Calcium/calmodulin (Ca2+/CaM)-dependent protein kinase II (CaMKII) couples increases in cellular Ca2+ to fundamental responses in excitable cells. CaMKII was identified over 20 years ago by activation dependence on Ca2+/CaM, but recent evidence shows that CaMKII activity is also enhanced by pro-oxidant conditions. Here we show that oxidation of paired regulatory domain methionine residues sustains CaMKII activity in the absence of Ca2+/CaM. CaMKII is activated by angiotensin II (AngII)-induced oxidation, leading to apoptosis in cardiomyocytes both in vitro and in vivo. CaMKII oxidation is reversed by methionine sulfoxide reductase A (MsrA), and MsrA-/- mice show exaggerated CaMKII oxidation and myocardial apoptosis, impaired cardiac function, and increased mortality after myocardial infarction. Our data demonstrate a dynamic mechanism for CaMKII activation by oxidation and highlight the critical importance of oxidation-dependent CaMKII activation to AngII and ischemic myocardial apoptosis.

Myocardial cell death is initiated by excessive mitochondrial Ca2+ entry, causing Ca2+ overload, mitochondrial permeability transition pore (mPTP) opening and dissipation of the mitochondrial inner membrane potential (ΔΨm)1,2. However, the signaling pathways that control mitochondrial Ca2+ entry through the inner membrane mitochondrial Ca2+ uniporter (MCU)3–5 are not known. The multifunctional Ca2+ and calmodulin-dependent protein kinase II (CaMKII) is activated in ischemia reperfusion (I/R), myocardial infarction (MI) and neurohumoral injury, common causes of myocardial death and heart failure, suggesting CaMKII could couple disease stress to mitochondrial injury. Here we show that CaMKII promotes mPTP opening and myocardial death by increasing MCU current (IMCU). Mitochondrial-targeted CaMKII inhibitory protein or cyclosporin A (CsA), an mPTP antagonist with clinical efficacy in I/R injury6, equivalently prevent mPTP opening, ΔΨm deterioration and diminish mitochondrial disruption and programmed cell death in response to I/R injury. Mice with myocardial and mitochondrial-targeted CaMKII inhibition are resistant to I/R injury, MI and neurohumoral injury, suggesting pathological actions of CaMKII are substantially mediated by increasing IMCU. Our findings identify CaMKII activity as a central mechanism for mitochondrial Ca2+ entry and suggest mitochondrial-targeted CaMKII inhibition could prevent or reduce myocardial death and heart failure dysfunction in response to common experimental forms of pathophysiological stress.

Excessive activation of β-adrenergic, angiotensin II, and aldosterone (Aldo) signaling pathways promotes mortality after myocardial infarction (MI), while antagonist drugs targeting these pathways are core therapies for treating post-MI patients. Catecholamines and angiotensin II activate the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMKII), and CaMKII inhibition prevents isoproterenol- and angiotensin II-mediated cardiomyopathy. Here we show that Aldo exerts direct toxic actions on myocardium by oxidative activation of CaMKII, causing cardiac rupture and increased mortality in mice after MI. Aldo oxidizes CaMKII by recruiting NADPH oxidase, and oxidized CaMKII promotes matrix metalloproteinase 9 (Mmp9) expression in cardiomyocytes. Myocardial CaMKII inhibition, over-expression of methionine sulfoxide reductase A, an enzyme that reduces oxidized CaMKII, or NADPH oxidase inhibition prevented Aldo-enhanced post-MI cardiac rupture. These findings show oxidized myocardial CaMKII mediates cardiotoxic effects of Aldo on cardiac matrix and establish CaMKII as a nodal signal for the neurohumoral pathways associated with poor outcomes after MI.

Diabetes increases oxidant stress and doubles the risk of dying after myocardial infarction, but the mechanisms underlying increased mortality are unknown. Mice with streptozotocin-induced diabetes developed profound heart rate slowing and doubled mortality compared with controls after myocardial infarction. Oxidized Ca 2+ /calmodulin-dependent protein kinase II (ox-CaMKII) was significantly increased in pacemaker tissues from diabetic patients compared with that in nondiabetic patients after myocardial infarction. Streptozotocin-treated mice had increased pacemaker cell ox-CaMKII and apoptosis, which were further enhanced by myocardial infarction. We developed a knockin mouse model of oxidation-resistant CaMKIIδ (MM-VV), the isoform associated with cardiovascular disease. Streptozotocin-treated MM-VV mice and WT mice infused with MitoTEMPO, a mitochondrial targeted antioxidant, expressed significantly less ox-CaMKII, exhibited increased pacemaker cell survival, maintained normal heart rates, and were resistant to diabetes-attributable mortality after myocardial infarction. Our findings suggest that activation of a mitochondrial/ox-CaMKII pathway contributes to increased sudden death in diabetic patients after myocardial infarction.

Sinus node dysfunction (SND) is a major public health problem that is associated with sudden cardiac death and requires surgical implantation of artificial pacemakers. However, little is known about the molecular and cellular mechanisms that cause SND. Most SND occurs in the setting of heart failure and hypertension, conditions that are marked by elevated circulating angiotensin II (Ang II) and increased oxidant stress. Here, we show that oxidized calmodulin kinase II (ox-CaMKII) is a biomarker for SND in patients and dogs and a disease determinant in mice. In wild-type mice, Ang II infusion caused sinoatrial nodal (SAN) cell oxidation by activating NADPH oxidase, leading to increased ox-CaMKII, SAN cell apoptosis, and SND. p47 --mice lacking functional NADPH oxidase and mice with myocardial or SAN-targeted CaMKII inhibition were highly resistant to SAN apoptosis and SND, suggesting that ox-CaMKII-triggered SAN cell death contributed to SND. We developed a computational model of the sinoatrial node that showed that a loss of SAN cells below a critical threshold caused SND by preventing normal impulse formation and propagation. These data provide novel molecular and mechanistic information to understand SND and suggest that targeted CaMKII inhibition may be useful for preventing SND in high-risk patients. IntroductionEach normal heart beat is initiated as an electrical impulse from a small number of highly specialized sinoatrial node (SAN) pacemaker cells that reside in the lateral right atrium. There is now general agreement that physiological SAN function requires a pacemaker current (I f ) (1) and spontaneous release of sarcoplasmic reticulum (SR) intracellular Ca 2+ that triggers depolarizing current through the Na + /Ca 2+ exchanger (I NCX ) (2, 3). The multifunctional Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) is essential for increasing SR Ca 2+ release in SAN cells in response to stress to cause physiological "fight-or-flight" heart rate (HR) increases (4). Although the physiological basis for SAN behavior is increasingly understood, very little is known about SAN disease. Severe SAN dysfunction (SND) is marked by irregular prolonged pauses between heart beats, pathologically slow HRs at rest, and inadequate activity-related increases in HR. At present, surgical implantation of permanent pacemakers is required for treatment of SND and costs $2 billion annually in the United States (5). SND commonly occurs in the setting of heart failure and hypertension (6-8), conditions characterized by excessive activation of renin-Ang II signaling (9) and elevated levels of ROS (10). Ang II increases ROS in ventricular myocardium by stimulating NADPH oxidase to cause activation of CaMKII (ox-CaMKII) by oxidation of Met281/282 in the CaMKII regulatory domain (11).