Role of SR-mitochondria interplay in calcium-dependent arrhythmias

Project Summary The release of Ca from the sarcoplasmic reticulum (SR) via the ryanodine receptors (RyR2) regulates the heartbeat. This Ca release process is tightly controlled in healthy hearts but goes awry in diseased hearts due to genetic or acquired defects of the RyR2 channel complex. These defects typically make the channel complex hyperactive or leaky, thus giving rise to aberrant Ca release (ACR). While the harmful role of RyR2-mediated ACR is well-established in a spectrum of cardiac pathologies, much less is known regarding how ACR is translated into a specific disease phenotype. For instance, ACR is associated with both arrhythmias and cell death in heart failure and metabolic heart diseases. However, in settings of catecholaminergic polymorphic ventricular tachycardia (CPVT), a genetic arrhythmia syndrome due to mutations in RyR2 or its accessory proteins, ACR results in arrhythmias without signs of pathological remodeling. Mitochondria are involved in myocyte Ca homeostasis to regulate energy production but also cell death. Intriguingly, our recent study provided evidence that mitochondria behave as an efficient Ca sink in the setting of CPVT, which appears to be critical to mitigate detrimental consequences of ACR. Interestingly, this protective Ca-absorbing role seems to be unique to CPVT mitochondria. In contrast, in wild type and several other disease models enhancing mitochondrial Ca uptake stimulates the emission of reactive oxygen species (ROS) and exacerbates RyR2 leak and arrhythmias. These studies suggest that mitochondria play a key role in translating ACR into a specific pathophenotype. They also raised important questions as to why mitochondria in CPVT act as a protective Ca sink and how that impacts Ca-dependent arrhythmias. Based on data in the literature and our preliminary results, we hypothesize that protective changes in CPVT mitochondria alter SR-mitochondria interplay to shape diastolic Ca signal and impact Ca-dependent arrhythmias. Specifically, we propose that in CPVT a dynamic phosphate (Pi)-based mitochondrial Ca handling mechanism converts mitochondria into a protective Ca sink so they can absorb ACR; in parallel, tethering (physical contacts) between mitochondria and SR, also known as mitochondria-associated-membranes (MAMs) are promoted in CPVT to facilitate SR-mitochondria Ca transfer. Thus, we propose that modulating the Pi-based mitochondrial Ca handling, as well as SR-mitochondria tethering both impacts arrhythmogenesis. We have designed multiscale studies (from molecule to whole animal) that employ novel genetic mice models/adeno-associated virus (AAV) 9-mediated gene transfer. We will utilize methods of cellular physiology, protein biochemistry, and in vivo cardiac functional assays to test our hypothesis. The proposed studies will help us gain a better understanding of protective molecular changes of CPVT mitochondria, thus not only benefitting the design of mechanism-based therapies for CPVT, but also providing clues for the development of therapies for a range of Ca-dependent cardiac dysfunctions that are linked to ACR.