Correlation between FKBP12/12.6 Bound to RyRs, Channel Opening to Maximal or Submaximal Conductance Levels and Myopathy

Abstract

Ryanodine receptor (RyR) Ca2+ release channels located in the membrane of sarcoplasmic reticulum Ca2+ stores provide the Ca2+ required for contraction in skeletal muscle and the heart. The normal function of the ion channels is necessary for optimal muscle function. Increases in channel activity in relaxed muscle lead to increased cytoplasmic Ca2+, skeletal myopathy and cardiac arrhythmia. The 12.0 and 12.6 KDa proteins that bind to the immunosuppressant drug FK506, FKBP12 and FKBP12.6, also bind to RyR channels and the binding is thought to be essential for normal channel function. Their dissociation from the RyR has been associated with the damaging increase in “leak” of Ca2+ from the SR when the muscle is relaxed. However the extent to which dissociation of the FKBPs from RyRs contributes to skeletal and cardiac myopathies caused by “leaky” RyR channels is unclear. In addition, there is controversy about the changes in RyR channel gating, in particular the prevalence of submaximal conductance opening, that contribute to channel “leak”. We have addressed this problem by examining association of the FKBP12 and FKBP12.6 in two myopathic conditions. Firstly in the presence of a mutant CLIC-2 protein, that activates RyR2 and whose interaction with RyR2 has been associated with cardiac hyperthrophy and intellectual deficit [1]. Secondly, with a cholesterol-reducing drug simvastatin that can produce a skeletal myopathy. In both cases we show increases in channel activity, particularly in the number and duration of submaximal conductance openings, as well significant FKBP12 and 12.6 dissociation from RyR2 and FKBP12 dissociation from RyR1. The results suggest that, in these situations, “leak” through RyR channels is facilitated by increased sub-conductance opening, due to dissociation or destabilisation of FKBP binding.Takano et al., Hum Mol Genet. 2012;21(20):4497-507.