The ryanodine receptors RyR2 have a key role in cardiac excitation-contraction coupling (ECC) and are regulated, among others, by FK506 binding proteins (FKBP). FKBP12 is more abundant than FKBP12.6 in cardiomyocytes with a low affinity for RyR2, whereas FKBP12.6 with stronger affinity is less abundant. In heart failure, FKBP expression decreases and RyR2 are hyperphosphorylated, both mechanisms involved in arrhythmia development. Cardiac FKBP12.6 overexpression protects against arrhythmias due to ß-adrenergic stimulation, but this mechanism is still unknown. Our hypothesis is that FKBP12.6 overexpression, with its stronger affinity for RyR2, displaces FKBP12, therefore reducing RyR2 sensitivity to sympathetic stimulation.

To address this issue, we developed 2 transgenic mouse lines with low (TG1) and high (TG2) levels of cardiac-specific FKBP12.6 overexpression. We characterized ECC of both lines in basal conditions and after ß-adrenergic stimulation to confirm the anti-adrenergic effects of FKBP12.6 overexpression in link with the expression levels and the stoichiometry of the 2 FKBP isoforms. ECC was characterized by confocal microscopy and expression levels of both isoforms by Western blot with commercial and homemade antibodies.

Our first results showed that TG1 mice had normal cardiac phenotype, whereas TG2 mice exhibited cardiac hypertrophy. In TG1 mice, basal Ca2+ transient (Catr) amplitude was slightly higher than that in wild type mice (WT), while time constant of Catr decay was unchanged, the latter being slower in TG2 mice. When 50 nM isoproterenol was added, the increase of Catr amplitude was smaller in TG1 (31%) than in WT (50%) and TG2 (60%) mice. Similarly, the acceleration of Catr decay was smaller in TG1 than in TG2 mice (−27% vs. −56%, respectively).

These preliminary results suggest that low FKBP12.6 overexpression protects partially against sympathetic stimulation, whereas high FKBP12.6 overexpression is associated with cardiomyopathy development.