Structure of the NH2-terminal variable region of cardiac troponin T determines its sensitivity to restrictive cleavage in pathophysiological adaptation

We previously reported that the NH2-terminal variable region of cardiac troponin T (cTnT) is removed by restrictive μ-calpain cleavage in myocardial ischemia–reperfusion [24]. Selective removal of the NH2-terminal variable region of cTnT had a compensatory effect on myocardial contractility [25]. Here we further studied this posttranslational modification under pathophysiological conditions. Thrombin perfusion of isolated mouse hearts and cardiomyocytes induced the production of NH2-terminal truncated cTnT (cTnT-ND), suggesting a role of calcium overloading. Ouabain treatment of primary cultures of mouse cardiomyocytes in hypokalemic media, another calcium overloading condition, also produced cTnT-ND. Exploring the molecular mechanisms, we found that cTnT phosphorylation was primarily in the NH2-terminal region and the level of cTnT phosphorylation did not change under the calcium overloading conditions. However, alternatively spliced cTnT variants differing in the NH2-terminal primary structure produced significantly different levels of cTnT-ND in vivo in transgenic mouse hearts. The results suggest that stress conditions involving calcium overloading may convey an increased sensitivity of cTnT to the restrictive μ-calpain proteolysis, in which structure of the NH2-terminal variable region may play a determining role.