From the Background to the Spotlight: TASK Channels in Pathological Conditions

Abstract

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.

INTRODUCTION

It was only about 10 years ago (1996–2003) that a whole new group of potassium channels named two‐pore domain potassium channels (K_2P channels) was identified. They contribute to the setting and modulation of the resting membrane potential and were initially regarded as background channels. As these channels are mostly time‐ and voltage‐independent, they are especially suited to carry a background K⁺ conductance at negative membrane potentials, whereas other potassium channels like voltage‐gated or inward‐rectifying K⁺ channels generally show less persistent activity [for recent general reviews about K_2P channels, see for example 30, 48, 65, 102, 125].

Compared to other K⁺ channels, the family of K_2P channels is unique in their membrane topology as their channel subunits display two‐pore and four‐transmembrane domains (2P/4TM). Functional K_2P channels consist of a homo‐ or heterodimer and the four‐pore domains build a K⁺‐selective pore region. The human K_2P family has 15 members—of which 12 were functionally expressed in expression systems—and can be further divided into six subgroups with regard to their phylogenetic relationships (Fig. 1; the widely accepted popular nomenclature will generally be used within this review). K_2P channels are under a complex control of a wide spectrum of chemical and physical stimuli as well as unusual pharmacological modulations and clinically relevant drugs [e.g. pH values 24, 40, O₂ partial pressure (15), membrane stretch (104), temperature (77), G proteins 89, 125, fatty acids 66, 79 and inhalation anesthetics 16, 85, 103, 120]. Along with their selective and differential expression pattern, K_2P channels are likely to be key players in a variety of physiological and pathological conditions.

Figure 1.

Organization of K_2P channels. K_2P channel members are displayed in a phylogenetic tree and sorted into six subfamilies. Nonfunctional members are displayed in grey and channels of special interest for this review are highlighted. The three common classification systems [popular name based on acronyms describing distinct properties of the channels, the KCNK classification of the Human Genome Organization (HUGO) and the K_2PX.1 nomenclature of the International Union of Pharmacology (IUPHAR)] are displayed on the right side. Abbreviations: TALK = TWIK‐related alkaline‐pH‐activated K⁺ channel; TASK = TWIK‐related acid‐sensitive K⁺ channel; THIK = tandem pore domain halothane‐inhibited K⁺ channel; TRAAK = TWIK‐related arachidonic‐acid‐stimulated K⁺ channel; TREK = TWIK‐related K⁺‐channel gene; TRESK =  TWIK‐related spinal‐cord K⁺ channel; TWIK = tandem of pore domains in a weak inwardly rectifying potassium channel.

This review will especially focus on pH‐sensitive K_2P channel family members [acid‐sensitive: tandem of pore domains in a weak inwardly rectifying potassium channel [TWIK]‐related acid‐sensitive K⁺ channel‐1 (TASK1, K_2P3.1) and TASK3 (K_2P9.1); alkaline‐activated: TASK2 (K_2P5.1)] in light of increased research effort during the last years, which provided us with a variety of interesting findings. TASK channels are among others inhibited by extracellular acidification, hypoxia, local anesthetics and the endocannabinoid anandamide and are activated by inhalational anesthetics, but experimental studies are often challenging due to the absence of selective blockers (3). This gap could at least partially be filled by usage of knockout animals. An involvement of TASK channels could, for example, be implicated in cell proliferation, ischemic and inflammatory tissue damage and epileptic activity. Here, we summarize findings concerning the role of TASK1, TASK2 and TASK3 as signal integrators under pathophysiological conditions like tumorigenesis, ischemia, (autoimmune) inflammation and epilepsia (see Table 1 for a selection of key findings discussed in this review). Other K_2P channels are only mentioned briefly when interesting findings increase the level of our knowledge and understanding of putative properties of TASK channels.

Table 1.

Literature overview. Abbreviations: TASK = tandem of pore domains in a weak inwardly rectifying potassium channel (TWIK)‐related acid‐sensitive K⁺ channel; TRESK = TWIK‐related spinal‐cord K⁺ channel.

Topic	Channel	Experimental finding	Reference
Oncology	TASK3	Overexpression in cancer samples	(91)
	TASK3	Anti‐apoptotic effect of TASK3 expression	75, 106
	TASK3	Implication in mitochondrial dysfunction	108, 116
Ischemia	TREK1	Neuroprotective effect of channel activation	37, 38
	TASK1	Increased ischemic damage in TASK1−/− mice	(88)
	TASK1	Neuroprotective effect of channel inhibition	(9)
Inflammation	TASK1,3	Critical role for T cell activation	9, 83
	TRESK	Putative role on T lymphocytes	(109)
	TASK2	Influences on T cell effector functions	(8)
Epilepsia	TASK1	Expression on interneurons dampens neuronal acitivity	(126)
	TASK1	Downregulation in astrocytes has an anti‐epileptic effect	(50)
	TASK2	Protective effect upon channel upregulation	(51)

TASK CHANNELS IN ONCOLOGY

Background

Potassium channels are critically involved in the signaling pathways that regulate cell proliferation and cell death 57, 58, 123, 131. Inhibition of these channels has been shown to decrease cell proliferation in human lymphocytes 70, 83, 111, embryonic stem cells (90), human melanoma cells (97), hepatocarcinoma cells (142), glioma cells (41), small cell lung (98), breast (133) and prostate (121) cancer cells. The mechanisms by which these channels might alter the proliferative state of cells has been an open question for several years and initial studies linked the functional role of K⁺ channels to their impact on the intracellular Ca²⁺ concentration 118, 132, to the regulation of the cell volume (114) or to direct activation of intracellular signaling cascades 131, 139. However, it has also become clear that K⁺‐currents play a necessary and pivotal role in apoptosis 11, 42 and oncogenesis (100).

Experimental data

Recently, one member of the K_2P family, namely TASK3, has been shown to be amplified 3–10‐fold in 10% of human breast cancer samples (91) and thus was identified as a novel proto‐oncogene in breast cancer. Furthermore, TASK3 was found to be overexpressed in 10%–50% of breast cancer, lung cancer and colorectal cancer cells 47, 91. Overexpression of TASK3 in NmuMG (cell line derived from mammary epithelial tissue) or NIH‐3T3 (fibroblasts) cells did not result in transformation of these cell lines. However, mice injected with NmuMG cells overexpressing TASK3 developed tumors within 3 months (91).

Other studies have investigated the oncogenic potential of TASK3 in more detail. Transfection of TASK3 in the partially transformed mouse embryonic fibroblast cell line C8 strengthens the oncogenic potential of C8 cells and especially increases cell survival under pro‐apoptotic conditions [low serum conditions, hypoxia; 75, 91]. Liu et al. additionally showed that overexpression of TASK3 as well as TASK1 and TASK2 enhanced cell viability of transfected C8 cells by inhibiting intracellular apoptosis pathways. In support of these findings, C8 cells transfected with a mutated TASK3 construct with abolished TASK3 channel activity showed reduced oncogenic properties, which directly link TASK3 channel function and tumorigenesis (106). These results are nicely extended by other studies focusing on K⁺ channel‐dependent cell death using high extracellular K⁺ concentrations 61, 86, which can be found in vivo under pathophysiological conditions with impaired K⁺ homeostasis 12, 63. In this model, rat granule neurons and several human glioma cell lines undergo cell death when cultured in a medium with physiological K⁺ concentration (5 mM), but cell survival is promoted by high extracellular K⁺ concentrations (25 mM). Using pharmacological modulators and changes in the extracellular pH value, this effect could be linked to TASK3 channel activity 61, 86. The biophysical properties of TASK3 (pH dependency, extracellular K⁺ concentration‐dependent conductance changes) may be especially relevant in the hypoxic and acidic micro milieu often occurring in the center of solid tumors (105): TASK1 and TASK3 channels are both inhibited by acidosis [pK values: TASK1: 7.3; TASK3: 6.7; (105)] and in addition TASK1 channels are directly inhibited by hypoxia. The conflicting data of O₂ sensitivity of TASK1 channels 44, 68 may be explained by a recent report (62) proposing NOX4 as O₂ sensor of TASK1 channels. Differential NOX4 expression could explain the inhomogeneous O₂ sensitivity of TASK1. However, other channels will also be affected by hypoxia like tandem pore domain halothane‐inhibited K⁺ channel (THIK1) (17), voltage‐gated potassium channels (101) and also a variety of other targets. Thus more research effort is needed to dissect the pleiotropic effects of hypoxia in tumors in vivo.

TASK3 channels have also been reported on melanoma cells 108, 116. Thorough investigations showed that TASK3 was expressed especially in mitochondria. Several potassium channels (124) have been shown to play a role in mitochondrial dysfunction. While the exact role of TASK3 channels in this context is yet unclear, their involvement in mitochondrial damage might be another piece of the puzzle toward understanding the role of TASK3 in apoptosis and tumorigenesis.

Interestingly, genomic investigations have revealed that TASK3 is an imprinted gene 76, 115, and proposing that the regulation and diversity of TASK3 channel function might even be more complex than thought before. Several diseases like Prader–Willi syndrome or Angelman syndrome have their cause in epigenetic dysfunction and a recent report of the maternally inherited so‐called Birk Barel mental retardation dysmorphism syndrome (2) revealed a similar hereditary disease for TASK3. As several tumors like Wilms tumor have also been linked to imprinting errors, it will be interesting to see this new evolvement of TASK channel research in the future.

Besides TASK3, differential expression of TASK1, another member of the K_2P family with comparable biophysical properties, could be correlated with prolonged survival of patients with advanced colorectal cancer (Dukes stages C and D) 5 years after surgery (18). This finding supports the hypothesis that both members of the K_2P family (TASK1 and TASK3) who are also able to form heterodimers 19, 49 can play a critical role in a variety of cancer entities (see Fig. 2).

Figure 2.

Involvement of TASK channels in the underlying pathophysiological mechanisms of A. oncology, B. ischemia, C. (autoimmune) inflammation and D. epilepsy. A. A schematic of a neuron is shown. 1. TASK3 channels influence cell survival and proliferation on different levels including cell volume, [Ca²⁺]_I and apoptosis. A low external pH value might block TASK3 channels. 2. TASK3 channels are also implicated in mitochondrial dysfunction. 3. In vitro experiments show that high local [K⁺]_e is associated with a reduced cell survival by blocking TASK3 channels. 4. Besides TASK3, other K_2P channels (TASK1, TREK1, TASK5 and THIK2) have been linked to tumor development. B. A neuron under ischemic conditions is shown. 1. TREK1 channels are activated by lower internal pH values, PUFAs, riluzole and inhalational anesthetics. 2. TASK1 channels are inhibited by anandamide, lower external pH values, hypoxia and activated by inhalational anesthetics. 3. Activation of potassium channels limits a harmful overload of the internal calcium concentration. 4. Depletion of potassium can, on the other hand, induce apoptosis. C. A number of ion channels are expressed on T lymphocytes. 1. TCR activation leads to an elevation of the internal calcium concentration necessary for further gene‐transcription and proliferation. 2. Potassium channels provide the counterbalancing driving force mandatory for this calcium influx. 3. TASK channels are inhibited by low external pH value and hypoxia, which might occur under inflammatory conditions. D. 1. TASK channels on interneurons may suppress intrinsic pacemaking and spontaneous firing in pyramidal cells. 2. An elevated TASK2 expression may dampen epileptic activity in response to the early seizure‐related alkalinization. 3. Neurotransmitters hyperpolarize pyramidal cells by activating TWIK1 and TREK2 channels. 4. TASK1 downregulation in astrocytes dampens seizure activity. Abbrevations: Ac = astrocyte; [Ca²⁺]_i = internal calcium concentration; CaM = Calmodulin; CRAC =  Calcium release‐activated calcium channel; GABA = gamma‐aminobutyric acid; 5‐HT = serotonin; IKCa1; intermediate calcium‐dependent potassium channel; IKIR = inward rectifier K⁺ channel; In = interneuron; [K⁺]_e = external potassium concentration; NA =  noradrenalin; pH↓ = lower pH value; pH↑ = increased pH value; PUFA = polyunsaturated fatty acids; Py = pyramidal cell; O₂↓ = hypoxia; TASK = TWIK‐related acid‐sensitive K⁺ channel; TCR = T‐cell receptor; THIK = tandem pore domain halothane‐inhibited K⁺ channel; TRESK =  TWIK‐related spinal‐cord K⁺ channel. Dashed lines indicate receptor‐mediated signaling pathways.

It should briefly be mentioned here that other members of the K_2P family as well have been linked to different cancer entities, like TWIK‐related K⁺‐channel gene‐1 (TREK1) to prostate carcinoma (130) and ALL(127), THIK2 to tubular breast carcinoma, synovial sarcoma and malignant peripheral nerve sheet tumor 94, 95, 113 and TASK5 to colon cancer and acute lymphoblastic leukemia (ALL) (119).

Taken together, the influence of TASK channels on tumors depends on the exact channel subunit, expressing cell type and surrounding pathophysiological stimuli (105). Increasing evidence points to an involvement of TASK channels in cell proliferation, apoptosis and tumorigenesis.

TASK CHANNELS AND ISCHEMIA

Background

Neuronal damage caused by ischemic or inflammatory events is an important cause for persistent disability and death in clinical practice. So far, the molecular mechanisms leading to neuronal death are poorly understood. Mild hypoxia or inflammation can, for example, induce neuroprotective signaling cascades preventing neuronal death 23, 34, 107. The role of K_2P channels during cellular stress remains unclear and is discussed controversially.

Activation of potassium channels results in membrane hyperpolarization, which has important implications for the function and survival of neurons: decreased neuronal activity and thereby lower metabolic demands could enhance neuronal survival under critical conditions. Moreover, the opening of voltage‐dependent Ca²⁺ channels in presynaptic neurons would be limited. Alongside an enhancement of the Mg²⁺ block of NMDA receptors (N‐methyl D‐aspartate) in postsynaptic neurons (80), this would protect against glutamate excitotoxicity 27, 39. Additionally, an immediate effect of neuronal depolarization might be a direct excitotoxic neuronal damage caused by prolonged influx of Ca²⁺ ions via different routes, including a reversal of the Na⁺/Ca²⁺ antiporter. The protective effect of K⁺ channels against an internal overload of Ca²⁺ ions, which would lead to apoptosis by activation of proteases, lipases and endonucleases, has been previously linked to other K⁺ channels [e.g. large conductance Ca²⁺‐activated K⁺ channels and adenosine triphosphate (ATP)‐sensitive K⁺ channels 36, 60]. This protective effect might therefore be true for different potassium channels remaining active during potentially harmful pathophysiological conditions.

Experimental data

So far, research has focused on a putative neuroprotective role of TREK and TASK channels. The open probability of TREK1 and TREK2 channels is strongly increased upon reduction of the intracellular pH value 1, 78, which can be observed in ischemia or inflammation (73). Polyunsaturated fatty acids (PUFAs)—known to provide neuroprotection from cerebral ischemia (59)—as well as riluzole—the first approved neuroprotective agent for the treatment of amyotrophic lateral sclerosis—exert their neuroprotective potential at least partially through activation of TREK1 channels 25, 38. Alpha linelonic acid and lysophosphatidylcholine (TREK1 activators) showed no additional effect in TREK1^−/− mice proofing the specificity of the protective effect. TREK1^−/− mice displayed significantly reduced neuronal survival rates in a model of cerebral ischemia (37).

These findings were supported by the observation of a worse outcome in a transient middle cerebral artery occlusion model of cerebral ischemia in TASK1^−/− mice compared to controls (88). TASK1 and 3 exert an important contribution to the standing outward current (I_SO) of neurons, which regulates, among others, the activity of neurons and shapes the action potential generation. In brain slices, pH reduction and O₂ deprivation led to a ∼40% reduction of the whole cell outward current resulting in membrane depolarization and concomitantly to an increase in neuronal activity. In vivo, TASK1^−/− mice showed significantly increased infarct volumes and this situation could be mimicked by the TASK1/3 inhibitor anandamide (88). Both TASK1 and TASK3 are inhibited by low extracellular pH values. TASK1 is also sensitive to hypoxic conditions (101), whereas reports are still inconsistent regarding TASK3 [(14) versus (35)]. Detailed single‐channel electrophysiological studies in rat glomus cells revealed recently that all TASK1, TASK3 and TASK1/3 heterodimers are inhibited by hypoxia (49). It was suggested that species differences might be responsible for different reports concerning TASK channels and oxygen sensitivity (136). Of notice, the phenotype of TASK2^−/− mice upon ischemic brain damage has not been reported so far, while the infarct volume in TASK3^−/− mice was unchanged (92).

This line of evidence is supported by experimental data describing a neuroprotective role of inhalational anesthetics, which is thought to be mediated by an inhibition of excitatory activity. Besides direct antagonistic mechanism on NMDA and amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid (AMPA) receptors and a potentiation of gamma‐aminobutyric acid (GABA) receptors, the neuroprotective effects of inhalation anesthetics have also been connected to an activation of K_2P channels 27, 54. Many but not all K_2P channels are opened by volatile anesthetics [TREK1, TREK2, TASK1, TASK3, TASK2, TWIK‐related alkaline‐pH‐activated K⁺ channel‐2 (TALK2) and TWIK‐related spinal‐cord K⁺ channel (TRESK) 29, 31, 67, 74, 81, 103]. Supportively, knockout of the predominantly neuronal expressed channels TREK1, TASK1 and TASK3 37, 71, 72 results in moderate resistance to inhalational anesthesia, while this effect is not observed in TASK2^−/− or KCNK7^−/− mice 28, 140.

On the other hand, an intracellular depletion of K⁺ is also a typical sign of early apoptosis 61, 75, resulting in neuronal cell death. This situation might especially occur under longer lasting pathophysiological conditions as in chronic (autoimmune) inflammation. Recently, TASK1^−/− mice were shown to be less susceptible to neuronal degeneration in myelin oligodendrocyte glycoprotein (MOG) peptide‐induced experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS) (9). Axonal loss was significantly reduced in TASK1^−/− mice. The chosen experimental model, however, does not allow to clearly dissect between central nervous system‐related neuroprotective effects of TASK1 deficiency and an indirect neuroprotection via an attenuated inflammation‐mediated neurodegeneration. Therefore, experiments were performed with acute brain slices from naïve wild‐type and TASK1^−/− mice which were cocultured with CD4⁺ T lymphocytes isolated from immunized wild‐type EAE mice at disease maximum. This approach ensured an identical inflammatory component under both conditions. A significant reduction of neuronal apoptosis in brain slices from TASK1^−/− mice argued in favor of a direct neuroprotective impact of TASK channel inhibition on neurons under these experimental conditions. As a note of caution, it should be kept in mind that the axonal degeneration in EAE is rather acute and mostly driven by T cell infiltration. Targeting the role of TASK1 in mouse models of Parkinson's, Alzheimer's or motor neuron disease should provide further insight into the role of TASK channels in the future.

The described phenotype is in contrast to the increased susceptibility of TASK1^−/− mice to acute ischemic brain damage as discussed above. It might be speculated that the diverse contribution of TASK channels on neuronal survival depends on the strength and duration of the pathophysiological stimulus. While TASK channel might contribute to neuronal hyperpolarization in the early phase of a pathological stimulus (acute ischemia), the situation might change as soon as a certain threshold of damage severity is reached (chronic inflammatory ischemia). Now the role of TASK channels switches to being a player in neuronal apoptosis. Therefore, inhibition of TASK channels might be neuroprotective in the long‐term course by preventing neuronal apoptosis. It should also be mentioned here that lowering of the extracellular pH value will not only affect TASK channels but also a variety of other targets as well, including several ion channels like transient receptor potential vanilloid 1 (TRPV1), acid sensing ion channels (ASICs) and NMDA receptors. This makes it difficult to predict the overall effect of acidosis in vivo.

TASK CHANNELS IN (AUTOIMMUNE) INFLAMMATION

Background

Human T lymphocytes are key players in the pathogenesis of autoimmune diseases like MS, rheumatoid arthritis (RA) or type I diabetes mellitus (7). The underlying pathophysiological processes of these diseases are still not completely understood but a major contribution of ion channels to T lymphocyte functions has been suggested. Therefore ion channels were appraised as potential target structures for future pharmacological approaches 53, 135. Upon T cell receptor activation, a longer lasting elevation of the intracellular calcium concentration is mandatory for transcription‐dependent steps of T cell activation 69, 96, 110 and potassium channels provide the counterbalancing hyperpolarizing current, which preserves the electrochemical driving force for an influx of Ca²⁺ ions (99).

So far, three different potassium currents could be identified to play an important role on T lymphocytes: a voltage‐gated K⁺ channel [K_V1.3 (20)], a Ca²⁺‐activated K⁺ channel [IKCa1 or K_Ca3.1 (64)] and the recently identified K_2P channels TASK1, TASK2, TASK3 and TRESK 8, 83, 109. Selective blockade of K_V1.3 influences T cell effector functions leading to reduced cytokine production and suppressed proliferation rates 5, 7. K_V1.3 knockout mice did not show the expected phenotype because of compensating chloride currents (55) but in vivo blockade of K_V1.3 in rats and mini swine significantly ameliorated the disease course of EAE and delayed‐type hypersensitivity response 6, 56. EAE animals showed a later disease onset and reduced disease maximum after immunization. Electrophysiological characterization of human T cell subtypes was used to investigate differential regulation of K_V1.3 and IKCa1 after antigen stimulation: Unstimulated T cells express about 200–300 copies of K_V1.3 and chronic T cell stimulation in vitro results in a selective and stable upregulation of K_V1.3 channels [1.500/cell (4)] on a special CD4⁺ T cell subtype named CCR7^‐CD45RA^‐ effector memory T cells (T_EM), which are thought to have an substantial impact in autoimmune diseases (134).

Experimental data

Most recently, three members of the K_2P family, TASK1, TASK2 and TASK3 could be identified and functionally characterized in the context of T cell activation 8, 9, 83. Human, rat and mouse T lymphocytes constitutively express TASK1, TASK2 and TASK3 and pharmacological inhibition leads to reduced cytokine production and proliferation after T cell receptor stimulation. Patch‐clamp experiments of stimulated T lymphocytes revealed an important anandamide‐sensitive component of the whole cell outward current and other current characteristics indicative of TASK channels. Based on these measurements and a numerical model analysis, it could be shown that around 40% of the total outward current is mediated by TASK channels. The in vivo relevance of TASK1/3 for T cell activation and effector functions could be confirmed in an active transfer EAE model in rats. Selective blockade of TASK channels on MBP specific T cells before i.v. transfer resulted in a reduced disease activity (83). Supportingly, TASK1^−/− mice showed a significantly ameliorated disease course upon MOG_35‐55 immunization, which was accompanied by a reduced activation of the immune system while naïve T lymphocytes per se were not severely impaired in TASK1^−/− mice. This EAE phenotype could be reproduced by the TASK channel inhibitor anandamide in wildtype mice when administered daily resulting in a delayed disease onset and reduced disease severity over 50 days. Remarkably, anandamide was equally effective when treatment only started after occurrence of first disease symptoms (9) or using a preventive treatment strategy (starting from the day of disease induction). Differential regulation of TASK1,3 during MOG‐induced EAE in rats was confirmed by immunohistochemistry and in situ hybridization revealing increased neuronal expression of TASK1,3 in the optic nerve and decreased expression in the spinal cord and thalamus. Lesion analysis of human brain autopsies displayed a similar downregulation of TASK channels emphasizing the regulatory role for TASK channels in neurons under pathophysiological conditions (87). Furthermore, human as well as rodent T lymphocytes functionally express TASK2 (see Fig. 2), which was found to be upregulated in MS patients with active disease.

A putative role of other members of the K_2P family in the immune system still needs to be addressed. Recently, an expression of the calcineurin‐regulated K_2P channel TRESK on Jurkat cells was postulated 109, 129 but a functional role for TRESK still lacks confirmation on primary T cells. This point is critical because of known differences in channel expression between leukemic and normal T cells [e.g. small conductance Ca²⁺‐activated K⁺ channels (SK2) 32, 43] and further studies are warranted to clarify the relevance of TRESK channel expression on immune cells in vivo. Additionally, an expression of TREK2 in mouse B lymphocytes has been described recently (141).

Taken together, TASK channels have been demonstrated to play a role in the regulation of the membrane potential of T lymphocytes and for immunmodulation in vitro and in vivo and might therefore be an interesting new molecular target for pharmacological approaches in autoimmune diseases. The rising plurality of different endogenous potassium channels on T lymphocytes, combined with multiple mechanisms of regulation, underlines the importance of a tightly controlled membrane potential of these cells. However, the picture is also becoming more complex demanding further studies on the role of ion channels in the immune system.

TASK CHANNELS IN EPILEPSY/SLEEP

Background

Epilepsy is a common chronic neurological condition that is characterized by the spontaneous occurrence of local or generalized episodes of neuronal discharges (112). These recurrent seizures are correlated with abnormal excessive and synchronous neuronal activity in the brain. Some types of frontal, generalized and infantile seizure syndromes have been associated with mutations in genes coding for protein subunits of ion channels, abnormal synaptic transmission, modulation of transmembrane K⁺ gradients and activity‐dependent shifts in extracellular pH values. The extracellular pH in the brain is tightly regulated and displays only minimal changes (45). However there are physiological as well as pathological conditions in which the extracellular pH of the brain changes. Seizure activity for example results in a biphasic pH shift, consisting of an initial extracellular alkalinization followed by a delayed acidification (138). Since K_2P channels are highly regulated by pH, several subtypes have been investigated in neurons as well as glia cells with respect to their possible involvement in the generation of and adaptation to epilepsy (see Fig. 2).

Experimental data

Most information is available for the role of TASK channels in hippocampal and entorhinal cortical regions representing key structures in temporal lobe epilepsy (93). In the rat hippocampus, TASK channels are differentially expressed in neurons and glia cells of different subregions 50, 52. In the CA1 region, TASK1 expression has been found to some extent in pyramidal cells and horizontal interneurons, while TASK3 was strongly expressed in pyramidal cells (126). The resulting hyperpolarizing effect of TASK channels may suppress intrinsic pacemaking and spontaneous firing in pyramidal cells, an electrophysiological behavior that is otherwise typical for interneurons (see Fig. 2). A second consequence may be the cell type‐specific sensitivity to extracellular acidification in conditions such as epilepsy. In addition, TASK2 expression was found in CA1‐3 pyramidal cells, dentate gyrus granula cells and perivascular astrocytes (51).

Astroglial expression of TASK1 was also described in the hippocampus of gerbils (51), a well‐established animal model of inherited epilepsy (13). In seizure‐sensitive strains, offspring of epileptic parents have a high probability of developing epilepsy in later life and individual seizures can be induced by mechanical stimulation of the back. Characteristic changes in TASK channel expression occur before and after the onset/induction of epileptic seizures. In gerbils, hippocampal expression of TASK1 and TASK2 was not different between young seizure‐sensitive and seizure‐resistant individuals (50). In adult seizure‐sensitive animals, the expression of TASK1 in astrocytes was higher compared to epilepsy‐free animals. After an individual seizure, TASK1 expression in astrocytes was downregulated. The general upregulation of TASK1 in astrocytes of seizure‐sensitive animals may diminish astroglial K⁺ buffering through an increased K⁺ outward rectification leading to slightly increased extracellular K⁺ levels and thus increased excitability. The rapid downregulation of TASK1 expression in astrocytes during an individual seizure is part of a fast adaptation process that helps to dampen seizure activity by reducing astroglial outward rectification. In this respect, it is interesting to note that several anti‐epileptic drugs reduce the expression of TASK1 channels in astrocytes (50). In rat hippocampus, experimentally induced temporal lobe epilepsy is associated with an increase of TASK2 expression in the CA3 pyramidal layer, dentate granule layer, and the endfeet of perivascular astrocytes (50). The elevated TASK2 expression in neurons probably represents a rapid adaptation that results in hyperpolarization and dampening of epileptic activity in response to the early seizure‐related alkalinization (see Fig. 2). The upregulated TASK2 expression in perivascular regions may contribute to hippocampal damage by contributing to abnormal blood flow and/or blood–brain barrier abnormalities.

Findings in interneurons of the entorhinal cortex add to our knowledge concerning the role of TASK3 channels in temporal lobe epilepsy. The entorhinal cortex receives strong serotonergic inputs from the raphe nuclei (10) and serotonin inhibits neuronal excitability (33). In GABA‐ergic interneurons of the entorhinal cortex, application of serotonin generates membrane depolarization mediated by inhibition of TASK3 channels and an increase in action potential firing (21). In consequence, the release of GABA by interneurons is increased, resulting in decreased pyramidal cell activity and inhibition of low‐Mg²⁺‐induced seizure activity. Two other types of K_2P channels present on entorhinal pyramidal cells, namely TWIK1 and TREK2, may be the molecular basis for the anti‐epileptic effects of monoamines (see Fig. 2). Serotonin and noradrenalin hyperpolarize entorhinal pyramidal cells by activating TWIK1 and TREK2 channels, respectively 22, 137. As a result, neuronal excitability in the entorhinal cortex and its hippocampal projection areas is inhibited.

The thalamocortical neuronal network is characterized by two fundamentally different states of activity (122): whereas low‐frequency (≤15 Hz) oscillatory activity occurs during natural sleep, deep anesthesia and absence epilepsy, high‐frequency oscillatory activity in the gamma range and tonic activity prevail during wakefulness. The switch between the two states of thalamic activity is accompanied by a depolarizing shift of the membrane potential of thalamocortical relay neurons and is governed by the neurotransmitter‐induced inhibition of TASK1 and TASK3 channels 82, 84. Therefore TASK3 has been regarded as a promising candidate gene for absence epilepsy in humans. After localizing the human TASK3 in the chromosomal region 8q24, a mutation analysis of the TASK3 gene in absence epilepsy patients revealed one exon‐2 polymorphism, which, however, was not associated with the disease (46). In another attempt to map a genetic cause of idiopathic generalized epilepsies in humans, a balanced translocation breakpoint involving chromosome 6p21 has been found (117). A sequence analysis revealed the genes of TALK1 and TALK2 at a distance of 3.5 kb downstream of the breakpoint. No malfunction of these K_2P channels was found.

In addition to the K_2P subtypes discussed above, TREK1 channels are involved in neuroprotection against epilepsy (37). TREK1‐deficient mice are hypersensitive to drug‐induced seizures and show no PUFA‐dependent neuroprotection against epilepsy.

It is concluded that TASK channels might have both an anti‐epileptic as well as a pro‐epileptic potential; the relative contribution of these opposing influences depend on their cell type‐specific expression and the exact conditions of the cellular environment. In addition genetic studies in humans were not able to demonstrate a coupling between mutated K_2P genes and epilepsy. In comparison to their role in ischemia, which involves a long‐term anti‐apoptotic effect, the influence of TASK channels on cellular excitability acts rather in the short term. Based on the findings discussed here the therapeutic potential of K_2P channels was emphasized in many cases 37, 50, 128.

CONCLUSION

Pathophysiological conditions of different etiology share comparable stimuli on the cellular level albeit their relative contribution to a specific disorder differs in strength and duration. It is particularly worth noting that oxygen deprivation, extra‐ and/or intracellular acidification, endogenous cannabinoids, disturbances of K⁺ homeostasis or actions by neurotransmitters (e.g. glutamate) belong to a multifarious repertoire of alterations that are particularly observed during potentially harmful processes. Based on their biophysical properties, expression pattern and complex physicochemical regulation, TASK channels represent target structures that are influenced by the stimuli mentioned above. Therefore, they have the potential to integrate these signals on the molecular level, resulting in modulation of basal cellular parameters (e.g. K⁺ homeostasis, resting membrane potential, action potential generation).

A dual influence of TASK channels on neuronal survival evolves during different pathophysiological situations: TASK channels contribute to neuronal hyperpolarization in the early phase of a moderate enduring pathophysiological stimulus (e.g. under ischemic condition), thus decreasing neuronal activity and preventing excessive Ca²⁺ overload. The situation changes once a certain threshold of damage severity is reached (e.g. long‐lasting inflammation). Now the role of TASK channels switches to being a player in neuronal apoptosis as K⁺ efflux can be a characteristic initial feature of early apoptosis. Therefore, inhibition of TASK channels might be neuroprotective in the long‐term course by preventing neuronal cell death.

TASK channels have also been demonstrated to be relevant for the regulation of the membrane potential of T lymphocytes and for immunmodulation in vitro and in vivo. Therefore they might be an interesting new molecular target for pharmacological approaches in autoimmune diseases. However many yet unsolved questions still await clarification: Possible off‐target effects of TASK channel modulation? Are TASK channels differently expressed on various immune cell subtypes (e.g. T effector cells, regulatory T cells)? How does antigen‐specific stimulation influence TASK channel expression? Are TASK channel functions regulated intracellularly in immune cells? What is the contribution of TASK channels in different autoimmune disorders (e.g. MS, RA)? Future research efforts will help to shed light on these and further unsolved questions (26).

Furthermore, TASK channels have an important influence on cellular excitability, thereby influencing epileptic conditions. In this context, they seem to have both an anti‐epileptic, as well as a pro‐epileptic potential. The relative contribution of these opposing influences most likely depends on their cell type‐specific expression and the exact micromilieu. However, genetic analysis has so far revealed no clear association between epilepsy and TASK channel expression and function.

Finally, we do not want to argue that TASK channels represent the Rosetta stone for diagnosis and treatment of neurological disorders. However, based on their expression pattern, biophysical regulation and their impact on basal cellular parameters like the resting membrane potential, they seem to be critically involved in pathophysiological scenarios shared by different neurological disorders. Highly specific inhibitors and activators of these channels (e.g. small peptide ligands and blocking antibodies), specific antibodies and further knockout animals (e.g. TRESK) are warranted to complete the scientific armamentarium for the investigation of the pathophysiological role of these interesting—potentially therapeutic and useful—molecular structures.