Correspondence to: Michael Pusch, Istituto di Cibernetica e Biofisica, Consiglio Nazionale delle Ricerche, Via de Marini 6, I-16149 Genova, Italy. Fax:39-0106475-500 E-mail:pusch{at}barolo.icb.ge.cnr.it.
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Abstract |
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We investigated in detail the mechanism of inhibition by the S(-) enantiomer of 2-(p-chlorophenoxy)butyric acid (CPB) of the Torpedo Cl- channel, ClC-0. The substance has been previously shown to inhibit the homologous skeletal muscle channel, CLC-1. ClC-0 is a homodimer with probably two independently gated protopores that are conductive only if an additional common gate is open. As a simplification, we used a mutant of ClC-0 (C212S) that has the common gate "locked open" (Lin, Y.W., C.W. Lin, and T.Y. Chen. 1999. J. Gen. Physiol. 114:112). CPB inhibits C212S currents only when applied to the cytoplasmic side, and single-channel recordings at voltages (V) between -120 and -80 mV demonstrate that it acts independently on individual protopores by introducing a long-lived nonconductive state with no effect on the conductance and little effect on the lifetime of the open state. Steady-state macroscopic currents at -140 mV are half-inhibited by 0.5 mM CPB, but the inhibition decreases with V and vanishes for V
40 mV. Relaxations of CPB inhibition after voltage steps are seen in the current responses as an additional exponential component that is much slower than the gating of drug-free protopores. For V
-40 mV, where a significant inhibition is observable for the CPB concentrations used in this study (
10 mM), the concentration dependence of its onset kinetics is consistent with CPB binding according to a bimolecular reaction. At all voltages, only the openings of drug-free protopores appear to contribute significantly to the current observed at any time. Lowering internal Cl- hastens significantly the apparent "on" rate, suggesting that internal Cl- antagonizes CPB binding to closed pores. Vice versa, lowering external Cl- reduces the apparent rate of CPB dissociation from open pores. We studied also the point mutant K519E (in the context of the C212S mutant) that has altered conduction properties and slower single protopore gating kinetics. In experiments with CPB, the mutant exhibited drastically slowed recovery from CPB inhibition. In addition, in contrast to WT (i.e., C212S), the mutant K519E showed also a significant CPB inhibition at positive voltages (
60 mV) with an IC50 of
3040 mM. Altogether, these findings support a model for the mechanism of CPB inhibition in which the drug competes with Cl- for binding to a site of the pore where it blocks permeation. CPB binds preferentially to closed channels, and thereby also strongly alters the gating of the single protopore. Since the affinity of CPB for open WT pores is extremely low, we cannot decide in this case if it acts also as an open pore blocker. However, the experiments with the mutant K519E strongly support this interpretation. CPB block may become a useful tool to study the pore of ClC channels. As a first application, our results provide additional evidence for a double-barreled structure of ClC-0 and ClC-1.
Key Words: ClC, slow gate, double barreled, anion channel, clofibric acid
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INTRODUCTION |
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Ion channel research has profited greatly from the existence of specific high affinity antagonists of ionic permeabilities. Unfortunately, only a few specific blockers exist for Cl- channels. Most "classical" Cl- channel inhibitors, like DIDS, anthracene-9-carboxylic acid (9AC),* and others, act rather unspecific and often only at elevated concentrations (
The voltage-dependent inhibition by S(-)-CPP (from now on simply called CPP) has been phenomenologically described as a "shift" of the popen(V) curve to more positive voltages (
Recently
In the present study, we investigated in detail the mechanism of action of a derivative of CPP, 2-(p-chlorophenoxy)butyric acid (CPB), that inhibits with almost equal potency both the WT ClC-0 and the slow gatedeficient mutant C212S. From single-channel recordings, we conclude that CPB acts by inhibiting independently the two single protopore conductances, and not by favoring or re-enabling the closure of a common permeation pathway. Furthermore, our analysis of the [CPB]- and voltage dependence of macroscopic currents suggests that, besides hindering the opening of single protopores, CPB binding is also blocking pore permeation, consistently with the peculiar feature of chloride channels of having strongly correlated gating and conduction mechanisms. The voltage dependence of CPB inhibition results from a much lower affinity of open protopores for the binding of CPB, which is strongly destabilized by permeant chloride ions.
A single-pore structure has been proposed for ClC-1, based on results of substituted cysteine accessibility and macroscopic current measurements (
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MATERIALS AND METHODS |
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Molecular Biology and Oocyte Expression
The point mutant C212S of ClC-0 (
Electrophysiology and Solutions
For patch-clamping, the vitelline membrane of the oocytes was removed manually after incubation in hypertonic medium. Patch-clamp experiments were performed using the inside-out or outside-out configuration using an EPC-7 amplifier (List). The solutions had the following composition: the standard intracellular solution contained (in mM) 100 NMDG-Cl, 2 MgCl2, 10 HEPES, and 2 EGTA at pH 7.3; the standard extracellular solution contained 100 NMDG-Cl, 5 MgCl2, and 10 HEPES at pH 7.3. In the solutions with lower Cl- concentrations, Cl- was replaced by glutamate. Liquid junction potentials (<10 mV) were not corrected for. Solutions were changed by a perfusion system consisting of 4 tubes of 200 µm diameter in which the tip of the patch pipette was inserted. The S(-) enantiomer of CPB was prepared as described by
Data Analysis
Open probability curves were obtained as described previously (
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(1) |
where Imax is the tail current after large prepulse depolarizations, z is the apparent gating charge, V1/2 is the voltage of half-maximal activation, and Imin is a constant offset. The apparent open probability, po(V), was obtained as the ratio I(V)/Imax; the residual open probability at negative voltage, pmin, was calculated as pmin = Imin/Imax. It reflects the (extrapolated) fraction of channels that do not close even at very negative voltages.
Single-channel analysis was performed as described by
Model Fitting
The kinetic model (Scheme 4) was fitted to the macroscopic experimental data using the procedure described below. For a given set of parameters defining the rate constants, the predictions of the model for the unbound probability pU(V) (Fig 9) and the slow time constant s (see Fig 10) were calculated. pU(V) is simply the sum of the steady-state probabilities of unbound pores that are either in the closed or in the open state, pC and pO, respectively. Since it comprises four states, the model predicts three time constants for gating relaxations, whereas only two time constants,
f and
s, could be distinguished experimentally. However, we found that for parameters that described the unbound probability reasonably well, only two of the time constants predicted for the relaxations in the actual experimental conditions had a significant weight, one of them being like
f independent of CPB concentration (i.e., representing the normal gating kinetics) and a much larger one that was identified with
s.
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The badness, b, of the model fitting of the data was calculated as:
where pUmeas, pUfit, and pU are the measured and fitted values of pU and the SEM of the measurement of pU;
meas,
fit and
are the measured and fitted values of
s and the SEM of the measurement of
s, respectively. The quantity b was minimized using the simplex algorithm. To reduce the number of free parameters of the model, we made the following assumptions as is also described later in the text. First, we assumed that CPB stabilized the closed state mainly through a reduction of the opening rate,
', whereas the closing rate ß' was set identical to ß, which is the closing rate of unbound channels. Second, we assumed exponential voltage dependence of all rate constants, apart from the opening rate,
, that was calculated as described below. Third, we assumed that binding and unbinding of CPB to open channels was only slightly voltage-dependent with an apparent gating valence of -0.15 in accordance with the results obtained with the mutant C212S/K519E. Fourth, we imposed microscopic reversibility at the reversal potential for Cl-. From this condition, we calculated the off rate constant from the open state at 0 mV, koffO(0) as a function of the other rate constants. Fifth, the opening rate,
, and closing rate, ß, of CPB-free channels were determined from the open probability, po, and the gating time constant,
f. The voltage dependence of po could be well described by a Boltzmann function with an offset:
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(2) |
with pmin = 0.092, V1/2 = -90 mV, and z = 0.96 (data not shown).
The voltage dependence of ß for the C212S channels was assumed to be of the same simple form derived from single-channel data (
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(3) |
The parameters ß0 and zß where determined by fitting f data according to the relation:
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(4) |
yielding ß0 = 6.8 s-1, zß =-0.36 (see Fig 10, solid line). The opening rate, , was consequently determined as:
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(5) |
Model calculations were performed by standard numerical methods implemented in Visual C++. The parameters obtained from the fits are listed in Table 1. For the fit, the four parameters marked with an asterisk in Table 1 were allowed to vary.
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RESULTS |
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Effect of CPB on a Slow Gatedeficient Mutant of ClC-0
Like for ClC-1 (
Single-channel Analysis of CPB Inhibition
CPB Acts on Single Protopores
CPB could inhibit the permeation of the single protopores of the double-barreled channel, or it could somehow shut down the conduction of both protopores simultaneously; e.g., by blocking a common pathway or by overcoming and reversing the effect of the C212S mutation and keeping the slow gate of ClC-0 closed. This question can be answered by single-channel recordings such as those illustrated in Fig 2. The figure shows short stretches of recordings of a single C212S channel at -80 mV (top traces) and -100 mV (bottom traces) in the absence of CPB (left traces) and with 1 mM CPB present (right traces). The double-barreled appearance of the channel is evident by the presence of two equally spaced nonzero current levels that are never observed singly (see also
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(6) |
as expected if the measured current arises from the random openings of two independent pores with open probability p = 0.37 at -80 mV and p = 0.21 at -100 mV (Fig 3A and Fig C, squares). Also in the presence of CPB, the occupation probabilities could be very well described by a binomial distribution (Fig 3B and Fig D, squares, also see legend), but using reduced single-pore open probabilities, p'= 0.15 at -80 mV and p'= 0.06 at -100 mV. This result strongly supports the idea that CPB acts independently on single protopores, because if CPB prevented the opening or the permeation of both pores simultaneously, the probability distribution would change quite differently. In that case, the current would be zero while the channel binds CPB, whereas the relative probability of the two current levels while the channel is drug-free would remain the same and so would the observed ratio, p2/p1. The experimental data are in strong contrast with this prediction: in the experiment of Fig 2, in the absence of CPB, the estimated values of p2/p1 were 0.27 at -80 mV and 0.11 at -100 mV, whereas in the presence of 1 mM CPB, the corresponding estimates were 0.10 and 0.02. A more direct qualitative demonstration that CPB acts on single protopores is the observation under drug application of numerous relatively long epochs containing only openings to the low conductance level (Fig 4 B). We interpret these epochs as periods during which only one of the two protopores is inhibited by CPB. The comparison with recordings from the same channel before CPB addition (Fig 4 A) shows that their spontaneous occurrence in the unaffected channel has a vanishingly small probability, and a simultaneous effect of CPB on both protopores cannot explain their appearance.
Kinetic Analysis of Single Channels
We performed a kinetic analysis of the single-channel records at -80, -100, and -120 mV in the absence and in the presence of 1 mM CPB. Fig 4 shows the results for a patch at -80 mV. The complete recording stretches used for the kinetic analysis are shown in Fig 4 A (no CPB) and Fig 4 B (1 mM CPB). From the traces, it can already be seen that the main qualitative effect of CPB is to introduce long closures that are not seen in the absence of CPB. We performed a quantitative dwell time analysis after idealization of the single-channel traces as described by 2 = 10.8 ms (double open; Fig 4 C, circles);
1 = 14.6 ms (single open; Fig 4 D, circles);
0 = 18.8 ms (closed times; Fig 4 E, circles). These values are in accordance with previous results (Chen and Miller; 1996;
, and closing rate, ß, of the protopore gate by:
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(7) |
These equations allow a check of consistency because and ß are overdetermined. From the values of
0 and
1, which are measured better than
2 from much larger samples of events, we estimate from the recordings of Fig 4 at -80 mV:
= 26.6 s-1 and ß= 43.2 s-1. From these estimates, we predict
2 = 11.6 ms (from Equation 7), in very good agreement with the measured value of 10.8 ms. As a further test of consistency, we verify that the quantity
/(
+ ß)= 0.38 coincides, as expected, with the single-pore open probability (p = 0.37) measured for the same experiment from the current-amplitude histogram (Fig 2 A and 3 A).
In the presence of 1 mM CPB, the dwell times of the double conducting states were not significantly different (Fig 4 C, squares) indicating that the rate of entry into CPB-inhibited states is much smaller than the normal closing rate, ß. The situation is more complex for the unit and zero conductance durations. If only one pore is conducting, the other could be either just regularly closed or in a long-lived inhibited state. Respectively, the duration of detectable single openings is expected to be governed by the normal escape rate (+ß) and by the lower rate ß. Indeed, the histogram of the unit conductance durations shows the presence of an additional larger time constant that is well approximated by 1/ß = 23.1 ms (Fig 4 D, squares and dashed line). If both protopores are nonconducting there are three possibilities: (1) both protopores are CPB-free and closed, a configuration with the normal escape rate 2
; (2) only one protopore is normally closed and stays so for a mean time 1/
, whereas the other is inhibited by CPB and is very unlikely to become conductive in any comparable time; and (3) both protopores bind a CPB molecule, in which case the transition rate to a conductive state is expected to be twice the effective rate of CPB dissociation. Indeed, the cumulative histogram of zero current durations is very well fitted with the sum of three exponentials, with two time constants fixed to 1/(2
) and 1/
, and a fitted additional time constant of
220 ms (Fig 4 E, squares and dashed line).
Macroscopic Currents Analysis
The Removal of Inhibition at Positive Voltages Is due to CPB Dissociation
Our single-channel data show that between -120 and -80 mV the binding of CPB induces a long-lived nonconducting state of single independent C212S protopores, but leave open the question of whether CPB blocks ion permeation or decreases the opening probability of the protopore to which it is bound. Since single-channel recordings at much less negative voltageswhere the switching of single drug-bound protopores to a conductive state might occur frequently enough and last long enough to be detectedare hampered by unfavorable signal to noise conditions, we have tried to answer this question by studying the voltage dependence of macroscopic current relaxations.
As shown qualitatively in Fig 1, the inhibition of chloride currents by CPB concentrations as high as 5 mM is reduced by long depolarizations at large positive voltages. We demonstrated the total absence of a steady-state inhibition for V 40 mV in experiments like that of Fig 5. A pulse protocol comprising a 0.5-s pulse to -140 mV followed by a similar pulse to 60 mV and return to 0 mV (Fig 5 A) was applied every 3 s to an inside-out macropatch and the perfusion was switched several times between control (no CPB) and test (5 mM CPB) solutions. Fig 5 B shows a sample record before application of CPB, in the presence of 5 mM CPB and after washout. It is seen that with CPB the current decays to a much smaller value at the end of the pulse to -140 mV compared with the control situation, indicative of a strong steady-state inhibition at -140 mV. Vice versa, the current reaches exactly the same level of control at the end of the following 1-s pulse to 60 mV. Is this because CPB-bound pores can open at sufficiently depolarized voltages or because these voltages favor the full dissociation of CPB from the pores? The second interpretation is supported by the experiments illustrated in Fig 6 and Fig 7. Fig 6 shows tail currents at Vt = -100 mV, after 700-ms prepulses to Vp from -160 mV to +120 mV, measured from the same inside-out patch before (A) and after (B) addition of 1 mM CPB to the normal perfusion solution. For clarity of illustration, only three of the records obtained for Vp = -160, -60, and +40 mV are displayed. The control records in A show the normal gating relaxations of C212S channels at -100 mV, which are very well described by a single exponential (smooth lines) with a Vp-independent time constant,
f, estimated in this experiment
19.5 ms. On the other hand, the initial amplitude of the tail current, I0, does depend on Vp, the relative amplitude of the following relaxation,
If /I0, being determined by the ratio of the steady-state open probabilities at V = Vp and V = Vt. Therefore, the combination of
f and
If /I0 provides a hallmark for the currents mediated by the normal gating of unmodified channels. Measurements of
If /I0 from four experiments are plotted as a function of Vp in Fig 6 C as circles. The tail currents recorded in 1 mM CPB (B) need to be described by the sum of two exponentials (smooth lines), with a fast relaxation that has the same time constant of control,
f
19.5 ms, and a slower component with a Vp-independent time constant,
s
253 ms. Although it is obvious that the slow relaxation must reflect the interaction of the C212S protopores with CPB, the observation that is most important for the present argument is that the relative amplitude of the fast relaxation (Fig 6 C) is not significantly different for any Vp from the respective quantity measured in CPB-free conditions. This implies that at the end of any prepulse all the conducting pores contributing to I0 undergo the same fast relaxation toward the normal open-close distribution at V = Vt as in drug-free conditions, i.e., they are all drug-free.
Also during the recovery from inhibition, it appears that no drug-bound hemichannels spend a significant time in a conductive state. This is shown by the experiment illustrated in Fig 7. In the presence of 5 mM CPB, the C212S channels were first maximally inhibited by a pulse to -140 mV; the inhibition was then relieved to various extents by a pulse to +60 mV of variable duration, Tp, after which the voltage was stepped to -100 mV. Fig 7 A shows records of the tail currents after 60-mV pulses with Tp = 10, 30, 50, and 110 ms, that reduced progressively the inhibition. In all cases, the tails were well fitted by a double exponential (smooth lines) with the same fast and slow time constants, and with the same relative amplitude of the fast relaxation (Fig 7 B). The same result was obtained in four different patches and shows, by the same argument used in the discussion of Fig 6, that drug-bound protopores do not spend even transiently any significant time in a conductive state.
Voltage Dependence of CPB Binding Relaxations
The experiments discussed above show that both at large negative and at large positive voltages the interaction of the C212S channels with CPB is characterized by relatively slow relaxations of the macroscopic currents that are clearly distinct from the normal gating of drug-free channels. A quantitative dissection of the two processes is in fact possible at all voltages, and was performed using the two types of tail protocols illustrated in Fig 8. In the first protocol (Fig 8 A), the channels are first almost completely freed from inhibition and fully activated by a prepulse to +60 mV; they are then allowed to relax for 2 s toward their steady-state condition at a variable test potential, Vp. Finally, the voltage is stepped back to +60 mV. This protocol allows measurements of the voltage dependencies of CPB "on-binding" relaxations (during the conditioning to Vp) and of the steady-state CPB-binding probabilities (from the early currents after the last step to +60 mV; see below). Sample recordings obtained with this protocol from a patch exposed to 5 mM CPB are shown in Fig 8 C. The second protocol (Fig 8 B and sample recordings from the same patch are shown in Fig 8 D) was designed to characterize "off-binding" relaxations. In this case, the channels were first almost maximally inhibited by a pulse to -140 mV, and then allowed to recover from inhibition at various more positive "tail" potentials.
The voltage dependence of the steady-state fraction of C212S channels that are not inhibited at a given CPB concentration can be estimated from the early currents after the last step to +60 mV in the pulse protocol of Fig 8 A. At this voltage, the channels that are drug-free at the end of the prepulse to Vp open completely within 1 ms (
f data in Fig 10) and contribute to an early current, I0(V), that appears almost "instantaneous" in comparison with the
100-fold slower successive relaxation (Fig 8 C). Therefore, the ratio:
where I (+60) is the steady-state current flowing when all the channels are fully open at +60 mV, provides a fair estimate of the steady-state probability of channels being unbound at V = Vp. The values of pU(Vp) from the experiment of Fig 8 are shown in Fig 8 E at three different CPB concentrations. From these curves, the increase of the inhibition at negative voltages can be clearly seen. These data provide a more informative description of the increasing inhibition of the currents at negative potentials than those shown in Fig 1 D. Most importantly they allow testing if such inhibition is consistent with a simple 1:1 binding of CPB. This consistency is shown by the plots of pU(Vp) estimates, obtained at any given Vp, as a function of CPB concentration, c (Fig 9 A). At all voltages at which a significant block occurs (Vp
-20 mV), these estimates were well fitted by the simple titration function (Fig 9 A, solid lines):
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(8) |
The fitted Kd values are plotted in Fig 9 B on a semilogarithmic scale as a function of voltage. It is seen that this apparent dissociation constant increases with voltage in the whole range from -140 to -20 mV, but much less steeply at increasing negative potentials. This rules out the simple idea that this voltage dependence arises solely from the movement of the charged ligand across a significant fraction of the transmembrane voltage upon binding. Some dependence of the binding on the channel state must also be involved.
The pulse protocol of Fig 8 A allows also the study of the kinetics of the onset of CPB inhibition by following the decay of the currents after stepping the voltage from +60 mV to any Vp -20 mV. As for the particular cases of Fig 5 Fig 6 Fig 7, the example of Fig 8 C shows that, at any Vp, this decay is well fitted (smooth lines in Fig 8 C) by the sum of a fast exponential relaxation, with a time constant,
f(Vp), very close to that of normal deactivation, and a much slower component that has a roughly 10-fold larger time constant,
s(Vp), mainly reflecting the kinetics of the CPB-binding reaction. Measurements of
f(Vp) and
s(Vp), obtained with this protocol for various CPB concentrations, are plotted in Fig 10 as solid symbols. Notice that, as stated above, the
f values do not show any dependence on CPB concentration and are not statistically distinguishable from the data in CPB-free conditions. In contrast, the
s values decrease with CPB concentration and we tentatively interpret them in the most simple way as time constants of a 1:1 binding relaxation:
where U denotes CPB-free channels in fast open-close equilibrium, B denotes nonconducting CPB-bound channels, and where the rates ron and roff are related to s and to the unbound probability, pU (Fig 8 E and 9 A) by:
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(9) |
According to our tentative interpretation, we expect roff to be independent of, and ron to be proportional to the CPB concentration, c:
with kon being the apparent second order association rate constant, and koff the first order dissociation rate constant of the effective binding reaction. An example of such analysis, for Vp = -140 mV, is shown in Fig 11 A where the plots of ron and roff as a function of c clearly reflect the consistency of the data with the above simple expectations. The same conclusion was reached from the analysis of "on" kinetics of CPB inhibition at any voltage in the range from -140 to -20 mV; plots of the resulting estimates of kon and koff as a function of Vp are shown in Fig 11 B. Notice that the apparent kon decreases with voltage much less steeply at large negative values than for Vp > -80 mV, where it decreases more than 10-fold for a 60-mV change. The voltage dependence of koff is weaker than that of kon in the whole range from -140 to -20 mV, but also the steepness of the increase of koff with voltage tends to vanish at increasing negative voltages.
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The apparent CPB-binding relaxations for Vp -80 mV were studied with the protocol of Fig 8 B. As for the on kinetics, also the decrease of the CPB inhibition induced by a conditioning prepulse at -140 mV is manifested by the presence of a much slower component in the time course of the current increase after the step to a more positive voltage (Fig 8 D). Measurements of the two time constants,
f(Vp) and
s(Vp), obtained from the double exponential fit of these "tail currents" are plotted in Fig 10 as open symbols. Notice that there is good agreement with the analogous on data (closed symbols), obtained with the other protocol described above, in the voltage range (-80 mV
Vp
-20 mV) where both protocols could be used. Notice also that
s(Vp) is independent of c for Vp > 0. This is consistent with the above interpretation of
s, since at these voltages pU
1 (Fig 8 E and 9 A) and
s is expected to be simply equal to the reciprocal of the first order dissociation rate constant, koff. Accordingly, 1/
s measurements for Vp > 0 are taken as koff estimates and plotted as such in Fig 11 B. This figure shows that the combined estimates of koff from on and off measurements tend to increase exponentially for Vp > -50 mV, whereas they tend to a nonzero level at negative voltages.
Dependence of CPB Inhibition on Cl- Concentrations
It is well-known that the fast gate of ClC-0 depends on the concentration of Cl-, both extracellular ([Cl]e;
The Effect of Extracellular Chloride
To study the effect of [Cl]e on CPB inhibition, we used outside-out patches to allow a change of the extracellular solution. Fig 12 A shows the effect of decreasing [Cl]e from 110 to 10 mM on the open probability of C212S channels in the absence of CPB. The po(V) curve is shifted to more positive voltages by 38 ± 3 mV (mean ± SD), with little change in the asymptotic residual open probability at very negative voltages. The analysis of ClC-0 single-channel data (f measurements according to Equation 4 and Equation 5 (data not shown) indicate that the shift of the po(V) curve is mainly due to a decrease of the opening rate,
, roughly by a factor of
2.5 for V
-100 mV that tends to unity at more negative voltages, with little change in the closing rate, ß. Fig 12 (BD) shows the effects of [Cl]e changes on the apparent values of the CPB-binding parameters, estimated as described above from pU and
s measurements at various CPB concentrations. Qualitatively, all parameters, i.e., the apparent Kd, the effective on rate constant, kon, and the effective off rate constant, koff, are shifted to more positive voltages in low [Cl]e by the same degree (38 mV) as the open probability in the absence of CPB (Fig 12, BD, dashed lines).
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The Effect of Intracellular Chloride
In accordance with previous reports for ClC-0 channels (f measurements according to Equation 4 and Equation 5 (data not shown) indicate that the main effect of lowering [Cl]i is an increase of the closing rate, ß, by roughly a constant factor of approximately five. In the same figure are shown the effects of the [Cl]i change on the apparent values of the CPB-binding parameters, estimated as described above from pU and
s measurements at various CPB concentrations. It is seen that both the apparent Kd and the effective on rate are changed by an almost constant factor of five (Fig 13B and Fig C, dashed lines), whereas the off rate is practically unchanged by a reduction of [Cl]i (Fig 13 D).
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These results indicate that intracellular chloride ions antagonize the binding of CPB, either by competing for the same site or by occupying an inner site in the permeation pathway from which they destabilize CPB binding in a manner similar to that proposed for potassium channels, in which peptide blockers are destabilized by K+ ions (
Open Channel Block of the Mutant K519E
We investigated the effects of the mutation K519E on the CPB inhibition. We chose this mutation because amino acid 519 is probably located at the intracellular pore entrance of the channel (
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From the fractional inhibition of steady-state currents at various concentrations of CPB, we derive the dissociation constant estimates shown in Fig 15 A as a function of voltage. It is seen that Kd increases steeply with voltage between -100 and 0 mV but the dependence is much flatter at both ends of the voltage range. The affinity of the mutant is slightly smaller than that of WT at negative voltages but higher at positive voltages, where it is unmeasurable for WT (compare with Fig 9 C). The values between +60 and +140 mV change with an equivalent valence of 0.15 (Fig 15 B, solid line).
Also, the kinetics of CPB effects are quite different for the double mutant C212S-K519E as compared with C212S, showing an interesting parallelism with differences in the gating kinetics of the two constructs. First, the relief from inhibition at positive voltages was significantly slower: s= 1,000 ± 100 ms at +60 compared with the WT value of 115 ± 4 ms. Most interestingly, the ratio of the slow off time constant and the fast gating time constant,
s /
f, at +60 mV is
100 for both WT and mutant (
f (mutant) = 10.2 ± 2.2 ms;
f (WT) = 0.93 ± 0.2 ms). Also the slow time constant at negative voltages that accompanies the onset of inhibition is larger in the mutant compared with WT (
s at 5 mM and -140 mV of the mutant K519E:
s = 590 ± 60 ms versus the WT value of 98 ± 5 ms). The ratio
s /
f is smaller at -140 mV, but again very similar for mutant and WT (
s /
f (mutant)
6.6;
s /
f (WT)
7.6).
The persisting inhibition at positive voltages, that is not seen with WT ClC-0, could be caused by three different mechanisms: (1) the maximal open probability of CPB-bound channels could be significantly smaller than one; (2) the open probability in the presence of CPB could be "shifted" very strongly; or (3) open channels could be directly blocked. The first possibility is unlikely because nonstationary noise analysis indicated a maximal open probability close to one (not shown). To distinguish between the latter possibilities, we measured the apparent open probability using the conventional tail current protocol (Fig 15 C). The resulting apparent po (Fig 15 D) clearly saturates for voltages +40 mV, indicating that the open probability in the presence of 5 mM CPB is maximal at these voltages where inhibition is substantial. The most likely explanation of these results is that CPB exerts an open channel block on the mutant and that the affinity of the open state is slightly voltage-dependent with an apparent valence of 0.15.
A Kinetic Model for CPB Inhibition
In all our measurements, for voltage steps between any two levels in the whole range from -140 to +140 mV, we observed macroscopic current relaxations described by single exponentials (no CPB) or double exponentials (with CPB) with time constants solely dependent on the final voltage level. This general feature, per se, is a strong indication that the independently gated protopores possess only two statistically significant states and that CPB binding only adds one significant state to this counting. Therefore, the most simple models that could describe our observations assume that CPB can either bind only to the open state, blocking ion permeation, or bind only to the closed state, preventing the opening transition:
In either one of the above models, the slow time constant of the single-channel closed state dwell time histogram or the slow component of macroscopic current relaxations would reflect the slow binding reaction of CPB to the appropriate protopore state. Both models are attractive for their simplicity and could be forced to fit the macroscopic and single-channel data for WT channels by choosing ad hoc voltage dependencies for the true association and dissociation rate-constants of CPB binding. However, both models predict that the measured koff is the true first order dissociation rate constant of CPB unbinding, expected as such to have a simple exponential dependence on voltage, which is contrary to what is actually observed (Fig 11 B, 12 D, and 13 D). Each scheme also has some other additional inconsistency with the data. Scheme 2 has the nice feature of predicting the steep decrease of kon with voltage, as a mere consequence of the decrease of closed-pore probability even in the absence of an intrinsic voltage dependence of CPB association rate; but it is inconsistent with the residual block of C212S-K519E channels at very large voltages that drive all channels to the open-pore conformation. Vice versa, in Scheme III, the strong voltage dependence of kon for V > -100 mV would reflect an intrinsic property of CPB association that is hardly reconcilable with its tendency to vanish at very negative voltages (Fig 11 B, 12 C, and 13 C).
A logical complication of any of the above models is to assume that CPB can bind to both conformations of the protopores, resulting in a 4-state model (Scheme 4):
The binding properties of the closed state are clearly different from those of the open state and we know already that the gating is strongly influenced by CPB. The results of Fig 6 and Fig 7 and the results obtained with the mutant K519E indicate that the open bound state OB is nonconducting, i.e., that CPB directly blocks the pore. The flattening out of the apparent on and off rates at negative voltages (Fig 11) suggest that binding of CPB to the closed state is little voltage-dependent. The experiments with the mutant K519E also indicate that binding of CPB to the open state is little voltage-dependent; the valence of the open channel block of the mutant was estimated to be 0.15. On the other hand, the effective rate of recovery from block at positive voltages shows a simple exponential voltage dependence with an apparent valence of
0.43 for voltages up to 120 mV (Fig 10). These properties strongly suggest that the recovery from block occurs through the state OB and that the opening rate
' is rate limiting for this process. This implies that the off rate from the open state, koffO, is large compared with
' at all voltages tested here. This interpretation of the slow recovery from block at positive voltages is supported by the following two observations. First, recovery from block of the mutant K519E is slowed by a factor of
10, practically by the same factor as the regular opening rate,
, in accordance with the hypothesis that the "opening" rate
' is reduced by the same factor in the mutant. Second, the reduction of extracellular chloride reduces the opening rate
and the effective off rate at positive voltages by about the same factor. On the other hand, the experiments with low intracellular chloride indicate that a reduction of the closing rate ß, and, thus, likely also of the rate ß', has almost no effect on the effective off rate constant (Fig 13).
Therefore, we fitted the predictions of the model to the unblock probability, pU(V) (Fig 9) and the slow time constants (Fig 10) using the following simplifying assumptions in accordance with the above considerations. First, we assume that the rate ß' is equal to ß. Second, we fixed the voltage dependence of the off rate from the open and from the closed state to 0.15 and that of the on rate to the open state to zero. Third, we fixed the voltage dependence of ' to 0.6, according to the slope of the fast opening time constant at positive voltages (see Fig 10). Fourth, we assumed that the on rate to the open state is identical to the on rate to the closed state.
The model was fitted to the pU(c, V) data (Fig 9 A) and to the slow time constants (Fig 10) as described in MATERIALS AND METHODS. The free parameters in the fit were: konC(0), koffC(0), zonC, '(0) (see Table 1). The model allows a reasonable description of the macroscopic data (see dashed lines in Fig 9 and Fig 10 and solid lines in Fig 11). The model captures the basic features of CPB inhibition that are experimentally observed. In the negative potential range, binding is relatively voltage-independent, and binding as well as unbinding steps occur primarily via the closed state; CPB binding strongly disfavors channel opening by reducing the opening rate, and vice versa the off rate from the open state is much larger than the off rate from the closed state, such that the affinity of the open state is very small. The latter two properties lead to the single exponential time course of the disinhibition at positive voltages.
Because of the difficulty in experimentally determining the Kd of the open state the parameters given in Table 1 are at best good guesses of the true values. Interestingly, caused by the large off rate from the open bound state, this state is only very little occupied. This implies that the model is practically equivalent to one in which the open bound state is conducting, i.e., a model in which CPB is a pure gating modifier.
What does the model predict under reduced chloride concentrations? Without CPB, the main effect of reducing [Cl]e is to decrease the opening rate with little effect on ß (
', is reduced in low [Cl]e. Indeed, the data show that the apparent kon(V) curve is shifted as much as the Po(V) curve and that the koff(V) at V > -50 mV (where unbinding occurs mainly via the CB
OB
O pathway) is decreased by the same factor as
. The results of the experiments in low [Cl]e could indeed be reasonably well reproduced by just reducing the opening rate
' by a factor of approximately two (Fig 13D and Fig E, solid lines, and parameters in Table 1). This suggests that a reduction of [Cl]e has little direct effect on the state-dependent binding of CPB, the overall effect being only influenced by the [Cl]e dependence of the relative probability of the two conformational states of the channels.
The [Cl]i dependence of CPB inhibition is also nicely consistent with our simple model. The main effect of lowering [Cl]i is known to be an increase of the closing rate ß ('. If the opening rate is not changed, we expect that koff is not changed at any voltage, as indeed observed. On the other hand, the ß increase at negative voltages is not expected to increase much the apparent kon. In this range, the observed change of kon is by a factor increasing from 5.5 to 9. The data are consistent with a main effect of competition between CPB and [Cl]i for the same binding site. Such competition would make:
Thus, this suggests that intracellular chloride strongly interferes with CPB binding.
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DISCUSSION |
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On purpose, we have described our results for C212S channels by stating that CPB-bound pores do not contribute appreciably to the chloride conductance, rather than stating that CPB-bound pores cannot open, because we could not exclude from the experiments with this construct that the dissociation of CPB from the protopore to which it is bound occurs at positive voltages preferentially via the intermediate step of pore opening. What our data indicate is that the dwelling in such a "transition state" is unmeasurably short at our time resolution, so that whether this state is conductive or not has no practical consequence on the effect of CPB inhibition. However, distinguishing between these two possibilities has important consequences for the interpretation of the inhibition mechanism. In one case we would conclude that the binding of CPB is allosterically interacting with the portions of the protopores whose structural changes gate the opening of the pore. In the other, we would have a strong indication that CPB binds to a site where it acts as a pore blocker and this would be nicely consistent with the competitive interaction of CPB with the permeant chloride ions. The mutant K519E changes several properties of the channel, including conductance and gating kinetics. However, the basic properties are similar to WT and introduction of various amino acids at position 519 lead to gradual changes in channel properties (
From our results, the following four basic conclusions can be drawn. First, CPB inhibits single protopores of the presumably double-barreled channel. Second, it binds preferentially to single protopores that have a closed fast gate. Third, the binding is influenced by internal Cl- in a manner consistent with a competition between the two anions for the occupation of the same or a closely located site; and fourth, when binding to the open conformation of the pore, CPB probably blocks the pore permeation.
To what extent are these results evidence in favor of a double-barreled architecture of the channel? The single protopore inhibition clearly shows that there are two binding sites on the channel that are practically independent of each other. Furthermore, the properties of the inhibition by CPB strongly suggest that the block occurs within the ion conducting pathway. Taken together, our results strongly favor a double pore architecture of ClC-0, in agreement with the recent projection structure of a prokaryotic ClC channel (
An interesting property of the CPB block is that even though the kinetics can be well described by a simple bimolecular reaction, the association rate-constant is far below that of a diffusion limited process. This means that a large free-energy "barrier" separates the intracellular fluid from the binding site. This finding is actually again in accordance with the idea that the binding site of CPB lies within the conduction pathway. The strongest evidence in favor of a binding of CPB in the pore comes from the experiments with the mutant K519E. The partial inhibition at voltages up to 140 mV strongly suggests an open pore block. However, more experiments are needed to further nail down this conclusion.
Our data can be well described by a simple model in which the closed state has a much larger affinity or CPB than the open state and in which binding itself is only slightly directly voltage dependent. The slow relief from inhibition at positive voltages does not directly reflect the unbinding of CPB but is governed by the slower and rate limiting opening step of the drug-bound channels, '. This small value of
' reflects the stabilization of the closed state by CPB. This interpretation is supported by the results with the mutant. Regular fast gating of mutant K519E is significantly slower than WT. The very slow relief from inhibition of the mutant results from a proportional slowing of the opening rate,
', of CPB-bound channels. The fact that also the apparent on rate at negative voltages is smaller in the mutant as compared with WT is more difficult to explain. One reason might be that the minimum open probability of the mutant is much larger than for WT (
The kinetic effects of mutation K519E on CPB unbinding appear to be largely caused indirectly by a slowed and otherwise changed gating kinetics of the mutant, and also the affinity of the mutant at negative voltages is not drastically changed. It is therefore unlikely that lysine 519 forms part of the binding site of CPB. We rather speculate that the binding site is further inside the pore, beyond the supposedly superficial location of K519.
The model can account quantitatively for the macroscopic measurements. Also the single-channel data are in good qualitative agreement with the model. Further single-channel measurements are needed, however, for a full comparison with the model.
The model is also in good agreement with our results obtained in low [Cl]e and [Cl]i. Whereas external Cl- appears not to directly interact with CPB, lowering of intracellular Cl- could be fitted only by adjusting mainly the on rates to the closed and open states. Based on this, we speculate that intracellular Cl- directly interferes with CPB binding, either by competition for a common binding site or by the occupation of closely located interacting binding sites. More experiments are clearly needed to explore this aspect of CPB block in more detail. The apparently complicated effects of CPB on gating are not at all surprising if CPB does bind in the pore because it is known that the gating of ClC-0 is tightly linked to permeation. Any agent that impedes the access of chloride as a channel opener will have profound effects on gating.
In conclusion, even though the block by CPB of the point mutant C212S of ClC-0 is of rather low affinity, it allowed us to obtain additional evidence for a double-barreled architecture of the channel. Furthermore, the drug shows very interesting interactions with the gating state of the channel and its study may therefore yield important insights into channel function. In particular, it will be important to identify the CPB binding site. If future experiments cement the conclusion that CPB is a pore blocker, the regions of the primary structure participating in the binding site may give a hint to where the pore is located in ClC channels, viewing from the inside.
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Footnotes |
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* Abbreviations used in this paper: 9AC, anthracene-9-carboxylic acid; [Cl]e, extracellular [Cl]; [Cl]i, intracellular [Cl]; CPB, 2-(p-chlorophenoxy) butyric acid; CPP, 2-(p-chlorophenoxy) propionic acid; DIDS, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid.
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Acknowledgements |
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We thank Dr. T.Y. Chen for providing the mutant C212S of ClC-0.
The work has been supported by Italian CNR PS No. 98-3265-74 and 99.2312.74 and partially by Telethon Italy, grant No. 1079 to M. PUSCH.
Submitted: 9 April 2001
Revised: 18 May 2001
Accepted: 21 May 2001
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References |
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