Dilated cardiomyopathy mutation (R174W) in troponin T attenuates the length-mediated increase in cross-bridge recruitment and myofilament Ca²⁺ sensitivity

Alterations in length-dependent activation (LDA) may constitute a mechanism by which cardiomyopathy mutations lead to deleterious phenotypes and compromised heart function, because LDA underlies the molecular basis by which the heart tunes myocardial force production on a beat-to-beat basis (Frank-Starling mechanism). In this study, we investigated the effect of DCM-linked mutation (R173W) in human cardiac troponin T (TnT) on myofilament LDA. R173W mutation is associated with left ventricular dilatation and systolic dysfunction and is found in multiple families. R173W mutation is in the central region (residues 80–180) of TnT, which is known to be important for myofilament cooperativity and cross-bridge (XB) recruitment. Steady-state and dynamic contractile parameters were measured in detergent-skinned guinea pig left ventricular muscle fibers reconstituted with recombinant guinea pig wild-type TnT (TnT WT ) or mutant TnT (TnT R174W ; guinea pig analog of human R173W mutation) at two different sarcomere lengths (SL): short (1.9 µm) and long (2.3 µm). TnT R174W decreased pCa 50 (−log [Ca 2+ ] free required for half-maximal activation) to a greater extent at long than at short SL; for example, pCa 50 decreased by 0.12 pCa units at long SL and by 0.06 pCa units at short SL. Differential changes in pCa 50 at short and long SL attenuated the SL-dependent increase in myofilament Ca 2+ sensitivity (ΔpCa 50 ) in TnT R174W fibers; ΔpCa 50 was 0.10 units in TnT WT fibers but only 0.04 units in TnT R174W fibers. Furthermore, TnT R174W blunted the SL-dependent increase in the magnitude of XB recruitment. Our observations suggest that the R173W mutation in human cardiac TnT may impair Frank-Starling mechanism. NEW & NOTEWORTHY This work characterizes the effect of dilated cardiomyopathy mutation in cardiac troponin T (TnT R174W ) on myofilament length-dependent activation. TnT R174W attenuates the length-dependent increase in cross-bridge recruitment and myofilament Ca 2+ sensitivity.