Contractile Function of Permeabilized Human Embryonic Stem Cell-Derived Cardiomyocytes with Defined Myosin Protein Isoform Expression

Abstract

Cardiomyocytes derived from human induced pluripotent or embryonic stem cells (hESC-CMs) are widely used for cellular disease models, tissue engineering and drug testing. However, the phenotype of such cardiomyocytes varies, depending not least on the sarcomeric proteins that are expressed. After differentiation, hESC-CMs typically express high levels of the fast atrial myosin heavy chain (α-MyHC). Yet, in the human ventricle, 95% slow β-MyHC-protein is found.Here we aimed to determine the effect of MyHC-protein isoform on contractile function of individual hESC-CMs with known cardiac myosin isoform composition.hESC-CMs with exclusive β-MyHC-containing sarcomeres were obtained by long-term maturation on laminin-coated glass coverslips. Treatment for one week with T3 (triiodothyronine) yielded pure α-MyHC-positive hESC-CMs. For functional assessment, hESC-CMs were chemically permeabilized and mounted in a nN-sensitive micromechanical setup, allowing precise control of activation and relaxation.hESC-CMs generated maximum isometric force of 42kN/m2, which was independent of MyHC isoform. Yet, the rate constants of force redevelopment (kTR) and of the isometric phase of force relaxation (kLIN) were significantly faster for cardiomyocytes with pure α- vs. pure β-MyHC-protein (kTR: 2.44±0.30s-1 and 0.67±0.10s-1, kLIN: 1.47±0.38s-1 and 0.30±0.13s-1 for α-MyHC-CMs and β-MyHC-CMs, respectively). The kinetic values of β-MyHC-CMs were very similar to myofibrils isolated from ventricular cardiomyocytes of human donor hearts (kTR=0.64±0.09s-1, kLIN=0.28±0.09s-1). The presence of some α-MyHC in the sarcomeres of mainly β-MyHC-positive hESC-CMs accelerated kTR and kLIN significantly. Importantly, similar maximum force but faster kLIN of α-MyHC-CMs yields much higher tension cost of α-MyHC-CMs compared to the energetically more economical β-MyHC-CMs, like in adult myocardium.We conclude that assessment of hESC-CM's contractile function together with sarcomere protein isoform composition at the single cell level are important for well-defined cardiac disease models and generation of artificial heart tissue.