Ablation of triadin causes loss of cardiac Ca2+ 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 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 beta-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.
Chopra
Nagesh
N
Division of Clinical Pharmacology, Department of Medicine, Vanderbilt University, Nashville, TN 37232, USA.
Yang
Tao
T
Asghari
Parisa
P
Moore
Edwin D
ED
Huke
Sabine
S
Akin
Brandy
B
Cattolica
Robert A
RA
Perez
Claudio F
CF
Hlaing
Thinn
T
Knollmann-Ritschel
Barbara E C
BE
Jones
Larry R
LR
Pessah
Isaac N
IN
Allen
Paul D
PD
Franzini-Armstrong
Clara
C
Knollmann
Björn C
BC
D000818:Animals; D001145:Arrhythmias, Cardiac / Q000235:genetics / Q000473:pathology / Q000503:physiopathology*; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D006321:Heart / Q000502:physiology / Q000503:physiopathology*; D047908:Intracellular Signaling Peptides and Proteins; D051379:Mice; D008817:Mice, Mutant Strains; D009124:Muscle Proteins / Q000235:genetics / Q000502:physiology*; D009200:Myocardial Contraction / Q000235:genetics; D009206:Myocardium / Q000378:metabolism / Q000648:ultrastructure; D012519:Sarcoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D017384:Sequence Deletion
C514350:CACNA1C protein, mouse; D020746:Calcium Channels, L-Type; D002352:Carrier Proteins; D047908:Intracellular Signaling Peptides and Proteins; D009124:Muscle Proteins; C498270:Trdn protein, mouse; D002118:Calcium