Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes as Models for Cardiac Channelopathies: A Primer for Non-Electrophysiologists

Abstract

Life threatening ventricular arrhythmias leading to sudden cardiac death are a major cause of morbidity and mortality. In the absence of structural heart disease, these arrhythmias, especially in the younger population, are often an outcome of genetic defects in specialized membrane proteins called ion channels. In the heart, exceptionally well-orchestrated activity of a diversity of ion channels mediates the cardiac action potential. Alterations in either the function or expression of these channels can disrupt the configuration of the action potential, leading to abnormal electrical activity of the heart that can sometimes initiate an arrhythmia. Understanding the pathophysiology of inherited arrhythmias can be challenging because of the complexity of the disorder and lack of appropriate cellular and in vivo models. Recent advances in human induced pluripotent stem cell technology have provided remarkable progress in comprehending the underlying mechanisms of ion channel disorders or channelopathies by modeling these complex arrhythmia syndromes in vitro in a dish. To fully realize the potential of induced pluripotent stem cells in elucidating the mechanistic basis and complex pathophysiology of channelopathies, it is crucial to have a basic knowledge of cardiac myocyte electrophysiology. In this review, we will discuss the role of the various ion channels in cardiac electrophysiology and the molecular and cellular mechanisms of arrhythmias, highlighting the promise of human induced pluripotent stem cell-cardiomyocytes as a model for investigating inherited arrhythmia syndromes and testing antiarrhythmic strategies. Overall, this review aims to provide a basic understanding of the electrical activity of the heart and related channelopathies, especially to clinicians or research scientists in the cardiovascular field with limited electrophysiology background.

Keywords: action potentials; channelopathies; electrophysiology; induced pluripotent stem cells; sudden cardiac death.

Figure 1.. Electrophysiological Basis of the Electrical Activity in the Heart.

(A) Schematic for ion channel proteins embedded in a lipid bilayer and ionic basis for resting membrane potential in a cardiac myocyte. Based on the depicted concentration of cations, the calculated equilibrium potential (E = (RT/zF) * ln [X]_out/[X]_inside) for K⁺, Na⁺ and Ca²⁺ would be approx. −90 mV, +50 mV and +130 mV at 25^oC, respectively. (B) Schematic depicting different phases (0–4) of a typical ventricular action potential (top) with various depolarizing (arrows down) and repolarizing (arrows up) ionic currents. Bottom panel shows the relative amount of different currents between an adult ventricular myocyte (gray) and a ventricular-like hiPSC-CM (red). The associated ion channel abnormalities are also shown. Purple circles represent loss-of-function mutations and blue circles represent gain-of-function mutations. (C) Excitation-contraction coupling in a cardiac myocyte. Ca²⁺ entering via plasma membrane channels activates ryanodine receptors (ryanodine receptor 2 [RYR2]) and initiates Ca²⁺-induced Ca²⁺-release mechanism (CICR). CASQ2 is the cardiac isoform of the high-capacity Ca²⁺-sequestering protein, calsequestrin present inside SR. Phospholamban (PLN, brown oval) is shown on top of Sarco/endoplasmic reticulum Ca²⁺-ATPase type-2a (SERCA 2a). Mutations in proteins associated with SR Ca²⁺ release or uptake are associated with CPVT.

Figure 2.. Key Electrophysiological Mechanisms of Cardiac Arrhythmia.

Ectopic impulse generation by (A) Enhanced automaticity. (B) APD prolongation (in blue) leading to the development of Phase 2 and Phase 3 EADs (red). (C) Development of delayed after depolarizations (DADs) that occur due to Ca²⁺ overload. (D) Reentry requires a vulnerable substrate, which can be caused by APD shortening or dispersion of refractoriness. Schematic shows a depolarizing wavefront around an anatomical obstacle. Under normal conditions, the depolarizing waves around the obstacle cancel each other out. Certain conditions e.g. ischemia might generate areas of unidirectional block, or sufficiently slow conduction to enable recovery of excitability in time for re-excitation by the depolarizing wavefront.

Figure 3.. Platforms for Functional Analysis of Human iPSC-CMs.

Graphical illustration summarizing different methods currently used for hiPSC-CM functionality analyses. (A) Patch Clamp (B) MEA (C) Fluorescence Imaging (D) Impedance. Representative MEA^, and Fluorescence Imaging^, traces reprinted with permission of the publisher.

Figure 4.

Diagram showing overlap between the genes associated with Brugada Syndrome (BrS), Short QT Syndrome (SQTS), Long QT syndrome (LQTS), Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) and Atrial Fibrillation (AFib).