Ablation of triadin causes loss of cardiac Ca 2+ release units, impaired excitation–contraction coupling, and cardiac arrhythmias

• DOI: 10.1073/pnas.0902919106
• Publication Date: 2009-04-22T02:34:59Z
• Authors: Nagesh Chopra; Tao Yang; Parisa Asghari; Edwin D. Moore; Sabine Huke; Brandy Akin; Robert A. Cattolica; Claudio F. Perez; Thinn Hlaing; Barbara E. C. Kno...; Larry R. Jones; Isaac N. Pessah; Paul D. Allen; Clara Franzini-Ar...; Björn C. Knollmann
• Keywords: 0301 basic medicine; Calcium Channels, L-Type; Myocardium; Intracellular Signaling Peptides and Proteins; Muscle Proteins; Arrhythmias, Cardiac; Heart; Myocardial Contraction; Mice, Mutant Strains; Mice; Sarcoplasmic Reticulum; 03 medical and health sciences; Animals; Calcium; Carrier Proteins; Sequence Deletion

Heart muscle excitation–contraction (E-C) coupling is governed by Ca 2+ release units (CRUs) whereby Ca 2+ influx via L-type Ca 2+ channels (Cav1.2) triggers Ca 2+ release from juxtaposed Ca 2+ release channels (RyR2) located in junctional sarcoplasmic reticulum (jSR). Although studies suggest that the jSR protein triadin anchors cardiac calsequestrin (Casq2) to RyR2, its contribution to E-C coupling remains unclear. Here, we identify the role of triadin using mice with ablation of the Trdn gene ( Trdn −/− ). The structure and protein composition of the cardiac CRU is significantly altered in Trdn −/− hearts. jSR proteins (RyR2, Casq2, junctin, and junctophilin 1 and 2) are significantly reduced in Trdn −/− hearts, whereas Cav1.2 and SERCA2a remain unchanged. Electron microscopy shows fragmentation and an overall 50% reduction in the contacts between jSR and T-tubules. Immunolabeling experiments show reduced colocalization of Cav1.2 with RyR2 and substantial Casq2 labeling outside of the jSR in Trdn −/− myocytes. CRU function is impaired in Trdn −/− myocytes, with reduced SR Ca 2+ release and impaired negative feedback of SR Ca 2+ release on Cav1.2 Ca 2+ currents (I Ca ). Uninhibited Ca 2+ influx via I Ca likely contributes to Ca 2+ overload and results in spontaneous SR Ca 2+ releases upon β-adrenergic receptor stimulation with isoproterenol in Trdn −/− myocytes, and ventricular arrhythmias in Trdn −/− mice. We conclude that triadin is critically important for maintaining the structural and functional integrity of the cardiac CRU; triadin loss and the resulting alterations in CRU structure and protein composition impairs E-C coupling and renders hearts susceptible to ventricular arrhythmias.