Skip Navigation

Cardiovascular Research 2003 60(2):235-241; doi:10.1016/j.cardiores.2003.08.002
© 2003 by European Society of Cardiology
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Thomas, D.
Right arrow Articles by Karle, C. A
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Thomas, D.
Right arrow Articles by Karle, C. A
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

Copyright © 2003, European Society of Cardiology

Defective protein trafficking in hERG-associated hereditary long QT syndrome (LQT2): molecular mechanisms and restoration of intracellular protein processing

Dierk Thomas*, Johann Kiehn, Hugo A Katus and Christoph A Karle

Department of Cardiology, Medical University Hospital Heidelberg, Bergheimerstrasse 58, D-69115 Heidelberg, Germany

*Corresponding author. Tel.: +49-6221-568476; fax: +49-6221-565515. Email address: dierk_thomas{at}med.uni-heidelberg.de

Received 4 July 2003; revised 29 July 2003; accepted 5 August 2003


   Abstract
Top
 Abstract
1. Introduction
 2. Molecular and cellular...
 3. Mechanisms of defective...
 4. Restoration of normal...
 5. Conclusions
 References
 
Human hereditary long QT syndrome is a cardiac disease characterized by prolongation of the QT interval and increased susceptibility to ventricular arrhythmias and sudden cardiac death. Mutations in the human-ether-a-go-go-related gene (hERG), encoding the protein underlying the repolarizing cardiac IKr potassium current, cause chromosome 7-linked long QT syndrome 2. Loss of function of mutant hERG channels may be caused by several mechanisms, including altered current kinetics, altered ion selectivity, or defective intracellular protein trafficking. Especially the latter category has become a focus of particular interest recently, because some of the mutant subunits display wild type current properties when normal trafficking is restored and channels are inserted in the cell membrane in vitro. This review summarizes the current knowledge on hERG channel trafficking under physiological and pathological conditions. In addition, therapeutic approaches to restore normal hERG trafficking in vitro and in vivo are discussed.

KEYWORDS Antiarrhythmic agents; Arrhythmia (mechanisms); Ion channels; K-channel; Long QT syndrome

Abbreviations: CF, cystic fibrosis • CFTR, cystic fibrosis transmembrane conductance regulator • HERG, human ether-a-go-go-related gene • MiRP1, mink-related peptide 1 • IKr, rapidly activating component of the cardiac delayed rectifier potassium current • LQTS, long QT syndrome


   1. Introduction
Top
 Abstract
 1. Introduction
2. Molecular and cellular...
 3. Mechanisms of defective...
 4. Restoration of normal...
 5. Conclusions
 References
 
Human hereditary long QT syndrome (LQTS) is a heterogeneous cardiac disorder characterized by a prolonged QT interval on the surface ECG and an increased risk for sudden cardiac death due to life-threatening "torsade de pointes" arrhythmias [1]. Characteristically, clinical manifestations of LQTS are already apparent in young patients. Management of LQTS patients [2] relies on reduction of physical and emotional stress in addition to pharmacotherapy with beta-blockers. Drugs that prolong cardiac repolarization should be avoided. Additional treatment options include left cardiac sympathetic denervation, implantation of an implantable cardioverter defibrillator (ICD), and cardiac pacing (in cases of bradycardia-dependent arrhythmias).

The human ether-a-go-go-related gene (hERG or KCNH2) [3] encodes the voltage-gated potassium channel {alpha}-subunit underlying IKr [4,5], a current contributing to the repolarization of the cardiac action potential. HERG potassium channels form homotetramers of identical six {alpha}-helical trans-membrane spanning domains, with a cluster of positive charges localized in the S4 domain acting as putative voltage sensor. It has been suggested that co-assembly of the regulatory β-subunit MiRP1 (minK-related peptide 1) with hERG {alpha}-subunits is required in order to reconstitute native IKr [6]. However, this issue is still discussed controversially. For example, Weerapura et al. [7] did not find any native IKr properties that were closer to properties of currents resulting from co-expression of hERG and MiRP1 compared with expression of hERG without MiRP1.

Mutations in hERG account for the second most common form of LQTS (LQT2 [4]). With only one exception [8], all hERG mutations characterized to date are loss-of-function mutations reducing IKr amplitudes (via haploinsufficiency or dominant-negative suppression of wild type channels) and consecutively prolonging cardiac repolarization [9] (Table 1). Especially the trafficking-deficient mutations (Class 4) have become a focus of interest recently, since most of these mutations give rise to channel proteins capable of conducting IKr current when intracellular trafficking is restored in vitro. Current knowledge on trafficking-deficient hERG mutations, their pathophysiological mechanisms and strategies for rescue of mutant channel proteins are reviewed in the present article.


View this table:
[in this window]
[in a new window]

  		Table 1 Phenotypes caused by hERG mutations associated with hereditary long QT syndrome [10]

 

   2. Molecular and cellular determinants of hERG protein trafficking
Top
 Abstract
 1. Introduction
 2. Molecular and cellular...
3. Mechanisms of defective...
 4. Restoration of normal...
 5. Conclusions
 References
 
2.1. Glycosylation
hERG channel proteins are synthesized in the endoplasmic reticulum before being transported to the cell surface among the Golgi apparatus. During this process, the hERG proteins undergo two crucial steps of glycosylation [15,16] (Fig. 1A). First, immature, newly synthesized hERG channels undergo asparagine (N)-linked core glycosylation in the ER. Consecutively, immature proteins are transported to the Golgi apparatus, where complex glycosylation occurs. Finally, the fully glycosylated form of the hERG wild type (WT) channel is inserted into the cell membrane. In Western blot analyses, hERG WT protein exhibits two characteristic bands. The core-glycosylated, immature precursor form is represented by a characteristic 135 kDa band, whereas the fully glycosylated mature protein is visible as 155 kDa band [11,15–17]. The hERG protein contains two N-linked glycosylation consensus sites (sequence: N-X-T/S; X may be any residue except proline), which are located in the extracellular S5-S6 linker (N598, N629) [15]. It has been demonstrated that hERG proteins are core-glycosylated by addition of high-mannose oligosaccharides at the N598 site, but not at position N629 [16]. The glycosylation process does not appear to be crucial for hERG protein assembly and trafficking, since the non-glycosylatable mutant hERG N598Q protein produced functional hERG channels. However, non-glycosylated hERG channels are degraded more rapidly than glycosylated channel proteins [16].


Figure 1
View larger version (22K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 1 (A) Mechanisms and sites of hERG protein glycosylation (see text for details). (B) Quality control mechanisms involved in hERG channel trafficking. Transient complexes are formed by core-glycosylated hERG proteins and the cytosolic chaperones Hsp70 and Hsp90 during protein maturation in the endoplasmic reticulum.

 
2.2. ER retention signals
Misfolded and incompletely assembled proteins are common side products of protein synthesis in the ER. These abnormal proteins are recognized by an ER quality control mechanism, resulting in retention followed by chaperone-induced correction or proteasomal degradation of the protein [18]. Recently, the amino acid sequence "R-X-R" (with any amino acid in position X) has been identified as a putative ER retention signal [19]. Analysis of the hERG sequence revealed that a putative ER retention signal is present in the C terminus of hERG (R-G-R at positions 1005–1007) [20]. Despite its presence in wild type hERG proteins, these channels are not trafficking-deficient. Thus, it is suggested that the C-terminal 104 amino acids mask and inactivate this retention signal in wild type channels, and that truncation of the C terminus leads to exposure of the R-G-R sequence, resulting in mistrafficking.

2.3. Cytosolic chaperones
In mammalian cells, one of the quality control mechanisms consists of ER-associated and cytoplasmic chaperones [18]. Ficker et al. [21] identified two cytosolic chaperones, heat shock protein (Hsp) 70 and Hsp90, interacting dynamically with the immature core-glycosylated ER-resident form of hERG, thereby forming a transient complex. During protein synthesis, large portions of the hERG protein are exposed to the cytoplasm and thus possible binding partners for chaperones. These partners include the N-terminal Per, Arnt, and Sim (PAS) domain, the putative C-terminal ER retention signal R-G-R, and the C-terminal cyclic nucleotide binding domain (cNBD), whereas small parts of the hERG tetramer point towards the ER lumen [3,22] (Fig. 1B). While Hsp70 participates in de novo protein folding by holding newly synthesized chains in a state competent for folding [23], Hsp90 is known to facilitate folding of proteins with complex conformations [24]. Thus, in case of hERG proteins, the correct folding of the large cytoplasmic termini including the PAS and cNBD domains might require Hsp90 binding. It is suggested that a large multi-chaperone complex with Hsp90 and Hsp70 at its core may be required for correct hERG folding in the ER (Fig. 1B), and hERG proteins that do not interact properly with Hsp90 may be diverted directly into the degradation pathway, while successful protein folding is accompanied by dissociation of the channel-chaperone complexes [21]. However, it is not yet understood how chaperones and hERG proteins interact on the molecular level, and how hERG-chaperone complexes and forward transport from the ER to the Golgi apparatus and into the plasma membrane are linked.

2.4. Golgi apparatus
The role of the Golgi apparatus within the machinery required for normal protein processing has been addressed by Roti Roti et al. [25]. The authors found that during maturation in the Golgi apparatus, hERG proteins associate with GM130/golgin-95, a Golgi-associated protein that is involved in vesicular transport. On the molecular level, these interactions seem to require integrity of certain C-terminal regions including the cNBD and the last ~100 amino acids of the hERG protein. Overexpression of GM130 suppressed hERG currents in vitro. Thus, GM130 may act as molecular checkpoint in hERG processing, slowing hERG channel trafficking when overexpressed. However, its role in glycosylation and maturation of hERG has yet to be determined.


   3. Mechanisms of defective protein trafficking
Top
 Abstract
 1. Introduction
 2. Molecular and cellular...
 3. Mechanisms of defective...
4. Restoration of normal...
 5. Conclusions
 References
 
3.1. Retention of defective hERG proteins in the ER
To date, various studies have shown that under physiological conditions (37 °C incubation temperature) many heterologously expressed mutant hERG channels are retained in the endoplasmic reticulum [10–14,21,25–28]. Some of these mutant channels (e.g. HERG R752W [10]) display no currents at all, whereas others (e.g. HERG G601S [12]) are hypomorphic (i.e. still capable of expressing greatly reduced, kinetically unaltered potassium currents at physiological temperatures). In general, the mutations are believed to give rise to abnormally folded and assembled channel proteins that are retained in the ER by quality control mechanisms. In addition, disruption of interactions between hERG and Golgi-associated proteins might also contribute to the pathogenesis of LQTS. The discovery of interactions between the chaperones Hsp70/Hsp90 and hERG proteins during maturation of hERG channels revealed that mutant (misfolded) hERG proteins display increased and prolonged association with this chaperone complex in the ER [21]. This observation might illustrate the attempt of the chaperone complex to arrest the mutant proteins in the ER (Hsp70) and re-fold them into a correct native conformation (Hsp90). Conversely, LQT2 mutations located in the cytoplasmic N- or C-terminal protein domains might interfere directly with chaperone binding, which might prevent chaperone-induced correction of folding defects, consecutively inducing more severe phenotypes. Moreover, the degree of chaperone-association was lower for hypomorphic channels than for non-functional mutants, which may provide a correlation to the mild phenotype observed with hypomorphic mutants (e.g. hERG G601S [21]). In contrast, trafficking-competent, but non-functional hERG628S channels [11] (Table 1, Class 3) displayed chaperone-association and maturation properties similar to hERG WT channels, indicating that this mutant protein possesses a conformation so close to the native hERG WT conformation that the quality control machinery does not recognize it as abnormal [21].

3.2. Dominant-negative current suppression caused by trafficking defects
Dominant-negative current suppression is a mechanism by which mutant hERG alleles cause reduction of wild type hERG channels in heterozygous patients, with the result that on co-assembly of hERG WT and mutant hERG channel subunits tetrameric hERG channel complexes are rendered non-conducting. In contrast to this "classical" dominant-negative mechanism, Ficker et al. [14] demonstrated that the trafficking-deficient (Class 4) hERG A651V mutant formed heterotetrameric channels that were retained in the ER to a great extent and degraded. Only few channel heterotetramers reached the cell surface, where they did not conduct any detectable currents. This serves as an example of dominant-negative wild type current suppression by an acquired hERG trafficking defect.

3.3. Interruption of interactions between hERG and GM130
Although the physiological role of the Golgi-associated protein GM130 in hERG maturation is not yet defined, its pathophysiological significance in hereditary long QT syndrome is illustrated by LQT2 mutations that selectively disrupt interactions between hERG and GM130 (hERG V822M, hERG S818P, and hERG R823W), as demonstrated by yeast two-hybrid assays [25]. These mutations are located in the cyclic nucleotide binding domain in the C-terminus of hERG. It may be hypothesized that GM130 serves as an example of a hERG trafficking defect caused by disruption of interactions with proteins along the intracellular hERG transport pathway. The exact mechanism of GM 130 interaction with hERG on a molecular level and whether it participates in hERG glycosylation, however, have not yet been determined.

3.4. Trafficking block by pharmacological inhibition of the cytosolic chaperone Hsp90
Drugs that block Hsp90 function (e.g. geldanamycin, radicicol) are being used as chemotherapeutic agents [29]. Since inhibition of Hsp90 blocks hERG maturation, it is speculated that pharmacological inhibition of Hsp90 may cause long QT syndrome by an acquired protein trafficking defect [21], providing a possible novel mechanism in human LQTS. On a molecular level, inhibition of Hsp90 increases multiubiquitination and subsequent degradation of hERG proteins, which are likely to be primarily consumed by the proteasome complex. However, it is not yet understood how the quality control machinery discriminates between misfolded mutant hERG proteins and wild type proteins that are in the process of folding. Moreover, the participation of other chaperones such as Hsp40, TRiC, BiP, or calnexin in hERG maturation remains to be investigated in future studies.

3.5. Lessons from cystic fibrosis: molecular mechanisms of defective CFTR trafficking
Defective protein trafficking has been discovered and studied extensively in cystic fibrosis (CF), and several approaches to restoring correct hERG protein trafficking have been adopted from previous results obtained in investigations of membrane trafficking in CF. The most common cystic fibrosis transmembrane conductance regulator (CFTR) mutation {Delta}F508 decreases the efficiency of CFTR protein folding, reduces the probability of its dislocation from molecular chaperones, and prevents its maturation during the intracellular processing pathway [30,31]. Similar to hERG channels, CFTR proteins are synthesized as precursors in the rough ER, where they are associated with the cytoplasmic chaperones Hsp40, Hsp70, and Hsp90, as well as with the ER luminal chaperone calnexin [31]. Coincident with its dissociation from Hsp70 and calnexin, CFTR matures to the Golgi apparatus, where it is converted to a stable mature protein, followed by integration into the plasma membrane. A crucial folding step within this procedure is CFTR maturation to the Golgi apparatus, which reflects the acquisition of a correct tertiary structure. The {Delta}F508 mutation interferes with this step, with the consequence that CFTR {Delta}F508 molecules remain in a partially folded or unfolded state in association with chaperones. In particular, the {Delta}F508 mutation affects the first cyclic nucleotide binding domain (cNBD1) in CFTR, with consequent self-aggregation of CFTR {Delta}F508 peptides. It is suggested that release from the ribosome or from the translocation machinery serves as an early quality control checkpoint, since misfolded CFTR {Delta}F508 molecules are degraded rapidly after synthesis in the ubiquitin-proteasome pathway [30]. Degradation of CFTR proteins is a multistep process that involves recognition, ubiquitin conjugation, dislocation across the ER membrane, and finally unfolding and degradation by the proteasome [31].

Overexpression of CFTR {Delta}F508 leads to the appearance of functional CFTR chloride channels in the plasma membrane, indicating that its phenotype is "leaky". This effect could be due to a mass-action effect or to a kinetic model of a small portion of CFTR {Delta}F508 molecules folding correctly and reaching the plasma membrane. The observations that CFTR {Delta}F508 maturation block is temperature sensitive and that chemical chaperones can stabilize the protein structure clearly argue in favour of the kinetic model. In addition, small, high-affinity compounds have been shown to influence the conformational maturation of the CFTR {Delta}F508 protein, thereby ameliorating the crucial folding defects [31]. This provides a starting point for pharmacological rescue of mutant CFTR proteins in vivo.


   4. Restoration of normal hERG trafficking
Top
 Abstract
 1. Introduction
 2. Molecular and cellular...
 3. Mechanisms of defective...
 4. Restoration of normal...
5. Conclusions
 References
 
In cystic fibrosis, the CFTR mutation {Delta}F508 which displays trafficking-deficiency can be rescued by lowering the incubation temperature or by application of chemical chaperones (e.g. glycerol, dimethyl sulfoxide/DMSO, trimethylamine oxide/TMO [32–34]). In addition, it has been shown that misprocessed mutants of the multidrug transporter P-glycoprotein can be rescued using specific substrates and inhibitors [35]. These results have initiated the search for similar rescue strategies in LQT2-associated hERG channel mutants.

4.1. Temperature effects
The majority of identified trafficking-deficient mutants have been shown to be temperature-sensitive [10,12,13,21,26,36,37]. Under in vitro conditions, lowering of the incubation temperature from 37 to 26 °C or 27 °C restored hERG protein trafficking, with mutant channels giving rise to detectable potassium currents in patch clamp experiments and mature proteins in Western blot analyses. The temperature-dependent induction of channel folding and processing in hERG R752W has been shown to be accompanied by a reduction in Hsp70 and Hsp90 association as well as by a markedly slowed synthesis and trafficking process. Thus, the improved channel folding at lower temperatures might be due to an increased ER retention time at lower temperatures. In addition, reversal of the mutant's effects on hERG currents could be due to inhibition of proteasomal degradation at reduced incubation temperatures.

4.2. Transcriptional regulators
To date, few studies have investigated the effects of a transcriptional regulator (4-phenylbutyrate) on hERG channel maturation [10,38]. Despite markedly enhanced synthesis of mutant hERG R752W protein at 37 °C upon application of 4-phenylbutyrate, transport of the mature protein to the cell surface could not be restored, indicating that the quality control machinery held the mutant protein in the ER, until it underwent degradation. Furthermore, in a study of five patients with hERG R752W-associated long QT-syndrome treated with 4-phenylbutyrate (19 g/day for 1 week; a dose that has proven successful in CFTR rescue in vivo), cardiac repolarization was not significantly improved [38]. These results suggest that enhanced protein expression may not be sufficient in restoring protein transport. Although data are limited, additional factors such as prolonged ER retention time (see above) and/or interactions with molecular or pharmacological chaperones seem to be required for successful rescue of hERG LQT2 mutants.

4.3. Non-specific chemical chaperones
The group of chemical chaperones consists of compounds with low molecular weight (e.g. glycerol, DMSO, TMO). Chemical chaperones are believed to stabilize protein conformations during their maturation [39], and restoration of trafficking by chemical chaperones such as glycerol has been demonstrated for hypomorphic hERG N470D channels [13]. However, hERG R752W trafficking could not be corrected by incubation with glycerol [10], indicating that rescue by chemical chaperones depends on the severity of the (Class 4) mutation in hERG, which is determined by the protein domain affected by a mutation and by properties of the amino acid residue introduced. Further investigations (e.g. mutagenesis studies on the effects of different amino acid substitutions at a certain position) need to be carried out to distinguish between amino acid- and domain-related success and failure of chemical chaperones. Finally, the high concentrations of chemical chaperones required for restoration of protein trafficking (e.g. 10% glycerol) restrict their clinical use.

In a different approach by Delisle et al. [36], trafficking of mutant G601S and F805C channels was restored by application of the sarcoplasmic/ER Ca2+-ATPase (SERCA) inhibitor thapsigargin, which did not cause hERG current inhibition. It is speculated that in this mechanistically different case, alterations in the activity of calcium-dependent chaperone proteins in the ER promote the relocation of intracellular hERG protein to the cell surface. The underlying mechanism requires an intact Golgi apparatus. Clearly, further studies are required to investigate the precise mechanism of rescue by thapsigargin and to identify the chaperones involved in this rescue pathway.

4.4. Specific pharmacological chaperones
Pharmacological rescue of trafficking-deficient hERG mutants appears to be the most promising approach to date. It has been revealed that trafficking of some Class 4 hERG mutants (e.g. hERG T65P, hERG N470D, hERG G601S) can be restored by application of high-affinity hERG channel ligands (i.e. inhibitors) such as the methanesulfonanilide E4031, the antihistamines astemizole and terfenadine, or the gastrointestinal prokinetic drug cisapride [13,21,26,28,37]. In contrast, attempts to rescue mutant hERG A561V, hERG R752W, hERG F805C or hERG R823W channels by incubation of pharmacological chaperones failed [10,14,26], suggesting that these channels represent a group of hERG LQT2 mutants which is resistant to pharmacological rescue. More recently, the molecular determinants and the binding site for hERG channel ligands serving as pharmacological chaperones have been investigated in detail [26]. The authors revealed that the efficacy for channel rescue is linked directly to current inhibition potency, and both channel rescue and current block require binding of the drug at a common binding site located inside the channel pore involving the aromatic residue F656, as reported previously for pharmacological hERG current inhibition [40]. Furthermore, Ficker et al. [26] confirmed that trafficking restoration with channel blockers is dependent on the protein domain affected by the mutation. In particular, mutations located in the cNBD of hERG appear to be insensitive to pharmacological rescue.

However, the molecular mechanism by which blocking molecules interact with certain mutant hERG proteins to stabilize their conformation and facilitate their intracellular processing remains to be elucidated. Compared to temperature-dependent restoration of hERG processing, rescue by specific hERG ligands seems to require a location of the underlying mutation outside a domain that is crucial for chaperone (i.e. Hsp70 and Hsp90) association. On the other hand, it may be speculated that drug binding to the hydrophobic inner vestibule of the pore region of hERG tetramers during maturation facilitates channel transport to the plasma membrane. Consecutively, impaired subunit assembly caused by point mutations might prevent binding of specific chemical chaperones, which could explain why some mutant proteins are not rescued by channel ligands. Indeed the hERG R752W and R823W mutants which are resistant to pharmacological rescue do not display dominant-negative suppression of wild type currents, suggesting improper assembly [10,41].

Nonetheless, pharmacological restoration of hERG channel maturation with high-affinity inhibitors of hERG currents requires application of the drug to the culture medium for a certain incubation time, followed by removal of the hERG-blocking drug prior to electrophysiological measurements, which prevents their clinical use. Surprisingly, Rajamani et al. [28] demonstrated that the less severe hypomorph hERG mutants N470D and G601S can be rescued by the terfenadine derivative fexofenadine, which displays low affinity for hERG channels but exhibits channel rescue with high efficacy, resulting in markedly different concentrations required for block and rescue (i.e., the half-maximal channel block concentration was {approx}350 fold higher than the half-maximal rescue concentration). Thus, although this mechanism might be restricted to hypomorph Class 4 hERG mutants, it represents a possible new therapeutic approach.


   5. Conclusions
Top
 Abstract
 1. Introduction
 2. Molecular and cellular...
 3. Mechanisms of defective...
 4. Restoration of normal...
 5. Conclusions
References
 
Trafficking defects are present in a significant group of hERG mutants associated with LQT2. Since many of the trafficking-deficient channels give rise to functional channels when inserted into the cell membrane, restoration of normal channel trafficking is an attractive strategy for treating patients carrying Class 4 hERG mutations. Among the strategies investigated to date, pharmacological restoration of hERG maturation with low-affinity channel antagonists seems to be most promising, although this hypothesis clearly requires detailed clinical evaluation. However, limitations may derive from the fact that in each individual LQTS patient, the underlying mutation will have to be analysed and characterized in vitro, and the efficacy of specific therapies will have to be evaluated for each individual mutation prior to treatment.


   Acknowledgements
 
This work was supported in part by grants from the Deutsche Forschungsgemeinschaft (project KI 663/1-1 to J.K.; project KA 1714/1-1 to C.A.K.), from the University of Heidelberg (AiP+F to D.T.), from the Novartis-Foundation (to D.T.), and from the Foundation Cardiology 2000 (Forssmann-Scholarship to D.T.).


   Notes
 
Time for primary review 19 days


   References
Top
 Abstract
 1. Introduction
 2. Molecular and cellular...
 3. Mechanisms of defective...
 4. Restoration of normal...
 5. Conclusions
 References
 

  1. Keating M.T., Sanguinetti M.C. Molecular and cellular mechanisms of cardiac arrhythmias. Cell (2001) 104:569–580.[CrossRef][ISI][Medline]
  2. Priori S., Aliot E., Blomstrom-Lundqvist C., et al. Task force on sudden cardiac death of the European Society of Cardiology. Eur. Heart J. (2001) 22:1374–1450.[Free Full Text]
  3. Warmke J.W., Ganetzky B. A family of potassium channel genes related to eag in Drosophila and mammals. Proc. Natl. Acad. Sci. U. S. A. (1994) 91:3438–3442.[Abstract/Free Full Text]
  4. Sanguinetti M.C., Jiang C., Curran M.E., Keating M.T. A mechanistic link between an inherited and an acquired cardiac arrhythmia: HERG encodes the IKr potassium channel. Cell (1995) 81:299–307.[CrossRef][ISI][Medline]
  5. Kiehn J., Lacerda A.E., Wible B.A., Brown A.M. Molecular physiology and pharmacology of HERG. Single-channel currents and block by dofetilide. Circulation (1996) 94:2572–2579.[Abstract/Free Full Text]
  6. Abbott G.W., Sesti F., Splawski I., et al. MiRP1 forms IKr potassium channels and is associated with cardiac arrhythmia. Cell (1999) 97:175–187.[CrossRef][ISI][Medline]
  7. Weerapura M., Nattel S., Chartier D., Caballero R., Hebert T.E. A comparison of currents carried by HERG, with and without coexpression of MiRP1, and the native rapid delayed rectifier current. Is MiRP1 the missing link? J. Physiol. (2002) 540:15–27.[Abstract/Free Full Text]
  8. Lees-Miller J.P., Duan Y., Teng G.Q., Thorstad K., Duff H.J. Novel gain-of-function mechanism in K(+) channel-related long-QT syndrome: altered gating and selectivity in the HERG1 N629D mutant. Circ. Res. (2000) 86:507–513.[Abstract/Free Full Text]
  9. Sanguinetti M.C., Curran M.E., Spector P.S., Keating M.T. Spectrum of HERG K+-channel dysfunction in an inherited cardiac arrhythmia. Proc. Natl. Acad. Sci. U. S. A. (1996) 93:2208–2212.[Abstract/Free Full Text]
  10. Ficker E., Thomas D., Viswanathan P.C., et al. Novel characteristics of a misprocessed mutant HERG channel linked to hereditary long QT syndrome. Am. J. Physiol. (2000) 279:H1748–H1756.[ISI]
  11. Zhou Z., Gong Q., Epstein M.L., January C.T. HERG channel dysfunction in human long QT syndrome: intracellular transport and functional defects. J. Biol. Chem. (1998) 273:21061–21066.[Abstract/Free Full Text]
  12. Furutani M., Trudeau M.C., Hagiwara N., et al. Novel mechanism associated with an inherited cardiac arrhythmia: defective protein trafficking of a mutant HERG (G601S) potassium channel. Circulation (1999) 99:2290–2294.[Abstract/Free Full Text]
  13. Zhou Z., Gong Q., January C.T. Correction of defective protein trafficking of a mutant HERG potassium channel in human long QT syndrome: pharmacological and temperature effects. J. Biol. Chem. (1999) 274:31123–31126.[Abstract/Free Full Text]
  14. Ficker E., Dennis A.T., Obejero-Paz C.A., Castaldo P., Taglialatela M., Brown A.M. Retention in the endoplasmic reticulum as a mechanism of dominant-negative current suppression in human long QT syndrome. J. Mol. Cell Cardiol. (2000) 32:2327–2337.[CrossRef][ISI][Medline]
  15. Petrecca K., Atanasiu R., Akhavan A., Shrier A. N-linked glycosylation sites determine HERG channel surface membrane expression. J. Physiol. (1999) 515:41–48.[Abstract/Free Full Text]
  16. Gong Q., Anderson C.L., January C.T., Zhou Z. Role of glycosylation in cell surface expression and stability of HERG potassium channels. Am. J. Physiol. (2002) 283:H77–H84.[ISI]
  17. Zhou Z., Gong Q., Ye B., Fan Z., Makielski J.C., Robertson G.A., et al. Properties of HERG channels stably expressed in HEK 293 cells studied at physiological temperature. Biophys. J. (1998) 74:230–241.[Abstract/Free Full Text]
  18. Ellgaard L., Molinari M., Helenius A. Setting the standards: quality control in the secretory pathway. Science (1999) 286:1882–1888.[Abstract/Free Full Text]
  19. Zerangue N., Schwappach B., Jan Y.N., Jan L.Y. A new ER trafficking signal regulates the subunit stoichiometry of plasma membrane K(ATP) channels. Neuron (1999) 22:537–548.[CrossRef][ISI][Medline]
  20. Kupershmidt S., Yang T., Chanthaphaychith S., Wang Z., Towbin J.A., Roden D.M. Defective human ether-a-go-go-related gene trafficking linked to an endoplasmic reticulum retention signal in the C terminus. J. Biol. Chem. (2002) 277:27442–27448.[Abstract/Free Full Text]
  21. Ficker E., Dennis A.T., Wang L., Brown A.M. Role of cytosolic chaperones Hsp70 and Hsp90 in maturation of the cardiac potassium channel hERG. Circ. Res. (2003) 92:87–100.[CrossRef]
  22. Morais-Cabral J.H., Lee A., Cohen S.L., Chait B.T., Li M., MacKinnon R. Crystal structure and functional analysis of the HERG potassium channel N terminus: a eukaryotic PAS domain. Cell (1998) 95:649–655.[CrossRef][ISI][Medline]
  23. Hartl F.U., Hayer-Hartl M. Molecular chaperones in the cytosol: from nascent chain to folded protein. Science (2002) 295:1852–1858.[Abstract/Free Full Text]
  24. Nathan D.F., Vos M.H., Lindquist S. In vivo functions of the Saccharomyces cerevisiae Hsp90 chaperone. Proc. Natl. Acad. Sci. U. S. A. (1997) 94:12949–12956.[Abstract/Free Full Text]
  25. Roti Roti E.C., Myers C.D., Ayers R.A., et al. Interaction with GM130 during HERG ion channel trafficking: disruption by type 2 congenital long QT syndrome mutations. J. Biol. Chem. (2002) 277:47779–47785.[Abstract/Free Full Text]
  26. Ficker E., Obejero-Paz C.A., Zhao S., Brown A.M. The binding site for channel blockers that rescue misprocessed human long QT syndrome 2 ether-a-go-go-related gene (HERG) mutations. J. Biol. Chem. (2002) 277:4989–4998.[Abstract/Free Full Text]
  27. Kagan A., Yu Z., Fishman G.I., McDonald T.V. The dominant negative LQT2 mutation A651V reduces wild-type HERG expression. J. Biol. Chem. (2000) 275:11241–11248.[Abstract/Free Full Text]
  28. Rajamani S., Anderson C.L., Anson B.D., January C.T. Pharmacological rescue of human K+ channel long-QT2 mutations: human ether-a-go-go-related gene rescue without block. Circulation (2002) 105:2830–2835.[Abstract/Free Full Text]
  29. Neckers L. Hsp90 inhibitors as novel chemotherapeutic agents. Trends Mol. Med. (2002) 8:S55–S61.[CrossRef][ISI][Medline]
  30. Kopito R.R. Biosynthesis and degradation of CFTR. Physiol .Rev. (1999) 79:S167–S173.[Medline]
  31. Gelman M.S., Kopito R.R. Rescuing protein conformation: prospects for pharmacological therapy in cystic fibrosis. J. Clin. Invest. (2002) 110:1591–1597.[CrossRef][ISI][Medline]
  32. Denning G.M., Anderson M.P., Amara J.F., Marshall J., Smith A.E., Welsh M.J. Processing of mutant cystic fibrosis transmembrane conductance regulator is temperature-sensitive. Nature (1992) 258:761–764.
  33. Brown C.R., Hong-Brown Q., Biwersi J., Verkman A.S., Welch W.J. Chemical chaperones correct the mutant phenotype of the delta F508 cystic fibrosis transmembrane conductance regulator protein. Cell Stress Chaperones (1996) 1:117–125.[CrossRef][ISI][Medline]
  34. Sato S., Ward C.L., Krouse M.E., Wine J.J., Kopito R.R. Glycerol reverses the misfolding phenotype of the most common cystic fibrosis mutation. J. Biol. Chem. (1996) 271:635–639.[Abstract/Free Full Text]
  35. Loo T.W., Clarke D.M. Correction of defective protein kinesis of human P-glycoprotein mutants by substrates and modulators. J. Biol. Chem. (1997) 272:709–712.[Abstract/Free Full Text]
  36. Delisle B.P., Anderson C.L., Balijepalli R.C., Anson B.D., Kamp T.J., January C.T. Thapsigargin selectively rescues the trafficking-defective LQT2 channels G601S and F805C. J. Biol. Chem. (2003) [JBC Papers in press, M305787200].
  37. Paulussen A., Raes A., Matthijs G., Snyders D.J., Cohen N., Aerssens J. A novel mutation (T65P) in the PAS domain of the human potassium channel HERG results in the long QT syndrome by trafficking deficiency. J. Biol. Chem. (2002) 50:48610–48616.
  38. Kaufman E.S., Ficker E. Is restoration of intracellular trafficking clinically feasible in the long QT syndrome? The example of HERG mutations. J. Cardiovasc. Electrophysiol. (2003) 14:320–322.[ISI][Medline]
  39. Smith D.F., Whitesell L., Katsanis E. Molecular chaperones: biology and prospects for pharmacological intervention. Pharmacol. Rev. (1998) 50:493–513.[Abstract/Free Full Text]
  40. Mitcheson J.S., Chen J., Lin M., Culberson C., Sanguinetti M.C. A structural basis for drug-induced long QT syndrome. Proc. Natl. Acad. Sci. U. S. A. (2000) 97:12329–12333.[Abstract/Free Full Text]
  41. Cui J., Kagan A., Qin D., Mathew J., Melman Y.F., McDonald T.V. Analysis of the cyclic nucleotide binding domain of the HERG potassium channel and interactions with KCNE2. J. Biol. Chem. (2001) 276:17244–17251.[Abstract/Free Full Text]

Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?


This article has been cited by other articles:


Home page
 Mol. Cell. Biol.Home page
L. Guasti, O. Crociani, E. Redaelli, S. Pillozzi, S. Polvani, M. Masselli, T. Mello, A. Galli, A. Amedei, R. S. Wymore, et al.
Identification of a Posttranslational Mechanism for the Regulation of hERG1 K+ Channel Expression and hERG1 Current Density in Tumor Cells
Mol. Cell. Biol., August 15, 2008; 28(16): 5043 - 5060.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Regul. Integr. Comp. Physiol.Home page
M. Hassinen, J. Haverinen, and M. Vornanen
Electrophysiological properties and expression of the delayed rectifier potassium (ERG) channels in the heart of thermally acclimated rainbow trout
Am J Physiol Regulatory Integrative Comp Physiol, July 1, 2008; 295(1): R297 - R308.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Thomas, D.
Right arrow Articles by Karle, C. A
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Thomas, D.
Right arrow Articles by Karle, C. A
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?