Differences in Contractile Function of Myofibrils within Human Embryonic Stem Cell-Derived Cardiomyocytes vs. Adult Ventricular Myofibrils Are Related to Distinct Sarcomeric Protein Isoforms

Bogdan Iorga; Kristin Schwanke; Natalie Weber; Meike Wendland; Stephan Greten; Birgit Piep; Cristobal G Dos Remedios; Ulrich Martin; Robert Zweigerdt; Theresia Kraft; Bernhard Brenner

doi:10.3389/fphys.2017.01111

Differences in Contractile Function of Myofibrils within Human Embryonic Stem Cell-Derived Cardiomyocytes vs. Adult Ventricular Myofibrils Are Related to Distinct Sarcomeric Protein Isoforms

Front Physiol. 2018 Jan 19:8:1111. doi: 10.3389/fphys.2017.01111. eCollection 2017.

Authors

Affiliations

¹ Department of Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany.
² Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, Bucharest, Romania.
³ Department of Cardiac, Thoracic, Transplantation and Vascular Surgery, Leibniz Research Laboratories for Biotechnology and Artificial Organs, REBIRTH-Center for Regenerative Medicine, Hannover Medical School, Hannover, Germany.
⁴ Department of Anatomy, Bosch Institute, University of Sydney, Sydney, NSW, Australia.

Abstract

Characterizing the contractile function of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) is key for advancing their utility for cellular disease models, promoting cell based heart repair, or developing novel pharmacological interventions targeting cardiac diseases. The aim of the present study was to understand whether steady-state and kinetic force parameters of β-myosin heavy chain (βMyHC) isoform-expressing myofibrils within human embryonic stem cell-derived cardiomyocytes (hESC-CMs) differentiated in vitro resemble those of human ventricular myofibrils (hvMFs) isolated from adult donor hearts. Contractile parameters were determined using the same micromechanical method and experimental conditions for both types of myofibrils. We identified isoforms and phosphorylation of main sarcomeric proteins involved in the modulation of force generation of both, chemically demembranated hESC-CMs (d-hESC-CMs) and hvMFs. Our results indicate that at saturating Ca²⁺ concentration, both human-derived contractile systems developed forces with similar rate constants (0.66 and 0.68 s^-1), reaching maximum isometric force that was significantly smaller for d-hESC-CMs (42 kPa) than for hvMFs (94 kPa). At submaximal Ca²⁺-activation, where intact cardiomyocytes normally operate, contractile parameters of d-hESC-CMs and hvMFs exhibited differences. Ca²⁺ sensitivity of force was higher for d-hESC-CMs (pCa₅₀ = 6.04) than for hvMFs (pCa₅₀ = 5.80). At half-maximum activation, the rate constant for force redevelopment was significantly faster for d-hESC-CMs (0.51 s^-1) than for hvMFs (0.28 s^-1). During myofibril relaxation, kinetics of the slow force decay phase were significantly faster for d-hESC-CMs (0.26 s^-1) than for hvMFs (0.21 s^-1), while kinetics of the fast force decay were similar and ~20x faster. Protein analysis revealed that hESC-CMs had essentially no cardiac troponin-I, and partially non-ventricular isoforms of some other sarcomeric proteins, explaining the functional discrepancies. The sarcomeric protein isoform pattern of hESC-CMs had features of human cardiomyocytes at an early developmental stage. The study indicates that morphological and ultrastructural maturation of βMyHC isoform-expressing hESC-CMs is not necessarily accompanied by ventricular-like expression of all sarcomeric proteins. Our data suggest that hPSC-CMs could provide useful tools for investigating inherited cardiac diseases affecting contractile function during early developmental stages.

Keywords: adult human ventricular myofibrils; cardiac contractile function; cross-bridge kinetics; hiPSC-CMs maturation; human cardiomyocytes development; human embryonic stem cell-derived cardiomyocytes; human pluripotent stem cell-derived cardiomyocytes; β-myosin isoform cross-bridge.