title: Modulation of SR Ca release by luminal Ca and calsequestrin in cardiac myocytes: effects of CASQ2 mutations linked to sudden cardiac death.
abstract: Cardiac calsequestrin (CASQ2) is an intrasarcoplasmic reticulum (SR) low-affinity Ca-binding protein, with mutations that 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, CASQ2(DEL)) or CASQ2 containing a point mutation (CASQ2(R33Q)), in canine ventricular myocytes and assessed their effects on Ca handling. We also measured CASQ2-CASQ2 variant interactions using fluorescence resonance transfer in a heterologous expression system, and evaluated CASQ2 interaction with triadin. We found that expression of CASQ2(DEL) or CASQ2(R33Q) altered myocyte Ca signaling through two different mechanisms. Overexpressing CASQ2(DEL) disrupted the CASQ2 polymerization required for high capacity Ca binding, whereas CASQ2(R33Q) 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 CASQ2(DEL)-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 CASQ2(R33Q) 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, lead to increased diastolic SR Ca release events and exhibit a similar CPVT disease phenotype.
authors.affiliation: Department of Physiology, Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University Medical Center, Columbus, Ohio 43210-1252, USA.
authors.forename: Dmitry
authors.initials: D
authors.lastname: Terentyev
authors.forename: Zuzana
authors.initials: Z
authors.lastname: Kubalova
authors.forename: Giorgia
authors.initials: G
authors.lastname: Valle
authors.forename: Alessandra
authors.initials: A
authors.lastname: Nori
authors.forename: Srikanth
authors.initials: S
authors.lastname: Vedamoorthyrao
authors.forename: Radmila
authors.initials: R
authors.lastname: Terentyeva
authors.forename: Serge
authors.initials: S
authors.lastname: Viatchenko-Karpinski
authors.forename: Donald M
authors.initials: DM
authors.lastname: Bers
authors.forename: Simon C
authors.initials: SC
authors.lastname: Williams
authors.forename: Pompeo
authors.initials: P
authors.lastname: Volpe
authors.forename: Sandor
authors.initials: S
authors.lastname: Gyorke
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002155:Calsequestrin / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured; D016757:Death, Sudden, Cardiac; D004285:Dogs; D006801:Humans; D032383:Myocytes, Cardiac / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
chemical_list: C443109:CASQ2 protein, human; D002155:Calsequestrin; D002118:Calcium