The T1 Region of Cardiac Troponin T is Important for Tuning the Crossbridge Recruitment Dynamics in the Cardiac Muscle

Abstract

The sequence heterogeneity in the T1 and T2 regions of cardiac TnT (cTnT) and fast skeletal TnT (fsTnT) suggests an important tissue-specific functional role. However, the physiological significance of such protein sequence heterogeneity remains elusive. We replaced T1 or T2 region of rat cTnT (RcT1 or RcT2) with its counterpart from rat fsTnT (RfsT1or RfsT2) to generate RfsT1-RcT2 and RcT1-RfsT2 recombinant chimeras. These chimeras were reconstituted into detergent-skinned rat cardiac myofibers for functional characterization. Dynamic contractile features of RfsT1-RcT2 and RcT1-RfsT2 reconstituted fibers were assessed by fitting the recruitment-distortion model to the force response of small amplitude (0.5%) muscle length changes. RfsT1-RcT2 fibers caused a significant decrease in both maximal tension (∼40%) and ATPase activity (∼44%), whereas, RcT1-RfsT2 fibers showed no effect. While the magnitude of crossbridge recruitment, E0, was significantly lower by ∼33%, the speed of crossbridge recruitment, b, was dramatically higher by ∼100% in RfsT1-RcT2 fibers. The rate of tension redevelopment, ktr, also increased by ∼20% in RfsT1-RcT2 reconstituted fibers. Furthermore, an increase in b shifted the frequency of minimum stiffness, fmin, to higher values in RfsT1-RcT2 fibers. On the other hand, b, ktr, and fmin were unaffected in RcT1-RfsT2 fibers. Our data suggests that cardiac-specific T1 plays an important role not only in the maximal activation of cardiac thin filament, but also plays a role in tuning the myocardial contraction such that the crossbridge recruitment dynamics matches with the heart rate.