cAMP Imaging at Ryanodine Receptors Reveals β2-Adrenoceptor Driven Arrhythmias

Rationale: 3',5'-cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger which, upon β-adrenergic receptor (β-AR) stimulation, acts in microdomains to regulate cardiac excitation-contraction coupling by activating phosphorylation of calcium handling proteins. One crucial microdomain is in vicinity of the cardiac ryanodine receptor type 2 (RyR2) which is associated with arrhythmogenic diastolic calcium leak from the sarcoplasmic reticulum (SR) often occurring in heart failure. Objective: We sought to establish a real time live cell imaging approach capable of directly visualizing cAMP in the vicinity of mouse and human RyR2 and to analyze its pathological changes in failing cardiomyocytes under β-AR stimulation. Methods and Results: We generated a novel targeted fluorescent biosensor Epac1-JNC for RyR2-associated cAMP and expressed it in transgenic mouse hearts as well in human ventricular myocytes using adenoviral gene transfer. In healthy cardiomyocytes, β 1 -AR but not β 2 -AR stimulation strongly increased local RyR2-associated cAMP levels. However, already in cardiac hypertrophy induced by aortic banding, there was a marked subcellular redistribution of phosphodiesterases (PDEs) 2, 3 and 4, which included a dramatic loss of the local pool of PDE4. This was also accompanied by measurableβ2-AR/AMP signals in the vicinity of RyR2 in failing mouse and human myocytes, increased β2-AR-dependent RyR2 phosphorylation, SR calcium leak and arrhythmia susceptibility. Conclusions: Our new imaging approach could visualize cAMP levels in the direct vicinity of cardiac RyR2. Unexpectedly, in mouse and human failing myocytes, it could uncover functionally relevant local arrhythmogenic β2-AR/cAMP signals which might be an interesting antiarrhythmic target for heart failure.