Cardiac muscle thin filament structures reveal calcium regulatory mechanism

Yamada, Yasuhiro; Namba, Keiichi; Fujii, Takashi

doi:10.1038/s41467-019-14008-1

Cited by 175 publications

(384 citation statements)

References 40 publications

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“…This provides evidence at the molecular level that a majority of thin filaments in these sarcomeres are strongly bound to thick filaments, allowing their relative sliding and, thus, sarcomere contraction. Moreover, the Ca 2+ bound state of the cardiac thin filament, which allows myosin head access but no strong binding (18), fitted best to the separate structure obtained from the data shown in Fig. 3D (fig.…”

Section: B-d Left)mentioning

confidence: 62%

“…10into the subtomogram average (B) reveals F-actin polarity at the level of the actin monomers (C). (D) Docking of pseudoatomic models in different states (10,18) into the map reveals that the thin filament is in the myosin state (see also fig. S9E).…”

Section: B-d Left)mentioning

confidence: 99%

“…4C and movie S5). Comparison with pseudoatomic models in different functional states (10,18) shows that the thin filament is in the fully activated ("myosin") state that allows strong binding of the myosin heads ( Fig. 4D and fig.…”

Section: B-d Left)mentioning

confidence: 99%

“…Analysis of the distances and in-plane rotation between refined subvolumes belonging to the same filament permitted to determine the mean rise and angular twist of the F-actin structure. Several pseudoatomic models containing F-actin in complex with Tpm in distinct functional states (10,18) were docked into the maps using rigid body-fitting in UCSF Chimera. The fit was limited to the actin region to reveal the functional state matching the Tpm density best.…”

Section: Subtomogram Averaging and Polarity Assessmentmentioning

confidence: 99%

“…S1A) (1,11,12,15,16). At the molecular level, Ca 2+ -binding to Tpn is thought to trigger a shift in the azimuthal position of Tpm on F-actin to uncover the myosin binding site which allows actomyosin interactions (17,18). Yet, none of the approaches used so far has permitted molecular-scale imaging of sarcomere organization inside unperturbed cellular environments.…”

Section: Introductionmentioning

confidence: 99%

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Molecular-scale visualization of sarcomere contraction within native cardiomyocytes

Burbaum

Schneider

Scholze

et al. 2020

Preprint

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Sarcomeres, the basic contractile units of striated muscle, produce the forces driving muscular contraction through cross-bridge interactions between actin-containing thin filaments and myosin II-based thick filaments. Until now, direct visualization of the molecular architecture underlying sarcomere contractility has remained elusive. Here, we use in situ cryo-electron to-mography to unveil sarcomere contraction in frozen-hydrated neonatal rat cardiomyocytes. We show that the hexagonal lattice of the thick filaments is already established at the neonatal stage, with an excess of thin filaments outside the trigonal positions. Structural assessment of actin polarity by subtomogram averaging reveals that thin filaments in the fully activated state form overlapping arrays of opposite polarity in the center of the sarcomere. Our approach provides direct evidence for thin filament sliding during muscle contraction and may serve as a basis for structural understanding of thin filament activation and actomyosin interactions inside unperturbed cellular environments.

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Section: B-d Left)mentioning

confidence: 62%

Section: B-d Left)mentioning

confidence: 99%

Section: B-d Left)mentioning

confidence: 99%

Section: Subtomogram Averaging and Polarity Assessmentmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Molecular-scale visualization of sarcomere contraction within native cardiomyocytes

Burbaum

Schneider

Scholze

et al. 2020

Preprint

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Cardiac troponin and tropomyosin bind to F‐actin cooperatively, as revealed by fluorescence microscopy

Solı́s

Robinson

2020

FEBS Open Bio

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In cardiac muscle, binding of troponin (Tn) and tropomyosin (Tpm) to filamentous (F)‐actin forms thin filaments capable of Ca2+‐dependent regulation of contraction. Tpm binds to F‐actin in a head‐to‐tail fashion, while Tn stabilizes these linkages. Valuable structural and functional information has come from biochemical, X‐ray, and electron microscopy data. However, the use of fluorescence microscopy to study thin filament assembly remains relatively underdeveloped. Here, triple fluorescent labeling of Tn, Tpm, and F‐actin allowed us to track thin filament assembly by fluorescence microscopy. It is shown here that Tn and Tpm molecules self‐organize on actin filaments and give rise to decorated and undecorated regions. Binding curves based on colocalization of Tn and Tpm on F‐actin exhibit cooperative binding with a dissociation constant Kd of ~ 0.5 µm that is independent of the Ca2+ concentration. Binding isotherms based on the intensity profile of fluorescently labeled Tn and Tpm on F‐actin show that binding of Tn is less cooperative relative to Tpm. Computational modeling of Tn‐Tpm binding to F‐actin suggests two equilibrium steps involving the binding of an initial Tn‐Tpm unit (nucleation) and subsequent recruitment of adjacent Tn‐Tpm units (elongation) that stabilize the assembly. The results presented here highlight the utility of employing fluorescence microscopy to study supramolecular protein assemblies.

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Molecular dynamics provides new insights into the mechanism of calcium signal transduction and interdomain interactions in cardiac troponin

Genchev

Kobayashi

et al. 2021

FEBS Open Bio

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Understanding the regulation of cardiac muscle contraction at a molecular level is crucial for the development of therapeutics for heart conditions. Despite the availability of atomic structures of the protein components of cardiac muscle thin filaments, detailed insights into their dynamics and response to calcium are yet to be fully depicted. In this study, we used molecular dynamic simulations of the core domains of the cardiac muscle protein troponin to characterize the equilibrium dynamics of cardiac troponin's calcium-bound and calcium-free forms, with a focus on elements of cardiac muscle contraction activation and deactivation; that is, calcium binding to the calcium sensor troponin C domain (TnC) and the release of the switch region of the troponin inhibitory domain I (TnI) from TnC. The process of calcium binding to the TnC binding site is described as a three-step process commencing with calcium capture by the binding site residues, followed by cooperative residue interplay bringing the calcium ion to the binding site, and finally, calcium-water exchange. Furthermore, we uncovered a set of TnC-TnI inter-domain interactions, critical for TnC N-lobe hydrophobic pocket dynamics. Absence of these interactions allows the closure of the TnC N-lobe hydrophobic pocket while the TnI switch region remains expelled, whereas if the interactions are maintained the hydrophobic pocket remains open. Modification of these interactions may fine-tune the ability of the TnC N-lobe hydrophobic pocket to close or remain open, modulate cardiac contractility, and present potential therapy-relevant targets.

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Cardiac muscle thin filament structures reveal calcium regulatory mechanism

Cited by 175 publications

References 40 publications

Molecular-scale visualization of sarcomere contraction within native cardiomyocytes

Molecular-scale visualization of sarcomere contraction within native cardiomyocytes

Cardiac troponin and tropomyosin bind to F‐actin cooperatively, as revealed by fluorescence microscopy

Molecular dynamics provides new insights into the mechanism of calcium signal transduction and interdomain interactions in cardiac troponin

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