Mechanistic Investigation of the Arrhythmogenic Role of Oxidized CaMKII in the Heart

Oxidative stress and calcium (Ca²⁺)/calmodulin (CaM)-dependent protein kinase II (CaMKII) both play important roles in the pathogenesis of cardiac disease. Although the pathophysiological relevance of reactive oxygen species (ROS) and CaMKII has been appreciated for some time, recent work has shown that ROS can directly oxidize CaMKII, leading to its persistent activity and an increase of the likelihood of cellular arrhythmias such as early afterdepolarizations (EADs). Because CaMKII modulates the function of many proteins involved in excitation-contraction coupling, elucidation of its role in cardiac function, in both healthy and oxidative stress conditions, is challenging. To investigate this role, we have developed a model of CaMKII activation that includes both the phosphorylation-dependent and the newly identified oxidation-dependent activation pathways. This model is incorporated into our previous local-control model of the cardiac myocyte that describes excitation-contraction coupling via stochastic simulation of individual Ca²⁺ release units and CaMKII-mediated phosphorylation of L-type Ca²⁺ channels (LCCs), ryanodine receptors and sodium (Na⁺) channels. The model predicts the experimentally measured slow-rate dependence of H₂O₂-induced EADs. Upon increased H₂O₂, simulations suggest that selective activation of late Na⁺ current (I_NaL), although it prolongs action potential duration, is not by itself sufficient to produce EADs. Similar results are obtained if CaMKII effects on LCCs and ryanodine receptors are considered separately. However, EADs emerge upon simultaneous activation of both LCCs and Na⁺ channels. Further modeling results implicate activation of the Na⁺-Ca²⁺ exchanger (NCX) as an important player in the generation of EADs. During bradycardia, the emergence of H₂O₂-induced EADs was correlated with a shift in the timing of NCX current reversal toward the plateau phase earlier in the action potential. Using the timing of NCX current reversal as an indicator event for EADs, the model identified counterintuitive ionic changes - difficult to experimentally dissect - that have the greatest influence on ROS-related arrhythmia propensity.