Tissue elasticity modulates cardiac pacemaker cell automaticity

Tissue elasticity is essential to a broad spectrum of cell biology and organ function including the heart. Routine cell culture models on rigid polystyrene dishes are limited in studying the impact of tissue elasticity in distinct regions of the myocardium such as the cardiac conduction system. Gelatin, a derivative of collagen, is a simple and tunable platform for modeling tissue elasticity. We sought to study the effects of increasing tissue stiffness on cardiac pacemaker cell function by employing transcription factor-reprogrammed pacemaker cells cultured on gelatin hydrogels with specific elasticity. Our data indicate that automaticity of the pacemaker cells, measured in rhythmic contractions and oscillating intracellular Ca 2+ transients, was enhanced when cultured on a stiffer matrix of 14 kPa. This was accompanied by increased expression of cardiac pacemaker ion channel, Hcn4, and a reciprocal decrease in Cx43 expression compared to control conditions. Propagation of Ca 2+ transients was slower in the pacemaker cell monolayers compared to control, which recapitulates a hallmark feature in the native pacemaker tissue. Ca 2+ transient propagation of pacemaker cell monolayers was slower on stiffer than on softer hydrogel, and this was dependent on enhanced proliferation of cardiac fibroblasts rather than differences in gap junctional coupling. Culturing the pacemaker cells on rigid plastic plates led to irregular or loss of synchronous contractions as well as unusually long Ca 2+ transient durations. Taken together, our data demonstrate that automaticity of pacemaker cells is augmented by stiffer ECM substrates within the elasticity range of the healthy myocardium. This simple approach presents a physiological in vitro model to study mechanoelectric feedback of cardiomyocytes including the conduction system cells.