LAUSR

The physical properties of the extracellular matrix (ECM), including stiffness and viscoelasticity, play a key role in the development of hypertensive and hypertrophic cardiac diseases at the cellular level, affecting the morphology and function of heart tissue. The ECM is a dynamic structure that is continuously changing in both normal and diseased tissue. In an extension of prior studies examining how substrates of fixed stiffness affect cardiac cells, we introduce a novel experimental model that allows exploration of cardiomyocyte functional alterations in response to rapid and reversible matrix stiffness modulation. We fabricated a biocompatible magneto-rheological elastomer (MRE) culture substrate by adding iron particles to a PDMS elastomer. In the presence of an applied magnetic field, this substrate stiffens reversibly and nearly instantaneously from 10 kPa to 50 kPa: the range observed in normal and diseased myocardium, respectively. Adult cardiomyocytes isolated from non-failing human hearts (unused organ donors) or adult rats were cultured for 24 - 48 hours on the tunable MREs. Cardiomyocytes cultured on a stiffened MRE (~50 kPa) exhibited time-dependent reductions in cell shortening and peak Ca 2+ transient amplitude compared with cardiomyocytes cultured on a soft MRE (~10 kPa). Rates of cardiomyocyte shortening and re-lengthening were also slowed by culture on 50 kPa substrates. Using super-resolution imaging, we found that cardiomyocytes cultured on a stiffened MRE exhibited an increased microtubule network density and demonstrated increased cellular stiffness and viscoelasticity, as measured by nanoindentation. These studies indicate that adult human and rat cardiomyocytes are acutely sensitive to changes in extracellular stiffness within the pathophysiological range and tend to adapt their own stiffness to match that of their surroundings via dynamic changes in microtubule architecture. These mechanisms may contribute to the regulation of mechanical homogeneity within the myocardium. Our method provides a unique in vitro platform for studying mechanosensing, mechanotransduction and mechanical memory in isolated adult cardiomyocytes.