| Cardiac troponin I (cTnI) is a key regulatory protein in cardiac muscle contraction and relaxation. Better understanding of the structure-function relationship of cTnI is crucial for drug development aimed to improve cardiac function in heart failure and/or to cure cardiac diseases associated with malfunction of cTnI. Accordingly, we determined the impact of cTnI subdomains, in particular, the C-terminal, the inhibitory, and the N-terminal region on cross-bridge cycling kinetics and myofilament functions.;Aim 1. The impact of C-terminal truncation on cross-bridge cycling kinetics was studied. Proteolytic cleavage of cTnI at the C-terminus (cTnI1-193) is associated with myocardial stunning. The mechanism(s) underlying depressed cardiac function by cTnI1-193 are not fully understood. We found that the presence of cTnI1-193 in the cardiac sarcomere results in decreased myofilament force, increased myofilament Ca 2+ sensitivity, decreased cooperativity of Ca2+ activation, and increased cross-bridge cycling kinetics. We conclude that decreased cardiac function in stunned myocardium may directly result, in part, from proteolytic degradation of cTnI, resulting in alteration in cross-bridge cycling kinetics.;Aim 2. The role of the inhibitory region of cTnI in myofilament length dependent activation (LDA), the mechanism underlying the Frank-Starling law of the heart, was investigated. We found that the presence of Thr at position 144 (in cTnI) or 112 (in ssTnI) is sufficient to impart length dependence onto the cardiac sarcomere. These data support the hypothesis that length sensing element(s) exist in the sarcomere to modulate myofilament LDA. This length sensor possibly resides in the inhibitory region of cTnI.;Aim 3. The role of N-terminal of cTnI on myofilament function was investigated. Proteolytic removal of N-terminal region of cTnI (cTnIND) was shown to be associated with hemodynamic adaptation to long term exposure to microgravity. We found that the effects of cTnI ND on myofilament function were to decrease myofilament force, decreased myofilament Ca2+ sensitivity, and increased cross-bridge detachment rate. The decrease in myofilament function induced by cTnIND as found in this study supports the hypothesis that proteolytic cleavage of N-terminal domain during long term exposure to microgravity may be an adaptive mechanism to reduce cardiac function when circulatory blood volume is reduced. |
