Objectives: 

The binding of FKBP12 to the skeletal ryanodine receptor (RyR1) is believed to be important for normal muscle function. Disruption of this interaction is suggested to play a role in the muscle weakness that is observed in conditions such as Duchenne muscular dystrophy and muscle aging. FKBP12.6 is structurally similar to FKBP12 and binds to RyR1 with similar affinity but because it is normally present in cells at very low levels, it is considered irrelevant to RyR1 regulation and its role in skeletal muscle has been ignored.

Materials: 

We have therefore compared the effects of FKBP12 and FKBP12.6 on the gating of single rabbit RyR1 channels incorporated into planar phosphatidylethanolamine lipid bilayers under voltage-clamp conditions.

Results: 

We found that FKBP12.6 was a potent and irreversible activator of RyR1. For example, 10 pM cytosolic FKBP12.6 significantly elevated open probability (Po) from 0.016±0.007 to 0.189±0.049 (SEM; n=5; P<0.05). Unlike FKBP12.6, 500 nM FKBP12 significantly decreased RyR1 Po from 0.023±0.075 to 0.007±0.003 (SEM; n=8; P<0.05). Importantly, pre-addition of FKBP12.6 prevented the inhibition of RyR1 by FKBP12 suggesting that the two isoforms compete for the same binding sites. Use of rapamycin (20 mM) to dissociate FKBPs from RyR1 and analysis of sub-conductance state gating behaviour demonstrated that there was no relationship between FKBP12/FKBP12.6 levels and the occurrence of sub-conductance gating states. Our results therefore refute the claim that FKBP12 binding to RyR1 is essential to prevent overt sub-conductance state gating that can destabilize excitation-contraction coupling.

Conclusions: 

We suggest that the activating effects of FKBP12.6 on RyR1 may be important for skeletal muscle function and that apparent changes in the binding of FKBP12 to RyR1 in disease states may reflect underlying alterations in the affinity and effectiveness of FKBP12.6 as activator of RyR1.

Supported by the BHF