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

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 juncto- philin 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.