Mechanism of inhibition of TREK-2 (K2P10.1) by the Gq-coupled M3 muscarinic receptor

Atomistic mechanism of coupling between cytosolic sensor domain and selectivity filter in TREK K2P channels

The two-pore domain potassium (K2P) channels TREK-1 and TREK-2 link neuronal excitability to a variety of stimuli including mechanical force, lipids, temperature and phosphorylation. This regulation involves the C-terminus as a polymodal stimulus sensor and the selectivity filter (SF) as channel gate. Using crystallographic up- and down-state structures of TREK-2 as a template for full atomistic molecular dynamics (MD) simulations, we reveal that the SF in down-state undergoes inactivation via conformational changes, while the up-state structure maintains a stable and conductive SF. This suggests an atomistic mechanism for the low channel activity previously assigned to the down state, but not evident from the crystal structure. Furthermore, experimentally by using (de-)phosphorylation mimics and chemically attaching lipid tethers to the proximal C-terminus (pCt), we confirm the hypothesis that moving the pCt towards the membrane induces the up-state. Based on MD simulations, we propose two gating pathways by which movement of the pCt controls the stability (i.e., conductivity) of the filter gate. Together, these findings provide atomistic insights into the SF gating mechanism and the physiological regulation of TREK channels by phosphorylation.

Activity of TREK-2-like Channels in the Pyramidal Neurons of Rat Medial Prefrontal Cortex Depends on Cytoplasmic Calcium

Simple Summary

The pyramidal neurons of rat prefrontal cortex express potassium channels identified as a non-canonical splice variant of the TREK-2 channel. The main function of TREK channels is to regulate the resting membrane potential. We showed that cytoplasmic Ca²⁺ upregulates the activity of TREK-2-like channels. Previous studies have indicated that the activation of TREK-2 channels is mediated by PI(4,5)P2, a polyanionic lipid in the inner leaflet of the plasma membrane. While TREK channels are believed to not be regulated by calcium, our work shows otherwise. We propose a model in which calcium ions enable the formation of PI(4,5)P2 nanoclusters, which stabilize active conformation of the channel.

Abstract

TREK-2-like channels in the pyramidal neurons of rat prefrontal cortex are characterized by a wide range of spontaneous activity—from very low to very high—independent of the membrane potential and the stimuli that are known to activate TREK-2 channels, such as temperature or membrane stretching. The aim of this study was to discover what factors are involved in high levels of TREK-2-like channel activity in these cells. Our research focused on the PI(4,5)P2-dependent mechanism of channel activity. Single-channel patch clamp recordings were performed on freshly dissociated pyramidal neurons of rat prefrontal cortexes in both the cell-attached and inside-out configurations. To evaluate the role of endogenous stimulants, the activity of the channels was recorded in the presence of a PI(4,5)P2 analogue (PI(4,5)P2DiC8) and Ca²⁺. Our research revealed that calcium ions are an important factor affecting TREK-2-like channel activity and kinetics. The observation that calcium participates in the activation of TREK-2-like channels is a new finding. We showed that PI(4,5)P2-dependent TREK-2 activity occurs when the conditions for PI(4,5)P2/Ca²⁺ nanocluster formation are met. We present a possible model explaining the mechanism of calcium action.

Are TREK Channels Temperature Sensors?

Internal human body normal temperature fluctuates between 36.5 and 37.5°C and it is generally measured in the oral cavity. Interestingly, most electrophysiological studies on the functioning of ion channels and their role in neuronal behavior are carried out at room temperature, which usually oscillates between 22 and 24°C, even when thermosensitive channels are studied. We very often forget that if the core of the body reached that temperature, the probability of death from cardiorespiratory arrest would be extremely high. Does this mean that we are studying ion channels in dying neurons? Thousands of electrophysiological experiments carried out at these low temperatures suggest that most neurons tolerate this aggression quite well, at least for the duration of the experiments. This also seems to happen with ion channels, although studies at different temperatures indicate large changes in both, neuron and channel behavior. It is known that many chemical, physical and therefore physiological processes, depend to a great extent on body temperature. Temperature clearly affects the kinetics of numerous events such as chemical reactions or conformational changes in proteins but, what if these proteins constitute ion channels and these channels are specifically designed to detect changes in temperature? In this review, we discuss the importance of the potassium channels of the TREK subfamily, belonging to the recently discovered family of two-pore domain channels, in the transduction of thermal sensitivity in different cell types.

Muscimol Directly Activates the TREK-2 Channel Expressed in GABAergic Neurons through Its N-Terminus

The two-pore domain K⁺ (K_2P) channel, which is involved in setting the resting membrane potential in neurons, is an essential target for receptor agonists. Activation of the γ-aminobutyric acid (GABA) receptors (GABA_AR and GABA_BR) reduces cellular excitability through Cl^- influx and K⁺ efflux in neurons. Relatively little is known about the link between GABA_AR and the K⁺ channel. The present study was performed to identify the effect of GABAR agonists on K_2P channel expression and activity in the neuroblastic B35 cells that maintain glutamic acid decarboxylase (GAD) activity and express GABA. TASK and TREK/TRAAK mRNA were expressed in B35 cells with a high level of TREK-2 and TRAAK. In addition, TREK/TRAAK proteins were detected in the GABAergic neurons obtained from GABA transgenic mice. Furthermore, TREK-2 mRNA and protein expression levels were markedly upregulated in B35 cells by GABA_AR and GABA_BR agonists. In particular, muscimol, a GABA_AR agonist, significantly increased TREK-2 expression and activity, but the effect was reduced in the presence of the GABA_AR antagonist bicuculine or TREK-2 inhibitor norfluoxetine. In the whole-cell and single-channel patch configurations, muscimol increased TREK-2 activity, but the muscimol effect disappeared in the N-terminal deletion mutant. These results indicate that muscimol directly induces TREK-2 activation through the N-terminus and suggest that muscimol can reduce cellular excitability by activating the TREK-2 channel and by inducing Cl^- influx in GABAergic neurons.

PIP2 Mediated Inhibition of TREK Potassium Currents by Bradykinin in Mouse Sympathetic Neurons

Bradykinin (BK), a hormone inducing pain and inflammation, is known to inhibit potassium M-currents (I_M) and to increase the excitability of the superior cervical ganglion (SCG) neurons by activating the Ca²⁺-calmodulin pathway. M-current is also reduced by muscarinic agonists through the depletion of membrane phosphatidylinositol 4,5-biphosphate (PIP₂). Similarly, the activation of muscarinic receptors inhibits the current through two-pore domain potassium channels (K2P) of the “Tandem of pore-domains in a Weakly Inward rectifying K⁺ channel (TWIK)-related channels” (TREK) subfamily by reducing PIP₂ in mouse SCG neurons (mSCG). The aim of this work was to test and characterize the modulation of TREK channels by bradykinin. We used the perforated-patch technique to investigate riluzole (RIL) activated currents in voltage- and current-clamp experiments. RIL is a drug used in the palliative treatment of amyotrophic lateral sclerosis and, in addition to blocking voltage-dependent sodium channels, it also selectively activates the K2P channels of the TREK subfamily. A cell-attached patch-clamp was also used to investigate TREK-2 single channel currents. We report here that BK reduces spike frequency adaptation (SFA), inhibits the riluzole-activated current (I_RIL), which flows mainly through TREK-2 channels, by about 45%, and reduces the open probability of identified single TREK-2 channels in cultured mSCG cells. The effect of BK on I_RIL was precluded by the bradykinin receptor (B₂R) antagonist HOE-140 (d-Arg-[Hyp³, Thi⁵, d-Tic⁷, Oic⁸]BK) but also by diC₈PIP₂ which prevents PIP₂ depletion when phospholipase C (PLC) is activated. On the contrary, antagonizing inositol triphosphate receptors (IP₃R) using 2-aminoethoxydiphenylborane (2-APB) or inhibiting protein kinase C (PKC) with bisindolylmaleimide did not affect the inhibition of I_RIL by BK. In conclusion, bradykinin inhibits TREK-2 channels through the activation of B₂Rs resulting in PIP₂ depletion, much like we have demonstrated for muscarinic agonists. This mechanism implies that TREK channels must be relevant for the capture of information about pain and visceral inflammation.

TREK-1 protects the heart against ischemia-reperfusion-induced injury and from adverse remodeling after myocardial infarction

The TWIK-related K+ channel (TREK-1) is a two-pore-domain potassium channel that produces background leaky potassium currents. TREK-1 has a protective role against ischemia-induced neuronal damage. TREK-1 is also expressed in the heart, but its role in myocardial ischemia-reperfusion (IR)-induced injury has not been examined. In the current study, we used a TREK-1 knockout (KO) mouse model to show that TREK-1 has a critical role in the cardiac I/R-induced injury and during remodeling after myocardial infarction (MI). At baseline, TREK-1 KO mice had similar blood pressure and heart rate as the wild-type (WT) mice. However, the lack of TREK-1 was associated with increased susceptibility to ischemic injury and compromised functional recovery following ex vivo I/R-induced injury. TREK-1 deficiency increased infarct size following permanent coronary artery ligation, resulting in greater systolic dysfunction than the WT counterpart. Electrocardiographic (ECG) analysis revealed QT interval prolongation in TREK-1 KO mice, but normal heart rate (HR). Acutely isolated TREK-1 KO cardiomyocytes exhibited prolonged Ca²⁺ transient duration associated with action potential duration (APD) prolongation. Our data suggest that TREK-1 has a protective effect against I/R-induced injury and influences the post-MI remodeling processes by regulating membrane potential and maintaining intracellular Ca²⁺ homeostasis. These data suggest that TREK-1 activation could be an effective strategy to provide cardioprotection against ischemia-induced damage.

A Soluble Fluorescent Binding Assay Reveals PIP2 Antagonism of TREK-1 Channels

Lipid regulation of ion channels by low-abundance signaling lipids phosphatidylinositol 4,5-bisphosphate (PIP₂) and phosphatidic acid (PA) has emerged as a central cellular mechanism for controlling ion channels and the excitability of nerves. A lack of robust assays suitable for facile detection of a lipid bound to a channel has hampered the probing of the lipid binding sites and measuring the pharmacology of putative lipid agonists for ion channels. Here, we show a fluorescent PIP₂ competition assay for detergent-purified potassium channels, including TWIK-1-related K⁺-channel (TREK-1). Anionic lipids PA and phosphatidylglycerol (PG) bind dose dependently (9.1 and 96 mM, respectively) and agonize the channel. Our assay shows PIP₂ binds with high affinity (0.87 mM) but surprisingly can directly antagonize TREK-1 in liposomes. We propose a model for TREK-1 lipid regulation where PIP₂ can compete with PA and PG agonism based on the affinity of the lipid for a site within the channel.

Activation of TREK-1, but Not TREK-2, Channel by Mood Stabilizers

Earlier studies have demonstrated that the tandem pore domain weak inward rectifying K⁺ channel (TWIK)-related K⁺ (TREK)-1 channel is inhibited by antidepressants and is associated with major depression. However, little is known about the effect of mood stabilizers that are commonly used for treatment of bipolar disorder on TREK channels, members of the two-pore domain K⁺ (K_2P) channel family. This study sought to investigate the effect of mood stabilizers on TREK-1 and TREK-2 channels. HEK-293A cells were transfected with human TREK-1 or TREK-2 DNA. The effect of mood stabilizers on TREK-1 and TREK-2 was studied using the patch clamp technique. Changes in TREK protein expression by mood stabilizers were studied in the HT-22 mouse hippocampal neuronal cells using western blot analysis. Lithium chloride (LiCl, 1 mM), gabapentin (100 μM), valproate (100 μM), and carbamazepine (100 μM) increased TREK-1 currents by 31 ± 14%, 25 ± 11%, 28 ± 12%, and 72 ± 12%, respectively, whereas they had no effect on TREK-2 channel activity. In addition, western blot analysis showed LiCl and carbamazepine slightly upregulated TREK-1 expression, but not TREK-2 in the HT-22 cells. These results suggest that TREK-1 could be a potential therapeutic target for treatment of bipolar disorders as well as depression, while TREK-2 is a target well suited for treatment of major depression.

Kinetic properties and adrenergic control of TREK-2-like channels in rat medial prefrontal cortex (mPFC) pyramidal neurons

TREK-2-like channels were identified on the basis of electrophysiological and pharmacological tests performed on freshly isolated and enzymatically/mechanically dispersed pyramidal neurons of the rat medial prefrontal cortex (mPFC). Single-channel currents were recorded in cell-attached configuration and the impact of adrenergic receptors (α₁, α₂, β) stimulation on spontaneously appearing TREK-2-like channel activity was tested. The obtained results indicate that noradrenaline decreases the mean open probability of TREK-2-like channel currents by activation of β₁ but not of α₁- and α₂-adrenergic receptors. Mean open time and channel conductance were not affected. The system of intracellular signaling pathways depends on the activation of protein kinase A. We also show that adrenergic control of TREK-2-like channel currents by adrenergic receptors was similar in pyramidal neurons isolated from young, adolescent, and adult rats. Immunofluorescent confocal scans of mPFC slices confirmed the presence of the TREK-2 protein, which was abundant in layer V pyramidal neurons. The role of TREK-2-like channel control by adrenergic receptors is discussed.

Voltage- and receptor-mediated activation of a non-selective cation channel in rat carotid body glomus cells

A recent study showed that hypoxia activates a Ca²⁺-sensitive, Na⁺-permeable non-selective cation channel (NSC) in carotid body glomus cells. We studied the effects of mitochondrial inhibitors that increase Ca²⁺ influx via Ca²⁺ channel (Ca_v), and receptor agonists that release Ca²⁺ from endoplasmic reticulum (ER) on NSC. Mitochondrial inhibitors (NaCN, FCCP, H₂S, NO) elevated [Ca²⁺]_i and activated NSC. Angiotensin II and acetylcholine that elevate [Ca²⁺]_i via the Gq-IP₃ pathway activated NSC. However, endothelin-1 (Gq) and 5-HT (Gq) showed little or no effect on [Ca²⁺]_i and did not activate NSC. Adenosine (Gs) caused a weak rise in [Ca²⁺]_i but did not activate NSC. Dopamine (Gs) and γ-aminobytyric acid (Gi) were ineffective in raising [Ca²⁺]_i and failed to activate NSC. Store-operated Ca²⁺ entry (SOCE) produced by depletion of Ca²⁺ stores with cyclopiazonic acid activated NSC. Our results show that Ca²⁺ entry via Ca_v, ER Ca²⁺ release and SOCE can activate NSC. Thus, NSC contributes to both voltage- and receptor-mediated excitation of glomus cells.

Two-pore Domain Potassium Channels in Astrocytes

Two-pore domain potassium (K_2P) channels have a distinct structure and channel properties, and are involved in a background K⁺ current. The 15 members of the K_2P channels are identified and classified into six subfamilies on the basis of their sequence similarities. The activity of the channels is dynamically regulated by various physical, chemical, and biological effectors. The channels are expressed in a wide variety of tissues in mammals in an isoform specific manner, and play various roles in many physiological and pathophysiological conditions. To function as channels, the K_2P channels form dimers, and some isoforms form heterodimers that provide diversity in channel properties. In the brain, TWIK1, TREK1, TREK2, TRAAK, TASK1, and TASK3 are predominantly expressed in various regions, including the cerebral cortex, dentate gyrus, CA1-CA3, and granular layer of the cerebellum. TWIK1, TREK1, and TASK1 are highly expressed in astrocytes, where they play specific cellular roles. Astrocytes keep leak K⁺ conductance, called the passive conductance, which mainly involves TWIK1-TREK1 heterodimeric channel. TWIK1 and TREK1 also mediate glutamate release from astrocytes in an exocytosis-independent manner. The expression of TREK1 and TREK2 in astrocytes increases under ischemic conditions, that enhance neuroprotection from ischemia. Accumulated evidence has indicated that astrocytes, together with neurons, are involved in brain function, with the K_2P channels playing critical role in these astrocytes.

Effects of analgesics and antidepressants on TREK-2 and TRESK currents

TWIK-related K⁺ channel-2 (TREK-2) and TWIK-related spinal cord K⁺ (TRESK) channel are members of two-pore domain K⁺ channel family. They are well expressed and help to set the resting membrane potential in sensory neurons. Modulation of TREK-2 and TRESK channels are involved in the pathogenesis of pain, and specifi c activators of TREK-2 and TRESK may be benefi cial for the treatment of pain symptoms. However, the effect of commonly used analgesics on TREK-2 and TRESK channels are not known. Here, we investigated the effect of analgesics on TREK-2 and TRESK channels. The effects of analgesics were examined in HEK cells transfected with TREK-2 or TRESK. Amitriptyline, citalopram, escitalopram, and fluoxetine significantly inhibited TREK-2 and TRESK currents in HEK cells (p<0.05, n=10). Acetaminophen, ibuprofen, nabumetone, and bupropion inhibited TRESK, but had no effect on TREK-2. These results show that all analgesics tested in this study inhibit TRESK activity. Further study is needed to identify the mechanisms by which the analgesics modulate TREK-2 and TRESK differently.

Inhibition of TREK-2 K(+) channels by PI(4,5)P2: an intrinsic mode of regulation by intracellular ATP via phosphatidylinositol kinase

TWIK-related two-pore domain K(+) channels 1 and 2 (TREKs) are activated under various physicochemical conditions. However, the directions in which they are regulated by PI(4,5)P2 and intracellular ATP are not clearly presented yet. In this study, we investigated the effects of ATP and PI(4,5)P2 on overexpressed TREKs (HEK293T and COS-7) and endogenously expressed TREK-2 (mouse astrocytes and WEHI-231 B cells). In all of these cells, both TREK-1 and TREK-2 currents were spontaneously increased by dialysis with ATP-free pipette solution for whole-cell recording (ITREK-1,w-c and ITREK-2w-c) or by membrane excision for inside-out patch clamping without ATP (ITREK-1,i-o and ITREK-2,i-o). Steady state ITREK-2,i-o was reversibly decreased by 3 mM ATP applied to the cytoplasmic side, and this reduction was prevented by wortmannin, a PI-kinase inhibitor. An exogenous application of PI(4,5)P2 inhibited the spontaneously increased ITREKs,i-o, suggesting that intrinsic PI(4,5)P2 maintained by intracellular ATP and PI kinase may set the basal activity of TREKs in the intact cells. The inhibition of intrinsic TREK-2 by ATP was more prominent in WEHI-231 cells than astrocytes. Interestingly, unspecific screening of negative charges by poly-L-lysine also inhibited ITREK-2,i-o. Application of PI(4,5)P2 after the poly-L-lysine treatment showed dose-dependent dual effects, initial activation and subsequent inhibition of ITREK-2,i-o at low and high concentrations, respectively. In HEK293T cells coexpressing TREK-2 and a voltage-sensitive PI(4,5)P2 phosphatase, sustained depolarization increased ITREK-2,w-c initially (<5 s) but then decreased the current below the control level. In HEK293T cells coexpressing TREK-2 and type 3 muscarinic receptor, application of carbachol induced transient activation and sustained suppression of ITREK-2,w-c and cell-attached ITREK-2. The inhibition of TREK-2 by unspecific electrostatic quenching, extensive dephosphorylation, or sustained hydrolysis of PI(4,5)P2 suggests the existence of dual regulatory modes that depend on PI(4,5)P2 concentration.

Activation of TRESK channels by the inflammatory mediator lysophosphatidic acid balances nociceptive signalling

In dorsal root ganglia (DRG) neurons TRESK channels constitute a major current component of the standing outward current IK_SO. A prominent physiological role of TRESK has been attributed to pain sensation. During inflammation mediators of pain e.g. lysophosphatidic acid (LPA) are released and modulate nociception. We demonstrate co-expression of TRESK and LPA receptors in DRG neurons. Heterologous expression of TRESK and LPA receptors in Xenopus oocytes revealed augmentation of basal K⁺ currents upon LPA application. In DRG neurons nociception can result from TRPV1 activation by capsaicin or LPA. Upon co-expression in Xenopus oocytes LPA simultaneously increased both depolarising TRPV1 and hyperpolarising TRESK currents. Patch-clamp recordings in cultured DRG neurons from TRESK[wt] mice displayed increased IK_SO after application of LPA whereas under these conditions IK_SO in neurons from TRESK[ko] mice remained unaltered. Under current-clamp conditions LPA application differentially modulated excitability in these genotypes upon depolarising pulses. Spike frequency was attenuated in TRESK[wt] neurons and, in contrast, augmented in TRESK[ko] neurons. Accordingly, excitation of nociceptive neurons by LPA is balanced by co-activation of TRESK channels. Hence excitation of sensory neurons is strongly controlled by the activity of TRESK channels, which therefore are good candidates for the treatment of pain disorders.

Muscarinic modulation of TREK currents in mouse sympathetic superior cervical ganglion neurons

Muscarinic receptors play a key role in the control of neurotransmission in the autonomic ganglia, which has mainly been ascribed to the regulation of potassium M-currents and voltage-dependent calcium currents. Muscarinic agonists provoke depolarization of the membrane potential and a reduction in spike frequency adaptation in postganglionic neurons, effects that may be explained by M-current inhibition. Here, we report the presence of a riluzole-activated current (IRIL ) that flows through the TREK-2 channels, and that is also inhibited by muscarinic agonists in neurons of the mouse superior cervical ganglion (mSCG). The muscarinic agonist oxotremorine-M (Oxo-M) inhibited the IRIL by 50%, an effect that was abolished by pretreatment with atropine or pirenzepine, but was unaffected in the presence of himbacine. Moreover, these antagonists had similar effects on single-channel TREK-2 currents. IRIL inhibition was unaffected by pretreatment with pertussis toxin. The protein kinase C blocker bisindolylmaleimide did not have an effect, and neither did the inositol triphosphate antagonist 2-aminoethoxydiphenylborane. Nevertheless, the IRIL was markedly attenuated by the phospholipase C (PLC) inhibitor ET-18-OCH3. Finally, the phosphatidylinositol-3-kinase/phosphatidylinositol-4-kinase inhibitor wortmannin strongly attenuated the IRIL , whereas blocking phosphatidylinositol 4,5-bisphosphate (PIP2 ) depletion consistently prevented IRIL inhibition by Oxo-M. These results demonstrate that TREK-2 currents in mSCG neurons are inhibited by muscarinic agonists that activate M1 muscarinic receptors, reducing PIP2 levels via a PLC-dependent pathway. The similarities between the signaling pathways regulating the IRIL and the M-current in the same neurons reflect an important role of this new pathway in the control of autonomic ganglia excitability.

The family of K2P channels: salient structural and functional properties

Potassium channels participate in many biological functions, from ion homeostasis to generation and modulation of the electrical membrane potential. They are involved in a large variety of diseases. In the human genome, 15 genes code for K(+) channels with two pore domains (K2P ). These channels form dimers of pore-forming subunits that produce background conductances finely regulated by a range of natural and chemical effectors, including signalling lipids, temperature, pressure, pH, antidepressants and volatile anaesthetics. Since the cloning of TWIK1, the prototypical member of this family, a lot of work has been carried out on their structure and biology. These studies are still in progress, but data gathered so far show that K2P channels are central players in many processes, including ion homeostasis, hormone secretion, cell development and excitability. A growing number of studies underline their implication in physiopathological mechanisms, such as vascular and pulmonary hypertension, cardiac arrhythmias, nociception, neuroprotection and depression. This review gives a synthetic view of the most noticeable features of these channels.

Differential phospholipase C-dependent modulation of TASK and TREK two-pore domain K+ channels in rat thalamocortical relay neurons

KEY POINTS

During the behavioural states of sleep and wakefulness thalamocortical relay neurons fire action potentials in high frequency bursts or tonic sequences, respectively. The modulation of specific K(+) channel types, termed TASK and TREK, allows these neurons to switch between the two modes of activity. In this study we show that the signalling lipids phosphatidylinositol 4,5-bisphosphate (PIP2) and diacylglycerol (DAG), which are components of their membrane environment, switch on and shut off TREK and TASK channels, respectively. These channel modulations contribute to a better understanding of the molecular basis of the effects of neurotransmitters such as ACh which are released by the brainstem arousal system. The present report introduces PIP2 and DAG as new elements of signal transduction in the thalamus. The activity of two-pore domain potassium channels (K2P ) regulates the excitability and firing modes of thalamocortical (TC) neurons. In particular, the inhibition of two-pore domain weakly inwardly rectifying K(+) channel (TWIK)-related acid-sensitive K(+) (TASK) channels and TWIK-related K(+) (TREK) channels, as a consequence of the stimulation of muscarinic ACh receptors (MAChRs) which are coupled to phosphoinositide-specific phospholipase C (PLCβ), induces a shift from burst to tonic firing. By using a whole cell patch-clamp approach, the contribution of the membrane-bound second messenger molecules phosphatidylinositol 4,5-bisphosphate (PIP2 ) and diacylglycerol (DAG) acting downstream of PLCβ was probed. The standing outward current (ISO ) was used to monitor the current through TASK and TREK channels in TC neurons. By exploiting different manoeuvres to change the intracellular PIP2 level in TC neurons, we here show that the scavenging of PIP2 (by neomycin) results in an increased muscarinic effect on ISO whereas increased availability of PIP2 (inclusion to the patch pipette; histone-based carrier) decreased muscarinic signalling. The degree of muscarinic inhibition specifically depends on phosphatidylinositol phosphate (PIP) and PIP2 but no other phospholipids (phosphatidic acid, phosphatidylserine). The use of specific blockers revealed that PIP2 is targeting TREK but not TASK channels. Furthermore, we demonstrate that the inhibition of TASK channels is induced by the application of the DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG). Under current clamp conditions the activation of MAChRs and PLCβ as well as the application of OAG resulted in membrane depolarization, while PIP2 application via histone carrier induced a hyperpolarization. These results demonstrate a differential role of PIP2 and DAG in K2P channel modulation in native neurons which allows a fine-tuned inhibition of TREK (via PIP2 depletion) and TASK (via DAG) channels following MAChR stimulation.

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.

Activation of neurotensin receptor 1 facilitates neuronal excitability and spatial learning and memory in the entorhinal cortex: beneficial actions in an Alzheimer's disease model

Neurotensin (NT) is a tridecapeptide distributed in the CNS, including the entorhinal cortex (EC), a structure that is crucial for learning and memory and undergoes the earliest pathological alterations in Alzheimer's disease (AD). Whereas NT has been implicated in modulating cognition, the cellular and molecular mechanisms by which NT modifies cognitive processes and the potential therapeutic roles of NT in AD have not been determined. Here we examined the effects of NT on neuronal excitability and spatial learning in the EC, which expresses high density of NT receptors. Brief application of NT induced persistent increases in action potential firing frequency, which could last for at least 1 h. NT-induced facilitation of neuronal excitability was mediated by downregulation of TREK-2 K(+) channels and required the functions of NTS1, phospholipase C, and protein kinase C. Microinjection of NT or NTS1 agonist, PD149163, into the EC increased spatial learning as assessed by the Barnes Maze Test. Activation of NTS1 receptors also induced persistent increases in action potential firing frequency and significantly improved the memory status in APP/PS1 mice, an animal model of AD. Our study identifies a cellular substrate underlying learning and memory and suggests that NTS1 agonists may exert beneficial actions in an animal model of AD.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

Protein kinase A and C regulate leak potassium currents in freshly isolated vascular myocytes from the aorta

We tested the hypothesis that protein kinase A (PKA) inhibits K2P currents activated by protein kinase C (PKC) in freshly isolated aortic myocytes. PDBu, the PKC agonist, applied extracellularly, increased the amplitude of the K2P currents in the presence of the “cocktail” of K⁺ channel blockers. Gö 6976 significantly reduced the increase of the K2P currents by PDBu suggesting the involvement of either α or β isoenzymes of PKC. We found that forskolin, or membrane permeable cAMP, did not inhibit K2P currents activated by the PKC. However, when PKA agonists were added prior to PDBu, they produced a strong decrease in the K2P current amplitudes activated by PKC. Inhibition of PDBu-elicited K2P currents by cAMP agonists was not prevented by the treatment of vascular smooth muscle cells with PKA antagonists (H-89 and Rp-cAMPs). Zn²⁺ and Hg²⁺ inhibited K2P currents in one population of cells, produced biphasic responses in another population, and increased the amplitude of the PDBu-elicited K⁺ currents in a third population of myocytes, suggesting expression of several K2P channel types. We found that cAMP agonists inhibited biphasic responses and increase of amplitude of the PDBu-elicited K2P currents produced by Zn²⁺ and Hg². 6-Bnz-cAMp produced a significantly altered pH sensitivity of PDBu-elicited K2P-currents, suggesting the inhibition of alkaline-activated K2P-currents. These results indicate that 6-Bnz-cAMP and other cAMP analogs may inhibit K2P currents through a PKA-independent mechanism. cAMP analogs may interact with unidentified proteins involved in K2P channel regulation. This novel cellular mechanism could provide insights into the interplay between PKC and PKA pathways that regulate vascular tone.

Role of Cholinergic Innervation and RGS2 in Atrial Arrhythmia

The heart receives sympathetic and parasympathetic efferent innervation as well as the ability to process information internally via an intrinsic cardiac autonomic nervous system (ICANS). For over a century, the role of the parasympathetics via vagal acetylcholine release was related to controlling primarily heart rate. Although in the late 1800s shown to play a role in atrial arrhythmia, the myocardium took precedence from the mid-1950s until in the last decade a resurgence of interest in the autonomics along with signaling cascades, regulators, and ion channels. Originally ignored as being benign and thus untreated, recent emphasis has focused on atrial arrhythmia as atrial fibrillation (AF) is the most common arrhythmia seen by the general practitioner. It is now recognized to have significant mortality and morbidity due to resultant stroke and heart failure. With the aging population, there will be an unprecedented increased burden on health care resources. Although it has been known for more than half a century that cholinergic stimulation can initiate AF, the classical concept focused on the M2 receptor and its signaling cascade including RGS4, as these had been shown to have predominant effects on nodal function (heart rate and conduction block) as well as contractility. However, recent evidence suggests that the M3 receptor may also playa role in initiation and perpetuation of AF and thus RGS2, a putative regulator of the M3 receptor, may be a target for therapeutic intervention. Mice lacking RGS2 (RGS2^−/−), were found to have significantly altered electrophysiological atrial responses and were more susceptible to electrically induced AF. Vagally induced or programmed stimulation-induced AF could be blocked by the selective M3R antagonist, darifenacin. These results suggest a potential surgical target (ICANS) and pharmacological targets (M3R, RGS2) for the management of AF.

Expression of K2P channels in sensory and motor neurons of the autonomic nervous system

Several types of neurons within the central and peripheral somatic nervous system express two-pore-domain potassium (K2P) channels, providing them with resting potassium conductances. We demonstrate that these channels are also expressed in the autonomic nervous system where they might be important modulators of neuronal excitability. We observed strong mRNA expression of members of the TRESK and TREK subfamilies in both the mouse superior cervical ganglion (mSCG) and the mouse nodose ganglion (mNG). Motor mSCG neurons strongly expressed mRNA transcripts for TRESK and TREK-2 subunits, whereas TASK-1 and TASK-2 subunits were only moderately expressed, with only few or very few transcripts for TREK-1 and TRAAK (TRESK ≈ TREK-2 > TASK-2 ≈ TASK-1 > TREK-1 > TRAAK). Similarly, the TRESK and TREK-1 subunits were the most strongly expressed in sensorial mNG neurons, while TASK-1 and TASK-2 mRNAs were moderately expressed, and fewer TREK-2 and TRAAK transcripts were detected (TRESK ≈ TREK-1 > TASK-1 ≈ TASK-2 > TREK-2 > TRAAK). Moreover, cell-attached single-channel recordings showed a major contribution of TRESK and TREK-1 channels in mNG. As the level of TRESK mRNA expression was not statistically different between the ganglia analysed, the distinct expression of TREK-1 and TREK-2 subunits was the main difference observed between these structures. Our results strongly suggest that TRESK and TREK channels are important modulators of the sensorial and motor information flowing through the autonomic nervous system, probably exerting a strong influence on vagal reflexes.

Molecular physiology of pH-sensitive background K(2P) channels

Background K(2P) channels are tightly regulated by different stimuli including variations of external and internal pH. pH sensitivity relies on proton-sensing residues that influence channel gating and activity. Gene inactivation in the mouse is a revealing implication of K(2P) channels in many physiological functions ranging from hormone secretion to central respiratory adaptation. Surprisingly, only a few phenotypic traits of these mice have yet been directly related to the pH sensitivity of K(2P) channels.

Covalent modification of a volatile anesthetic regulatory site activates TASK-3 (KCNK9) tandem-pore potassium channels

TASK-3 (KCNK9) tandem-pore potassium channels provide a volatile anesthetic-activated and Gα(q) protein- and acidic pH-inhibited potassium conductance important in neuronal excitability. Met-159 of TASK-3 is essential for anesthetic activation and may contribute to the TASK-3 anesthetic binding site(s). We hypothesized that covalent occupancy of an anesthetic binding site would irreversibly activate TASK-3. We introduced a cysteine at residue 159 (M159C) and studied the rate and effect of Cys-159 modification by N-ethylmaleimide (NEM), a cysteine-selective alkylating agent. TASK-3 channels were transiently expressed in Fischer rat thyroid cells, and their function was studied in an Ussing chamber. NEM irreversibly activated M159C TASK-3, with minimal effects on wild-type TASK-3. NEM-modified M159C channels were resistant to inhibition by both acidic pH and active Gα(q) protein. M159C channels that were first inhibited by Gα(q) protein were more-slowly activated by NEM, which suggests protection of Cys-159, and similar results were observed with isoflurane activation of wild-type TASK-3. M159W and M159F TASK-3 mutants behaved like NEM-modified M159C channels, with increased basal currents and resistance to inhibition by active Gα(q) protein or acidic pH. TASK-3 wild-type/M159C dimers expressed as a single polypeptide demonstrated that modification of a single Cys-159 was sufficient for TASK-3 activation, and M159F/M159C and M159W/M159C dimers provided evidence for cross-talk between subunits. The data are consistent with residue 159 contributing to an anesthetic regulatory site or sites, and they suggest that volatile anesthetics, through perturbations at a single site, increase TASK-3 channel activity and disrupt its regulation by active Gα(q) protein, a determinant of central nervous system arousal and consciousness.

Molecular regulations governing TREK and TRAAK channel functions

K+ channels with two-pore domain (K2p) form a large family of hyperpolarizing channels. They produce background currents that oppose membrane depolarization and cell excitability. They are involved in cellular mechanisms of apoptosis, vasodilatation, anesthesia, pain, neuroprotection and depression. This review focuses on TREK-1, TREK-2 and TRAAK channels subfamily and on the mechanisms that contribute to their molecular heterogeneity and functional regulations. Their molecular diversity is determined not only by the number of genes but also by alternative splicing and alternative initiation of translation. These channels are sensitive to a wide array of biophysical parameters that affect their activity such as unsaturated fatty acids, intra- and extracellular pH, membrane stretch, temperature, and intracellular signaling pathways. They interact with partner proteins that influence their activity and their plasma membrane expression. Molecular heterogeneity, regulatory mechanisms and protein partners are all expected to contribute to cell specific functions of TREK currents in many tissues.

Colitis decreases mechanosensitive K2P channel expression and function in mouse colon sensory neurons

TREK-1, TREK-2 and TRAAK are mechanosensitive two-pore domain K(+) (K(2P)) channels thought to be involved in the attenuation of mechanotransduction. Because colon inflammation is associated with colon mechanohypersensitivity, we hypothesized that the role of these channels in colon sensory (dorsal root ganglion, DRG) neurons would be reduced by colon inflammation. Accordingly, we studied the functional expression of mechanosensitive K(2P) channels in colon sensory neurons in both thoracolumbar (TL) and lumbosacral (LS) DRG that represent the splanchnic and pelvic nerve innervations of the colon, respectively. In colon DRG neurons identified by retrograde tracer previously injected into the colon wall, 62% of TL neurons and 83% of LS neurons expressed at least one of three K(2P) channel mRNAs; the proportion of neurons expressing the TREK-1 gene was greater in LS than in TL DRG. In electrophysiological studies, single-channel activities of TREK-1a, TREK-1b, TREK-2, and TRAAK-like channels were detected in cultured colon DRG neuronal membranes. After trinitrobenzene sulfonic acid-induced colon inflammation, we observed significant decreases in the amount of TREK-1 mRNA, in the response of TREK-2-like channels to membrane stretch, and in the whole cell outward current during osmotic stretch in LS colon DRG neurons. These findings document that the majority of DRG neurons innervating the mouse colon express mechanosensitive K(2P) channels and suggest that a decrease in their expression and activities contributes to the increased colon mechanosensitivity that develops in inflammatory bowel conditions.

Activation of TREK currents by the neuroprotective agent riluzole in mouse sympathetic neurons

Background K2P channels play a key role in stabilizing the resting membrane potential, thereby modulating cell excitability in the central and peripheral somatic nervous system. Whole-cell experiments revealed a riluzole-activated current (I(RIL)), transported by potassium, in mouse superior cervical ganglion (mSCG) neurons. The activation of this current by riluzole, linoleic acid, membrane stretch, and internal acidification, its open rectification and insensitivity to most classic potassium channel blockers, indicated that I(RIL) flows through channels of the TREK [two-pore domain weak inwardly rectifying K channel (TWIK)-related K channel] subfamily. Whole-ganglia and single-cell reverse transcription-PCR demonstrated the presence of TREK-1, TREK-2, and TRAAK (TWIK-related arachidonic acid-activated K(+) channel) mRNA, and the expression of these three proteins was confirmed by immunocytochemistry in mSCG neurons. I(RIL) was enhanced by zinc, inhibited by barium and fluoxetine, but unaffected by quinine and ruthenium red, strongly suggesting that it was carried through TREK-1/2 channels. Consistently, a channel with properties identical with the heterologously expressed TREK-2 was recorded in most (75%) cell-attached patches. These results provide the first evidence for the expression of K2P channels in the mammalian autonomic nervous system, and they extend the impact of these channels to the entire nervous system.

Distinct modes of modulation of GABAergic transmission by Group I metabotropic glutamate receptors in rat entorhinal cortex

Activation of metabotropic glutamate receptors (mGluRs) modulates synaptic transmission, whereas the roles of mGluRs in GABAergic transmission in the entorhinal cortex (EC) are elusive. Here, we examined the effects of mGluRs on GABAergic transmission onto the principal neurons in the superficial layers of the EC. Bath application of DHPG, a selective Group I mGluR agonist, increased the frequency and amplitude of spontaneous IPSCs (sIPSCs) whereas application of DCG-IV, an agonist for Group II mGluRs or L-AP4, an agonist for Group III mGluRs failed to change significantly sIPSC frequency and amplitude. Bath application of DHPG failed to change significantly the frequency and amplitude of miniature IPSCs (mIPSCs) recorded in the presence of tetradotoxin but significantly reduced the amplitude of IPSCs evoked by extracellular field stimulation or in synaptically connected interneuron-pyramidal neuron pairs in layer III of the EC. DHPG increased the frequency but reduced the amplitude of APs recorded from entorhinal interneurons. Bath application of DHPG generated membrane depolarization and increased the input resistance of GABAergic interneurons. DHPG-mediated depolarization of GABAergic interneurons was mediated by inhibition of background K(+) channels which are insensitive to extracellular Cs(+), TEA, 4-AP, and Ba(2+). DHPG-induced facilitation of sIPSCs was mediated by mGluR(5) and required the function of Galphaq but was independent of phospholipase C activity. Elevation of synaptic glutamate concentration by bath application of glutamate transporter inhibitors significantly increased sIPSC frequency and amplitude demonstrating a physiological role of mGluRs in GABAergic transmission. Our results provide a cellular and molecular mechanism to explain the physiological and pathological roles of mGluRs in the EC.

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.

AMP-activated protein kinase inhibits TREK channels

AMP-activated protein kinase (AMPK) is a serine/threonine kinase activated by conditions that increase the AMP : ATP ratio. In carotid body glomus cells, AMPK is thought to link changes in arterial O(2) with activation of glomus cells by inhibition of unidentified background K(+) channels. Modulation by AMPK of individual background K(+) channels has not been described. Here, we characterize effects of activated AMPK on recombinant TASK-1, TASK-3, TREK-1 and TREK-2 background K(+) channels expressed in HEK293 cells. We found that TREK-1 and TREK-2 channels but not TASK-1 or TASK-3 channels are inhibited by AMPK. AMPK-mediated inhibition of TREK involves key serine residues in the C-terminus that are also known to be important for PKA and PKC channel modulation; inhibition of TREK-1 requires Ser-300 and Ser-333 and inhibition of TREK-2 requires Ser-326 and Ser-359. Metabolic inhibition by sodium azide can also inhibit both TREK and TASK channels. The effects of azide on TREK occlude subsequent channel inhibition by AMPK and are attenuated by expression of a dominant negative catalytic subunit of AMPK (dnAMPK), suggesting that metabolic stress modulates TREK channels by an AMPK mechanism. By contrast, inhibition of TASK channels by azide was unaffected by expression of dnAMPK, suggesting an AMPK-independent mechanism. In addition, prolonged exposure (6-7 min) to hypoxia ( = 11 +/- 1 mmHg) inhibits TREK channels and this response was blocked by expression of dnAMPK. Our results identify a novel modulation of TREK channels by AMPK and indicate that select residues in the C-terminus of TREK are points of convergence for multiple signalling cascades including AMPK, PKA and PKC. To the extent that carotid body O(2) sensitivity is dependent on AMPK, our finding that TREK-1 and TREK-2 channels are inhibited by AMPK suggests that TREK channels may represent the AMPK-inhibited background K(+) channels that mediate activation of glomus cells by hypoxia.

Targeting TASK-1 channels as a therapeutic approach

The voltage-independent background two-pore domain K(+) channel TASK-1 sets the resting membrane potential in excitable cells and renders these cells sensitive to a variety of vasoactive factors. There is clear evidence for TASK-1 in human pulmonary artery smooth muscle cells and TASK-1 channels are likely to regulate the pulmonary vascular tone through their regulation by hypoxia, pH, inhaled anesthetics, and G protein-coupled pathways. Furthermore, TASK-1 is a strong candidate to play a role in hypoxic pulmonary vasoconstriction. On the other hand, consistent with the activation of TASK-1 channels by volatile anesthetics, TASK-1 contributes to the anesthetic-induced pulmonary vasodilation. TASK-1 channels are unique among K(+) channels because they are regulated by both, increases and decreases from physiological pH, thus contributing to their protective effect on the pulmonary arteries. Moreover, TASK-1 may also have a critical role in mediating the vasoactive response of G protein-coupled pathways in resistance arteries which can offer promising therapeutic solutions to target diseases of the pulmonary circulation.

GABA(B) receptor activation inhibits neuronal excitability and spatial learning in the entorhinal cortex by activating TREK-2 K+ channels

The entorhinal cortex (EC) is regarded as the gateway to the hippocampus and thus is essential for learning and memory. Whereas the EC expresses a high density of GABA(B) receptors, the functions of these receptors in this region remain unexplored. Here, we examined the effects of GABA(B) receptor activation on neuronal excitability in the EC and spatial learning. Application of baclofen, a specific GABA(B) receptor agonist, inhibited significantly neuronal excitability in the EC. GABA(B) receptor-mediated inhibition in the EC was mediated via activating TREK-2, a type of two-pore domain K(+) channels, and required the functions of inhibitory G proteins and protein kinase A pathway. Depression of neuronal excitability in the EC underlies GABA(B) receptor-mediated inhibition of spatial learning as assessed by Morris water maze. Our study indicates that GABA(B) receptors exert a tight control over spatial learning by modulating neuronal excitability in the EC.

Extracellular acidification exerts opposite actions on TREK1 and TREK2 potassium channels via a single conserved histidine residue

Mechanosensitive K(+) channels TREK1 and TREK2 form a subclass of two P-domain K(+) channels. They are potently activated by polyunsaturated fatty acids and are involved in neuroprotection, anesthesia, and pain perception. Here, we show that acidification of the extracellular medium strongly inhibits TREK1 with an apparent pK near to 7.4 corresponding to the physiological pH. The all-or-none effect of pH variation is steep and is observed within one pH unit. TREK2 is not inhibited but activated by acidification within the same range of pH, despite its close homology with TREK1. A single conserved residue, H126 in TREK1 and H151 in TREK2, is involved in proton sensing. This histidine is located in the M1P1 extracellular loop preceding the first P domain. The differential effect of acidification, that is, activation for TREK2 and inhibition for TREK1, involves other residues located in the P2M4 loop, linking the second P domain and the fourth membrane-spanning segment. Structural modeling of TREK1 and TREK2 and site-directed mutagenesis strongly suggest that attraction or repulsion between the protonated side chain of histidine and closely located negatively or positively charged residues in P2M4 control outer gating of these channels. The differential sensitivity of TREK1 and TREK2 to external pH variations discriminates between these two K(+) channels that otherwise share the same regulations by physical and chemical stimuli, and by hormones and neurotransmitters.

Modulation of GABAergic transmission by muscarinic receptors in the entorhinal cortex of juvenile rats

Whereas the entorhinal cortex (EC) receives profuse cholinergic innervations from the basal forebrain and activation of cholinergic receptors has been shown to modulate the activities of the principal neurons and promote the intrinsic oscillations in the EC, the effects of cholinergic receptor activation on GABAergic transmission in this brain region have not been determined. We examined the effects of muscarinic receptor activation on GABA(A) receptor-mediated synaptic transmission in the superficial layers of the EC. Application of muscarine dose-dependently increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) recorded from the principal neurons in layer II/III via activation of M(3) muscarinic receptors. Muscarine slightly reduced the frequency but had no effects on the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. Muscarine reduced the amplitude of IPSCs evoked by extracellular field stimulation and by depolarization of GABAergic interneurons in synaptically connected interneuron and pyramidal neuron pairs. Application of muscarine generated membrane depolarization and increased action potential firing frequency but reduced the amplitude of action potentials in GABAergic interneurons. Muscarine-induced depolarization of GABAergic interneurons was mediated by inhibition of background K(+) channels and independent of phospholipase C, intracellular Ca(2+) release, and protein kinase C. Our results demonstrate that activation of muscarinic receptors exerts diverse effects on GABAergic transmission in the EC.

Identification of the large-conductance background K+ channel in mouse B cells as TREK-2

Mouse B cells and their cell line (WEHI-231) express large-conductance background K(+) channels (LK(bg)) that are activated by arachidonic acids, characteristics similar to TREK-2. However, there is no evidence to identify the molecular nature of LK(bg); some properties of LK(bg) were partly different from the reported results of TREK type channels. In this study, we compared the properties of cloned TREK-2 and LK(bg) in terms of their sensitivities to ATP, phosphatidylinositol 4,5-bisphosphate (PIP(2)), intracellular pH (pH(i)), and membrane stretch. Similar to the previous findings of LK(bg), TREK-2 showed spontaneous activation after membrane excision (i-o patch) and were inhibited by MgATP or by PIP(2). The inhibition by MgATP was prevented by wortmannin, suggesting membrane-delimited regulation of TREKs by phosphoinositide (PI) kinase. The same was observed with the property of LK(bg); the activation of TREK-2 by membrane stretch was suppressed by U73122 (PLC inhibitor). As with the known properties of TREK-2, LK(bg) were activated by acidic pH(i) and inhibited by PKC activator. Finally, we confirmed the expression of TREK-2 in WEHI-231 by using RT-PCR and immunoblot analyses. The amplitude of background K(+) current and the TREK-2 expression in WEHI-231 were commonly decreased by genetic knockdown of TREK-2 using small interfering RNA. The downregulation of TREK-2 attenuated Ca(2+)-influx induced by arachidonic acid in WEHI-231. As a whole, these results strongly indicate that TREK-2 encodes LK(bg) in mouse B cells. We also newly suggest that the low activity of TREK-2 in intact cells is due to the inhibition by intrinsic PIP(2).

Endothelin-1 inhibits background two-pore domain channel TASK-1 in primary human pulmonary artery smooth muscle cells

Endothelin (ET)-1 causes long-lasting vasoconstriction and vascular remodeling by interacting with specific G-protein-coupled receptors in pulmonary artery smooth muscle cells (PASMCs), and thus plays an important role in the pathophysiology of pulmonary arterial hypertension. The two-pore domain K(+) channel, TASK-1, controls the resting membrane potential in human PASMCs (hPASMCs), and renders these cells sensitive to a variety of vasoactive factors, as previously shown. ET-1 may exert its vasoconstrictive effects in part by targeting TASK-1. To clarify this, we analyzed the ET-1 signaling pathway related to TASK-1 in primary hPASMCs. We employed the whole-cell patch-clamp technique combined with TASK-1 small interfering RNA (siRNA) in hPASMC and the isolated, perfused, and ventilated mouse lung model. We found that ET-1 depolarized primary hPASMCs by phosphorylating TASK-1 at clinically relevant concentrations. The ET sensitivity of TASK-1 required ET(A) receptors, phospholipase C, phosphatidylinositol 4,5-biphosphate, diacylglycerol, and protein kinase C in primary hPASMCs. The ET-1 effect on membrane potential and TASK-1 was abrogated using TASK-1 siRNA. This is the first time that the background K(+) channel, TASK-1, has been identified in the ET-1-mediated depolarization in native hPASMC, and might represent a novel pathologic mechanism related to pulmonary arterial hypertension.

Control of the single channel conductance of K2P10.1 (TREK-2) by the amino-terminus: role of alternative translation initiation

TREK-2 expressed in mammalian cells exhibits small ( approximately 52 pS) and large ( approximately 220 pS) unitary conductance levels. Here we tested the role of the N-terminus (69 amino acids long) in the control of the unitary conductance, and role of the alternative translation initiation as a mechanism that produces isoforms of TREK-2 that show different conductance levels. Deletion of the first half (Delta1-36) of the N-terminus had no effect. However, deletion of most of the N-terminus (Delta1-66) resulted in the appearance of only the large-conductance channel ( approximately 220 pS). In support of the critical function of the distal half of the N-terminus, the deletion mutants Delta1-44 and Delta1-54 produced approximately 90 pS and 188 pS channels, respectively. In Western blot analysis, TREK-2 antibody detected two immunoreactive bands at approximately 54 kDa and approximately 60 kDa from cells expressing wild-type TREK-2 that has three potential translation initiation sites (designated M(1)M(2)M(3)) within the N-terminus. Mutation of the second and third initiation sites from Met to Leu (M(1)L(2)L(3)) produced only the approximately 60 kDa isoform and the small-conductance channel ( approximately 52 pS). Mutants designed to produce translation from the second (M(2)L(3)) or third (M(3)) initiation site produced the approximately 54 kDa isoform, and the large conductance channel ( approximately 185-224 pS). M(1)L(2)L(3), M(2)L(3) and M(3) were relatively selectively permeable to K(+), as judged by the 51-55 mV shifts in reversal potential following a 10-fold change in [K(+)](o). P(Na)/P(K) values were also similar for M(1)L(2)L(3) ( approximately 0.02), M(2)L(3) ( approximately 0.02) and M(3) ( approximately 0.03). Arachidonic acid, proton and membrane stretch activated, whereas dibutyryl-cAMP inhibited all three isoforms of TREK-2, indicating that deletion of the N-terminus does not abolish modulation. These results show that the small and large conductance TREK-2 channels are produced as a result of alternative translation initiation, producing isoforms with long and short N-termini, and that the distal half of the N-terminus controls the unitary conductance.

Regulation of K(+) channels may enhance wound healing in the skin

In the process of promoting wound healing, epidermal growth factor (EGF) activates protein kinase C, protein tyrosine kinase and ERK MAPK (mitogen-activated protein kinase). The activation of these mediators in signal pathways can regulate the operation of K(+) channels. In addition, the K(+) channel is involved with cell migration and proliferation, both of which are requisite for wound healing. Recent studies, although not conducted on skin wounds, have found that the K(+) channel is associated with wound healing and that wound healing can be promoted by regulating the K(+) channels. Therefore, the authors hypothesize that healing of skin wounds could be promoted by regulating K(+) channel distribution in skin keratinocytes or fibroblasts. We plan to conduct a study of the promotion of skin wound healing using K(+) channel regulators.

Phosphorylation-dependent binding of 14-3-3 proteins controls TRESK regulation

The two-pore domain K(+) channel, TRESK (TWIK-related spinal cord K(+) channel) is reversibly activated by the calcium/calmodulin-dependent protein phosphatase, calcineurin. In the present study, we report that 14-3-3 proteins directly bind to the intracellular loop of TRESK and control the kinetics of the calcium-dependent regulation of the channel. Coexpression of 14-3-3eta with TRESK blocked, whereas the coexpression of a dominant negative form of 14-3-3eta accelerated the return of the K(+) current to the resting state after the activation mediated by calcineurin in Xenopus oocytes. The direct action of 14-3-3 was spatially restricted to TRESK, since 14-3-3eta was also effective, when it was tethered to the channel by a flexible polyglutamine-containing chain. The effect of both the coexpressed and chained 14-3-3 was alleviated by the microinjection of Ser(P)-Raf259 phosphopeptide that competes with TRESK for binding to 14-3-3. The gamma and eta isoforms of 14-3-3 controlled TRESK regulation, whereas the beta, zeta, epsilon, sigma, and tau isoforms failed to influence the mechanism significantly. Phosphorylation of serine 264 in mouse TRESK was required for the binding of 14-3-3eta. Because 14-3-3 proteins are ubiquitous, they are expected to control the duration of calcineurin-mediated TRESK activation in all the cell types that express the channel, depending on the phosphorylation state of serine 264. This kind of direct control of channel regulation by 14-3-3 is unique within the two-pore domain K(+) channel family.

TWIK-related two-pore domain potassium channel TREK-1 in carotid endothelium of normotensive and hypertensive mice

AIMS

Potassium channels are essential elements of endothelial function. Recently, evidence emerged that the TWIK (tandem of P domains in a weak inwardly rectifying K+ channel)-related K+ channel (TREK-1) of the two-pore domain potassium channel gene family (K2P) may be involved in the regulation of vascular tone. However, the functional and molecular characterization of vascular TREK-1 is incomplete. In this study, we therefore analysed the functional expression of TREK-1 in the endothelium. Moreover, we hypothesized that changes in channel expression may contribute to altered endothelial vasodilator response under conditions of elevated blood pressure.

METHODS AND RESULTS

Gene expression and function of endothelial TREK-1 were analysed by single-cell RT-PCR, the patch-clamp technique and pressure myography in murine carotid arteries (CA). K+ outward currents displaying the characteristics of TREK-1 were observed following various TREK-1-activating stimuli such as membrane stretch, intracellular acidosis, polyunsaturated fatty acids, isoflurane (ISOFL), riluzole, and acetylcholine (ACh). In K(Ca)3.1(-/-) mice exhibiting elevated blood pressure, endothelial TREK-1 currents and TREK-1 mRNA expression were enhanced as compared with normotensive control mice. TREK-1-mediated vasodilator responses to alpha-linolenic acid, ISOFL, or ACh were increased. A similar up-regulation of endothelial TREK-1 was observed in spontaneously hypertensive rats.

CONCLUSION

We have found that TREK-1 is an endothelial K+ channel capable of producing hyperpolarization and vasodilation. A correlation between hypertension and up-regulation of TREK-1 was observed in two different animal models of elevated blood pressure. Thus, TREK-1 may play a protective role in the cardiovascular system by providing a novel type of endothelial hyperpolarization-mediated vasodilator response.

Expression of thermosensitive two-pore domain K+ channels in human keratinocytes cell line HaCaT cells

Recent studies have shown that keratinocytes can sense temperature via thermo-transient receptor potential (TRP) channels. It is not known whether other thermosensitive ion channels such as TREK-1, TREK-2 and TRAAK (TREKs/TRAAK) that are members of the two-pore domain K(+) (K(2P)) channel family are expressed in human keratinocytes. Here, we identified the expression of TREKs/TRAAK in human keratinocytes-derived cell line HaCaT cells using RT-PCR, immunocytochemistry, Western blot analysis and patch-clamp technique. RT-PCR showed that all six K(2P) channels tested (TASK-1, TASK-3, TREK-1, TREK-2, TRAAK and TASK-2) were expressed in HaCaT cells, as well as in skin and dorsal root ganglion (DRG) of rat. The expression of TREKs/TRAAK mRNA identified by RT-PCR was further studied at the protein level. Using anti-TREK-1, -TREK-2 and -TRAAK, bands of approximately 46, approximately 60 and approximately 43 kDa, respectively, were observed at plasma membrane of HaCaT cells. Immunostaining also showed that TREK-1, TREK-2 and TRAAK were expressed in all area of cells including plasma membrane. Whole-cell K(+) currents recorded from HaCaT cells were activated by arachidonic acid and heat. These results suggest that TREKs/TRAAK channels could act as thermosensors in human keratinocytes.

Lamotrigine inhibits TRESK regulated by G-protein coupled receptor agonists

Dorsal root ganglion (DRG) neurons express mRNAs for numerous two-pore domain K(+) (K(2P)) channels and G-protein coupled receptors (GPCR). Recent studies have shown that TRESK is a major background K(+) channel in DRG neurons. Here, we demonstrate the pharmacological properties of TRESK, including GPCR agonist-induced effects on DRG neurons. TRESK mRNA was highly expressed in DRG compared to brain and spinal cord. Similar to cloned TRESK, native TRESK was inhibited by acid and arachidonic acid (AA), but not zinc. Native TRESK was also activated by GPCR agonists such as acetylcholine, glutamate, and histamine. The glutamate-activated TRESK was blocked by lamotrigine in DRG neurons. In COS-7 cells transfected with mouse TRESK, 30 microM lamotrigine inhibited TRESK by approximately 50%. Since TRESK is target of modulation by acid, AA, GPCR agonists, and lamotrigine, it is likely to play an active role in the regulation of excitability in DRG neurons.

mGlu4 potentiation of K(2P)2.1 is dependant on C-terminal dephosphorylation

Two-pore domain potassium (K(2P)) channels are proposed to underlie the background or leak current found in many excitable cells. Extensive studies have been performed investigating the inhibition of K(2P)2.1 by Galpha(q)- and Galpha(s)-coupled G-protein-coupled receptors (GPCRs), whereas in the present study we investigate the mechanisms underlying Galpha(i)/Galpha(o)-coupled GPCR increases in K(2P)2.1 activity. Activation of mGlu4 increases K(2P)2.1 activity, with pharmacological inhibition of protein kinases and phosphatases revealing the involvement of PKA whereas PKC, PKG or protein phosphatases play no role. Mutational analysis of potential C-terminal phosphorylation sites indicates S333 to control approximately 70%, with S300 controlling approximately 30% of the increase in K(2P)2.1 activity following mGlu4 activation. These data reveal that activation of mGlu4 leads to an increase in K(2P)2.1 activity through a reduction in C-terminal phosphorylation, which represents a novel mechanism by which group III mGlu receptors may regulate cell excitability and synaptic activity.

Recent advance and possible future in TREK-2: a two-pore potassium channel may involved in the process of NPP, brain ischemia and memory impairment

TREK-2, a new member of the mechanosensitive tandem-pore K+ channel family, share 65% amino acid sequence identity and some similar basic electrophysiological and pharmacological properties with TREK-1. It also has some specific regulatory pathway and tissue distribution contrasted with TREK-1 and TRAAK. TREK-2 distributes extensively in CNS and periphery tissue. It can be regulated by G-protein-coupled receptor (GPCR) and may involve in several of physiological and pathophysiological conditions. The long-chain unsaturated free fatty acids such as arachidonic acid (AA), PHi, pressure and temperature can increase the activity of TREK-2. The purpose of this review is to present the recent study and possible importance of TREK-2 in neuropathic pain, thereby emphasizing TREK-2 as one of the important mechanisms underlying. This information should be very useful and prospective for effective chronic pain therapy and future analgesic drug development. This review also further predicts the role of TREK-2 in brain ischemia, memory and other tissue. The specific location and function of TREK-2 in these tissues need further study.

Properties of single two-pore domain TREK-2 channels expressed in mammalian cells

TREK-2 (K2P10.1), a member of the two-pore domain K+ (K2P) channel family, provides the background K+ conductance in many cell types, and is a target of neurotransmitters that act on receptors coupled to Gs and Gq. We report here that TREK-2 exhibits small (TREK-2S) and large (TREK-2L) conductance phenotypes when expressed in mammalian cell lines (COS-7, HEK293, HeLa) and in Xenopus oocytes. TREK-2S phenotype shows a noisy open state with a mean conductance of 54 pS (+40 mV). TREK-2L phenotype shows a full open state (202 pS) with several short-lived sub-conductance levels. Both phenotypes were strongly activated by arachidonic acid, membrane stretch (-40 mmHg) and intracellular acidification (pH 6.4). Phosphorylation of TREK-2 produced by treatment of cells with activators of protein kinases A and C, and okadaic acid (a serine/threonine phosphatase inhibitor) decreased the current contributed by TREK-2S and TREK-2L, and caused partial switching of conductance levels from those of TREK-2S and TREK-2L to more intermediate values. Under this condition, TREK-2 exhibited six conducting levels and one closed level. TREK-2 mutants in which putative protein kinases A and C phosphorylation sites were mutated to alanines (S326A, S359A, S326A/S359A) displayed mostly TREK-2S and TREK-2L phenotypes. However, S326D and S359D mutants (as well as the double mutants) that mimic the phosphorylated state showed all six conducting levels and low channel activity. The S326A and S359A mutants did not significantly affect the intrinsic voltage dependence of TREK-2 in Mg2+-free solution. Phenotypes resembling TREK-2S and TREK-2L were also observed in cerebellar granule neurons that express TREK-2 mRNA. These results show that TREK-2 exhibits two primary modes of gating that give rise to two channel phenotypes under dephosphorylated conditions, and that its phosphorylation shifts the gating mode to include intermediate conducting levels. This represents a novel mechanism by which receptor agonists modulate the function of a K+ channel to alter cell excitability.

Elevated resting [Ca2+]iin myotubes expressing malignant hyperthermia RyR1 cDNAs is partially restored by modulation of passive calcium leak from the SR

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle triggered in susceptible individuals by inhalation anesthetics and depolarizing skeletal muscle relaxants. This syndrome has been linked to a missense mutation in the type 1 ryanodine receptor (RyR1) in more than 50% of cases studied to date. Using double-barreled Ca2+microelectrodes in myotubes expressing wild-type RyR1 (WTRyR1) or RyR1 with one of four common MH mutations (MHRyR1), we measured resting intracellular Ca2+concentration ([Ca2+]i). Changes in resting [Ca2+]iproduced by several drugs known to modulate the RyR1 channel complex were investigated. We found that myotubes expressing any of theMHRyR1s had a 2.0- to 3.7-fold higher resting [Ca2+]ithan those expressingWTRyR1. Exposure of myotubes expressingMHRyR1s to ryanodine (500 μM) or (2,6-dichloro-4-aminophenyl)isopropylamine (FLA 365; 20 μM) had no effects on their resting [Ca2+]i. However, when myotubes were exposed to bastadin 5 alone or to a combination of ryanodine and bastadin 5, the resting [Ca2+]iwas significantly reduced ( P < 0.01). Interestingly, the percent decrease in resting [Ca2+]iin myotubes expressingMHRyR1s was significantly greater than that forWTRyR1. From these data, we propose that the high resting myoplasmic [Ca2+]iinMHRyR1 expressing myotubes is due in part to a related structural conformation ofMHRyR1s that favors “passive” calcium leak from the sarcoplasmic reticulum.

Gαq-Mediated Regulation of TASK3 Two-Pore Domain Potassium Channels: The Role of Protein Kinase C

The TASK subfamily of two pore domain potassium channels (K2P) gives rise to leak potassium currents, which contribute to the resting membrane potential of many neurons and regulate their excitability. K2P channels are highly regulated by phosphorylation and by G protein-mediated pathways. In this study, we show that protein kinase C (PKC) inhibits recombinant TASK3 channels. Inhibition by PKC is blocked by the PKC inhibitors bisindolylmaleimide 1 hydrochloride (BIM) and 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole (Gö6976). Gene-silencing experiments with a validated small interfering RNA sequence against PKCalpha ablates the effect of PKC. PKC acts directly on hTASK3 channels to phosphorylate an identified amino acid in the C terminus region (Thr341), thereby reducing channel current. PKC also inhibits mTASK3 channels despite their having a quite different C-terminal structure to hTASK3 channels. Activation of M(3) muscarinic receptors inhibits both hTASK3 channels expressed in tsA-201 cells and standing outward potassium current (IK(SO)) in mouse cerebellar granule neurons through the activation of the G protein Galpha(q), because both effects are abolished by the selective Galpha(q) antagonist YM-254890 (J Biol Chem 279:47438-47445, 2004). This inhibition is not directly transduced through activation of PKC because inhibition persists in mutated PKC-insensitive hTASK3 channels. Instead, inhibition seems to occur through a direct action of Galpha(q) on the channel. Nevertheless, preactivation of PKC blocks muscarinic inhibition of both TASK3 channels and IK(SO). Our results suggest that activation of PKC (via phospholipase C) has a role in opposing inhibition after M(3) receptor activation rather than transducing it and may act as a negative regulator of G protein modulation to prevent prolonged current inhibition.