Phosphoproteomics Study Based on In Vivo Inhibition Reveals Sites of Calmodulin‐Dependent Protein Kinase II Regulation in the Heart

BACKGROUND: The multifunctional Ca(2+)‐ and calmodulin‐dependent protein kinase II (CaMKII) is a crucial mediator of cardiac physiology and pathology. Increased expression and activation of CaMKII has been linked to elevated risk for arrhythmic events and is a hallmark of human heart failure. A useful approach to determining CaMKII's role therein is large‐scale analysis of phosphorylation events by mass spectrometry. However, current large‐scale phosphoproteomics approaches have proved inadequate for high‐fidelity identification of kinase‐specific roles. The purpose of this study was to develop a phosphoproteomics approach to specifically identify CaMKII's downstream effects in cardiac tissue. METHODS AND RESULTS: To identify putative downstream CaMKII targets in cardiac tissue, animals with myocardial‐delimited expression of the specific peptide inhibitor of CaMKII (AC3‐I) or an inactive control (AC3‐C) were compared using quantitative phosphoproteomics. The hearts were isolated after isoproterenol injection to induce CaMKII activation downstream of β‐adrenergic receptor agonist stimulation. Enriched phosphopeptides from AC3‐I and AC3‐C mice were differentially quantified using stable isotope dimethyl labeling, strong cation exchange chromatography and high‐resolution LC‐MS/MS. Phosphorylation levels of several hundred sites could be profiled, including 39 phosphoproteins noticeably affected by AC3‐I‐mediated CaMKII inhibition. CONCLUSIONS: Our data set included known CaMKII substrates, as well as several new candidate proteins involved in functions not previously implicated in CaMKII signaling.