Elsevier

Heart Rhythm

Volume 6, Issue 12, December 2009, Pages 1802-1809
Heart Rhythm

Experimental–genetic
Changes in microRNA-1 expression and IK1 up-regulation in human atrial fibrillation

https://doi.org/10.1016/j.hrthm.2009.08.035Get rights and content

Background

Atrial fibrillation (AF) is associated with increased inward-rectifier current activity that may stabilize atrial rotors maintaining the arrhythmia. Left atrial (LA) structures are important for AF maintenance, but previous studies have mostly evaluated changes in the right atrium. MicroRNA-1 (miR-1) reciprocally regulates inwardly rectifying potassium channel (Kir)2.1 expression in coronary disease, contributing to arrhythmogenesis.

Objectives

This study sought to evaluate changes in miR-1 and Kir2 subunit expression in relation to IK1 alterations in LA of patients with persistent AF.

Methods

Atrial tissue was obtained from 62 patients (31 with AF) undergoing mitral valve repair or bypass grafting. Currents were recorded from isolated cells. Proteins were quantified from immunoblots. mRNA and miR-1 levels were measured with real-time polymerase chain reaction. Immunohistochemistry was applied to localize connexin (Cx) 43.

Results

IK1 density was increased in LA cells from patients with AF (at −100 mV: −5.9 ± 1.3 vs. −2.7 ± 0.7 sinus rhythm, P <.05). There was a corresponding increase in Kir2.1 protein expression, but no change in other Kir or Cx proteins. Expression of inhibitory miR-1 was reduced by approximately 86% in tissue samples of AF patients. Kir2.1 mRNA was significantly increased. No change in Cx43 localization occurred. Ex vivo tachystimulation of human atrial slices up-regulated Kir2.1 and down-regulated miR-1, suggesting a primary role of atrial rate in miR-1 down-regulation and IK1 up-regulation.

Conclusion

miR-1 levels are greatly reduced in human AF, possibly contributing to up-regulation of Kir2.1 subunits, leading to increased IK1. Because up-regulation of inward-rectifier currents is important for AF maintenance, these results provide potential new insights into molecular mechanisms of AF with potential therapeutic implications.

Introduction

Atrial fibrillation (AF) maintenance is importantly related to changes in atrial electrical and structural properties.1 Electrical remodeling occurs early during the course of AF and leads to characteristic changes in action potential shape and duration, whereas structural alterations develop more slowly.2

Up-regulation of inward rectifier currents (IK1 and IKACh,c) is a hallmark of AF-associated electrical remodeling.3 AF-related inward current changes have consistently been shown in animal models4 and in human disease.5, 6 Inward rectifier current up-regulation is an important contributor to stabilization of atrial rotors and thus of AF persistence caused by electrical remodeling.7 In line with these concepts, overexpression of inwardly rectifying potassium channel (Kir)2.1 channels (underlying IK1) generates stable and very fast rotors in mouse ventricles, providing direct evidence for a role of IK1 in reentry stabilization.8

In addition, primary changes in electrical properties (e.g., in genetically determined arrhythmia syndromes) may occasionally create a specific substrate for AF. Such changes include gain-of-function mutations of the Kir2.1-encoding KCNJ2 gene associated with familial AF or the short QT syndrome.9, 10 Affected patients may present with AF at an unusually young age, suggesting that IK1 up-regulation may be a causative factor rather than a (mal-)adaptive response in these cases.

IK1 decrease may similarly be pro-arrhythmic and has been reported in patients with chronic coronary artery disease (CAD).11 Recent evidence suggests a potential regulatory role of microRNA in reduction of human Kir2.1 in this setting.11 Expression of the inhibitory microRNA 1 (miR-1) was increased in patients with CAD and could exacerbate experimental arrhythmogenesis by slowing conduction and depolarizing cell membranes through a post-transcriptional repression of KCNJ2 (encoding Kir2.1) and GJA1 (encoding connexin [Cx] 43).11

Whether miR-1 expression changes with AF is unknown. Furthermore, the role of atrial rate in any AF-related miR-1 changes and any concomitant Kir2.1 expression changes is also unknown. The present study was designed to investigate changes of Kir subunits in relation to alterations in miR-1 expression in human AF, and to use in vitro paced human atrial tissue slices to evaluate the specific role of activation rate.

Section snippets

Patient recruitment

All patients provided written informed consent to participate. Left atrial (LA) tissue samples were collected from patients undergoing mitral valve repair in accordance with the local ethics committee at the Goethe-Universität, Frankfurt (Nr. 54/05). Patients older than 18 years with preserved systolic left ventricular function (ejection fraction >40%) and an indication for surgical repair of mitral valve disease were asked to participate. Exclusion criteria were: paroxysmal AF (as documented

Patient characteristics

Patient characteristics are presented in Table 3. AF patients had greater LA diameters than those with SR. Otherwise, patient characteristics were similar; in particular, mean age, the degree of mitral valve regurgitation, and systolic left ventricular function were comparable.

Electrophysiological recordings

Ionic currents were recorded with a ramp protocol (3-s duration; from −100 to +40 mV) from a holding potential of −40 mV. Inward currents at −100 mV elicited from LA cells of patients with AF (N = 5 cells, 4 patients)

Discussion

The results of our study show increased IK1 in LA cardiomyocytes of AF patients, which was associated with increased Kir2.1 expression and reduced levels of inhibitory miR-1. Similar changes in Kir2.1 and miR-1 expression were reproduced by atrial tachypacing. Reduced levels of miR-1 may play a role in increased expression of Kir2.1. There were no changes in Cx43 expression or localization, suggesting differential regulatory pathways for Kir2.1 and Cx43 expression.

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  • Cited by (0)

    Dr. Ehrlich received support from Hans und Gertie Fischer–Stiftung and Adolf Messer–Stiftung. Dr Nattel was supported by the Canadian Institutes of Health Research (MOP 44365). Dr. Biliczki was the recipient of a postdoctoral fellowship of the Deutsche Gesellschaft für Kardiologie. Mrs. Girmatsion received an award from the August Scheidel–Stiftung.

    Drs. Girmatsion and Biliczki contributed equally to this work.

    The authors thank Sabine Harenkamp for expert technical assistance.

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