Engineering Cardiac Contractility from the Sarcomere to Tissue-Scale

Abstract

Engineering myocardium with specific contractile properties is a major goal of cardiac regenerative medicine, both for in vivo therapy and in vitro disease models. Yet maintaining the multiscale coupling and uniaxial orientation from nanometer-scale actin-myosin motors to centimeter-scale muscle tissue remains a major obstacle. Inspired by the collagen fibrillar network and capillary beds in myocardium, we hypothesized that these microscale heterogeneities act as boundary conditions that direct cardiomyocyte alignment and functional coupling. Specifically, our interest is the role these boundary conditions play in the sub-cellular alignment of the sarcomeres. To tackle this problem we developed a cardiac tissue engineering methodology that allows us to control and quantify sub-micron sarcomere orientation and measure macroscale contractile force and electrical conduction. We evaluate the deformation of 2-dimensional (2D) myocardial sheets that mimic the lamellar layers of the ventricular wall. Cardiomyocytes are grown onto a free-standing film of polydimethylsiloxane elastomer, referred to as a muscular thin film (MTF). Microscale heterogeneities are created using microcontact printing to direct 2D myogenesis, either (i) isotropic with no cell or sarcomere alignment, (ii) anisotropic with uniaxial cell and sarcomere alignment or (iii) an array of 20 micrometer wide myocardial strands with enhanced uniaxial cell and sarcomere alignment. For contractility experiments, MTFs are fashioned into cantilevers, mounted in an organ bath and electrically stimulated at 0.5 Hz. Results demonstrate that isotropic myocardium generates ~1 kPa peak systolic stress. In stark contrast the anisotropic myocardium generates an order-of-magnitude greater peak systolic stress of ~10 kPa. The 1D myocardial strands generate the greatest peak systolic stress of ~17 kPa, or ~35 kPa when normalized for muscle mass. This demonstrates that microscale heterogeneities can have a profound effect on sarcomere alignment and muscle contractility, defining new strategies for optimizing electro-mechanical function in engineered muscle.