- Email: [email protected]
Properties and modulation of mammalian 2P domain K+ channels
Review
8 Lennie, P. (1981) The physiological basis of variations in visual latency. Vis. Res. 21, 815–824 9 Maunsell, J.H.R. and Gibson, J.R. (1992) Visual response latencies in striate cortex of the macaque monkey. J. Neurophysiol. 68, 1332–1344 10 Purushothaman, G. et al. (1998) Moving ahead through differential visual latency. Nature 396, 424 11 Whitney, D. et al. (2000) Illusory spatial offset of a flash relative to a moving stimulus is caused by differential latencies for moving and flashed stimuli. Vis. Res. 40, 137–149 12 Eagleman, D.M. and Sejnowski, T.J. (2000) Motion integration and postdiction in visual awareness. Science 287, 2036–2038 13 Sheth, B.R. (2000) Changing objects lead briefly flashed ones. Nat. Neurosci. 3, 489–495 14 Baldo, M.V.C. and Klein, S.A. (1995) Extrapolation or attention shift? Nature 378, 565–566 15 Brenner, E. and Smeets, J.B.J. (2000) Motion extrapolation is not responsible for the flash-lag effect. Vis. Res. 40, 1645–1648 16 Bachmann, T. (1999) Twelve spatiotemporal phenomena and one explanation. In Cognitive Contributions to the Perception of Spatial and Temporal Events (Aschersleben, G. et al., eds), pp. 173–206, Elsevier Science 17 Khurana, B. et al. (2000) The role of attention in motion extrapolation: are moving objects
TRENDS in Neurosciences Vol.24 No.6 June 2001
18
19 20 21 22 23 24 25
26
27
‘corrected’ or flashed objects attentionally delayed? Perception 29, 675–692 Krekelberg, B. and Lappe, M. (2000) A model of the perceived relative positions of moving objects based upon a slow averaging process. Vis. Res. 40, 201–215 Krekelberg, B. (2001) The persistence of position. Vis. Res. 41, 529–539 Eagleman, D.M. and Sejnowski, T.J. (2000) Reply to Patel et al. Science 290, 1051 Krekelberg, B. and Lappe, M. (2000) The position of moving objects. Science 289, 1107 Whitney, D. and Cavanagh, P. (2000) Reply to Eagleman and Sejnowski. Science 289, 1107 Eagleman, D.M. and Sejnowski, T.J. (2000) Reply to Krekelberg et al. Science 289, 1107 Patel, S.S. et al. (2000) Flash-lag effect: differential latency, not postdiction. Science 290, 1051 Bishop, P.O. et al. (1971) Responses to visual contours: spatio-temporal aspects of excitation in the receptive fields of simple striate neurones. J. Physiol. 219, 625–657 Orban, G.A. et al. (1985) Velocity selectivity in the cat visual system. I. Responses of LGN cells to moving bar stimuli: a comparison with cortical areas 17 and 18. J. Neurophysiol. 54, 1026–1049 Raiguel, S. et al. (1989) Response latencies of visual cells in macaque area V1; V2 and V5. Brain Res. 493, 155–159
339
28 Berry, M.J. et al. (1999) Anticipation of moving stimuli by the retina. Nature 398, 334–338 29 Jancke, D. et al. (1999) Parametric population representation of retinal location: neuronal interaction dynamics in cat primary visual cortex. J. Neurosci. 19, 9016–9028 30 Spillmann, L. and Werner., J.S. (1996) Longrange interactions in visual perception. Trends Neurosci. 19, 428–434 31 Kirschfeld, K. and Kammer, T. (1999) The Fröhlich effect: a consequence of the interaction of visual focal attention and metacontrast. Vis. Res. 39, 3702–3709 32 Bringuier, V. et al. (1999) Horizontal propagation of visual activity in the synaptic integration field of area 17 neurons. Science 283, 695–699 33 Schlag, J. et al. (2000) Extrapolating movement without retinal motion. Nature 403, 38–39 34 Cai, R.H. et al. (2000) Vestibular signals can distort the perceived spatial relationship of retinal stimuli. Exp. Brain Res. 135, 275–278 35 Mateeff, S. and Hohnsbein, J. (1988) Perceptual latencies are shorter for motion towards the fovea than for motion away. Vis. Res. 28, 711–719 36 van Beers, R. et al. (2001) Sensorimotor integration compensates for visual localization errors during smooth pursuit eye movements. J. Neurophysiol. (in press)
Properties and modulation of mammalian 2P domain K+ channels Amanda J. Patel and Eric Honoré Mammalian 2P domain K+ channels are responsible for background or ‘leak’ K+ currents. These channels are regulated by various physical and chemical stimuli, including membrane stretch, temperature, acidosis, lipids and inhalational anaesthetics. Furthermore, channel activity is tightly controlled by membrane receptor stimulation and second messenger phosphorylation pathways. Several members of this novel family of K+ channels are highly expressed in the central and peripheral nervous systems in which they are proposed to play an important physiological role. The pharmacological modulation of this novel class of ion channels could be of interest for both general anaesthesia and ischaemic neuroprotection.
Amanda J. Patel Eric Honoré* Institut de Pharmacologie Moléculaire et Cellulaire, CNRS-UMR6097, 660 route des Lucioles, Sophia Antipolis, 06560 Valbonne, France. *e-mail: honore@ ipmc.cnrs.fr
Leak or background K+ selective channels – defined by a lack of voltage- and time-dependency, and with a linear current to voltage relationship in a symmetrical K+ gradient – play an essential role in setting the resting membrane potential, tuning the action potential duration and modulating the responsiveness to synaptic inputs. Regulation of background K+ channels by neurotransmitters and second messengers is central for synaptic function1,2. The most extensively studied native background K+ channel is the S channel in the marine snail Aplysia sensory neurones1. Closing of the S-type background
K+ channel by 5-HT receptor activation is involved in presynaptic sensitization, a simple form of learning2. Additionally, neuronal background K+ channels are the targets of an important class of pharmacological agents, the volatile general anaesthetics3–7. Mammalian K+ channel subunits (~80 genes) can be divided into three main structural classes comprising two transmembrane segments (TMS), four-TMS or six-TMS (Ref. 8). The common feature of all K+ channels is the presence of a conserved motif called the P domain, which is part of the K+ conduction pathway9. The two-TMS channels comprise a single P domain and encode the inward rectifiers. These K+ channels, which operate at negative membrane potentials, contribute to the setting of the resting membrane potential. The sixTMS channels, including the voltage-gated and the Ca2+-activated K+ channels, similarly comprise a single P domain. These channels, which are usually activated at depolarized membrane potentials, mostly contribute to the repolarization of the action potential. By contrast, the most recently discovered class of four-TMS subunits is characterized by the
http://tins.trends.com 0166-2236/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0166-2236(00)01810-5
Review
340
TRENDS in Neurosciences Vol.24 No.6 June 2001
(a)
(b)
TASK-1
Loop
TASK-3 THIK-1
KT 3-3 P1
P2 TWIK-2
THIK-2
TWIK-1 1
2
3
TRAAK
4
TREK-1
KCNK7
TREK-2
TALK-2
NH2
COOH
(c)
TALK-1
Cerebellum
Brain h
m
TWIK-1 TWIK-2 KCNK7 TREK-1 TREK-2 TRAAK TASK-1 TASK-3 TASK-2 THIK-1 THIK-2
KCNK1 KCNK6 KCNK7 KCNK2 KCNK10 KCNK4 KCNK3 KCNK9 KCNK5 KCNK13 KCNK12
+ – + + + + + – –
+ – – +
TALK-1 TALK-2
KCNK16 KCNK17
– –
r –
+ + +
+ +
– + +
h
m
+ – + – + – + + –
+ – – +
Spinal cord r –
+ + +
0.1
TASK-2
+ +
–
h + + + + + – + – +
m
r
– + + +
+
– +
TRENDS in Neurosciences
Fig. 1. Neuronal expression of mammalian 2P domain K+ channels. (a) Topology of a 2P domain K+ channel. (b) Dendrogram of human 2P domain K+ channels established with ClustalW and Treeview. KT3.3 accession number is NM 022358. (c) Tissue specific expression of the 2P domain K+ channels in human, mouse and rat as determined by northern blot, RT-PCR, in situ hybridization and immunohistochemistry. Expression is indicated by a plus sign; lack of or low, expression is indicated by a minus sign. It is evident from these results that important inter-species differences exist between the pattern of expression of the 2P domain K+ channels. TASK-1, TREK-1 and TRAAK are localized at both synaptic and nonsynaptic sites41,51,62,63.
presence of a tandem of P domains10–12. The leak activity of the 2P domain K+ channels suggests that they will influence both the resting membrane potential and the action potential duration. Functional K+ channels are tetramers of pore-forming subunits for the two- and six-TMS classes and are likely to be dimers for the four-TMS class13. Although the 2P domain K+ channel subunits display the same structural motifs with four-TMS – an extended M1P1 extracellular loop and both aminoand carboxy-termini in the intracellular domains – they share moderate sequence homology outside their P regions (Fig. 1a). The 14 human 2P domain K+ channels identified so far have been classified into five structural subgroups: (1) TWIK-1, TWIK-2 and KCNK7 (KCNK7 is not functional)10,14–18; (2) TASK-1, TASK-3 and KT 3-3 (the functional expression of KT 33 has not yet been reported)19–24 ; (3) TREK-1, TREK-2 and TRAAK (Refs 25–31); (4) TASK-2, TALK-1 and TALK-2 (also called TASK-4) (Refs 32–35); (5) THIK-1 http://tins.trends.com
and THIK-2 (THIK-2 is not functional)36 (Fig. 1b). With the exception of TWIK-2, TASK-2, TASK-3, TALK-1 and TALK-2, the 2P domain K+ channels are highly expressed in the human brain (Fig. 1c)14,32,35,37. No evidence for heteromultimerization has been reported for the 2P domain K+ channels. The number of studies concerning native background K+ channel currents in mammalian cells is rather limited. This is as a result of their leak characteristics, the modest amplitude of these currents, in addition to the lack of specific pharmacology. 2P domain subunits encode background K+ channels which are insensitive to most classical K+ channel blockers including tetraethylammonium (TEA) and 4aminopyridine (4-AP) (Table 1)10,14–17,19–33,36. Based on these criteria and considering their overlapping expression, it is still a delicate issue to establish a close relationship between cloned and native background K+ channels. However, recent studies with recombinant channels reveal that their activity is modulated by an unusual variety of physical and chemical stimuli. Consequently, it is now possible to extrapolate to native tissues and try to evaluate their physiological role. In the future transgenic and knockout animals will be extremely valuable to confirm these findings. This review focuses on the specific functional and regulation properties of the cloned mammalian 2P domain K+ channels expressed in the central and peripheral nervous systems. TREK-1, TREK-2 and TRAAK are mechano-gated K+ channels
TREK-1, TREK-2 and TRAAK channel activity is elicited by increasing the mechanical pressure applied to the cell membrane and is independent of intracellular Ca2+ (Refs 25,28,31,38–40; Fig. 2). In the inside–out patch configuration, positive pressure is significantly less effective compared with negative pressure in opening channels suggesting that a specific membrane deformation (convex curving) preferentially opens these channels38,40. At the wholecell level, TREK-1 and TRAAK are modulated by cellular volume, for example, hyperosmolarity closes the channels38,41. Both the number of active channels and the sensitivity to mechanical stretch are strongly enhanced after treating the cell-attached patches with the cytoskeleton disrupting agents, colchicine and cytochalasin D (Ref. 40). These data suggest that mechanical force might be transmitted directly to the channel via the lipid bilayer and does not require the integrity of the cytoskeleton38,40. Both TREK-1 and TRAAK are blocked by amiloride and Gd3+, blockers of stretch-sensitive ion channels40,42. TREK-1 behaves as a cold sensor in thermo-sensitive neurones
The application of heat gradually and reversibly opens TREK-1 with a sevenfold increase in current amplitude corresponding to a temperature jump of 10°C (Q10) (Ref. 41). The high expression levels of
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
341
Table 1. Biophysical and pharmacological properties of the mammalian 2P domain Kⴙ channelsa Conductance 155 mM Kⴙ
Rectification 155 mM Kⴙ
Activation kinetics
Inactivation kinetics
Ba2ⴙ IC50
Refs
TWIK-1 human
34 pS
Inward
None
None
0.1 mM
10
TWIK-2 human rat
⬍5 pS
Inward Inward
None None
None Inactivating
0.1 mM
16 14
101 pS
Outward Outward
None None
None None
Resistant Resistant
29,30 26,38
100 pS 110 pS
Outward Linear
Delayed None
None None
Resistant ⬎ 2 mM
28 25
Outward Linear
None
None
Resistant 1 mM
31 27
None None None, delayed
None None None
0.35 mM
16 pS
Linear Linear Linear
19 22,51 23,64
27 pS 27 pS
Outward Linear Outward
None None Delayed
None None None
3 mM 0.29 mM
24 20 21
60 pS
Channel
TREK-1 human mouse
Conductance 5 mM Kⴙ
48 pS
TREK-2 human rat TRAAK human mouse TASK-1 human rat mouse
45 pS
40 pS
TASK-3 human rat guinea pig TASK-2 human
Outward
Delayed
None
Resistant
32,33
THIK-1 rat
14 pS
Inward
Delayed
None
1 mM
36
TALK-1 human
Linear
None
None
1 mM
35
TALK-2 human
Outward
None
None
1 mM
34,35
single channel conductance, measured in the linear range of the I–Vs, was determined in both physiological and symmetrical K⫹ gradients. The direction of the whole cell rectification (inward, linear or outward) was determined in a symmetrical K⫹ gradient. The kinetic of activation is instantaneous, delayed or occurring in two phases64. Currents are either time-independent or inactivating. The Ba2⫹ sensitivity was determined in a physiological K⫹ gradient. The 2P domain K⫹ channels are resistant to millimolar concentrations of TEA and 4-AP. aThe
TREK-1 in the small and medium-size diameter dorsal root ganglion (DRG) sensory neurones as well as in the preoptic and anterior hypothalamus makes this background K+ channel an attractive candidate for a temperature sensor41 (Box 1; Fig. 2a,b). At the protein level TREK-1 is present at both synaptic and nonsynaptic sites41. Thermal activation of TREK-1 requires cell integrity suggesting that a cytosolic factor might be involved in channel regulation41. Activation of TREK-1 by temperature is specific because TASK-1 and THIK-1 only display a Q10 of 2 and 1.6, respectively36,41. By contrast, TWIK2, a weak inwardly rectifying background K+ channel, inactivates with increasing temperature14. At the CNS level, cold-sensitive neurones in the anterior and preoptic hypothalamus, which control body heat loss, heat retention and heat production, respond to cooling by enhanced action potential firing43. Prostaglandin E2 (PGE2) and its second messenger cAMP are proposed to be prime candidates in the genesis of fever44–47. TREK-1 is strongly inhibited by PGE2 receptor activation via the protein kinase A (PKA) phosphorylation pathway (see below)41. http://tins.trends.com
Inhibition of TREK-1 by either cold temperature or PGE2 stimulation might contribute to trigger the heat production and retention programme, leading to correction of the hypothermia or production of fever. Halothane and other inhalational anaesthetics impair normal thermoregulatory processes48,49. Interestingly, TREK-1 is directly opened by volatile general anaesthetics (see below)29. These pharmacological observations further suggest an important role of TREK-1 in central thermo-regulative processes41. Nevertheless, the functional role of a background K+ channel current in hypothalamic cold-sensitive neurones remains to be confirmed. Intracellular and extracellular acidosis differentially modulate 2P domain K+ channels
Lowering intracellular pH shifts the pressureactivation relationship of TREK-1 and TREK-2, but not of TRAAK, toward positive values and ultimately leads to channel opening at atmospheric pressure39 (Table 2). Essentially, acidosis converts a TREK mechano-gated channel into a constitutively active channel28,39. Deletional analysis demonstrates that
342
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
(a)
(c)
F
F
C
C
F
Br
COOH F Arachidonic acid
Cl
Halothane
P
Heat
LPC
(b) Stretch
P NH2
P
COOH
COOH
pHi
PKA and PKC TRENDS in Neurosciences
Fig. 2. TREK-1 is a polymodal background K+ channel. (a) TREK-1 mRNA expression pattern in the mouse brain (parasagittal section). (b) Immunolocalization of mouse TREK-1 in a dorsal root ganglion (DRG) with a polyclonal antibody. A higher magnification (inset) shows that TREK-1 is highly expressed in small and medium-size DRG neurones. (c) Polymodal activation of TREK-1 by membrane stretch, long chain anionic polyunsaturated fatty acids including arachidonic acid (AA), neutral lysophospholipids including LPC, volatile general anaesthetics including chloroform, warm temperature and finally intracellular acidosis. TREK-1 shares the functional properties of the Aplysia S and Lymnea KAn background K+ channels1,3. TREK-1 is inhibited by protein kinase A (PKA) and protein kinase C (PKC)mediated phosphorylation (red arrow). Adapted and reproduced, with permission, from Ref. 41.
the carboxy terminus, but not the amino terminus and the extracellular M1P1 loop, is crucial for activation of TREK-1 by stretch and intracellular acidosis39. By contrast, TWIK-1 and TWIK-2 channel activities are downregulated by intracellular acidosis induced by CO2 or dinitrophenol10,16. However, this Box 1. Cold transduction and background K+ channels Peripheral sensory cold fibres discharge slowly at the resting skin temperature but evoke strong action potential discharge following a decrease in temperaturea,b. Recent evidence supports the role of a background K+ conductance in cold transduction of rat primary sensory neuronesc. Cooling from 32 to 20°C induces a large depolarization (>8 mV), increases input resistance and triggers action potential firing in 25% of capsaicin-resistant DRG neurones (L1–S1) (Ref. c). In cold-sensitive neurones, cooling reversibly inhibits a time-independent TREK-1-like tetraethylammonium (TEA) and 4-AP resistant background K+ current (called Icold) (Ref. c). References a Darian-Smith, I. (1984) Thermal sensibility. In Handbook of Physiology. The Nervous System III, pp. 879–913 b Spray, D.C. (1986) Cutaneous temperature receptors. Annu. Rev. Physiol. 48, 625–638 c Reid, G. and Flonta, M. (2001) Cold transduction by inhibition of a background potassium conductance in rat primary sensory neurons. Neurosci. Lett. 297, 171–174
http://tins.trends.com
effect is indirect because intracellular acidosis fails to affect TWIK-1 after excision of the patches10. TASK-1, TASK-2 and TASK-3 are sensitive to variations of extracellular pH in the physiological range (Table 2 and Fig. 3c)19–24,32,33. 50% of TASK-1, TASK-2 and TASK-3 channels are closed at pHs of 7.3, 7.8 and 6.6, respectively. A histidine residue at position 98 located near the GYG sequence of P1 confers the extracellular pH sensitivity of TASK-3 (Refs 20,21). This crucial histidine is conserved in the P1 sequence of TASK-1, but is absent in TASK-2 (Refs 19,21,32). Endogenous, acid-sensitive TASK-1-like K+ currents have been described in cerebellar granule neurones, rat somatic motoneurones and locus coerulus neurones7,50–52. Additionally, an oxygen- and acid-sensitive TASK-1-like background K+ current was recently described in the chemo-sensitive type I carotid body cells53 (Fig. 3). The primary sensory cells of the carotid body respond to acidosis and hypoxia with a depolarization initiating electrical activity and neurosecretion, thus triggering the reflex increase in respiration54,55. 2P domain K+ channels are regulated by cellular lipids
TREK-1, TREK-2 and TRAAK are reversibly opened by polyunsaturated fatty acids (PUFA) including arachidonic acid (AA)25,27,28,38 (Fig. 2c; Table 2). Although the activation is independent of AA metabolism27,38. The extent of saturation of the PUFA, the length of the carbonyl chain and charge of the molecule are crucial because saturated fatty acids (FA), a short carbonyl chain and neutral lipids are ineffective27,38. Direct activation of TREK and TRAAK channels by PUFA occurs by interacting either with the channel protein or by partitioning into the lipid bilayer27,38,40,42. AA-sensitive mechano-gated
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
343
Table 2. Regulation of the mammalian 2P domain K⫹ channelsa Channel
pHi acidic
TWIK-1 human
Inhibition
TWIK-2 human rat
Inhibition
pHo 6.4
AA 10 m
Halothane
cAMP
PMA
Refs
Resistant
Stimulated
10
Resistant ⫺36%
⫹76%
⫺27% (0.5 mM)
Resistant Resistant
Stimulated Stimulated
16 14
⫺55%b
⫹430% ⫹530%
⫹40% (1 mM) ⫹90% (1 mM)
Inhibited
Inhibited
29,30 26,38,39,42
⫹840% ⫹600%
⫹170% (1 mM)
Inhibited
Resistant
⫹630% ⫹660%
Resistant (1 mM)
Resistant
Resistant
⫺45%
⫹50% (1 mM)
Resistant Inhibited
Resistant Resistant
Resistant
Resistant
Resistant
Resistant, stimulated 32,33
TREK-1 human mouse
Activation
TREK-2 human rat
Activation Activation
TRAAK human mouse
Resistant
⫺35%b
TASK-1 human rat mouse
Resistant Inhibited
⫺95% ⫺95% ⫺95%
TASK-3 human rat guinea pig
Resistant Resistant
⫺74% ⫺74% ⫺30%
⫺59%
TASK-2 human
Inhibition
⫺95%
Resistant
⫹40% (1 mM)
THIK-1 rat
Resistant
⫺10%
⫹90%
⫺56% (5 mM)
TALK-1 human
Resistant
⫺50%
Resistant
⫺27% (0.8 mM)
Resistant
Resistant
35
TALK-2 human
Resistant
Resistant
Resistant
⫺56% (0.8 mM)
Resistant
Resistant
34,35
⫺40%
28 25 31 27,39 7,19,29,50 22 23,64 24 20 21
36
, DNP, HCO3⫺, acid injection and NH4Cl washout in the whole-cell configuration. Extracellular acidosis from pH 7.4 to pH 6.4 was investigated in the whole-cell configuration in a physiological K⫹ gradient. The effect of CPT cAMP (300–500 m) and PMA (30–100 nM) were investigated in the whole-cell configuration. bThe values for TREK-1 and TRAAK are personal observations. aIntracellular acidification was induced using various protocols including CO
2
TREK-like K+ channels have been identified in mesencephalic and hypothalamic neurones, cardiomyocytes and stomach smooth muscle56–59. Lysophospholipids (LP) including lysophosphatidylcholine (LPC), but not phospholipids, open TREK and TRAAK channels28,42 (Fig. 2c). At low doses, AA and LPC produce additive activation. The effect of LP is crucially dependent on the length of the carbonyl chain (longer than ten carbons) and the presence of a large polar head (choline or inositol)42. Activation is independent of the saturation status of the lipid, the charge of the polar head or the presence of an acetyl group at position 2 (platelet activating factor)42. Patch excision produces a progressive loss of channel activation by LPC, whereas AA still produces maximal opening demonstrating that cellular integrity is crucially required for LP, but not for AA, activation42. LP and PUFA activation of TREK and TRAAK channels thus clearly involve different mechanisms. Deletional analysis indicates that the carboxy terminus of TREK-1, but not the amino terminus and the extracellular loop M1P1, is crucial for both AA and LPC activation38,42. The same region was http://tins.trends.com
previously found to be important for stretch activation and suggests that chemical and mechanical activation might share a common molecular pathway (Fig. 2)39. THIK-1 is insensitive to LPC but is significantly and reversibly stimulated by AA (Ref. 36). The concentration dependence of the effect of AA on recombinant THIK-1 is described by a Kd of ~1 µM and a Hill coefficient of 2. However, it should be noticed that the magnitude of the effect is weak in comparison to that observed with TREK-1, TREK-2 and TRAAK (Table 2)25,27,31,38. Similarly, AA weakly stimulates TWIK-2 current amplitude14. By contrast to the other 2P domain K+ channels, TASK-1 is blocked by an amide derivative of AA, the endocannabinoid anandamide50. The block is fully reversible and occurs at submicromolar concentrations. The effect of anandamide is direct and independent of the cannabinoid receptors50. 2P domain K+ channels are sensitive to volatile general anaesthetics
Both TREK-1 and TREK-2 are opened by chloroform, diethyl ether, halothane and isoflurane28,29 (Table 2).
344
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
(a)
(b) (i)
(ii)
Type I
(c) (i) V (mV) 0
pH 6.4
–20
(ii) I (pA) 12
(iii) I (nA) pH 7.4
pH 8.4
1.5
8
1.0 pH 7.4
–40
4
pH 6.4
0.5
–60
pH 6.4
0 –80 20 s
–100
–80
–60
V (mV)
–40
–100
–50
0
50
100
V (mV) TRENDS in Neurosciences
Fig. 3. The acid-sensitive background K+ channel TASK-1 is expressed in chemo-sensitive type I carotid body cells. (a) Innervation of the carotid body by the sinus nerve afferent endings. (b)(i) TASK-1 mRNA (shown in blue) is highly expressed in both type I and type II carotid body cells. (b)(ii) The chemo-sensitive type I carotid body cells (expressing tyrosine hydroxylase are illustrated in red) release neurotransmitters during hypoxia and acidosis. (c)(i) Chemo-sensitive type I carotid body cells depolarize following extracellular acidosis from pH 7.4 to pH 6.4. (c)(ii) The oxygen-sensitive TASK-1-like background K+ current in type I carotid body cells is reversibly inhibited by extracellular acidosis to pH 6.4. (c)(iii) TASK-1 recorded in a transfected COS cell is similarly sensitive to extracellular acidosis. Adapted and reproduced, with permission, from Ref. 53.
Interestingly, the other structurally and functionally related 2P domain K+ channel TRAAK is insensitive to volatile anaesthetics29. Deletional and chimera analysis demonstrate that the carboxy terminus, but not the amino terminus, of TREK-1 is crucial for anaesthetic activation29. TREK-1 and TREK-2 share all the functional properties of the anaesthetic-sensitive background K+ channels (KAn) in Lymnea pacemaker neurones and Aplysia sensory neurones3–5,60. The lack of effect of volatile anaesthetics on TRAAK, another mechanogated 2P domain K+ subunit, suggests that an indirect membrane effect (bilayer couple hypothesis) is unlikely29. The effect of TASK-1 opening by halothane is greater than with isoflurane, whereas it is insensitive http://tins.trends.com
to chloroform and partially inhibited by diethylether7,29 (Table 2). Halothane opens TASK-1 in the excised patch configuration in spite of a large decrease of channel activity following excision29. Again the carboxy terminus of TASK-1 is crucial for anaesthetic sensitivity29. Halothane, unlike chloroform, opens the endogenous background TASK1-like K+ channels in type I carotid body cells53 (Fig. 3). This effect might be partially responsible for the suppression of hypoxic ventilatory drive under general anaesthesia53. In rat somatic motoneurones, locus coeruleus neurones and cerebellar granule neurones, inhalational anaesthetics similarly activate a background TASK-1-like conductance, causing membrane hyperpolarization and suppressing action potential discharge7,50. External acidosis to a pH of 6.5 completely blocks the currentactivated by anaesthetics7,50. In motoneurones and cerebellar granule neurones opening of TASK-1 probably contributes to anaesthetic-induced immobilization, whereas in the locus coeruleus, it might support analgesic and hypnotic actions7,50. Application of clinical concentrations of volatile anaesthetics causes an increase in TASK-2 current amplitude33 (Table 2). TASK-2 is more sensitive to halothane compared with isoflurane. By contrast with TASK-1, TASK-2 is also stimulated by chloroform33. Opening of TASK-2 in the motoneurones of the spinal
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
345
Box 2. Neuroprotection and lipid-sensitive 2P domain K+ channels During brain ischaemia, activation of cytosolic and secretory phospholipases A2 leads to the release of polyunsaturated fatty acids (PUFA) and lysophospholipids (LP) from phospholipidsa,b. Interestingly, PUFA, but not saturated fatty acid (FA), prevent neuronal death in animal models of transient global ischaemia even when administrated after the insultc,d. The protective effects of PUFA on cerebellar granule neurones are strongly dependent on external K+ concentrationsc. Moreover, a Gd3+-sensitive 86Rb+ efflux in cerebellar granule neurones is strongly stimulated by PUFA, but not by PUFA methyl derivatives and saturated FA (Ref. c). Additionally, patch-clamp experiments revealed the existence of TREK-like channels in cerebellar granule neuronesc. Cell swelling and intracellular acidosis, in addition to the release of cellular lipids, will contribute to the opening of TREK and TRAAK channels during ischaemiae. Activation of TREK and TRAAK channels will hyperpolarize neurones and consequently reduce Ca2+ influx via voltage-gated Ca2+ channels and NMDA receptors, and thus could represent an important
cord could also contribute to the depression of mobility that is induced by anaesthetics. The stimulation of channel activity by volatile anaesthetics is specific to TREK-1, TREK-2, TASK-1 and TASK-2. By contrast, halothane reversibly inhibits both TWIK-2 and THIK-1 (Refs 14,36; Table 2). This effect is agent specific because chloroform inhibits TWIK-2 but fails to alter THIK-1. Together with the known modulation of neurotransmitter receptors, opening of the 2P domain K+ channels could thus explain some of the actions of volatile anaesthetics at both pre- and postsynaptic levels29.
neuroprotective switch. Interestingly, other activators of TREK and TRAAK channels including riluzole, volatile anaesthetics and heat are also neuroprotectivef–h. References a Bonventre, J.V. (1997) Roles of phospholipases A2 in brain cell and tissue injury associated with ischemia and excitotoxicity. J. Lipid Mediat. Cell Signal 16, 199–208 b Lauritzen, I. et al. (1994) Expression of group II phospholipase A2 in rat brain after severe forebrain ischemia and in endotoxic shock. Brain Res. 651, 353–356 c Lauritzen, I. et al. (2000) Poly-unsaturated fatty acids are potent neuroprotectors. EMBO J. 19, 1784–1793 d Leaf, A. et al. (1999) The antiarrhythmic and anticonvulsant effects of dietary N-3 fatty acids. J. Membr. Biol. 172, 1–11 e Maingret, F. et al. (1999) Mechano- or acid stimulation, two interactive modes of activation of the TREK-1 potassium channel. J. Biol. Chem. 274, 26691–26696 f Maingret, F. et al. (2000) TREK-1 is a heat-activated background K+ channel. EMBO J. 19, 2483–2491 g Patel, A.J. et al. (1999) Inhalational anaesthetics activate twopore-domain background K+ channels. Nat. Neurosci. 2, 422–426 h Duprat, F. et al. (2000) The neuroprotective agent riluzole activates the two P domain K+ channels TREK-1 and TRAAK. Mol. Pharmacol. 57, 906–912
TASK-1, in addition to its functional correlate in rat cerebellar granule neurones and motoneurones, is inhibited by activation of receptors coupled to Gq proteins including the muscarinic M3 receptor51,52. TASK-1 is insensitive to intracellular inositol (1,4,5)trisphosphate [Ins(1,4,5)P3] injection, treatment with phorbol esters, stimulation of PKA and intracellular Ca2+ buffering19. The endocannabinoid anandamide, a product of phospholipase D, is a potent blocker of TASK-1 and is worth considering as a strong candidate second messenger50. Conclusions and directions for future research
Receptor-mediated regulation of 2P domain K+ channels
Acknowledgements We are grateful to M. Lazdunski for continual support and to all members of the laboratory for their help during the time course of this study. M. Jodar, V. Lopez and F. Aguila are acknowledged for excellent technical assistance. This work was supported by the Centre National de la Recherche Scientifique (CNRS).
TREK-1 and TREK-2 share most of the properties of the Aplysia S channel1,2. When co-expressed with the 5-HT4 serotonin receptor, serotonin inhibits TREK-1 and TREK-2 (Refs 31,38). This effect is mimicked by a membrane permeant cAMP derivative28,38 (Table 2). Protein kinase A (PKA)-mediated phosphorylation of Ser333 in the carboxy terminus is responsible for closing TREK-1 (Ref. 38) (Fig. 2c). TREK-1, unlike TREK-2 and TRAAK, is strongly inhibited by protein kinase C stimulation26,42. The opening of TREK-1 by either LPC or AA is completely reversed by treatment with the phorbol ester PMA (phorbol 12-myristate 13-acetate) (Ref. 42). By contrast, TWIK-1 is stimulated by PKC activation, although this effect is indirect10 (Table 2).
References 1 Belardetti, F. and Siegelbaum, S.A. (1988) Upand down-modulation of single K+ channel function by distinct second messengers. Trends Neurosci. 11, 232–238
http://tins.trends.com
Polymodal activation of 2P domain K+ channels by physical and chemical stimuli is of functional importance in the brain. The S-type TREK channels might be responsible for presynaptic modulation, a mechanism involved in learning2. The temperature sensitivity of TREK-1 suggests that it could be implicated in central and peripheral thermoregulatory functions41. At the pathophysiological level, lipid-sensitive and mechano-gated 2P domain K+ channels might be important in neuroprotection during brain ischaemia (Box 2)61. Opening of specific 2P domain K+ channels by inhalational anaesthetics indicates that they are possibly involved in the mechanism of general anaesthesia7,29. These recent results might help to design novel anaesthetic and neuroprotective agents.
2 Hawkins, R.D. et al. (1993) Learning to modulate transmitter release: themes and variations in synaptic plasticity. Annu. Rev. Neurosci. 16, 625–665 3 Franks, N.P. and Lieb, W.R. (1988) Volatile general anaesthetics activate a novel neuronal K+
current. Nature 333, 662–664 4 Lopes, C.M.B. et al. (1998) Actions of general anaesthetics and arachidonic acid pathway inhibitors on K+ currents activated by volatile anaesthetics and FMRFamide in molluscan
346
Review
neurones. Br. J. Pharmacol. 125, 309–318 5 Winegar, B.D. et al. (1996) Volatile general anesthetics produce hyperpolarization of Aplysia neurons by activation of a discrete population of baseline potassium channels. Anesthesiology 85, 889–900 6 Sirois, J.E. et al. (1998) Multiple ionic mechanisms mediate inhibition of rat motoneurones by inhalation anaesthetics. J. Physiol. 512.3, 851–862 7 Sirois, J.E. et al. (2000) The TASK-1 two-pore domain K+ channel is a molecular substrate for neuronal effects of inhalational anesthetics. J. Neurosci. 20, 6347–6354 8 Coetzee, W.A. et al. (1999) Molecular diversity of K+ channels. Ann. New York Acad. Sci. 868, 233–285 9 Doyle, D.A. et al. (1998) The structure of the potassium channel: molecular basis of K+ conduction and selectivity. Science 280, 69–77 10 Lesage, F. et al. (1996) TWIK-1, a ubiquitous human weakly inward rectifying K+ channel with a novel structure. EMBO J. 15, 1004–1011 11 Goldstein, S.A. et al. (1996) ORK1, a potassiumselective leak channel with two pore domains cloned from Drosophila melanogaster by expression in Saccharomyces cerevisiae. Proc. Natl. Acad. Sci. U. S. A. 93, 13256–13261 12 Wei, A. et al. (1996) Eight potassium channel families revealed by the C. elegans genome project. Neuropharmacology 35, 805–829 13 Lesage, F. et al. (1996) Dimerization of TWIK-1 K+ channel subunits via a disulfide bridge. EMBO J. 15, 6400–6407 14 Patel, A.J. et al. (2000) TWIK-2, an inactivating 2P domain K+ channel. J. Biol. Chem. 275, 28722–28730 15 Pountney, D.J. et al. (1999) Identification and cloning of TWIK-originated similarity sequence (TOSS): a novel human 2-pore K+ channel principal subunit. FEBS Lett. 450, 191–196 16 Chavez, R.A. et al. (1999) TWIK-2, a new weak inward rectifying member of the tandem pore domain potassium channel family. J. Biol. Chem. 274, 7887–7892 17 Salinas, M. et al. (1999) Cloning of a new mouse two-P domain channel subunit and a human homologue with a unique pore structure. J. Biol. Chem. 274, 11751–11760 18 Lesage, F. et al. (1997) The structure, function and distribution of the mouse TWIK-1 K+ channel. FEBS Lett. 402, 28–32 19 Duprat, F. et al. (1997) TASK, a human background K+ channel to sense external pH variations near physiological pH. EMBO J. 16, 5464–5471 20 Kim, Y. et al. (2000) TASK-3, a new member of the tandem pore K+ channel family. J. Biol. Chem. 275, 9340–9347 21 Rajan, S. et al. (2000) TASK-3, a novel tandem pore-domain acid-sensitive K+ channel: an extracellular histidine as pH sensor. J. Biol. Chem. 275, 16650–16657 22 Leonoudakis, D. et al. (1998) An open rectifier potassium channel with two pore domains in tandem cloned from rat cerebellum. J. Neurosci. 18, 868–877 23 Kim, D. et al. (1998) Cloning and functional expression of a novel cardiac two-pore background K+ channel (cTBAK-1). Circ. Res. 82, 513–518 24 Chapman, C.G. et al. (2000) Cloning, localisation and functional expression of a novel human cerebellum specific, two pore domain potassium channel. Brain Res. Mol. Brain Res. 82, 74–83
http://tins.trends.com
TRENDS in Neurosciences Vol.24 No.6 June 2001
25 Bang, H. et al. (2000) TREK-2, a new member of the mechanosensitive tandem pore K+ channel family. J. Biol. Chem. 275, 17412–17419 26 Fink, M. et al. (1996) Cloning, functional expression and brain localization of a novel unconventional outward rectifier K+ channel. EMBO J. 15, 6854–6862 27 Fink, M. et al. (1998) A neuronal two P domain K+ channel actvitated by arachidonic acid and polyunsaturated fatty acid. EMBO J. 17, 3297–3308 28 Lesage, F. et al. (2000) Human TREK2, a 2P domain mechano-sensitive K+ channel with multiple regulations by polyunsaturated fatty acids, lysophospholipids, and Gs, Gi, and Gq proteincoupled receptors. J. Biol. Chem. 275, 28398–28405 29 Patel, A.J. et al. (1999) Inhalational anaesthetics activate two-pore-domain background K+ channels. Nat. Neurosci. 2, 422–426 30 Meadows, H.J. et al. (2000) Cloning, localisation and functional expression of the human orthologue of the TREK-1 potassium channel. Pflügers Arch. 439, 714–722 31 Lesage, F. et al. (2000) Cloning and expression of human TRAAK, a polyunsaturated fatty acidsactivated and mechano-sensitive K+ channel. FEBS Lett. 471, 137–140 32 Reyes, R. et al. (1998) Cloning and expression of a novel pH-sensitive two pore domain potassium channel from human kidney. J. Biol. Chem. 273, 30863–30869 33 Gray, A.T. et al. (2000) Volatile anesthetics activate the human tandem pore domain baseline K+ channel KCNK5. Anesthesiology 92, 1722–1730 34 Decher, N. et al. (2001) Characterization of TASK-4, a novel member of the pH-sensitive, two-pore domain potassium channel family. FEBS Lett. 492, 84–89 35 Girard, C. et al. (2001) Genomic and functional characteristics of novel human pancreatic 2P domain potassium channels. Biochem. Biophys. Res. Commun. 23, 249–256 36 Rajan, S. et al. (2000) THIK-1 and THIK-2, a novel subfamily of tandem pore domain K+ channels. J. Biol. Chem. 276, 7302–7311 37 Medhurst, A.D. et al. (2001) Distribution analysis of human two pore domain potassium channels in tissues of the central nervous system and periphery. Brain Res. Mol. Brain Res. 86, 101–114 38 Patel, A.J. et al. (1998) A mammalian two pore domain mechano-gated S-like K+ channel. EMBO J. 17, 4283–4290 39 Maingret, F. et al. (1999) Mechano- or acid stimulation, two interactive modes of activation of the TREK-1 potassium channel. J. Biol. Chem. 274, 26691–26696 40 Maingret, F. et al. (1999) TRAAK is a mammalian neuronal mechano-gated K+ channel. J. Biol. Chem. 274, 1381–1387 41 Maingret, F. et al. (2000) TREK-1 is a heatactivated background K+ channel. EMBO J. 19, 2483–2491 42 Maingret, F. et al. (2000) Lysophospholipids open the two P domain mechano-gated K+ channels TREK-1 and TRAAK. J. Biol. Chem. 275, 10128–10133 43 Boulant, J.A. (1998) Cellular mechanisms of temperature sensitivity in hypothalamic neurons. Prog. Brain Res. 115, 3–8 44 Philipp-Dormston, W.K. (1976) Evidence for the involvement of adenosine 3′,5′-cyclic monophosphate in fever genesis. Pflügers Arch. 362, 223–227
45 Scammell, T.E. et al. (1996) Ventromedial preoptic prostaglandin E2 activates fever-producing autonomic pathways. J. Neurosci. 16, 6246–6254 46 Saper, C.B. (1998) Neurobiological basis of fever. Ann. New York Acad. Sci. 856, 90–94 47 Cesare, P. et al. (1999) Ion channels gated by heat. Proc. Natl. Acad. Sci. U. S. A. 96, 7658–7663 48 Farber, N.E. et al. (1995) Halothane modulates thermosensitive hypothalamic neurons in rat brain slices. Anesthesiology 83, 1241–1253 49 Poterack, K.A. et al. (1991) The effect of halothane on thermosensitive neurons in the preoptic region of the anterior hypothalamus in acutely instrumented cats. Anesthesiology 75, 625–633 50 Maingret, F. et al. (2001) The endocannabinoid anandamide is a direct and selective blocker of the background K+ channel TASK-1. EMBO J. 20, 47–54 51 Millar, J.A. et al. (2000) A functional role for the two-pore domain potassium channel TASK-1 in cerebellar granule neurons. Proc. Natl. Acad. Sci. U. S. A. 97, 3614–3618 52 Talley, E.M. et al. (2000) TASK-1, a two-pore domain K+ channel, is modulated by multiple neurotransmitters in motoneurons. Neuron 25, 399–410 53 Buckler, K. et al. (2000) An oxygen-, acid- and anaesthetic-sensitive TASK-like background potassium channel in rat arterial chemoreceptor cells. J. Physiol. 525, 135–142 54 Lopez-Barneo, J. (1996) Oxygen-sensing by ion channels and the regulation of cellular functions. Trends Neurosci. 19, 435–440 55 Peers, C. (1997) Oxygen-sensitive ion channels. Trends Pharmacol. Sci. 18, 405–408 56 Kim, D. and Clapham, D.E. (1989) Potassium channels in cardiac cells activated by arachidonic acid and phospholipids. Science 244, 1174–1176 57 Kim, D.H. et al. (1995) Arachidonic acid activation of a new family of K+ channels in cultured rat neuronal cells. J. Physiol. 484, 643–660 58 Ordway, R.W. et al. (1989) Arachidonic acid and other fatty acids directly activate potassium channels in smooth muscle cells. Science 244, 1176–1179 59 Ordway, R.W. et al. (1991) Stretch activation of a toad smooth muscle K+ channel may be mediated by fatty acids. J. Physiol. 484, 331–337 60 Franks, N.P. and Lieb, W.R. (1991) Stereospecific effects of inhalational general anesthetic optical isomers on nerve ion channels. Science 254, 427–430 61 Lauritzen, I. et al. (2000) Poly-unsaturated fatty acids are potent neuroprotectors. EMBO J. 19, 1784–1793 62 Reyes, R. et al. (2000) Immunolocalization of the arachidonic-acid and mechano-sensitive baseline TRAAK potassium channel in the nervous system. Neuroscience 95, 893–901 63 Kindler, C.H. et al. (2000) Localization of the tandem pore domain K+ channel TASK-1 in the rat. Brain Res. Mol. Brain Res. 80, 99–108 64 Lopes, C.M. et al. (2000) Proton block and voltage gating are potassium-dependent in the cardiac leak channel Kcnk3. J. Biol. Chem. 275, 16969–16978 65 Medhurst, A.D. et al. (2001) Distribution analysis of human twopore domain potassium channels in tissues of the central nervous system and periphery. Brain Res. Mol. Brain. Res. 86, 101–114. Note added in proof A recent study65 describes the distribution of human two pore domain K+ channels in tissues of the CNS and periphery.
8 Lennie, P. (1981) The physiological basis of variations in visual latency. Vis. Res. 21, 815–824 9 Maunsell, J.H.R. and Gibson, J.R. (1992) Visual response latencies in striate cortex of the macaque monkey. J. Neurophysiol. 68, 1332–1344 10 Purushothaman, G. et al. (1998) Moving ahead through differential visual latency. Nature 396, 424 11 Whitney, D. et al. (2000) Illusory spatial offset of a flash relative to a moving stimulus is caused by differential latencies for moving and flashed stimuli. Vis. Res. 40, 137–149 12 Eagleman, D.M. and Sejnowski, T.J. (2000) Motion integration and postdiction in visual awareness. Science 287, 2036–2038 13 Sheth, B.R. (2000) Changing objects lead briefly flashed ones. Nat. Neurosci. 3, 489–495 14 Baldo, M.V.C. and Klein, S.A. (1995) Extrapolation or attention shift? Nature 378, 565–566 15 Brenner, E. and Smeets, J.B.J. (2000) Motion extrapolation is not responsible for the flash-lag effect. Vis. Res. 40, 1645–1648 16 Bachmann, T. (1999) Twelve spatiotemporal phenomena and one explanation. In Cognitive Contributions to the Perception of Spatial and Temporal Events (Aschersleben, G. et al., eds), pp. 173–206, Elsevier Science 17 Khurana, B. et al. (2000) The role of attention in motion extrapolation: are moving objects
TRENDS in Neurosciences Vol.24 No.6 June 2001
18
19 20 21 22 23 24 25
26
27
‘corrected’ or flashed objects attentionally delayed? Perception 29, 675–692 Krekelberg, B. and Lappe, M. (2000) A model of the perceived relative positions of moving objects based upon a slow averaging process. Vis. Res. 40, 201–215 Krekelberg, B. (2001) The persistence of position. Vis. Res. 41, 529–539 Eagleman, D.M. and Sejnowski, T.J. (2000) Reply to Patel et al. Science 290, 1051 Krekelberg, B. and Lappe, M. (2000) The position of moving objects. Science 289, 1107 Whitney, D. and Cavanagh, P. (2000) Reply to Eagleman and Sejnowski. Science 289, 1107 Eagleman, D.M. and Sejnowski, T.J. (2000) Reply to Krekelberg et al. Science 289, 1107 Patel, S.S. et al. (2000) Flash-lag effect: differential latency, not postdiction. Science 290, 1051 Bishop, P.O. et al. (1971) Responses to visual contours: spatio-temporal aspects of excitation in the receptive fields of simple striate neurones. J. Physiol. 219, 625–657 Orban, G.A. et al. (1985) Velocity selectivity in the cat visual system. I. Responses of LGN cells to moving bar stimuli: a comparison with cortical areas 17 and 18. J. Neurophysiol. 54, 1026–1049 Raiguel, S. et al. (1989) Response latencies of visual cells in macaque area V1; V2 and V5. Brain Res. 493, 155–159
339
28 Berry, M.J. et al. (1999) Anticipation of moving stimuli by the retina. Nature 398, 334–338 29 Jancke, D. et al. (1999) Parametric population representation of retinal location: neuronal interaction dynamics in cat primary visual cortex. J. Neurosci. 19, 9016–9028 30 Spillmann, L. and Werner., J.S. (1996) Longrange interactions in visual perception. Trends Neurosci. 19, 428–434 31 Kirschfeld, K. and Kammer, T. (1999) The Fröhlich effect: a consequence of the interaction of visual focal attention and metacontrast. Vis. Res. 39, 3702–3709 32 Bringuier, V. et al. (1999) Horizontal propagation of visual activity in the synaptic integration field of area 17 neurons. Science 283, 695–699 33 Schlag, J. et al. (2000) Extrapolating movement without retinal motion. Nature 403, 38–39 34 Cai, R.H. et al. (2000) Vestibular signals can distort the perceived spatial relationship of retinal stimuli. Exp. Brain Res. 135, 275–278 35 Mateeff, S. and Hohnsbein, J. (1988) Perceptual latencies are shorter for motion towards the fovea than for motion away. Vis. Res. 28, 711–719 36 van Beers, R. et al. (2001) Sensorimotor integration compensates for visual localization errors during smooth pursuit eye movements. J. Neurophysiol. (in press)
Properties and modulation of mammalian 2P domain K+ channels Amanda J. Patel and Eric Honoré Mammalian 2P domain K+ channels are responsible for background or ‘leak’ K+ currents. These channels are regulated by various physical and chemical stimuli, including membrane stretch, temperature, acidosis, lipids and inhalational anaesthetics. Furthermore, channel activity is tightly controlled by membrane receptor stimulation and second messenger phosphorylation pathways. Several members of this novel family of K+ channels are highly expressed in the central and peripheral nervous systems in which they are proposed to play an important physiological role. The pharmacological modulation of this novel class of ion channels could be of interest for both general anaesthesia and ischaemic neuroprotection.
Amanda J. Patel Eric Honoré* Institut de Pharmacologie Moléculaire et Cellulaire, CNRS-UMR6097, 660 route des Lucioles, Sophia Antipolis, 06560 Valbonne, France. *e-mail: honore@ ipmc.cnrs.fr
Leak or background K+ selective channels – defined by a lack of voltage- and time-dependency, and with a linear current to voltage relationship in a symmetrical K+ gradient – play an essential role in setting the resting membrane potential, tuning the action potential duration and modulating the responsiveness to synaptic inputs. Regulation of background K+ channels by neurotransmitters and second messengers is central for synaptic function1,2. The most extensively studied native background K+ channel is the S channel in the marine snail Aplysia sensory neurones1. Closing of the S-type background
K+ channel by 5-HT receptor activation is involved in presynaptic sensitization, a simple form of learning2. Additionally, neuronal background K+ channels are the targets of an important class of pharmacological agents, the volatile general anaesthetics3–7. Mammalian K+ channel subunits (~80 genes) can be divided into three main structural classes comprising two transmembrane segments (TMS), four-TMS or six-TMS (Ref. 8). The common feature of all K+ channels is the presence of a conserved motif called the P domain, which is part of the K+ conduction pathway9. The two-TMS channels comprise a single P domain and encode the inward rectifiers. These K+ channels, which operate at negative membrane potentials, contribute to the setting of the resting membrane potential. The sixTMS channels, including the voltage-gated and the Ca2+-activated K+ channels, similarly comprise a single P domain. These channels, which are usually activated at depolarized membrane potentials, mostly contribute to the repolarization of the action potential. By contrast, the most recently discovered class of four-TMS subunits is characterized by the
http://tins.trends.com 0166-2236/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0166-2236(00)01810-5
Review
340
TRENDS in Neurosciences Vol.24 No.6 June 2001
(a)
(b)
TASK-1
Loop
TASK-3 THIK-1
KT 3-3 P1
P2 TWIK-2
THIK-2
TWIK-1 1
2
3
TRAAK
4
TREK-1
KCNK7
TREK-2
TALK-2
NH2
COOH
(c)
TALK-1
Cerebellum
Brain h
m
TWIK-1 TWIK-2 KCNK7 TREK-1 TREK-2 TRAAK TASK-1 TASK-3 TASK-2 THIK-1 THIK-2
KCNK1 KCNK6 KCNK7 KCNK2 KCNK10 KCNK4 KCNK3 KCNK9 KCNK5 KCNK13 KCNK12
+ – + + + + + – –
+ – – +
TALK-1 TALK-2
KCNK16 KCNK17
– –
r –
+ + +
+ +
– + +
h
m
+ – + – + – + + –
+ – – +
Spinal cord r –
+ + +
0.1
TASK-2
+ +
–
h + + + + + – + – +
m
r
– + + +
+
– +
TRENDS in Neurosciences
Fig. 1. Neuronal expression of mammalian 2P domain K+ channels. (a) Topology of a 2P domain K+ channel. (b) Dendrogram of human 2P domain K+ channels established with ClustalW and Treeview. KT3.3 accession number is NM 022358. (c) Tissue specific expression of the 2P domain K+ channels in human, mouse and rat as determined by northern blot, RT-PCR, in situ hybridization and immunohistochemistry. Expression is indicated by a plus sign; lack of or low, expression is indicated by a minus sign. It is evident from these results that important inter-species differences exist between the pattern of expression of the 2P domain K+ channels. TASK-1, TREK-1 and TRAAK are localized at both synaptic and nonsynaptic sites41,51,62,63.
presence of a tandem of P domains10–12. The leak activity of the 2P domain K+ channels suggests that they will influence both the resting membrane potential and the action potential duration. Functional K+ channels are tetramers of pore-forming subunits for the two- and six-TMS classes and are likely to be dimers for the four-TMS class13. Although the 2P domain K+ channel subunits display the same structural motifs with four-TMS – an extended M1P1 extracellular loop and both aminoand carboxy-termini in the intracellular domains – they share moderate sequence homology outside their P regions (Fig. 1a). The 14 human 2P domain K+ channels identified so far have been classified into five structural subgroups: (1) TWIK-1, TWIK-2 and KCNK7 (KCNK7 is not functional)10,14–18; (2) TASK-1, TASK-3 and KT 3-3 (the functional expression of KT 33 has not yet been reported)19–24 ; (3) TREK-1, TREK-2 and TRAAK (Refs 25–31); (4) TASK-2, TALK-1 and TALK-2 (also called TASK-4) (Refs 32–35); (5) THIK-1 http://tins.trends.com
and THIK-2 (THIK-2 is not functional)36 (Fig. 1b). With the exception of TWIK-2, TASK-2, TASK-3, TALK-1 and TALK-2, the 2P domain K+ channels are highly expressed in the human brain (Fig. 1c)14,32,35,37. No evidence for heteromultimerization has been reported for the 2P domain K+ channels. The number of studies concerning native background K+ channel currents in mammalian cells is rather limited. This is as a result of their leak characteristics, the modest amplitude of these currents, in addition to the lack of specific pharmacology. 2P domain subunits encode background K+ channels which are insensitive to most classical K+ channel blockers including tetraethylammonium (TEA) and 4aminopyridine (4-AP) (Table 1)10,14–17,19–33,36. Based on these criteria and considering their overlapping expression, it is still a delicate issue to establish a close relationship between cloned and native background K+ channels. However, recent studies with recombinant channels reveal that their activity is modulated by an unusual variety of physical and chemical stimuli. Consequently, it is now possible to extrapolate to native tissues and try to evaluate their physiological role. In the future transgenic and knockout animals will be extremely valuable to confirm these findings. This review focuses on the specific functional and regulation properties of the cloned mammalian 2P domain K+ channels expressed in the central and peripheral nervous systems. TREK-1, TREK-2 and TRAAK are mechano-gated K+ channels
TREK-1, TREK-2 and TRAAK channel activity is elicited by increasing the mechanical pressure applied to the cell membrane and is independent of intracellular Ca2+ (Refs 25,28,31,38–40; Fig. 2). In the inside–out patch configuration, positive pressure is significantly less effective compared with negative pressure in opening channels suggesting that a specific membrane deformation (convex curving) preferentially opens these channels38,40. At the wholecell level, TREK-1 and TRAAK are modulated by cellular volume, for example, hyperosmolarity closes the channels38,41. Both the number of active channels and the sensitivity to mechanical stretch are strongly enhanced after treating the cell-attached patches with the cytoskeleton disrupting agents, colchicine and cytochalasin D (Ref. 40). These data suggest that mechanical force might be transmitted directly to the channel via the lipid bilayer and does not require the integrity of the cytoskeleton38,40. Both TREK-1 and TRAAK are blocked by amiloride and Gd3+, blockers of stretch-sensitive ion channels40,42. TREK-1 behaves as a cold sensor in thermo-sensitive neurones
The application of heat gradually and reversibly opens TREK-1 with a sevenfold increase in current amplitude corresponding to a temperature jump of 10°C (Q10) (Ref. 41). The high expression levels of
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
341
Table 1. Biophysical and pharmacological properties of the mammalian 2P domain Kⴙ channelsa Conductance 155 mM Kⴙ
Rectification 155 mM Kⴙ
Activation kinetics
Inactivation kinetics
Ba2ⴙ IC50
Refs
TWIK-1 human
34 pS
Inward
None
None
0.1 mM
10
TWIK-2 human rat
⬍5 pS
Inward Inward
None None
None Inactivating
0.1 mM
16 14
101 pS
Outward Outward
None None
None None
Resistant Resistant
29,30 26,38
100 pS 110 pS
Outward Linear
Delayed None
None None
Resistant ⬎ 2 mM
28 25
Outward Linear
None
None
Resistant 1 mM
31 27
None None None, delayed
None None None
0.35 mM
16 pS
Linear Linear Linear
19 22,51 23,64
27 pS 27 pS
Outward Linear Outward
None None Delayed
None None None
3 mM 0.29 mM
24 20 21
60 pS
Channel
TREK-1 human mouse
Conductance 5 mM Kⴙ
48 pS
TREK-2 human rat TRAAK human mouse TASK-1 human rat mouse
45 pS
40 pS
TASK-3 human rat guinea pig TASK-2 human
Outward
Delayed
None
Resistant
32,33
THIK-1 rat
14 pS
Inward
Delayed
None
1 mM
36
TALK-1 human
Linear
None
None
1 mM
35
TALK-2 human
Outward
None
None
1 mM
34,35
single channel conductance, measured in the linear range of the I–Vs, was determined in both physiological and symmetrical K⫹ gradients. The direction of the whole cell rectification (inward, linear or outward) was determined in a symmetrical K⫹ gradient. The kinetic of activation is instantaneous, delayed or occurring in two phases64. Currents are either time-independent or inactivating. The Ba2⫹ sensitivity was determined in a physiological K⫹ gradient. The 2P domain K⫹ channels are resistant to millimolar concentrations of TEA and 4-AP. aThe
TREK-1 in the small and medium-size diameter dorsal root ganglion (DRG) sensory neurones as well as in the preoptic and anterior hypothalamus makes this background K+ channel an attractive candidate for a temperature sensor41 (Box 1; Fig. 2a,b). At the protein level TREK-1 is present at both synaptic and nonsynaptic sites41. Thermal activation of TREK-1 requires cell integrity suggesting that a cytosolic factor might be involved in channel regulation41. Activation of TREK-1 by temperature is specific because TASK-1 and THIK-1 only display a Q10 of 2 and 1.6, respectively36,41. By contrast, TWIK2, a weak inwardly rectifying background K+ channel, inactivates with increasing temperature14. At the CNS level, cold-sensitive neurones in the anterior and preoptic hypothalamus, which control body heat loss, heat retention and heat production, respond to cooling by enhanced action potential firing43. Prostaglandin E2 (PGE2) and its second messenger cAMP are proposed to be prime candidates in the genesis of fever44–47. TREK-1 is strongly inhibited by PGE2 receptor activation via the protein kinase A (PKA) phosphorylation pathway (see below)41. http://tins.trends.com
Inhibition of TREK-1 by either cold temperature or PGE2 stimulation might contribute to trigger the heat production and retention programme, leading to correction of the hypothermia or production of fever. Halothane and other inhalational anaesthetics impair normal thermoregulatory processes48,49. Interestingly, TREK-1 is directly opened by volatile general anaesthetics (see below)29. These pharmacological observations further suggest an important role of TREK-1 in central thermo-regulative processes41. Nevertheless, the functional role of a background K+ channel current in hypothalamic cold-sensitive neurones remains to be confirmed. Intracellular and extracellular acidosis differentially modulate 2P domain K+ channels
Lowering intracellular pH shifts the pressureactivation relationship of TREK-1 and TREK-2, but not of TRAAK, toward positive values and ultimately leads to channel opening at atmospheric pressure39 (Table 2). Essentially, acidosis converts a TREK mechano-gated channel into a constitutively active channel28,39. Deletional analysis demonstrates that
342
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
(a)
(c)
F
F
C
C
F
Br
COOH F Arachidonic acid
Cl
Halothane
P
Heat
LPC
(b) Stretch
P NH2
P
COOH
COOH
pHi
PKA and PKC TRENDS in Neurosciences
Fig. 2. TREK-1 is a polymodal background K+ channel. (a) TREK-1 mRNA expression pattern in the mouse brain (parasagittal section). (b) Immunolocalization of mouse TREK-1 in a dorsal root ganglion (DRG) with a polyclonal antibody. A higher magnification (inset) shows that TREK-1 is highly expressed in small and medium-size DRG neurones. (c) Polymodal activation of TREK-1 by membrane stretch, long chain anionic polyunsaturated fatty acids including arachidonic acid (AA), neutral lysophospholipids including LPC, volatile general anaesthetics including chloroform, warm temperature and finally intracellular acidosis. TREK-1 shares the functional properties of the Aplysia S and Lymnea KAn background K+ channels1,3. TREK-1 is inhibited by protein kinase A (PKA) and protein kinase C (PKC)mediated phosphorylation (red arrow). Adapted and reproduced, with permission, from Ref. 41.
the carboxy terminus, but not the amino terminus and the extracellular M1P1 loop, is crucial for activation of TREK-1 by stretch and intracellular acidosis39. By contrast, TWIK-1 and TWIK-2 channel activities are downregulated by intracellular acidosis induced by CO2 or dinitrophenol10,16. However, this Box 1. Cold transduction and background K+ channels Peripheral sensory cold fibres discharge slowly at the resting skin temperature but evoke strong action potential discharge following a decrease in temperaturea,b. Recent evidence supports the role of a background K+ conductance in cold transduction of rat primary sensory neuronesc. Cooling from 32 to 20°C induces a large depolarization (>8 mV), increases input resistance and triggers action potential firing in 25% of capsaicin-resistant DRG neurones (L1–S1) (Ref. c). In cold-sensitive neurones, cooling reversibly inhibits a time-independent TREK-1-like tetraethylammonium (TEA) and 4-AP resistant background K+ current (called Icold) (Ref. c). References a Darian-Smith, I. (1984) Thermal sensibility. In Handbook of Physiology. The Nervous System III, pp. 879–913 b Spray, D.C. (1986) Cutaneous temperature receptors. Annu. Rev. Physiol. 48, 625–638 c Reid, G. and Flonta, M. (2001) Cold transduction by inhibition of a background potassium conductance in rat primary sensory neurons. Neurosci. Lett. 297, 171–174
http://tins.trends.com
effect is indirect because intracellular acidosis fails to affect TWIK-1 after excision of the patches10. TASK-1, TASK-2 and TASK-3 are sensitive to variations of extracellular pH in the physiological range (Table 2 and Fig. 3c)19–24,32,33. 50% of TASK-1, TASK-2 and TASK-3 channels are closed at pHs of 7.3, 7.8 and 6.6, respectively. A histidine residue at position 98 located near the GYG sequence of P1 confers the extracellular pH sensitivity of TASK-3 (Refs 20,21). This crucial histidine is conserved in the P1 sequence of TASK-1, but is absent in TASK-2 (Refs 19,21,32). Endogenous, acid-sensitive TASK-1-like K+ currents have been described in cerebellar granule neurones, rat somatic motoneurones and locus coerulus neurones7,50–52. Additionally, an oxygen- and acid-sensitive TASK-1-like background K+ current was recently described in the chemo-sensitive type I carotid body cells53 (Fig. 3). The primary sensory cells of the carotid body respond to acidosis and hypoxia with a depolarization initiating electrical activity and neurosecretion, thus triggering the reflex increase in respiration54,55. 2P domain K+ channels are regulated by cellular lipids
TREK-1, TREK-2 and TRAAK are reversibly opened by polyunsaturated fatty acids (PUFA) including arachidonic acid (AA)25,27,28,38 (Fig. 2c; Table 2). Although the activation is independent of AA metabolism27,38. The extent of saturation of the PUFA, the length of the carbonyl chain and charge of the molecule are crucial because saturated fatty acids (FA), a short carbonyl chain and neutral lipids are ineffective27,38. Direct activation of TREK and TRAAK channels by PUFA occurs by interacting either with the channel protein or by partitioning into the lipid bilayer27,38,40,42. AA-sensitive mechano-gated
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
343
Table 2. Regulation of the mammalian 2P domain K⫹ channelsa Channel
pHi acidic
TWIK-1 human
Inhibition
TWIK-2 human rat
Inhibition
pHo 6.4
AA 10 m
Halothane
cAMP
PMA
Refs
Resistant
Stimulated
10
Resistant ⫺36%
⫹76%
⫺27% (0.5 mM)
Resistant Resistant
Stimulated Stimulated
16 14
⫺55%b
⫹430% ⫹530%
⫹40% (1 mM) ⫹90% (1 mM)
Inhibited
Inhibited
29,30 26,38,39,42
⫹840% ⫹600%
⫹170% (1 mM)
Inhibited
Resistant
⫹630% ⫹660%
Resistant (1 mM)
Resistant
Resistant
⫺45%
⫹50% (1 mM)
Resistant Inhibited
Resistant Resistant
Resistant
Resistant
Resistant
Resistant, stimulated 32,33
TREK-1 human mouse
Activation
TREK-2 human rat
Activation Activation
TRAAK human mouse
Resistant
⫺35%b
TASK-1 human rat mouse
Resistant Inhibited
⫺95% ⫺95% ⫺95%
TASK-3 human rat guinea pig
Resistant Resistant
⫺74% ⫺74% ⫺30%
⫺59%
TASK-2 human
Inhibition
⫺95%
Resistant
⫹40% (1 mM)
THIK-1 rat
Resistant
⫺10%
⫹90%
⫺56% (5 mM)
TALK-1 human
Resistant
⫺50%
Resistant
⫺27% (0.8 mM)
Resistant
Resistant
35
TALK-2 human
Resistant
Resistant
Resistant
⫺56% (0.8 mM)
Resistant
Resistant
34,35
⫺40%
28 25 31 27,39 7,19,29,50 22 23,64 24 20 21
36
, DNP, HCO3⫺, acid injection and NH4Cl washout in the whole-cell configuration. Extracellular acidosis from pH 7.4 to pH 6.4 was investigated in the whole-cell configuration in a physiological K⫹ gradient. The effect of CPT cAMP (300–500 m) and PMA (30–100 nM) were investigated in the whole-cell configuration. bThe values for TREK-1 and TRAAK are personal observations. aIntracellular acidification was induced using various protocols including CO
2
TREK-like K+ channels have been identified in mesencephalic and hypothalamic neurones, cardiomyocytes and stomach smooth muscle56–59. Lysophospholipids (LP) including lysophosphatidylcholine (LPC), but not phospholipids, open TREK and TRAAK channels28,42 (Fig. 2c). At low doses, AA and LPC produce additive activation. The effect of LP is crucially dependent on the length of the carbonyl chain (longer than ten carbons) and the presence of a large polar head (choline or inositol)42. Activation is independent of the saturation status of the lipid, the charge of the polar head or the presence of an acetyl group at position 2 (platelet activating factor)42. Patch excision produces a progressive loss of channel activation by LPC, whereas AA still produces maximal opening demonstrating that cellular integrity is crucially required for LP, but not for AA, activation42. LP and PUFA activation of TREK and TRAAK channels thus clearly involve different mechanisms. Deletional analysis indicates that the carboxy terminus of TREK-1, but not the amino terminus and the extracellular loop M1P1, is crucial for both AA and LPC activation38,42. The same region was http://tins.trends.com
previously found to be important for stretch activation and suggests that chemical and mechanical activation might share a common molecular pathway (Fig. 2)39. THIK-1 is insensitive to LPC but is significantly and reversibly stimulated by AA (Ref. 36). The concentration dependence of the effect of AA on recombinant THIK-1 is described by a Kd of ~1 µM and a Hill coefficient of 2. However, it should be noticed that the magnitude of the effect is weak in comparison to that observed with TREK-1, TREK-2 and TRAAK (Table 2)25,27,31,38. Similarly, AA weakly stimulates TWIK-2 current amplitude14. By contrast to the other 2P domain K+ channels, TASK-1 is blocked by an amide derivative of AA, the endocannabinoid anandamide50. The block is fully reversible and occurs at submicromolar concentrations. The effect of anandamide is direct and independent of the cannabinoid receptors50. 2P domain K+ channels are sensitive to volatile general anaesthetics
Both TREK-1 and TREK-2 are opened by chloroform, diethyl ether, halothane and isoflurane28,29 (Table 2).
344
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
(a)
(b) (i)
(ii)
Type I
(c) (i) V (mV) 0
pH 6.4
–20
(ii) I (pA) 12
(iii) I (nA) pH 7.4
pH 8.4
1.5
8
1.0 pH 7.4
–40
4
pH 6.4
0.5
–60
pH 6.4
0 –80 20 s
–100
–80
–60
V (mV)
–40
–100
–50
0
50
100
V (mV) TRENDS in Neurosciences
Fig. 3. The acid-sensitive background K+ channel TASK-1 is expressed in chemo-sensitive type I carotid body cells. (a) Innervation of the carotid body by the sinus nerve afferent endings. (b)(i) TASK-1 mRNA (shown in blue) is highly expressed in both type I and type II carotid body cells. (b)(ii) The chemo-sensitive type I carotid body cells (expressing tyrosine hydroxylase are illustrated in red) release neurotransmitters during hypoxia and acidosis. (c)(i) Chemo-sensitive type I carotid body cells depolarize following extracellular acidosis from pH 7.4 to pH 6.4. (c)(ii) The oxygen-sensitive TASK-1-like background K+ current in type I carotid body cells is reversibly inhibited by extracellular acidosis to pH 6.4. (c)(iii) TASK-1 recorded in a transfected COS cell is similarly sensitive to extracellular acidosis. Adapted and reproduced, with permission, from Ref. 53.
Interestingly, the other structurally and functionally related 2P domain K+ channel TRAAK is insensitive to volatile anaesthetics29. Deletional and chimera analysis demonstrate that the carboxy terminus, but not the amino terminus, of TREK-1 is crucial for anaesthetic activation29. TREK-1 and TREK-2 share all the functional properties of the anaesthetic-sensitive background K+ channels (KAn) in Lymnea pacemaker neurones and Aplysia sensory neurones3–5,60. The lack of effect of volatile anaesthetics on TRAAK, another mechanogated 2P domain K+ subunit, suggests that an indirect membrane effect (bilayer couple hypothesis) is unlikely29. The effect of TASK-1 opening by halothane is greater than with isoflurane, whereas it is insensitive http://tins.trends.com
to chloroform and partially inhibited by diethylether7,29 (Table 2). Halothane opens TASK-1 in the excised patch configuration in spite of a large decrease of channel activity following excision29. Again the carboxy terminus of TASK-1 is crucial for anaesthetic sensitivity29. Halothane, unlike chloroform, opens the endogenous background TASK1-like K+ channels in type I carotid body cells53 (Fig. 3). This effect might be partially responsible for the suppression of hypoxic ventilatory drive under general anaesthesia53. In rat somatic motoneurones, locus coeruleus neurones and cerebellar granule neurones, inhalational anaesthetics similarly activate a background TASK-1-like conductance, causing membrane hyperpolarization and suppressing action potential discharge7,50. External acidosis to a pH of 6.5 completely blocks the currentactivated by anaesthetics7,50. In motoneurones and cerebellar granule neurones opening of TASK-1 probably contributes to anaesthetic-induced immobilization, whereas in the locus coeruleus, it might support analgesic and hypnotic actions7,50. Application of clinical concentrations of volatile anaesthetics causes an increase in TASK-2 current amplitude33 (Table 2). TASK-2 is more sensitive to halothane compared with isoflurane. By contrast with TASK-1, TASK-2 is also stimulated by chloroform33. Opening of TASK-2 in the motoneurones of the spinal
Review
TRENDS in Neurosciences Vol.24 No.6 June 2001
345
Box 2. Neuroprotection and lipid-sensitive 2P domain K+ channels During brain ischaemia, activation of cytosolic and secretory phospholipases A2 leads to the release of polyunsaturated fatty acids (PUFA) and lysophospholipids (LP) from phospholipidsa,b. Interestingly, PUFA, but not saturated fatty acid (FA), prevent neuronal death in animal models of transient global ischaemia even when administrated after the insultc,d. The protective effects of PUFA on cerebellar granule neurones are strongly dependent on external K+ concentrationsc. Moreover, a Gd3+-sensitive 86Rb+ efflux in cerebellar granule neurones is strongly stimulated by PUFA, but not by PUFA methyl derivatives and saturated FA (Ref. c). Additionally, patch-clamp experiments revealed the existence of TREK-like channels in cerebellar granule neuronesc. Cell swelling and intracellular acidosis, in addition to the release of cellular lipids, will contribute to the opening of TREK and TRAAK channels during ischaemiae. Activation of TREK and TRAAK channels will hyperpolarize neurones and consequently reduce Ca2+ influx via voltage-gated Ca2+ channels and NMDA receptors, and thus could represent an important
cord could also contribute to the depression of mobility that is induced by anaesthetics. The stimulation of channel activity by volatile anaesthetics is specific to TREK-1, TREK-2, TASK-1 and TASK-2. By contrast, halothane reversibly inhibits both TWIK-2 and THIK-1 (Refs 14,36; Table 2). This effect is agent specific because chloroform inhibits TWIK-2 but fails to alter THIK-1. Together with the known modulation of neurotransmitter receptors, opening of the 2P domain K+ channels could thus explain some of the actions of volatile anaesthetics at both pre- and postsynaptic levels29.
neuroprotective switch. Interestingly, other activators of TREK and TRAAK channels including riluzole, volatile anaesthetics and heat are also neuroprotectivef–h. References a Bonventre, J.V. (1997) Roles of phospholipases A2 in brain cell and tissue injury associated with ischemia and excitotoxicity. J. Lipid Mediat. Cell Signal 16, 199–208 b Lauritzen, I. et al. (1994) Expression of group II phospholipase A2 in rat brain after severe forebrain ischemia and in endotoxic shock. Brain Res. 651, 353–356 c Lauritzen, I. et al. (2000) Poly-unsaturated fatty acids are potent neuroprotectors. EMBO J. 19, 1784–1793 d Leaf, A. et al. (1999) The antiarrhythmic and anticonvulsant effects of dietary N-3 fatty acids. J. Membr. Biol. 172, 1–11 e Maingret, F. et al. (1999) Mechano- or acid stimulation, two interactive modes of activation of the TREK-1 potassium channel. J. Biol. Chem. 274, 26691–26696 f Maingret, F. et al. (2000) TREK-1 is a heat-activated background K+ channel. EMBO J. 19, 2483–2491 g Patel, A.J. et al. (1999) Inhalational anaesthetics activate twopore-domain background K+ channels. Nat. Neurosci. 2, 422–426 h Duprat, F. et al. (2000) The neuroprotective agent riluzole activates the two P domain K+ channels TREK-1 and TRAAK. Mol. Pharmacol. 57, 906–912
TASK-1, in addition to its functional correlate in rat cerebellar granule neurones and motoneurones, is inhibited by activation of receptors coupled to Gq proteins including the muscarinic M3 receptor51,52. TASK-1 is insensitive to intracellular inositol (1,4,5)trisphosphate [Ins(1,4,5)P3] injection, treatment with phorbol esters, stimulation of PKA and intracellular Ca2+ buffering19. The endocannabinoid anandamide, a product of phospholipase D, is a potent blocker of TASK-1 and is worth considering as a strong candidate second messenger50. Conclusions and directions for future research
Receptor-mediated regulation of 2P domain K+ channels
Acknowledgements We are grateful to M. Lazdunski for continual support and to all members of the laboratory for their help during the time course of this study. M. Jodar, V. Lopez and F. Aguila are acknowledged for excellent technical assistance. This work was supported by the Centre National de la Recherche Scientifique (CNRS).
TREK-1 and TREK-2 share most of the properties of the Aplysia S channel1,2. When co-expressed with the 5-HT4 serotonin receptor, serotonin inhibits TREK-1 and TREK-2 (Refs 31,38). This effect is mimicked by a membrane permeant cAMP derivative28,38 (Table 2). Protein kinase A (PKA)-mediated phosphorylation of Ser333 in the carboxy terminus is responsible for closing TREK-1 (Ref. 38) (Fig. 2c). TREK-1, unlike TREK-2 and TRAAK, is strongly inhibited by protein kinase C stimulation26,42. The opening of TREK-1 by either LPC or AA is completely reversed by treatment with the phorbol ester PMA (phorbol 12-myristate 13-acetate) (Ref. 42). By contrast, TWIK-1 is stimulated by PKC activation, although this effect is indirect10 (Table 2).
References 1 Belardetti, F. and Siegelbaum, S.A. (1988) Upand down-modulation of single K+ channel function by distinct second messengers. Trends Neurosci. 11, 232–238
http://tins.trends.com
Polymodal activation of 2P domain K+ channels by physical and chemical stimuli is of functional importance in the brain. The S-type TREK channels might be responsible for presynaptic modulation, a mechanism involved in learning2. The temperature sensitivity of TREK-1 suggests that it could be implicated in central and peripheral thermoregulatory functions41. At the pathophysiological level, lipid-sensitive and mechano-gated 2P domain K+ channels might be important in neuroprotection during brain ischaemia (Box 2)61. Opening of specific 2P domain K+ channels by inhalational anaesthetics indicates that they are possibly involved in the mechanism of general anaesthesia7,29. These recent results might help to design novel anaesthetic and neuroprotective agents.
2 Hawkins, R.D. et al. (1993) Learning to modulate transmitter release: themes and variations in synaptic plasticity. Annu. Rev. Neurosci. 16, 625–665 3 Franks, N.P. and Lieb, W.R. (1988) Volatile general anaesthetics activate a novel neuronal K+
current. Nature 333, 662–664 4 Lopes, C.M.B. et al. (1998) Actions of general anaesthetics and arachidonic acid pathway inhibitors on K+ currents activated by volatile anaesthetics and FMRFamide in molluscan
346
Review
neurones. Br. J. Pharmacol. 125, 309–318 5 Winegar, B.D. et al. (1996) Volatile general anesthetics produce hyperpolarization of Aplysia neurons by activation of a discrete population of baseline potassium channels. Anesthesiology 85, 889–900 6 Sirois, J.E. et al. (1998) Multiple ionic mechanisms mediate inhibition of rat motoneurones by inhalation anaesthetics. J. Physiol. 512.3, 851–862 7 Sirois, J.E. et al. (2000) The TASK-1 two-pore domain K+ channel is a molecular substrate for neuronal effects of inhalational anesthetics. J. Neurosci. 20, 6347–6354 8 Coetzee, W.A. et al. (1999) Molecular diversity of K+ channels. Ann. New York Acad. Sci. 868, 233–285 9 Doyle, D.A. et al. (1998) The structure of the potassium channel: molecular basis of K+ conduction and selectivity. Science 280, 69–77 10 Lesage, F. et al. (1996) TWIK-1, a ubiquitous human weakly inward rectifying K+ channel with a novel structure. EMBO J. 15, 1004–1011 11 Goldstein, S.A. et al. (1996) ORK1, a potassiumselective leak channel with two pore domains cloned from Drosophila melanogaster by expression in Saccharomyces cerevisiae. Proc. Natl. Acad. Sci. U. S. A. 93, 13256–13261 12 Wei, A. et al. (1996) Eight potassium channel families revealed by the C. elegans genome project. Neuropharmacology 35, 805–829 13 Lesage, F. et al. (1996) Dimerization of TWIK-1 K+ channel subunits via a disulfide bridge. EMBO J. 15, 6400–6407 14 Patel, A.J. et al. (2000) TWIK-2, an inactivating 2P domain K+ channel. J. Biol. Chem. 275, 28722–28730 15 Pountney, D.J. et al. (1999) Identification and cloning of TWIK-originated similarity sequence (TOSS): a novel human 2-pore K+ channel principal subunit. FEBS Lett. 450, 191–196 16 Chavez, R.A. et al. (1999) TWIK-2, a new weak inward rectifying member of the tandem pore domain potassium channel family. J. Biol. Chem. 274, 7887–7892 17 Salinas, M. et al. (1999) Cloning of a new mouse two-P domain channel subunit and a human homologue with a unique pore structure. J. Biol. Chem. 274, 11751–11760 18 Lesage, F. et al. (1997) The structure, function and distribution of the mouse TWIK-1 K+ channel. FEBS Lett. 402, 28–32 19 Duprat, F. et al. (1997) TASK, a human background K+ channel to sense external pH variations near physiological pH. EMBO J. 16, 5464–5471 20 Kim, Y. et al. (2000) TASK-3, a new member of the tandem pore K+ channel family. J. Biol. Chem. 275, 9340–9347 21 Rajan, S. et al. (2000) TASK-3, a novel tandem pore-domain acid-sensitive K+ channel: an extracellular histidine as pH sensor. J. Biol. Chem. 275, 16650–16657 22 Leonoudakis, D. et al. (1998) An open rectifier potassium channel with two pore domains in tandem cloned from rat cerebellum. J. Neurosci. 18, 868–877 23 Kim, D. et al. (1998) Cloning and functional expression of a novel cardiac two-pore background K+ channel (cTBAK-1). Circ. Res. 82, 513–518 24 Chapman, C.G. et al. (2000) Cloning, localisation and functional expression of a novel human cerebellum specific, two pore domain potassium channel. Brain Res. Mol. Brain Res. 82, 74–83
http://tins.trends.com
TRENDS in Neurosciences Vol.24 No.6 June 2001
25 Bang, H. et al. (2000) TREK-2, a new member of the mechanosensitive tandem pore K+ channel family. J. Biol. Chem. 275, 17412–17419 26 Fink, M. et al. (1996) Cloning, functional expression and brain localization of a novel unconventional outward rectifier K+ channel. EMBO J. 15, 6854–6862 27 Fink, M. et al. (1998) A neuronal two P domain K+ channel actvitated by arachidonic acid and polyunsaturated fatty acid. EMBO J. 17, 3297–3308 28 Lesage, F. et al. (2000) Human TREK2, a 2P domain mechano-sensitive K+ channel with multiple regulations by polyunsaturated fatty acids, lysophospholipids, and Gs, Gi, and Gq proteincoupled receptors. J. Biol. Chem. 275, 28398–28405 29 Patel, A.J. et al. (1999) Inhalational anaesthetics activate two-pore-domain background K+ channels. Nat. Neurosci. 2, 422–426 30 Meadows, H.J. et al. (2000) Cloning, localisation and functional expression of the human orthologue of the TREK-1 potassium channel. Pflügers Arch. 439, 714–722 31 Lesage, F. et al. (2000) Cloning and expression of human TRAAK, a polyunsaturated fatty acidsactivated and mechano-sensitive K+ channel. FEBS Lett. 471, 137–140 32 Reyes, R. et al. (1998) Cloning and expression of a novel pH-sensitive two pore domain potassium channel from human kidney. J. Biol. Chem. 273, 30863–30869 33 Gray, A.T. et al. (2000) Volatile anesthetics activate the human tandem pore domain baseline K+ channel KCNK5. Anesthesiology 92, 1722–1730 34 Decher, N. et al. (2001) Characterization of TASK-4, a novel member of the pH-sensitive, two-pore domain potassium channel family. FEBS Lett. 492, 84–89 35 Girard, C. et al. (2001) Genomic and functional characteristics of novel human pancreatic 2P domain potassium channels. Biochem. Biophys. Res. Commun. 23, 249–256 36 Rajan, S. et al. (2000) THIK-1 and THIK-2, a novel subfamily of tandem pore domain K+ channels. J. Biol. Chem. 276, 7302–7311 37 Medhurst, A.D. et al. (2001) Distribution analysis of human two pore domain potassium channels in tissues of the central nervous system and periphery. Brain Res. Mol. Brain Res. 86, 101–114 38 Patel, A.J. et al. (1998) A mammalian two pore domain mechano-gated S-like K+ channel. EMBO J. 17, 4283–4290 39 Maingret, F. et al. (1999) Mechano- or acid stimulation, two interactive modes of activation of the TREK-1 potassium channel. J. Biol. Chem. 274, 26691–26696 40 Maingret, F. et al. (1999) TRAAK is a mammalian neuronal mechano-gated K+ channel. J. Biol. Chem. 274, 1381–1387 41 Maingret, F. et al. (2000) TREK-1 is a heatactivated background K+ channel. EMBO J. 19, 2483–2491 42 Maingret, F. et al. (2000) Lysophospholipids open the two P domain mechano-gated K+ channels TREK-1 and TRAAK. J. Biol. Chem. 275, 10128–10133 43 Boulant, J.A. (1998) Cellular mechanisms of temperature sensitivity in hypothalamic neurons. Prog. Brain Res. 115, 3–8 44 Philipp-Dormston, W.K. (1976) Evidence for the involvement of adenosine 3′,5′-cyclic monophosphate in fever genesis. Pflügers Arch. 362, 223–227
45 Scammell, T.E. et al. (1996) Ventromedial preoptic prostaglandin E2 activates fever-producing autonomic pathways. J. Neurosci. 16, 6246–6254 46 Saper, C.B. (1998) Neurobiological basis of fever. Ann. New York Acad. Sci. 856, 90–94 47 Cesare, P. et al. (1999) Ion channels gated by heat. Proc. Natl. Acad. Sci. U. S. A. 96, 7658–7663 48 Farber, N.E. et al. (1995) Halothane modulates thermosensitive hypothalamic neurons in rat brain slices. Anesthesiology 83, 1241–1253 49 Poterack, K.A. et al. (1991) The effect of halothane on thermosensitive neurons in the preoptic region of the anterior hypothalamus in acutely instrumented cats. Anesthesiology 75, 625–633 50 Maingret, F. et al. (2001) The endocannabinoid anandamide is a direct and selective blocker of the background K+ channel TASK-1. EMBO J. 20, 47–54 51 Millar, J.A. et al. (2000) A functional role for the two-pore domain potassium channel TASK-1 in cerebellar granule neurons. Proc. Natl. Acad. Sci. U. S. A. 97, 3614–3618 52 Talley, E.M. et al. (2000) TASK-1, a two-pore domain K+ channel, is modulated by multiple neurotransmitters in motoneurons. Neuron 25, 399–410 53 Buckler, K. et al. (2000) An oxygen-, acid- and anaesthetic-sensitive TASK-like background potassium channel in rat arterial chemoreceptor cells. J. Physiol. 525, 135–142 54 Lopez-Barneo, J. (1996) Oxygen-sensing by ion channels and the regulation of cellular functions. Trends Neurosci. 19, 435–440 55 Peers, C. (1997) Oxygen-sensitive ion channels. Trends Pharmacol. Sci. 18, 405–408 56 Kim, D. and Clapham, D.E. (1989) Potassium channels in cardiac cells activated by arachidonic acid and phospholipids. Science 244, 1174–1176 57 Kim, D.H. et al. (1995) Arachidonic acid activation of a new family of K+ channels in cultured rat neuronal cells. J. Physiol. 484, 643–660 58 Ordway, R.W. et al. (1989) Arachidonic acid and other fatty acids directly activate potassium channels in smooth muscle cells. Science 244, 1176–1179 59 Ordway, R.W. et al. (1991) Stretch activation of a toad smooth muscle K+ channel may be mediated by fatty acids. J. Physiol. 484, 331–337 60 Franks, N.P. and Lieb, W.R. (1991) Stereospecific effects of inhalational general anesthetic optical isomers on nerve ion channels. Science 254, 427–430 61 Lauritzen, I. et al. (2000) Poly-unsaturated fatty acids are potent neuroprotectors. EMBO J. 19, 1784–1793 62 Reyes, R. et al. (2000) Immunolocalization of the arachidonic-acid and mechano-sensitive baseline TRAAK potassium channel in the nervous system. Neuroscience 95, 893–901 63 Kindler, C.H. et al. (2000) Localization of the tandem pore domain K+ channel TASK-1 in the rat. Brain Res. Mol. Brain Res. 80, 99–108 64 Lopes, C.M. et al. (2000) Proton block and voltage gating are potassium-dependent in the cardiac leak channel Kcnk3. J. Biol. Chem. 275, 16969–16978 65 Medhurst, A.D. et al. (2001) Distribution analysis of human twopore domain potassium channels in tissues of the central nervous system and periphery. Brain Res. Mol. Brain. Res. 86, 101–114. Note added in proof A recent study65 describes the distribution of human two pore domain K+ channels in tissues of the CNS and periphery.