Abstract
Background
Cardiac arrhythmias, abnormal heartbeats produced by disruption to the electrical excitation of the heart, are a fatal complication of various cardiovascular diseases (CVDs), and are one of the leading causes of death worldwide. The pathogenesis of many arrhythmias can be attributed to dysregulation in the cardiac ryanodine receptor type Ⅱ (RyR2). In disease RyR2 becomes overly active, often referred to as gain-of-function (GOF), resulting in uncoordinated Ca2+ leak which triggers arrhythmias. Paradoxically, it has also been shown that loss-of-function (LOF) changes can also lead to arrhythmias. Thus, it is apparent that therapeutic approaches will need to ensure a balance between GOF and LOF changes in RyR2 activity. Recently, it has been shown that a novel cause of GOF (Ca2+ leak) and arrhythmias is the loss of RyR2
phosphorylation by casein kinase 2 (CK2). This suggests that increasing CK2 phosphorylation of RyR2 might be a therapeutic option. However, it is not yet known if excessive RyR2 phosphorylation by CK2 induces LOF changes and arrhythmias. Interestingly, RyR2 appears to be very highly phosphorylated by CK2 at rest, and phosphorylation is reduced in cardiac diseases such as heart failure. This suggests that promoting CK2 phosphorylation of RyR2 might not trigger LOF changes. Therefore, this study examines the effect of maximal CK2 phosphorylation of RyR2 on cardiac rhythm, function, and structure. We hypothesise, that maximal CK2 phosphorylation (hyperphosphorylation) of RyR2 will not have any effect on cardiac rhythm, function, or structure.
Methods
Wildtype controls (WT -/-) and transgenic mice mimicking increased and 100% RyR2 phosphorylation by CK2 (mutant heterozygous +/- and mutant homozygous +/+, respectively) were studied at 5 and 8 months of age. Each mouse was subjected to an electrocardiogram (ECG) to detect arrhythmic activity, and an echocardiogram (ECHO) to observe the functional and structural parameters of the heart. Masson’s trichrome staining was also performed to examine changes in the ultrastructure and the presence of collagen.
Results & Conclusions
No arrhythmic activity was observed in any of the mice groups, irrespective of age or genotype. Similarly, there were no differences in cardiac function between genotypes. Consistent with this, there were also no significant differences in cardiac structure or ultrastructure between genotypes. These results show that maximal phosphorylation of RyR2 by CK2 does not induce a LOF phenotype and that cardiac rhythm and structure remain normal as the mice age. Thereby, promoting, or maintaining high levels of CK2 phosphorylation of RyR2 represents a novel viable therapeutic avenue to prevent the generation of cardiac arrhythmias.