Heart muscle is characterized by a regular array of proteins and structures that form a repeating functional unit identified as the sarcomere. This regular structure enables tight coupling between electrical activity and Ca ²⁺ signaling. In heart failure, multiple cellular defects develop, including reduced contractility, altered Ca ²⁺ signaling, and arrhythmias; however, the underlying causes of these defects are not well understood. Here, in ventricular myocytes from spontaneously hypertensive rats that develop heart failure, we identify fundamental changes in Ca ²⁺ signaling that are related to restructuring of the spatial organization of the cells. Myocytes display both a reduced ability to trigger sarcoplasmic reticulum Ca ²⁺ release and increased spatial dispersion of the transverse tubules (TTs). Remodeled TTs in cells from failing hearts no longer exist in the regularly organized structures found in normal heart cells, instead moving within the sarcomere away from the Z-line structures and leaving behind the sarcoplasmic reticulum Ca ²⁺ release channels, the ryanodine receptors (RyRs). These orphaned RyRs appear to be responsible for the dyssynchronous Ca ²⁺ sparks that have been linked to blunted contractility and, probably, Ca ²⁺ -dependent arrhythmias in diverse models of heart failure. We conclude that the increased spatial dispersion of the TTs and orphaned RyRs lead to the loss of local control and Ca ²⁺ instability in heart failure.

Increased mitochondrial calcium entry has been implicated in myocardial death, heart failure and related conditions. Here, the authors show that inhibition of the multifunctional Ca2+- and calmodulin-dependent protein kinase II (CaMKII) in a mouse model of ischaemia reperfusion injury reduces infarct size and mitochondrial-triggered cell death and dysfunction. This is due to reduced mitochondrial Ca2+ entry through the mitochondrial calcium uniporter and enhanced Ca2+ tolerance of the mitochondrial permeability transition pore. This suggests that CaMKII inhibition could reduce adverse responses to common forms of pathological myocardial stress. 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 signalling pathways that control mitochondrial Ca2+ entry through the inner membrane mitochondrial Ca2+ uniporter (MCU)3,4,5 are not known. The multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMKII) is activated in ischaemia reperfusion, myocardial infarction and neurohumoral injury, common causes of myocardial death and heart failure; these findings suggest that 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, an mPTP antagonist with clinical efficacy in ischaemia reperfusion injury6, equivalently prevent mPTP opening, ΔΨm deterioration and diminish mitochondrial disruption and programmed cell death in response to ischaemia reperfusion injury. Mice with myocardial and mitochondrial-targeted CaMKII inhibition have reduced IMCU and are resistant to ischaemia reperfusion injury, myocardial infarction and neurohumoral injury, suggesting that pathological actions of CaMKII are substantially mediated by increasing IMCU. Our findings identify CaMKII activity as a central mechanism for mitochondrial Ca2+ entry in myocardial cell death, and indicate that mitochondrial-targeted CaMKII inhibition could prevent or reduce myocardial death and heart failure in response to common experimental forms of pathophysiological stress.