Abstract
The congenital short QT syndrome (SQTS) is a cardiac condition that leads to abbreviated ventricular repolarization and an increased susceptibility to arrhythmia and sudden death. The SQT3 form of the syndrome is due to mutations to the KCNJ2 gene that encodes Kir2.1, a critical component of channels underlying cardiac inwardly rectifying K+ current, IK1. The first reported SQT3 KCNJ2 mutation gives rise to the D172N Kir2.1 mutation, the consequences of which have been studied on recombinant channels in vitro and in ventricular cell and tissue simulations. The aim of this study was to establish the effects of the D172N mutation on ventricular repolarization through real-time replacement of IK1 using the dynamic clamp technique. Whole-cell patch-clamp recordings were made from adult guinea-pig left ventricular myocytes at physiological temperature. Action potentials (APs) were elicited at 1 Hz. Intrinsic IK1 was inhibited with a low concentration (50 µM) of Ba2+ ions, which led to AP prolongation and triangulation, accompanied by a ∼6 mV depolarization of resting membrane potential. Application of synthetic IK1 through dynamic clamp restored AP duration, shape and resting potential. Replacement of wild-type (WT) IK1 with heterozygotic (WT-D172N) or homozygotic (D172N) mutant formulations under dynamic clamp significantly abbreviated AP duration (APD90) and accelerated maximal AP repolarization velocity, with no significant hyperpolarization of resting potential. Across stimulation frequencies from 0.5 to 3 Hz, the relationship between APD90 and cycle length was downward shifted, reflecting AP abbreviation at all stimulation frequencies tested. In further AP measurements at 1 Hz from hiPSC cardiomyocytes, the D172N mutation produced similar effects on APD and repolarization velocity; however, resting potential was moderately hyperpolarized by application of mutant IK1 to these cells. Overall, the results of this study support the major changes in ventricular cell AP repolarization with the D172N predicted from prior AP modelling and highlight the potential utility of using adult ventricular cardiomyocytes for dynamic clamp exploration of functional consequences of Kir2.1 mutations.
Highlights
Cardiac action potential (AP) repolarization depends on the dynamic interplay between multiple ionic currents, including those carried by a number of key potassium channels (Varro et al, 2020)
Multiple studies have used dynamic clamp to compensate for the deficit of intrinsic IK1 in stem-cell derived cardiomyocytes (e.g., Bett et al, 2013; Meijer van Putten et al, 2015; Goversen et al, 2017; Verkerk et al, 2017; Fabbri et al, 2019; Becker et al, 2020) there is relatively little published work in which IK1 has been supplied to isolated adult cardiomyocytes from human-relevant model species using this technique
Dynamic clamp has been used in the exploration of regional differences in IK1 shape and magnitude in the canine heart (Cordeiro et al, 2015)
Summary
Cardiac action potential (AP) repolarization depends on the dynamic interplay between multiple ionic currents, including those carried by a number of key potassium channels (Varro et al, 2020). Application of the AP voltage clamp technique to rabbit ventricular myocytes showed that IK1 is suppressed at AP plateau voltages, but increases steeply during the late phase of the AP, peaking negative to ∼ −60 mV, before declining as the AP voltage approaches the K+ equilibrium potential (Shimoni et al, 1992). The importance of Kir2.1 in contributing to native IK1 is demonstrated by the fact that loss-of-function KCNJ2 mutations have been implicated in Andersen-Tawil syndrome, which causes one form of long QT syndrome (long QT type 7) as well as extra-cardiac abnormalities (Tristani-Firouzi and Etheridge, 2010; Perez-Riera et al, 2021). Gain-of-function KCNJ2 mutations have been implicated in human familial atrial fibrillation (Xia et al, 2005) and the SQT3 variant of the short QT syndrome (SQTS; Priori et al, 2005; Hattori et al, 2012; Deo et al, 2013; Ambrosini et al, 2014)
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