A hallmark feature of myosin-II is that it can spontaneously self-assemble into bipolar synthetic thick filaments (STFs) in low ionic strength buffers, thereby serving as a reconstituted in-vitro model for muscle thick filament. While these STFs have been extensively used for structural characterization, their functional evaluation has been limited.  In this report, we show that myosins in STFs mirror the more electrostatic and cooperative interactions that underlie the energy-sparing super-relaxed (SRX) state, which are not seen using shorter myosin sub-fragments, heavy meromyosin (HMM) and myosin subfragment-1 (S1). Using these STFs, we show several pathophysiological insults in hypertrophic cardiomyopathy, including the R403Q myosin mutation, phosphorylation of myosin light chains, and increased ADP:ATP ratio destabilize the SRX population. Furthermore, wild-type myosin containing STFs, but not S1, HMM, or STFs-containing R403Q myosin, recapitulated the ADP-induced destabilization of the SRX state. Studies involving a clinical-stage small molecule inhibitor, mavacamten, showed that it is not only more effective in increasing myosin SRX population in STFs than in S1 or HMM ,  but it also increases myosin SRX population equally well in STFs made of healthy and disease-causing R403Q myosin. Importantly, we also found that pathophysiological perturbations   such as elevated ADP concentration weakens the mavacamten’s ability to increase the myosin SRX population, suggesting that mavacamten-bound myosin heads are not permanently protected in the SRX state but can be recruited   into action. These findings collectively emphasize that STFs serve as a valuable tool to provide novel insights into the myosin SRX state in healthy, disease, and therapeutic conditions.

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Cardiac troponin T (cTnT) is a key component of contractile regulatory proteins. cTnT is characterized by a ∼32 amino acid N-terminal extension (NTE), the function of which remains unknown. To understand its function, we generated a transgenic (TG) mouse line that expressed a recombinant chimeric cTnT in which the NTE of mouse cTnT was removed by replacing its 1-73 residues with the corresponding 1-41 residues of mouse fast skeletal TnT. Detergent-skinned papillary muscle fibers from non-TG (NTG) and TG mouse hearts were used to measure tension, ATPase activity, Ca(2+) sensitivity (pCa(50)) of tension, rate of tension redevelopment, dynamic muscle fiber stiffness, and maximal fiber shortening velocity at sarcomere lengths (SLs) of 1.9 and 2.3 μm. Ca(2+) sensitivity increased significantly in TG fibers at both short SL (pCa(50) of 5.96 vs. 5.62 in NTG fibers) and long SL (pCa(50) of 6.10 vs. 5.76 in NTG fibers). Maximal cross-bridge turnover and detachment kinetics were unaltered in TG fibers. Our data suggest that the NTE constrains cardiac thin filament activation such that the transition of the thin filament from the blocked to the closed state becomes less responsive to Ca(2+). Our finding has implications regarding the effect of tissue- and disease-related changes in cTnT isoforms on cardiac muscle function.

Given the differential impact of α- and β-myosin heavy chain (MHC) isoforms on how troponin T (TnT) modulates contractile dynamics, we hypothesized that the effects of dilated cardiomyopathy (DCM) mutations in TnT would be altered differently by α- and β-MHC. We characterized dynamic contractile features of normal (α-MHC) and transgenic (β-MHC) mouse cardiac muscle fibers reconstituted with a mouse TnT analog (TnTR144W) of the human DCM R141W mutation. TnTR144W did not alter maximal tension but attenuated myofilament Ca(2+) sensitivity (pCa50) to a similar extent in α- and β-MHC fibers. TnTR144W attenuated the speed of cross-bridge (XB) distortion dynamics (c) by 24% and the speed of XB recruitment dynamics (b) by 17% in α-MHC fibers; however, both b and c remained unaltered in β-MHC fibers. Likewise, TnTR144W attenuated the rates of XB detachment (g) and tension redevelopment (ktr) only in α-MHC fibers. TnTR144W also decreased the impact of strained XBs on the recruitment of new XBs (γ) by 30% only in α-MHC fibers. Because c, b, g, ktr, and γ are strongly influenced by thin filament-based cooperative mechanisms, we conclude that the TnTR144W- and β-MHC-mediated changes in the thin filament interact to produce a less severe functional phenotype, compared with that brought about by TnTR144W and α-MHC. These observations provide a basis for lower mortality rates of humans (β-MHC) harboring the TnTR141W mutant compared with transgenic mouse studies. Our findings strongly suggest that some caution is necessary when extrapolating data from transgenic mouse studies to human hearts.

Omecamtiv mecarbil (OM) is a pharmacological agent that augments cardiac contractile function by enhancing myofilament Ca sensitivity. Given that interventions that increase myofilament Ca sensitivity have the potential to alter length-dependent activation (LDA) of cardiac myofilaments, we tested the influence of OM on this fundamental property of the heart. This is significant not only because LDA is prominent in cardiac muscle but also because it contributes to the Frank-Starling law, a mechanism by which the heart increases stroke volume in response to an increase in venous return. We measured steady-state and dynamic contractile indices in detergent-skinned guinea pig (Cavia porcellus) cardiac muscle fibers in the absence and presence of 0.3 and 3.0 μM OM at two different sarcomere lengths (SLs), short SL (1.9 μm) and long SL (2.3 μm). Myofilament Ca sensitivity, as measured by pCa (-log of [Ca] concentration required for half-maximal activation), increased significantly at both short and long SLs in OM-treated fibers when compared to untreated fibers; however, the magnitude of increase in pCa was twofold greater at short SL than at long SL. A consequence of this greater increase in pCa at short SL was that pCa did not increase any further at long SL, suggesting that OM abolished the SL dependency of pCa. Furthermore, the SL dependency of rate constants of cross-bridge distortion dynamics (c) and force redevelopment (k) was abolished in 0.3-μM-OM-treated fibers. The negative impact of OM on the SL dependency of pCa, c, and k was also observed in 3.0-μM-OM-treated fibers, indicating that cooperative mechanisms linked to LDA were altered by the OM-mediated effects on cardiac myofilaments.