© 2000 American Heart Association, Inc.
Editorials |
LQT2
Amplitude Reduction and Loss of Selectivity in the Tail That Wags the HERG Channel
From the Department of Physiology and Cardiovascular Research Center, University of Wisconsin – Madison Medical School, Madison, Wis.
Correspondence to Gail Robertson, Department of Physiology, University of Wisconsin – Madison Medical School, 1300 University Ave, Madison, WI 53706. E-mail robertson{at}physiology.wisc.edu
Key Words: LQT2 • HERG • selectivity • K+ channels
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Introduction |
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Without the ion selectivity of voltage-gated ion channels, excitable cells could not generate more than a few millivolts of resting potential, much less produce action potentials. Even when the loss of selectivity occurs in just one type of K+ channel in the heart, the outcome may be catastrophic, suggests a report in this issue of Circulation Research.1 A defect in K+ selectivity caused by a mutation near the "signature sequence" in the pore of HERG channel subunits may contribute as a new mechanism to the prolongation of the action potential and the associated susceptibility to life-threatening torsades de pointes arrhythmias.
Since chromosome 7-linked long-QT syndrome (LQT2) was first mapped to HERG,2 a relative of the Drosophila and mammalian eag genes,3 we have learned much about the pathology of the disease and the underlying physiological mechanisms that have gone awry. In heterologous expression systems, the subunits encoded by the wild-type HERG gene assemble to form channels with the functional properties of IKr,4 5 an unusual repolarizing current first identified by its sensitivity to the antiarrhythmic agent E-4031.6 Our understanding of how IKr participates in repolarization has emerged largely from voltage-clamp analyses of the remarkable tail currents dominating the HERG current profile. At the positive voltages typically reached at the peak or plateau of the ventricular action potential, much of the current is suppressed by a rapid inactivation mechanism.4 5 7 8 9 10 As repolarization ensues, HERG channels recover from inactivation and linger in a highly stable open state before closing.11 The result is a "resurgent current," a term coined for Na+ current in cerebellar Purkinje neurons arising from an analogous gating process.12 HERG mutations causing long-QT syndrome generally reduce the magnitude of this resurgent current by altering channel properties13 14 15 or as a consequence of trafficking defects.16 17
Add to the mounting list of familial HERG mutations N629D,18 situated conspicuously in a guilty-by-association position adjacent to three residues thought to single-handedly ensure high rates of K+ conduction through the pore while excluding Na+ entry.19 Whereas most K+ channels exhibit the signature gly-tyr-gly (GYG),20 HERG and other Eag-related channels have GFG at the corresponding site,3 with a phenylalanine substituting for tyrosine. Mutations at the adjacent residue in Shaker channels (GYGD) are not well tolerated,20 but when engineered into heteromeric dimers, they reduce the degree of selectivity for K+ over Na+ and alter the rate of C-type inactivation.21 As Duff and colleagues1 show, a disease mutation at the corresponding site in HERG (GFGN to D) eliminates selectivity of K+ over Na+ and disrupts C-type inactivation as well, underscoring the influence of residues at this site in the closely related processes of selectivity and C-type inactivation.22 23
It may seem surprising that a mutation that removes inactivation, which would increase the relative amplitude of HERG currents during the peak and plateau phases of the action potential, could prolong the QT interval. However, in the absence of an inactivated state from which to recover, the resurgent current is eliminated and thus cannot contribute to terminal repolarization.8 9 10 What is left is a small, rapid tail current unable to exert much of a physiological effect. If that were not enough, the authors observe, the tail currents of the selectivity-impaired N629D mutant are inward and may be surprisingly effective at further delaying repolarization and contributing to arrhythmogenesis. That even a small inward current could be proarrhythmic is supported by the modeling study of Clancy and Rudy,24 who show that the persistent inward current through mutant Na+ channels associated with LQT325 is capable of prolonging the QT interval, even though it is a mere 2% of its normal peak amplitude. Whether the inward Na+ current tail in the HERG mutant N629D prolongs the QT interval over and above the simple loss of the resurgent current could be put to the test using the same model.
As fascinating as the biophysical defects in channel properties are, one must ask whether long QT in families carrying the N629D mutation might actually be due to a HERG protein trafficking defect. The data indicate that the mutant N629D RNA does not express as well as wild type in oocytes, with one example showing mutant current amplitudes less than 10% of those of wild type, despite the injection of twice as much RNA. It would not be the first case in which biophysical defects in channel properties were of secondary importance to failed transport of channels to the plasma membrane in accounting for the disease process.17 Furthermore, trafficking defects can be exacerbated at physiological temperatures,26 27 potentially limiting the inferences about disease mechanisms that can be drawn from experiments conducted in Xenopus oocytes at room temperature. Given the growing importance of trafficking defects in LQT2, the burden of proof is on the investigator to eliminate this possibility before the disease process can be attributed solely to a defect in channel properties, however compelling the biophysical phenotype may be.
An interesting and paradoxical result of the present study is that an aspartate adjacent to the signature sequence can sabotage selectivity in HERG channels whereas it may serve as part of the normal selectivity filter in Shaker.21 However, no such K+ binding site can be inferred from the superimposed electron density and rubidium difference maps reported for the KcsA bacterial channel structure, which bears the GYGD sequence.19 More information should come to light in the next generation of K+ channel structures, which, with luck, will include a view of HERG at atomic resolution. Whether its curious GFGN signature translates into three-dimensional differences compared with its K+ channel brethren remains to be seen.
This study by Duff and colleagues1 is emblematic of the explosive synergy of basic and clinical research over little more than a single decade, punctuated in this case by such milestones as the cloning of the Shaker28 and eag29 channel genes from Drosophila, the mapping of disease loci on the human chromosome,2 the structural resolution of a K+ channel pore,19 and innumerable structure-function studies along the way. No less significant are the emerging models30 that will soon encompass every element of excitability in different regions of the heart, revealing the mechanistic underpinnings of the long-QT syndromes at the molecular, cellular, and systems levels.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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TopIntroductionReferences |
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