Role of Calsequestrin and Sorcin in the Regulation of Cardiac Excitation Contraction Coupling: A Transcriptional and Physiological Study

Abstract

In the heart, Ca2+ release through the Ryanodine Receptors (RyR2) is a high integrated process that can be regulated by several accessory proteins. Sorcin (SOR) and Calsequestrin (CSQ2) are two Ca2+ binding proteins that can interact with RyR2 modulating the kinetic properties of this channel. However, at the cellular level the molecular mechanism that underlies this possible regulation is still unclear. In order to address this question we implemented an integrated experimental approach to assess the hypothesis that SOR and CSQ2 regulate the SR Ca2+ release process under physiological conditions. Specifically, we generated transgenic mice by breeding SOR-/- and CSQ2-/- animals. A completed qPCR transcriptional study was carried out in the entire population in order to address if the mRNA levels for other genes were altered during the generation of the murine strains. Interestingly, we found that the mRNA's level for RyR2 and Phospholamban were significantly decreased in CSQ-/- animals. Additionally, subcellular Ca2+ dynamics were evaluated by measuring Ca2+ sparks, Ca2+ transients and restitution experiments in isolated myocytes by means of confocal microscopy under physiological temperatures. Animals in which SOR and CSQ2 genes were simultaneously ablated presented a dramatic increase in the time to peak of Ca2+ sparks (41.2±0.30 vs. 18.9±0.30 ms) and Ca2+ transients (37.8±0.09 vs. 19.5±0.12 ms). Moreover, the double ablation of SOR and CSQ2 significantly changed the refractoriness of Ca2+ release. Summarizing, the data presented here indicates that although the SOR-/- itself cannot produce any changes in excitation contraction coupling, in combination with the CSQ2-/- can promote significant changes in the Ca2+. However, most of the alterations observed in the double KOs’ were not significantly different than those observed for the CSQ2-/- itself. Supported by R01-HL-084487 to AE.