Calcium Signaling among RyR2s within a Calcium Release Unit during a Calcium Spark

Abstract

Although Ca2+ sparks were discovered more than two decades ago, very little is known about the interactions among the different RyR2s within a dyad (i.e. a Ca2+ release unit, CRU) during a Ca2+ spark. Examination of Ca2+ signaling within cardiac ventricular myocytes by targeting the nanoscopic “subspace” of cardiac dyads (using a triadin or junctin-targeted Ca2+ sensor) was carried out in conjunction with simultaneous visualization of Ca2+ dynamics within the cytosol (using rhod-2) and within the sarcoplasmic reticulum (using Fluo-5N). This combination of Ca2+ signals enabled us to unravel the sequence of activation of the different RyR2s within a dyad during a Ca2+ spark. A Ca2+ spark is first initiated by the activation of a single RyR2 on the edge of the array of RyR2s that will in turn activate several RyR2s within the array which activation will propagate by local CICR to the rest of the array. The order of activation of the RyR2s is stochastic, meaning that every Ca2+ sparks are different. Furthermore, the fewer RyR2s that were activated within the cluster at the CRU, the smaller the width of sparks. Increasing the Ca2+ buffer capacity with EGTA will both narrow the Ca2+ spark width and also decrease the frequency. In this context, Quarky Ca2+ release events (QCRs) correspond to the activation of one or a very few RyR2s. Taken together, these results suggest that the intra-CRU communication between RyR2s during a Ca2+ spark may be very important for our understanding of diseases associated with altered Ca2+ signaling such as arrhythmogenesis and heart failure.