Probing the regulation of TASK potassium channels by PI4,5P₂ with switchable phosphoinositide phosphatases

A versatile functional interaction between electrically silent KV subunits and KV7 potassium channels

Voltage-gated K+ (KV) channels govern K+ ion flux across cell membranes in response to changes in membrane potential. They are formed by the assembly of four subunits, typically from the same family. Electrically silent KV channels (KVS), however, are unable to conduct currents on their own. It has been assumed that these KVS must obligatorily assemble with subunits from the KV2 family into heterotetrameric channels, thereby giving rise to currents distinct from those of homomeric KV2 channels. Herein, we show that KVS subunits indeed also modulate the activity, biophysical properties and surface expression of recombinant KV7 isoforms in a subunit-specific manner. Employing co-immunoprecipitation, and proximity labelling, we unveil the spatial coexistence of KVS and KV7 within a single protein complex. Electrophysiological experiments further indicate functional interaction and probably heterotetramer formation. Finally, single-cell transcriptomic analyses identify native cell types in which this KVS and KV7 interaction may occur. Our findings demonstrate that KV cross-family interaction is much more versatile than previously thought-possibly serving nature to shape potassium conductance to the needs of individual cell types.

Two-Pore Domain Potassium Channel in Neurological Disorders

K2P channel is the leaky potassium channel that is critical to keep up the negative resting membrane potential for legitimate electrical conductivity of the excitable tissues. Recently, many substances and medication elements are discovered that could either straightforwardly or in a roundabout way influence the 15 distinctive K⁺ ion channels including TWIK, TREK, TASK, TALK, THIK, and TRESK. Opening and shutting of these channels or any adjustment in their conduct is thought to alter the pathophysiological condition of CNS. There is no document available till now to explain in detail about the molecular mechanism of agents acting on K2P channel. Accordingly, in this review we cover the current research and mechanism of action of these channels, we have also tried to mention the detailed effect of drugs and how the channel behavior changes by focusing on recent advances regarding activation and modulation of ion channels.

Voltage-Sensing Phosphatases: Biophysics, Physiology, and Molecular Engineering

Voltage-sensing phosphatase (VSP) contains a voltage sensor domain (VSD) similar to that in voltage-gated ion channels, and a phosphoinositide phosphatase region similar to phosphatase and tensin homolog deleted on chromosome 10 (PTEN). The VSP gene is conserved from unicellular organisms to higher vertebrates. Membrane depolarization induces electrical driven conformational rearrangement in the VSD, which is translated into catalytic enzyme activity. Biophysical and structural characterization has revealed details of the mechanisms underlying the molecular functions of VSP. Coupling between the VSD and the enzyme is tight, such that enzyme activity is tuned in a graded fashion to the membrane voltage. Upon VSP activation, multiple species of phosphoinositides are simultaneously altered, and the profile of enzyme activity depends on the history of the membrane potential. VSPs have been the obvious candidate link between membrane potential and phosphoinositide regulation. However, patterns of voltage change regulating VSP in native cells remain largely unknown. This review addresses the current understanding of the biophysical biochemical properties of VSP and provides new insight into the proposed functions of VSP.

Depletion of plasma membrane-associated phosphoinositides mimics inhibition of TRPM7 channels by cytosolic Mg2+, spermine, and pH

Transient receptor potential cation channel subfamily M member 7 (TRPM7) is an ion channel/protein kinase belonging to the TRP melastatin and eEF2 kinase families. Under physiological conditions, most native TRPM7 channels are inhibited by cytoplasmic Mg²⁺, protons, and polyamines. Currents through these channels (I_TRPM7) are robustly potentiated when the cell interior is exchanged with low Mg²⁺-containing buffers. I_TRPM7 is also potentiated by phosphatidyl inositol bisphosphate (PI(4,5)P₂) and suppressed by its hydrolysis. Here we characterized internal Mg²⁺- and pH-mediated inhibition of TRPM7 channels in HEK293 cells overexpressing WT voltage-sensing phospholipid phosphatase (VSP) or its catalytically inactive variant VSP-C363S. VSP-mediated depletion of membrane phosphoinositides significantly increased channel sensitivity to Mg²⁺ and pH. Proton concentrations that were too low to inhibit I_TRPM7 when the VSP-C363S variant was expressed (pH 8.2) became inhibitory in WT VSP-expressing cells. At pH 6.5, protons inhibited I_TRPM7 both in WT and VSP C363S-expressing cells but with a faster time course in the WT VSP-expressing cells. Inhibition by 150 μm Mg²⁺ was also significantly faster in the WT VSP-expressing cells. Cellular PI(4,5)P₂ depletion increased the sensitivity of TRPM7 channels to the inhibitor 2-aminoethyl diphenyl borinate, which acidifies the cytosol. Single substitutions at Ser-1107 of TRPM7, reducing its sensitivity to Mg²⁺, also decreased its inhibition by spermine and acidic pH. Furthermore, these channel variants were markedly less sensitive to VSP-mediated PI(4,5)P₂ depletion than the WT. We conclude that the internal Mg²⁺-, polyamine-, and pH-mediated inhibition of TRPM7 channels is not direct but, rather, reflects electrostatic screening and resultant disruption of PI(4,5)P₂-channel interactions.

Kv12.1 channels are not sensitive to GqPCR-triggered activation of phospholipase Cβ

K_v12.1 K⁺ channels are expressed in several brain areas, but no physiological function could be attributed to these subunits so far. As genetically-modified animal models are not available, identification of native K_v12.1 currents must rely on characterization of distinct channel properties. Recently, it was shown in Xenopus laevis oocytes that K_v12.1 channels were modulated by membrane PI(4,5)P₂. However, it is not known whether these channels are also sensitive to physiologically-relevant PI(4,5)P₂ dynamics. We thus studied whether K_v12.1 channels were modulated by activation of phospholipase C β (PLCβ) and found that they were insensitive to receptor-triggered depletion of PI(4,5)P₂. Thus, K_v12.1 channels add to the growing list of K⁺ channels that are insensitive to PLCβ signaling, although modulated by PI(4,5)P₂ in Xenopus laevis oocytes.

The N-terminal homology (ENTH) domain of Epsin 1 is a sensitive reporter of physiological PI(4,5)P2 dynamics

Phospholipase Cβ (PLCβ)-induced depletion of phosphatidylinositol-(4,5)-bisphosphate (PI(4,5)P₂) transduces a plethora of signals into cellular responses. Importance and diversity of PI(4,5)P₂-dependent processes led to strong need for biosensors of physiological PI(4,5)P₂ dynamics applicable in live-cell experiments. Membrane PI(4,5)P₂ can be monitored with fluorescently-labelled phosphoinositide (PI) binding domains that associate to the membrane depending on PI(4,5)P₂ levels. The pleckstrin homology domain of PLCδ1 (PLCδ1-PH) and the C-terminus of tubby protein (tubby_CT) are two such sensors widely used to study PI(4,5)P₂ signaling. However, certain limitations apply to both: PLCδ1-PH binds cytoplasmic inositol-1,4,5-trisphosphate (IP₃) produced from PI(4,5)P₂ through PLCβ, and tubby_CT responses do not faithfully report on PLCβ-dependent PI(4,5)P₂ dynamics. In searching for an improved biosensor, we fused N-terminal homology domain of Epsin1 (ENTH) to GFP and examined use of this construct as genetically-encoded biosensor for PI(4,5)P₂ dynamics in living cells. We utilized recombinant tools to manipulate PI or G_q protein-coupled receptors (G_qPCR) to stimulate PLCβ signaling and characterized PI binding properties of ENTH-GFP with total internal reflection (TIRF) and confocal microscopy. ENTH-GFP specifically recognized membrane PI(4,5)P₂ without interacting with IP₃, as demonstrated by dialysis of cells with the messenger through a patch pipette. Utilizing Ci-VSP to titrate PI(4,5)P₂ levels, we found that ENTH-GFP had low PI(4,5)P₂ affinity. Accordingly, ENTH-GFP was highly sensitive to PLCβ-dependent PI(4,5)P₂ depletion, and in contrast to PLCδ1-PH, overexpression of ENTH-GFP did not attenuate G_qPCR signaling. Taken together, ENTH-GFP detects minute changes of PI(4,5)P₂ levels and provides an important complementation of experimentally useful reporters of PI(4,5)P₂ dynamics in physiological pathways.

Beyond voltage-gated ion channels: Voltage-operated membrane proteins and cellular processes

Voltage-gated ion channels were believed to be the only voltage-sensitive proteins in excitable (and some non-excitable) cells for a long time. Emerging evidence indicates that the voltage-operated model is shared by some other transmembrane proteins expressed in both excitable and non-excitable cells. In this review, we summarize current knowledge about voltage-operated proteins, which are not classic voltage-gated ion channels as well as the voltage-dependent processes in cells for which single voltage-sensitive proteins have yet to be identified. Particularly, we will focus on the following. (1) Voltage-sensitive phosphoinositide phosphatases (VSP) with four transmembrane segments homologous to the voltage sensor domain (VSD) of voltage-gated ion channels; VSPs are the first family of proteins, other than the voltage-gated ion channels, for which there is sufficient evidence for the existence of the VSD domain; (2) Voltage-gated proton channels comprising of a single voltage-sensing domain and lacking an identified pore domain; (3) G protein coupled receptors (GPCRs) that mediate the depolarization-evoked potentiation of Ca²⁺ mobilization; (4) Plasma membrane (PM) depolarization-induced but Ca²⁺ -independent exocytosis in neurons. (5) Voltage-dependent metabolism of phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P₂ , PIP₂ ) in the PM. These recent discoveries expand our understanding of voltage-operated processes within cellular membranes.

Regulation of neural ion channels by muscarinic receptors

The excitable behaviour of neurons is determined by the activity of their endogenous membrane ion channels. Since muscarinic receptors are not themselves ion channels, the acute effects of muscarinic receptor stimulation on neuronal function are governed by the effects of the receptors on these endogenous neuronal ion channels. This review considers some principles and factors determining the interaction between subtypes and classes of muscarinic receptors with neuronal ion channels, and summarizes the effects of muscarinic receptor stimulation on a number of different channels, the mechanisms of receptor - channel transduction and their direct consequences for neuronal activity. Ion channels considered include potassium channels (voltage-gated, inward rectifier and calcium activated), voltage-gated calcium channels, cation channels and chloride channels. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

TASK-1 (KCNK3) channels in the lung: from cell biology to clinical implications

TWIK-related acid-sensitive potassium channel 1 (TASK-1 encoded by KCNK3) belongs to the family of two-pore domain potassium channels. This gene subfamily is constitutively active at physiological resting membrane potentials in excitable cells, including smooth muscle cells, and has been particularly linked to the human pulmonary circulation. TASK-1 channels are sensitive to a wide array of physiological and pharmacological mediators that affect their activity such as unsaturated fatty acids, extracellular pH, hypoxia, anaesthetics and intracellular signalling pathways. Recent studies show that modulation of TASK-1 channels, either directly or indirectly by targeting their regulatory mechanisms, has the potential to control pulmonary arterial tone in humans. Furthermore, mutations in KCNK3 have been identified as a rare cause of both familial and idiopathic pulmonary arterial hypertension. This review summarises our current state of knowledge of the functional role of TASK-1 channels in the pulmonary circulation in health and disease, with special emphasis on current advancements in the field.

Phosphatidylinositol (4,5)-bisphosphate dynamically regulates the K2P background K+ channel TASK-2

Two-pore domain K_2P K⁺ channels responsible for the background K⁺ conductance and the resting membrane potential, are also finely regulated by a variety of chemical, physical and physiological stimuli. Hormones and transmitters acting through Gq protein-coupled receptors (GqPCRs) modulate the activity of various K_2P channels but the signalling involved has remained elusive, in particular whether dynamic regulation by membrane PI(4,5)P₂, common among other classes of K⁺ channels, affects K_2P channels is controversial. Here we show that K_2P K⁺ channel TASK-2 requires PI(4,5)P₂ for activity, a dependence that accounts for its run down in the absence of intracellular ATP and its full recovery by addition of exogenous PI(4,5)P₂, its inhibition by low concentrations of polycation PI scavengers, and inhibition by PI(4,5)P₂ depletion from the membrane. Comprehensive mutagenesis suggests that PI(4,5)P₂ interaction with TASK-2 takes place at C-terminus where three basic aminoacids are identified as being part of a putative binding site.

Dynamic NHERF interaction with TRPC4/5 proteins is required for channel gating by diacylglycerol

The activation mechanism of the classical transient receptor potential channels TRPC4 and -5 via the G_q/11 protein-phospholipase C (PLC) signaling pathway has remained elusive so far. In contrast to all other TRPC channels, the PLC product diacylglycerol (DAG) is not sufficient for channel activation, whereas TRPC4/5 channel activity is potentiated by phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion. As a characteristic structural feature, TRPC4/5 channels contain a C-terminal PDZ-binding motif allowing for binding of the scaffolding proteins Na⁺/H⁺ exchanger regulatory factor (NHERF) 1 and 2. PKC inhibition or the exchange of threonine for alanine in the C-terminal PDZ-binding motif conferred DAG sensitivity to the channel. Altogether, we present a DAG-mediated activation mechanism for TRPC4/5 channels tightly regulated by NHERF1/2 interaction. PIP₂ depletion evokes a C-terminal conformational change of TRPC5 proteins leading to dynamic dissociation of NHERF1/2 from the C terminus of TRPC5 as a prerequisite for DAG sensitivity. We show that NHERF proteins are direct regulators of ion channel activity and that DAG sensitivity is a distinctive hallmark of TRPC channels.

Direct modulation of TRPM4 and TRPM3 channels by the phospholipase C inhibitor U73122

BACKGROUND AND PURPOSE

Signalling through phospholipase C (PLC) controls many cellular processes. Much information on the relevance of this important pathway has been derived from pharmacological inhibition of the enzymatic activity of PLC. We found that the most frequently employed PLC inhibitor, U73122, activates endogenous ionic currents in widely used cell lines. Given the extensive use of U73122 in research, we set out to identify these U73122-sensitive ion channels.

EXPERIMENTAL APPROACH

We performed detailed biophysical analysis of the U73122-induced currents in frequently used cell lines.

KEY RESULTS

At concentrations required to inhibit PLC, U73122 modulated the activity of transient receptor potential melastatin (TRPM) channels through covalent modification. U73122 was shown to be a potent agonist of ubiquitously expressed TRPM4 channels and activated endogenous TRPM4 channels in CHO cells independently of PLC and of the downstream second messengers PI(4,5)P2 and Ca(2+) . U73122 also potentiated Ca(2) (+) -dependent TRPM4 currents in human Jurkat T-cells, endogenous TRPM4 in HEK293T cells and recombinant human TRPM4. In contrast to TRPM4, TRPM3 channels were inhibited whereas the closely related TRPM5 channels were insensitive to U73122, showing that U73122 exhibits high specificity within the TRPM channel family.

CONCLUSIONS AND IMPLICATIONS

Given the widespread expression of TRPM4 and TRPM3 channels, these actions of U73122 must be considered when interpreting its effects on cell function. U73122 may also be useful for identifying and characterizing TRPM channels in native tissue, thus facilitating the analysis of their physiology.

A Polybasic Plasma Membrane Binding Motif in the I-II Linker Stabilizes Voltage-gated CaV1.2 Calcium Channel Function

L-type voltage-gated Ca(2+) channels (LTCCs) regulate many physiological functions like muscle contraction, hormone secretion, gene expression, and neuronal excitability. Their activity is strictly controlled by various molecular mechanisms. The pore-forming α1-subunit comprises four repeated domains (I-IV), each connected via an intracellular linker. Here we identified a polybasic plasma membrane binding motif, consisting of four arginines, within the I-II linker of all LTCCs. The primary structure of this motif is similar to polybasic clusters known to interact with polyphosphoinositides identified in other ion channels. We used de novo molecular modeling to predict the conformation of this polybasic motif, immunofluorescence microscopy and live cell imaging to investigate the interaction with the plasma membrane, and electrophysiology to study its role for Cav1.2 channel function. According to our models, this polybasic motif of the I-II linker forms a straight α-helix, with the positive charges facing the lipid phosphates of the inner leaflet of the plasma membrane. Membrane binding of the I-II linker could be reversed after phospholipase C activation, causing polyphosphoinositide breakdown, and was accelerated by elevated intracellular Ca(2+) levels. This indicates the involvement of negatively charged phospholipids in the plasma membrane targeting of the linker. Neutralization of four arginine residues eliminated plasma membrane binding. Patch clamp recordings revealed facilitated opening of Cav1.2 channels containing these mutations, weaker inhibition by phospholipase C activation, and reduced expression of channels (as quantified by ON-gating charge) at the plasma membrane. Our data provide new evidence for a membrane binding motif within the I-II linker of LTCC α1-subunits essential for stabilizing normal Ca(2+) channel function.

Ion channel regulation by phosphoinositides analyzed with VSPs-PI(4,5)P2 affinity, phosphoinositide selectivity, and PI(4,5)P2 pool accessibility

The activity of many proteins depends on the phosphoinositide (PI) content of the membrane. E.g., dynamic changes of the concentration of PI(4,5)P₂ are cellular signals that regulate ion channels. The susceptibility of a channel to such dynamics depends on its affinity for PI(4,5)P₂. Yet, measuring affinities for endogenous PIs has not been possible directly, but has relied largely on the response to soluble analogs, which may not quantitatively reflect binding to native lipids. Voltage-sensitive phosphatases (VSPs) turn over PI(4,5)P₂ to PI(4)P when activated by depolarization. In combination with voltage-clamp electrophysiology VSPs are useful tools for rapid and reversible depletion of PI(4,5)P₂. Because cellular PI(4,5)P₂ is resynthesized rapidly, steady state PI(4,5)P₂ changes with the degree of VSP activation and thus depends on membrane potential. Here we show that titration of endogenous PI(4,5)P₂ with Ci-VSP allows for the quantification of relative PI(4,5)P₂ affinities of ion channels. The sensitivity of inward rectifier and voltage-gated K⁺ channels to Ci-VSP allowed for comparison of PI(4,5)P₂ affinities within and across channel subfamilies and detected changes of affinity in mutant channels. The results also reveal that VSPs are useful only for PI effectors with high binding specificity among PI isoforms, because PI(4,5)P₂ depletion occurs at constant overall PI level. Thus, Kir6.2, a channel activated by PI(4,5)P₂ and PI(4)P was insensitive to VSP. Surprisingly, despite comparable PI(4,5)P₂ affinity as determined by Ci-VSP, the Kv7 and Kir channel families strongly differed in their sensitivity to receptor-mediated depletion of PI(4,5)P₂. While Kv7 members were highly sensitive to activation of PLC by Gq-coupled receptors, Kir channels were insensitive even when PI(4,5)P₂ affinity was lowered by mutation. We hypothesize that different channels may be associated with distinct pools of PI(4,5)P₂ that differ in their accessibility to PLC and VSPs.

Two-pore domain potassium channels in the adrenal cortex

The physiological control of steroid hormone secretion from the adrenal cortex depends on the function of potassium channels. The "two-pore domain K(+) channels" (K2P) TWIK-related acid sensitive K(+) channel 1 (TASK1), TASK3, and TWIK-related K(+) channel 1 (TREK1) are strongly expressed in adrenocortical cells. They confer a background K(+) conductance to these cells which is important for the K(+) sensitivity as well as for angiotensin II and adrenocorticotropic hormone-dependent stimulation of aldosterone and cortisol synthesis. Mice with single deletions of the Task1 or Task3 gene as well as Task1/Task3 double knockout mice display partially autonomous aldosterone synthesis. It appears that TASK1 and TASK3 serve different functions: TASK1 affects cell differentiation and prevents expression of aldosterone synthase in the zona fasciculata, while TASK3 controls aldosterone secretion in glomerulosa cells. TREK1 is involved in the regulation of cortisol secretion in fasciculata cells. These data suggest that a disturbed function of K2P channels could contribute to adrenocortical pathologies in humans.

A method to control phosphoinositides and to analyze PTEN function in living cells using voltage sensitive phosphatases

Voltage sensitive phosphatases (VSPs), including engineered voltage sensitive PTEN, are excellent tools to rapidly and reversibly alter the phosphoinositide (PI) content of the plasma membrane in vivo and study the tumor suppressor PTEN. However, widespread adoption of these tools is hampered by the requirement for electrophysiological instrumentation to control the activity of VSPs. Additionally, monitoring and quantifying the PI changes in living cells requires sophisticated microscopy equipment and image analysis. Here we present methods that bypass these obstacles. First, we explore technically simple means for activation of VSPs via extracellularly applied agents or light. Secondly, we characterize methods to monitor PI(4,5)P₂ and PI(3,4,5)P₃ levels using fluorescence microscopy or photometry in conjunction with translocation or FRET based PI probes, respectively. We then demonstrate the application of these techniques by characterizing the effect of known PTEN mutations on its enzymatic activity, analyzing the effect of PTEN inhibitors, and detecting in real time rapid inhibition of protein kinase B following depletion of PI(3,4,5)P₃. Thus, we established an approach that does not only allow for rapidly manipulating and monitoring PI(4,5)P₂ and PI(3,4,5)P₃ levels in a population of cells, but also facilitates the study of PTEN mutants and pharmacological targeting in mammalian cells.

PIPs in neurological diseases

Phosphoinositide (PIP) lipids regulate many aspects of cell function in the nervous system including receptor signalling, secretion, endocytosis, migration and survival. Levels of PIPs such as PI4P, PI(4,5)P2 and PI(3,4,5)P3 are normally tightly regulated by phosphoinositide kinases and phosphatases. Deregulation of these biochemical pathways leads to lipid imbalances, usually on intracellular endosomal membranes, and these changes have been linked to a number of major neurological diseases including Alzheimer's, Parkinson's, epilepsy, stroke, cancer and a range of rarer inherited disorders including brain overgrowth syndromes, Charcot-Marie-Tooth neuropathies and neurodevelopmental conditions such as Lowe's syndrome. This article analyses recent progress in this area and explains how PIP lipids are involved, to varying degrees, in almost every class of neurological disease. This article is part of a Special Issue entitled Brain Lipids.

TASK channels in arterial chemoreceptors and their role in oxygen and acid sensing

Arterial chemoreceptors play a vital role in cardiorespiratory control by providing the brain with information regarding blood oxygen, carbon dioxide, and pH. The main chemoreceptor, the carotid body, is composed of sensory (type 1) cells which respond to hypoxia or acidosis with a depolarising receptor potential which in turn activates voltage-gated calcium entry, neurosecretion and excitation of adjacent afferent nerves. The receptor potential is generated by inhibition of Twik-related acid-sensitive K(+) channel 1 and 3 (TASK1/TASK3) heterodimeric channels which normally maintain the cells' resting membrane potential. These channels are thought to be directly inhibited by acidosis. Oxygen sensitivity, however, probably derives from a metabolic signalling pathway. The carotid body, isolated type 1 cells, and all forms of TASK channel found in the type 1 cell, are highly sensitive to inhibitors of mitochondrial metabolism. Moreover, type1 cell TASK channels are activated by millimolar levels of MgATP. In addition to their role in the transduction of chemostimuli, type 1 cell TASK channels have also been implicated in the modulation of chemoreceptor function by a number of neurocrine/paracrine signalling molecules including adenosine, GABA, and serotonin. They may also be instrumental in mediating the depression of the acute hypoxic ventilatory response that occurs with some general anaesthetics. Modulation of TASK channel activity is therefore a key mechanism by which the excitability of chemoreceptors can be controlled. This is not only of physiological importance but may also offer a therapeutic strategy for the treatment of cardiorespiratory disorders that are associated with chemoreceptor dysfunction.

Diacylglycerol mediates regulation of TASK potassium channels by Gq-coupled receptors

The two-pore domain potassium (K2P) channels TASK-1 (KCNK3) and TASK-3 (KCNK9) are important determinants of background K(+) conductance and membrane potential. TASK-1/3 activity is regulated by hormones and transmitters that act through G protein-coupled receptors (GPCR) signalling via G proteins of the Gαq/11 subclass. How the receptors inhibit channel activity has remained unclear. Here, we show that TASK-1 and -3 channels are gated by diacylglycerol (DAG). Receptor-initiated inhibition of TASK required the activity of phospholipase C, but neither depletion of the PLC substrate PI(4,5)P2 nor release of the downstream messengers IP3 and Ca(2+). Attenuation of cellular DAG transients by DAG kinase or lipase suppressed receptor-dependent inhibition, showing that the increase in cellular DAG-but not in downstream lipid metabolites-mediates channel inhibition. The findings identify DAG as the signal regulating TASK channels downstream of GPCRs and define a novel role for DAG that directly links cellular DAG dynamics to excitability.

The role of acid-sensitive two-pore domain potassium channels in cardiac electrophysiology: focus on arrhythmias

The current kinetics of two-pore domain potassium (K2P) channels resemble those of the steady-state K(+) currents being active during the plateau phase of cardiac action potentials. Recent studies support that K2P channels contribute to these cardiac currents and thereby influence action potential duration in the heart. Ten of the 15 K2P channels present in the human genome are sensitive to variations of the extracellular and/or intracellular pH value. This review focuses on a set of K2P channels which are inhibited by extracellular protons, including the subgroup of tandem of P domains in a weak inward-rectifying K(+) (TWIK)-related acid-sensitive potassium (TASK) and TWIK-related alkaline-activated K(+) (TALK) channels. The role of TWIK-1 in the heart is also discussed since, after successful expression, an extracellular pH dependence, similar to that of TASK-1, was described as a hallmark of TWIK-1. The expression profile in cardiac tissue of different species and the functional data in the heart are summarized. The distinct role of the different acid-sensitive K2P channels in cardiac electrophysiology, inherited forms of arrhythmias and pharmacology, and their role as drug targets is currently emerging and is the subject of this review.

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.

New experimental trends for phosphoinositides research on ion transporter/channel regulation

Phosphoinositides(4,5)-bisphosphates [PI(4,5)P2] critically controls membrane excitability, the disruption of which leads to pathophysiological states. PI(4,5)P2 plays a primary role in regulating the conduction and gating properties of ion channels/transporters, through electrostatic and hydrophobic interactions that allow direct associations. In recent years, the development of many molecular tools have brought deep insights into the mechanisms underlying PI(4,5)P2-mediated regulation. This review summarizes the methods currently available to manipulate the cell membrane PI(4,5)P2 level including pharmacological interventions as well as newly designed molecular tools. We concisely introduce materials and experimental designs suitable for the study of PI(4,5)P2-mediated regulation of ion-conducting molecules, in order to assist researchers who are interested in this area. It is our further hope that the knowledge introduced in this review will help to promote our understanding about the pathology of diseases such as cardiac arrhythmias, bipolar disorders, and Alzheimer's disease which are somehow associated with a disruption of PI(4,5)P2 metabolism.

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.

Phosphatidylinositol 4,5-bisphosphate depletion fails to affect neurosteroid modulation of GABAA receptor function

RATIONALE

Neurosteroids and likely other lipid modulators access transmembrane sites on the GABAA receptor (GABAAR) by partitioning into and diffusing through the plasma membrane. Therefore, specific components of the plasma membrane may affect the potency or efficacy of neurosteroid-like modulators. Here, we tested a possible role for phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid that governs activity of many channels and transporters, in modulation or function of GABAARs.

OBJECTIVES

In these studies, we sought to deplete plasma-membrane PIP2 and probe for a change in the strength of potentiation by submaximal concentrations of the neurosteroid allopregnanolone (3α5αP) and other anesthetics, including propofol, pentobarbital, and ethanol. We also tested for a change in the behavior of negative allosteric modulators pregnenolone sulfate and dipicrylamine.

METHODS

We used Xenopus oocytes expressing the ascidian voltage-sensitive phosphatase (Ci-VSP) to deplete PIP2. Voltage pulses to positive membrane potentials were used to deplete PIP2 in Ci-VSP-expressing cells. GABAARs composed of α1β2γ2L and α4β2δ subunits were challenged with GABA and 3α5αP or other modulators before and after PIP2 depletion. KV7.1 channels and NMDA receptors (NMDARs) were used as positive controls to verify PIP2 depletion.

RESULTS

We found no evidence that PIP2 depletion affected modulation of GABAARs by positive or negative allosteric modulators. By contrast, Ci-VSP-induced PIP2 depletion depressed KV7.1 activation and NMDAR activity.

CONCLUSIONS

We conclude that despite a role for PIP2 in modulation of a wide variety of ion channels, PIP2 does not affect modulation of GABAARs by neurosteroids or related compounds.

Orexin/hypocretin receptor signalling cascades

Orexin (hypocretin) peptides and their two known G-protein-coupled receptors play essential roles in sleep-wake control and powerfully influence other systems regulating appetite/metabolism, stress and reward. Consequently, drugs that influence signalling by these receptors may provide novel therapeutic opportunities for treating sleep disorders, obesity and addiction. It is therefore critical to understand how these receptors operate, the nature of the signalling cascades they engage and their physiological targets. In this review, we evaluate what is currently known about orexin receptor signalling cascades, while a sister review (Leonard & Kukkonen, this issue) focuses on tissue-specific responses. The evidence suggests that orexin receptor signalling is multifaceted and is substantially more diverse than originally thought. Indeed, orexin receptors are able to couple to members of at least three G-protein families and possibly other proteins, through which they regulate non-selective cation channels, phospholipases, adenylyl cyclase, and protein and lipid kinases. In the central nervous system, orexin receptors produce neuroexcitation by postsynaptic depolarization via activation of non-selective cation channels, inhibition of K⁺ channels and activation of Na⁺/Ca²⁺ exchange, but they also can stimulate the release of neurotransmitters by presynaptic actions and modulate synaptic plasticity. Ca²⁺ signalling is also prominently influenced by these receptors, both via the classical phospholipase C-Ca²⁺ release pathway and via Ca²⁺ influx, mediated by several pathways. Upon longer-lasting stimulation, plastic effects are observed in some cell types, while others, especially cancer cells, are stimulated to die. Thus, orexin receptor signals appear highly tunable, depending on the milieu in which they are operating.

Golgi and plasma membrane pools of PI(4)P contribute to plasma membrane PI(4,5)P2 and maintenance of KCNQ2/3 ion channel current

Plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] regulates the activity of many ion channels and other membrane-associated proteins. To determine precursor sources of the PM PI(4,5)P2 pool in tsA-201 cells, we monitored KCNQ2/3 channel currents and translocation of PHPLCδ1 domains as real-time indicators of PM PI(4,5)P2, and translocation of PHOSH2×2, and PHOSH1 domains as indicators of PM and Golgi phosphatidylinositol 4-phosphate [PI(4)P], respectively. We selectively depleted PI(4)P pools at the PM, Golgi, or both using the rapamycin-recruitable lipid 4-phosphatases. Depleting PI(4)P at the PM with a recruitable 4-phosphatase (Sac1) results in a decrease of PI(4,5)P2 measured by electrical or optical indicators. Depleting PI(4)P at the Golgi with the 4-phosphatase or disrupting membrane-transporting motors induces a decline in PM PI(4,5)P2. Depleting PI(4)P simultaneously at both the Golgi and the PM induces a larger decrease of PI(4,5)P2. The decline of PI(4,5)P2 following 4-phosphatase recruitment takes 1-2 min. Recruiting the endoplasmic reticulum (ER) toward the Golgi membranes mimics the effects of depleting PI(4)P at the Golgi, apparently due to the trans actions of endogenous ER Sac1. Thus, maintenance of the PM pool of PI(4,5)P2 appears to depend on precursor pools of PI(4)P both in the PM and in the Golgi. The decrease in PM PI(4,5)P2 when Sac1 is recruited to the Golgi suggests that the Golgi contribution is ongoing and that PI(4,5)P2 production may be coupled to important cell biological processes such as membrane trafficking or lipid transfer activity.

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.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Regulation of voltage-gated potassium channels by PI(4,5)P2

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) regulates activities of numerous ion channels including inwardly rectifying potassium (K_ir) channels, KCNQ, TRP, and voltage-gated calcium channels. Several studies suggest that voltage-gated potassium (K_V) channels might be regulated by PI(4,5)P₂. Wide expression of K_V channels in different cells suggests that such regulation could have broad physiological consequences. To study regulation of K_V channels by PI(4,5)P₂, we have coexpressed several of them in tsA-201 cells with a G protein–coupled receptor (M₁R), a voltage-sensitive lipid 5-phosphatase (Dr-VSP), or an engineered fusion protein carrying both lipid 4-phosphatase and 5-phosphatase activity (pseudojanin). These tools deplete PI(4,5)P₂ with application of muscarinic agonists, depolarization, or rapamycin, respectively. PI(4,5)P₂ at the plasma membrane was monitored by Förster resonance energy transfer (FRET) from PH probes of PLCδ1 simultaneously with whole-cell recordings. Activation of Dr-VSP or recruitment of pseudojanin inhibited K_V7.1, K_V7.2/7.3, and K_ir2.1 channel current by 90–95%. Activation of M₁R inhibited K_V7.2/7.3 current similarly. With these tools, we tested for potential PI(4,5)P₂ regulation of activity of K_V1.1/K_Vβ1.1, K_V1.3, K_V1.4, and K_V1.5/K_Vβ1.3, K_V2.1, K_V3.4, K_V4.2, K_V4.3 (with different KChIPs and DPP6-s), and hERG/KCNE2. Interestingly, we found a substantial removal of inactivation for K_V1.1/K_Vβ1.1 and K_V3.4, resulting in up-regulation of current density upon activation of M₁R but no changes in activity upon activating only VSP or pseudojanin. The other channels tested except possibly hERG showed no alteration in activity in any of the assays we used. In conclusion, a depletion of PI(4,5)P₂ at the plasma membrane by enzymes does not seem to influence activity of most tested K_V channels, whereas it does strongly inhibit members of the K_V7 and K_ir families.

Dual effect of phosphatidylinositol (4,5)-bisphosphate PIP(2) on Shaker K(+) [corrected] channels

Phosphatidylinositol (4,5)-bisphosphate (PIP(2)) is a phospholipid of the plasma membrane that has been shown to be a key regulator of several ion channels. Functional studies and more recently structural studies of Kir channels have revealed the major impact of PIP(2) on the open state stabilization. A similar effect of PIP(2) on the delayed rectifiers Kv7.1 and Kv11.1, two voltage-gated K(+) channels, has been suggested, but the molecular mechanism remains elusive and nothing is known on PIP(2) effect on other Kv such as those of the Shaker family. By combining giant-patch ionic and gating current recordings in COS-7 cells, and voltage-clamp fluorimetry in Xenopus oocytes, both heterologously expressing the voltage-dependent Shaker channel, we show that PIP(2) exerts 1) a gain-of-function effect on the maximal current amplitude, consistent with a stabilization of the open state and 2) a loss-of-function effect by positive-shifting the activation voltage dependence, most likely through a direct effect on the voltage sensor movement, as illustrated by molecular dynamics simulations.

A human phospholipid phosphatase activated by a transmembrane control module

In voltage-sensitive phosphatases (VSPs), a transmembrane voltage sensor domain (VSD) controls an intracellular phosphoinositide phosphatase domain, thereby enabling immediate initiation of intracellular signals by membrane depolarization. The existence of such a mechanism in mammals has remained elusive, despite the presence of VSP-homologous proteins in mammalian cells, in particular in sperm precursor cells. Here we demonstrate activation of a human VSP (hVSP1/TPIP) by an intramolecular switch. By engineering a chimeric hVSP1 with enhanced plasma membrane targeting containing the VSD of a prototypic invertebrate VSP, we show that hVSP1 is a phosphoinositide-5-phosphatase whose predominant substrate is PI(4,5)P(2). In the chimera, enzymatic activity is controlled by membrane potential via hVSP1's endogenous phosphoinositide binding motif. These findings suggest that the endogenous VSD of hVSP1 is a control module that initiates signaling through the phosphatase domain and indicate a role for VSP-mediated phosphoinositide signaling in mammals.

Opposite Effects of the S4-S5 Linker and PIP(2) on Voltage-Gated Channel Function: KCNQ1/KCNE1 and Other Channels

Voltage-gated potassium (Kv) channels are tetramers, each subunit presenting six transmembrane segments (S1-S6), with each S1-S4 segments forming a voltage-sensing domain (VSD) and the four S5-S6 forming both the conduction pathway and its gate. S4 segments control the opening of the intracellular activation gate in response to changes in membrane potential. Crystal structures of several voltage-gated ion channels in combination with biophysical and mutagenesis studies highlighted the critical role of the S4-S5 linker (S4S5(L)) and of the S6 C-terminal part (S6(T)) in the coupling between the VSD and the activation gate. Several mechanisms have been proposed to describe the coupling at a molecular scale. This review summarizes the mechanisms suggested for various voltage-gated ion channels, including a mechanism that we described for KCNQ1, in which S4S5(L) is acting like a ligand binding to S6(T) to stabilize the channel in a closed state. As discussed in this review, this mechanism may explain the reverse response to depolarization in HCN-like channels. As opposed to S4S5(L), the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)), stabilizes KCNQ1 channel in an open state. Many other ion channels (not only voltage-gated) require PIP(2) to function properly, confirming its crucial importance as an ion channel cofactor. This is highlighted in cases in which an altered regulation of ion channels by PIP(2) leads to channelopathies, as observed for KCNQ1. This review summarizes the state of the art on the two regulatory mechanisms that are critical for KCNQ1 and other voltage-gated channels function (PIP(2) and S4S5(L)), and assesses their potential physiological and pathophysiological roles.

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.

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.

Identification of the muscarinic pathway underlying cessation of sleep-related burst activity in rat thalamocortical relay neurons

Modulation of the standing outward current (I (SO)) by muscarinic acetylcholine (ACh) receptor (MAChR) stimulation is fundamental for the state-dependent change in activity mode of thalamocortical relay (TC) neurons. Here, we probe the contribution of MAChR subtypes, G proteins, phospholipase C (PLC), and two pore domain K(+) (K(2P)) channels to this signaling cascade. By the use of spadin and A293 as specific blockers, we identify TWIK-related K(+) (TREK)-1 channel as new targets and confirm TWIK-related acid-sensitve K(+) (TASK)-1 channels as known effectors of muscarinic signaling in TC neurons. These findings were confirmed using a high affinity blocker of TASK-3 and TREK-1, namely, tetrahexylammonium chloride. It was found that the effect of muscarinic stimulation was inhibited by M(1)AChR-(pirenzepine, MT-7) and M(3)AChR-specific (4-DAMP) antagonists, phosphoinositide-specific PLCβ (PI-PLC) inhibitors (U73122, ET-18-OCH(3)), but not the phosphatidylcholine-specific PLC (PC-PLC) blocker D609. By comparison, depleting guanosine-5'-triphosphate (GTP) in the intracellular milieu nearly completely abolished the effect of MAChR stimulation. The block of TASK and TREK channels was accompanied by a reduction of the muscarinic effect on I (SO). Current-clamp recordings revealed a membrane depolarization following MAChR stimulation, which was sufficient to switch TC neurons from burst to tonic firing under control conditions but not during block of M(1)AChR/M(3)AChR and in the absence of intracellular GTP. These findings point to a critical role of G proteins and PLC as well as TASK and TREK channels in the muscarinic modulation of thalamic activity modes.