CamkII Phosphorylation of RyRs: a Mechanistic Mathematical Model

Abstract

During heart failure (HF), the ability of the sarcoplasmic reticulum (SR) to store Ca2+ is severely impaired resulting in abnormal Ca2+ cycling and excitation-contraction (EC) coupling. While it has been demonstrated that SR Ca2+ ATPase function is reduced and Na+/Ca2+ exchanger function is up-regulated in HF, recently it has been proposed that “leaky” ryanodine receptors (RyRs) also contribute to diminished Ca2+ levels in the SR. Various groups have experimentally investigated the effects of RyR phosphorylation mediated by Ca2+/calmodulin dependent kinase II (CaMKII) and other kinases on RyR behavior. Some of these results are inconsistent, and are difficult to interpret since RyR gating is modulated by many external proteins and ions, including Ca2+. Here, we present a mathematical model representing CaMKII-RyR interaction in the canine ventricular myocyte. This is an extension of our previous model which characterized CaMKII phosphorylation of L-type Ca2+ channels (LCCs) in the cardiac dyad. In this model, it is assumed that upon phosphorylation, RyR Ca2+-sensitivity is increased. Individual RyR phosphorylation is modeled as a function of dyadic CaMKII activity, which is modulated by local Ca2+ levels. The model is constrained by experimental measurements of Ca2+ spark frequency and steady state RyR phosphorylation. It replicates steady state RyR (leak) fluxes in the range measured in experiments without the addition of a separate leak flux pathway. Interestingly, simulation results suggest that CaMKII phosphorylation of LCCs, but not RyRs, significantly increases RyR flux; i.e., increasing trigger Ca2+ has a stronger impact on RyR flux than phosphorylation-induced increases in RyR open probability under physiological conditions. We also show that phosphorylation of LCCs decreases EC coupling gain significantly. These results suggest that LCC phosphorylation sites may be a more effective target than RyR sites in modulating RyR flux and regulating abnormal Ca2+ cycling.