Abstract
Arrhythmia occurs in a number of heart diseases including heart failure (HF) and Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT). It has been shown that Store Overload Induced Calcium Release (SOICR) is a common mechanism underlying many types of arrhythmia. SOICR occurs due to the inappropriate opening of the cardiac ryanodine receptor (RyR2) once calcium in the sarcoplasmic reticulum (SR) reaches a certain threshold. RyR2 forms a large macromolecular complex with other proteins. One such protein is triadin; which is known to be lost from the RyR2 complex in HF and CPVT. The loss of triadin is also linked to an increase in arrhythmias, suggesting its loss may lead to SOICR. However, triadin has also been shown to have a role in maintaining the ultrastructure of cardiomyocytes, therefore it is unclear whether arrhythmia occurs as a direct result of the loss of interaction between the RyR2 and triadin or simply due to structural changes in the cell. In the present study, we investigated whether the direct interaction between RyR2 and triadin alters the propensity for SOICR.
Single cell cytosolic imaging in HEK 293 cells stably expressing RyR2 with or without triadin, using the high affinity calcium indicator Fluo4-AM, showed that the presence of triadin reduced the propensity of SOICR events, suggesting an inhibitory role for triadin. This was confirmed by single cell luminal calcium imaging, using the endoplasmic reticulum (ER) targeted Ca2+ indicator protein D1ER which showed that the expression of triadin increased ER calcium threshold at which SOICR occurred.
These results provide evidence that the direct interaction between RyR2 and triadin reduces the propensity for SOICR by increasing the threshold that SR calcium must reach to trigger SOICR. Conversely, this suggests that loss of the direct interaction between RyR2 and triadin in HF and CPVT will be arrhythmogenic. Therefore, stablising this interaction in patients susceptible to arrhythmias is likely to be therapeutic.