Abstract
During physical exercise or stress, the sympathetic system stimulates cardiac contractility via β-adrenergic receptor (β-AR) activation, resulting in protein kinase A (PKA)-mediated phosphorylation of the cardiac ryanodine receptor RyR2. PKA-dependent "hyperphosphorylation" of the RyR2 channel has been proposed as a major impairment that contributes to progression of heart failure. However, the sites of PKA phosphorylation and their phosphorylation status in cardiac diseases are not well defined. Among the known RyR2 phosphorylation sites, serine 2030 (S2030) remains highly controversial as a site of functional impact. We examined the contribution of RyR2-S2030 to Ca2+ signaling and excitation-contraction coupling (ECC) in a transgenic mouse with an ablated RyR2-S2030 phosphorylation site (RyR2-S2030A+/+). We assessed ECC gain by using whole-cell patch-clamp recordings and confocal Ca2+ imaging during β-ARs stimulation with isoproterenol (Iso) and consistent SR Ca2+ loading and L-type Ca2+ current (I Ca) triggering. Under these conditions, ECC gain is diminished in mutant compared with WT cardiomyocytes. Resting Ca2+ spark frequency (CaSpF) with Iso is also reduced by mutation of S2030. In permeabilized cells, when SR Ca2+ pump activity is kept constant (using 2D12 antibody against phospholamban), cAMP does not change CaSpF in S2030A+/+ myocytes. Using Ca2+ spark recovery analysis, we found that mutant RyR Ca2+ sensitivity is not enhanced by Iso application, contrary to WT RyRs. Furthermore, ablation of RyR2-S2030 prevents acceleration of Ca2+ waves and increases latency to the first spontaneous Ca2+ release after a train of stimulations during Iso treatment. Together, these results suggest that phosphorylation at S2030 may represent an important step in the modulation of RyR2 activity during β-adrenergic stimulation and a potential target for the development of new antiarrhythmic drugs.