Modulation of SR Ca Release by Luminal Ca and Calsequestrin in Cardiac Myocytes: Effects of CASQ2 Mutations Linked to Sudden Cardiac Death
Dmitry Terentyev 1, Zuzana Kubalova 2, Giorgia Valle 3, Alessandra Nori 3, Srikanth Vedamoorthyrao 1, Radmila Terentyeva 1, Serge Viatchenko-Karpinski 1, Donald M. Bers 4, Simon C. Williams 5, Pompeo Volpe 3 and Sandor Gyorke 1*
1 The Ohio State University
2 Institute of Molecular Physiology and Genetics, Slovak Academy of Sciences
3 Univ of Padova
4 Loyola University Med. Ctr.
5 Texas Tech University
* To whom correspondence should be addressed. E-mail: sandor.gyorke{at}osumc.edu.
Submitted on December 26, 2007
Revised on January 23, 2008
Accepted on 8 April 2008
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Abstract |
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Cardiac calsequestrin (CASQ2) is an intra-sarcoplasmic reticulum (SR) low-affinity Ca-binding protein, mutations in which are associated with catecholamine-induced polymorphic ventricular tachycardia (CPVT). To better understand how CASQ2 mutants cause CPVT, we expressed two CPVT-linked CASQ2 mutants, a truncated protein (at G112+5X, CASQ2DEL) or CASQ2 containing a point mutation (CASQ2R33Q), in canine ventricular myocytes and assessed their effects on Ca handling. We also measured CASQ2-CASQ2 variant interactions using fluorescence resonance transfer (FRET) in a heterologous expression system, and evaluated CASQ2 interaction with triadin. We found that expression of CASQ2DEL or CASQ2R33Q altered myocyte Ca signaling through two different mechanisms. Overexpressing CASQ2DEL disrupted the CASQ2 polymerization required for high capacity Ca binding, while CASQ2R33Q compromised the ability of CASQ2 to control ryanodine receptor (RyR2) channel activity. Despite profound differences in SR Ca buffering strengths, local Ca release terminated at the same free luminal [Ca] in control cells, cells overexpressing wild type CASQ2 and CASQ2DEL –expressing myocytes, suggesting that a decline in [Ca]SR is a signal for RyR2 closure. Importantly, disrupting interactions between the RyR2 channel and CASQ2 by expressing CASQ2R33Q markedly lowered the [Ca]SR threshold for Ca release termination. We conclude that CASQ2 in the SR determines the magnitude and duration of Ca release from each SR terminal by providing both a local source of releasable Ca and by effects on luminal Ca-dependent RyR2 gating. Furthermore, two CPVT-inducing CASQ2 mutations, which cause mechanistically different defects in CASQ2 and RyR2 function, both lead to increased diastolic SR Ca release events and exhibit a similar CPVT disease phenotype.
Key Words:
Ca-Induced Ca Release, Calsequestrin, Catecholaminergic Polymorphic Ventricular Tachycardia, Ryanodine Receptor, Sarcoplasmic Reticulum
