A non-inactivating K+ current sensitive to muscarinic receptor activation in rat cultured cerebellar granule neurons

A Direct Interaction between Cyclodextrins and TASK Channels Decreases the Leak Current in Cerebellar Granule Neurons

Simple Summary

Cyclodextrins are cyclic oligosaccharides used to deplete cholesterol from cellular membranes. The effects of methyl-β-cyclodextrin (MβCD) on cellular functions originate principally from reductions in cholesterol levels. In this study, using immunocytochemistry, heterologous expression of K2P channels, and cholesterol-depleting maneuvers, we provide evidence of expression in cultured rat cerebellar granule neurons (CGNs) of TWIK-1 (K2P1), TASK-1 (K2P3), TASK-3 (K2P9), and TRESK (K2P18) channels and their association with lipid rafts using the specific lipids raft markers. In addition, we show a direct blocking with MβCD of TASK-1 and TASK-3 channels as well as for the covalently concatenated heterodimer TASK-1/TASK-3.

Abstract

Two pore domain potassium channels (K2P) are strongly expressed in the nervous system (CNS), where they play a central role in excitability. These channels give rise to background K⁺ currents, also known as IK_SO (standing-outward potassium current). We detected the expression in primary cultured cerebellar granule neurons (CGNs) of TWIK-1 (K2P1), TASK-1 (K2P3), TASK-3 (K2P9), and TRESK (K2P18) channels by immunocytochemistry and their association with lipid rafts using the specific lipids raft markers flotillin-2 and caveolin-1. At the functional level, methyl-β-cyclodextrin (MβCD, 5 mM) reduced IK_SO currents by ~40% in CGN cells. To dissect out this effect, we heterologously expressed the human TWIK-1, TASK-1, TASK-3, and TRESK channels in HEK-293 cells. MβCD directly blocked TASK-1 and TASK-3 channels and the covalently concatenated heterodimer TASK-1/TASK-3 currents. Conversely, MβCD did not affect TWIK-1- and TRESK-mediated K+ currents. On the other hand, the cholesterol-depleting agent filipin III did not affect TASK-1/TASK-3 channels. Together, the results suggest that neuronal background K⁺ channels are associated to lipid raft environments whilst the functional activity is independent of the cholesterol membrane organization.

Glia-derived ATP inversely regulates excitability of pyramidal and CCK-positive neurons

Astrocyte responds to neuronal activity with calcium waves and modulates synaptic transmission through the release of gliotransmitters. However, little is known about the direct effect of gliotransmitters on the excitability of neuronal networks beyond synapses. Here we show that selective stimulation of astrocytes expressing channelrhodopsin-2 in the CA1 area specifically increases the firing frequency of CCK-positive but not parvalbumin-positive interneurons and decreases the firing rate of pyramidal neurons, phenomena mimicked by exogenously applied ATP. Further evidences indicate that ATP-induced increase and decrease of excitability are caused, respectively, by P2Y1 receptor-mediated inhibition of a two-pore domain potassium channel and A1 receptor-mediated opening of a G-protein-coupled inwardly rectifying potassium channel. Moreover, the activation of ChR2-expressing astrocytes reduces the power of kainate-induced hippocampal ex vivo gamma oscillation. Thus, through distinct receptor subtypes coupled with different K⁺ channels, astrocyte-derived ATP differentially modulates the excitability of different types of neurons and efficiently controls the activity of neuronal network.

14-3-3 Proteins regulate K2P 5.1 surface expression on T lymphocytes

K_2P 5.1 channels (also called TASK-2 or Kcnk5) have already been shown to be relevant in the pathophysiology of autoimmune disease because they are known to be upregulated on peripheral and central T lymphocytes of multiple sclerosis (MS) patients. Moreover, overexpression of K_2P 5.1 channels in vitro provokes enhanced T-cell effector functions. However, the molecular mechanisms regulating intracellular K_2P 5.1 channel trafficking are unknown so far. Thus, the aim of the study is to elucidate the trafficking of K_2P 5.1 channels on T lymphocytes. Using mass spectrometry analysis, we have identified 14-3-3 proteins as novel binding partners of K_2P 5.1 channels. We show that a non-classical 14-3-3 consensus motif (R-X-X-pT/S-x) at the channel's C-terminus allows the binding between K_2P 5.1 and 14-3-3. The mutant K_2P 5.1/S266A diminishes the protein-protein interaction and reduces the amplitude of membrane currents. Application of a non-peptidic 14-3-3 inhibitor (BV02) significantly reduces the number of wild-type channels in the plasma membrane, whereas the drug has no effect on the trafficking of the mutated channel. Furthermore, blocker application reduces T-cell effector functions. Taken together, we demonstrate that 14-3-3 interacts with K_2P 5.1 and plays an important role in channel trafficking.

The CNS under pathophysiologic attack--examining the role of K₂p channels

Members of the two-pore domain K(+) channel (K2P) family are increasingly recognized as being potential targets for therapeutic drugs and could play a role in the diagnosis and treatment of neurologic disorders. Their broad and diverse expression pattern in pleiotropic cell types, importance in cellular function, unique biophysical properties, and sensitivity toward pathophysiologic parameters represent the basis for their involvement in disorders of the central nervous system (CNS). This review will focus on multiple sclerosis (MS) and stroke, as there is growing evidence for the involvement of K2P channels in these two major CNS disorders. In MS, TASK1-3 channels are expressed on T lymphocytes and are part of a signaling network regulating Ca(2+)- dependent pathways that are mandatory for T cell activation, differentiation, and effector functions. In addition, TASK1 channels are involved in neurodegeneration, resulting in autoimmune attack of CNS cells. On the blood-brain barrier, TREK1 channels regulate immune cell trafficking under autoinflammatory conditions. Cerebral ischemia shares some pathophysiologic similarities with MS, including hypoxia and extracellular acidosis. On a cellular level, K2P channels can have both proapoptotic and antiapoptotic effects, either promoting neurodegeneration or protecting neurons from ischemic cell death. TASK1 and TREK1 channels have a neuroprotective effect on stroke development, whereas TASK2 channels have a detrimental effect on neuronal survival under ischemic conditions. Future research in preclinical models is needed to provide a more detailed understanding of the contribution of K2P channel family members to neurologic disorders, before translation to the clinic is an option.

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.

Differential expression of two-pore domain potassium channels in rat cerebellar granule neurons

Two pore domain potassium (K2P) channels are mostly present in the central nervous system (CNS) where they play important roles in modulating neuronal excitability. K2P channels give rise to background K(+) currents (IKSO) a key component in setting and maintaining the resting membrane potential in excitable cells. Here, we studied the expression and relative abundances of K2P channels in cerebellar granule neurons (CGNs), combining molecular biology, electrophysiology and immunologic techniques. The CGN IKSO was very sensitive to external pH, as previously reported. Quantitative determination of mRNA expression level demonstrated the existence of an accumulation pattern of transcripts in CGN that encode K2P9>K2P1>K2P3>K2P18>K2P2=K2P10>K2P4>K2P5 subunits. The presence of the major K2P subunits expressed was then confirmed by Western blot and immunofluorescence analysis, demonstrating robust expression of K2P1 (TWIK-1), K2P3 (TASK-1), K2P9 (TASK-3) and K2P18 (TRESK) channel protein. Based, on these results, it is concluded that K2P1, -3, -9 and -18 subunits represent the majority component of IKSO current in CGN.

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

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

Enhancement of TWIK-related acid-sensitive potassium channel 3 (TASK3) two-pore domain potassium channel activity by tumor necrosis factor α

TASK3 two-pore domain potassium (K2P) channels are responsible for native leak K channels in many cell types which regulate cell resting membrane potential and excitability. In addition, TASK3 channels contribute to the regulation of cellular potassium homeostasis. Because TASK3 channels are important for cell viability, having putative roles in both neuronal apoptosis and oncogenesis, we sought to determine their behavior under inflammatory conditions by investigating the effect of TNFα on TASK3 channel current. TASK3 channels were expressed in tsA-201 cells, and the current through them was measured using whole cell voltage clamp recordings. We show that THP-1 human myeloid leukemia monocytes, co-cultured with hTASK3-transfected tsA-201 cells, can be activated by the specific Toll-like receptor 7/8 activator, R848, to release TNFα that subsequently enhances hTASK3 current. Both hTASK3 and mTASK3 channel activity is increased by incubation with recombinant TNFα (10 ng/ml for 2-15 h), but other K2P channels (hTASK1, hTASK2, hTREK1, and hTRESK) are unaffected. This enhancement by TNFα is not due to alterations in levels of channel expression at the membrane but rather to an alteration in channel gating. The enhancement by TNFα can be blocked by extracellular acidification but persists for mutated TASK3 (H98A) channels that are no longer acid-sensitive even in an acidic extracellular environment. TNFα action on TASK3 channels is mediated through the intracellular C terminus of the channel. Furthermore, it occurs through the ASK1 pathway and is JNK- and p38-dependent. In combination, TNFα activation and TASK3 channel activity can promote cellular apoptosis.

Novel TASK channels inhibitors derived from dihydropyrrolo[2,1-a]isoquinoline

TASK channels belong to the family of K(+) channels with 4 transmembrane segments and 2 pore domains (4TM/2P) per subunit. These channels have been related to apoptosis in cerebellar granule neurons (CGN), as well as cancer in other tissues. TASK current is regulated by hormones, neurotransmitters, anesthetics and divalent cations, which are not selective. Recently, there has been found some organic compounds that inhibit TASK current selectively. In order to find other modulators, we report here a group of five dihydropyrrolo[2,1-a]isoquinolines (DPIs), four of them with putative anticancer activity, that were evaluated on TASK-1 and TASK-3 channels. The compounds 1, 2 and 3 showed IC50 < 320 μM on TASK-1 and TASK-3, intermediate activity on TASK-1/TASK-3 heterodimer, moderate effect over hslo and TREK-1 (500 μM), and practically not inhibition on Shaker-IR, herg and IRK2.1 potassium channels, when they were expressed heterologously in Xenopus laevis oocytes. In rat CGN, 500 μM of these three compounds induced a decrement by >39% of the TASK-carried leak current. Finally, only compound 1 showed significant protection (∼36%) against apoptotic death of CGN induced by K(+) deprivation. These results suggest that DPI compounds could be potential candidates for designing new selective inhibitors of TASK channels.

A new TASK for Dipeptidyl Peptidase-like Protein 6

Dipeptidyl Peptidase-like Protein 6 (DPP6) is widely expressed in the brain where it co-assembles with Kv4 channels and KChIP auxiliary subunits to regulate the amplitude and functional properties of the somatodendritic A-current, I_SA. Here we show that in cerebellar granule (CG) cells DPP6 also regulates resting membrane potential and input resistance by increasing the amplitude of the I_K(SO) resting membrane current. Pharmacological analysis shows that DPP6 acts through the control of a channel with properties matching the K_2P channel TASK-3. Heterologous expression and co-immunoprecipitation shows that DPP6 co-expression with TASK-3 results in the formation of a protein complex that enhances resting membrane potassium conductance. The co-regulation of resting and voltage-gated channels by DPP6 produces coordinate shifts in resting membrane potential and A-current gating that optimize the sensitivity of I_SA inactivation gating to subthreshold fluctuations in resting membrane potential.

SUMOylation silences heterodimeric TASK potassium channels containing K2P1 subunits in cerebellar granule neurons

The standing outward K(+) current (IKso) governs the response of cerebellar granule neurons to natural and medicinal stimuli including volatile anesthetics. We showed that SUMOylation silenced half of IKso at the surface of cerebellar granule neurons because the underlying channels were heterodimeric assemblies of K2P1, a subunit subject to SUMOylation, and the TASK (two-P domain, acid-sensitive K(+)) channel subunits K2P3 or K2P9. The heterodimeric channels comprised the acid-sensitive portion of IKso and mediated its response to halothane. We anticipate that SUMOylation also influences sensation and homeostatic mechanisms in mammals through TASK channels formed with K2P1.

A new signalling pathway for parallel fibre presynaptic type 4 metabotropic glutamate receptors (mGluR4) in the rat cerebellar cortex

In the rodent cerebellum, pharmacological activation of mGluR4 acutely depresses excitatory synaptic transmission at parallel fibre–Purkinje cell synapses. This depression involves the inhibition of presynaptic calcium (Ca2+) influx that ultimately controls glutamate release. In this study, we investigate the molecular basis of mGluR4-mediated inhibition of presynaptic Ca2+ transients. Our results demonstrate that the mGluR4 effect does not depend on selective inhibition of a specific type of presynaptic voltage-gated Ca2+ channel, but rather involves modulation of all classes of Ca2+ channels present in the presynaptic terminals. In addition, this inhibitory effect does not involve the activation of G protein-activated inwardly rectifying potassium channels, TEA-sensitive potassium channels or two-pore-domain potassium channels. Furthermore, this inhibition does not require pertussis toxin-sensitive G proteins, and is independent of any effect on adenylyl cyclases, protein kinase A, mitogen-activated protein kinases or phosphoinositol-3 kinase activity. Interestingly we found that mGluR4 inhibition of presynaptic Ca2+ influx employs a newly defined signalling pathway, notably that involving the activation of phospholipase C and ultimately protein kinase C.

A Role for K2P Channels in the Operation of Somatosensory Nociceptors

The ability to sense mechanical, thermal, and chemical stimuli is critical to normal physiology and the perception of pain. Contact with noxious stimuli triggers a complex series of events that initiate innate protective mechanisms designed to minimize or avoid injury. Extreme temperatures, mechanical stress, and chemical irritants are detected by specific ion channels and receptors clustered on the terminals of nociceptive sensory nerve fibers and transduced into electrical information. Propagation of these signals, from distant sites in the body to the spinal cord and the higher processing centers of the brain, is also orchestrated by distinct groups of ion channels. Since their identification in 1995, evidence has emerged to support roles for K2P channels at each step along this pathway, as receptors for physiological and noxious stimuli, and as determinants of nociceptor excitability and conductivity. In addition, the many subtypes of K2P channels expressed in somatosensory neurons are also implicated in mediating the effects of volatile, general anesthetics on the central and peripheral nervous systems. Here, I offer a critical review of the existing data supporting these attributes of K2P channel function and discuss how diverse regulatory mechanisms that control the activity of K2P channels act to govern the operation of nociceptors.

Amoxapine inhibits delayed outward rectifier K(+) currents in cerebellar granule cells via dopamine receptor and protein kinase A activation

BACKGROUND

Although tricyclic antidepressants amoxapine is proposed to target 5-HT and D2 receptors, very few studies have addressed the effect of amoxapine on molecular and cellular mechanisms via receptor pathways. In this study, we test the effect of amoxapine on rat cerebellar granule neurons (CGNs) to address this possibility.

METHODS

CGNs cell culture, whole-cell current recording using a patch-clamp technique, western blot and non-radioactive detection analysis of phosphorylated protein kinase A (PKA) were used.

RESULTS

Amoxapine inhibits delayed rectifier potassium (I(K)) current in a dose-dependent manner and modulates inactivation properties in CGNs. Those effects were not eliminated by preincubation with 5-HT or 5-HT receptor antagonists, but abolished by dopamine and D1/D5 receptor antagonists. Application of GTPγ-S and inhibitor of the Gs signalling cascade abolished the amoxapine-induced effect on I(K). The application of forskolin or dibutyryl-cAMP mimicked the inhibitory effect of amoxapine on I(K). Western blotting for phosphorylated PKA revealed that amoxapine significantly increased the intracellular levels of phosphorylated PKA, a marker of PKA activation.

CONCLUSION

Amoxapine inhibits I(K) currents in rat CGNs via cAMP/PKA-dependent pathways, as in mouse cortical neurons we reported earlier, but that involves D1-like receptors instead of 5-HT receptors.

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

TASK channels are background K+ channels that contribute to the resting conductance in many neurons. A key feature of TASK channels is the reversible inhibition by Gq-coupled receptors, thereby mediating the dynamic regulation of neuronal activity by modulatory transmitters. The mechanism that mediates channel inhibition is not fully understood. While it is clear that activation of Gαq is required, the immediate signal for channel closure remains controversial. Experimental evidence pointed to either phospholipase C (PLC)-mediated depletion of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) as the cause for channel closure or to a direct inhibitory interaction of active Gαq with the channel. Here, we address the role of PI(4,5)P2 for G-protein-coupled receptor (GPCR)-mediated TASK inhibition by using recently developed genetically encoded tools to alter phosphoinositide (PI) concentrations in the living cell.When expressed in CHO cells, TASK-1- and TASK-3-mediated currents were not affected by depletion of plasma membrane PI(4,5)P2 either via the voltage-activated phosphatase Ci-VSP or via chemically triggered recruitment of a PI(4,5)P2-5'-phosphatase. Depletion of both PI(4,5)P2 and PI(4)P via membrane recruitment of a novel engineered dual-specificity phosphatase also did not inhibit TASK currents. In contrast, each of these methods produced robust inhibition of the bona fide PI(4,5)P2-dependent channel KCNQ4. Efficient depletion of PI(4,5)P2 and PI(4)P was further confirmed with a fluorescent phosphoinositide sensor. Moreover, TASK channels recovered normally from inhibition by co-expressed muscarinic M1 receptors when resynthesis of PI(4,5)P2 was prevented by depletion of cellular ATP. These results demonstrate that TASK channel activity is independent of phosphoinositide concentrations within the physiological range. Consequently, Gq-mediated inhibition of TASK channels is not mediated by depletion of PI(4,5)P2.

Two pore domain potassium channels in cerebral ischemia: a focus on K2P9.1 (TASK3, KCNK9)

Background

Recently, members of the two-pore domain potassium channel family (K_2P channels) could be shown to be involved in mechanisms contributing to neuronal damage after cerebral ischemia. K_2P3.1^-/- animals showed larger infarct volumes and a worse functional outcome following experimentally induced ischemic stroke. Here, we question the role of the closely related K_2P channel K_2P9.1.

Methods

We combine electrophysiological recordings in brain-slice preparations of wildtype and K_2P9.1^-/- mice with an in vivo model of cerebral ischemia (transient middle cerebral artery occlusion (tMCAO)) to depict a functional impact of K_2P9.1 in stroke formation.

Results

Patch-clamp recordings reveal that currents mediated through K_2P9.1 can be obtained in slice preparations of the dorsal lateral geniculate nucleus (dLGN) as a model of central nervous relay neurons. Current characteristics are indicative of K_2P9.1 as they display an increase upon removal of extracellular divalent cations, an outward rectification and a reversal potential close to the potassium equilibrium potential. Lowering extracellular pH values from 7.35 to 6.0 showed comparable current reductions in neurons from wildtype and K_2P9.1^-/- mice (68.31 ± 9.80% and 69.92 ± 11.65%, respectively). These results could be translated in an in vivo model of cerebral ischemia where infarct volumes and functional outcomes showed a none significant tendency towards smaller infarct volumes in K_2P9.1^-/- animals compared to wildtype mice 24 hours after 60 min of tMCAO induction (60.50 ± 17.31 mm³ and 47.10 ± 19.26 mm³, respectively).

Conclusions

Together with findings from earlier studies on K_2P2.1^-/- and K_2P3.1^-/- mice, the results of the present study on K_2P9.1^-/- mice indicate a differential contribution of K_2P channel subtypes to the diverse and complex in vivo effects in rodent models of cerebral ischemia.

M(1)-like muscarinic acetylcholine receptors regulate fast-spiking interneuron excitability in rat dentate gyrus

Cholinergic transmission through muscarinic acetylcholine receptors (mAChRs) plays a key role in cortical oscillations. Although fast-spiking (FS), parvalbumin-expressing basket cells (BCs) are proposed to be the cellular substrates of gamma oscillations, previous studies reported that FS nonpyramidal cells in neocortical areas are unresponsive to cholinergic modulation. Dentate gyrus (DG) is an independent gamma oscillator in the hippocampal formation. However, in contrast to other cortical regions, the direct impact of mAChR activation on FS BC excitability in this area has not been investigated. Here, we show that bath-applied muscarine or carbachol, two mAChR agonists, depolarize DG BCs in the acute brain slices, leading to action potential firing in the theta-gamma bands in the presence of blockers of ionotropic glutamate and gamma-aminobutyric acid type A receptors at physiological temperatures. The depolarizing action persists in the presence of tetrodotoxin, a voltage-gated Na(+) channel blocker. In voltage-clamp recordings, muscarine markedly reduces background K(+) currents. These effects are mimicked by oxotremorine methiodide, an mAChR-specific agonist, and largely reversed by atropine, a non-selective mAChR antagonist, or pirenzepine, an M(1) receptor antagonist, but not by gallamine, an M(2/4) receptor antagonist. Interestingly, in contrast to M(1)-receptor-mediated depolarization, M(2) receptor activation by the specific agonist arecaidine but-2-ynyl ester tosylate down-regulates GABA release at BC axons-the effect is occluded by gallamine, an M(2) receptor antagonist. Overall, muscarinic activation results in a net increase in phasic inhibitory output to the target cells. Thus, cholinergic activation through M(1)-like receptor enhances BC activity and promotes the generation of nested theta and gamma rhythms, thereby enhancing hippocampal function and associated performance.

Two-pore potassium ion channels are inhibited by both G(q/11)- and G(i)-coupled P2Y receptors

Two-pore potassium (K(2P)) ion channels and P2Y receptors modulate the activity of neurones and are targets for the treatment of neuronal disorders. Here we have characterised their interaction. In cells coexpressing the Galpha(i)-coupled hP2Y(12) receptor, ADP and ATP significantly inhibited hK(2P)2.1 currents. This was abolished by pertussis toxin (PTX), the hP2Y(12) antagonist AR-C69931MX, the hP2Y(1) antagonist MRS2179 and by mutating potential PKA/PKC phosphorylation sites in the channel C terminal. In cells coexpressing the Galpha(q/11)-coupled hP2Y(1) receptor, ADP and ATP also inhibited hK(2P)2.1 currents, which were abolished by MRS2179, but unaffected by AR-C69931MX and PTX. When both receptors were coexpressed with K(2P)2.1 channels, ADP-induced inhibition was antagonised by AR-C69913MX and MRS2179, but not PTX. Thus, both Galpha(q/11)- and Galpha(i)-coupled P2Y receptors inhibit K(2P) channels and the action of hP2Y(12) receptors appears to involve co-activation of endogenous hP2Y(1) receptors. This represents a novel mechanism by which P2Y receptors may modulate neuronal activity.

Paradoxical effect of ethanol on potassium channel currents and cell survival in cerebellar granule neurons

Transient exposure to ethanol (EtOH) results in a massive neurodegeneration in the developing brain leading to behavioral and cognitive deficits observed in fetal alcohol syndrome. There is now compelling evidence that K+ channels play an important role in the control of programmed cell death. The aim of the present work was to investigate the involvement of K+ channels in the EtOH-induced cerebellar granule cell death and/or survival. At low and high concentrations, EtOH evoked membrane depolarization and hyperpolarization, respectively. Bath perfusion of EtOH (10 mM) depressed the I(A) (transient K+ current) potassium current whereas EtOH (400 mM) provoked a marked potentiation of the specific I(K) (delayed rectifier K+ current) current. Pipette dialysis with GTPgammaS or GDPbetaS did not modify the effects of EtOH (400 mM) on both membrane potential and I(K) current. In contrast, the reversible depolarization and slowly recovering inhibition of I(A) induced by EtOH (10 mM) became irreversible in the presence of GTPgammaS. EtOH (400 mM) induced prodeath responses whereas EtOH (10 mM) and K+ channel blockers promoted cell survival. Altogether, these results indicate that in cerebellar granule cells, EtOH mediates a dual effect on K+ currents partly involved in the control of granule cell death.

The neuroprotective impact of the leak potassium channel TASK1 on stroke development in mice

Oxygen depletion (O(2)) and a decrease in pH are initial pathophysiological events in stroke development, but secondary mechanisms of ischemic cell death are incompletely understood. By patch-clamp recordings of brain slice preparations we show that TASK1 and TASK3 channels are inhibited by pH-reduction (42+/-2%) and O(2) deprivation (36+/-5%) leading to membrane depolarization, increased input resistance and a switch in action potential generation under ischemic conditions. In vivo TASK blockade by anandamide significantly increased infarct volumes at 24 h in mice undergoing 30 min of transient middle cerebral artery occlusion (tMCAO). Moreover, blockade of TASK channels accelerated stroke development. Supporting these findings TASK1(-/-) mice developed significantly larger infarct volumes after tMCAO accompanied by worse outcome in functional neurological tests compared to wild type mice. In conclusion, our data provide evidence for an important role of functional TASK channels in limiting tissue damage during cerebral ischemia.

PLC-dependent intracellular Ca2+ release was associated with C6-ceramide-induced inhibition of Na+ current in rat granule cells

In this report, the effects of C(6)-ceramide on the voltage-gated inward Na(+) currents (I(Na)), two types of main K(+) current [outward rectifier delayed K(+) current (I(K)) and outward transient K(+) current (I(A))], and cell death in cultured rat cerebellar granule cells were investigated. At concentrations of 0.01-100 microM, ceramide produced a dose-dependent and reversible inhibition of I(Na) without alteration of the steady-state activation and inactivation properties. Treatment with C(2)-ceramide caused a similar inhibitory effect on I(Na). However, dihydro-C(6)-ceramide failed to modulate I(Na). The effect of C(6)-ceramide on I(Na) was abolished by intracellular infusion of the Ca(2+)-chelating agent, 1,2-bis (2-aminophenoxy) ethane-N, N, N9, N9-tetraacetic acid, but was mimicked by application of caffeine. Blocking the release of Ca(2+) from the sarcoplasmic reticulum with ryanodine receptor blocker induced a gradual increase in I(Na) amplitude and eliminated the effect of ceramide on I(Na). In contrast, the blocker of the inositol 1,4,5-trisphosphate-sensitive Ca(2+) receptor did not affect the action of C(6)-ceramide. Intracellular application of GTPgammaS also induced a gradual decrease in I(Na) amplitude, while GDPbetaS eliminated the effect of C(6)-ceramide on I(Na). Furthermore, the C(6)-ceramide effect on I(Na) was abolished after application of the phospholipase C (PLC) blockers and was greatly reduced by the calmodulin inhibitors. Fluorescence staining showed that C(6)-ceramide decreased cell viability and blocking I(Na) by tetrodotoxin did not mimic the effect of C(6)-ceramide, and inhibiting intracellular Ca(2+) release by dantrolene could not decrease the C(6)-ceramide-induced cell death. We therefore suggest that increased PLC-dependent Ca(2+) release through the ryanodine-sensitive Ca(2+) receptor may be responsible for the C(6)-ceramide-induced inhibition of I(Na), which does not seem to be associated with C(6)-ceramide-induced granule neuron death.

Acid-sensitive channel inhibition prevents fetal alcohol spectrum disorders cerebellar Purkinje cell loss

Ethanol is now considered the most common human teratogen. Educational campaigns have not reduced the incidence of ethanol-mediated teratogenesis, leading to a growing interest in the development of therapeutic prevention or mitigation strategies. On the basis of the observation that maternal ethanol consumption reduces maternal and fetal pH, we hypothesized that a pH-sensitive pathway involving the TWIK-related acid-sensitive potassium channels (TASKs) is implicated in ethanol-induced injury to the fetal cerebellum, one of the most sensitive targets of prenatal ethanol exposure. Pregnant ewes were intravenously infused with ethanol (258+/-10 mg/dl peak blood ethanol concentration) or saline in a "3 days/wk binge" pattern throughout the third trimester. Quantitative stereological analysis demonstrated that ethanol resulted in a 45% reduction in the total number of fetal cerebellar Purkinje cells, the cell type most sensitive to developmental ethanol exposure. Extracellular pH manipulation to create the same degree and pattern of pH fall caused by ethanol (manipulations large enough to inhibit TASK 1 channels), resulted in a 24% decrease in Purkinje cell number. We determined immunohistochemically that TASK 1 channels are expressed in Purkinje cells and that the TASK 3 isoform is expressed in granule cells of the ovine fetal cerebellum. Pharmacological blockade of both TASK 1 and TASK 3 channels simultaneous with ethanol effectively prevented any reduction in fetal cerebellar Purkinje cell number. These results demonstrate for the first time functional significance of fetal cerebellar two-pore domain pH-sensitive channels and establishes them as a potential therapeutic target for prevention of ethanol teratogenesis.

Biophysical, pharmacological, and functional characteristics of cloned and native mammalian two-pore domain K+ channels

The mammalian family of two-pore domain K+ (K2P) channel proteins are encoded by 15 KCNK genes and subdivided into six subfamilies on the basis of sequence similarities: TWIK, TREK, TASK, TALK, THIK, and TRESK. K2P channels are expressed in cells throughout the body and have been implicated in diverse cellular functions including maintenance of the resting potential and regulation of excitability, sensory transduction, ion transport, and cell volume regulation, as well as metabolic regulation and apoptosis. In recent years K2P channel isoforms have been identified as important targets of several widely employed drugs, including: general anesthetics, local anesthetics, neuroprotectants, and anti-depressants. An important goal of future studies will be to identify the basis of drug actions and channel isoform selectivity. This goal will be facilitated by characterization of native K2P channel isoforms, their pharmacological properties and tissue-specific expression patterns. To this end the present review examines the biophysical, pharmacological, and functional characteristics of cloned mammalian K2P channels and compares this information with the limited data available for native K2P channels in order to determine criteria which may be useful in identifying ionic currents mediated by native channel isoforms and investigating their pharmacological and functional characteristics.

Delayed rectifier outward K+ current mediates the migration of rat cerebellar granule cells stimulated by melatonin

Melatonin (MT) may work as a neuromodulator through the associated MT receptors in the central nervous system. Previously, our studies have shown that MT increased the I(K) current via a G protein-related pathway. In the present study, patch-clamp whole-cell recording, transwell migration assays and organotypic cerebellar slice cultures were used to examine the effect of MT on granule cell migration. MT increased the I(K) current amplitude and migration of granule cells. Meanwhile, TEA, the I(K) channel blocker, decreased the I(K) current and slowed the migration of granule cells. Furthermore, the effects of MT on the I(K) current and cell migration were not abolished by pre-incubation with P7791, a specific antagonist of MT(3)R, but were eliminated by the application of the MT(2)R antagonists K185 and 4-P-PDOT. I(K) current and cell migration were decreased by the application of dibutyryl cyclic AMP (dbcAMP), which was in contrast to the MT effect on the I(K) current and cell migration. Incubation with dbcAMP essentially blocked the MT-induced increasing effect. Moreover, incubation of isolated cell cultures in the MT-containing medium also decreased the cAMP immunoreactivity in the granule cells. It is concluded, therefore, that I(K) current, downstream of a cAMP transduction pathway, mediates the migration of rat cerebellar granule cells stimulated by MT.

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.

cAMP/protein kinase A signalling pathway protects against neuronal apoptosis and is associated with modulation of Kv2.1 in cerebellar granule cells

Previously, we have reported that apoptosis of cerebellar granular neurons induced by incubation in 5 mm K(+) and serum-free medium (LK-S) was associated with an increase in the delayed rectifier K(+) current (I(K)). Here, we show that I(K) associated with apoptotic neurons is mainly encoded by a Kv2.1 subunit. Silencing Kv2.1 expression by small interfering RNA reduces I(K) and increases neuron viability. Forskolin is able to decrease the I(K) amplitude recording from neurons of both the LK-S and control group, and prevents apoptosis of granule cells that are induced by LK-S. Dibutyryl cAMP mimicks the effect of forskolin on the modulation of I(K) and, accordingly, the inhibitor of protein kinase A, H-89, aborts the neuron-protective effect induced by forskolin. Whereas the expression of Kv2.1 was silenced by Kv2.1 small interfering RNA, the inhibition of forskolin on the current amplitude was significantly reduced. Quantitative RT-PCR and whole-cell recording revealed that the expression of Kv2.1 was elevated in the apoptotic neurons, and forskolin significantly depressed the expression of Kv2.1. We conclude that the protection against apoptosis via the protein kinase A pathway is associated with a double modulation on I(K) channel properties and its expression of alpha-subunit that is mainly encoded by the Kv2.1 gene.

Neuronal two-pore-domain potassium channels and their regulation by G protein-coupled receptors

Leak potassium currents in the nervous system are often carried through two-pore-domain potassium (K2P) channels. These channels are regulated by a number of different G protein-coupled receptor (GPCR) pathways. The TASK subfamily of K2P channels are inhibited following activation of the G protein Galpha(q). The mechanism(s) that transduce this inhibition have yet to be established but there is evidence to support a role of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis products, depletion of PIP2 itself from the membrane, or a direct action of activated Galpha(q) on TASK channels. It seems possible that more than one pathway may act in parallel to transduce inhibition. By contrast, TRESK channels are stimulated following activation of Galpha(q). This is due to stimulation of the protein phosphatase, calcineurin, which dephosphorylates TRESK channels and enhances their activity. TREK channels are the most widely regulated of the K2P channel subfamilies being inhibited following activation of Galpha(q) and Galpha(s) but enhanced following activation of Galpha(i). The multiple pathways activated and the apparent promiscuous coupling of at least some K2P channel types to different G protein regulatory pathways suggests that the excitability of neurons that express K2P channels will be profoundly sensitive to variations in GPCR activity.

The background K+ channel TASK-3 is regulated at both the transcriptional and post-transcriptional levels

The K(+) channel TASK-3 is highly expressed in cerebellar granule neurons where it encodes the K(+) current IKso. Besides the role of TASK-3 in controlling cellular excitability and shaping neuronal responses, it has recently been proposed to contribute to the development and maturation of neurons in the cerebellum. K(+) dependent apoptosis and tumorigenesis have also been attributed to TASK-3 over-expression. Transcription of TASK-3 is strongly dependent on depolarization-induced Ca(2+)-entry. To understand the mechanisms involved in TASK-3 regulation, we have characterized a minimal promoter which specifically expresses in cellular backgrounds expressing endogenous TASK-3. Moreover, we have cloned and characterized the 5' and 3' untranslated regions of TASK-3. Both regions contribute to inhibit expression of a reporter gene. Given the direct consequence of membrane potential on TASK-3 expression, this is an important first step towards the understanding of the complex regulation of this gene.

Membrane Resting Potential of Thalamocortical Relay Neurons Is Shaped by the Interaction Among TASK3 and HCN2 Channels

By combining molecular biological, electrophysiological, immunological, and computer modeling techniques, we here demonstrate a counterbalancing contribution of TASK channels, underlying hyperpolarizing K+ leak currents, and HCN channels, underlying depolarizing Ih, to the resting membrane potential of thalamocortical relay (TC) neurons. RT-PCR experiments revealed the expression of TASK1, TASK3, and HCN1–4. Quantitative determination of mRNA expression levels and immunocytochemical staining demonstrated that TASK3 and HCN2 channels represent the dominant thalamic isoforms and are coexpressed in TC neurons. Extracellular acidification, a standard procedure to inhibit TASK channels, blocked a TASK current masked by additional action on HCN channels. Only in the presence of the HCN blocker ZD7288 was the pH-sensitive component typical for a TASK current, i.e., outward rectification and current reversal at the K+ equilibrium potential. In a similar way extracellular acidification was able to shift the activity pattern of TC neurons from burst to tonic firing only during block of Ih or genetic knock out of HCN channels. A single compartmental computer model of TC neurons simulated the counterbalancing influence of TASK and HCN on the resting membrane potential. It is concluded that TASK3 and HCN2 channels stabilize the membrane potential by a mutual functional interaction, that the most efficient way to regulate the membrane potential of TC neurons is the converse modulation of TASK and HCN channels, and that TC neurons are potentially more resistant to insults accompanied by extracellular pH shifts in comparison to other CNS regions.

Membrane potential-regulated transcription of the resting K+ conductance TASK-3 via the calcineurin pathway

The 2P domain K(+) channel TASK-3 is highly expressed in cerebellar granule neurons where it has been proposed to underlie the K(+) leak conductance, IKso. In a previous work we showed that expression of TASK-3 increases in cerebellar granule neurons as they mature in culture. Here we show that within the cerebellum, levels of TASK-3 mRNA increase as granule neurons migrate to their adult positions and receive excitatory mossy fiber input. To understand the mechanism of this increase in TASK-3 expression we used an in vitro model culturing the neurons in either depolarizing conditions mimicking neuronal activity (25K, 25 mm KCl) or in conditions which approach deafferentation (5K, 5 mm KCl). An important increase in TASK-3 mRNA is uniquely observed in 25K and is specific since other background K(+) channel levels remain unchanged or decrease. The rise in TASK-3 mRNA leads to an increase in TASK-3 protein and the IKso conductance resulting in hyperpolarization. Blocking L-type calcium channels or their downstream effector calcineurin, abrogates TASK-3 expression and IKso, leading to hyperexcitability. This is the first study demonstrating that depolarization-induced Ca(2+) entry can directly regulate cellular excitability by dynamically regulating the transcription of a resting K(+) conductance. The appearance of this conductance may play an important role in the transition of depolarized immature neurons to their mature hyperpolarized state during neuronal development.

Modifying the subunit composition of TASK channels alters the modulation of a leak conductance in cerebellar granule neurons

Two-pore domain potassium (K2P) channel expression is believed to underlie the developmental emergence of a potassium leak conductance [IK(SO)] in cerebellar granule neurons (CGNs), suggesting that K2P function is an important determinant of the input conductance and resting membrane potential. To investigate the role that different K2P channels may play in the regulation of CGN excitability, we generated a mouse lacking TASK-1, a K2P channel known to have high expression levels in CGNs. In situ hybridization and real-time PCR studies in wild-type and TASK-1 knock-outs (KOs) demonstrated that the expression of other K2P channels was unaltered in CGNs. TASK-1 knock-out mice were healthy and bred normally but exhibited compromised motor performance consistent with altered cerebellar function. Whole-cell recordings from adult cerebellar slice preparations revealed that the resting excitability of mature CGNs was no different in TASK-1 KO and littermate controls. However, the modulation of IK(SO) by extracellular Zn2+, ruthenium red, and H+ was altered. The IK(SO) recorded from TASK-1 knock-out CGNs was no longer sensitive to alkalization and was blocked by Zn2+ and ruthenium red. These results suggest that a TASK-1-containing channel population has been replaced by a homodimeric TASK-3 population in the TASK-1 knock-out. These data directly demonstrate that TASK-1 channels contribute to the properties of IK(SO) in adult CGNs. However, TASK channel subunit composition does not alter the resting excitability of CGNs but does influence sensitivity to endogenous modulators such as Zn2+ and H+.

Endogenous Regulator of G-Protein Signaling Proteins Regulate the Kinetics of Gαq/11-Mediated Modulation of Ion Channels in Central Nervous System Neurons

Slow synaptic potentials are generated when metabotropic G-protein-coupled receptors activate heterotrimeric G-proteins, which in turn modulate ion channels. Many neurons generate excitatory postsynaptic potentials mediated by G-proteins of the Galphaq/11 family, which in turn activate phospholipase C-beta. Accessory GTPase-activating proteins (GAPs) are thought to be required to accelerate GTP hydrolysis and rapidly turn off G-proteins, but the involvement of GAPs in neuronal Galphaq/11 signaling has not been examined. Here, we show that regulator of G-protein signaling (RGS) proteins provide necessary GAP activity at neuronal Galphaq/11 subunits. We reconstituted inhibition of native 2-pore domain potassium channels in cerebellar granule neurons by expressing chimeric Galpha subunits that are activated by Galphai/o-coupled receptors, thus bypassing endogenous Galphaq/11 subunits. RGS-insensitive variants of these chimeras mediated inhibition of potassium channels that developed and recovered more slowly than inhibition mediated by RGS-sensitive (wild-type) chimeras or native Galphaq/11 subunits. These changes were not accompanied by a change in agonist sensitivity, as might be expected if RGS proteins acted primarily as effector antagonists. The slowed recovery from potassium channel inhibition was largely reversed by an additional mutation that mimics the RGS-bound state. These results suggest that endogenous RGS proteins regulate the kinetics of rapid Galphaq/11-mediated signals in central nervous system neurons by providing GAP activity.

Is chemical neurotransmission altered specifically during methylmercury-induced cerebellar dysfunction?

Methylmercury (MeHg) is an important environmental neurotoxicant that is present in seafood and affects the developing and mature nervous system. The neurotoxicity induced by MeHg is a concern, particularly for fish-eating populations and pregnant or nursing women. During MeHg-induced neurotoxicity, degeneration of the granule cell layer in the cerebellum occurs, which leads to deficits in motor function. I suggest that the action of MeHg on specific neurotransmitter receptors contributes to the selective vulnerability of granule cells. MeHg appears to stimulate M(3) muscarinic acetylcholine receptors and to inhibit GABA(A) receptor subtypes preferentially on cerebellar granule cells. This could lead to the loss of tonic inhibition of granule cells as a result of antagonism of GABA(A) receptors, and a M(3)-receptor-mediated increase in the intracellular concentration of Ca(2+) and block of a K(+)-dependent leak current. The net result would be increased spontaneous release of glutamate, which, coupled with a MeHg-induced impairment of glutamate uptake by astrocytes, could cause Ca(2+)-mediated cytotoxicity.

Elevation of intracellular Ca2+ modulates A-currents in rat cerebellar granule neurons

In the brain, the transient-inactivating voltage-gated potassium channel currents (called I(K(A)) or A-currents) are activated at subthreshold membrane potentials to control the excitability of neurons. In the current study, the effect of intracellular calcium on the A-current and the action mechanism of intracellular calcium was investigated by using the whole-cell voltage-clamp technique. Elevation of intracellular calcium by addition of 2 mM CaCl2 in the pipette solution significantly modulated both the peak amplitude and the kinetics of the A-current in rat granule neurons. The peak amplitudes of the A-current were 1,060 +/- 87 pA and 1,972 +/- 16 pA under conditions of no Ca2+ and elevated intracellular Ca2+, respectively. The time to peak, the time course of fast inactivation, and the steady-state inactivation property of the A-current were all significantly altered by elevating the intracellular Ca2+. Replacement of the Ca2+ in the pipette solution with the same concentration of Co2+ did not mimic the effects of intracellular Ca2+ on the A-current amplitude and kinetics. These effects are similar to the behavior of the reconstituted Kv4/KChIP (K(V) channel-interacting proteins) current induced by expression of KChIP and Kv4 together in a cell expression system. Application of 10 microM arachidonic acid, which can bind to the Kv4/KChIP complex, inhibited the A-current and eliminated the effects of intracellular Ca2+ on the A-current, suggesting that KChIP may be involved in the effects of Ca2+ on the A-current. Collectively, our results indicate that elevated intracellular Ca2+ modulates the amplitude, fast activation, and steady-state inactivation characteristics of the A-current in rat cerebellar granule neurons, and this may occur via KChIP.

Diclofenac, a nonsteroidal anti-inflammatory drug, activates the transient outward K+ current in rat cerebellar granule cells

Diclofenac, a nonsteroidal anti-inflammatory drug (NSAID), has been widely investigated in terms of its pharmacological action, but less is known about its direct effect on ion channels. Here, the effect of diclofenac on voltage-dependent transient outward K+ currents (I(A)) in cultured rat cerebellar granule cells was investigated using the whole-cell voltage-clamp technique. At concentrations of 10(-5)-10(-3) M, diclofenac reversibly increased the I(A) amplitude in a dose-dependent manner and significantly modulated the steady-state inactivation properties of the I(A) channels, but did not alter the steady-state activation properties. Furthermore, diclofenac treatment resulted in a slightly accelerated recovery from I(A) channel inactivation. Intracellular application of diclofenac could mimic the effects induced by extracellular application, although once the intracellular response reached a plateau, extracellular application of diclofenac could induce further increases in the current. These observations indicate that diclofenac might exert its effects on the channel protein at both the inner and outer sides of the cell membrane. Our data provide the first evidence that diclofenac is able to activate transient outward potassium channels in neurons. Although further work will be necessary to define the exact mechanism of diclofenac-induced I(A) channel activation, this study provides evidence that the nonsteroidal anti-inflammatory drug, diclofenac, may play a novel neuronal role that is worthy of future study.

A TASK-like pH- and amine-sensitive ‘leak’ K+ conductance regulates neonatal rat facial motoneuron excitability in vitro

A 'leak' potassium (K+) conductance (gK(Leak)) modulated by amine neurotransmitters is a major determinant of neonatal rat facial motoneuron excitability. Although the molecular identity of gK(Leak) is unknown, TASK-1 and TASK-3 channel mRNA is found in facial motoneurons. External pH, across the physiological range (pH 6-8), and noradrenaline (NA) modulated a conductance that displayed a relatively linear current/voltage relationship and reversed at the K+ equilibrium potential, consistent with inhibition of gK(Leak). The pH-sensitive current (I(pH)), was maximal around pH 8, fully inhibited near pH 6 and was described by a modified Hill equation with a pK of 7.1. The NA-induced current (I(NA)) was occluded at pH 6 and enhanced at pH 7.7. The TASK-1 selective inhibitor anandamide (10 microM), its stable analogue methanandamide (10 microM), the TASK-3 selective inhibitor ruthenium red (10 microM) and Zn2+ (100-300 microM) all failed to alter facial motoneuron membrane current or block I(NA) or I(pH). Isoflurane, a volatile anaesthetic that enhances heteromeric TASK-1/TASK-3 currents, increased gK(Leak). Ba2+, Cs+ and Rb+ blocked I(NA) and I(pH) voltage-dependently with maximal block at hyperpolarized potentials. 4-Aminopyridine (4-AP, 4 mM) voltage-independently blocked I(NA) and I(pH). In summary, gK(Leak) displays some of the properties of a TASK-like conductance. The linearity of gK(Leak) and an independence of activation on external [K+] suggests against pH-sensitive inwardly rectifying K+ channels. Our results argue against principal contributions to gK(Leak) by homomeric TASK-1 or TASK-3 channels, while the potentiation by isoflurane supports a predominant role for heterodimeric TASK-1/TASK-3 channels.

2-iodomelatonin prevents apoptosis of cerebellar granule neurons via inhibition of A-type transient outward K+ currents

Compelling evidence indicates that excessive K+ efflux and intracellular K+ depletion are key early steps in apoptosis. Previously, we reported that apoptosis of cerebellar granular neurons induced by incubation under low K+ (5 mM) conditions was associated with an increase in delayed rectifier outward K+ current (IK) amplitude and caspase-3 activity. Moreover, the melatonin receptor antagonist 4P-PDOT abrogated the effects of 2-iodomelatonin on IK augmentation, caspase-3 activity and apoptosis. Here, we show that incubation under low K+/serum-free conditions for 6 hr led to a dramatic increase in the A-type transient outward K+ current (IA) (a 27% increase; n=31); in addition, fluorescence staining showed that under these conditions, cell viability decreased by 30% compared with the control. Treatment with 2-iodomelatonin inhibited the IA amplitude recorded from control and apoptotic cells in a concentration-dependent manner and modified the IA channel activation kinetics of cells under control conditions. Moreover, 2-iodomelatonin increased the viability of cell undergoing apoptosis. Interestingly, 4P-PDOT did not abrogate the effect of 2-iodomelatonin on IA augmentation under these conditions; in the presence of 4P-PDOT (100 microm), 2-iodomelatonin reduced the average IA by 41+/-4%, which was similar to the effect of 2-iodomelatonin alone. These results suggest that the neuroprotective effects of 2-idomelatonin are not only because of its antioxidant or receptor-activating properties, but rather that 2-iodomelatonin may inhibit IA channels by acting as a channel blocker.

Endogenous RGS proteins regulate presynaptic and postsynaptic function: functional expression of RGS-insensitive Galpha subunits in central nervous system neurons

Regulators of G-protein signaling (RGS)-insensitive (RGSi) G-protein alpha subunits can be used to indirectly determine the function of endogenous RGS proteins in native cells. This article describes the application of RGSi Galpha subunits to the study of endogenous RGS function in central nervous system (CNS) neurons. Presynaptic inhibition of neurotransmitter release was reconstituted in primary neurons using RGSi Galpha(i/o) subunits, whereas postsynaptic regulation of potassium channels was reconstituted using RGSi chimeras of Galpha(q) and Galpha(i). These studies have shown that endogenous RGS proteins are essential for the rapid termination of some G-protein-mediated signals in CNS neurons, whereas these proteins are much less important for the regulation of other signals. Together, these techniques have helped reveal the complexity of RGS regulation of CNS function.

Selective block of the human 2-P domain potassium channel, TASK-3, and the native leak potassium current, IKSO, by zinc

Background potassium channels control the resting membrane potential of neurones and regulate their excitability. Two-pore-domain potassium (2-PK) channels have been shown to underlie a number of such neuronal background currents. Currents through human TASK-1, TASK-2 and TASK-3 channels expressed in Xenopus oocytes were inhibited by extracellular acidification. For TASK-3, mutation of histidine 98 to aspartate or alanine considerably reduced this effect of pH. Zinc was found to be a selective blocker of TASK-3 with virtually no effect on TASK-1 or TASK-2. Zinc had an IC(50) of 19.8 microM for TASK-3, at +80 mV, with little voltage dependence associated with this inhibition. TASK-3 H98A had a much reduced sensitivity to zinc suggesting this site is important for zinc block. Surprisingly, TASK-1 also has histidine in position 98 but is insensitive to zinc block. TASK-3 and TASK-1 differ at position 70 with glutamate for TASK-3 and lysine for TASK-1. TASK-3 E70K also had a much reduced sensitivity to zinc while the corresponding reverse mutation in TASK-1, K70E, induced zinc sensitivity. A TASK-3-TASK-1 concatamer channel was comparatively zinc insensitive. For TASK-3, it is concluded that positions E70 and H98 are both critical for zinc block. The native cerebellar granule neurone (CGN) leak current, IK(SO), is sensitive to block by zinc, with current reduced to 0.58 of control values in the presence of 100 microM zinc. This suggests that TASK-3 channels underlie a major component of IK(SO). It has recently been suggested that zinc is released from inhibitory synapses onto CGNs. Therefore it is possible that inhibition of IK(SO) in cerebellar granule cells by synaptically released zinc may have important physiological consequences.

Identification of native rat cerebellar granule cell currents due to background K channel KCNK5 (TASK-2)

The TWIK-related, Acid Sensing K (TASK-2; KCNK5) potassium channel is a member of the tandem pore (2P) family of potassium channels and mediates an alkaline pH-activated, acid pH-inhibited, outward-rectified potassium conductance. In previous work, we demonstrated TASK-2 protein expression in newborn rat cerebellar granule neurons (CGNs). In this study, we demonstrate TASK-2 functional expression in CGNs as a component of the pH-sensitive, volatile anesthetic-potentiated, standing-outward potassium conductance (I(K,SO)). Using excised, inside-out patch-clamp technique, we studied CGNs grown in primary culture. We identified four distinct, noninactivating single channel potassium conductances, Types 1-4. Types 1-3 have previously been attributed to TASK-1 (KCNK3), TASK-3 (KCNK9) and TASK-1/TASK-3 heteromers, and TREK-2 (KCNK10) 2P potassium channel function, respectively; however, the Type 4 conductance is currently unassigned. Previous studies demonstrated that Type 4 single channel activity is potentiated by extracellular, alkaline pH and cytoplasmic arachidonic acid (10-20 microM) and inhibited by cytoplasmic tetraethylammonium (TEA; 1 mM). We determined that heterologously expressed TASK-2 channels have single channel gating, conductance properties and pH sensitivity identical to the Type 4 conductance. Additionally, we found that TASK-2 single channel activity, like the Type 4 conductance is potentiated by cytoplasmic arachidonic acid (20 microM) and inhibited by cytoplasmic TEA (1 mM). We conclude that TASK-2 mediates the Type 4 single channel conductance in CGNs as a component of I(K,SO).

Single-channel properties and pH sensitivity of two-pore domain K+ channels of the TALK family

The two-pore K2P channel family comprises TASK, TREK, TWIK, TRESK, TALK, and THIK subfamilies, and TALK-1, TALK-2, and TASK-2 are functional members of the TALK subfamily. Here we report for the first time the single-channel properties of TALK-2 and its pHo sensitivity, and compare them to those of TALK-1 and TASK-2. In transfected COS-7 cells, the three TALK K2P channels could be identified easily by their differences in single-channel conductance and gating kinetics. The single-channel conductances of TALK-1, TALK-2, and TASK-2 in symmetrical 150 mM KCl were 21, 33, and 70 pS (-60 mV), respectively. TALK-2 was sensitive mainly to the alkaline range (pH 7-10), whereas TALK-1 and TASK-2 were sensitive to a wider pHo range (6-10). The effect of pH changes was mainly on the opening frequency. Thus, members of the TALK family expressed in native tissues may be identified based on their single-channel kinetics and pHo sensitivity.

Contribution of TWIK-related acid-sensitive K+ channel 1 (TASK1) and TASK3 channels to the control of activity modes in thalamocortical neurons

The thalamocortical network is characterized by rhythmic burst activity during natural sleep and tonic single-spike activity during wakefulness. The change between these two activity modes is partially governed by transmitters acting on leak K+ currents in the thalamus, although the nature of the constituting ion channels is not yet known. In the present study, the contribution of members of the two-pore domain K+ channel family to the leak current was investigated using whole-cell patch-clamp techniques and molecular biological techniques. RT-PCR and in situ hybridization revealed the expression of TWIK-related acid-sensitive K+ channel 1 (TASK 1) and TASK3 channels in the rat dLGN. Voltage-clamp recordings of thalamocortical relay neurons in slice preparations demonstrated the existence of a current component sensitive to the TASK channel blocker bupivacaine, which reversed at the presumed K+ equilibrium potential, showed outward rectification, and contributed approximately 40% to the standing outward current at depolarized values of the membrane potential (-28 mV). The pharmacological profile was indicative of TASK channels, in that the current was sensitive to changes in extracellular pH, reduced by muscarine and increased by halothane, and these effects were occluded by a near-maximal action of bupivacaine. Pharmacological manipulation of this current under current-clamp conditions resulted in a shift between burst and tonic firing modes. It is concluded that TASK1 and TASK3 channels contribute to the muscarine- and halothane-sensitive conductance in thalamocortical relay neurons, thereby contributing to the change in the activity mode of thalamocortical networks observed during the sleep-wake cycle and on application of inhalational anesthetics.

K+-dependent cerebellar granule neuron apoptosis. Role of task leak K+ channels

Rat mature cerebellar granule, unlike hippocampal neurons, die by apoptosis when cultured in a medium containing a physiological concentration of K+ but survive under high external K+ concentrations. Cell death in physiological K+ parallels the developmental expression of the TASK-1 and TASK-3 subunits that encode the pH-sensitive standing outward K+ current IKso. Genetic transfer of the TASK subunits in hippocampal neurons, lacking IKso, induces cell death, while their genetic inactivation protects cerebellar granule neurons. Neuronal death of cultured rat granule neurons is also prevented by conditions that specifically reduce K+ efflux through the TASK-3 channels such as extracellular acidosis and ruthenium red. TASK leak K+ channels thus play an important role in K+-dependent apoptosis of cerebellar granule neurons in culture.

What are the roles of the many different types of potassium channel expressed in cerebellar granule cells?

Potassium (K) channels have a key role in the regulation of neuronal excitability. Over a hundred different subunits encoding distinct K channel subtypes have been identified so far. A major challenge is to relate these many different channel subunits to the functional K currents observed in native neurons. In this review, we have concentrated on cerebellar granule neurons (CGNs). We have considered each of the three principal super families of K channels in turn, namely, the six transmembrane domain, voltage-gated super family, the two transmembrane domain, inward-rectifier super family and the four transmembrane domain, leak channel super family. For each super family, we have identified the subunits that are expressed in CGNs and related the properties of these expressed channel subunits to the functional currents seen in electrophysiological recordings from these neurons. In some cases, there are strong molecular candidates for proteins underlying observed currents. In other cases the correlation is less clear. We show that at least 26 potassium channel alpha subunits are moderately or strongly expressed in CGNs. Nevertheless, a good empirical model of CGN function has been obtained with just six distinct K conductances. The transient KA current in CGNs, seems due to expression of Kv4.2 channels or Kv4.2/4.3 heteromers, while the KCa current is due to expression of large-conductance slo channels. The G-protein activated KIR current is probably due to heteromeric expression of KIR3.1 and KIR3.2. Perhaps KIR2.2 subunits underlie the KIR current when it is constitutively active. The leak conductance can be attributed to TASK-1 and or TASK-3 channels. With less certainty, the IK-slow current may be due to expression of one or more members of the KCNQ or EAG family. Lastly, the delayed-rectifier Kv current has as many as six different potential contributors from the extensive Kv family of alpha subunits. Since many of these subunits are highly regulated by neurotransmitters, physiological regulators and, often, auxiliary subunits, the resulting electrical properties of CGNs may be highly dynamic and subject to constant fine-tuning.

Muscarine-induced increase in frequency of spontaneous EPSCs in Purkinje cells in the vestibulo-cerebellum of the rat

Cholinergic projections are relatively sparse in the cerebellum compared with other parts of the brain. However, some mossy fibers in the vestibulo-cerebellum are known to be cholinergic. To clarify the functional roles of cholinergic mossy fibers in the vestibulo-cerebellum, we investigated the effects of acetylcholine (ACh) on the membrane electrical properties of both granule cells and Purkinje cells in slices of the cerebellar vermis of the rat using whole-cell patch-clamp techniques. The bath application of ACh induced a marked increase in the frequency of spontaneous EPSCs (sEPSCs) in Purkinje cells specifically in the vestibulo-cerebellum. This effect of ACh was mimicked by muscarine but not by nicotine. It was abolished by application of either tetrodotoxin or the antagonist of AMPA receptors, indicating that the ACh-induced enhancement of sEPSCs occurred indirectly via the activation of neurons sending glutamatergic projections to Purkinje cells. In approximately 15% of granule cells tested in the vestibulo-cerebellum, muscarine elicited membrane depolarization accompanied by a decrease in membrane conductance and increased the neuronal excitability. The muscarine-induced depolarization of granule cells in the vestibulo-cerebellum was attributable to the inhibition of standing-outward K+ currents (IKSO) most likely via the activation of muscarinic M3 receptors. Taken together, these results indicate that ACh increases the firing frequency of granule cells by inhibiting IKSO, which in turn increases the frequency of sEPSCs in Purkinje cells in the rat vestibulo-cerebellum.

Properties and modulation of the G protein-coupled K+ channel in rat cerebellar granule neurons: ATP versus phosphatidylinositol 4,5-bisphosphate

Cerebellar granule (CG) neurons express a G protein-gated K+ current (GIRK) that is involved in the neurotransmitter regulation of the excitatory input to the Purkinje fibres of the cerebellum. Here, we characterized the single-channel behaviour of GIRK in CG neurons, and examined the effects of several known modulators of GIRK and their putative physiological roles. Whole-cell GIRKs were activated by baclofen, a GABAB receptor agonist. In cell-attached patches, baclofen activated GIRK with a single-channel conductance of 34 pS and a mean open time of 0.5 ms. In inside-out patches, application of GTPgammaS to the cytoplasmic side activated GIRK with similar kinetic properties. Addition of 2 mM ATP resulted in a marked increase in GIRK activity and induced longer-lived openings with a mean open time of 2.3 ms (ATP-dependent gating). Brain cytosolic fraction or free fatty acids inhibited this effect of ATP, and this was reversed by addition of purified recombinant brain fatty acid binding protein. Applying phosphatidylinositol 4,5-bisphosphate (PIP2) to inside-out patches in place of ATP also increased GIRK activity; however, only an increase in the frequency of opening was observed. The stimulatory effect of PIP2 on GIRK activity was not inhibited by the cytosolic fraction. Following maximal activation by PIP2, ATP caused an additional 2.2-fold increase in GIRK activity. These results show that GIRKs in CG neurons are regulated by positive and negative modulators that affect frequency as well as open time duration. The net effect is that the ligand-activated GIRK is in the 'low activity' state associated with short-lived openings, mainly due to strong action of the cytosolic inhibitor of ATP-dependent gating. Our results also show that intracellular ATP modulates GIRK via pathways different from that of PIP2 in CG neurons.

Contribution of TWIK-related acid-sensitive K+ channel 1 (TASK1) and TASK3 channels to the control of activity modes in thalamocortical neurons

The thalamocortical network is characterized by rhythmic burst activity during natural sleep and tonic single-spike activity during wakefulness. The change between these two activity modes is partially governed by transmitters acting on leak K+ currents in the thalamus, although the nature of the constituting ion channels is not yet known. In the present study, the contribution of members of the two-pore domain K+ channel family to the leak current was investigated using whole-cell patch-clamp techniques and molecular biological techniques. RT-PCR and in situ hybridization revealed the expression of TWIK-related acid-sensitive K+ channel 1 (TASK 1) and TASK3 channels in the rat dLGN. Voltage-clamp recordings of thalamocortical relay neurons in slice preparations demonstrated the existence of a current component sensitive to the TASK channel blocker bupivacaine, which reversed at the presumed K+ equilibrium potential, showed outward rectification, and contributed approximately 40% to the standing outward current at depolarized values of the membrane potential (-28 mV). The pharmacological profile was indicative of TASK channels, in that the current was sensitive to changes in extracellular pH, reduced by muscarine and increased by halothane, and these effects were occluded by a near-maximal action of bupivacaine. Pharmacological manipulation of this current under current-clamp conditions resulted in a shift between burst and tonic firing modes. It is concluded that TASK1 and TASK3 channels contribute to the muscarine- and halothane-sensitive conductance in thalamocortical relay neurons, thereby contributing to the change in the activity mode of thalamocortical networks observed during the sleep-wake cycle and on application of inhalational anesthetics.