Cyclical shortening promotes maturation of atrial engineered heart tissues.

Abstract

Mechanotransduction plays a central role in cardiovascular physiology and pathophysiology, and pathways by which mechanical load is sensed by cardiomyocytes have been extensively studied. However, the responses of atrial cardiomyocytes to mechanical load have been less thoroughly examined than those of ventricular cardiomyocytes. Here, we sought to examine the effects of physiologically-inspired atrial loading regimes on the maturation and contractile properties of atrial engineered heart tissues (EHTs). We used a custom-designed bioreactor to apply precise strain transients to atrial EHTs that mimic left atrial strain in vivo. The strain transient faithfully recapitulates the three phases of the atrial cycle: reservoir, conduit, and boost. iPSCs were differentiated into atrial cardiomyocytes by biphasic Wnt signaling with retinoic acid enrichment, and were seeded on decellularized porcine ECM to form EHTs. After three weeks in culture, EHTs were subject to mechanical testing and Western blot analysis. We found that atrial EHTs that were subject to cyclical mechanical loading trended towards faster contraction kinetics (Time to peak: 187 ms vs 137 ms, p = 0.056, n = 4-5 per group) and displayed significantly faster relaxation kinetics (Time to 50% relaxation: 97 ms vs. 73 ms, p = 0.026). EHTs that were cyclically stretched also displayed enhanced Frank-Starling gain (1.7-fold increase in peak force vs. 3-fold increase at 10% strain, two-way ANOVA interaction p = 0.052, mechanical strain effect p = 0.0097). Western blotting revealed that cyclical strain was associated with reduced levels of α-smooth muscle actin, a marker of both cardiomyocyte immaturity and myofibroblast activation (0.44-fold change with cyclical strain, p < 0.0001, n = 5-6 per group). In conclusion, cyclical mechanical strain of atrial engineered heart tissues promotes contractile maturity, which may help develop more faithful models of atrial physiology and disease.