Abstract
The present study investigated the functional consequences of the human hypertrophic cardiomyopathy (HCM) mutation A28V in cardiac troponin T (TnT). The A28V mutation is located within the NH(2) terminus of TnT, a region known to be important for full activation of cardiac thin filaments. The functional consequences of the A28V mutation in TnT remain unknown. Given how α- and β-myosin heavy chain (MHC) isoforms differently alter the functional effect of the NH(2) terminus of TnT, we hypothesized that the A28V-induced effects would be differently modulated by α- and β-MHC isoforms. Recombinant wild-type mouse TnT (TnT(WT)) and the mouse equivalent of the human A28V mutation (TnT(A30V)) were reconstituted into detergent-skinned cardiac muscle fibers extracted from normal (α-MHC) and transgenic (β-MHC) mice. Dynamic and steady-state contractile parameters were measured in reconstituted muscle fibers. Step-like length perturbation experiments demonstrated that TnT(A30V) decreased the magnitude of the muscle length-mediated recruitment of new force-bearing cross bridges (E(R)) by 30% in α-MHC fibers. In sharp contrast, TnT(A30V) increased E(R) by 55% in β-MHC fibers. Inferences drawn from other dynamic contractile parameters suggest that directional changes in E(R) in TnT(A30V) + α-MHC and TnT(A30V) + β-MHC fibers result from a divergent impact on cross bridge-regulatory unit (troponin-tropomyosin complex) cooperativity. TnT(A30V)-mediated effects on Ca(2+)-activated maximal tension and instantaneous muscle fiber stiffness (E(D)) were also divergently affected by α- and β-MHC. Our study demonstrates that TnT(A30V) + α-MHC and TnT(A30V) + β-MHC fibers show contrasting contractile phenotypes; however, only the observations from β-MHC fibers are consistent with the clinical data for A28V in humans. NEW & NOTEWORTHY: The differential impact of α- and β-myosin heavy chain (MHC) on contractile dynamics causes a mutant cardiac troponin T (TnT(A30V)) to differently modulate cardiac contractile function. TnT(A30V) attenuated Ca(2+)-activated maximal tension and length-mediated cross-bridge recruitment against α-MHC but augmented these parameters against β-MHC, suggesting divergent contractile phenotypes.