From the Molecular Medicine Laboratories, Institute for Drug Discovery Research, Yamanouchi Pharmaceutical Co., Ltd., 21 Miyukigaoka, Tsukuba, Ibaraki, 305-8585, Japan
Received for publication, July 9, 2002 , and in revised form, May 16, 2003.
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ABSTRACT |
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INTRODUCTION |
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The most recently identified K+ channels were two-pore domain K+ channels, which have four transmembrane domains (TM14), two pore-forming domains (P1, P2), cytoplasmic N- and C-terminal regions, and an extended extracellular loop between TM1 and P1, which are evolutionarily conserved (7). Especially, the pore-forming domain has a consensus amino acid motif of TXG(Y/F)G. In mammals the first gene identified as coding for the two-pore domain K+ channel was named TWIK-1, and to date 14 genes have been isolated as members of the subfamily (825). In addition to mammals, two-pore domain K+ channels have been isolated and characterized in Drosophila, Caenorhabditis elegans, and plants (2628). Most clones of these genes form functional K+ channels when expressed in Xenopus oocytes or in mammalian cell lines and exhibit the properties of a background K+ current. Functional characterizations show that these channels have the following sensitivities: (i) mechanical stress, TREK-1, TREK2, and TRAAK; (ii) changes in pH, TASK-1, TASK-2, TASK-3, THIK-1, TREK-2, TALK-1, and TALK-2; (iii) heat stress, TWIK-2, TREK-1; (iv) fatty acids, TASK-1, TASK-3, TREK-1, TREK-2, TRAAK, and THIK-1 (824). However, it is not clear whether these sensitivities are involved in cellular function.
The human genome project was completed in 2001, and the genome sequence is now open to the public (29, 30). As a result, many genes whose existence had not previously been reported have now been isolated by using the human draft sequencing data base. Many channel genes have been isolated and analyzed for their cellular mechanisms. This has been especially true for two-pore domain K+ channel genes. In this paper we describe the cloning of a novel two-pore domain K+ channel, TRESK, from the human draft sequencing data base and describe its tissue distribution and electrophysiological and pharmacological properties.
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EXPERIMENTAL PROCEDURES |
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Finally, to verify that human TRESK cDNA including the entire open-reading frame could be cloned from an independent source, human spinal cord poly(A)+ RNA (Clontech) was used for the reverse transcription (RT)1-PCR. The RT reaction was primed with random hexamer. The primers for PCR were 5'-TCAGGGACGATGGAGGTCT-3' and 5'-AAAACCAGCTCAGCCAGTG-3'. PCR was performed for 35 cycles at 98 °C for 15 s, 60 °C for 30 s, and 72 °C for 2 min with Taq polymerase (TAKARA). The amplified fragment was subcloned into pCR-TOPO vector sequenced on both strands.
RT-PCR AnalysisMultiple human poly(A)+ RNA (Clontech) was reverse-transcribed to generate first-strand cDNA with an Advantage RT-for-PCR kit (Clontech). PCR was carried out with this first-strand cDNA, corresponding to 5 ng of poly(A)+ RNA. PCR primers were designed from the human TRESK cDNA sequence, 5'-TGCTTTCTGGTGACCTACGC-3' and 5'-GCTGCACAGGAAATGTAGGC-3'. PCR was performed for 45 cycles at 98 °C for 15 s and 70 °C for 1 min using Taq polymerase. The PCR products were run on 1.5% polyacrylamide gels. Ethidium bromide-stained gels were examined under UV illumination. Amplified fragments were subcloned into the pCR-TOPO vector and sequenced on both strands to confirm the PCR product as human TRESK. As a control, glyceraldehyde-3-phosphate dehydrogenase was amplified using specific primers (Clontech).
Transfection of TRESKThe previously amplified cDNA of TRESK was cloned into the pcDNA3.1 vector (Invitrogen). L929 cells were transfected with TRESK cDNA for electrophysiological studies as described elsewhere (3133). Cells were co-transfected with the TRESK expression vector described previously and the green fluorescent protein expression vector phGFP S65T (Clontech) using FuGENE 6 (Roche Applied Science). The cells were cultured in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum. Transfected cells were identified by observing green fluorescent protein fluorescence with an epifluorescence microscope. Electrophysiological studies were carried out 13 days after transfection.
Electrophysiological StudyElectrophysiological recordings of TRESK transfectants were performed using a voltage-clamp technique (34). Recordings were made with an Axopatch 1D amplifier (Axon Instruments) using patch electrodes with a resistance of 35 megaohms. The internal pipette solution contained 149 mM KCl, 1.8 mM MgCl2, 4.5 mM EGTA, and 9 mM HEPES-K (pH 7.3). The external solution contained 149 mM NaCl, 5 mM KCl, 2 mM MgCl2, and 10 mM HEPES-Na (pH 7.3). For the inside-out patch experiments, both the pipette and bath solutions contained 150 mM KCl, 2 mM MgCl2, and 10 mM HEPES-K (pH 7.3). All recordings were performed at room temperature (22 °C). Analysis was carried out on a personal computer using pCLAMP6 software (Axon Instruments). All values are expressed as means ± S.E. Statistical significance was tested with Student's t test.
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RESULTS |
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Searching the GenBankTM data base with the BLAST sequence alignment program revealed that the full amino acid sequence of the new clone shows the greatest similarity to human TREK2, but even so, it still shares only 34% identity. Alignment of the amino acid sequence of the new clone with other two-pore domain K+ channels shows low homology, 13.519.7%. Also, phylogenetic analysis suggested that this clone has a common ancestor with previously reported two-pore domain K+ channels but represents an additional branch in this family of the channels (Fig. 2). These results indicate that this new clone is a novel two-pore domain K+ channel that is included in a novel subfamily. Thus, we named this new clone the TWIK-related spinal cord K+ channel, TRESK.
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Tissue Distribution of TRESKTo determine the tissue distribution of TRESK in humans, RT-PCR analysis was carried out using a specific primer set, one primer from the first exon and the other from the third exon of TRESK, with human multiple tissue poly(A)+ RNA. The expected 800-bp PCR product of TRESK, confirmed by sequencing, was amplified only in the spinal cord (Fig. 3). To be sure of this result, we repeated the same experiment with another specific primer set, one primer from the first exon and the other from the third exon of TRESK, and another set of human multiple tissue poly(A)+ RNA. Again, the expected PCR product was amplified only in the spinal cord (data not shown). Northern blot analysis was performed using human multiple tissue blots (MTN Blots, Clontech), but no specific signal was detected in the 23 tissues (brain, heart, skeletal muscle, colon, thymus, spleen, kidney, liver, small intestine, placenta, lung, peripheral blood leukocytes, adrenal gland, bladder, bone marrow, lymph node, mammary gland, prostate, spinal cord, stomach, thyroid, trachea, and uterus) (data not shown). The reason is thought to be that the TRESK transcript level is very low and/or expressed in a specific region of the spinal cord.
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Basic Electrophysiological Properties of TRESKTo characterize the electrophysiological properties of TRESK using the whole-cell voltage clamp technique, L929 cells were transiently transfected with the TRESK expression vector. The cells were clamped at a holding potential of 80 mV and were depolarized to voltages between 120 and +100 mV. In TRESK-transfected cells, large currents were recorded (Fig. 4A) that were not present in the control cells (data not shown). The activation kinetics of TRESK current was rapid. Depolarization steps induced a two-step current composed of large instantaneous and delayed components (Fig. 4A). The ion selectivity of TRESK was studied by changing the concentration of K+ in the external solution from 5 to 150 mM while maintaining the pipette [K+] constant at 150 mM. The current-voltage (I-V) relationship is outwardly rectifying, and almost no inward currents were recorded in an external medium containing 5 mM K+ (Fig. 4B). The plots of the reversal potential as a function of [K+] showed that the slope was 49.0 ± 0.8 mV/decade (n = 3), which is close to the calculated Nernst value of 58 mV, as expected for a highly selective K+ channel (Fig. 4C). As previously reported for other two-pore domain K+ channels, the membrane potential of TRESK-expressed L929 cells with a high current density was strongly polarized (Fig. 4D). In contrast, the membrane potential of TRESK-expressed cells that had a low current density was not polarized (Fig. 4D).
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Pharmacological Properties of TRESKThe effect of various pharmacological agents on currents elicited by voltage pulse to 0 mV have been studied in TRESK-transfected cells using whole-cell recording. The classical K+ channel blocker, Ba2+, applied extracellularly at 3 mM inhibited TRESK current. This inhibition was gradually released by depolarization to voltages from 20 to +80 mV (Fig. 5A). To clarify the inhibition by Ba2+, we tested the effect of Ba2+ at the start and end of pulses at different voltages. The inhibition by Ba2+ at the end of the pulse was significantly decreased at depolarized potentials. On the other hand, the inhibition at the start of the pulse was not affected by the depolarization (Fig. 5B). These results suggest that Ba2+ acts as an open channel blocker. TRESK was insensitive to 4-aminopyridine (non-selective K+ channel blocker), paxilline, penitrem A (Ca2+-activated K+ channel blocker), tolazamide, and glipizide (ATP-sensitive K+ channel blocker) when applied extracellularly at 100 µM. Propafenone, glyburide (ATP-sensitive K+ channel blocker), lidocaine (Na+ channel blocker), quinine, quinidine, and triethanolamine (non-selective K+ channel blocker) inhibited TRESK currents by 95.3 ± 1.8% (n = 3), 76.8 ± 8.6% (n = 4), 75.7 ± 5.4% (n = 5), 75.2 ± 7.1% (n = 4), 90.6 ± 1.5% (n = 4), and 34.4 ± 1.0% (n = 3), respectively (Fig. 5C). Because these compounds were reported to inhibit K+ channels, the results suggest that TRESK has the property of K+ channels.
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Inhibition of TRESK by Free Fatty AcidsOutside-out patches were used to test whether TRESK is sensitive to free fatty acids. Arachidonic acid, which is a polyunsaturated free fatty acid, inhibited TRESK current when applied to the extracellular side of outside-out patches (Fig. 6A). We also tested whether arachidonic acid could inhibit TRESK from the cytoplasmic side of the membrane. In inside-out patches, intracellular application of arachidonic acid inhibits TRESK current (Fig. 6B). This inhibition was expressed in a concentration-dependent manner (Fig. 6C). The onset of inhibition was generally rapid (within 1 min), and steady-state inhibition was observed within 1 min after application. The relative channel current was plotted as a function of the arachidonic acid concentration in Fig. 6D. With the reasonable assumption that 50 µM arachidonic acid produces maximal inhibition of TRESK, the averaged data were fit to a Hill equation of the form y = 1/(1 + (K1/2/[arachidonic acid])n), where K1/2 is the apparent concentration of arachidonic acid that produces half-maximal inhibition, and n is the Hill coefficient (apparent K1/2 = 6.6 µM, Hill coefficient = 1.3, n = 3). We tested whether other free fatty acids, i.e. saturated and unsaturated free fatty acids, were also able to inhibit TRESK current. When applied to the intracellular side, unsaturated free fatty acids such as docosahexaenoic acid, linoleic acid, and oleatic acid significantly inhibited TRESK current at 20 µM (Fig. 6, E and F). Saturated free fatty acids such as palmitic acid and stearic acid had almost no effect on TRESK current at 20 µM (Fig. 6, E and G, n = 3 and 4, respectively). These results show that unsaturated free fatty acids inhibit TRESK current.
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Regulation of TRESK Current by pHWe tested whether TRESK is sensitive to changes in the extracellular and intracellular pH. We first used whole-cell recording to test whether TRESK is sensitive to changes in the extracellular pH. The cells were clamped at a holding potential of 40 mV and then depolarized to voltages between 80 and +80 mV. TRESK currents were measured at different extracellular pH values (Fig. 7A). Changes in the extracellular pH from 7.3 to 5.6 and 8.9 resulted in inhibition (80 ± 7%, n = 3) and enhancement (120 ± 5%, n = 3) of TRESK currents, respectively, from the basal channel activity observed at pH 7.3 (Fig. 7B). Thus, the TRESK channel is sensitive to changes in the extracellular pH, acid, and alkali, but those sensitivities are not remarkable compared with other K+ channels. To test the effect of changes in the intracellular pH, we used inside-out patches. Fig. 7C shows that the TRESK current was remarkably inhibited by a change in the intracellular pH from 7.3 to 5.6. Changes in the intracellular pH from 7.3 to 5.6 and 8.9 resulted in inhibition (39 ± 4%, n = 3) and enhancement (140 ± 12%, n = 3) of TRESK currents, respectively, from the basal channel activity observed at pH 7.3 (Fig. 7D). These effects, especially at pH 5.6, were greater than those of the extracellular pH. These results show that TRESK is much more sensitive to changes in intracellular pH than extracellular pH.
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We also tested whether TRESK is sensitive to changes in the temperature. Several two-pore K+ channels were also reported to be sensitive to changes in the temperature (16, 19). We recorded the TRESK current in whole-cell recording at 15, 22, 39, and 43 °C. Lowering the temperature from 22 to 16 °C had no effect on the TRESK current at 40 mV (102.7 ± 11.5%, n = 3). Raising the temperature from 22 to 39 or 43 °C also had no effect on the TRESK current (106.3 ± 5.6%, n = 3; 118.8 ± 6.4%, n = 3, respectively). These results show that TRESK is insensitive to changes in temperature.
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DISCUSSION |
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Electrophysiological Behavior of TRESKElectrophysiological studies demonstrated many profiles of the TRESK channel. The reversal potential of the current induced by expression of TRESK-cDNA in mammalian cell lines followed the calculated K+ equilibrium potential when the external [K+] was changed. This suggests that the TRESK channel selectively permeates K+. The whole-cell current-voltage I-V relationship in a symmetric K+ solution showed that TRESK induced outward rectification. When TRESK is transiently expressed in mammalian cell lines, TRESK currents are observed at all membrane potentials, including the resting membrane potential. This channel activity is also seen in other two-pore domain K+ channels. The basal current that is present at the resting membrane potential is thought to drive the membrane potential toward the K+ equilibrium potential. TRESK current is activated instantaneously by a voltage step and shows no voltage threshold for activation. Moreover, the membrane potential of TRESK-expressing cells is strongly polarized. These properties suggest that TRESK may function as a background K+ channel that sets the resting membrane potential.
The pharmacological profiles of the TRESK channel are similar to those of previously reported two-pore domain K+ channels, which are inhibited by K+ channel inhibitors. We analyzed the detailed mechanism of the inhibition by Ba2+. The inhibition by Ba2+ at the end of the pulse was significantly decreased at depolarized potentials, but the inhibition at the start of the pulse was not affected. These properties suggest that Ba2+ inhibited the open state of TRESK, and this blockage was released by depolarized stimulus.
Free Fatty Acid and pH Sensitivity of TRESKTo test for functional similarities with other members of the two-pore domain K+ channel family, we studied modulation of the TRESK channel by various experimental interventions such as application of free fatty acids and changes in the pH.
Like TASK-1 and TASK-3, TRESK was inhibited by intracellular and extracellular application of arachidonic acid. This inhibition was shown by intracellular application of not only arachidonic acid but also other unsaturated free fatty acids, such as docosahexaenoic acid, linoleic acid, and oleatic acid. However, our studies also showed that saturated free fatty acids such as palmitic acid and stearic acid did not significantly affect the TRESK current. Thus, the TRESK channel is sensitive only to unsaturated free fatty acids, and it seems that there may be an unsaturated free fatty acid-selective site in the TRESK protein.
Furthermore, we demonstrated here that TRESK currents were inhibited and enhanced by extreme changes in the pH values from 7.3 to 5.6 and 8.9, respectively. Regulation of the channel activity by the pH is an important functional property of two-pore domain K+ channels. TRESK exhibited greater sensitivity to acidic conditions than alkaline conditions. Our studies also show that TRESK is more sensitive to changes in the intracellular pH than changes in the extracellular pH. This property has also been reported for TASK-3 and TREK-2 (18, 20).
Predicted Physiological Role of TRESK in the Spinal Cord Until now, some two-pore domain K+ channels were reported to be expressed in the spinal cord, but they were not specific to the spinal cord (11). On the other hand, the expression of TRESK mRNA is specifically localized in the spinal cord, indicating that TRESK may play a unique role in the spinal cord that is different from other two-pore domain K+ channels. Actually, we revealed that the TRESK channel is regulated by free fatty acids, which have been reported to participate in the function of the spinal cord. Spinal cord trauma is known to cause a marked release of free fatty acids, especially arachidonic acid, from the cell membrane. These released free fatty acids are suggested to lead to the secondary injury to spinal cord neurons that is observed in spinal cord trauma (35). However, the specific effects of released arachidonic acid in relation to the mechanisms of spinal cord injury remain to be established. One would suspect that Ca2+ influx, which is known to be one of the most common cellular signaling factors, is involved in spinal cord injury. Therefore, if injured neurons show TRESK currents, released free fatty acids may cause Ca2+ influx via a voltage-gated Ca2+ channel by depolarizing the membrane potential through changing the TRESK current.
In summary, we have isolated TRESK, an outward rectifying K+ channel that belongs to the two-pore domain K+ channel family. The functional properties of TRESK show that it is sensitive to unsaturated free fatty acids and extreme changes in extracellular and intracellular pH. Furthermore, its tissue distribution and basic electrophysiological properties suggest that TRESK may be involved in regulation of the resting membrane potential in spinal cord-specific cells.
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FOOTNOTES |
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* The costs of publication of this article were defrayed in part by the
payment of page charges. This article must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section 1734
solely to indicate this fact.
These authors contributed equally to this work.
To whom correspondence should be addressed. Tel.: 81-29-852-5111; Fax:
81-29-852-5444; E-mail:
sano.yorikata{at}yamanouchi.co.jp.
1 The abbreviations used are: RT, reverse transcription; TM, transmembrane
domain.
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ACKNOWLEDGMENTS |
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REFERENCES |
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