Troponin I Serine 150 Phosphorylation Inhibits pH-Induced Troponin Calcium Desensitization

Abstract

A hallmark of cardiac ischemia is decreased intracellular pH which can affect a number of cellular processes. Such an acidic environment alters cardiac troponin (Tn) myofilament regulation to decrease the Ca2+ sensitivity of force production. Tn also undergoes cardiac ischemic-induced AMPK troponin I (TnI) Ser-150 phosphorylation. We recently characterized the effect of TnI Ser-150 phosphorylation demonstrating it to increase myofilament Ca2+ sensitivity and blunt TnI PKA phosphorylation-induced desensitization. The effect of an acidic environment on Tn myofilament regulation in the presence of the ischemic-induced TnI Ser-150 phosphorylation is unknown. In this study we investigate the effect of pH on myofilament regulation in the presence of TnI Ser-150 phosphorylation. Myofilament regulation was determined by measuring troponin C Ca2+ binding properties at normal and acidic pH. Results demonstrate acidic pH decreases steady-state Ca2+ binding to troponin C in thin filaments reconstituted with either WT or Ser-150 pseudo-phosphorylated TnI (Tn S150D) such that Tn S150D Ca2+ sensitivity at pH 6.5 is similar to WT at pH 7. Furthermore, while Tn S150D slowed Ca2+ dissociation compared to Tn WT at pH 7.0, decreasing the pH to 6.5 did not further alter isolated Tn S150D Ca2+ dissociation indicating a lack of pH to modulate Tn Ca2+ dissociation kinetics. Thin filament dissociation kinetics are ongoing. We conclude that TnI Ser-150 phosphorylation imparts resistance to acidic pH induced myofilament Ca2+ desensitization similar to that present during ischemia without alteration of Tn Ca2+ dissociation. Future investigations are aimed at examining the effect of TnI Ser-150 phosphorylation in the presence of TnI Ser-23/24 phosphorylation at low pH.