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

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 ERG1 K+ Channel and Its Role in Neuronal Health and Disease

The ERG1 potassium channel, encoded by KCNH2, has long been associated with cardiac electrical excitability. Yet, a growing body of work suggests that ERG1 mediates physiology throughout the human body, including the brain. ERG1 is a regulator of neuronal excitability, ERG1 variants are associated with neuronal diseases (e.g., epilepsy and schizophrenia), and ERG1 serves as a potential therapeutic target for neuronal pathophysiology. This review summarizes the current state-of-the-field regarding the ERG1 channel structure and function, ERG1’s relationship to the mammalian brain and highlights key questions that have yet to be answered.

Fly seizure EEG: field potential activity in the Drosophila brain

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

Prolonged Exposure to Microgravity Reduces Cardiac Contractility and Initiates Remodeling in Drosophila

Understanding the effects of microgravity on human organs is crucial to exploration of low-earth orbit, the moon, and beyond. Drosophila can be sent to space in large numbers to examine the effects of microgravity on heart structure and function, which is fundamentally conserved from flies to humans. Flies reared in microgravity exhibit cardiac constriction with myofibrillar remodeling and diminished output. RNA sequencing (RNA-seq) in isolated hearts revealed reduced expression of sarcomeric/extracellular matrix (ECM) genes and dramatically increased proteasomal gene expression, consistent with the observed compromised, smaller hearts and suggesting abnormal proteostasis. This was examined further on a second flight in which we found dramatically elevated proteasome aggregates co-localizing with increased amyloid and polyQ deposits. Remarkably, in long-QT causing sei/hERG mutants, proteasomal gene expression at 1g, although less than the wild-type expression, was nevertheless increased in microgravity. Therefore, cardiac remodeling and proteostatic stress may be a fundamental response of heart muscle to microgravity.

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.

Pyridox (am) ine 5'-phosphate oxidase deficiency induces seizures in Drosophila melanogaster

Pyridox (am) ine 5'-phosphate oxidase (PNPO) is a rate-limiting enzyme in converting dietary vitamin B6 (VB6) to pyridoxal 5'-phosphate (PLP), the biologically active form of VB6 and involved in the synthesis of neurotransmitters including γ-aminobutyric acid (GABA), dopamine, and serotonin. In humans, PNPO mutations have been increasingly identified in neonatal epileptic encephalopathy and more recently also in early-onset epilepsy. Till now, little is known about the neurobiological mechanisms underlying PNPO-deficiency-induced seizures due to the lack of animal models. Previously, we identified a c.95 C>A missense mutation in sugarlethal (sgll)-the Drosophila homolog of human PNPO (hPNPO)-and found mutant (sgll95) flies exhibiting a lethal phenotype on a diet devoid of VB6. Here, we report the establishment of both sgll95 and ubiquitous sgll knockdown (KD) flies as valid animal models of PNPO-deficiency-induced epilepsy. Both sgll95 and sgll KD flies exhibit spontaneous seizures before they die. Electrophysiological recordings reveal that seizures caused by PNPO deficiency have characteristics similar to that in flies treated with the GABA antagonist picrotoxin. Both seizures and lethality are associated with low PLP levels and can be rescued by ubiquitous expression of wild-type sgll or hPNPO, suggesting the functional conservation of the PNPO enzyme between humans and flies. Results from cell type-specific sgll KD further demonstrate that PNPO in the brain is necessary for seizure prevention and survival. Our establishment of the first animal model of PNPO deficiency will lead to better understanding of VB6 biology, the PNPO gene and its mutations discovered in patients, and can be a cost-effective system to test therapeutic strategies.

Increased Smooth Muscle Kv11.1 Channel Expression in Pulmonary Hypertension and Protective Role of Kv11.1 Channel Blocker Dofetilide

Kv11.1 potassium channels are essential for heart repolarization. Prescription medication that blocks Kv11.1 channels lengthens the ventricular action potential and causes cardiac arrhythmias. Surprisingly little is known about the Kv11.1 channel expression and function in the lung tissue. Here we report that Kv11.1 channels were abundantly expressed in the large pulmonary arteries (PAs) of healthy lung tissues from humans and rats. Kv11.1 channel expression was increased in the lungs of humans affected by chronic obstructive pulmonary disease-associated pulmonary hypertension and in the lungs of rats with pulmonary arterial hypertension (PAH). In healthy lung tissues from humans and rats, Kv11.1 channels were confined to the large PAs. In humans with chronic obstructive pulmonary disease-associated pulmonary hypertension and in rats with PAH, Kv11.1 channels were expressed in both the large and small PAs. The increase in Kv11.1 channel expression closely followed the time-course of the development of pulmonary vascular remodeling in PAH rats. Treatment of PAH rats with dofetilide, an Kv11.1 channel blocker approved by the US Food and Drug Administration for use in the treatment of arrythmia, inhibited PAH-associated pulmonary vascular remodeling. Taken together, the findings from this study uncovered a novel role of Kv11.1 channels in lung function and their potential as new drug targets in the treatment of pulmonary hypertension. The protective effect of dofetilide raises the possibility of repurposing this antiarrhythmic drug for the treatment of patients with pulmonary hypertension.

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.

Preclinical study of a Kv11.1 potassium channel activator as antineoplastic approach for breast cancer

Potassium ion (K⁺) channels have been recently found to play a critical role in cancer biology. Despite that pharmacologic manipulation of ion channels is recognized as an important therapeutic approach, very little is known about the effects of targeting of K⁺ channels in cancer. In this study, we demonstrate that use of the Kv11.1 K⁺ channel activator NS1643 inhibits tumor growth in an in vivo model of breast cancer.

Tumors exposed to NS1643 had reduced levels of proliferation markers, high expression levels of senescence markers, increased production of ROS and DNA damage compared to tumors of untreated mice. Importantly, mice treated with NS1643 did not exhibit significant cardiac dysfunction. In conclusion, pharmacological stimulation of Kv11.1 activity produced arrested TNBC-derived tumor growth by generating DNA damage and senescence without significant side effects. We propose that use of Kv11.1 channels activators could be considered as a possible pharmacological strategy against breast tumors.

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, we show here 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. We conclude 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.

We have used the fruit fly cardiac model to show that seizure, the fly homolog of the human ether a go-go K⁺ channel hERG, is functional in the fly heart. This channel plays a major role in cardiac repolarization in humans but not in adult rodent hearts. Loss of channel function in the fly causes bradycardia, electrical arrhythmia and altered myofibrillar structure. Gene expression analysis indicates that Wnt signaling is affected and we show a genetic interaction between sei and pygopus, a Wnt pathway component, on heart function.

Drosophila in the Heart of Understanding Cardiac Diseases: Modeling Channelopathies and Cardiomyopathies in the Fruitfly

Cardiovascular diseases and, among them, channelopathies and cardiomyopathies are a major cause of death worldwide. The molecular and genetic defects underlying these cardiac disorders are complex, leading to a large range of structural and functional heart phenotypes. Identification of molecular and functional mechanisms disrupted by mutations causing channelopathies and cardiomyopathies is essential to understanding the link between an altered gene and clinical phenotype. The development of animal models has been proven to be efficient for functional studies in channelopathies and cardiomyopathies. In particular, the Drosophila model has been largely applied for deciphering the molecular and cellular pathways affected in these inherited cardiac disorders and for identifying their genetic modifiers. Here we review the utility and the main contributions of the fruitfly models for the better understanding of channelopathies and cardiomyopathies. We also discuss the investigated pathological mechanisms and the discoveries of evolutionarily conserved pathways which reinforce the value of Drosophila in modeling human cardiac diseases.

Ether-à-go-go family voltage-gated K+ channels evolved in an ancestral metazoan and functionally diversified in a cnidarian-bilaterian ancestor

We examined the evolutionary origins of the ether-à-go-go (EAG) family of voltage-gated K(+) channels, which have a strong influence on the excitability of neurons. The bilaterian EAG family comprises three gene subfamilies (Eag, Erg and Elk) distinguished by sequence conservation and functional properties. Searches of genome sequence indicate that EAG channels are metazoan specific, appearing first in ctenophores. However, phylogenetic analysis including two EAG family channels from the ctenophore Mnemiopsis leidyi indicates that the diversification of the Eag, Erg and Elk gene subfamilies occurred in a cnidarian/bilaterian ancestor after divergence from ctenophores. Erg channel function is highly conserved between cnidarians and mammals. Here we show that Eag and Elk channels from the sea anemone Nematostella vectensis (NvEag and NvElk) also share high functional conservation with mammalian channels. NvEag, like bilaterian Eag channels, has rapid kinetics, whereas NvElk activates at extremely hyperpolarized voltages, which is characteristic of Elk channels. Potent inhibition of voltage activation by extracellular protons is conserved between mammalian and Nematostella EAG channels. However, characteristic inhibition of voltage activation by Mg(2+) in Eag channels and Ca(2+) in Erg channels is reduced in Nematostella because of mutation of a highly conserved aspartate residue in the voltage sensor. This mutation may preserve sub-threshold activation of Nematostella Eag and Erg channels in a high divalent cation environment. mRNA in situ hybridization of EAG channels in Nematostella suggests that they are differentially expressed in distinct cell types. Most notable is the expression of NvEag in cnidocytes, a cnidarian-specific stinging cell thought to be a neuronal subtype.

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.

A new model to study sleep deprivation-induced seizure

BACKGROUND AND STUDY OBJECTIVES

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

DESIGN

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

SETTING

Laboratory.

PARTICIPANTS

Drosophila melanogaster.

INTERVENTIONS

Sleep deprivation.

MEASUREMENTS AND RESULTS

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

CONCLUSIONS

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

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.

Genome-wide screen for modifiers of Na (+) /K (+) ATPase alleles identifies critical genetic loci

Background

Mutations affecting the Na⁺/ K⁺ATPase (a.k.a. the sodium-potassium pump) genes cause conditional locomotor phenotypes in flies and three distinct complex neurological diseases in humans. More than 50 mutations have been identified affecting the human ATP1A2 and ATP1A3 genes that are known to cause rapid-onset Dystonia Parkinsonism, familial hemiplegic migraine, alternating hemiplegia of childhood, and variants of familial hemiplegic migraine with neurological complications including seizures and various mood disorders. In flies, mutations affecting the ATPalpha gene have dramatic phenotypes including altered longevity, neural dysfunction, neurodegeneration, myodegeneration, and striking locomotor impairment. Locomotor defects can manifest as conditional bang-sensitive (BS) or temperature-sensitive (TS) paralysis: phenotypes well-suited for genetic screening.

Results

We performed a genome-wide deficiency screen using three distinct missense alleles of ATPalpha and conditional locomotor function assays to identify novel modifier loci. A secondary screen confirmed allele-specificity of the interactions and many of the interactions were mapped to single genes and subsequently validated. We successfully identified 64 modifier loci and used classical mutations and RNAi to confirm 50 single gene interactions. The genes identified include those with known function, several with unknown function or that were otherwise uncharacterized, and many loci with no described association with locomotor or Na⁺/K⁺ ATPase function.

Conclusions

We used an unbiased genome-wide screen to find regions of the genome containing elements important for genetic modulation of ATPalpha dysfunction. We have identified many critical regions and narrowed several of these to single genes. These data demonstrate there are many loci capable of modifying ATPalpha dysfunction, which may provide the basis for modifying migraine, locomotor and seizure dysfunction in animals.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-014-0089-3) contains supplementary material, which is available to authorized users.

The enigmatic cytoplasmic regions of KCNH channels

KCNH channels are expressed across a vast phylogenetic and evolutionary spectrum. In humans, they function in a wide range of tissues and serve as biomarkers and targets for diseases such as cancer and cardiac arrhythmias. These channels share a general architecture with other voltage-gated ion channels but are distinguished by the presence of an N-terminal PAS (Per-Arnt-Sim) domain and a C-terminal domain with homology to cyclic nucleotide binding domains (referred to as the CNBh domain). Cytosolic regions outside these domains show little conservation between KCNH families but are strongly conserved across species within a family, likely reflecting variability that confers specificity to individual channel types. PAS and CNBh domains participate in channel gating, but at least twice in evolutionary history, the PAS domain has been lost and it is omitted by alternate transcription to create a distinct channel subunit in one family. In this focused review, we present current knowledge of the structure and function of these cytosolic regions, discuss their evolution as modular domains and provide our perspective on the important questions moving forward.

Functional evolution of Erg potassium channel gating reveals an ancient origin for IKr

Mammalian Ether-a-go-go related gene (Erg) family voltage-gated K(+) channels possess an unusual gating phenotype that specializes them for a role in delayed repolarization. Mammalian Erg currents rectify during depolarization due to rapid, voltage-dependent inactivation, but rebound during repolarization due to a combination of rapid recovery from inactivation and slow deactivation. This is exemplified by the mammalian Erg1 channel, which is responsible for IKr, a current that repolarizes cardiac action potential plateaus. The Drosophila Erg channel does not inactivate and closes rapidly upon repolarization. The dramatically different properties observed in mammalian and Drosophila Erg homologs bring into question the evolutionary origins of distinct Erg K(+) channel functions. Erg channels are highly conserved in eumetazoans and first evolved in a common ancestor of the placozoans, cnidarians, and bilaterians. To address the ancestral function of Erg channels, we identified and characterized Erg channel paralogs in the sea anemone Nematostella vectensis. N. vectensis Erg1 (NvErg1) is highly conserved with respect to bilaterian homologs and shares the IKr-like gating phenotype with mammalian Erg channels. Thus, the IKr phenotype predates the divergence of cnidarians and bilaterians. NvErg4 and Caenorhabditis elegans Erg (unc-103) share the divergent Drosophila Erg gating phenotype. Phylogenetic and sequence analysis surprisingly indicates that this alternate gating phenotype arose independently in protosomes and cnidarians. Conversion from an ancestral IKr-like gating phenotype to a Drosophila Erg-like phenotype correlates with loss of the cytoplasmic Ether-a-go-go domain. This domain is required for slow deactivation in mammalian Erg1 channels, and thus its loss may partially explain the change in gating phenotype.

Regulation of hERG and hEAG channels by Src and by SHP-1 tyrosine phosphatase via an ITIM region in the cyclic nucleotide binding domain

Members of the EAG K⁺ channel superfamily (EAG/Kv10.x, ERG/Kv11.x, ELK/Kv12.x subfamilies) are expressed in many cells and tissues. In particular, two prototypes, EAG1/Kv10.1/KCNH1 and ERG1/Kv11.1/KCNH2 contribute to both normal and pathological functions. Proliferation of numerous cancer cells depends on hEAG1, and in some cases, hERG. hERG is best known for contributing to the cardiac action potential, and for numerous channel mutations that underlie ‘long-QT syndrome’. Many cells, particularly cancer cells, express Src-family tyrosine kinases and SHP tyrosine phosphatases; and an imbalance in tyrosine phosphorylation can lead to malignancies, autoimmune diseases, and inflammatory disorders. Ion channel contributions to cell functions are governed, to a large degree, by post-translational modulation, especially phosphorylation. However, almost nothing is known about roles of specific tyrosine kinases and phosphatases in regulating K⁺ channels in the EAG superfamily. First, we show that tyrosine kinase inhibitor, PP1, and the selective Src inhibitory peptide, Src40-58, reduce the hERG current amplitude, without altering its voltage dependence or kinetics. PP1 similarly reduces the hEAG1 current. Surprisingly, an ‘immuno-receptor tyrosine inhibitory motif’ (ITIM) is present within the cyclic nucleotide binding domain of all EAG-superfamily members, and is conserved in the human, rat and mouse sequences. When tyrosine phosphorylated, this ITIM directly bound to and activated SHP-1 tyrosine phosphatase (PTP-1C/PTPN6/HCP); the first report that a portion of an ion channel is a binding site and activator of a tyrosine phosphatase. Both hERG and hEAG1 currents were decreased by applying active recombinant SHP-1, and increased by the inhibitory substrate-trapping SHP-1 mutant. Thus, hERG and hEAG1 currents are regulated by activated SHP-1, in a manner opposite to their regulation by Src. Given the widespread distribution of these channels, Src and SHP-1, this work has broad implications in cell signaling that controls survival, proliferation, differentiation, and other ERG1 and EAG1 functions in many cell types.

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.

Tuning of EAG K(+) channel inactivation: molecular determinants of amplification by mutations and a small molecule

Ether-à-go-go (EAG) and EAG-related gene (ERG) K⁺ channels are close homologues but differ markedly in their gating properties. ERG1 channels are characterized by rapid and extensive C-type inactivation, whereas mammalian EAG1 channels were previously considered noninactivating. Here, we show that human EAG1 channels exhibit an intrinsic voltage-dependent slow inactivation that is markedly enhanced in rate and extent by 1–10 µM 3-nitro-N-(4-phenoxyphenyl) benzamide, or ICA105574 (ICA). This compound was previously reported to have the opposite effect on ERG1 channels, causing an increase in current magnitude by inhibition of C-type inactivation. The voltage dependence of 2 µM ICA-induced inhibition of EAG1 current was half-maximal at −73 mV, 62 mV negative to the half-point for channel activation. This finding suggests that current inhibition by the drug is mediated by enhanced inactivation and not open-channel block, where the voltage half-points for current inhibition and channel activation are predicted to overlap, as we demonstrate for clofilium and astemizole. The mutation Y464A in the S6 segment also induced inactivation of EAG1, with a time course and voltage dependence similar to that caused by 2 µM ICA. Several Markov models were investigated to describe gating effects induced by multiple concentrations of the drug and the Y464A mutation. Models with the smallest fit error required both closed- and open-state inactivation. Unlike typical C-type inactivation, the rate of Y464A- and ICA-induced inactivation was not decreased by external tetraethylammonium or elevated [K⁺]_e. EAG1 channel inactivation introduced by Y464A was prevented by additional mutation of a nearby residue located in the S5 segment (F359A) or pore helix (L434A), suggesting a tripartite molecular model where interactions between single residues in S5, S6, and the pore helix modulate inactivation of EAG1 channels.

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.

KCNH2 gene mutation: a potential link between epilepsy and long QT-2 syndrome

Long QT syndrome (LQTS) is closely associated with syncope, seizure, and sudden death but LQTS is frequently misdiagnosed as epilepsy. LQTS and epilepsy both belong to the group of ion channelopathies that manifest in the heart and brain. Therefore, genetic analysis of genes associated with potassium and sodium homeostasis and electrical disorders may reveal a link between epilepsy and lethal cardiac arrhythmia. Here, the authors report a young woman who suffered recurrent seizure episodes and syncopes that occurred while walking and also during rest. She showed electroencephalogram abnormalities and a pathological prolonged QTc interval in electrocardiogram. The patient and the patient's asymptomatic family members underwent genetic screening of the three genes most frequently associated with LQTS: KCNQ1, KCNH2, and SCN5A. The patient and the family members did not show DNA alterations in the genes KCNQ1 and SCN5A associated with LQT-1 and LQT-3, respectively. However, the patient showed a de novo mutation 2587T→C in exon 10 of KCNH2 gene associated with LQT-2. The mutation caused a stop codon substitution (R863X) in the HERG channel, leading to a 296-amino acid deletion. The patient's asymptomatic relatives did not show the KCNH2 gene mutation. R863X alteration in HERG channel may be involved in both prolonged QTc interval and epilepsy. This fact raises the possibility that R863X alteration in KCNH2-encoded potassium channel may confer susceptibility for epilepsy and cardiac LQT-2 arrhythmia.

Animal models

Epilepsy accounts for a significant portion of the dis-ease burden worldwide. Research in this field is fundamental and mandatory. Animal models have played, and still play, a substantial role in understanding the patho-physiology and treatment of human epilepsies. A large number and variety of approaches are available, and they have been applied to many animals. In this chapter the in vitro and in vivo animal models are discussed,with major emphasis on the in vivo studies. Models have used phylogenetically different animals - from worms to monkeys. Our attention has been dedicated mainly to rodents.In clinical practice, developmental aspects of epilepsy often differ from those in adults. Animal models have often helped to clarify these differences. In this chapter, developmental aspects have been emphasized.Electrical stimulation and chemical-induced models of seizures have been described first, as they represent the oldest and most common models. Among these models, kindling raised great interest, especially for the study of the epileptogenesis. Acquired focal models mimic seizures and occasionally epilepsies secondary to abnormal cortical development, hypoxia, trauma, and hemorrhage.Better knowledge of epileptic syndromes will help to create new animal models. To date, absence epilepsy is one of the most common and (often) benign forms of epilepsy. There are several models, including acute pharmacological models (PTZ, penicillin, THIP, GBL) and chronic models (GAERS, WAG/Rij). Although atypical absence seizures are less benign, thus needing more investigation, only two models are so far available (AY-9944,MAM-AY). Infantile spasms are an early childhood encephalopathy that is usually associated with a poor out-come. The investigation of this syndrome in animal models is recent and fascinating. Different approaches have been used including genetic (Down syndrome,ARX mutation) and acquired (multiple hit, TTX, CRH,betamethasone-NMDA) models.An entire section has been dedicated to genetic models, from the older models obtained with spontaneous mutations (GEPRs) to the new engineered knockout, knocking, and transgenic models. Some of these models have been created based on recently recognized patho-genesis such as benign familial neonatal epilepsy, early infantile encephalopathy with suppression bursts, severe myoclonic epilepsy of infancy, the tuberous sclerosis model, and the progressive myoclonic epilepsy. The contribution of animal models to epilepsy re-search is unquestionable. The development of further strategies is necessary to find novel strategies to cure epileptic patients, and optimistically to allow scientists first and clinicians subsequently to prevent epilepsy and its consequences.

Eag and HERG potassium channels as novel therapeutic targets in cancer

Voltage gated potassium channels have been extensively studied in relation to cancer. In this review, we will focus on the role of two potassium channels, Ether à-go-go (Eag), Human ether à-go-go related gene (HERG), in cancer and their potential therapeutic utility in the treatment of cancer. Eag and HERG are expressed in cancers of various organs and have been implicated in cell cycle progression and proliferation of cancer cells. Inhibition of these channels has been shown to reduce proliferation both in vitro and vivo studies identifying potassium channel modulators as putative inhibitors of tumour progression. Eag channels in view of their restricted expression in normal tissue may emerge as novel tumour biomarkers.

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.

Drosophila melanogaster as a model organism of brain diseases

Drosophila melanogaster has been utilized to model human brain diseases. In most of these invertebrate transgenic models, some aspects of human disease are reproduced. Although investigation of rodent models has been of significant impact, invertebrate models offer a wide variety of experimental tools that can potentially address some of the outstanding questions underlying neurological disease. This review considers what has been gleaned from invertebrate models of neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, metabolic diseases such as Leigh disease, Niemann-Pick disease and ceroid lipofuscinoses, tumor syndromes such as neurofibromatosis and tuberous sclerosis, epilepsy as well as CNS injury. It is to be expected that genetic tools in Drosophila will reveal new pathways and interactions, which hopefully will result in molecular based therapy approaches.

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.

Emerging epilepsy models: insights from mice, flies, worms and fish

PURPOSE OF REVIEW

Animal models provide a means to investigate fundamental mechanisms of abnormal electrical discharge (i.e., seizures). Understanding the pathogenesis of epilepsy and therapy development have greatly benefited from these models. Here we review recent mouse mutants featuring spontaneous seizures and simpler organisms.

RECENT FINDINGS

New genetically engineered mice provide additional insights to cellular mechanisms underlying seizure generation (BK calcium-activated potassium channels and interneuron-expressed sodium channels), genetic interactions that exacerbate seizure phenotype (Scn2a, Kcnq2 and background) and neurodevelopmental influences (Dlx transcription factors). Mutants for neuronal nicotinic acetylcholine receptors, Glut-1 deficiency and aquaporin channels highlight additional seizure phenotypes in mice. Additional models in Caenorhabditis elegans (Lis-1) and Danio rerio (pentylenetetrazole) highlight a reductionist approach. Taking further advantage of 'simple' organisms, antiepileptic drugs and genetic modifiers of seizure activity are being uncovered in Drosophila.

SUMMARY

Studies of epilepsy in mutant mice provide a framework for understanding critical features of the brain that regulate excitability. These, and as yet undiscovered, mouse mutants will continue to serve as the foundation for basic epilepsy research. Interestingly, an even greater potential for analyzing epileptic phenotypes may lie in the more widespread use of genetically tractable organisms such as worms, flies and zebrafish.

Heteropolymeric triplex-based genomic assay to detect pathogens or single-nucleotide polymorphisms in human genomic samples

Human genomic samples are complex and are considered difficult to assay directly without denaturation or PCR amplification. We report the use of a base-specific heteropolymeric triplex, formed by native duplex genomic target and an oligonucleotide third strand probe, to assay for low copy pathogen genomes present in a sample also containing human genomic duplex DNA, or to assay human genomic duplex DNA for Single Nucleotide Polymorphisms (SNP), without PCR amplification. Wild-type and mutant probes are used to identify triplexes containing FVL G1691A, MTHFR C677T and CFTR mutations. The specific triplex structure forms rapidly at room temperature in solution and may be detected without a separation step. YOYO-1, a fluorescent bis-intercalator, promotes and signals the formation of the specific triplex. Genomic duplexes may be assayed homogeneously with single base pair resolution. The specific triple-stranded structures of the assay may approximate homologous recombination intermediates, which various models suggest may form in either the major or minor groove of the duplex. The bases of the stable duplex target are rendered specifically reactive to the bases of the probe because of the activity of intercalated YOYO-1, which is known to decondense duplex locally 1.3 fold. This may approximate the local decondensation effected by recombination proteins such as RecA in vivo. Our assay, while involving triplex formation, is sui generis, as it is not homopurine sequence-dependent, as are “canonical triplexes”. Rather, the base pair-specific heteropolymeric triplex of the assay is conformation-dependent. The highly sensitive diagnostic assay we present allows for the direct detection of base sequence in genomic duplex samples, including those containing human genomic duplex DNA, thereby bypassing the inherent problems and cost associated with conventional PCR based diagnostic assays.

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.

High-conductance potassium channels of the SLO family

High-conductance, 'big' potassium (BK) channels encoded by the Slo gene family are among the largest and most complex of the extended family of potassium channels. The family of SLO channels apparently evolved from voltage-dependent potassium channels, but acquired a large conserved carboxyl extension, which allows channel gating to be altered in response to the direct sensing of several different intracellular ions, and by other second-messenger systems, such as those activated following neurotransmitter binding to G-protein-coupled receptors (GPCRs). This versatility has been exploited to serve many cellular roles, both within and outside the nervous system.

wasted away, a Drosophila mutation in triosephosphate isomerase, causes paralysis, neurodegeneration, and early death

To identify genes required for maintaining neuronal viability, we screened our collection of Drosophila temperature-sensitive paralytic mutants for those exhibiting shortened lifespan and neurodegeneration. Here, we describe the characterization of wasted away (wstd), a recessive, hypomorphic mutation that causes progressive motor impairment, vacuolar neuropathology, and severely reduced lifespan. We demonstrate that the affected gene encodes the glycolytic enzyme, triosephosphate isomerase (Tpi). Mutations causing Tpi deficiency in humans are also characterized by progressive neurological dysfunction, neurodegeneration, and early death. In Tpi-deficient flies and humans, a decrease in ATP levels did not appear to cause the observed phenotypes because ATP levels remained normal. We also found no genetic evidence that the mutant Drosophila Tpi was misfolded or involved in aberrant protein-protein associations. Instead, we favor the hypothesis that mutations in Tpi lead to an accumulation of methylglyoxal and the consequent enhanced production of advanced glycation end products, which are ultimately responsible for the death and dysfunction of Tpi-deficient neurons. Our results highlight an essential protective role of Tpi and support the idea that advanced glycation end products may also contribute to pathogenesis of other neurological disorders.

Temperature-sensitive paralytic mutants: insights into the synaptic vesicle cycle

Forward genetic screens have identified numerous proteins with critical roles in neurotransmission. One particularly fruitful screening target in Drosophila has been TS (temperature-sensitive) paralytic mutants, which have revealed proteins acutely required in neuronal signalling. In the present paper, we review recent insights and current questions from one recently cloned TS paralytic mutant, rbo (rolling blackout). The rbo mutant identifies a putative integral lipase of the pre-synaptic plasma membrane that is required for the SV (synaptic vesicle) cycle. Identification of this mutant adds to a growing body of evidence that lipid-modifying enzymes locally control specialized lipid microenvironments and lipid signalling pathways with key functions regulating neurotransmission strength. The RBO protein is absolutely required for phospholipase C signalling in phototransduction. We posit that RBO might be required to regulate the availability of fusogenic lipids such as phosphatidylinositol 4,5-bisphosphate and diacylglycerol that may directly modify membrane properties and/or activate lipid-binding fusogenic proteins mediating SV exocytosis.

Species diversity and peptide toxins blocking selectivity of ether-a-go-go-related gene subfamily K+ channels in the central nervous system

The ether-à-go-go-related gene (erg) K+ channels are known to be crucial for life in Caenorhabditis elegans (mating), Drosophila melanogaster (seizure), and humans (LQT syndrome). The erg genes known to date (erg1, erg2, and erg3) are highly expressed in various areas of the rat and mouse central nervous system (CNS), and ERG channel blockers alter firing accommodation. To assign physiological roles to each isoform, it is necessary to design pharmacological strategies to distinguish individual currents. To this purpose, we have investigated the blocking properties of specific peptide inhibitors of hERG1 channels on the human and rat isoforms. In particular, we have tested ErgTx1 (from the scorpion Centruroides noxious), BeKm-1 (from the scorpion Buthus eupeus), and APETx1 (from the sea anemone Anthopleura elegantissima). Because these peptides had different species-specific effects on the six different channels, we have also carried out a biophysical characterization of hERG2 and hERG3 channels that turned out to be different from the rat homologs. It emerged that APETx1 is exquisitely selective for ERG1 and does not compete with the other two toxins. BeKm-1 discriminates well among the three rat members. ErgTx1 is unable to block hERG2, but blocks rERG2 and has the lowest KD for hERG3. BeKm-1 and ErgTx1 compete for hERG3 but not for rERG2 blockade. Our findings should be helpful for structure-function studies and for novel CNS ERG-specific drug design.

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.

Genome-Wide Transcriptional Changes Associated with Enhanced Activity in the Drosophila Nervous System

Neuronal plasticity is an important feature of the developing brain and requires neuronal circuits to reconfigure their functional connectivity depending upon activity patterns. To explore changes in neuronal function that occur downstream of altered activity, we performed an expression analysis in Drosophila mutants with acute or chronic alterations in neuronal activity. We find that seizure induction leads to an overproliferation of synaptic connections, indicating that activity-dependent neuronal rewiring occurs in Drosophila. To analyze transcriptional recoding during altered neuronal activity, we performed genome-wide DNA microarray analysis following multiple seizure induction and recovery paradigms. Approximately 250 genes implicated in cell adhesion, membrane excitability, and cellular signaling are differentially regulated, including the Kek 2 neuronal cell adhesion protein, which, as we demonstrate, functions as a regulator of synaptic growth. These data identify a collection of activity-regulated transcripts that may link changes in neuronal firing patterns to transcription-dependent modulation of brain function, including activity-dependent synaptic rewiring.

Nervous Wreck, an SH3 Adaptor Protein that Interacts with Wsp, Regulates Synaptic Growth in Drosophila

We describe the isolation and characterization of nwk (nervous wreck), a temperature-sensitive paralytic mutant that causes excessive growth of larval neuromuscular junctions (NMJs), resulting in increased synaptic bouton number and branch formation. Ultrastructurally, mutant boutons have reduced size and fewer active zones, associated with a reduction in synaptic transmission. nwk encodes an FCH and SH3 domain-containing adaptor protein that localizes to the periactive zone of presynaptic terminals and binds to the Drosophila ortholog of Wasp (Wsp), a key regulator of actin polymerization. wsp null mutants display synaptic overgrowth similar to nwk and enhance the nwk morphological phenotype in a dose-dependent manner. Evolutionarily, Nwk belongs to a previously undescribed family of adaptor proteins that includes the human srGAPs, which regulate Rho activity downstream of Robo receptors. We propose that Nwk controls synapse morphology by regulating actin dynamics downstream of growth signals in presynaptic terminals.

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.

Distribution and functional properties of human KCNH8 (Elk1) potassium channels

The Elk subfamily of the Eag K+ channel gene family is represented in mammals by three genes that are highly conserved between humans and rodents. Here we report the distribution and functional properties of a member of the human Elk K+ channel gene family, KCNH8. Quantitative RT-PCR analysis of mRNA expression patterns showed that KCNH8, along with the other Elk family genes, KCNH3 and KCNH4, are primarily expressed in the human nervous system. KCNH8 was expressed at high levels, and the distribution showed substantial overlap with KCNH3. In Xenopus oocytes, KCNH8 gives rise to slowly activating, voltage-dependent K+ currents that open at hyperpolarized potentials (half-maximal activation at -62 mV). Coexpression of KCNH8 with dominant-negative KCNH8, KCNH3, and KCNH4 subunits led to suppression of the KCNH8 currents, suggesting that Elk channels can form heteromultimers. Similar experiments imply that KCNH8 subunits are not able to form heteromultimers with Eag, Erg, or Kv family K+ channels.

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.