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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS
Departments of 1Ophthalmology and Visual Sciences and 2Molecular and Integrative Physiology, University of Michigan, Ann Arbor, Michigan
Submitted 30 August 2007 ; accepted in final form 17 December 2007
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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patch clamp; Xenopus oocyte; mutagenesis
Biochemical, molecular biological, and biophysical evidence indicates that Kir7.1 channels are the basis for the mild inwardly rectifying K+ conductance in the RPE (21). Immunohistochemisty has demonstrated that Kir7.1 channels are abundantly expressed in the apical microvilli (29), where they operate in conjunction with Na+-K+-ATPase to regulate the concentration of K+ in the extracellular space that separates the RPE from the rod and cone photoreceptor outer segments (5). Previously, we showed that the Kir conductance of native bovine RPE cells is modulated by both extracellular pH (pHo) and intracellular pH (pHi) (30). Kir conductance in these cells is relatively independent of pHo in the range from 6.5 to 9.0, but it is strongly inhibited at pHo below 6.0. In contrast, the Kir conductance is highly sensitive to pHi within the physiological range: conductance is augmented by mild intracellular acidification but is inhibited by either strong acidification or alkalinization. A similar bell-shaped relationship between conductance and pHi was obtained when the charge carrier was Rb+, which increases Kir7.1 channel conductance, suggesting that this pH sensitivity reflects a property of Kir7.1 channels.
In the present study, we investigated the sensitivity of cloned human Kir7.1 channels expressed in Xenopus oocytes to changes in extracellular and intracellular pH. We find that the macroscopic Kir7.1 conductance in intact oocytes is relatively insensitive to pHo in the range 6.0–9.0 but is inhibited when the extracellular solution is acidified to pHo < 6.0. Using excised inside-out macropatches, we demonstrate that the Kir7.1 conductance is maximally active at about pHi 7.0 and is inhibited by either alkalinization or strong acidification. Site-directed mutagenesis of histidine residues in the NH2 and COOH termini followed by functional analysis led to the identification of a histidine residue in the NH2 terminus that is critical to the pHi sensitivity of Kir7.1 channels.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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All procedures with animals were designed to minimize pain and suffering and conformed to the guidelines of the National Institutes of Health. The University of Michigan Committee on the Use and Care of Animals reviewed and approved the protocols for animal use. Xenopus laevis oocytes were surgically removed from deeply anesthetized (topical 0.15% tricane methane sulfonate) adult females, and they were defolliculated by incubating clusters of oocytes in 0.2% collagenase (type IV, Sigma Chemical, St. Louis, MO) in calcium-free ND96 solution (96 mM NaCl, 2 mM KCl, 1 mM MgCl2, 10 mM Na-HEPES, 300 µg/ml gentamycin, 300 µg/ml anakacin, and 550 µg/ml sodium pyruvate, pH 7.4). Healthy-looking stage V–VI oocytes were injected with 10–20 ng wild-type or mutant Kir7.1 cRNA in 50 nl DEPC-treated water, and maintained at 18°C in incubation solution plus 1 mM CaCl2 for up to 72 h before recording was performed.
Solutions. The standard bath solution for recording intact oocytes (ND96) consisted of (in mM) 96 NaCl, 2 KCl, 10 Na-HEPES, 1.0 CaCl2, and 1.0 MgCl2, pH 7.4. In some experiments, NaCl was replaced with KCl (98 mM K+ solution) or both NaCl and KCl were replaced with RbCl (98 mM Rb+ solution). To test the effect of changes in extracellular pH, we used the membrane-impermeable buffer phthalate (10 mM) in place of HEPES and titrated with either HCl or NaOH to the desired pH. To prevent channel rundown in excised macropatches due to depletion of membrane phosphatidylinositol 4,5-bisphosphate (PIP2) (18), the cytoplasmic side was bathed with a modified FVPP solution containing (in mM) 100 KCl, 5 HEPES, 5 EGTA, 4 NaF, 3 Na3VO4, and 10 Na2P2O7. Poly-L-lysine (4–15 kDa) and other chemicals were purchased from Sigma Chemical.
Solutions were delivered to the recording chamber (
300-µl volume) by gravity feed at a rate of 2 ml/min and were changed by a combination of six-way and slider valves. In experiments measuring the time course of poly-L-lysine-induced channel rundown, the solution outside the excised patch was rapidly changed with a perfusion fast-step device (model SF-77B, Warner Instruments, Hamden, CT).
Electrophysiology.
Whole cell currents were recorded using the two-electrode voltage-clamp technique (23). Microelectrodes, pulled from thick-wall borosilicate glass (outer diameter = 1.0 mm; inner diameter = 0.5 mm) with a multistage programmable puller (Sutter Instruments, San Rafael, CA) and having an impedance of 0.5–1.5 M
when filled with 3 M KCl, were used as voltage-sensing and current-passing electrodes. Signals from the current-passing electrode were amplified with a GeneClamp 500 amplifier (Molecular Devices, Sunnyvale, CA) with the built-in low pass filter set to 0.5 or 1 kHz and were stored in the computer for later analysis. Data acquisition and analysis were performed with pCLAMP 8.0 or 9.0 software (Molecular Devices).
For excised-patch recordings, the vitelline membrane was manually removed after oocytes were exposed to a hypertonic solution [400 mosmol/kgH2O: 220 mM N-methyl-D-glucamine (NMDG)-Cl, 5 mM EGTA-KOH, 1 mM MgCl2 and 10 mM HEPES] for 5–10 min. Patch pipettes were pulled from 7052 glass tubing (outer diameter = 1.65 mm; inner diameter = 1.2 mm, Garner Glass, Claremont, CA), heat polished to a resistance in the range 0.5–1.5 M, and filled with pipette solution containing 10 mM HEPES-NMDG-free base and 98 mM RbCl (pH 7.4). Pipette tips were sealed on the plasma membrane close to the site of cRNA injection to obtain maximal current density (13). Currents were recorded with a GeneClamp 500 amplifier at a sampling rate of 2 kHz with the built-in low-pass filter set to 0.5 kHz.
Voltage-clamp protocols and analysis. Current-voltage (I-V) relationships were determined from currents elicited by either a voltage-step or voltage-ramp protocol. In the voltage-step protocol, the membrane potential was held at 0 or –100 mV and was stepped for 0.5–1 s to voltages ranging from +50 to –150 mV in 10-mV steps. In the voltage-ramp protocol, the membrane potential was held at 0 mV and was ramped every 5 to 15 s from +50 to –150 mV over a 1-s period. Currents elicited by repeated voltage ramps were used to reconstruct the time course of changes in current at specific voltages.
Measurement of intracellular pH in oocytes.
Intracellular pH was measured with pH-sensitive microelectrodes essentially as described by Leipziger et al. (11). Intracellular pH measurement involved simultaneous intracellular recordings with a pH-sensitive microelectrode and a conventional microelectrode. To make pH-selective microelectrodes, thick-wall borosilicate glass tubing was pulled as described above, baked in a glass petri dish on a hot plate at 120°C for 30 min, and then silanized by adding a few drops of hexamethyldisilazane (Fluka, Ronkonkoma, NY) into the petri dish and baking for an additional hour. The tips of the silanized microelectrodes were filled with a drop of hydrogen ionophore II, cocktail A (Fluka), and were back filled with a phosphate buffer containing (in mM) 40 KH2PO4, 23 NaOH, and 150 NaCl. pH-sensitive microelectrodes were calibrated with phosphate buffers titrated to pH 6.5, pH 7.5, and pH 8.5 and had slopes in the range 54–60 mV. The pH-sensitive and conventional microelectrodes were connected to a high-impedance amplifier (Axoprobe 1A, Molecular Devices) and were adjusted for offsets in the perfusion solution before oocyte impalement. pHi was calculated from the relationship:
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VH/pH unit). Statistical analysis. Data are given as means ± SE and represent four or more measurements made from two more batches of oocytes. Statistical significance was calculated by ANOVA or the t-test, as appropriate. Data were fitted empirically to the Hill equation with the use of computer software (SigmaPlot version 8.0, SPSS, Chicago, IL).
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Histidine 26 in the NH2 terminus is critical for Kir7.1 channel activity at acidic pHi. In an attempt to identify residues involved in the pHi sensitivity of Kir7.1, we performed site-directed mutagenesis on potentially titratable residues of histidine, an amino acid whose side chain pKa (6.04) is closest to the optimum pHi for Kir7.1 channel activity. Six histidines—histidine 26 (H26), H180, H225, H255, H264, and H296 (Fig. 3A) —were targeted on the basis of their locations in the NH2 and COOH termini that are presumed to lie in contact with the cytoplasm and their lack of conservation among the majority of members of the Kir channel family. Point mutants with alanine substituted for histidine at position 225, 259, 264, or 296 gave rise to whole cell currents in oocytes bathed in ND96 that were similar to those of wild-type Kir7.1 in terms of amplitude, degree of rectification, and kinetics (data not illustrated). Also in common with wild-type Kir7.1, H225A, H259A, H264A, and H296A channels exhibited weak dependence of current on external K+ and roughly a 10-fold increase in inward current in the presence of high external Rb+ (Fig. 4). Thus, none of these four mutations appeared to have altered permeation properties. The amplitudes of Rb+ current associated with the H259A, H264A, and H296A channels were not significantly different from that of wild-type Kir7.1 (P > 0.05, t-test). In the case of the H225A, however, Rb+ current was significantly larger (P < 0.001, t-test), nearly twice that associated with Kir7.1. It is unclear whether this difference reflects enhanced trafficking of H225A to the plasma membrane or alterations in gating.
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10 min and reversible. In the absence of butyrate, acidification of phthalate-buffered ND96 did not alter pHi (Fig. 5D, bottom). The effect of intracellular acidification on wild-type and mutant Kir7.1 channel activities was determined by comparing currents recorded in ND96 and after 10 min of exposure to butyrate solution at pH 6.4. Exposure of oocytes expressing wild-type Kir7.1 channels to pH 6.4 butyrate solution increased both inward and outward currents (Fig. 5A); in 11 cells, the butyrate-induced increase in current at –150 mV averaged 27.5 ± 3.0% (Fig. 5E). By comparison, bath acidification to pH 6.4 in the absence of butyrate increased inward wild-type Kir7.1 current by only 16.4 ± 1.8% (n = 12; P < 0.05, t-test; Fig. 5E), indicating that a component of the butyrate-induced current activation resulted from intracellular acidification. Similar experiments were performed on the mutant channels. For the H225A, H259A, H264A, and H296A mutants, exposure to pH 6.4 butyrate solution resulted in an increase in inward current that was significantly (P < 0.05, t-test) larger than that produced by pH 6.4 phthalate solution (Fig. 5E). In contrast, when the H26A and H26R mutants were tested, intracellular acidification with pH 6.4 butyrate solution inhibited both inward and outward current (Fig. 5, B and C). The butyrate-induced decrease in inward current at –150 mV averaged –26.6 ± 3.8% for H26A and –38.5 ± 5.4% for H26R (Fig. 5E), suggesting a role for H26 in pHi-induced modulation of Kir7.1 channel activity. Experiments with inside-out macropatches revealed that the substitution of H26 with alanine eliminated the requirement of protons for channel activation, resulting in spontaneous activity at alkaline pHi, and increased sensitivity to proton-induced inhibition. Consistent with the results obtained in whole cell recordings, the amplitude of inward Rb+ current associated with H26A channels in macropatches bathed with pHi 7.4 solution was smaller on average than that mediated by wild-type Kir7.1 channels (–486 ± 63 pA at –125 mV for H26A vs. –1,511 ± 105 pA for wild-type Kir7.1; n = 10–12; P < 0.02, t-test). Figure 6A shows the time course of changes in inward H26A current produced by switching the pH of the solution perfusing the intracellular surface of a macropatch from pHi 7.4 to various test pHi values. Although the current amplitude in this macropatch was larger than average, the responses to changes in pHi are representative of H26A. In contrast with wild-type Kir7.1 current, inward H26A current was activated by alkalinization from pHi 7.4 (Fig. 6B) and was inhibited by acidification (Fig. 6C). Similar results were obtained in six other macropatches containing H26A channels and are summarized in Fig. 6D, which plots normalized inward H26A current as a function of pHi. H26A current was saturated at about pH 8.5 and was decreased monotonically in response to acidification. The mean data were fitted to the Hill equation, which gave an apparent pKa value of 7.71 and Hill coefficient of 1.4.
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H26 and strength of Kir7.1 channel-PIP2 interaction. Amino acid sequence comparison of the NH2 termini of Kir7.1 and other Kir channels revealed that Kir2.1 has a histidine (H53) at the position corresponding to H26 in Kir7.1 (Fig. 3B). Other investigators have shown that H53 is one of several residues that contribute to strong PIP2-channel interactions in Kir2.1 (14). Mutation of H23 to glutamine dramatically decreased the apparent binding affinity between Kir2.1 channels and membrane PIP2 (14), which stems from electrostatic interactions between positively charged residues in the channel protein and the negatively charged phosphates of PIP2. In other studies, mutation of a residue involved in PIP2 binding was found to shift the pH sensitivity of ROMK1 (Kir1.1) toward more alkaline pH (12). Because Kir7.1 channels are also gated by membrane PIP2 (18), it seemed possible that the mutation of H26 to alanine may alter sensitivity to proton-induced inhibition by modifying PIP2-channel binding affinity.
To test this possibility, we assessed the influence of pHi on PIP2-Kir7.1 affinity by measuring the rate of current rundown induced by poly-L-lysine, which competes with Kir7.1 for binding to the negatively charged phosphates of PIP2. This method has been widely used in the study of PIP2 interaction with other Kir channels and has been validated by comparison to alternative functional and biochemical approaches (4, 14, 18). The results of a representative experiment are depicted in Fig. 7A, which shows the superimposed records of inward Kir7.1 current rundown induced by exposure to poly-L-lysine (300 µg/ml) in FVPP solution titrated to pH 6.0, pH 7.4, or pH 8.0. The poly-L-lysine-induced rundown was rapid and complete within several seconds at each of the test pHi values. Similar results were obtained in seven other patches and are summarized in Fig. 7B, which shows that the half-time of poly-L-lysine-induced rundown of Kir7.1 current was similar at pH 6.0, 7.4, and 8.0. This finding indicates that the binding of Kir7.1 channels to PIP2 is not influenced by pHi. To investigate the role of H26 in PIP2-Kir7.1 channel binding affinity, we measured the rate of poly-L-lysine-induced rundown of H26A and H26R currents. These experiments were performed at pHi 8.0 to evoke maximal baseline currents before poly-L-lysine exposure. For both mutants, the half-time of rundown was not significantly different from that obtained for wild-type Kir7.1 (Fig. 7C), indicating that H26 is not involved in PIP2-Kir7.1 channel interactions.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Effects of extracellular pH. Extracellular acidification from pHo 7.4 to pHo values in the range 7.0–6.0 elicited a modest activation of Kir7.1 current in intact oocytes, but further acidification to pHo 5.5 produced strong inhibition. These results are consistent with the idea that protons act at two separate regulatory sites on the extracellular face of the Kir7.1 channel, one with a threshold of about pHo 7.0 that triggers activation and the other with a threshold of about pHo 6.0 that causes inhibition. It could be argued that the effects of extracellular acidification may be mediated by secondary changes in intracellular pH acting on pH-sensitive regulatory sites located on the cytoplasmic face of the channel, but this possibility can be ruled out because extracellular pH was manipulated with the impermeant buffer phthalate, which recordings with pH-sensitive microelectrodes showed produced no change in intracellular pH (see Fig. 5D). Thus we conclude that the inhibition of Kir7.1 channels by extracellular acidification is likely a direct effect of external protons.
Effects of intracellular pH. The pHi dependency of other Kir channel subtypes has been investigated in intact oocytes using CO2 (26), HCO3– (3), or weak acids such as acetate (19) or butyrate (11) to acidify intracellular pH. We used the butyrate method, because this gave more reproducible results in our hands. Exposing oocytes to 3 mM butyrate at pHo 6.4 in the presence of 2 mM external K+ acidified the cytoplasm from pHi 7.2 to pHi 6.8 and produced a 28% increase in Kir7.1 current, which is significantly larger than the 16% increase produced by exposure to pHo 6.4 solution buffered with phthalate, a maneuver that did not alter pHi. Together, these results indicate that mild acidification of the cytoplasm in the physiological range activates Kir7.1 channels.
Results obtained from inside-out macropatches revealed that the pHi regulation of Kir7.1 is complex. Intracellular alkalinization from pHi 7.4 caused a relatively rapid inhibition of Kir7.1 current that was reversible. In contrast, when the cytoplasmic pH was acidified to pHi 6.0 or 5.5, Kir7.1 current underwent a transient activation followed by inhibition. The bell-shaped relationship between steady-state Kir7.1 current and pHi could be fitted by the sum of two Hill equations, suggesting that there are two components to pHi-dependent modulation of Kir7.1 channels: proton-induced activation with an apparent pKa of 8.3 and proton-induced inhibition with an apparent pKa of 6.5. This pHi-sensitivity profile is qualitatively similar to what we determined previously for the inwardly rectifying K+ channel in intact bovine RPE cells (30), a channel that molecular, immunohistochemical, and functional evidence indicates is mediated by Kir7.1 (9, 21, 29). It is also in general agreement with the data of Jiang et al. (5a), who previously reported a biphasic response of Kir7.1 channels to intracellular acidification with pKa values for proton-induced activation and inhibition of 7.7 and 6.7, respectively. Although the pKa value for acid-induced inhibition reported in the previous study is nearly identical to that obtained in the present study, the pKa value for acid-induced activation determined from our data is more alkaline. Another difference between the two studies is the degree of inhibition at strong acidic pHi: in the study of Jiang et al. (5a), Kir7.1 channels were nearly completely inhibited at pHi 6.0, whereas we observed <50% inhibition at this pHi. The reasons for these discrepancies are unclear but may be related to differences in experimental conditions.
We used Rb+ as the permeant cation in our excised-patch experiments because macroscopic Rb+ currents through Kir7.1 channels are 10-fold larger than K+ currents (21, 25). Studies by other groups have shown that the intracellular pH sensitivity of another Kir channel, ROMK1, is influenced by external monovalent cations (19, 20). Because of low-current amplitude, we were unable to study systematically Kir7.1 currents in excised patches under more physiological conditions with ND96 (2 mM K+) in the pipette. Hence, the present experiments do not allow us to specify whether the pHi sensitivity of Kir7.1 is affected by extracellular cations. It is worth noting, however, that native Kir7.1 channels in intact bovine RPE cells bathed with 5 mM K+ Ringer solution displayed a pHi sensitivity profile that was similar to that obtained with 135 mM Rb+ solution in the bath (30).
Role of H26. As a first step toward identifying the pHi sensor of Kir7.1, we individually mutated six nonconserved histidine residues in the NH2 and COOH termini to alanine and tested these mutants for functional changes in intact Xenopus oocytes. We found that the substitution of alanine for histidine at position 26 resulted in whole cell currents that were of smaller amplitude than those of wild-type Kir7.1 and that exhibited inhibition rather than activation in response to mild intracellular acidification. Measurements in inside-out macropatches showed that H26A current was a sigmoidal function of pHi, with maximal current at alkaline pHi and inhibition at acidic pHi with an apparent pKa value of about pH 7.7. Similar results were obtained with H26R channels, except in this mutant the apparent pKa value for acid-induced inhibition was shifted in the acidic direction to about pH 7.1, resulting in whole cell currents at resting pHi that were of comparable amplitude to those of wild-type Kir7.1. Thus, the replacement of histidine at position 26 with either alanine or arginine eliminated the requirement of protons for channel activation and increased sensitivity to proton-induced inhibition. This suggests that H26 plays an important role in the activity of Kir7.1 channels at neutral and mildly acidic pHi, but it is unclear whether this residue acts as a pH sensor or is a link between the pH sensor and the physical gate.
In addition to affecting proton-induced activation, H26 also appears to modulate proton-induced inhibition. The apparent pKa value for proton-induced inhibition was 7.1 for H26R and 7.7 for H26A. Thus, positive charge at position 26 seems to decrease sensitivity to inhibition by acidic pH, perhaps by influencing the environment of the pH sensor. If H26 were titratable, an intermediate pKa for proton-induced inhibition may be expected in wild-type Kir7.1 channels, but instead a more acidic value of 6.6 was obtained.
ROMK1 channels are inhibited by intracellular acidification and are dependent on direct interaction with membrane PIP2 for activation. Mutation of an arginine in the COOH terminus (A188) disrupted PIP2-ROMK1 interaction and increased the sensitivity to pHi (12). Amino acid sequence alignment of the NH2 termini of Kir7.1 and other Kir channels (Fig. 3B) revealed that Kir2.1 has a histidine (H53) at the position equivalent to H26 in Kir7.1. In a scanning-mutagenesis study of NH2 and COOH termini amino acids influencing the interaction between Kir2.1 and PIP2, Lopes et al. (14) identified H53 as one of 12 positively charged amino acids that contribute to strong PIP2-Kir2.1 channel binding. Like other members of the Kir channel family, Kir7.1 is gated by PIP2, but PIP2-Kir7.1 channel binding affinity is weaker than it is for Kir2.1 and most other Kir channels (18). Nevertheless, we considered the possibility that H26 may contribute to PIP2-Kir7.1 interaction and that disruption of this interaction by the H26A mutation may contribute to altered pH sensitivity. Using the poly-L-lysine method to assess the strength of PIP2-channel binding, we found no evidence for an influence of pHi on the interaction of between membrane PIP2 and wild-type Kir7.1 channels. Moreover, the apparent PIP2 binding affinities of the H26A and H26R mutants did not differ from that of wild-type Kir7.1. Together, these results indicate that H26 is not involved in PIP2-Kir7.1 channel interactions and points to some other explanation for the difference in pH sensitivity between the H26A and H26R mutants.
Comparison with other pH-sensitive Kir channels. Intracellular acidification inhibits several members of the Kir channel family, including Kir1 (3, 11), Kir2.3 (17, 31), Kir4.1 (22, 28), and heteromeric Kir4.1-Kir5.1 channels (24). None of the histidine or lysine residues identified as contributing to the pH-sensing domains of these channels is conserved in Kir7.1. Superficially, the pHi sensitivity profile of Kir7.1 resembles that of Kir6 channels, which, in excised patches, exhibit activation by mild acidification and inhibition by strong acidification (7, 10). Studies on Kir6.2 (26) have shown that channel activation is the direct result of proton binding to a histidine residue (H175) that is conserved in Kir6.1 and Kir6.2 among different species but not in other members of the Kir channel family, including Kir7.1. Other histidine residues in the COOH terminus (H186, H193, and H216) were implicated in the inhibition of Kir6.2 channels at extremely low pH (27), and these, too, are absent at equivalent positions in Kir7.1. The region(s) of Kir7.1 that determine proton-induced inhibition remain to be determined.
In summary, we have characterized the extracellular and intracellular pH sensitivity of Kir7.1 and demonstrated the requirement of a histidine residue in the NH2 terminus of the channel protein for its distinct bell-shaped intracellular pH sensitivity profile. The presence of histidine at position 26 appears to be important for channel activity at physiological pHi, and its replacement with alanine or arginine results in spontaneous activity at alkaline pH and renders the channel more susceptible to inhibition by protons. The distinct pH sensitivity of Kir7.1 implies that K+ conductance in epithelia expressing this channel, such as choroid plexus and RPE, will be increased by mild acidification but inhibited by strong acidification. Because Kir7.1 channels are functionally coupled to the Na+-K+ pump, Kir7.1 channel inhibition could be an important mechanism for diminished Na+-K+-ATPase and transport function in some epithelia during ischemia, hypoxia, or hypercapnia.
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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. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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