Article

Mechanisms of pharmacological rescue of trafficking-defective hERG mutant channels in human long QT syndrome

Division of Cardiovascular Medicine, Department of Medicine, Oregon Health & Science University, Portland, 97239, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 03/2006; 281(7):4069-74. DOI: 10.1074/jbc.M511765200
Source: PubMed

ABSTRACT

Long QT syndrome type 2 is caused by mutations in the human ether-a-go-go-related gene (hERG). We previously reported that the N470D mutation is retained in the endoplasmic reticulum (ER) but can be rescued to the plasma membrane by hERG channel blocker E-4031. The mechanisms of ER retention and how E-4031 rescues the N470D mutant are poorly understood. In this study, we investigated the interaction of hERG channels with the ER chaperone protein calnexin. Using coimmunoprecipitation, we showed that the immature forms of both wild type hERG and N470D associated with calnexin. The association required N-linked glycosylation of hERG channels. Pulse-chase analysis revealed that N470D had a prolonged association with calnexin compared with wild type hERG and E-4031 shortened the time course of calnexin association with N470D. To test whether the prolonged association of N470D with calnexin is due to defective folding of mutant channels, we studied hERG channel folding using the trypsin digestion method. We found that N470D and the immature form of wild type hERG were more sensitive to trypsin digestion than the mature form of wild type hERG. In the presence of E-4031, N470D became more resistant to trypsin even when its ER-to-Golgi transport was blocked by brefeldin A. These results suggest that defective folding of N470D contributes to its prolonged association with calnexin and ER retention and that E-4031 may restore proper folding of the N470D channel leading to its cell surface expression.

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    • "Several different chaperone and co-chaperone proteins that interact with Kv11.1a proteins have been identified. They include the ER lectin chaperone calnexin, the ER transmembrane chaperone 38-kDa FK506-binding protein FKBP8 (FKBP38), cytosolic heat shock proteins 40, 70, and 90 (Hsp40, Hsp70, and Hsp90), the cytosolic heat shock cognate protein 70 (Hsc70), the cytosolic cochaperone Hsp-organizing protein (HOP), the BCL-associated athanogene 2 (BAG-2), and the E3-ubiquitin ligase CHIP444546. Ficker and colleagues (2002) were the first to demonstrate that Hsp90 is crucial for the maturation and trafficking of Kv11.1a channels to the cell membrane[44]. "
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    ABSTRACT: The molecular mechanisms underlying congenital long QT syndrome (LQTS) are now beginning to be understood. New insights into the etiology and therapeutic strategies are emerging from heterologous expression studies of LQTS-linked mutant proteins, as well as inducible pluripotent stem cell derived cardiomyocytes (iPSC-CMs) from LQTS patients. This review focuses on the major molecular mechanism that underlies LQTS type 2 (LQT2). LQT2 is caused by loss of function (LOF) mutations in KCNH2 (also known as the human Ether-à-go-go-Related Gene or hERG). Most LQT2-linked mutations are missense mutations and functional studies suggest that ~90% of them disrupt the intracellular transport (trafficking) of KCNH2-encoded Kv11.1 proteins to the cell membrane. Trafficking deficient LQT2 mutations disrupt Kv11.1 protein folding and misfolded Kv11.1 proteins are retained in the endoplasmic reticulum (ER) until they are degraded in the ER associated degradation pathway (ERAD). This review focuses on the quality control mechanisms in the ER that contribute to the folding and ERAD of Kv11.1 proteins; the mechanism for ER export of Kv11.1 proteins in the secretory pathway; different subclasses of trafficking deficient LQT2 mutations; and strategies being developed to mitigate or correct trafficking deficient LQT2-related phenotypes.
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    • "The data showed that BFA alone resulted in a time-dependent reduction of the 155-kDa band (Fig. 3A), reflecting the dynamic degradation of mature hERG protein [29]. Ang II further significantly accelerated the degradation of the mature hERG protein (Fig. 3B). "
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    ABSTRACT: We investigated the effects of AT1 receptor stimulation by angiotensin II (Ang II) on human ether-a-go-go-related gene (hERG) potassium channel protein in a heterogeneous expression system with the human embryonic kidney (HEK) 293 cells which stably expressed hERG channel protein and were transiently transfected with the human AT1 receptors (HEK293/hERG). Western-blot analysis showed that Ang II significantly decreased the expression of mature hERG channel protein (155-kDa band) in a time- and dose-dependent manner without affecting the level of immature hERG channel protein (135-kDa band). The relative intensity of 155-kDa band was 64.7 ± 6.8% of control (P < 0.01) after treatment of Ang II at 100 nM for 24 h. To investigate the effect of Ang II on the degradation of mature hERG channel protein, we blocked forward trafficking from ER to Golgi with a Golgi transit inhibitor brefeldin A (10 μM). Ang II significantly enhanced the time-dependent reduction of mature hERG channel protein. In addition, the proteasomal inhibitor lactacystin (5 μM) inhibited Ang II-mediated the reduction of mature hERG channel protein, but the lysosomal inhibitor bafilomycin A1 (1 μM) had no effect on the protein. The protein kinase C (PKC) inhibitor bisindolylmaleimide 1 (1 μM) antagonized the reduction of mature hERG channel protein induced by Ang II. The results indicate that sustained stimulation of AT1 receptors by Ang II reduces the mature hERG channel protein via accelerating channel proteasomal degradation involving the PKC pathway.
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    • "However, the drugs also bind many of the trafficking-defective LQT2 hERG forms mutated in the pore-forming region, to stabilize them and partially rescue their trafficking. In contrast, the drugs have no effects on trafficking-defective hERG with mutations in its independently folded N- or C-terminal regions (Anderson et al., 2006; Gong et al., 2006; Harley et al., 2012). "
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