FKBPs facilitate the termination of spontaneous Ca2+ release in wild-type RyR2 but not CPVT mutant RyR2

Abstract

FK506-binding proteins 12.6 (FKBP12.6) and 12 (FKBP12) tightly associate with the cardiac ryanodine receptor (RyR2). Studies suggest that dissociation of FKBP12.6 from mutant forms of RyR2 contributes to store overload-induced Ca(2+) release (SOICR) and Ca(2+)-triggered arrhythmias. However, these findings are controversial. Previous studies focused on the effect of FKBP12.6 on the initiation of SOICR and did not explore changes in the termination of Ca(2+) release. Less is known about FKBP12. We aimed to determine the effect of FKBP12.6 and FKBP12 on the termination of SOICR. Using single-cell imaging, in cells expressing wild-type RyR2, we found that FKBP12.6 and FKBP12 significantly increase the termination threshold of SOICR without changing the activation threshold of SOICR. This effect, dependent on the association of each FKBP with RyR2, reduced the magnitude of Ca(2+) release but had no effect on the propensity for SOICR. In contrast, neither FKBP12.6 nor FKBP12 was able to regulate an arrhythmogenic variant of RyR2, despite a conserved protein interaction. Our results suggest that both FKBP12.6 and FKBP12 play critical roles in regulating RyR2 function by facilitating the termination of SOICR. The inability of FKBPs to mediate a similar effect on the mutant RyR2 represents a novel mechanism by which mutations within RyR2 lead to arrhythmia.

Molecular and tissue mechanisms of catecholaminergic polymorphic ventricular tachycardia

“…However, others (Xiao et al 2007), and at the present time it seems unlikely that loss of FKBP 12.6 is responsible for CPVT. Rather, FKBPs still bind to mutant RyR2 but fail to inhibit them (Zhang et al 2016). The most widely held hypothesis states that CPVT mutations sensitize RyR2 channels to SR luminal calcium, causing them to open at a lower intra-SR calcium concentration, termed store overload-induced calcium release (SOICR) (Jiang et al 2004).…”

Activation of RyR2 by class I kinase inhibitors

“…Two images were collected by a CoolSNAP HQ2 CCD camera (Photometrics, AZ, USA) using a dual-channel imaging system (DV2). The amount of FRET was determined from the ratio of the emissions at 535 and 480 nm (YFP : CFP) as previously described Zhang et al, 2016). From each trace, several parameters were determined.…”

“…the F SOICR (Kong et al, 2008;Waddell et al, 2016). Therefore, we next explored the effect of class I and class II kinase inhibitors on F SOICR (which is expressed as a percentage of the total store) using the luminally expressed Ca 2+ sensor D1ER as previously described Zhang et al, 2016). We also examined the termination threshold (the intracellular Ca 2+ store level, expressed as a percentage of the total store), at which SOICR events terminate (F Termi ), controlled by the inactivation of RyR2 due to partial depletion of SR Ca 2+ (Zima et al, 2008;Tang et al, 2012) or luminal Ca 2+ -dependent deactivation of RyR2 (Lukyanenko et al, 1998).…”

Control of cardiac ryanodine receptor by sarcoplasmic reticulum luminal Ca2+

“…FKBP was originally shown to decrease RyR2 opening ( Marx et al, 2000 ; Marks, 2002 ); however, more recent studies suggest that this is not the case ( Hunt et al, 2007 ; Xiao et al, 2007 ; Guo et al, 2010 ). Intriguingly, rather than affecting activation, both FKBP and calmodulin have now been shown to promote earlier termination of Ca 2+ release, suggesting that they may alter the closing of the channel ( Tian et al, 2013 ; Zhang et al, 2016 ). However, given their respective binding sites (both far from the channel pore), the mechanism by which these proteins promote RyR2 inactivation is unclear.…”