Ion channel expression and electrophysiology of singular human (primary and induced pluripotent stem cell derived) cardiomyocytes
Authors: Schmid Christina , Abi-Gerges Najah , Zellner Dietmar , Rast Georg 
Published: March 2021

SUMMARYHuman induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and primary human cardiomyocytes are used forin vitrocardiac safety testing. hiPSC-CMs have been associated with a vast heterogeneity regarding single-cell morphology, beating behavior and action potential duration, prompting a systematic analysis of single-cell characteristics. Previously published hiPSC-CM studies revealed action potentials with nodal-, atrial- or ventricular-like morphology, although ion channel expression of singular hiPSC-CMs is not fully understood. Other studies used single-cell RNA-sequencing, however, these studies did not extensively focus on expression patterns of cardiac ion channels or failed to detect ion channel transcripts. Thus, the current study used a single-cell patch-clamp-RT-qPCR approach to get insights into single-cell electrophysiology (capacitance, action potential duration at 90% of repolarization, upstroke velocity, spontaneous beat rate, and sodium-driven fast inward current) and ion channel expression (HCN4, CACNA1G, CACNA1D, KCNA5, KCNJ4, SCN5A, KCNJ2, CACNA1D, and KCNH2), the combination of both within individual cells, and their correlations in single cardiomyocytes. We used commercially available hiPSC-CMs (iCell cardiomyocytes, atrial and ventricular Pluricytes) and primary human adult atrial and ventricular cardiomyocytes. Recordings of electrophysiological parameters revealed differences between the cell groups and variation within the hiPSC-CMs groups as well as within primary ventricular cardiomyocytes. Expression analysis on mRNA level showed no-clear-cut discrimination between primary cardiac subtypes and revealed both similarities and differences between all cell groups. Higher expression of atrial-associated ion channels in primary atrial cardiomyocytes and atrial Pluricytes compared to their ventricular counterpart indicates a successful chamber-specific hiPSC differentiation. Interpretation of correlations between the single-cell parameters was challenging, as the total data set is complex, particularly for parameters depending on multiple processes, like the spontaneous beat rate. Yet, for example, expression of SCN5A correlated well with the fast inward current amplitude for all three hiPSC-CM groups. To further enhance our understanding of the physiology and composition of the investigated hiPSC-CMs, we compared beating and non-beating cells and assessed distributions of single-cell data. Investigating the single-cell phenotypes of hiPSC-CMs revealed a combination of attributes which may be interpreted as a mixture of traits of different adult cardiac cell types: (i) nodal-related pacemaking attributes are spontaneous generation of action potentials and high HCN4 expression; and (ii) non-nodal attributes: cells have a prominent INa-driven fast inward current, a fast upstroke velocity and a high expression of SCN5A. In conclusion, the combination of nodal- and non-nodal attributes in single hiPSC-CMs may hamper the interpretation of drug effects on complex electrophysiological parameters like beat rate and action potential duration. However, the proven expression of specific ion channels enables the evaluation of drug effects on ionic currents in a more realistic environment than in recombinant systems.