Tandem pore domain K(+)-channel TASK-3 (KCNK9) and idiopathic absence epilepsies

Thalamocortical circuits in generalized epilepsy: Pathophysiologic mechanisms and therapeutic targets

Generalized epilepsy affects 24 million people globally; at least 25% of cases remain medically refractory. The thalamus, with widespread connections throughout the brain, plays a critical role in generalized epilepsy. The intrinsic properties of thalamic neurons and the synaptic connections between populations of neurons in the nucleus reticularis thalami and thalamocortical relay nuclei help generate different firing patterns that influence brain states. In particular, transitions from tonic firing to highly synchronized burst firing mode in thalamic neurons can cause seizures that rapidly generalize and cause altered awareness and unconsciousness. Here, we review the most recent advances in our understanding of how thalamic activity is regulated and discuss the gaps in our understanding of the mechanisms of generalized epilepsy syndromes. Elucidating the role of the thalamus in generalized epilepsy syndromes may lead to new opportunities to better treat pharmaco-resistant generalized epilepsy by thalamic modulation and dietary therapy.

Contribution of Neuronal and Glial Two-Pore-Domain Potassium Channels in Health and Neurological Disorders

Two-pore-domain potassium (K2P) channels are widespread in the nervous system and play a critical role in maintaining membrane potential in neurons and glia. They have been implicated in many stress-relevant neurological disorders, including pain, sleep disorder, epilepsy, ischemia, and depression. K2P channels give rise to leaky K⁺ currents, which stabilize cellular membrane potential and regulate cellular excitability. A range of natural and chemical effectors, including temperature, pressure, pH, phospholipids, and intracellular signaling molecules, substantially modulate the activity of K2P channels. In this review, we summarize the contribution of K2P channels to neuronal excitability and to potassium homeostasis in glia. We describe recently discovered functions of K2P channels in glia, such as astrocytic passive conductance and glutamate release, microglial surveillance, and myelin generation by oligodendrocytes. We also discuss the potential role of glial K2P channels in neurological disorders. In the end, we discuss current limitations in K2P channel researches and suggest directions for future studies.

KCNK9 imprinting syndrome-further delineation of a possible treatable disorder

Patients with KCNK9 imprinting syndrome demonstrate congenital hypotonia, variable cleft palate, normal MRIs and EEGs, delayed development, and feeding problems. Associated facial dysmorphic features include dolichocephaly with bitemporal narrowing, short philtrum, tented upper lip, palatal abnormalities, and small mandible. This disorder maps to chromosomal region 8q24, and it is caused by a specific missense mutation 770G>A in exon 2, replacing glycine at position 236 by arginine (G236R) in the maternal copy of KCNK9 within this locus. KCNK9 (also called TASK3) encodes a member of the two pore- domain potassium channel (K2P) subfamily. This gene is normally imprinted with paternal silencing, thus a mutation in the maternal copy of the gene will result in disease, whereas a mutation in the paternal copy will have no effect. Exome sequencing in four new patients with developmental delay and central hypotonia revealed de novo G236R mutations. Older members of a previously reported Arab-Israeli family have intellectual disability of variable severity, persistent feeding difficulties in infancy with dysphagia of liquids and dysphonia with a muffled voice in early adulthood, generalized hypotonia, weakness of proximal muscles, elongated face with narrow bitemporal diameter, and reduced facial movements. We describe the clinical features in four recently recognized younger patients and compare them with those found in members of the originally reported Arab-Israeli family and suggest this may be a treatable disorder. © 2016 Wiley Periodicals, Inc.

Involvement of leak K+ channels in neurological disorders

TWIK-related acid-sensitive K⁺ (TASK) channels give rise to leak K⁺ currents which influence the resting membrane potential and input resistance. The wide expression of TASK1 and TASK3 channels in the central nervous system suggests that these channels are critically involved in neurological disorders. It has become apparent in the past decade that TASK channels play critical roles for the development of various neurological disorders. In this review, I describe evidence for their roles in ischemia, epilepsy, learning/memory/cognition and apoptosis.

Targeting two-pore domain K(+) channels TREK-1 and TASK-3 for the treatment of depression: a new therapeutic concept

Depression is a disease that is particularly frequent, affecting up to 20% of the population in Western countries. The origins of this pathology involve multiple genes as well as environmental and developmental factors leading to a disorder that remains difficult to treat. Several therapies for depression have been developed and these mainly target monoamine neurotransmitters. However, these treatments are not only associated with numerous adverse effects, but they are also ineffective for more than one-third of patients. Therefore, the need to develop new concepts to treat depression is crucial. Recently, studies using knockout mouse models have provided evidence for a crucial role of two members of the two-pore domain potassium channel (K2P ) family, tandem P-domain weak inward rectifying K(+) (TWIK)-related K(+) channel 1 (TREK-1) and TWIK-related acid-sensitive K(+) channel 3 (TASK-3) in the pathophysiology of depression. It is believed that TREK-1 and TASK-3 antagonists could lead to the development of new antidepressants. Herein, we describe the discovery of spadin, a natural peptide released from the maturation of the neurotensin receptor-3 (also known as sortilin), which specifically blocks the activity of the TREK-1 channel and displays particular antidepressant properties, with a rapid onset of action and the absence of adverse effects. The development of such molecules may open a new era in the field of psychiatry.

The role of two-pore-domain background K⁺ (K₂p) channels in the thalamus

The thalamocortical system is characterized by two fundamentally different activity states, namely synchronized burst firing and tonic action potential generation, which mainly occur during the behavioral states of sleep and wakefulness, respectively. The switch between the two firing modes is crucially governed by the bidirectional modulation of members of the K2P channel family, namely tandem of P domains in a weakly inward rectifying K(+) (TWIK)-related acid-sensitive K(+) (TASK) and TWIK-related K(+) (TREK) channels, in thalamocortical relay (TC) neurons. Several physicochemical stimuli including neurotransmitters, protons, di- and multivalent cations as well as clinically used drugs have been shown to modulate K2P channels in these cells. With respect to modulation of these channels by G-protein-coupled receptors, PLCβ plays a unique role with both substrate breakdown and product synthesis exerting important functions. While the degradation of PIP2 leads to the closure of TREK channels, the production of DAG induces the inhibition of TASK channels. Therefore, TASK and TREK channels were found to be central elements in the control of thalamic activity modes. Since research has yet focused on identifying the muscarinic pathway underling the modulation of TASK and TREK channels in TC neurons, future studies should address other thalamic cell types and members of the K2P channel family.

Immunocytochemical localization of TASK-3 protein (K2P9.1) in the rat brain

Among all K2P channels, TASK-3 shows the most widespread expression in rat brain, regulating neuronal excitability and transmitter release. Using a recently purified and characterized polyclonal monospecific antibody against TASK-3, the entire rat brain was immunocytochemically analyzed for expression of TASK-3 protein. Besides its well-known strong expression in motoneurons and monoaminergic and cholinergic neurons, TASK-3 expression was found in most neurons throughout the brain. However, it was not detected in certain neuronal populations, and neuropil staining was restricted to few areas. Also, it was absent in adult glial cells. In hypothalamic areas, TASK-3 was particularly strongly expressed in the supraoptic and suprachiasmatic nuclei, whereas other hypothalamic nuclei showed lower protein levels. Immunostaining of hippocampal CA1 and CA3 pyramidal neurons showed strongest expression, together with clear staining of CA3 mossy fibers and marked staining also in the dentate gyrus granule cells. In neocortical areas, most neurons expressed TASK-3 with a somatodendritic localization, most obvious in layer V pyramidal neurons. In the cerebellum, TASK-3 protein was found mainly in neurons and neuropil of the granular cell layer, whereas Purkinje cells were only faintly positive. Particularly weak expression was demonstrated in the forebrain. This report provides a comprehensive overview of TASK-3 protein expression in the rat brain.

Recovery of current through mutated TASK3 potassium channels underlying Birk Barel syndrome

TASK3 (TWIK-related acid-sensitive K(+) channel 3) potassium channels are members of the two-pore-domain potassium channel family. They are responsible for background leak potassium currents found in many cell types. TASK3 channels are genetically imprinted, and a mutation in TASK3 (G236R) is responsible for Birk Barel mental retardation dysmorphism syndrome, a maternally transmitted developmental disorder. This syndrome may arise from a neuronal migration defect during development caused by dysfunctional TASK3 channels. Through the use of whole-cell electrophysiologic recordings, we have found that, although G236R mutated TASK3 channels give rise to a functional current, this current is significantly smaller in an outward direction when compared with wild-type (WT) TASK3 channels. In contrast to WT TASK3 channels, the current is inwardly rectifying. Furthermore, the current through mutated channels is differentially sensitive to a number of regulators, such as extracellular acidification, extracellular zinc, and activation of Gαq-coupled muscarinic (M3) receptors, compared with WT TASK3 channels. The reduced outward current through mutated TASK3_G236R channels can be overcome, at least in part, by both a gain-of-function additional mutation of TASK3 channels (A237T) or by application of the nonsteroidal anti-inflammatory drug flufenamic acid (FFA; 2-{[3-(trifluoromethyl)phenyl]amino}benzoic acid). FFA produces a significantly greater enhancement of current through mutated channels than through WT TASK3 channels. We propose that pharmacologic enhancement of mutated TASK3 channel current during development may, therefore, provide a potentially useful therapeutic strategy in the treatment of Birk Barel syndrome.

Mutation Screening of the γ-Aminobutyric Acid Type-A Receptor Subunit γ2 Gene in Korean Patients with Childhood Absence Epilepsy

Background and Purpose

Since the γ-aminobutyric acid type-A receptor subunit γ2 gene (GABRG2) mutation was discovered in an Australian family with childhood absence epilepsy (CAE) and febrile convulsions, a few screening studies for the GABRG2 mutation have been conducted in sporadic individuals with CAE from other ethnic groups. The aim of this study was to determine whether or not the previously reported genetic mutations and single-nucleotide polymorphisms (SNPs) of GABRG2 can be reproduced in sporadic Korean individuals with CAE, compared to healthy Korean individuals.

Methods

Thirty-five children with CAE in Chonnam National University Hospital and healthy controls (n=207) were enrolled, and the medical records of patients with CAE were reviewed. CAE was diagnosed according to the Classification and Terminology of the International League Against Epilepsy. All nine exons of GABRG2 were directly sequenced. In addition, the two SNPs found in our CAE patients were analyzed: C315T in exon 3 (E3) and C588T in exon 5 (E5). The frequencies of the two SNPs in the CAE patients were compared with data from healthy controls (for E3 and E5) and from previously reported Korean population data (only for E3).

Results

No mutation of GABRG2 was found in our CAE patients. In addition, the allele and genotype frequencies of the two polymorphisms did not differ significantly between CAE patients, healthy controls, and the Korean general population (p>0.05).

Conclusions

Our study of sporadic Korean individuals with CAE found no evidence that GABRG2 contributes to the genetic basis of CAE.

From the background to the spotlight: TASK channels in pathological conditions

TWIK-related acid-sensitive potassium channels (TASK1-3) belong to the family of two-pore domain (K(2P) ) potassium channels. Emerging knowledge about an involvement of TASK channels in cancer development, inflammation, ischemia and epilepsy puts the spotlight on a leading role of TASK channels under these conditions. TASK3 has been especially linked to cancer development. The pro-oncogenic potential of TASK3 could be shown in cell lines and in various tumor entities. Pathophysiological hallmarks in solid tumors (e.g. low pH and oxygen deprivation) regulate TASK3 channels. These conditions can also be found in (autoimmune) inflammation. Inhibition of TASK1,2,3 leads to a reduction of T cell effector function. It could be demonstrated that TASK1(-/-) mice are protected from experimental autoimmune inflammation while the same animals display increased infarct volumes after cerebral ischemia. Furthermore, TASK channels have both an anti-epileptic as well as a pro-epileptic potential. The relative contribution of these opposing influences depends on their cell type-specific expression and the conditions of the cellular environment. This indicates that TASK channels are per se neither protective nor detrimental but their functional impact depends on the "pathophysiological" scenario. Based on these findings TASK channels have evolved from "mere background" channels to key modulators in pathophysiological conditions.

Molecular background of leak K+ currents: two-pore domain potassium channels

Two-pore domain K(+) (K(2P)) channels give rise to leak (also called background) K(+) currents. The well-known role of background K(+) currents is to stabilize the negative resting membrane potential and counterbalance depolarization. However, it has become apparent in the past decade (during the detailed examination of the cloned and corresponding native K(2P) channel types) that this primary hyperpolarizing action is not performed passively. The K(2P) channels are regulated by a wide variety of voltage-independent factors. Basic physicochemical parameters (e.g., pH, temperature, membrane stretch) and also several intracellular signaling pathways substantially and specifically modulate the different members of the six K(2P) channel subfamilies (TWIK, TREK, TASK, TALK, THIK, and TRESK). The deep implication in diverse physiological processes, the circumscribed expression pattern of the different channels, and the interesting pharmacological profile brought the K(2P) channel family into the spotlight. In this review, we focus on the physiological roles of K(2P) channels in the most extensively investigated cell types, with special emphasis on the molecular mechanisms of channel regulation.

Computational and experimental identification of novel human imprinted genes

Imprinted genes are essential in embryonic development, and imprinting dysregulation contributes to human disease. We report two new human imprinted genes: KCNK9 is predominantly expressed in the brain, is a known oncogene, and may be involved in bipolar disorder and epilepsy, while DLGAP2 is a candidate bladder cancer tumor suppressor. Both genes lie on chromosome 8, not previously suspected to contain imprinted genes. We identified these genes, along with 154 others, based on the predictions of multiple classification algorithms using DNA sequence characteristics as features. Our findings demonstrate that DNA sequence characteristics, including recombination hot spots, are sufficient to accurately predict the imprinting status of individual genes in the human genome.

A TASK3 channel (KCNK9) mutation in a genetic model of absence epilepsy

Childhood absence epilepsy is an idiopathic, generalized, nonconvulsive epilepsy with a multifactorial genetic etiology. The KCNK9 gene coding for the TASK3 (Twik-like acid-sensitive K</U)+) channel is present on chromosome 8 at position 8q24, a locus that has shown positive linkage to the human absence epilepsy phenotype. Sequencing of the KCNK9 gene in the genetic absence epilepsy rats from Strasbourg (GAERS), a well established genetic model of this disease, reveals an additional alanine residue in a polyalanine tract within the C-terminal intracellular domain. This additional alanine is absent in the inbred nonepileptic control (NEC) strain, Wistar, and Wistar albino Glaxo strain bred in Rijswijk, another inbred rat model of absence epilepsy. Expression of the mutant channel in CHO cells produces a K+ current that is blocked by acidic pH and millimolar concentrations of barium or ruthenium red and is not different from the wild-type channel. In brain slices, thalamic neurons display a prominent pH-sensitive tonic K+ current, but no difference was observed between GAERS and NEC or Wistar rats. Ruthenium red had no effect in cortical, reticular thalamic, or sensory thalamic neurons in either GAERS or NEC, indicating that the TASK3 homodimer is not present in these structures. Twik-like acid-sensitive K+(TASK3) channels, therefore, are probably associated with TASK1 to form ruthenium red-insensitive heterodimers in these neurons. Finally, no difference was found between GAERS and NEC rats in the modulation of the leak K+ current following activation of muscarinic receptors. These studies describe the first mutation found in a genetic model of absence epilepsy. Although our experiments showed no difference in the leak K+ current between GAERS and NEC rats, further work is needed to ascertain whether this mutation contributes to the generation of absence seizures, possibly by mechanisms related to the expansion of the polyalanine run.

Genetic Association Studies in Epilepsy: "The Truth Is Out There"

Success has been achieved in identifying many mutations in rare monogenic epilepsy syndromes by using linkage analysis, but dissecting the genetic basis of common epilepsy syndromes has proven more difficult. Common epilepsies are genetically complex disorders believed to be influenced by variation in several susceptibility genes. Association studies can theoretically identify these genes, but despite more than 50 association studies in epilepsy, no consistent or convincing susceptibility genes have emerged, leading to scepticism about the association-study approach. We review the results of existing association studies in focal epilepsies, generalized epilepsies, febrile seizures, and epilepsy pharmacogenetics. By using an illustrative example, we discuss how methodologic issues of sample size, selection of appropriate controls, population stratification, and significance thresholds can lead to bias and false-positive associations; the importance of biologic plausibility also is emphasized. Newer methodologic refinements for association studies, such as use of two control groups, genomic control, haplotyping, and use of two independent datasets, are discussed. A summary of existing guidelines and a checklist for planning and appraising such association studies in epilepsy is presented. We remain cautiously optimistic that with methodologic refinements and multicenter collaborations with large sample sizes, association studies will ultimately be useful in dissecting the genetic basis of common epilepsy syndromes.

Characterization of a 6p21 translocation breakpoint in a family with idiopathic generalized epilepsy

The idiopathic generalized epilepsies (IGE), for which a genetic cause is widely accepted, account for 20-30% of all epilepsies. Mapping these epilepsies is difficult, but progress in the positional cloning of idiopathic epilepsy genes responsible for monogenic forms provide emerging evidence that many idiopathic epilepsies are caused by mutations in genes coding for ion channels. Here, we show the characterization of a balanced translocation present in three members of a nuclear family, two of them affected with IGE. The translocation involved chromosome 6p21 [t(4;6) (q35;p21)], a region in which a susceptibility locus for IGE (EJM1) has been reported. Fluorescence in situ hybridization analysis with YACs and PACs resulted in the identification of a PAC clone that included the 6p21 translocation breakpoint. The genomic sequence of this PAC clone contains two 2-pore potassium channel genes, TALK-1 and TALK-2. We characterized the genomic organization of both genes, including three different isoforms of TALK-1, and investigated them in IGE patients, finding some polymorphisms in the coding sequence of TALK-1A.

Pharmacology of neuronal background potassium channels

Background or leak conductances are a major determinant of membrane resting potential and input resistance, two key components of neuronal excitability. The primary structure of the background K(+) channels has been elucidated. They form a family of channels that are molecularly and functionally divergent from the voltage-gated K(+) channels and inward rectifier K(+) channels. In the nervous system, the main representatives of this family are the TASK and TREK channels. They are relatively insensitive to the broad-spectrum K(+) channel blockers tetraethylammonium (TEA), 4-aminopyridine (4-AP), Cs(+), and Ba(2+). They display very little time- or voltage-dependence. Open at rest, they are involved in the maintenance of the resting membrane potential in somatic motoneurones, brainstem respiratory and chemoreceptor neurones, and cerebellar granule cells. TASK and TREK channels are also the targets of many physiological stimuli, including intracellular and extracellular pH and temperature variations, hypoxia, bioactive lipids, and neurotransmitter modulation. Integration of these different signals has major effects on neuronal excitability. Activation of some of these channels by volatile anaesthetics and by other neuroprotective agents, such as riluzole and unsaturated fatty acids, illustrates how the neuronal background K(+) conductances are attractive targets for the development of new drugs.

Childhood absence epilepsy: genes, channels, neurons and networks

Childhood absence epilepsy is an idiopathic, generalized non-convulsive epilepsy with a multifactorial genetic aetiology. Molecular-genetic analyses of affected human families and experimental models, together with neurobiological investigations, have led to important breakthroughs in the identification of candidate genes and loci, and potential pathophysiological mechanisms for this type of epilepsy. Here, we review these results, and compare the human and experimental phenotypes that have been investigated. Continuing efforts and comparisons of this type will help us to elucidate the multigenetic traits and pathophysiology of this form of generalized epilepsy.