Sensitized signalling between L-type Ca2+ channels and ryanodine receptors in the absence or inhibition of FKBP12.6 in cardiomyocytes

• Published in: Cardiovascular research 2017-03, Vol.113 (3), p.332-342
• Main Authors: Zhao, Yan-Ting, Guo, Yun-Bo, Gu, Lei, Fan, Xue-Xin, Yang, Hua-Qian, Chen, Zheng, Zhou, Peng, Yuan, Qi, Ji, Guang-Ju, Wang, Shi-Qiang
• Format: Article
• Language: English
• Subjects: Animals; Arrhythmias, Cardiac - genetics; Arrhythmias, Cardiac - metabolism; Arrhythmias, Cardiac - physiopathology; Arrhythmias, Cardiac - prevention & control; Calcium Channels, L-Type - chemistry; Calcium Channels, L-Type - drug effects; Calcium Channels, L-Type - metabolism; Calcium Signaling - drug effects; Genotype; Isoproterenol - pharmacology; Kinetics; Male; Membrane Potentials; Mice, Knockout; Microscopy, Confocal; Models, Molecular; Myocytes, Cardiac - drug effects; Myocytes, Cardiac - metabolism; Original; Patch-Clamp Techniques; Phenotype; Protein Binding; Protein Interaction Domains and Motifs; Receptor Cross-Talk - drug effects; Ryanodine Receptor Calcium Release Channel - chemistry; Ryanodine Receptor Calcium Release Channel - drug effects; Ryanodine Receptor Calcium Release Channel - metabolism; Sirolimus - pharmacology; Stochastic Processes; Tacrolimus - pharmacology; Tacrolimus Binding Proteins - antagonists & inhibitors; Tacrolimus Binding Proteins - chemistry; Tacrolimus Binding Proteins - deficiency; Tacrolimus Binding Proteins - genetics
• ISSN: 0008-6363; 1755-3245
• DOI: 10.1093/cvr/cvw247

The heart contraction is controlled by the Ca2+-induced Ca2+ release (CICR) between L-type Ca2+ channels and ryanodine receptors (RyRs). The FK506-binding protein FKBP12.6 binds to RyR subunits, but its role in stabilizing RyR function has been debated for long. Recent reports of high-resolution RyR structure show that the HD2 domain that binds to the SPRY2 domain of neighbouring subunit in FKBP-bound RyR1 is detached and invisible in FKBP-null RyR2. The present study was to test the consequence of FKBP12.6 absence on the in situ activation of RyR2. Using whole-cell patch-clamp combined with confocal imaging, we applied a near threshold depolarization to activate a very small fraction of LCCs, which in turn activated RyR Ca2+ sparks stochastically. FKBP12.6-knockout and FK506/rapamycin treatments increased spark frequency and LCC-RyR coupling fidelity without altering LCC open probability. Neither FK506 nor rapamycin further altered LCC-RyR coupling fidelity in FKBP12.6-knockout cells. In loose-seal patch-clamp experiments, the LCC-RyR signalling kinetics, indexed by the delay for a LCC sparklet to trigger a RyR spark, was accelerated after FKBP12.6 knockout and FK506/rapamycin treatments. These results demonstrated that RyRs became more sensitive to Ca2+ triggers without FKBP12.6. Isoproterenol (1 μM) further accelerated the LCC-RyR signalling in FKBP12.6-knockout cells. The synergistic sensitization of RyRs by catecholaminergic signalling and FKBP12.6 dysfunction destabilized the CICR system, leading to chaotic Ca2+ waves and ventricular arrhythmias. FKBP12.6 keeps the RyRs from over-sensitization, stabilizes the potentially regenerative CICR system, and thus may suppress the life-threatening arrhythmogenesis.