Cardiomyopathy-related mutation (A30V) in mouse cardiac troponin T divergently alters the magnitude of stretch activation in α- and β-myosin heavy chain fibers

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 NH₂ 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 NH₂ 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 Ca²⁺-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 Ca²⁺-activated maximal tension and length-mediated cross-bridge recruitment against α-MHC but augmented these parameters against β-MHC, suggesting divergent contractile phenotypes.