FKBP12 is a High Affinity, Reversible Activator of RyR2, and FKBP12.6 Antagonises Its Actions

Abstract

FKBP12.6 binds tightly to RyR2 and evidence suggests that it plays a vital physiological role in regulating channel activity. Moreover, changes in FKBP12.6/RyR2 interactions have been implicated in heart failure. Controversy exists, however, as to how FKBP12.6 affects the single-channel behaviour of RyR2. Furthermore, although higher levels of FKBP12 than FKBP12.6 are present in cardiac cells, the effects of FKBP12 on RyR2 are virtually unresearched. We have therefore compared the effects of FKBP12 and FKBP12.6 on the single-channel function of sheep RyR2 incorporated into bilayers under voltage-clamp conditions. We find that FKBP12 increases RyR2 open probability (Po) in a dose-dependant, reversible manner with an EC50 of 51 nM. In the presence of 10 μM cytosolic Ca2+, physiological levels of FKBP12 (3 μM) increased Po from 0.187±0.051 in controls to 0.657±0.111 (SEM; n=14; P<0.001). In contrast, under identical experimental conditions, FKBP12.6 did not significantly increase or decrease RyR2 Po, however, it was able to antagonise the actions of FKBP12, shifting the EC50 value for FKBP12 to 4 μM. Our experiments demonstrate that FKBP12 has high affinity for RyR2 and that at physiological concentrations (1-3 μM) is an effective activator of the channel thereby suggesting that FKBP12 may have a more important role in cardiac excitation-contraction coupling than previously thought. We hypothesise that FKBP12.6 is a very low efficacy (but high affinity) partial agonist of RyR2 and that the balance between the effects of FKBP12 and FKBP12.6 is crucial for normal EC-coupling in cardiac cells. Supported by the British Heart Foundation