Increased Ca <sup>2+</sup> Transient Underlies RyR2-Related Left Ventricular Noncompaction
Mingke Ni, Yanhui Li, Jinhong Wei, Zhenpeng Song, Hui Wang, Jinjing Yao, Yong-Xiang Chen, Darrell D. Belke, John Paul Estillore, Ruiwu Wang, Alexander Vallmitjana, Raúl Benítez, Leif Hove‐Madsen, Wei Feng, Ju Chen, Thomas M. Roston, Shubhayan Sanatani, Anna Lehman, S.R. Wayne Chen
Abstract
Background: A loss-of-function cardiac ryanodine receptor (RyR2) mutation, I4855M +/– , has recently been linked to a new cardiac disorder termed RyR2 Ca 2+ release deficiency syndrome (CRDS) as well as left ventricular noncompaction (LVNC). The mechanism by which RyR2 loss-of-function causes CRDS has been extensively studied, but the mechanism underlying RyR2 loss-of-function-associated LVNC is unknown. Here, we determined the impact of a CRDS-LVNC-associated RyR2-I4855M +/– loss-of-function mutation on cardiac structure and function. Methods: We generated a mouse model expressing the CRDS-LVNC-associated RyR2-I4855M +/– mutation. Histological analysis, echocardiography, ECG recording, and intact heart Ca 2+ imaging were performed to characterize the structural and functional consequences of the RyR2-I4855M +/– mutation. Results: As in humans, RyR2-I4855M +/– mice displayed LVNC characterized by cardiac hypertrabeculation and noncompaction. RyR2-I4855M +/– mice were highly susceptible to electrical stimulation–induced ventricular arrhythmias but protected from stress-induced ventricular arrhythmias. Unexpectedly, the RyR2-I4855M +/– mutation increased the peak Ca 2+ transient but did not alter the L-type Ca 2+ current, suggesting an increase in Ca 2+ -induced Ca 2+ release gain. The RyR2-I4855M +/– mutation abolished sarcoplasmic reticulum store overload–induced Ca 2+ release or Ca 2+ leak, elevated sarcoplasmic reticulum Ca 2+ load, prolonged Ca 2+ transient decay, and elevated end-diastolic Ca 2+ level upon rapid pacing. Immunoblotting revealed increased level of phosphorylated CaMKII (Ca 2+ -calmodulin dependent protein kinases II) but unchanged levels of CaMKII, calcineurin, and other Ca 2+ handling proteins in the RyR2-I4855M +/– mutant compared with wild type. Conclusions: The RyR2-I4855M +/– mutant mice represent the first RyR2-associated LVNC animal model that recapitulates the CRDS-LVNC overlapping phenotype in humans. The RyR2-I4855M +/– mutation increases the peak Ca 2+ transient by increasing the Ca 2+ -induced Ca 2+ release gain and the end-diastolic Ca 2+ level by prolonging Ca 2+ transient decay. Our data suggest that the increased peak-systolic and end-diastolic Ca 2+ levels may underlie RyR2-associated LVNC.