Moderate FKBP12.6 Overexpression Mitigates Β-Adrenergic-associated Pro-Arrhythmogenic Ca2+ Events, but a Higher Expression Level Leads to a Cardiomyopathic Phenotype

Cardiac ryanodine receptors RyR2 have a crucial role in excitation-contraction coupling by releasing Ca2+ from the sarcoplasmic reticulum (SR). In cardiomyocytes, two FK506 binding proteins (FKBP) of 12 and 12.6 kDa bind and stabilize RyR2 in a closed state. Despite its lower concentration, FKBP12.6 has a stronger affinity for RyR2 than FKBP12. In heart failure (HF), the expression of both isoforms decreases, associated with a high incidence of arrhythmias. By contrast, cardiac FKBP12.6 overexpression protects against β-adrenergic-activated arrhythmias, but the underlying molecular mechanism remains elusive. We wanted to understand the anti-arrhythmic mechanism of FKBP12.6 overexpression, with the hypothesis of dose-effect anti-adrenergic properties. Accordingly, we developed two transgenic mouse lines with cardiac-specific moderate- ( × 10, TG1) and high- ( × 40, TG2) FKBP12.6 overexpression. We characterized cardiac function, [Ca2 + ]i cycling, and its response to sympathetic stimulation in both mouse lines. TG2 mice died prematurely and exhibited marked cardiac hypertrophy with nonsignificant pulmonary congestion, whereas TG1 mice had a normal phenotype. At 2-3 months, TG1 mice had a normal cardiac function, whereas TG2 had a collapsed heart rate and a dilated and hypertrophied left ventricle with unaltered fractional shortening. TG1 cardiomyocytes had higher [Ca2 + ]i transient amplitude and less pro-arrhythmogenic Ca2+ release events than WT mice, without modification of the SR Ca2+ load, and an attenuated response to β-adrenergic stimulation. In contrast, TG2 mice showed [Ca2 + ]i handling characteristics similar to HF with slower [Ca2 + ]i transient relaxation, although pro-arrhythmogenic Ca2+ release events were reduced. These results confirm that FKBP12.6 overexpression interferes with pro-arrhythmic sympathetic signaling, and demonstrate that high level of FKBP12.6 has a deleterious cardiac effect.