Role of the N-terminal region of troponin T in cardiac muscle contractile activation

Cardiac muscle contraction is the result of coordinated interactions between the contractile regulatory proteins of the thick and thin filaments, and a detailed understanding of how they interact is central to advancing treatments for human heart diseases. Coordinated interactions between the regulatory proteins mediate Ca2+-, crossbridge (XB)-, and sarcomere length (SL)-dependent cardiac contractile activation. The thick filaments contain myosin, while the thin filament proteins include actin, tropomyosin (Tm), cardiac troponin C, cardiac troponin I, and cardiac troponin T (cTnT). cTnT is a key player in cardiac contraction since it interacts with all other proteins on the thin filament. In particular, cTnT has an N-terminal end that extends into the neighboring structural unit (SU; 7actin:1Tm:1Tn), indicating that this region has a cardiac-specific functional role. However, there is a lack of knowledge regarding the role of the N-terminal end in cardiac contraction. Thus, the overall objective of this dissertation is to determine the role of specific regions in the N-terminal end of cTnT in regulating Ca2+-, XB-, and SL-mediated cardiac contraction. The central hypothesis is that the functional role of specific regions in the N-terminal end of cTnT is modulated by shifts in the isoforms of Tm and myosin heavy chain (MHC). We tested our hypothesis by deleting specific regions in the N-terminal end of cTnT and measuring the contractile function in fibers reconstituted with the cTnT variants. Specific Aim 1 was designed to determine the cardiac-specific role of the N-terminal end of cTnT. Specific Aim 2 was designed to determine how changes in Tm affect the role of the N-terminal end of cTnT. Specific Aim 3 was designed to determine how shifts in MHC isoforms affect the role of the N-terminal end of cTnT. Our findings reveal that specific regions of in the N-terminal end of cTnT have distinct functional impact, and that changes in Tm and MHC modify the impact of the N-terminal end of cTnT in cardiac contraction. New insights from our study will have a positive impact because they further our understanding regarding the role of the N-terminal end of cTnT under physiological and diseased states.