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 using 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 Ca²⁺ 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 with control conditions. Propagation of Ca²⁺ transients was slower in the pacemaker cell monolayers compared with control, which recapitulates a hallmark feature in the native pacemaker tissue. Ca²⁺ transient propagation of pacemaker cell monolayer 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 Ca²⁺ transient durations. Taken together, our data demonstrate that automaticity of pacemaker cells is augmented by stiffer extracellular matrix 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.<b>NEW & NOTEWORTHY</b> The major achievement of this work is development of a robust and straightforward approach to model cardiac conduction system cells with a range of cardiac tissue elasticity with a goal to understand the impact of tissue stiffness on cardiac pacing. Our data provide a framework for further investigation of the heart rhythm in health and disease in the context of fibrosis.