Mechanistic insights into effects of the cardiac myosin activator omecamtiv mecarbil from mechanokinetic modelling

Figures

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  FIGURE 1. Key transitions and model states. (A). Schematic illustration of myosin head states including their coarse-grained structure in interaction with actin. The model states are encoded by letters where A and M denote actin and myosin respectively and T, D and P denote substrate (ATP) and products (ADP and Pi), respectively (see further text). The subscripts PP and PiR denote a pre-power-stroke state, and a Pi-release state respectively as defined previously (Llinas et al., 2015; Rahman et al., 2018). The subscripts L and H are defined in the text. Upper case and lower-case letters for parameters, refer to equilibrium constants and rate constants, respectively. The argument x indicates strain dependence. (B) Free energies of the states defined in A as function of the position coordinate, x where x = 0 nm is defined as the distance between the myosin origin on the thick filament backbone and the actin site where force = 0 pN in the AMDPPP state. Parameters x1, x2 and x3 are indicated.

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  FIGURE 2. Force-velocity and power stroke data simulated for human cardiac muscle at 25°C using model in Figure 1 and parameter values in Tables 1, 2. (A) Force-velocity data simulated using parameter values in the third column of Tables 1, 2 (open black symbols; 1a in Supplementary Table S1) or optimized parameter values in 2nd column of Tables 1, 2 (filled black symbols, 5 in Supplementary Table S1). Simulations compared to experimental data of Tang et al. (Tang et al., 2016) (top) and Blair et al. (2020) (bottom). (B) Displacements during power-strokes simulated using parameter values in 3rd column of Tables 1, 2 (“1a”; see further Supplementary Table S1) or optimized parameter values in 2nd column of Tables 1, 2 (“5”, see further Supplementary Table S1) assuming stiff conditions (e.g., stiff optical trap; see Materials and Methods).

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  FIGURE 3. Simulation of effects on the force-velocity relationship, the power-stroke and the actin-activated ATPase of combined reductions in ΔGLH and kLH- without changes in other parameter values. (A) Force-velocity relationship. (B) Force-velocity relationship as in A but with a logarithmic scale for the velocity axis to more clearly visualize the low-velocity range. (C) Number of attached myosin heads in isometric contraction (filled symbols, left vertical axis) normalized to the control value and actin activated ATPase (open symbols, right vertical axis) vs. ΔGLH for a constant value of kLH- = 0.1 s−1. (D) Effects on displacement associated with power-stroke of reduction in ΔGLH from control value of 11 kBT down to 0 kBT and as indicated by numbers in figure with kLH- = 0.1 s−1. Stiff optical trap (>>3 pN/nm) assumed. (E) Effects on displacement associated with power-stroke of reduction in ΔGLH from control value of 11 kBT down to 0 kBT as indicated by numbers in figure with kLH- = 0.1 s−1. Soft optical trap (0.07 pN/nm) assumed. (F) Free energy diagrams from Figure 1 with the effects of OM in the simulations indicated by dashed lines.

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  FIGURE 4. Model with reduced value of ΔGPiR compared to Figure 3. (A) Simulation (black) of control force-velocity data assuming ΔGPiR = 1 kBT instead of ΔGPiR = 5 kBT. The parameter values used are otherwise given in the second column of Tables 1, 2. Simulations by solution of differential equations in state probabilities (filled circles) compared to the same experimental data (Tang et al., 2016; Blair et al., 2020) as reproduced in Figure 2. (B) Simulation of OM effects on the force-velocity relationship compared to control data (black) reproduced from panel (A). In all simulations of the OM effects (red, blue, orange) the following changes from the second column in Tables 1, 2 are assumed: ΔGLH = 1 kBT, kLH- = 0.1 s−1, Kc = 100 mM, ΔGon = 4 kBT. For red data, ΔGPiR = 1 kBT. For blue and orange data ΔGPiR = 5.5 kBT. In addition, ε = 1 nm for orange data but ε = 0 nm for all other simulations. (C) Simulation of OM effects on the displacement associated with the power-stroke under conditions similar to those of orange and blue data points in B with control data simulated on the assumption that ΔGPiR = 1 kBT and all other parameter values as in Tables 1, 2 (2nd column). Simulation of…

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  FIGURE 5. Model allowing either lever-arm swing or 50 kD cleft closure to occur first in myosin force-generation. (A) Schematic (tentative) activation energy diagram for lever arm swing and 50 kD cleft-closure in the absence of load. The diagram in the presence (red and blue) and absence (blue) of OM assumes that OM primarily increases the activation energy for the lever arm swing. (B) Likely path between states 1–4 in the model in A under conditions of high and low load in the absence and presence of OM. This means that a substantial reduction of kLH- (corresponding to increased activation energy for the lever arm swing) would make the path 1 to 3 to dominate in the presence of OM even in the absence of load, corresponding to a lower average free energy in the pre-power-stroke state (increased Kc-value) together with reduced value of ΔGLH.

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  FIGURE 6. Molecular mechanisms of action of OM. All mechanisms considered above are numbered from 1 to 6 starting in the upper left corner. OM binding occurs to the MDP state and is assumed to leave its binding pocket upon completion of the power-stroke or after detachment via an “escape” route. The OM mechanisms are: 1. Shift of the hydrolysis equilibrium, 2. Increased attachment rate, 3. Increased affinity in the Pi-release state (AMDPPiR), 4. Overpriming of the lever arm in a pre-power-stroke, 5. Reduced Pi-affinity, 6 (major mechanism): almost complete inhibition of power-stroke by reduction of kLH- and ΔGLH in the model. The escape route is likely to exist also in the absence of OM but would not be relevant when the power-stroke is very fast.