Beschreibung:
Introduction: Heart failure (HF) is the number one killer in the US and new treatments are needed. Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)-2a is a crucial cardiac protein regulating contractility and its downregulation is a molecular hallmark of chronic HF. Its activation underlies β1-adrenergic receptor (AR)-induced cardiac contractility. Agonist-bound β1ARs undergo functional desensitization/internalization due to the actions of βarrestin1 or -2. The two βarrestins are universal receptor adapter proteins and signal transducers thanks to protein scaffolding. Among the cellular processes they can regulate, is protein SUMO (small ubiquitin-like modifier)-ylation, generally increasing protein stability/levels. In the heart, βarrestin1 appears detrimental, whereas βarrestin2 beneficial, for structure and function post-myocardial infarction (MI). Post-MI βarrestin1 knockout mice display elevated SERCA2a activity and better contractility than post-MI wild type (WT) mice. In addition, reduced cardiac SERCA2a SUMOylation is known to underlie its downregulation in HF, decreasing cardiac contractility. Hypothesis: Cardiac β1AR-activated βarrestin2 promotes SERCA2a SUMOylation and activity, thereby regulating contractility. Methods & Results: By studying individual βarrestin knockout heart extracts, we found that βarrestin2, but not βarrestin1, interacts with SERCA2a in the mouse heart in vivo, promoting the latter`s SUMOylation and activity. This interaction is direct, as indicated by pulldown and FRET experiments. In vitro studies in the cardiomyocyte line h9c2 indicate that this interaction is both β1AR-, and β-agonist-specific, and leads to increased Ubc9-dependent SERCA2a SUMOylation, which, in turn, acutely enhances SERCA2a activity. This results in significantly enhanced cardiac function in post-MI HF mice in vivo (Ejection Fraction of WT post-MI mice overexpressing βarrestin2 in the heart vs. control post-MI mice: 46+2.8% vs. 34+2.1%, respectively, p<0.05, n=5). Conclusions: These results suggest that βarrestin2, presumed, like βarrestin1, to decrease cardiac function by desensitizing βARs, may actually (directly) enhance cardiac contractility, thereby opposing βarrestin1 in that regard.