An integrated picture of the structural pathways controlling the heart performance

Figures

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  Fig. 1.. Overview of the sarcomere with its protein content. (A) Schematic representation of two consecutive sarcomeres at full overlap (~2.2 μm sarcomere length). Yellow, thin filament; orange, array of myosin motors; light blue, thick filament. (B) Protein disposition on the thin (yellow) and thick (light blue) filaments in the half-sarcomere at rest at 2.2 μm SL. M-line on the Right, Z-line on the Left. Motors (49 crowns) on the thick filament numbered starting from the M-line; thin filament with tropomyosin and troponin complex; MyBP-C (green) from crown 7 to 30. Titin (magenta) with, in the I-band, PEVK segment (dark magenta) and, in the A-band, the segments characterized by C-type (from crown 4 to 36) and D-type (from crown 37 to 49) superrepeats. HBZ, half-bare zone. P-, C-, and D-zones: proximal, MyBP-C-containing and distal zones. Only two of the six titin molecules and only one of the three series of MyBP-C molecules per half-thick filament are represented for clarity. (C) Enlarged scale of overlap region showing: i) on the thin filament, the double-stranded helix formed by actin monomers, the regulatory proteins tropomyosin (red) and the troponin complex (TnC, brown, TnT, gray,…

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  Fig. 2.. X-ray signals showing the effects of cooling on the structure of thick filament at rest. (A) Superimposed intensity distribution in the reciprocal space (R) of the ML1 layer line at different temperatures (red, 35 °C; orange, 27 °C; green, 20 °C; blue, 15 °C; violet, 10 °C). (B and C) Superimposed intensity distributions of the meridional reflections (same color code as in A). M, myosin and T, troponin series reflections. (D) IML1, relative to the value at 35 °C. (E) Enlargement of the M3 profiles in (B) (same color code as in A). L, M, and H low, medium, and high angle peaks. (F) IM3. (G and H) SM3 (G) and SM6 (H) as percentage change relative to the value at 35 °C. (I) IDH. Gray line linear fit on the data for guiding the eye. In (A–C and E) each trace is the sum of two exposures 10 ms each. a.u., arbitrary units. In (D and F) the intensities are normalized for their value at 35 °C. Mean ± SEM, N = 3-7, three preparations. (J) Fraction of motors in the OFF conformation (fOFF) determined from the square root of either IM3 (filled circles, from F) or IML1 (open circles from D). Values relative to fOFF at 35 °C. Mean ± SEM. Gray dashed line, simulated relation from K. (K) Model…

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  Fig. 3.. Protocols used to change Tp and the dependence on Tp of the relevant structural parameters marking the regulatory state of the thick filament. (A) Tp changes following the start of pacing at 0.5 Hz, SL ~2.1 μm. The arrows identify the twitches shown superimposed in the Lower panel (violet ST, orange PT). (B) Tp at different [Ca2+]o as indicated by the numbers (mM) above the bars, SL ~2.1 μm (violet, ST; orange, PT). (C) Force-sarcomere length relation at the peak of the twitch in 1 mM [Ca2+]o (violet, ST; orange, PT). Averaged data pooled from twitches in either FE or LC conditions, range of SL before stimulus 1.9 to 2.3 µm. N range 6 to 12 (PT), 6 to 51 (ST). (D) Superimposed force traces for single twitches in FE (black, 2.0 μm SL at Tp), and LC (gray, 2.15 μm SL at Tp). SL before stimulus, 2.19 μm; [Ca2+]o = 1 mM. (E–H) Superimposed intensity axial distribution of the ML1 (E), M3 (F), M5 (G), and M6 (H) at rest (blue) and at Tp 23 kPa (green) and 64 kPa (magenta). (I–P) Plots of the relevant structural parameters versus Tp, resting value on the ordinate axis. (I) IML1. (J) IM5. (K) Superimposed IML1 (filled circles) and IM5 (open circles) changes relative to their resting…

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  Fig. 4.. The source of the interference fine structure of the M3 reflection at different Tp. (A) Axial intensity distribution of the M3 reflection at rest and at different Tp as indicated by the color code (the same color code is used in panels B–D). Vertical lines mark the position of the three component peaks. (B) Superimposed intensity distributions from A, after normalization for the intensity of the highest peak (M for rest and Tp ≤ 41 kPa, L for Tp > 41 kPa). (C) ID versus Tp. (D) Pictorial representation (colored arrows) of the ID of the two arrays of diffractors at rest and at different Tp in relation to the location of the two arrays on the thick filament.

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  Fig. 5.. Model 1 simulation of motor fractional occupancy of the OFF, ON, and attached states. (A) Schematic representation of the probability of motors in each layer along the half-thick filament to be in the OFF (cyan), ON (gray), or attached (red) state at 27 °C. From Top to Bottom rest, 13 kPa, 41 kPa, and 89 kPa. Motor layers numbered starting from the center of the thick filament. The fraction of attached motors is defined according to the assumptions of model 1 (see text). The three fractional occupancies in each layer are represented through the length of the corresponding color in the bar. (B) Model 1 prediction of the Tp dependence of the fraction of motors in the OFF state relative to the fraction at rest at 27 °C (fOFF/fOFF,Rest) within the C-zone (filled diamonds) and outside the C-zone (open diamonds). (C) Superimposed IML1–Tp relations: orange, observed (filled circles from Fig. 3K); cyan, predicted by Model 1.

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  Fig. 6.. Interactions among titin, MyBP-C, and myofilaments and their function in the regulation of the cardiac output. Rise in internal [Ca2+] triggers activation of the thin filament (yellow) and, through a still undetermined process, activation of titin (magenta) that switches ON the thick filament (cyan) and then the N terminus of MyBP-C (violet) accounting for initial availability of actin for motor attachments from the C-zone. The ensuing force development and titin-mediated mechanosensing complete the thick filament activation. At subsaturating level of internal [Ca2+] near-neighbor cooperative activation of thin filament by attached motors allows further attachments from the more peripherical regions of the half thick filament.