The Cardiac Troponin T Mutant Missing the N-Terminal Extension Causes Dose-Dependent Effects on Cardiac Function and Remodeling in Transgenic Mice

The N-terminal extension (NTE; residues 42-73) of mouse cardiac troponin T (TnT) desensitizes cardiac myofilaments to Ca2+ by stabilizing thin filaments in the blocked-state. We arrived at this conclusion using detergent-skinned muscle from transgenic (TG) mouse hearts that expressed 54% of chimeric TnT (residues 1-73 of mouse cardiac TnT were replaced by residues 1-41 of mouse fast skeletal TnT). Here, we extended our investigation to include higher dose effects of the modified TnT on cardiac myofilament function/phenotype using detergent-skinned fiber studies and echocardiography measurements in two different TG mouse lines (TG-55 and TG-64 that expressed 55% and 64% of chimeric TnT, respectively). Both TG-55 and TG-64 mice showed a similar increase in myofilament Ca2+ sensitivity at sarcomere lengths (SL) of 1.9 and 2.3 μm. However, Ca2+-activated maximal tension increased significantly only in TG-64 mice at either SL. There was a progressive decrease in the overall heart size and heart-to-body weight ratios in both TG-55 and TG-64 mice. Left ventricular diastolic functional parameters (isovolumic relaxation time and E-wave deceleration time) showed a graded increase in TG-55 and TG-64 mice; however, such effects were only significant in TG-64 mice, suggesting impaired relaxation. Systolic functional parameters (stroke volume, ejection fraction and fractional shortening) were unaffected in TG-55 mice, but significantly decreased in TG-64 mice. Thus, higher levels of chimeric TnT (64%) depressed both diastolic and systolic function significantly in TG-64 mice. We will discuss the link between the effects of the modified N-terminus of TnT on cardiac myofilament function and the resultant pathological remodeling of the heart. Our findings have pathological relevance because a growing number of disease-related mutations are found both in and near the NTE of cardiac TnT.