Activation of endogenous protein phosphatase 1 enhances the calcium sensitivity of the ryanodine receptor type 2 in murine ventricular cardiomyocytes

Key points Increased protein phosphatase 1 (PP‐1) activity has been found in end stage human heart failure. Although PP‐1 has been extensively studied, a detailed understanding of its role in the excitation–contraction coupling mechanism, in normal and diseased hearts, remains elusive. The present study investigates the functional effect of the PP‐1 activity on local Ca2+ release events in ventricular cardiomyocytes, by using an activating peptide (PDP3) for the stimulation of the endogenous PP‐1 protein. We report that acute de‐phosphorylation may increase the sensitivity of RyR2 channels to Ca2+ in situ, and that the RyR2‐serine2808 phosphorylation site may mediate such a process. Our approach unmasks the functional importance of PP‐1 in the regulation of RyR2 activity, suggesting a potential role in the generation of a pathophysiological sarcoplasmic reticulum Ca2+ leak in the diseased heart. Changes in cardiac ryanodine receptor (RyR2) phosphorylation are considered to be important regulatory and disease related post‐translational protein modifications. The extent of RyR2 phosphorylation is mainly determined by the balance of the activities of protein kinases and phosphatases, respectively. Increased protein phosphatase‐1 (PP‐1) activity has been observed in heart failure, although the regulatory role of this enzyme on intracellular Ca2+ handling remains poorly understood. To determine the physiological and pathophysiological significance of increased PP‐1 activity, we investigated how the PP‐1 catalytic subunit (PP‐1c) alters Ca2+ sparks in permeabilized cardiomyocytes and we also applied a PP‐1‐disrupting peptide (PDP3) to specifically activate endogenous PP‐1, including the one anchored on the RyR2 macromolecular complex. We compared wild‐type and transgenic mice in which the usually highly phosphorylated site RyR2‐S2808 has been ablated to investigate its involvement in RyR2 modulation (S2808A+/+). In wild‐type myocytes, PP‐1 increased Ca2+ spark frequency by two‐fold, followed by depletion of the sarcoplasmic reticulum Ca2+ store. Similarly, PDP3 transiently increased spark frequency and decreased sarcoplasmic reticulum Ca2+ load. RyR2 Ca2+ sensitivity, which was assessed by Ca2+ spark recovery analysis, was increased in the presence of PDP3 compared to a negative control peptide. S2808A+/+ cardiomyocytes did not respond to both PP‐1c and PDP3 treatment. Our results suggest an increased Ca2+ sensitivity of RyR2 upon de‐phosphorylation by PP‐1. Fur