Calmodulin Regulation of Ryanodine Receptors (RyR2) Differs in Failing and Non-Faling Human Hearts due to Differences in RyR2 Phosphorylation

Abstract

The RyR2 is a macromolecular complex comprising a Ca2+ channel and many accessory proteins that regulate channel activity. In animal models, the binding of intracellular calmodulin (CaM, ∼45 nM in the cytoplasm) partially inhibits calcium release in cardiomyocytes due to its ability to inhibit RyR2 channel opening. Single channel studies of RyR2 in lipid bilayers find that CaM inhibits RyR2 from animal hearts with an IC50 of 100 nM.In this study, RyR2 was isolated from healthy human donor hearts and hearts with ischemic cardiomyopathy (ICM) with ethics approval (UoN H-2009-0369; Univ Sydney #2012/2814; QUT EC28114) and incorporated into lipid bilayers. We found that RyR2 from healthy human hearts were not inhibited by CaM at concentrations up to 500 nM whereas for RyR2 from human hearts with ICM, CaM caused a reduction in open probability of 57% ± 6%. Dephosphorylating these RyR2 channels with Protein Phosphatase 1 prior to experiments abolished the effect of CaM, suggesting that increased RyR2 phosphorylation in ICM hearts mediates CaM inhibition in humans.