Cross-bridge dependent regulation of cardiac muscle affinity for calcium

Despite decades of study, the structural basis for striated muscle strong cross-bridge-dependent Ca2+ sensitivity and cooperative activation remains to be established. We have developed a model of muscle cell activation, which incorporates strong cross bridge-induced retention of Ca2+ (tight coupling) and nearest-neighbor-derived cooperative influences. The model qualitatively reproduces a number of published results and provides a mechanistic basis for muscle fiber activation. We hypothesized that cardiac troponin I (cTnI) might play a critical role in both Ca2+- and strong cross-bridge-induced activation. To test this hypothesis, we characterized structural changes occurring in two adjacent regions of TnI during activation: the inhibitory (TnI-I) and regulatory regions (TnI-R). We used Forster resonance energy transfer-derived distance constraints to construct a model of Ca2+-induced internal movements in cTnI-I and cTnI-R. We developed a method to uniquely define the position of Tn residues on the actin filament. We then used this method to define the Ca2+-induced movement of TnI-R with respect to the actin filament. The studies culminated with experiments, defining the role of cTnI in tight coupling and cooperative activation. The data indicate that strong cross-bridges, by modulating the cTnI-R affinity for the N-domain of cardiac troponin C (cTnC), dynamically alter Ca2+ affinity for cTnC. TnI-I binding to actin in neighboring regulatory units appears to enhance the rate of TnI-I switching-off. Based on these results, we propose that cooperativity in the Ca2+-switch results from a release of cooperative influences during activation.