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Abstract

The small calcium (Ca2+) binding protein S100A1 is as a critical regulator of cardiomyocyte Ca2+ handling thereby enhancing cardiac performance in vivo and in vitro. Our previous studies demonstrated that the positive inotropic effects of S100A1 are due to enhanced Ca2+ transients and sarcoplasmic reticulum (SR) Ca2+ load in isolated adult cardiomyocytes. These effects are independent of and in addition to cAMP-dependent positive inotropic mechanisms. However, inotropic interventions come at the risk of arrhythmogenic diastolic Ca2+ leakage when the SR Ca2+ content exceeds the threshold for spontaneous diastolic Ca2+ release. In a more recent study we could demonstrate that enhanced Ca2+ transients after S100A1 overexpression are associated with a reduced incidence of diastolic Ca2+ sparks and Ca2+ waves. These results favor the assumption that S100A1 might reduce the diastolic RyR2 leak, thereby impeding the development of pro-arrhythmogenic events. Thus, the aim of this work was to investigate the effect of S100A1 on diastolic Ca2+ handling and on the impact of Ca2+-triggered arrhythmias in a multicellular system. For this reason, the 3-dimensional tissue culture model of Engineered Heart Tissue (EHT) was chosen. Due to its syncytial architecture, EHT closely mimics functional alterations, intercellular communication and reverse remodeling of whole hearts in vivo despite eased handling and pharmacological as well as therapeutic manipulations. Pharmacological stimulation of EHT with endothelin-1 resulted in a heart failure-like phenotype with strong impairment of contractile performance. Adenoviral-mediated S100A1 overexpression was able to rescue failing EHT and resulted in superior contractility in normal EHT. Triggered contraction abnormalities, referred to as after-contractions, were induced by Ca2+ and β-AR stimulation and served as a surrogate of SOICR (store-overload-induced-Ca2+-release). S100A1 overexpression significantly protected against Ca2+ and β-adrenergic receptor (β-AR) triggered after-contractions in normal and failing EHT. Despite persistent abnormal phosphorylation-dependent changes at the RyR2 and altered complex formation with accessory proteins, S100A1 overexpression enhanced S100A1/RyR2 stoichiometry, which seems to be key for S100A1’s effects, combining inotropic and anti-arrhythmic potency.