Role of FK506-binding protein in Ca²⁺ spark regulation

The elementary Ca²⁺ release events, Ca²⁺ sparks, has been found for a quarter of century. However, the molecular regulation of the spark generator, the ryanodine receptor (RyR) on the sarcoplasmic reticulum, remains obscure. Although each subunit of the RyR homotetramer has a site for FK506-binding protein (FKBP), the role of FKBPs in modifying RyR Ca²⁺ sparks has been debated for long. One of the reasons behind the controversy is that most previous studies detect spontaneous sparks, where the mixture with out-of-focus events and local wavelets prevents an accurate characterization of Ca²⁺ sparks. In the present study, we detected Ca²⁺ sparks triggered by single L-type Ca²⁺ channels (LCCs) under loose-seal patch clamp conditions in FK506-treated or FKBP12.6 knockout cardiomyocytes. We found that FKBP dissociation both by FK506 and by rapamycin decreased the Ca²⁺ spark amplitude in ventricular cardiomyocytes. This change was neither due to decreased releasable Ca²⁺ in the sarcoplasmic reticulum, nor explained by changed RyR sensitivity. Actually FK506 increased the LCC-RyR coupling probability and curtailed the latency for an LCC to trigger a RyR Ca²⁺ spark. FKBP12.6 knockout had similar effects as FK506/rapamycin treatment, indicating that the decreased spark amplitude was attributable to the dissociation of FKBP12.6 rather than FKBP12. We also explained how decreased amplitude of spontaneous sparks after FKBP dissociation sometimes appears to be increased or unchanged due to inappropriate data processing. Our results provided firm evidence that without the inter-RyR coordination by functional FKBP12.6, the RyR recruitment during a Ca²⁺ spark would be compromised despite the sensitization of individual RyRs.