1Universitätsklinikum Heidelberg Klinik für Innere Med. III, Kardiologie, Angiologie u. Pneumologie Heidelberg, Deutschland; 2Universitätsklinikum Heidelberg Innere Medizin VIII, Institut für Experimentelle Kardiologie Heidelberg, Deutschland
Background: The two-pore domain potassium channel TASK-1 has recently been described to play a crucial role in the pathophysiology of atrial fibrillation (AF). In the human heart, it is predominantly expressed in the atria and significantly upregulated in patients suffering from AF. Moreover, it has been described as a promising new target for antiarrhythmic therapy, as inhibition of TASK-1 in atrial cardiomyocytes was shown to counteract the action potential (AP) shortening observed during AF and application of a TASK-1 inhibitor led to restoration of sinus rhythm (SR) in a pig model of AF. However, the transcriptional mechanisms underlying TASK-1 regulation are still unclear. A few years ago, it was reported that the transcription factor ETV1 elicits atrial remodeling and arrhythmia. Like TASK-1, ETV1 is almost exclusively expressed in the atria and upregulated in AF patients. Therefore, investigating possible interactions between ETV1 and TASK-1 is important to further understand atrial arrhythmogenesis and develop new antiarrhythmic strategies.
Purpose: The purpose the present study was to scrutinize the effects of ETV1 regulation on TASK-1 expression. For that, electrophysiological effects of ETV1 modulation on TASK-1 currents and action potential formation were studied on HL-1 cells.
Methods: Modulation of ETV1 was achieved either through application of the pharmacological ETV1-inhibitor BRD32048 (1 µM) or transfection of HL-1 cells with a specific ETV1-siRNA. Electrophysiological effects of ETV1 inhibition were assessed using two-electrode voltage clamp experiments on Xenopus laevis oocytes heterologously expressing TASK-1 as well as whole-cell patch clamp measurements on HL-1 cells after pharmacological- and siRNA-mediated ETV1 inhibition. Chromatin immunoprecipitation combined with real-time qPCR (ChIP-qPCR) and ATAC-seq experiments were performed to identify the epigenetic interaction between ETV1 and the KCNK3 gene encoding TASK-1. Above that, HL-1 cardiomyocytes were induced to rapid tachypacing at 300 bpm before and after ETV1 knockdown and effects were quantified via immunoblot and qPCR.
Results: Upon ETV1 inhibition, TASK-1 currents were noticeably reduced in Xenopus laevis oocytes. Furthermore, both pharmacological inhibition and siRNA-mediated knockdown of ETV1 caused a significant decrease of TASK-1 currents in HL-1 cells by 35.8% and 39.3%, respectively. These effects correlated with a statistically significant prolongation of the action potential duration at 90% repolarization (APD90) by 28.5% after functional inhibition and 24.9% after siRNA-knockdown. This was associated with a significant decrease of TASK-1 expression on mRNA and protein level. Additionally, ATAC-seq data and ChIP-qPCR revealed an enrichment of ETV1 binding within accessible regulatory elements at the KCNK3 gene locus. Rapid tachypacing of HL-1 cells lead to significant upregulation of TASK-1 over the course of 24 h, while ETV1-inhibition showed protective effects as it prevented this tachypacing-induced remodeling.
Conclusions: Taken together, ETV1-knockdown shows strong inhibitory effects on TASK-1 while ChIP-qPCR and ATAC-seq data suggest that ETV1 acts as a direct transcriptional activator of KCNK3. Therefore, we suppose that ETV1 plays a significant role in the regulation of atrial TASK-1 currents. These results contribute to a better understanding underlying atrial fibrillation and will help to advance TASK-1-based AF therapy in the future.