Differential Twitch Kinetics in Engineered Cardiac Tissue Expressing Human Cardiac Myosins

Congestive heart failure is a debilitating disease in which the principal pathology is impaired ventricular contractility leading to diminished cardiac output, and previous work indicates that reduced contractility is based in part on the ratio of myosin heavy chain (MyHC) isoforms, α- and β-MyHC, expressed in the ventricles. Normal human ventricles express ∼10% of the fast α-MyHC on a background of the slower β-MyHC, while in failing hearts α-MyHC is reduced to virtually undetectable levels with complete replacement by β-MyHC. Data from permeabilized myocardial preparations suggests that this isoform switch may be partly responsible for reduced myocardial twitch force and pressure development by failing ventricles since β-MyHC is a slower motor protein, yet most experiments have used non-human myosins and experimental conditions in which preparations were steadily activated, thus little is known about the response of human myosins to a time-varying Ca2+ transient. To address these limitations, we recently developed a human 3D engineered cardiac tissue (hECT) system in which we can express recombinant human muscle myosin motors. Using commercially available cloning and adenoviral expression systems, α- or β-MyHC isoform expressing adenoviral particles were used to transduce human cardiomyocytes produced from human iPS cells and construct hECTs. Preparations displayed well-defined cellular structure with elongated morphology aligned in the direction of preparation shortening during electrical pacing, while histological analysis of hECT revealed appropriate protein expression and localization within the sarcomere. In response to a Ca2+ transient, the time-course of twitch force development was accelerated in hECT expressing α-MyHC compared to β-MyHC, while peak twitch force was greater in hECT expressing α-MyHC. These results demonstrate the relative contribution of myosin isoforms to myocardial twitch kinetics in human engineered cardiac tissue constructs expressing a stable background of myofibrillar proteins.