Ablation of triadin causes loss of cardiac Ca2+ release units, impaired excitation-contraction coupling, and cardiac arrhythmias

Division of Clinical Pharmacology, Department of Medicine, Vanderbilt University, Nashville, TN 37232, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 05/2009; 106(18):7636-41. DOI: 10.1073/pnas.0902919106
Source: PubMed

ABSTRACT 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.

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Available from: Clara Franzini-Armstrong, Feb 03, 2015
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    • "In contrast, the frequency and extent of JMCs are largely preserved in Casq2-null cardiomyocytes, although SR cisternae are significantly enlarged in these cells [21]. In Trdnnull mice, the number and the size of dyadic junctions are reduced in cardiomyocytes [22], while triad alteration is subtler in skeletal muscles [23]. Specifically, triad orientation is modified and calsequestrin is occasionally mis-localized in triadin-deficient skeletal muscle cells [23]. "
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    ABSTRACT: Excitable cells typically possess junctional membrane complexes (JMCs) constructed by the plasma membrane and the endo/sarcoplasmic reticulum (ER/SR) for channel crosstalk. These JMCs are termed triads in skeletal muscle, dyads in cardiac muscle, peripheral couplings in smooth and developing striated muscles, and subsurface cisterns in neurons. Junctophilin subtypes contribute to the formation and maintenance of JMCs by serving as a physical bridge between the plasma membrane and ER/SR membrane in different cell types. In muscle cells, junctophilin deficiency prevents JMC formation and functional crosstalk between cell-surface Ca2+ channels and ER/SR Ca2+ release channels. Human genetic mutations in junctophilin subtypes are linked to congenital hypertrophic cardiomyopathy and neurodegenerative diseases. Furthermore, growing evidence suggests that dysregulation of junctophilins induces pathological alterations in skeletal and cardiac muscle.
    Cell Calcium 01/2015; DOI:10.1016/j.ceca.2015.01.007 · 4.21 Impact Factor
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    • "Thereby, lowering the SR Ca 2+ concentration during a spark would make the RyRs insensitive for Ca 2+ on the cytosolic side of the channels, which causes their deactivation. Based on observations in SR vesicles, RyRs in lipid bilayers and cells overexpressing calsequestrin, deactivation has been suggested to occur via a retrograde signal mediated by allosteric interactions between calsequestrin (acting as the Ca 2+ sensor) and junctin and/or triadin and the RyR [2] [8] [49] [50]. This mode of spark termination could be stabilized by a reinforcing mechanism that has been proposed recently based on model predictions. "
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    Biochimica et Biophysica Acta 08/2012; 1833(4). DOI:10.1016/j.bbamcr.2012.08.016 · 4.66 Impact Factor
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    • "In these animals, many clusters of Ca v 1.2 are no longer adjacent to RyR2, and the Ca 2+ -dependent inactivation of Ca v 1.2 is measurably delayed. Loss of rapid inactivation delays membrane repolarization, which prolongs the duration of the action potential (Chopra et al. 2009). These electrical perturbations are arrhythmogenic and potentially lethal. "
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    Protoplasma 11/2011; 249 Suppl 1:S31-8. DOI:10.1007/s00709-011-0347-5 · 3.17 Impact Factor
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