Rat cardiac troponin T mutation (F72L)-mediated impact on thin filament cooperativity is divergently modulated by α- and β-myosin heavy chain isoforms

Abstract

The primary causal link between disparate effects of human hypertrophic cardiomyopathy (HCM)-related mutations in troponin T (TnT) and α- and β-myosin heavy chain (MHC) isoforms on cardiac contractile phenotype remains poorly understood. Given the divergent impact of α- and β-MHC on the NH2-terminal extension (44-73 residues) of TnT, we tested if the effects of the HCM-linked mutation (TnTF70L) were differentially altered by α- and β-MHC. We hypothesized that the emergence of divergent thin filament cooperativity would lead to contrasting effects of TnTF70L on contractile function in the presence of α- and β-MHC. The rat TnT analog of the human F70L mutation (TnTF72L) or the wild-type rat TnT (TnTWT) was reconstituted into demembranated muscle fibers from normal (α-MHC) and propylthiouracil-treated (β-MHC) rat hearts to measure steady-state and dynamic contractile function. TnTF72L-mediated effects on tension, myofilament Ca(2+) sensitivity, myofilament cooperativity, rate constants of cross-bridge (XB) recruitment dynamics, and force redevelopment were divergently modulated by α- and β-MHC. TnTF72L increased the rate of XB distortion dynamics by 49% in α-MHC fibers but had no effect in β-MHC fibers; these observations suggest that TnTF72L augmented XB detachment kinetics in α-MHC, but not β-MHC, fibers. TnTF72L increased the negative impact of strained XBs on the force-bearing XBs by 39% in α-MHC fibers but had no effect in β-MHC fibers. Therefore, TnTF72L leads to contractile changes that are linked to dilated cardiomyopathy in the presence of α-MHC. On the other hand, TnTF72L leads to contractile changes that are linked to HCM in the presence of β-MHC.

The effect of cardiomyopathy mutation (R97L) in mouse cardiac troponin T on the muscle length-mediated recruitment of crossbridges is modified divergently by α- and β-myosin heavy chain

“…Post-hoc t -tests confirmed that TnT R97L had no effect on E D in either α-MHC ( P =0.44; Table 1) or β-MHC fibers ( P =0.54; Table 1). We have previously demonstrated that E D is strongly correlated to maximal tension [13, 25, 32, 36]. Similar observations in E D and maximal tension (Table 1) substantiate our conclusion that TnT R97L does not affect maximal activation regardless of the MHC isoform.…”

“…The composition of HR solution is listed in our previous works [11, 13]. Left ventricular (LV) papillary muscle bundles were extracted and further dissected into smaller muscle bundles (~0.15 mm in cross-section and 2.0 mm in length) and chemically-skinned overnight at 4°C in HR solution containing 1% Triton X-100.…”

“…The muscle fibers were then exposed to various Ca 2+ solutions with ranging from 9.0 to 4.3 in a constantly-stirred chamber and the corresponding steady-state force values were measured. The compositions of various pCa solutions were based on the program by Fabiato and Fabiato [31], and these were listed in our earlier works [11, 13]. The pH and ionic strength of pCa solutions were adjusted to 7.0 and 180 mM, respectively.…”

“…A nonlinear recruitment-distortion (NRD) model was fitted to family of force responses to estimate five NRD model parameters: the magnitude of instantaneous change in stiffness caused by a sudden ML change ( E D ); the rate by which the sudden ML-induced strain within force-bearing XBs dissipates to a steady-state level ( c ); the parameter that characterizes the XB strain-mediated negative impact on other force-bearing XBs (γ); the rate by which new XBs are recruited into the force-bearing state due to a change in ML ( b ); and the magnitude of change in the steady-state stiffness caused by the ML-mediated change in the number of newly-recruited force-bearing XBs ( E R ). More details on step-like ML perturbation protocol and the significance of NRD model parameters are in our previously published articles [13, 28, 32]. …”

“…Previous studies have demonstrated that the CR of TnT tunes both the magnitude and dynamics of myofilament activation by modulating the Tm-dependent functions, as well as the crossbridge (XB)-mediated effects on the thin filament [7–9]. Because there is significant interplay between the TnT- and XB-mediated effects on the thin filament [10–13], the expression of different types of cardiac MHC isoforms in different animal models adds another level of complexity to our understanding of HCM in humans. Different forms of XBs — as in α- and β-MHC — are likely to have different effects on contractile dynamics because α- and β-MHC have been shown to divergently modify the ML-mediated activation of cardiac myofilaments [14–16].…”