consequent reduction in adenosine triphosphate (ATP) levels. Transgenic-rescue experiments with a genomic construct containing the entire Aldolase gene confirm that paralysis, reduced lifespan, and neurodegeneration all result from the same mutation. Tissue-specific rescue and RNA interference (RNAi) knockdown experiments indicate that Aldolase function (and presumably glycolysis) is important both in neurons and in glia for normal lifespan and neuronal maintenance over time. Impaired glycolysis in neurons can apparently be rescued in part by glycolytically active glia. However, this rescue may depend on the exact physiological state of the neurons and may also vary in different subsets of neurons. Further studies of m4 and related mutants in Drosophila should help elucidate the connections between energy production and utilization in glia and neurons and lead to better understanding of how metabolic defects impair neuronal function and maintenance.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

A Drosophila behavioral mutant, down and out (dao), is defective in an essential regulator of Erg potassium channels

To signal properly, excitable cells must establish and maintain the correct balance of various types of ion channels that increase or decrease membrane excitability. The mechanisms by which this balance is regulated remain largely unknown. Here, we describe a regulatory mechanism uncovered by a Drosophila behavioral mutant, down and out (dao). At elevated temperatures, dao loss-of-function mutants exhibit seizures associated with spontaneous bursts of neural activity. This phenotype closely resembles that of seizure mutations, which impair activity of ether-a-go-go-related gene (Erg)-type potassium channels. Conversely, neural over-expression of wild-type Dao confers dominant temperature-sensitive paralysis with kinetics reminiscent of paralytic sodium-channel mutants. The over-expression phenotype of dao is suppressed in a seizure mutant background, suggesting that Dao acts by an effect on Erg channels. In support of this hypothesis, functional expression of Erg channels in a heterologous system is dependent on the presence of Dao. These results indicate that Dao has an important role in establishing the proper level of neuronal membrane excitability by regulating functional expression of Erg channels.

Identification of ion channel genes in the Acyrthosiphon pisum genome

Aphids are major pests of crops, causing hundreds of millions of dollars worth of damage annually. Ion channel proteins are often the targets of modern insecticides and mutations in ion channel genes can lead to resistance to many leading classes of insecticides. The sequencing of the pea aphid, Acyrthosiphon pisum, genome has now allowed detailed in silico analysis of the aphid ion channels. The study has revealed significant differences in the composition of the ion channel families between the aphid and other insects. For example A. pisum does not appear to contain a homologue of the nACh receptor alpha 5 gene whilst the calcium channel beta subunit has been duplicated. These variations could result in differences in function or sensitivity to insecticides. The genome sequence will allow the study of aphid ion channels to be accelerated, leading to a better understanding of the function of these economically important channels. The potential for identifying novel insecticide targets within the aphid is now a step closer.

Activation of ERG2 potassium channels by the diphenylurea NS1643

Three members of the ERG potassium channel family have been described (ERG1-3 or Kv 11.1-3). ERG1 is by far the best characterized subtype and it constitutes the molecular component of the cardiac I(Kr) current. All three channel subtypes are expressed in neurons but their function remains unclear. The lack of functional information is at least partly due to the lack of specific pharmacological tools. The compound NS1643 has earlier been reported as an ERG1 channel activator. We found that NS1643 also activates the ERG2 channel; however, the molecular mechanism of the activation differs between the ERG1 and ERG2 channels. This is surprising since ERG1 and ERG2 channels have very similar biophysical and structural characteristics. For ERG2, NS1643 causes a left-ward shift of the activation curve, a faster time-constant of activation and a slower time-constant of inactivation as well as an increased relative importance for the fast component of deactivation to the total deactivation. In contrast, for ERG1, NS1643 causes a right-ward shift in the voltage-dependent release from inactivation but does not affect time-constants of deactivation. Because of these differences in the responses of ERG1 and ERG2 to NS1643, NS1643 can be used as a pharmacological tool to address ERG channel function. It may be useful for revealing physiological functions of ERG channels in neuronal tissue as well as to elucidate the structure-function relationships of the ERG channels.

A novel family of transmembrane proteins interacting with beta subunits of the Na,K-ATPase

We characterized a family consisting of four mammalian proteins of unknown function (NKAIN1, 2, 3 and 4) and a single Drosophila ortholog dNKAIN. Aside from highly conserved transmembrane domains, NKAIN proteins contain no characterized functional domains. Striking amino acid conservation in the first two transmembrane domains suggests that these proteins are likely to function within the membrane bilayer. NKAIN family members are neuronally expressed in multiple regions of the mouse brain, although their expression is not ubiquitous. We demonstrate that mouse NKAIN1 interacts with the beta1 subunit of the Na,K-ATPase, whereas Drosophila ortholog dNKAIN interacts with Nrv2.2, a Drosophila homolog of the Na,K-ATPase beta subunits. We also show that NKAIN1 can form a complex with another beta subunit-binding protein, MONaKA, when binding to the beta1 subunit of the Na,K-ATPase. Our results suggest that a complex between mammalian NKAIN1 and MONaKA is required for NKAIN function, which is carried out by a single protein, dNKAIN, in Drosophila. This hypothesis is supported by the fact that dNKAIN, but not NKAIN1, induces voltage-independent amiloride-insensitive Na(+)-specific conductance that can be blocked by lanthanum. Drosophila mutants with decreased dNKAIN expression due to a P-element insertion in the dNKAIN gene exhibit temperature-sensitive paralysis, a phenotype also caused by mutations in the Na,K-ATPase alpha subunit and several ion channels. The neuronal expression of NKAIN proteins, their membrane localization and the temperature-sensitive paralysis of NKAIN Drosophila mutants strongly suggest that this novel protein family may be critical for neuronal function.

Modulation of ERG Channels by XE991

In neuronal tissue, KCNQ2-5 channels conduct the physiologically important M-current. In some neurones, the M-current may in addition be conducted partly by ERG potassium channels, which have widely overlapping expression with the KCNQ channel subunits. XE991 and linopiridine are known to be standard KCNQ potassium channel blockers. These compounds have been used in many different tissues as specific pharmacological tools to discern native currents conducted by KCNQ channels from other potassium currents. In this article, we demonstrate that ERG1-2 channels are also reversibly inhibited by XE991 in the micromolar range (EC(50) 107 microM for ERG1). The effect has been characterized in Xenopus laevis oocytes expressing ERG1-2 and in the mammalian HEK293 cell line stably expressing ERG1 channels. The IC(50) values for block of KCNQ channels by XE991 range 1-65 microM. In conclusion, great care should be taken when choosing the concentration of XE991 to use for experiments on native potassium channels or animal studies in order to be able to conclude on selective KCNQ channel-mediated effects.

Probing the outer mouth structure of the HERG channel with peptide toxin footprinting and molecular modeling

Previous studies have shown that the unusually long S5-P linker lining human ether a-go-go related gene's (hERG's) outer vestibule is critical for its channel function: point mutations at high-impact positions here can interfere with the inactivation process and, in many cases, also reduce the pore's K+ selectivity. Because no data are available on the equivalent region in the available K channel crystal structures to allow for homology modeling, we used alternative approaches to model its three-dimensional structure. The first part of this article describes mutant cycle analysis used to identify residues on hERG's outer vestibule that interact with specific residues on the interaction surface of BeKm-1, a peptide toxin with known NMR structure and a high binding affinity to hERG. The second part describes molecular modeling of hERG's pore domain. The transmembrane region was modeled after the crystal structure of KvAP pore domain. The S5-P linker was docked to the transmembrane region based on data from previous NMR and mutagenesis experiments, as well as a set of modeling criteria. The models were further restrained by contact points between hERG's outer vestibule and the bound BeKm-1 toxin molecule deduced from the mutant cycle analysis. Based on these analyses, we propose a working model for the open conformation of the outer vestibule of the hERG channel, in which the S5-P linkers interact with the pore loops to influence ion flux through the pore.

Toxins interacting with ether-à-go-go-related gene voltage-dependent potassium channels

The critical role that ether-à-go-go-related gene (erg) K(+) channels play in mating in Caenorhabditis elegans, neuronal seizures in Drosophila and cardiac action potential repolarization in humans has been well documented. Three erg genes (erg1, erg2 and erg3) have been identified and characterized. A structurally diverse number of compounds block these channels, but do not display specificity among the different channel isoforms. In this review we describe the blocking properties of several peptides, purified from scorpion, sea anemone and spider venoms, which are selective for certain members of the ERG family of channels. These peptides do not behave as classical pore blockers and appear to modify the gating properties of the channel. Genomic studies predict the existence of many other novel peptides with the potential of being more selective for ERG channels than those discussed here.

Drosophila melanogaster Scramblases modulate synaptic transmission

Scramblases are a family of single-pass plasma membrane proteins, identified by their purported ability to scramble phospholipids across the two layers of plasma membrane isolated from platelets and red blood cells. However, their true in vivo role has yet to be elucidated. We report the generation and isolation of null mutants of two Scramblases identified in Drosophila melanogaster. We demonstrate that flies lacking either or both of these Scramblases are not compromised in vivo in processes requiring scrambling of phospholipids. Instead, we show that D. melanogaster lacking both Scramblases have more vesicles and display enhanced recruitment from a reserve pool of vesicles and increased neurotransmitter secretion at the larval neuromuscular synapses. These defects are corrected by the introduction of a genomic copy of the Scramb 1 gene. The lack of phenotypes related to failure of scrambling and the neurophysiological analysis lead us to propose that Scramblases play a modulatory role in the process of neurotransmission.

Interactions between charged residues in the transmembrane segments of the voltage-sensing domain in the hERG channel

Studies on voltage-gated K channels such as Shaker have shown that positive charges in the voltage-sensor (S4) can form salt bridges with negative charges in the surrounding transmembrane segments in a state-dependent manner, and different charge pairings can stabilize the channels in closed or open states. The goal of this study is to identify such charge interactions in the hERG channel. This knowledge can provide constraints on the spatial relationship among transmembrane segments in the channel's voltage-sensing domain, which are necessary for modeling its structure. We first study the effects of reversing S4's positive charges on channel activation. Reversing positive charges at the outer (K525D) and inner (K538D) ends of S4 markedly accelerates hERG activation, whereas reversing the 4 positive charges in between either has no effect or slows activation. We then use the 'mutant cycle analysis' to test whether D456 (outer end of S2) and D411 (inner end of S1) can pair with K525 and K538, respectively. Other positive charges predicted to be able, or unable, to interact with D456 or D411 are also included in the analysis. The results are consistent with predictions based on the distribution of these charged residues, and confirm that there is functional coupling between D456 and K525 and between D411 and K538.

Expression pattern of the ether-a-go-go-related (ERG) family proteins in the adult mouse central nervous system: evidence for coassembly of different subunits

Voltage-dependent K+ channels are the main determinants in controlling cellular excitability within the central nervous system. Among voltage-dependent K+ channels, the ERG subfamily is deeply involved in the control of cellular excitability, both in mammals and in invertebrates. ERG channels are encoded by different genes: the erg1 gene, which can generate two alternative transcripts (erg1a and erg1b), erg2 and erg3. The aim of the present study was to determine the expression pattern and cellular localization of ERG proteins (ERG1, ERG2, and ERG3) in the mouse CNS, differentiating, for the first time, the ERG1A and ERG1B isoforms. To this purpose, novel specific antibodies were raised against the various channel proteins and their specificity and immunoreactivity tested. It emerged that: 1) all the erg genes were indeed translated in neuronal tissue; 2) ERG proteins distribution in the mouse CNS often overlapped, and only in specific areas each ERG protein showed a distinct pattern of expression; and 3) ERG proteins were generally expressed in neuronal soma, but dendritic and/or white matter labeling could be detected in specific areas. The finding that ERG proteins often have an overlapping expression suggests that neuronal ERG currents could be determined, at least in part, by heterotetrameric ERG channels. This suggestion is demonstrated to occur for ERG1A/ERG1B by showing that the two isoforms coassemble in mouse brain.

Postmortem molecular screening in unexplained sudden death

OBJECTIVES

We examined the prevalence of defects in arrhythmia-related candidate genes among patients with unexplained sudden cardiac death (SCD).

BACKGROUND

Patients with unexplained sudden death may constitute up to 5% of overall SCD cases. For such patients, systematic postmortem genetic analysis of archived tissue, using a candidate gene approach, may identify etiologies of SCD.

METHODS

We performed analysis of KCNQ1 (KVLQT1), KCNH2 (HERG), SCN5A, KCNE1, and KCNE2 defects in a subgroup of 12 adult subjects with unexplained sudden death, derived from a 13-year, 270-patient autopsy series of SCD. Archived, paraffin-embedded myocardial tissue blocks obtained at the original postmortem examination were the source of deoxyribonucleic acid for genetic analysis.

RESULTS

Two patients were found to have the same HERG defect, a missense mutation in exon 7 (nucleotide change G1681A, coding effect A561T). The mutation was heterozygous in Patient 1, but Patient 2 appeared to be homozygous for the defect. Patch-clamp recordings showed that the A561T mutant channel expressed in human embryonic kidney cells failed to generate HERG current. Western blot analysis implicated a trafficking defect in the protein, resulting in loss of post-translational processing from the immature to the mature form of HERG. No mutations were detected among the remaining four candidate genes.

CONCLUSIONS

In this autopsy series, only 2 of 12 patients with unexplained sudden death were observed to have a defect in HERG among five candidate genes tested. It is likely that elucidation of SCD mechanisms in such patients will await the discovery of multiple, novel arrhythmia-causing gene defects.

Autosomal recessive idiopathic epilepsy in an inbred family from Turkey: identification of a putative locus on chromosome 9q32-33

PURPOSE

The study describes the clinical features of an inbred family from Turkey with three members affected by seizures and tests possible autosomal recessive (AR) inheritance by means of linkage analysis.

METHODS

Personal and family history was obtained from each subject, and general physical, neurologic, and EEG examinations were performed. A set of 382 fluorescence-labeled markers was used for the initial genome-wide search. A further set of 83 markers was used to map the locus precisely and to exclude the remaining genome.

RESULTS

Twelve individuals from three generations were examined. Two subjects were affected by idiopathic epilepsy, whereas, their brother experienced a single unprovoked generalized seizure. Two siblings affected by idiopathic epilepsy and their unaffected sister showed a photoparoxysmal response to photic stimulation. Nine family members reported migraine. The genome-wide search led to the identification of a unique homozygous, 15.1-cM region shared by subjects with seizures on chromosome 9q32-33 and providing a lod score of 2.9. This locus, however, was not associated with migraine in this pedigree.

CONCLUSIONS

The study suggests that idiopathic epileptic traits with AR inheritance might be underestimated in the general population and that inbred pedigrees may represent powerful tools for the identification of AR genes.

Expression pattern of the ether-a-gogo-related (ERG) K+ channel-encoding genes ERG1, ERG2, and ERG3 in the adult rat central nervous system

Voltage-dependent K(+) channels play a pivotal role in controlling cellular excitability within the nervous system. The aim of the present study was to investigate the expression in the adult rat brain of the three ether-a-gogo-related gene (ERG) family members ERG1, ERG2, and ERG3, encoding for K(+) channel subunits. To this aim, the distribution of ERG transcripts was studied by means of reverse-transcription polymerase chain reaction (RT-PCR) and nonradioactive in situ hybridization histochemistry (NR-ISH). Furthermore, ERG1 subunit distribution was studied by immunohistochemical analysis. RT-PCR analysis revealed ERG1, ERG2, and ERG3 expression in the olfactory bulb, cerebral cortex, hippocampus, hypothalamus, and cerebellum. NR-ISH experiments detected transcripts encoded by all three ERG genes in the cerebral cortex and in all CA subfields and in the granular cell layer of the dentate gyrus of the hippocampus; strong ERG1 signals were also detected in scattered large elements throughout the oriens, pyramidal, and radiatum layers, and in the hilus of the dentate gyrus. In the thalamus, positively labeled neurons were detected in the reticular nucleus with ERG1 and ERG3 and in the anterodorsal nucleus with ERG2 riboprobes. Transcripts for ERG1 and, to a lesser degree, also for ERG3, were detected in the basal ganglia and in several brainstem nuclei. All three ERG genes appeared to be expressed in cerebellar Purkinje cells. Finally, ERG1 expression was also revealed in non-neuronal elements such as ependymal and subependymal cells along the ventricular walls and hippocampal astrocytes. These results suggest that the K(+) channel isoforms of the ERG family appear to be expressed in different central nervous system regions where they might differentially control the firing of neurons engaged in several networks.

Caenorhabditis elegans UNC-103 ERG-like potassium channel regulates contractile behaviors of sex muscles in males before and during mating

During mating behavior the Caenorhabditis elegans male must regulate periodic and prolonged protractor muscle contractions to insert his copulatory spicules into his mate. The protractors undergo periodic contractions to allow the spicules to reattempt insertion if a previous thrust failed to breach the vulva. When the spicule tips penetrate the vulva, the protractors undergo prolonged contraction to keep the spicules inside the hermaphrodite until sperm transfer is complete. To understand how these contractions are regulated, we isolated EMS-induced mutations that cause males to execute prolonged contraction inappropriately. Loss-of-function mutations in the unc-103 ERG-like K(+) channel gene cause the protractor muscles to contract in the absence of mating stimulation. unc-103-induced spicule protraction can be suppressed by killing the SPC motor neurons and the anal depressor muscle: cells that directly contact the protractors. Also, reduction in acetylcholine suppresses unc-103-induced protraction, suggesting that UNC-103 keeps cholinergic neurons from stimulating the protractors before mating behavior. UNC-103 also regulates the timing of spicule protraction during mating behavior. unc-103 males that do not display mating-independent spicule protraction show abnormal spicule insertion behavior during sex. In contrast to wild-type males, unc-103 mutants execute prolonged contractions spontaneously within sequences of periodic protractor contractions. The premature prolonged contractions cause the spicules to extend from the male tail before the spicule tips penetrate the vulva. These observations demonstrate that unc-103 controls various aspects of spicule function.

Neural dysfunction and neurodegeneration in Drosophila Na+/K+ ATPase alpha subunit mutants

The Na+/K+ ATPase asymmetrically distributes sodium and potassium ions across the plasma membrane to generate and maintain the membrane potential in many cell types. Although these pumps have been hypothesized to be involved in various human neurological disorders, including seizures and neurodegeneration, direct genetic evidence has been lacking. Here, we describe novel mutations in the Drosophila gene encoding the alpha (catalytic) subunit of the Na+/K+ ATPase that lead to behavioral abnormalities, reduced life span, and severe neuronal hyperexcitability. These phenotypes parallel the occurrence of extensive, age-dependent neurodegeneration. We have also discovered that the ATPalpha transcripts undergo alternative splicing that substantially increases the diversity of potential proteins. Our data show that maintenance of neuronal viability is dependent on normal sodium pump activity and establish Drosophila as a useful model for investigating the role of the pump in human neurodegenerative and seizure disorders.

The functional properties of the human ether-à-go-go-like (HELK2) K+ channel

The voltage-dependent K+ channels belonging to the ether-à-go-go family (eag, erg, elk) are widely expressed in the mammalian CNS. Their neuronal function, however, is poorly understood. Among the elk clones, elk2 is the most abundantly expressed in the brain. We have characterized the human ELK2 channel (HELK2) expressed in mammalian cell lines. Moreover, we have detected helk2 mRNA and ELK2-like currents in freshly dissociated human astrocytoma cells. HELK2 was inhibited by Cs+ in a voltage-dependent way (Kd was 0.7 mm, at -120 mV). It was not affected by Way 123398 (5 micro m), dofetilide (10 micro m), quinidine (10 micro m), verapamil (20 micro m), haloperidol (2 micro m), astemizole (1 micro m), terfenadine (1 micro m) and hydroxyzine (30 micro m), compounds known to inhibit the biophysically related HERG channel. The crossover of the activation and inactivation curves produced a steady state 'window' current with a peak around -20 mV and considerably broader than it usually is in voltage-dependent channels, including HERG. Similar features were observed in the ELK2 clone from rat, in the same experimental conditions. Thus, ELK2 channels are active within a wide range of membrane potentials, both sub- and suprathreshold. Moreover, the kinetics of channel deactivation and removal of inactivation was about one order of magnitude quicker in HELK2, compared to HERG. Overall, these properties suggest that ELK2 channels are very effective at dampening the neuronal excitability, but less so at producing adaptation of action potential firing frequency. In addition, we suggest experimental ways to recognize HELK2 currents in vivo and raise the issue of the possible function of these channels in astrocytoma.

Regulation of an ERG K+ current by Src tyrosine kinase

The human "ether-a-go-go"-related gene (HERG) K(+) channel, and its homologues are present in heart, neuronal tissue, some cancer cells, and the MLS-9 rat microglia cell line (Zhou, W., Cayabyab, F. S., Pennefather, P. S., Schlichter, L. C., and DeCoursey, T. E. (1998) J. Gen. Physiol. 111, 781-794). Despite its importance, there are few studies of ERG modulation. In this first report of regulation by tyrosine phosphorylation we show that MLS-9 cells express transcripts for r-erg1 (rat homologue of HERG) and r-erg2, and an immunoreactive doublet was identified using an anti-HERG antibody. The constitutive tyrosine phosphorylation of the ERG1 protein, detected by co-immunoprecipitation, was reduced by the protein-tyrosine kinase inhibitors, lavendustin A, herbimycin A, or genistein (but not daidzein). The whole cell ERG current was reduced by protein-tyrosine kinase inhibitors or the Src-selective inhibitory peptide, src40-58, but not by a scrambled peptide. Conversely, the current was increased by the Src-activating peptide, srcpY, but not by an inactive analogue. Activating endogenous Src or transfecting constitutively active v-Src altered the voltage dependence and deactivation kinetics to produce more current at negative potentials. Co-immunoprecipitation identified an association between the channel protein and Src. Thus, r-ERG1 and Src tyrosine kinase appear to exist in a signaling complex that is well positioned to modulate this K(+) channel and affect its contribution to cellular functions.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

A novel extracellular calcium sensing mechanism in voltage-gated potassium ion channels

Potassium (K(+)) channels influence neurotransmitter release, burst firing rate activity, pacing, and critical dampening of neuronal circuits. Internal and external factors that further modify K(+) channel function permit fine-tuning of neuronal circuits. Human ether-à-go-go-related gene (HERG) K(+) channels are unusually sensitive to external calcium concentration ([Ca(2+)](o)). Small changes in [Ca(2+)](o) shift the voltage dependence of channel activation to more positive membrane potentials, an effect that cannot be explained by nonspecific surface charge screening or channel pore block. The HERG-calcium concentration-response relationship spans the physiological range for [Ca(2+)](o). The modulatory actions of calcium are attributable to differences in the Ca(2+) affinity between rested and activated channels. Adjacent extracellular, negatively charged amino acids (E518 and E519) near the S4 voltage sensor influence both channel gating and Ca(2+) dependence. Neutralization of these charges had distinct effects on channel gating and calcium sensitivity. A change in the degree of energetic coupling between these amino acids on transition from closed to activated channel states reveals movement in this region during channel gating and defines a molecular mechanism for protein state-dependent ligand interactions. The results suggest a novel extracellular [Ca(2+)](o) sensing mechanism coupled to allosteric changes in channel gating and a mechanism for fine-tuning cell repolarization.

Differential effects of amino-terminal distal and proximal domains in the regulation of human erg K(+) channel gating

The participation of amino-terminal domains in human ether-a-go-go (eag)-related gene (HERG) K(+) channel gating was studied using deleted channel variants expressed in Xenopus oocytes. Selective deletion of the HERG-specific sequence (HERG Delta138-373) located between the conserved initial amino terminus (the eag or PAS domain) and the first transmembrane helix accelerates channel activation and shifts its voltage dependence to hyperpolarized values. However, deactivation time constants from fully activated states and channel inactivation remain almost unaltered after the deletion. The deletion effects are equally manifested in channel variants lacking inactivation. The characteristics of constructs lacking only about half of the HERG-specific domain (Delta223-373) or a short stretch of 19 residues (Delta355-373) suggest that the role of this domain is not related exclusively to its length, but also to the presence of specific sequences near the channel core. Deletion-induced effects are partially reversed by the additional elimination of the eag domain. Thus the particular combination of HERG-specific and eag domains determines two important HERG features: the slow activation essential for neuronal spike-frequency adaptation and maintenance of the cardiac action potential plateau, and the slow deactivation contributing to HERG inward rectification.

Genetic analysis of ion channel dysfunction in Drosophila

To dissect the molecular mechanisms of electrical activity in the nervous system, an extensive collection of mutations affecting various types of voltage-gated ion channels was identified and characterized in Drosophila. Most of these mutations were generated by chemical mutagenesis and were recognized on the basis of defects in motor behavior. These were the first genetically determined ion channelopathies to be characterized in any multicellular organism. Drosophila is a particularly attractive model system for such studies because of the availability of powerful genetic, electrophysiological, and molecular techniques for generating new mutations, characterizing their phenotypes, and cloning the genes thus defined. Consequently, a number of ion channels, including various types of K+ channels that had not yielded previously to biochemical approaches, were first identified via a genetic strategy in Drosophila. Evolutionary conservation of these genes enabled subsequent isolation of the corresponding genes from various mammals, including humans. Several of these human homologues have been found to be associated with heritable neuromuscular disorders. Studies of ion channel mutations in Drosophila have thus provided important biological information concerning the molecular and functional diversity of ion channels, their evolutionary relationships, and their in vivo functions in the nervous system. Similar studies of additional new mutations should now facilitate the analysis of ion channel regulatory mechanisms.

Two types of K(+) channel subunit, Erg1 and KCNQ2/3, contribute to the M-like current in a mammalian neuronal cell

The potassium M current was originally identified in sympathetic ganglion cells, and analogous currents have been reported in some central neurons and also in some neural cell lines. It has recently been suggested that the M channel in sympathetic neurons comprises a heteromultimer of KCNQ2 and KCNQ3 (Wang et al., 1998) but it is unclear whether all other M-like currents are generated by these channels. Here we report that the M-like current previously described in NG108-15 mouse neuroblastoma x rat glioma cells has two components, "fast" and "slow", that may be differentiated kinetically and pharmacologically. We provide evidence from PCR analysis and expression studies to indicate that these two components are mediated by two distinct molecular species of K(+) channel: the fast component resembles that in sympathetic ganglia and is probably carried by KCNQ2/3 channels, whereas the slow component appears to be carried by merg1a channels. Thus, the channels generating M-like currents in different cells may be heterogeneous in molecular composition.

New ether-à-go-go K(+) channel family members localized in human telencephalon

A cDNA encoding a novel voltage-gated K(+) channel protein was isolated from human brain. This protein, termed BEC1, is 46% identical to rat elk in the ether-à-go-go K(+) channel family. The BEC1 gene maps to the 12q13 region of the human genome. Northern blot analysis indicates that BEC1 is exclusively expressed in human brain, where the expression is concentrated in the telencephalic areas such as the cerebral cortex, amygdala, hippocampus, and striatum. By in situ hybridization, BEC1 is detected in the CA1-CA3 pyramidal cell layers and the dentate gyrus granule cell layers of the hippocampus. Specific signals are also found in neocortical neurons. Transfection of mammalian L929 and Chinese hamster ovary cells with BEC1 cDNA induces a voltage-gated outward current with a fast inactivation component. This current is insensitive to tetraethylammonium and quinidine. Additionally, a second related gene BEC2 was isolated from human brain. BEC2 is also brain-specific, located in the neocortex and the striatum, and functional as a channel gene. Phylogenetic analysis indicates that BEC1 and BEC2 constitute a subfamily, together with elk, in the ether-à-go-go family. The two genes may be involved in cellular excitability of restricted neurons in the human central nervous system.

Subfamily-specific posttranscriptional mechanism underlies K(+) channel expression in a developing neuronal blastomere

Na(+) and K(+) channels are the two key proteins that shape the action potentials in neurons. However, little is known about how the expression of these two channels is coordinated. To address this issue, we cloned a Shab-related K(+) channel gene from ascidian Halocynthia roretzi (TuKv2). In this animal, a blastomere of neuronal lineage isolated from the 8-cell embryo expresses single Na(+) channel and K(+) channel genes after neural induction. Expression of a dominant negative form of TuKv2 eliminated the native delayed rectifier K(+) currents, indicating that the entire delayed rectifier K(+) current of the neuronal blastomere is exclusively encoded by TuKv2. TuKv2 transcripts are expressed more broadly than Na(+) channel transcripts, which are restricted to the neuronal lineages. There is also a temporal mismatch in the expression of TuKv2 transcript and the K(+) current; TuKv2 transcripts are present throughout development, whereas delayed rectifier K(+) currents only appear after the tailbud stage, suggesting that the functional expression of the TuKv2 transcript is suppressed during the early embryonic stages. To test if this suppression occurs by a mechanism specific to the TuKv2 channel protein, an ascidian Shaker-related gene, TuKv1, was misexpressed in neural blastomeres. A TuKv1-encoded current was expressed earlier than the TuKv2 current. Furthermore, the introduction of the TuKv2-expressing plasmid into noninduced cells did not lead to the current expression. These results raise the possibility that the expression of TuKv2 is post-transcriptionally controlled through a mechanism that is dependent on neural induction.

Unmasking of a novel potassium current in Drosophila by a mutation and drugs

The delayed rectifier potassium current plays a critical role in cellular physiology. This current (I(K)) in Drosophila larvae is believed to be a single current. However, a likely null mutation in the Shab K(+) channel gene (Shab(3)) reduces I(K) but does not eliminate it. This raises a question as to whether or not the entire I(K) passes through channels encoded by one gene. Similarly, an incomplete blockade of I(K) by high concentrations of quinidine, a selective I(K) blocker, raises a question as to whether I(K) consists of two components that are differentially sensitive to quinidine. We have addressed these questions by a combined use of genetics, pharmacology, and physiology. The current component removed by the Shab(3) mutation differed from the remaining component in activation kinetics, inactivation kinetics, threshold of activation, and voltage dependence. The two components showed strong differences in sensitivity to quinidine. Physiological properties of the current component removed by the Shab(3) mutation were similar to those of the quinidine-sensitive fraction of I(K). Complementary to this, properties of the current component remaining in the Shab(3) mutant muscles were similar to those of the quinidine-resistant fraction of I(K). These observations strongly suggest that, in contrast to the current belief, I(K) consists of two components in Drosophila, which are genetically, pharmacologically, and physiologically distinct. These components are being called I(KS) and I(KF). I(KS) is carried via Shab-encoded channels. I(KF) defines a new voltage-activated K(+) current in Drosophila.

The eag family of K+ channels in Drosophila and mammals

Mutations of eag, first identified in Drosophila on the basis of their leg-shaking phenotype, cause repetitive firing and enhanced transmitter release in motor neurons. The encoded EAG polypeptide is related both to voltage-gated K+ channels and to cyclic nucleotide-gated cation channels. Homology screens identified a family of eag-related channel polypeptides, highly conserved from nematodes to humans, comprising three subfamilies: EAG, ELK, and ERG. When expressed in frog oocytes, EAG channels behave as voltage-dependent, outwardly rectifying K(+)-selective channels. Mutations of the human eag-related gene (HERG) result in a form of cardiac arrhythmia that can lead to ventricular fibrillation and sudden death. Electrophysiological and pharmacological studies have provided evidence that HERG channels specify one component of the delayed rectifier, IKr, that contributes to the repolarization phase of cardiac action potentials. An important role for HERG channels in neuronal excitability is also suggested by the expression of these channels in brain tissue. Moreover, mutations of ERG-type channels in the Drosophila sei mutant cause temperature-induced convulsive seizures associated with aberrant bursting activity in the flight motor pathway. The in vivo function of ELK channels has not yet been established, but when these channels are expressed in frog oocytes, they display properties intermediate between those of EAG- and ERG-type channels. Coexpression of the K(+)-channel beta subunit encoded by Hk with EAG in oocytes dramatically increases current amplitude and also affects the gating and modulation of these currents. Biochemical evidence indicates a direct physical interaction between EAG and HK proteins. Overall, these studies highlight the diverse properties of the eag family of K+ channels, which are likely to subserve diverse functions in vivo.

Functional role of the slow activation property of ERG K+ channels

ERG (ether-à-go-go-related gene) K+ channels are crucial in human heart physiology (h-ERG), but are also found in neuronal cells and are impaired in Drosophila 'seizure' mutants. Their biophysical properties include the relatively fast kinetics of the inactivation gate and much slower kinetics of the activation gate. In order to elucidate how the complex time- and voltage-dependent activation properties of ERG channels underlies distinct roles in excitability, we investigated different types of ERG channels intrinsically present in cells or heterologously expressed in mammalian cells or Xenopus oocytes. Voltage-dependent activation curves were highly dependent on the features of the eliciting protocols. Only very long preconditioning times produced true steady-state relationships, a fact that has been largely neglected in the past, hampering the comparison of published data on ERG channels. Beyond this technical aspect, the slow activation property of ERG can be responsible for unsuspected physiological roles. We found that around the midpoint of the activation curve, the time constant of ERG open-close kinetics is of the order of 10-15 s. During sustained trains of depolarizations, e.g. those produced in neuronal firing, this leads to the use-dependent accumulation of open-state ERG channels. Accumulation is not observed in a mutant with a fast activation gate. In conclusion, it is well established that other K+ channels (i.e. Ca2+-activated and M) control the spike-frequency adaptation, but our results support the notion that the purely voltage-dependent activation property of ERG channels would allow a slow inhibitory physiological role in rapid neuronal signalling.

Cloning and functional expression of rat ether-à-go-go-like K+ channel genes

1. Screening of rat cortex cDNA resulted in cloning of two complete and one partial orthologue of the Drosophila ether-à-go-go-like K+ channel (elk). 2. Northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed predominant expression of rat elk mRNAs in brain. Each rat elk mRNA showed a distinct, but overlapping expression pattern in different rat brain areas. 3. Transient transfection of Chinese hamster ovary (CHO) cells with rat elk1 or rat elk2 cDNA gave rise to voltage-activated K+ channels with novel properties. 4. RELK1 channels mediated slowly activating sustained potassium currents. The threshold for activation was at -90 mV. Currents were insensitive to tetraethylammonium (TEA) and 4-aminopyridine (4-AP), but were blocked by micromolar concentrations of Ba2+. RELK1 activation kinetics were not dependent on prepulse potential like REAG-mediated currents. 5. RELK2 channels produced currents with a fast inactivation component and HERG-like tail currents. RELK2 currents were not sensitive to the HERG channel blocker E4031.

A Ca2+/calmodulin-dependent protein kinase modulates Drosophila photoreceptor K+ currents: a role in shaping the photoreceptor potential

Light activation of Drosophila photoreceptors leads to the generation of a depolarizing receptor potential via opening of transient receptor potential and transient receptor potential-like cationic channels. Counteracting the light-activated depolarizing current are two voltage-gated K+ conductances, IA and IK, that are expressed in these sensory neurons. Here we show that Drosophila photoreceptors IA and IK are regulated by calcium-calmodulin (Ca2+/calmodulin) via a Ca2+/calmodulin-dependent protein kinase (CaM kinase), with IK being far more sensitive than IA. Inhibition of Ca2+/calmodulin by N-(6 aminohexyl)-5-chloro-1-naphthalenesulfonamide or trifluoperazine markedly reduced the K+ current amplitudes. Likewise, inhibition of CaM kinases by KN-93 potently depressed IK and accelerated its C-type inactivation kinetics. The effect of KN-93 was specific because its structurally related but functionally inactive analog KN-92 was totally ineffective. In Drosophila photoreceptor mutant ShKS133, which allows isolation of IK, we demonstrate by current-clamp recording that inhibition of IK by quinidine or tetraethylammonium increased the amplitude of the photoreceptor potential, depressed light adaptation, and slowed down the termination of the light response. Similar results were obtained when CaM kinases were blocked by KN-93. These findings place photoreceptor K+ channels as an additional target for Ca2+/calmodulin and suggest that IK is well suited to act in concert with other components of the signaling machinery to sharpen light response termination and fine tune photoreceptor sensitivity during light adaptation.

Cloning of a mammalian elk potassium channel gene and EAG mRNA distribution in rat sympathetic ganglia

1. Three new members of the EAG potassium channel gene family were identified in rat and the complete coding sequence of one of these genes (elk1) was determined by cDNA cloning. 2. The elk1 gene, when expressed in Xenopus oocytes, encodes a slowly activating and slowly deactivating potassium channel. 3. The elk1 gene is expressed in sympathetic ganglia and is also expressed in sciatic nerve. 4. Six of the seven known EAG genes were found to be expressed in rat sympathetic ganglia, suggesting an important functional role for these channels in the sympathetic nervous system.

Human ether-a-gogo related gene (HERG) K+ channels as pharmacological targets: present and future implications

Electrophysiological and molecular biology techniques have widely expanded our knowledge of the diverse functions where K+ channels are implicated as potential and proven pharmacological targets. The aim of the present commentary is to review the recent progress in the understanding of the functional role of the K+ channels encoded by the human ether-a-gogo related gene (HERG), with particular emphasis on their direct pharmacological modulation by drugs, or on their regulation by pharmacologically relevant phenomena. About 3 years have passed since the cloning, expression, and description of the pathophysiological role of HERG K+ channels in human cardiac repolarization. Despite this short lapse of time, these K+ channels have already gained considerable attention as pharmacological targets. In fact, interference with HERG K+ channels seems to be the main mechanism explaining both the therapeutic actions of the class III antiarrhythmics and the potential cardiotoxicity of second-generation H1 receptor antagonists such as terfenadine and astemizole, as well as of psychotropic drugs such as some antidepressants and neuroleptics. It seems possible to anticipate that the main tasks for future investigation will be, on the one side, the better understanding of the intimate mechanism of action of HERG K+ channel-blocking drugs in order to elucidate the conditions regulating the delicate balance between antiarrhythmic and proarrhythmic potential and, on the other, to unravel the pathophysiological role of this K+ channel in the function of the brain and of other excitable tissues.

Genetic dissection of functional contributions of specific potassium channel subunits in habituation of an escape circuit in Drosophila

Potassium channels have been implicated in central roles in activity-dependent neural plasticity. The giant fiber escape pathway of Drosophila has been established as a model for analyzing habituation and its modification by memory mutations in an identified circuit. Several genes in Drosophila encoding K+ channel subunits have been characterized, permitting examination of the contributions of specific channel subunits to simple conditioning in an identified circuit that is amenable to genetic analysis. Our results show that mutations altering each of four K+ channel subunits (Sh, slo, eag, and Hk) have distinct effects on habituation at least as strong as those of dunce and rutabaga, memory mutants with defective cAMP metabolism (). Habituation, spontaneous recovery, and dishabituation of the electrically stimulated long-latency giant fiber pathway response were shown in each mutant type. Mutations of Sh (voltage-gated) and slo (Ca2+-gated) subunits enhanced and slowed habituation, respectively. However, mutations of eag and Hk subunits, which confer K+-current modulation, had even more extreme phenotypes, again enhancing and slowing habituation, respectively. In double mutants, Sh mutations moderated the strong phenotypes of eag and Hk, suggesting that their modulatory functions are best expressed in the presence of intact Sh subunits. Nonactivity-dependent responses (refractory period and latency) at two stages of the circuit were altered only in some mutants and do not account for modifications of habituation. Furthermore, failures of the long-latency response during habituation, which normally occur in labile connections in the brain, could be induced in the thoracic circuit stage in Hk mutants. Our work indicates that different K+ channel subunits play distinct roles in activity-dependent neural plasticity and thus can be incorporated along with second messenger "memory" loci to enrich the genetic analysis of learning and memory.

Interaction of the K channel beta subunit, Hyperkinetic, with eag family members

Assembly of K channel alpha subunits of the Shaker (Sh) family occurs in a subfamily specific manner. It has been suggested that subfamily specificity also applies in the association of beta subunits with Sh channels (Rhodes, K. J., Keilbaugh, S. A., Barrezueta, N. X., Lopez, K. L., and Trimmer, J. S. (1995) J. Neurosci. 15, 5360-5371; Sewing, S., Roeper, J. and Pongs, O. (1996) Neuron 16, 455-463; Yu, W., Xu, J., and Li, M. (1996) Neuron 16, 441-453). Here we show that the Drosophila beta subunit homologue Hyperkinetic (Hk) associates with members of the ether go-go (eag), as well as Sh, families. Anti-EAG antibody coprecipitates EAG and HK indicating a physical association between proteins. Heterologously expressed Hk dramatically increases the amplitudes of eag currents and also affects gating and modulation by progesterone. Through their ability to interact with a range of alpha subunits, the beta subunits of voltage-gated K channels are likely to have a much broader impact on the signaling properties of neurons and muscle fibers than previously suggested.