Aims Diastolic Ca release (DCR) from sarcoplasmic reticulum (SR) Ca release channel ryanodine receptor (RyR2) has been linked to multiple cardiac pathologies, but its exact role in shaping divergent cardiac pathologies remains unclear. We hypothesize that the SR-mitochondria interplay contributes to disease phenotypes by shaping Ca signaling. Methods and Results A genetic model of catecholaminergic polymorphic ventricular tachycardia (CPVT2 model of CASQ2 knockout) and a pre-diabetic cardiomyopathy model of fructose fed mice (FFD), both marked by DCR, are employed in this study. Mitochondria Ca (mCa) is modulated by pharmacologically targeting mitochondria Ca uniporter (MCU) or permeability transition pore (mPTP), mCa uptake and extrusion mechanisms, respectively. An MCU activator abolished Ca waves in CPVT2 but exacerbated waves in FFD cells. Mechanistically this is ascribed to mitochondria’s function as a Ca buffer or source of reactive oxygen species (mtROS) to exacerbate RyR2 functionality, respectively. Enhancing mCa uptake reduced and elevated mtROS production in CPVT2 and FFD respectively. In CPVT2, mitochondria took up more Ca in permeabilized cells, and had higher level of mCa content in intact cells vs FFD. Conditional ablation of MCU in the CPVT2 model caused lethality and cardiac remodeling, but reduced arrhythmias in the FFD model. In parallel, CPVT2 mitochondria also employ upregulated mPTP-mediated Ca efflux to avoid mCa overload, as seen by elevated incidence of MitoWinks (an indicator of mPTP-mediated Ca efflux) vs FFD. Both pharmacological and genetic inhibition of mPTP promoted mtROS production and exacerbation of myocyte Ca handling in CPVT2. Further, genetic inhibition of mPTP exacerbated arrhythmias in CPVT2. Conclusion In contrast to FFD, which is more susceptible to mtROS-dependent RyR2 leak, in CPVT2 mitochondria buffer SR-derived DCR to mitigate Ca-dependent pathological remodeling and rely on mPTP-mediated Ca efflux to avoid mCa overload. SR-mitochondria interplay contributes to the divergent pathologies by disparately shaping intracellular Ca signaling. Translational Perspective It is well-established that RyR2 dysfunction is involved in a spectrum of pathological conditions including cardiac arrhythmias. In this study, two disease models marked by RyR2 dysfunction were employed to explore how the interplay between SR and mitochondria contributes to divergent cardiac pathologies. We found mitochondria act as essential Ca buffer to absorb SR-derived Ca to mitigate pathological remodeling in the genetic arrhythmic syndrome CPVT, but they are more susceptible to Ca overload or ROS-related exacerbation of RyR2 dysfunction in pre-diabetic cardiomyopathy. Thus, tailored therapies should be developed to target SR-mitochondria interplay in the aims of treating these diseases.

matching mitochondrial ATP production to the myocyte's energetic requirements. Here we tested whether elevated activity of RyR2 resulted in selfimposed exacerbation of SR Ca 2þ leak, via altered SR-mitochondrial Ca 2þ transfer and increased mito-ROS emission. In ventricular myocytes from rats and catecholaminergic polymorphic ventricular tachycardia (CPVT) mice, fluorescent indicators and organelle-targeted redox biosensors revealed that pharmacologically and genetically enhanced RyR2 activity led to elevated ROS emission during b-adrenergic stimulation. Mitochondrial-targeted Ca 2þ biosensor mtRCamp1h showed that mitochondria has a reduced ability to uptake and accumulate Ca 2þ . Voltage-sensitive dye TMRM revealed this was due to depolarization of the mitochondrial matrix. Although this offers protection against deleterious mitochondrial Ca 2þ overload, mito-ROS emission was significantly increased by enhanced SR Ca 2þ leak and resulted in RyR2 oxidation, thus exacerbating augmented channel activity. Application of mito-ROS scavenger mitoTEMPO attenuated the effects of increased RyR2 Ca 2þ release, reducing the incidence of proarrhythmic spontaneous Ca 2þ waves. Furthermore, the hypothesis that transfer of Ca 2þ from the SR to mitochondria is essential to mito-ROS emission was supported by experiments with dominant-negative paralog of the mitochondrial Ca 2þ uniporter (MCU), whereby it was reduced by inhibition of mitochondrial Ca 2þ uptake. These findings suggest that in a detrimental feedback cycle, hyperactivity of RyR2 channels and SR-mitochondrial Ca 2þ transfer results in increased channel oxidation via mito-ROS. Increased leak therefore induces leak, further augmenting proarrhythmic SR Ca 2þ release in diseased myocytes.

The brief opening mode of the mitochondrial permeability transition pore (mPTP) serves as a calcium (Ca2+) release valve to prevent mitochondrial Ca2+ (mCa2+) overload. Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a stress-induced arrhythmic syndrome due to mutations in the Ca2+ release channel complex of ryanodine receptor 2 (RyR2). We hypothesize that inhibiting the mPTP opening in CPVT exacerbates the disease phenotype. By crossbreeding a CPVT model of CASQ2 knockout (KO) with a mouse missing CypD, an activator of mPTP, a double KO model (DKO) was generated. Echocardiography, cardiac histology, and live-cell imaging were employed to assess the severity of cardiac pathology. Western blot and RNAseq were performed to evaluate the contribution of various signaling pathways. Although exacerbated arrhythmias were reported, the DKO model did not exhibit pathological remodeling. Myocyte Ca2+ handling was similar to that of the CASQ2 KO mouse at a low pacing frequency. However, increased ROS production, activation of the CaMKII pathway, and hyperphosphorylation of RyR2 were detected in DKO. Transcriptome analysis identified altered gene expression profiles associated with electrical instability in DKO. Our study provides evidence that genetic inhibition of mPTP exacerbates RyR2 dysfunction in CPVT by increasing activation of the CaMKII pathway and subsequent hyperphosphorylation of RyR2.

The rhythmic contraction of the heart relies on tightly regulated calcium (Ca) release from the sarcoplasmic reticulum (SR) Ca release channel, Ryanodine receptor (RyR2). Genetic mutations in components of the calcium release unit such as RyR2, cardiac calsequestrin and other proteins have been shown to cause a genetic arrhythmic syndrome known as catecholaminergic polymorphic ventricular tachycardia (CPVT). This book chapter will focus on the following: (1) to describing CPVT as a stress-induced cardiac arrhythmia syndrome and its genetic causes. (2) Discussing the regulation of SR Ca release, and how dysregulation of Ca release contributes to arrhythmogenesis. (3) Discussing molecular mechanisms of CPVT with a focus on impaired Ca signaling refractoriness as a unifying mechanism underlying different genetic forms of CPVT. (4) Discussing pharmacological approaches as CPVT treatments as well as other potential future therapies. Since dysregulated SR Ca release has been implicated in multiple cardiac disorders including heart failure and metabolic heart diseases, knowledge obtained from CPVT studies will also shed light on the development of therapeutic approaches for these devastating cardiac dysfunctions as a whole.