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CELLULAR |
Center for Cellular and Systems Electrophysiology,
1 Department of Gynecology and Obstetrics
2 Department of Physiology and Biophysics, School of Medicine and Biomedical Sciences, 124 Sherman Hall, State University of New York at Buffalo, Buffalo, NY 14214-3005, USA
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Abstract |
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(Received 26 June 2006;
accepted after revision 3 August 2006;
first published online 3 August 2006)
Corresponding author G. C. L. Bett: Department of Gynecology and Obstetrics, Department of Physiology and Biophysics, 124 Sherman Hall, State University of NY at Buffalo, Buffalo, NY 14214, USA. Email: bett{at}buffalo.edu
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Introduction |
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The electrical excitability of these tissues varies widely, as do the characteristics of the ionic currents associated with the KCNQ1 subunit identified in the native cells. Some tissues that contain KCNQ1 exhibit high electrical excitability, but some other KCNQ1-containing tissues, such as epithelia, exhibit little electrical excitability. Although tetramers of the alpha-subunit of the KCNQ1 ion channel can independently form a rapidly activating voltage-dependent potassium channel, when KCNQ1 is co-expressed with ancillary subunits, the behaviour of the channel is dramatically transformed, and can even become voltage insensitive (Sanguinetti et al. 1996; Melman et al. 2001).
Although the details of the interactions between the KCNQ1 channel and KCNE subunits are not fully understood, KCNE subunits clearly have an effect on the mechanism of gating. KCNE3 (MiRP2) appears to ‘lock’ the channel in an open position, resulting in a constitutively open channel (Melman et al. 2001). KCNE1 (MinK) appears to prevent the KCNQ1 from entering the inactivated state which is seen at positive potentials with pure KCNQ1 channels (Seebohm et al. 2005). In both cases, the presence of the subunit has greatly reduced or eliminated a particular gating transition.
The ability of KCNE subunits to alter gating behaviour and alter the allowable physical conformations of the channel can result in changes in the drug binding ability of the channels. Drug binding is strongly dependent upon channel conformational states, and frequently on the open state (Wang et al. 2003; Seebohm et al. 2003a; Bett & Rasmusson, 2004). In the same way that mutations that affect gating can alter drug binding (Wang et al. 1997; Bett & Rasmusson, 2004), ancillary subunits that alter the gating characteristics and conformational states of a channel can also alter drug–channel interactions (Bett et al. 2006). In this study, we used site directed mutagenesis to investigate the correspondence between subunit-mediated changes in the gating behaviour of KCNQ1 and changes in drug potency.
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Methods |
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(when filled with 3 M KCl) were fabricated from 1.5 mm o.d. borosilicate glass tubing (TW150-4, World Precision Instruments) using a two-stage puller (Kopf Instruments, CA, USA) and filled with 3 M KCl. The control extracellular solution (2 mM K+) contained (mM): 96 NaCl, 2 KCl, 1 MgCl2, 1.8 CaCl2, 10 Hepes, pH 7.4. Voltage clamp protocols used are described as appropriate in the text, and unless otherwise stated, raw two-electrode voltage clamp data traces were not leak or capacitance subtracted. Data were digitized and analysed using pCLAMP 6.0–9.2 (Axon Instruments). Further analysis was performed using Clampfit 9.2 (Axon Instruments), Excel (Microsoft Corp.) and Origin (Microcal Software Inc., MA, USA). Data were filtered at 2 kHz. Data are shown as mean ± S.E.M. Confidence levels were calculated using Student's t test and ANOVA.
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Results |
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The electrophysiological characteristics of the KCNQ1 alpha-subunit are dramatically altered by the presence of KCNE ancillary subunits. Figure 2 shows representative traces obtained from two electrode voltage clamp experiments on KCNQ1 channels, KCNQ1/KCNE1 channels, and KCNQ1/KCNE3 channels. A 2 s depolarizing pulse was applied from the holding potential of –80 mV to a range of voltages between –90 and +60 mV in 10 mV steps. This was followed by a 1.5 s pulse to –60 mV. The oocyte was then returned to the holding potential of –80 mV, and allowed to rest for 20 s before the next depolarizing pulse was applied. Figure 2 shows that the presence of ancillary subunits substantially altered the kinetics of the KCNQ1 current, as has been previously noted (Schroeder et al. 2000). Figure 2A shows that KCNQ1 channels, when expressed alone, exhibit rapid activation. At positive potentials, there was slight inactivation of the current. The presence of mild inactivation can be detected in the P2 pulse to –60 mV, where the tail current clearly increases before deactivation occurs, i.e. recovery from inactivation is readily observable. Co-expression of KCNQ1 channels with the ancillary subunit KCNE1 (KvLQT1/MinK) is thought to be the molecular basis of the slowly activating delayed rectifier current found in the heart, IKs (Sanguinetti et al. 1996). Figure 2B shows that the presence of the KCNE1 subunit alters the kinetics of gating. KCNQ1/KCNE1 channels have very slow activation, with a sigmoidal onset. The channel continues to activate slowly, and activation is not complete, even at the end of a 3 s pulse. The very slow increase in current may reflect the channel entering a second open state (Pusch et al. 2001). KCNQ1/KCNE1 channels do not display inactivation, even at very positive potentials. On repolarization to –60 mV, KCNQ1/KCNE1 channels deactivate.
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We determined the effect of the chromanol 293B on KCNQ1 channels, and how this interaction was modulated by the presence of ancillary subunits. 293B is an open pore blocker which is selective for KCNQ1 channels (Bleich et al. 1997). Figure 3 shows representative traces from two-electrode voltage-clamp experiments showing the effect of 293B on the channels. A 3 s depolarizing pulse was applied from the holding potential of –90 mV to a range of voltages between –90 and +40 mV. This was followed by a 1.5 s pulse to –60 mV. The oocyte was then returned to the holding potential of –90 mV, and allowed to rest for 30 s before the next depolarizing pulse was applied. Figure 3 shows representative traces in control and the presence of drug. KCNQ1 was the least responsive to chromanol 293B, with 10 µM only slightly reducing the magnitude of the current, and having no effect on the time course of channel gating. Figure 3A therefore shows the effect of 50 µM 293B on KCNQ1. KCNQ1/KCNE1 channels were more sensitive to chromanol. Application of 10 µM 293B not only reduced the current, but also altered the apparent time course of channel activation, but did not affect the threshold for voltage-dependent activation. This could be due to direct modification of gating, such as the inhibition of the slowly developing component of block. In theory, it could also occur by a coincidence of the development of a slow open channel block that exactly matches and cancels the slow component of activation. As discussed below, we favour the former explanation. The slowly developing component of activation was apparently reduced by 10 µM 293B, and the apparent time course of activation appeared to have reached a steady state at the end of a 3 s pulse. KCNQ1/KCNE3 currents were most sensitive to 293B, which substantially reduced the current in a time-independent manner. Data from the effect of 293B on multiple oocytes are summarized in Fig. 4.
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100-fold change in potency for the drug. Chromanol 293B is an open channel blocker, and the opening of KCNQ1 channels is time dependent. We therefore examined the time dependence of drug binding to KCNQ1 channels, and whether it was affected by the presence of ancillary subunits. Time-dependent changes in the fraction of current block during a step depolarization from –90 to +40 mV were calculated by determining the ratio of the current elicited during a 3 s pulse in the presence of drug to a 3 s control pulse, i.e. Iratio = 1 – Idrug/Icontrol, throughout the pulse duration. Figure 5A shows average current ratios recorded during a 3 s depolarization from –90 to +40 mV. When KCNQ1 channels were depolarized in the presence of 50 µM 293B, there was a rapid onset of 293B block, which is consistent with 293B being an open channel blocker. After the initial onset, the degree of block changed little over the 3 s depolarization. After the rapid onset of block as the channel opens, the initial KCNQ1 channel block was 40.0 ± 6.2% of control. This was similar to the final percentage of block, which was 43.5 ± 5.3% of control (n = 8). When KCNQ1/KCNE3 channels were depolarized in the presence of 1 µM 293B the block was apparent from the very beginning of the pulse, i.e. there was no rapid initial onset of block (Fig. 5C). This is consistent with the notion that KCNQ1/KCNE3 channels are constitutively open. The degree of block at the beginning of the depolarizing pulse was 61.6 ± 5.8% which is not significantly different from the block of 59.0 ± 5.2% of control observed at the end of the 3 s pulse (n = 5).
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The significant time dependence of chromanol 293B block on KCNQ1/KCNE1 channels raises interesting questions about how the interaction of the drug with KCNQ1 channels varies with stimulation frequency (Seebohm et al. 2001a). We therefore subjected KCNQ1 and KCNQ1/KCNE1 channels to different pacing protocols after a period of 30 s without stimulation. Figure 6 shows results from using a 500 ms depolarizing pulse applied from –90 to +50 mV with either a 5 s inter-pulse interval or a 500 ms inter-pulse interval. KCNQ1 channels were tested in the presence of 50 µM 293B and KCNQ1/KCNE1 channels were tested in the presence of 10 µM 293B as these two concentrations of drug result in a similar fraction of current block for the respective currents. Figure 6A shows that with a 5 s inter-pulse interval there was little change in the peak outward current elicited by repetitive depolarization, measured at the end of the 500 ms pulse, in the presence of 293B for either KCNQ1 or KCNQ1/KCNE1 channels. In addition, when a 3 s pulse was applied from –90 mV to +50 mV with a 3 s inter-pulse interval there was little change in the ratio of peak outward current with and without drug (data not shown). However, when the inter-pulse interval was reduced to 500 ms, there was a clear difference in the use dependence of 293B on the different channels (Fig. 6B). KCNQ1 channels showed no use dependence in the presence of 50 µM chromanol 293B with a 500 ms inter-pulse interval. Conversely, there was a clear and rapid use dependence to block of KCNQ1/KCNE1 channels by 10 µM 293B when stimulated at 1 Hz. The maximum current at the end of the first 500 ms depolarizing pulse in the presence of 10 µM 293B was 68.7 ± 5.0% of control (n = 4). However, the maximum current at the end of the 20th depolarization applied with a frequency of 1 Hz was only 43.0 ± 1.7% of control (n = 4). This shows that 293B interacts with KCNQ1/KCNE1 in a use-dependent manner.
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fast = 1.07 ± 0.04 s and slow = 7.66 ± 0.43 s (n = 4). The fast time constant is dominant, with a ratio of the amplitudes Afast/Aslow = 4.78 ± 0.16 (n = 4). In the presence of 10 µM 293B, the maximum KCNQ1/KCNE1 current is still increased by repetitive stimulation, but the degree of potentiation is reduced. The time course of the potentiation is also altered, and the increase in current is no longer a bi-exponential process, but instead is well fitted with a single exponential: 293B = 0.87 ± 0.16 s (n = 4). This monoexponential increase is not significantly different from the fast component of potentiation measured in the absence of 293B. The use dependence of 293B block of KCNQ1/KCNE1 channels seen in Fig. 6 therefore appears to result from the elimination of the slow component of potentiation which is usually seen on repetitive stimulation of the channels. Figure 7C shows representative current traces from the 1st and 20th depolarizations of KCNQ1/KCNE1 subjected to the 500 ms depolarization from –90 to +50 mV with a 500 ms inter-pulse interval, and the 1st and 20th sweeps of KCNQ1/KCNE1 with 10 µM 293B. The current elicited by the first sweep shows the typical slow activation of KCNQ1/KCNE1, with the current slowly increasing during the 500 ms depolarization. There is a clear time dependence to the development of block by 293B.
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A triplet of amino acids on the KCNE subunit play a critical role in the resulting behaviour of the channel formed on co-expression with KCNQ1 (Melman et al. 2002). In particular, the mutation of the valine at position 72 of KCNE3 introduces voltage dependence to the KCNQ1/KCNE3[V72T] channel (Melman et al. 2002). We studied how this mutation altered the modulatory effects of ancillary subunits on drug binding and frequency dependence to examine if activation gating was tightly linked to apparent drug affinity.
Figure 8 shows the current elicited when a standard two-pulse depolarization protocol is applied to KCNQ1/KCNE3[V72T]. Activation is clearly voltage dependent, and on repolarization to –60 mV deactivation is rapid. The presence of 10 µM 293B is sufficient to abolish most of the current. Addition of 1 µM 293B reduced current to 37.4 ± 5.8% (n = 6), which is not significantly different from the block of KCNQ1/KCNE3 channels (41.4 ± 6.1%, n = 5). Figure 8D shows an average of the degree of block of KCNQ1/KCNE3[V72T] current during a step depolarization from –90 to +40 mV. There is a large increase in block during the pulse, as it rises from 33.7 ± 6.7% to 64.5 ± 5.3% (n = 6). Clearly, apparent affinity for 293B is unchanged, despite the introduction of time-dependent activation gating.
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Discussion |
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The two subunits appear to have oppositely directed effects on the activation process. KCNQ1/KCNE1 activation is delayed, and channel opening is slow. In contrast, KCNQ1/KCNE3 co-expression results in a constitutively open channel (see Fig. 10). However, both KCNQ1/KCNE1 and KCNQ1/KCNE3 have higher apparent affinity for 293B than KCNQ1 alone, which suggests that modulation of activation/deactivation is not the sole determinant of modulation of 293B potency by ancillary subunits.
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KCNQ1/KCNE3 channels are constitutively open, which suggests that the KCNE3 subunit locks the channel in an open position. No time- or voltage-dependent component of chromanol binding was observed in KCNQ1/KCNE3 channels, which is consistent with the lack of time- and voltage-dependent gating behaviour of the KCNQ1/KCNE3 combination. Our data suggest that chromanol has the highest apparent affinity for the constitutively open KCNQ1/KCNE3 channel. This suggests that chromanol may act as an open channel blocker, or at least have a preference for the open state of the channel. The presence of an open channel blocking mechanism is further reinforced by the time-dependent development of block observed for the KCNQ1/KCNE1. Although the time course of development of KCNQ1/KCNE1 block occurs relatively slowly following channel opening, it has some other properties which suggest a more complicated mechanism. The activation time course of KCNQ1 alone has a fast early time course of activation, and reaches a stable steady state. In contrast, the activation time course of the KCNQ1/KCNE1 channel is relatively complex, with sigmoidal activation, and a second very slowly developing component. The initial changes in closed pre-activated states during the activation process may contribute to a mild relief of block by 293B through interactions which are not understood (Fig. 5B). However, chromanol appears to inhibit the development of the more slowly developing component of KCNQ1/KCNE1, or blocks it completely relative to earlier open steps in the activation of the channel. The result suggests that 293B has differing levels of interaction with different open states of the channel.
Seebohm et al. (2003b) proposed that the complex activation time courses of KCNQ1 and KCNQ1/KCNE1 are a result of these channels having multiple open states. These open states are kinetically distinct, but both presumably present an open pore with differing stability. One explanation for our results is that these states may be pharmacologically distinct (i.e. they can be distinguished by their differing affinities for drugs). While the binding site for the drug is presumably the open pore, different residues may line the pore in each putative open state. If so, each open state may have a different affinity for drugs such as 293B. The balance between the open states may depend on the presence of associated subunits. Thus, the differing apparent affinity for the open pore with different subunits may represent the effects the subunits have on the balance between these different open states. This case represents a re-balancing of drug binding states which are identical to those open channel states seen in the KCNQ1 channel. Conversely, the subunits may force the channel to adopt a completely new open state or states. The initial rapid block of KCNQ1/KCNE1 is similar to the time-independent block of KCNQ1 channels. However, block of KCNQ1/KCNE1 increases during the depolarization, possibly inhibiting the channel from entering a second, very slowly developing, open state.
How can the subunits produce distinct binding conformations with different affinities for chromanol? Many lipophilic drugs bind in the open intracellular pore of potassium channels. The major regions involved in drug binding have been identified as residing on the intracellular half of S6 (Yeola et al. 1996; Zhou et al. 2001). Melman et al. (2004) demonstrated that KCNE subunits strongly interact with the pore, and particularly with S6, to regulate gating activity. In Kv1 channels, the end of the short helical segment just before the GYG selectivity filter, has also been implicated as being important in drug binding. In Kv1 channels this segment has the sequence TMTT, with the M being the critical residue for interaction with quinidine (Tamkun et al. 1994). The analogous region in KCNQ1 channels has the sequence TVTT. Significantly, Seebohm et al. (2005) demonstrated that this valine is critical in producing the gating phenotype of KCNQ1 channels and KCNQ1/KCNE1 channels. The orientation of this valine (V310) is modulated by interactions with both S5 and S6. These and other structural experiments all indicate that open channel block at the intracellular pore should be strongly modulated by KCNE subunit expression (Panaghie et al. 2006).
In general, the activation gate of potassium channels appears to involve opening of the inner mouth of the pore, i.e. the lower half of the S6 domain (Seebohm et al. 2006). The fact that KCNE subunits interact with this region and strongly modify activation gating is consistent with this mechanism. Since binding of hydrophobic drugs in this region is ubiquitous, the strong effect of KCNE subunits on chromanol potency also seems consistent with this mechanism. However, our results with the mutant channel KCNQ1/KCNE3[V72T] may appear to contradict this close relationship between modification of the activation gate and drug binding. The apparent affinity of the open state of KCNQ1/KCNE3[V72T] for 293B is the same as for KCNQ1/KCNE3, even though there has been a dramatic alteration in the activation gating kinetics. However, these data can be reconciled by considering the possibility of multiple open states, e.g. KCNQ1/KCNE3[V72T] may have an open state similar to one of the KCNQ1/KCNE1 open states. There are several lines of evidence to support this idea. First, rapid stimulation potentiates the KCNQ1/KCNE3[V72T] current, with a time course identical to that of the 293B-insenstive open state of KCNQ1/KCNE1. In addition, the open state of KCNQ1/KCNE3[V72T] is insensitive to the presence of 293B, similar to the 293B-insenstive open state of KCNQ1/KCNE1. Finally, development of 293B block of KCNQ1/KCNE3[V72T] channels during a step depolarization has a similar time course to the development of 293B block of KCNQ1/KCNE1, except for an initial unbinding of drug in KCNQ1/KCNE1 (the mild relief of block seen in Fig. 5B). The V72T mutation alters activation gating, but does not change the apparent affinity for the drug. Importantly, even though the apparent affinity is not changed, the mutation does alter the time-dependent properties of drug binding. This is consistent with the residue at position 72 influencing the transitions of the activation gate and accessibility to the binding site. However, other residues in the KCNE subunit must be involved in holding the binding site for chromanol in higher and lower affinity conformations.
Our experiments demonstrate that KCNE co-expression does indeed strongly modulate open channel block by drugs. In this sense, chromanol serves as a pharmacological probe of the ability of KCNE subunits to change pore structure and provides confirmation of the structural hypotheses developed in the preceding biophysical studies based on kinetic analysis of site directed mutations. In physiological terms, this study demonstrates that the pharmacology of KCNQ1 channels is strongly dependent on the expression of ancillary subunits. This subunit-specific pharmacology suggests that drugs that are targeted to KCNQ in specific organs can be made more specific by considering subunit characteristics. Furthermore, the kinetic differences in gating and the corresponding differences in drug use-dependence suggest that the response of KCNQ1 channels to a drug will depend on the patterns of electrical activity of a particular organ or cell type, providing the opportunity for additional specificity of action. Understanding these molecular properties will help in the development of organ-specific pharmacological therapies, and the reduction of adverse side-effects associated with current pharmacological interventions.
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References |
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); KCNQ1/KCNE1, 16.1 ± 1.8 µM (•); KCNQ1/KCNE3, 0.72 ± 0.02 µM (
). For each point, n = 3 – 8.
