Elsevier

Heart Rhythm

Volume 13, Issue 10, October 2016, Pages 2004-2011
Heart Rhythm

Febrile temperature facilitates hERG/IKr degradation through an altered K+ dependence

https://doi.org/10.1016/j.hrthm.2016.06.019Get rights and content

Background

Dysfunction of the rapidly activating delayed rectifier K+ channel (IKr) encoded by the human ether-à-go-go-related gene (hERG) is the primary cause of acquired long QT syndrome (LQTS). Fever has been reported to trigger LQTS in various conditions.

Objective

We aim to clarify the effect and underlying mechanisms of febrile temperature on hERG expressed in HEK cells, IKr in neonatal rat ventricular myocytes, and the QT interval in rabbits.

Methods

Western blot analysis was used to determine the expression of hERG channel protein in stably transfected HEK 293 cells. Immunocytochemistry was used to visualize the localization of hERG channels. The whole-cell patch clamp technique was used to record hERG K+ current (IhERG) in hERG expressing HEK 293 cells, as well as IKr, transient outward K+ current (Ito), and L-type Ca2+ current (ICa) in neonatal rat ventricular myocytes. Electrocardiographic recordings were performed in an in vivo rabbit model.

Results

Compared with culture at 37°C, culture at 40°C reduced the mature hERG expression and IhERG in an extracellular K+ concentration–dependent manner. Point mutations that remove the K+ dependence of hERG—S624T and F627Y—also abolished the febrile temperature–induced hERG reduction. In neonatal rat ventricular myocytes, febrile temperature prolonged the action potential duration and selectively reduced IKr in a manner similar to low K+ culture. In an in vivo rabbit model, fever and hypokalemia synergistically prolonged the QT interval.

Conclusion

Febrile temperature facilitates the development of LQTS by expediting hERG degradation through altered K+ dependence.

Introduction

The rapidly activating delayed rectifier K+ channel (IKr), encoded by the human ether-à-go-go-related gene (hERG), is important for the repolarization of cardiac action potentials, and its dysfunction is the primary cause of acquired long QT syndrome (LQTS).1 Since LQTS predisposes affected individuals to the ventricular arrhythmia torsades de pointes and sudden death,1 it is important to understand how hERG is modulated in various conditions.

Evidence suggests that fever can trigger LQTS and associated arrhythmias.2, 3 Amin et al2 investigated the repetitive occurrence of fever-induced polymorphic ventricular tachycardia and ventricular fibrillation in patients with the A558P missense mutation in hERG and indicated fever as a potential trigger of life-threatening arrhythmias in patients with hERG mutations. Furthermore, fever-precipitated LQTS was reported in patients without a history of cardiac disease.3, 4 Although the effects of reduced temperature culture on hERG channels in various conditions have been demonstrated,5, 6 the chronic effects of elevated temperature on wild-type (WT) hERG or native IKr have not been reported.

In the present study, we investigated the effects of febrile temperature culture on hERG channels expressed in HEK 293 cells and on IKr in cardiomyocytes. We also examined the effects of fever on QT intervals in a rabbit model. Our data show that febrile temperature chronically decreases hERG K+ current (IhERG) as well as IKr and prolongs the QT interval in rabbits especially in hypokalemic conditions.

Section snippets

Animal experiments

The rabbit in vivo model7, 8 and neonatal rat ventricular myocyte isolation and culture9 have been described previously and are available in Online Supplemental Materials.

Molecular biology

WT hERG complementary DNA, the stable hERG expressing HEK 293 (hERG-HEK) cell line, hERG mutations, and their respective stable HEK cell lines have been described previously10 and are available in Online Supplemental Materials.

Electrophysiological recordings

Cells after culture at different temperatures were used for the electrophysiological analysis of

Febrile temperature culture reduces the expression of mature hERG channels

Figure 1A shows the effect of cultured hERG-HEK cells for 12 hours at 27°C, 37°C, or 40°C on the expression of hERG channels. Upon Western blot analysis, hERG proteins display 2 bands with molecular masses of 155 and 135 kDa, representing the mature fully glycosylated form expressed in the plasma membrane and the immature core-glycosylated form expressed in the endoplasmic reticulum, respectively.5, 7, 8, 9, 10, 11 Compared with culture at 37°C, culture at 27°C increased whereas culture at 40°C

Discussion

Dysfunction of hERG is a common cause of acquired LQTS.1 Previous studies have demonstrated that febrile temperature prolongs cardiac repolarization in conditions where hERG/IKr function is compromised.2, 16 Furthermore, fever-induced QTc prolongation has been reported in individuals in whom a lengthened repolarization was not identified.3, 4 In the present study, we demonstrated that cell culture at febrile temperature (40°C) reduces the expression and current of WT hERG channels expressed in

Conclusion

Febrile temperature chronically reduces mature hERG expression through accelerated hERG degradation due to an altered [K+]o dependence of the channel. Fever is more likely to induce LQTS in conditions where hERG function is partially compromised, such as hypokalemia. Our finding provides mechanistic insights into fever-associated LQTS and arrhythmias.

References (20)

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This work was supported by the Canadian Institutes of Health Research (grant no. MOP 72911) to Dr Zhang, who is a recipient of the Heart and Stroke Foundation of Ontario Career Investigator Award.

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