Dilated cardiomyopathy mutation (R174W) in troponin T attenuates the length-mediated increase in cross-bridge recruitment and myofilament Ca 2+ 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.