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Department of Pharmacology, College of Physicians and Surgeons of Columbia University, New York, New York 10032
Correspondence: Address reprint requests to R. S. Kass, PhD, Dept. of Pharmacology, College of Physicians and Surgeons of Columbia University, 630 W. 168th St. PH 7W 318, New York, NY 10032. Tel.: 212-305-7444; Fax: 212-342-2703; E-mail: rsk20{at}columbia.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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; Jentsch, 2000; Jentsch et al., 2000; Schwake et al., 2001; Lossin et al., 2002; Jurkat-Rott and Lehmann-Horn, 2001; Kullmann, 2002). Expression of ion channels in heterologous systems allows for investigation of inherited ion channel defects at the single protein and cellular level to directly identify the disease-associated alteration in ion channel function (Clancy and Kass, 2002). However, despite a wealth of experimental evidence documenting the functional consequences of the disease-linked mutations in ion channels, understanding the physical mechanisms affected by the mutations at the structural level has been lacking. This is due in large part to the difficulty in obtaining structural information with high-resolution experimental techniques such as x-ray crystallography and multidimensional NMR methods in integral membrane proteins. To date, structural information is available for only a limited number of ion channel proteins, despite the fact that ion channels regulate key physiological cellular and subcellular functions in health and disease (Miller, 2000a,b).
Computational methods that predict three-dimensional structures from amino acid sequences have become increasingly accurate and have provided insights into structure-function relationships for proteins without structural information (Yang and Honig, 2000; Yang, 2002; Yang and Wang, 2002). Although the information required to generate a protein structure is expected to be embedded in its amino acid sequence, current computational methodologies that unravel how three-dimensional information is mapped onto a linear sequence predict low-resolution structures at best when close homologous structural templates from protein structural databases are absent. The accuracy of computational structural models requires experimental approaches for validation.
Here we report direct testing of the predictions of a structural model reported by us of the C-terminal domain of the human heart Na+ channel (Cormier et al., 2002). We focused on understanding the structural basis for the unique effects of an inherited C-terminal mutation (Y1795C (YC)) that is associated with variant 3 of the long QT syndrome (LQT-3) and has pronounced effects on the entry of Na+ channels into a nonconducting inactivated state (Rivolta et al., 2002; Clancy et al., 2002). The model structure of the C-terminal domain was based on a remote relationship of the C-terminal domain and calmodulin structure: the C-terminal domain was predicted to have a calmodulin-like fold with one pair of EF-hands packed against each other as in calmodulin. In this article, we provide evidence that the naturally occurring mutation, in which a cysteine replaces a tyrosine at position 1795 (Y1795C), enables the formation of disulfide bonds with a partner cysteine in the channel. Using the predictions of the model, we identify the cysteine and show that three-dimensional information contained in the sequence for the channel protein is necessary to understand the structural basis of the effects of the mutation. The experimental evidence supports the accuracy of the predicted structural model of the C-terminal domain in the human heart Na+ channel and provides insight into a structural basis for the mutation-induced altered channel function underlying the disease phenotype.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). All constructs were transiently expressed in HEK293 cells. Transient transfection was carried out using lipofectamine (Invitrogen) according to the protocol suggested by the manufacturer. Cells were transfected with equal amounts of the 
-subunit and hß1-subunit (SCNB1), subcloned individually into the pcDNA3.1 (Invitrogen) vector. In addition, the same amount of CD8 cDNA was cotransfected as a reporter gene (EBo-pCD vector, American Type Culture Collection, total cDNA 2.5 µg). CD8 positive cells were patch clamped 48 h after transfection. Expression of CD8 did not alter the channel properties, compared with those of stably expressed channel (data not shown). Control experiments in which CD8 and hß1 were transfected together using a bicistronic vector (pIRES, Clontech, Palo Alto, CA) were used to confirm coexpression of hß1 and -subunits as described above. Expressed channel biophysical properties were independent of the above conditions (data not shown).
Electrophysiology
Membrane currents were measured using whole cell and single channel patch-clamp procedures with Axopatch 200B amplifiers and Pclamp 8 (Axon Instruments, Foster City, CA). All measurements were obtained at room temperature (22°C). Single channel recordings employed the following bath solution (mM): 140 KCl, 5 HEPES, 1 MgCl2, and pH adjusted to 7.4. Whole cell procedures have been published previously (Abriel et al., 2001). For single channel experiments, pipettes were coated with Sylgard (Dow Chemical, Midland, MI) to decrease noise and capacitance of the glass. Electrode resistance was typically 5–7 M
when filled with internal solution (110 mM NaCl, 10 mM HEPES, pH adjusted to 7.4). In single channel experiments, after establishing the cell-attached configuration (seal resistance > 10 G), the membrane was held at a holding potential of -120 mV. Test pulses (-30 mV, 100 ms) were applied every 0.5 s. Single channel currents were filtered by a low pass filter in the clamp amplifier with a cutoff frequency of 5 kHz and digitized for storage on computer at a sampling frequency of 20 kHz.
DTT (Dithiothreitol; Sigma, St. Louis, MO) was dissolved in DMSO at a stock concentration of 1 M. Stock solutions were kept no longer than 1 week. DTT stock solution was diluted into extracellular (bath) solutions to a final concentration of 10 mM immediately before experimental use. Because DTT is membrane permeable and can be used to modify candidate intracellular sites by external application (Nassir et al., 2003), cells were first incubated in DTT-containing solutions for at least 30 min at room temperature before patch-clamp experiments. DTT (10 mM) was maintained in external solutions during each experiment, and KCl (3 M) was used as a reference electrode to compensate for junction potentials caused by 10 mM DTT. Alternatively, in specified experiments, DTT (50 mM) was added to the external solution and then applied by perfusion to patched cells. Effects on membrane currents reached steady state and were recorded in the presence of DTT after 10-min exposure periods. These effects were reversible upon the return to DTT-free external solutions (within 10 min). The effects recorded on the decay of Na+ channel currents were the same for each method of DTT application.
Data analysis
Capacitative and leak currents were eliminated by digital subtraction of averaged null sweeps. Idealization of single channel currents and the measurement of open time were carried out with the program SKM (QUB suit, U. Buffalo; Premkumar et al., 1997; Qin et al., 1996). Further analysis was carried out using Excel (Microsoft, Seattle, WA) and Origin (Microcal Software, Northampton, MA). Data are represented as mean ± SE. Dannet's t-test was used to compare statistical significance among four groups; p < 0.05 was considered statistically significant.
Model
Details of the model used to predict the C-terminal domain of the Na+ channel have been published previously by us (Cormier et al., 2002).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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-subunit) cytoplasmic linker between domains III and IV of the channel that occurs within milliseconds of membrane depolarization (Stuhmer et al., 1989; West et al., 1992; Vassilev et al., 1988, 1989), is evident as brief channel openings followed by quiescent periods in single channel recordings and in the decay of peak inward whole cell or macroscopic averaged currents. We previously reported that the inherited mutations Y1795C (associated with long QT variant 3) and Y1795H (associated with Brugada Syndrome) had opposite effects on both single channel gating kinetics and the time course of open state inactivation measured in whole cell recordings (Liu et al., 2002). Here we varied systematically the amino acid substitution for Tyr1795 to determine whether a physical chemical property such as charge or molecular size could be causally linked to the altered rate of open state inactivation. Fig. 1 illustrates the experiments by showing the effects of replacement of Tyr1795 by Ser (Y1795S (YS)). This mutation does not significantly alter channel mean open time (MOT) or open state inactivation kinetics (right traces and histogram, Fig. 1), in contrast to the effects of the Y1795C mutation and the previously reported effects of the Y1795H mutation. What properties of the Cys in this position underlie the apparently distinct slowing of inactivation and prolongation of channel mean open time?
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) mutation slows open state inactivation over the range of voltages tested. Importantly, the slowing (increase in mean open time) is most apparent at voltages positive to -10 mV, where there is little change in the time course of inactivation with further depolarization, i.e., where there is a weak voltage dependence to inactivation kinetics. The effect of this mutation on the kinetics of the onset of inactivation appears to be linked to the presence of the cysteine thiol group, because replacement of Tyr1795 by either alanine or serine, amino acids with similar structures but lacking sulfur atoms, fails to slow inactivation kinetics (Fig. 2, A and B), and none of the mutations affected the voltage dependence of activation (Table 1). We found similar results when we replaced Tyr1795 by Arg, Glu, and His (see Table 1) and, importantly, found no effect on gating kinetics when the tyrosine was replaced by phenylalanine. As summarized in Table 1, the time to half decay of peak whole cell current varies with the following order for residues substituted at codon 1795: C >> A > Y (WT) = F > S > H > E = R. Alteration in inactivation gating does not vary systematically with either charge or size of substituted residues at position 1795, suggesting that mutation of Tyr1795 disrupts a structural motif of the C-terminal tail, which, in turn, plays a key role in controlling channel gating. Furthermore, because of the unique effects of the cysteine substitution, our results raise the possibility that the slowing of inactivation by the Y1795C mutation may be uniquely related to the presence of a Cys residue at this location. This suggests that disulfide bonds may form in the channel C-terminus when Tyr1795 is replaced by Cys in the disease-associated Y1795C mutation. Further, the data suggest that putative disulfide bond formation may impede transitions from the open to inactivated states (slow the onset of inactivation and prolongs channel mean open time).
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-helices (H1 and H4; see Fig. 4, B and C), which cross in a region where two pairs of EF-hands are predicted to be packed against each other. The model also predicts that residue F1794, adjacent to Y1795 in the linear sequence, is far removed from C1850 (see Fig. 4 A), and thus the mutation F1794C would not be likely to form a disulfide bond with residue C1850 according to predictions of the model.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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; West et al., 1992; Vassilev et al., 1988, 1989; Catterall, 1995). Disruption of inactivation by inherited mutation of the III/IV linker (Bennett et al., 1995) or its docking sites, underlie channelopathies in skeletal muscle, neuronal tissue, and in the heart (Green et al., 1998; Lossin et al., 2002; Keating and Sanguinetti, 2001; Balser, 2002; Lerche et al., 2001; Moxley III, 2000; Steinlein and Noebels, 2000). The Y1795C mutation also disrupts inactivation, but not by direct alteration of the inactivation gate or its previously identified docking sites. This mutation, and several others associated both with LQT-3 and Brugada Syndrome, occur in an acid rich region of the C-terminal tail of the channel (An et al., 1998; Bezzina et al., 1999; Veldkamp et al., 2000; Akai et al., 2000; Wei et al., 1999; Ackerman et al., 2001; Rivolta et al., 2001; Baroudi and Chahine, 2000; Deschenes et al., 2000; Wehrens et al., 2000).
Mechanistic basis for altered function: insight from molecular modeling
How might inherited C-terminal mutations, and in particular the Y1795C LQT-3 mutation, alter channel inactivation? Without a solved structure for Nav1.5 (SCN5A), it is difficult to reconcile the functional changes caused by the Y1795C mutation with information that is based solely on its cDNA sequence. However, the experimental results presented in this work provide insight into the plausible structure of the C-terminal domain, and thus can be used to understand how this mutation may alter channel inactivation. In a previous study, we constructed a molecular model for the first half of the C-terminal domain with a remote homologous structural template calmodulin. Based on this structural model, the distance between the C atoms of Y1795 and C1850 is estimated to be 6.8 Å. In general, a disulfide bond involves two cysteines for which the average C distance is 7 Å. Y1795C and C1850 are thus expected to be in range to form a disulfide bridge, provided that the molecular model is reasonably accurate (see Fig. 4). We have demonstrated with CD spectroscopy that the first half of the C-terminal domain is indeed consistent with the calmodulin structure in that the C-terminal domain forms a structure comprised of mostly -helices (Cormier et al., 2002). In this work, we have further confirmed that the predicted head-to-tail distance for part of the model structure with 56 consecutive residues is in range to form a disulfide bond between C1850 and C1795. This head-to-tail distance is extremely sensitive to the tertiary structure of the intervening residues; any structural discrepancies in the orientation and packing of the helices between these two residues would introduce a large range of variation in this head-to-tail distance. The implication of the observed disulfide bond formation is that the tertiary structure for the C-terminal domain that consists of a pair of EF-hand structural motifs must be a reasonable model for the first half of the C-terminal domain.
The structural model has provided insights into the question as to how a disulfide bond in the C-terminal domain might introduce such a drastic changes in inactivation kinetics involving the III-IV loop and the pore domain. Mutational analysis revealed distinct effects of the Y1795C mutation that are particularly significant at positive voltages. At voltages positive to 0 mV, the kinetics of the onset of open state inactivation are significantly and uniquely slowed by the Y1795C mutation independent of test pulse voltage. Intuitively, this voltage independent effect may involve intradomain interactions, as the disulfide bond formation observed in this work. Two speculative models may be considered that involve the C-domain in the inactivation mechanism: 1), the C-terminal domain competes with the III-IV loop binding site in the opened pore domain, such that the III-IV loop can not effectively bind to the pore domain to inactive (block) the channel; or 2), the C-terminal domain interacts with the III-IV loop to hinder the interaction of the III-IV loop with the opened pore domain. A C-terminal domain stabilized by the interhelix disulfide bond can enhance both mechanisms and in turn slow down the inactivation kinetics. Moreover, the fact that polar residues in position 1795 speed up the kinetics of the inactivation is consistent with the possibility that regions near the mutation site may be close to the interaction interface of the C-terminal domain. This region is hydrophobic (see Fig. 4 B) and is likely to interact with the hydrophobic surface exposed in the opened pore domain or the isoleucine-phenylalanine-methionine hydrophobic motif in the III-IV loop. The experimental work so far has not provided direct evidence to support either of the models above. Since the III-IV loop is highly positively charged, the second model also suggests that long range electrostatic interactions involving the negative charge near the mutation site (see Fig. 4 B) can significantly affect the interactions and in turn the kinetics of the inactivation mechanism. These possibilities, which need to be tested with further experimental work, offer a structural basis for life threatening arrhythmias that accompany inherited mutations of the C-terminal domain of the human heart Na+ channel.
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FOOTNOTES |
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Submitted on August 29, 2003; accepted for publication November 7, 2003.
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
) was mutated to Cys (Y1795C, C, 
), Phen (Y1795F, F, •), and Ser (Y1795S, S, 
). (A) The plot shows the time for peak inward current elicited by the test voltages to decay to 50% of peak (t1/2) plotted versus test pulse voltage for each construct. Data shown are mean ± SE: n = 8 (WT); n = 7 (C); n = 6 (S); n = 8 (A). (B) Summary plot of time to half decay (t1/2) measured at +10 mV versus channel construct. Experimental numbers are as in A. *p < 0.01 versus WT; **NS versus WT.
, 
