Dual effect of cardiac FKBP12.6 overexpression on excitation-contraction coupling and the incidence of ventricular arrhythmia depending on its expression level

FKBP12.6, a binding protein to the immunosuppressant FK506, which also binds the ryanodine receptor (RyR2) in the heart, has been proposed to regulate RyR2 function and to have antiarrhythmic properties. However, the level of FKBP12.6 expression in normal hearts remains elusive and some controversies still persist regarding its effects, both in basal conditions and during β-adrenergic stimulation. We quantified FKBP12.6 in the left ventricles (LV) of WT (wild-type) mice and in two novel transgenic models expressing distinct levels of FKBP12.6, using a custom-made specific anti-FKBP12.6 antibody and a recombinant protein. FKBP12.6 level in WT LV was very low (0.16 ± 0.02 nmol/g of LV), indicating that <15% RyR2 monomers are bound to the protein. Mice with 14.1 ± 0.2 nmol of FKBP12.6 per g of LV (TG1) had mild cardiac hypertrophy and normal function and were protected against epinephrine/caffeine-evoked arrhythmias. The ventricular myocytes showed higher [Ca2+]i transient amplitudes than WT myocytes and normal SR-Ca2+ load, while fewer myocytes showed Ca2+ sparks. TG1 cardiomyocytes responded to 50 nM Isoproterenol increasing these [Ca2+]i parameters and producing RyR2-Ser2808 phosphorylation. Mice with more than twice the TG1 FKBP12.6 value (TG2) showed marked cardiac hypertrophy with calcineurin activation and more arrhythmias than WT mice during β-adrenergic stimulation, challenging the protective potential of high FKBP12.6. RyR2R420Q CPVT mice overexpressing FKBP12.6 showed fewer proarrhythmic events and decreased incidence and duration of stress-induced bidirectional ventricular tachycardia. Our study, therefore, quantifies for the first time endogenous FKBP12.6 in the mouse heart, questioning its physiological relevance, at least at rest due its low level. By contrast, our work demonstrates that with caution FKBP12.6 remains an interesting target for the development of new antiarrhythmic therapies.

P3510Cardiac function and Ca2+-cycling are different according to the level of cardiac-specific FKBP12.6 overexpression

Cardiac ryanodine receptors (RyR2) have a key role in excitation-contraction coupling by releasing Ca2+ from sarcoplasmic reticulum (SR). In cardiomyocytes, two FK506 binding protein (FKBP) isoforms have been shown to bind and to stabilize RyR2 opening: FKBP12 and FKBP12.6, the later having a stronger affinity for RyR2 despite its lower abundance. Cardiac-specific FKBP12.6 overexpressing mice have fewer arrhythmias induced by β-adrenergic stimulation than wild type (WT) mice, suggesting an implication of FKBP12.6 in an antiarrhythmic mechanism. Heart failure (HF) syndrome has a high incidence of arrhythmias, which may be explained by a decrease of FKBP isoform expression.

The precise mechanism of the antiarrhythmic effect of FKBP12.6 overexpression remains unknown.

To gain insight into this mechanism, we developed 2 transgenic mouse lines with cardiac-specific moderate- (TG1) and high- (TG2) FKBP12.6 overexpression levels. We characterized cardiac function, [Ca2+]i cycling and its response to β-adrenergic stimulation in both mouse lines.

TG1 and TG2 mice developed mild and marked cardiac hypertrophy, respectively, associated with basal cardiac function increase in TG1 mice only. In stimulated cardiomyocytes, [Ca2+]i transient amplitude, measured by confocal microscopy, was higher in TG1 than in WT mice, without a significant difference in their SR Ca2+ content. The effect of β-adrenergic stimulation (50 nM isoproterenol) was attenuated in TG1 mice compared to WT mice, in association with the prevention of pro-arrhythmogenic Ca2+ release events, such as Ca2+ waves. In contrast, TG2 mice showed [Ca2+]i handling characteristics similar to HF, with slower [Ca2+]i transient relaxation. Interestingly, and contrary to HF, pro-arrhythmogenic Ca2+ release events were also reduced in TG2.

These results indicate that the level of FKBP12.6 overexpression has distinct effects on cardiac function and on Ca2+-cycling and its response to β-adrenergic stimulation.

1178Unsuspected role of the cardiac PKA type I in excitation-contraction coupling and in heart failure development

The cAMP-dependent protein kinase (PKA) consists of two regulatory (R) and two catalytic (C) subunits and comprises two subtypes, PKAI and PKAII, defined by the nature of their regulatory subunits, RIα and RIIα respectively. Whereas PKAII is thought to play a key role in β-adrenergic (β-AR) regulation of cardiac contractility, the function of PKAI is unclear. To address this question, we generated mice with cardiomyocyte-specific and conditional invalidation of the RIα subunit of PKA. Tamoxifen injection in 8 weeks-old mice resulted in a >70% decrease in RIα protein without modification of other PKA subunits, which was associated with ∼2-fold increased basal PKA activity in RIα-KO mice (p<0.05, N=6/group). This translated into enhanced cardiac contraction and relaxation, as observed in vivo by increased fractional shortening and E-wave velocity (p<0.05, N=10/group) and ex vivo by increased LV pressure and maximal rate of contraction and relaxation (p<0.05, N=9/group). L-type Ca2+ current density was increased in ventricular myocytes from RIα-KO, and β-AR stimulation was decreased by ∼50% (p<0.05, n=38 cells for WT, and, n=40 for RIα-KO). Consistently, Ca2+ transients amplitude and relaxation kinetics were increased, along with increased occurrence of Ca2+ sparks and waves (p<0.05, n=44 cells for WT, and, n=50 for RIα KO). Phosphorylation of Ca2+ channels (CaV1.2), PLB, RyR2 and cMyBP-C at PKA sites was increased >2-fold (p<0.05, N=6/group) in RIα KO without modification of total protein expression. With age, these mice developed a congestive heart failure (HF) phenotype with massive hypertrophy and fibrosis which eventually led to death in 50% of RIα-KO mice at 50 weeks (versus 0% in WT, p<0.01). These results reveal a previously unsuspected role of PKA type I in cardiac excitation-contraction coupling and HF.