1
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Barry ME, Rynkiewicz MJ, Pavadai E, Viana A, Lehman W, Moore JR. Glutamate 139 of tropomyosin is critical for cardiac thin filament blocked-state stabilization. J Mol Cell Cardiol 2024; 188:30-37. [PMID: 38266978 DOI: 10.1016/j.yjmcc.2024.01.004] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/16/2023] [Revised: 12/14/2023] [Accepted: 01/20/2024] [Indexed: 01/26/2024]
Abstract
The cardiac thin filament proteins troponin and tropomyosin control actomyosin formation and thus cardiac contractility. Calcium binding to troponin changes tropomyosin position along the thin filament, allowing myosin head binding to actin required for heart muscle contraction. The thin filament regulatory proteins are hot spots for genetic mutations causing heart muscle dysfunction. While much of the thin filament structure has been characterized, critical regions of troponin and tropomyosin involved in triggering conformational changes remain unresolved. A poorly resolved region, helix-4 (H4) of troponin I, is thought to stabilize tropomyosin in a position on actin that blocks actomyosin interactions at low calcium concentrations during muscle relaxation. We have proposed that contact between glutamate 139 on tropomyosin and positively charged residues on H4 leads to blocking-state stabilization. In this study, we attempted to disrupt these interactions by replacing E139 with lysine (E139K) to define the importance of this residue in thin filament regulation. Comparison of mutant and wild-type tropomyosin was carried out using in-vitro motility assays, actin co-sedimentation, and molecular dynamics simulations to determine perturbations in troponin-tropomyosin function caused by the tropomyosin mutation. Motility assays revealed that mutant thin filaments moved at higher velocity at low calcium with increased calcium sensitivity demonstrating that tropomyosin residue 139 is vital for proper tropomyosin-mediated inhibition during relaxation. Similarly, molecular dynamic simulations revealed a mutation-induced decrease in interaction energy between tropomyosin-E139K and troponin I (R170 and K174). These results suggest that salt-bridge stabilization of tropomyosin position by troponin IH4 is essential to prevent actomyosin interactions during cardiac muscle relaxation.
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Affiliation(s)
- Meaghan E Barry
- Department of Biological Sciences, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, United States of America
| | - Michael J Rynkiewicz
- Department of Pharmacology, Physiology & Biophysics, Boston University Chobanian and Avedisan School of Medicine, 700 Albany Street, W-408E, Boston, MA 02118, United States of America
| | - Elumalai Pavadai
- Department of Pharmacology, Physiology & Biophysics, Boston University Chobanian and Avedisan School of Medicine, 700 Albany Street, W-408E, Boston, MA 02118, United States of America
| | - Alex Viana
- Department of Biological Sciences, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, United States of America
| | - William Lehman
- Department of Pharmacology, Physiology & Biophysics, Boston University Chobanian and Avedisan School of Medicine, 700 Albany Street, W-408E, Boston, MA 02118, United States of America
| | - Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, United States of America.
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2
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Nabiev SR, Kochurova AM, Nikitina LV, Beldiia EA, Matyushenko AM, Yampolskaya DS, Bershitsky SY, Kopylova GV, Shchepkin DV. N-Terminal Fragment of Cardiac Myosin Binding Protein-C Increases the Duration of Actin-Myosin Interaction. Bull Exp Biol Med 2024; 176:324-327. [PMID: 38336971 DOI: 10.1007/s10517-024-06017-y] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2023] [Indexed: 02/12/2024]
Abstract
Cardiac myosin binding protein-C (cMyBP-C) located in the C-zone of myocyte sarcomere is involved in the regulation of myocardial contraction. Its N-terminal domains C0, C1, C2, and the m-motif between C1 and C2 can bind to the myosin head and actin of the thin filament and affect the characteristics of their interaction. Measurements using an optical trap showed that the C0-C2 fragment of cMyBP-C increases the interaction time of cardiac myosin with the actin filament, while in an in vitro motility assay, it dose-dependently reduces the sliding velocity of actin filaments. Thus, it was found that the N-terminal part of cMyBP-C affects the kinetics of the myosin cross-bridge.
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Affiliation(s)
- S R Nabiev
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Yekaterinburg, Russia.
| | - A M Kochurova
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Yekaterinburg, Russia
| | - L V Nikitina
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Yekaterinburg, Russia
| | - E A Beldiia
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Yekaterinburg, Russia
| | - A M Matyushenko
- A. N. Bakh Institute of Biochemistry, Federal Research Center Fundamentals of Biotechnology, Russian Academy of Sciences, Moscow, Russia
| | - D S Yampolskaya
- A. N. Bakh Institute of Biochemistry, Federal Research Center Fundamentals of Biotechnology, Russian Academy of Sciences, Moscow, Russia
| | - S Yu Bershitsky
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Yekaterinburg, Russia
| | - G V Kopylova
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Yekaterinburg, Russia
| | - D V Shchepkin
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Yekaterinburg, Russia
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3
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Liu C, Ruppel KM, Spudich JA. Motility Assay to Probe the Calcium Sensitivity of Myosin and Regulated Thin Filaments. Methods Mol Biol 2024; 2735:169-189. [PMID: 38038849 PMCID: PMC10773985 DOI: 10.1007/978-1-0716-3527-8_10] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2023]
Abstract
Calcium-dependent activation of the thin filament mediated by the troponin-tropomyosin complex is key in the regulation of actin-myosin based muscle contraction. Perturbations to this system, either physiological (e.g., phosphorylation of myosin light chains) or pathological (e.g., mutations that cause familial cardiomyopathies), can alter calcium sensitivity and thus have important implications in human health and disease. The in vitro motility assay provides a quantitative and precise method to study the calcium sensitivity of the reconstituted myosin-thin filament motile system. Here we present a simple and robust protocol to perform calcium-dependent motility of β-cardiac myosin and regulated thin filaments. The experiment is done on a multichannel microfluidic slide requiring minimal amounts of proteins. A complete velocity vs. calcium concentration curve is produced from one experiment in under 1 h.
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Affiliation(s)
- Chao Liu
- Department of Biochemistry, Beckman Center B405, Stanford University School of Medicine, Stanford, CA, USA
- Cardiovascular Institute, Stanford University, Stanford, CA, USA
- Biosciences and Biotechnology Division, Lawrence Livermore National Laboratory, Livermore, CA, USA
| | - Kathleen M Ruppel
- Department of Biochemistry, Beckman Center B405, Stanford University School of Medicine, Stanford, CA, USA.
- Cardiovascular Institute, Stanford University, Stanford, CA, USA.
- Department of Pediatrics (Cardiology), Stanford University, Stanford, CA, USA.
| | - James A Spudich
- Department of Biochemistry, Beckman Center B405, Stanford University School of Medicine, Stanford, CA, USA.
- Cardiovascular Institute, Stanford University, Stanford, CA, USA.
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4
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Clippinger Schulte SR, Scott B, Barrick SK, Stump WT, Blackwell T, Greenberg MJ. Single-molecule mechanics and kinetics of cardiac myosin interacting with regulated thin filaments. Biophys J 2023; 122:2544-2555. [PMID: 37165621 PMCID: PMC10323011 DOI: 10.1016/j.bpj.2023.05.008] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2023] [Revised: 04/18/2023] [Accepted: 05/05/2023] [Indexed: 05/12/2023] Open
Abstract
The cardiac cycle is a tightly regulated process wherein the heart generates force to pump blood to the body during systole and then relaxes during diastole. Disruption of this finely tuned cycle can lead to a range of diseases including cardiomyopathies and heart failure. Cardiac contraction is driven by the molecular motor myosin, which pulls regulated thin filaments in a calcium-dependent manner. In some muscle and nonmuscle myosins, regulatory proteins on actin tune the kinetics, mechanics, and load dependence of the myosin working stroke; however, it is not well understood whether or how thin-filament regulatory proteins tune the mechanics of the cardiac myosin motor. To address this critical gap in knowledge, we used single-molecule techniques to measure the kinetics and mechanics of the substeps of the cardiac myosin working stroke in the presence and absence of thin filament regulatory proteins. We found that regulatory proteins gate the calcium-dependent interactions between myosin and the thin filament. At physiologically relevant ATP concentrations, cardiac myosin's mechanics and unloaded kinetics are not affected by thin-filament regulatory proteins. We also measured the load-dependent kinetics of cardiac myosin at physiologically relevant ATP concentrations using an isometric optical clamp, and we found that thin-filament regulatory proteins do not affect either the identity or magnitude of myosin's primary load-dependent transition. Interestingly, at low ATP concentrations at both saturating and physiologically relevant subsaturating calcium concentrations, thin-filament regulatory proteins have a small effect on actomyosin dissociation kinetics, suggesting a mechanism beyond simple steric blocking. These results have important implications for the modeling of cardiac physiology and diseases.
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Affiliation(s)
- Sarah R Clippinger Schulte
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri
| | - Brent Scott
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri
| | - Samantha K Barrick
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri
| | - W Tom Stump
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri
| | - Thomas Blackwell
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri
| | - Michael J Greenberg
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri.
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5
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Clippinger Schulte SR, Scott B, Barrick SK, Stump WT, Blackwell T, Greenberg MJ. Single Molecule Mechanics and Kinetics of Cardiac Myosin Interacting with Regulated Thin Filaments. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.01.09.522880. [PMID: 36711892 PMCID: PMC9881944 DOI: 10.1101/2023.01.09.522880] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
The cardiac cycle is a tightly regulated process wherein the heart generates force to pump blood to the body during systole and then relaxes during diastole. Disruption of this finely tuned cycle can lead to a range of diseases including cardiomyopathies and heart failure. Cardiac contraction is driven by the molecular motor myosin, which pulls regulated thin filaments in a calcium-dependent manner. In some muscle and non-muscle myosins, regulatory proteins on actin tune the kinetics, mechanics, and load dependence of the myosin working stroke; however, it is not well understood whether or how thin filament regulatory proteins tune the mechanics of the cardiac myosin motor. To address this critical gap in knowledge, we used single-molecule techniques to measure the kinetics and mechanics of the substeps of the cardiac myosin working stroke in the presence and absence of thin filament regulatory proteins. We found that regulatory proteins gate the calcium-dependent interactions between myosin and the thin filament. At physiologically relevant ATP concentrations, cardiac myosin's mechanics and unloaded kinetics are not affected by thin filament regulatory proteins. We also measured the load-dependent kinetics of cardiac myosin at physiologically relevant ATP concentrations using an isometric optical clamp, and we found that thin filament regulatory proteins do not affect either the identity or magnitude of myosin's primary load-dependent transition. Interestingly, at low ATP concentrations, thin filament regulatory proteins have a small effect on actomyosin dissociation kinetics, suggesting a mechanism beyond simple steric blocking. These results have important implications for both disease modeling and computational models of muscle contraction. Significance Statement Human heart contraction is powered by the molecular motor β-cardiac myosin, which pulls on thin filaments consisting of actin and the regulatory proteins troponin and tropomyosin. In some muscle and non-muscle systems, these regulatory proteins tune the kinetics, mechanics, and load dependence of the myosin working stroke. Despite having a central role in health and disease, it is not well understood whether the mechanics or kinetics of β-cardiac myosin are affected by regulatory proteins. We show that regulatory proteins do not affect the mechanics or load-dependent kinetics of the working stroke at physiologically relevant ATP concentrations; however, they can affect the kinetics at low ATP concentrations, suggesting a mechanism beyond simple steric blocking. This has important implications for modeling of cardiac physiology and diseases.
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6
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Al Azzam OY, Watts JC, Reynolds JE, Davis JE, Reinemann DN. Myosin II Adjusts Motility Properties and Regulates Force Production Based on Motor Environment. Cell Mol Bioeng 2022; 15:451-465. [PMID: 36444350 PMCID: PMC9700534 DOI: 10.1007/s12195-022-00731-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2022] [Accepted: 08/01/2022] [Indexed: 11/27/2022] Open
Abstract
Introduction Myosin II has been investigated with optical trapping, but single motor-filament assay arrangements are not reflective of the complex cellular environment. To understand how myosin interactions propagate up in scale to accomplish system force generation, we devised a novel actomyosin ensemble optical trapping assay that reflects the hierarchy and compliancy of a physiological environment and is modular for interrogating force effectors. Methods Hierarchical actomyosin bundles were formed in vitro. Fluorescent template and cargo actin filaments (AF) were assembled in a flow cell and bundled by myosin. Beads were added in the presence of ATP to bind the cargo AF and activate myosin force generation to be measured by optical tweezers. Results Three force profiles resulted across a range of myosin concentrations: high force with a ramp-plateau, moderate force with sawtooth movement, and baseline. The three force profiles, as well as high force output, were recovered even at low solution concentration, suggesting that myosins self-optimize within AFs. Individual myosin steps were detected in the ensemble traces, indicating motors are taking one step at a time while others remain engaged in order to sustain productive force generation. Conclusions Motor communication and system compliancy are significant contributors to force output. Environmental conditions, motors taking individual steps to sustain force, the ability to backslip, and non-linear concentration dependence of force indicate that the actomyosin system contains a force-feedback mechanism that senses the local cytoskeletal environment and communicates to the individual motors whether to be in a high or low duty ratio mode. Supplementary Information The online version contains supplementary material available at 10.1007/s12195-022-00731-1.
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Affiliation(s)
- Omayma Y. Al Azzam
- Department of Chemical Engineering, University of Mississippi, University, MS 38677 USA
| | - Janie C. Watts
- Department of Chemical Engineering, University of Mississippi, University, MS 38677 USA
| | - Justin E. Reynolds
- Department of Biomedical Engineering, University of Mississippi, University, MS 38677 USA
| | - Juliana E. Davis
- Department of Biomedical Engineering, University of Mississippi, University, MS 38677 USA
| | - Dana N. Reinemann
- Department of Chemical Engineering, University of Mississippi, University, MS 38677 USA
- Department of Biomedical Engineering, University of Mississippi, University, MS 38677 USA
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7
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Debold EP. Mini‐ review: Recent insights into the relative timing of myosin’s powerstroke and release of phosphate. Cytoskeleton (Hoboken) 2022; 78:448-458. [DOI: 10.1002/cm.21695] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2022] [Revised: 02/25/2022] [Accepted: 03/08/2022] [Indexed: 11/07/2022]
Affiliation(s)
- Edward P. Debold
- Department of Kinesiology University of Massachusetts Amherst Massachusetts
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8
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Smith QM, Inchingolo AV, Mihailescu MD, Dai H, Kad NM. Single-molecule imaging reveals the concerted release of myosin from regulated thin filaments. eLife 2021; 10:69184. [PMID: 34569933 PMCID: PMC8476120 DOI: 10.7554/elife.69184] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2021] [Accepted: 09/11/2021] [Indexed: 11/13/2022] Open
Abstract
Regulated thin filaments (RTFs) tightly control striated muscle contraction through calcium binding to troponin, which enables tropomyosin to expose myosin-binding sites on actin. Myosin binding holds tropomyosin in an open position, exposing more myosin-binding sites on actin, leading to cooperative activation. At lower calcium levels, troponin and tropomyosin turn off the thin filament; however, this is antagonised by the high local concentration of myosin, questioning how the thin filament relaxes. To provide molecular details of deactivation, we used single-molecule imaging of green fluorescent protein (GFP)-tagged myosin-S1 (S1-GFP) to follow the activation of RTF tightropes. In sub-maximal activation conditions, RTFs are not fully active, enabling direct observation of deactivation in real time. We observed that myosin binding occurs in a stochastic step-wise fashion; however, an unexpectedly large probability of multiple contemporaneous detachments is observed. This suggests that deactivation of the thin filament is a coordinated active process.
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Affiliation(s)
- Quentin M Smith
- School of Biosciences, University of Kent, Canterbury, United Kingdom
| | | | | | - Hongsheng Dai
- Department of Mathematical Sciences, University of Essex, Colchester, United Kingdom
| | - Neil M Kad
- School of Biosciences, University of Kent, Canterbury, United Kingdom
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9
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Stewart TJ, Murthy V, Dugan SP, Baker JE. Velocity of myosin-based actin sliding depends on attachment and detachment kinetics and reaches a maximum when myosin-binding sites on actin saturate. J Biol Chem 2021; 297:101178. [PMID: 34508779 PMCID: PMC8560993 DOI: 10.1016/j.jbc.2021.101178] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2021] [Revised: 09/02/2021] [Accepted: 09/07/2021] [Indexed: 11/22/2022] Open
Abstract
Molecular motors such as kinesin and myosin often work in groups to generate the directed movements and forces critical for many biological processes. Although much is known about how individual motors generate force and movement, surprisingly, little is known about the mechanisms underlying the macroscopic mechanics generated by multiple motors. For example, the observation that a saturating number, N, of myosin heads move an actin filament at a rate that is influenced by actin–myosin attachment and detachment kinetics is accounted for neither experimentally nor theoretically. To better understand the emergent mechanics of actin–myosin mechanochemistry, we use an in vitro motility assay to measure and correlate the N-dependence of actin sliding velocities, actin-activated ATPase activity, force generation against a mechanical load, and the calcium sensitivity of thin filament velocities. Our results show that both velocity and ATPase activity are strain dependent and that velocity becomes maximized with the saturation of myosin-binding sites on actin at a value that is 40% dependent on attachment kinetics and 60% dependent on detachment kinetics. These results support a chemical thermodynamic model for ensemble motor mechanochemistry and imply molecularly explicit mechanisms within this framework, challenging the assumption of independent force generation.
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Affiliation(s)
- Travis J Stewart
- Department of Pharmacology, University of Nevada, Reno School of Medicine, Reno, Nevada, USA
| | - Vidya Murthy
- Department of Biomedical Engineering, University of Nevada, Reno, Nevada, USA
| | - Sam P Dugan
- Department of Pharmacology, University of Nevada, Reno School of Medicine, Reno, Nevada, USA
| | - Josh E Baker
- Department of Pharmacology, University of Nevada, Reno School of Medicine, Reno, Nevada, USA.
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10
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Vander Roest AS, Liu C, Morck MM, Kooiker KB, Jung G, Song D, Dawood A, Jhingran A, Pardon G, Ranjbarvaziri S, Fajardo G, Zhao M, Campbell KS, Pruitt BL, Spudich JA, Ruppel KM, Bernstein D. Hypertrophic cardiomyopathy β-cardiac myosin mutation (P710R) leads to hypercontractility by disrupting super relaxed state. Proc Natl Acad Sci U S A 2021. [PMID: 34117120 DOI: 10.1073/pnas.2025030118/suppl_file/pnas.2025030118.sm02.avi] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/13/2023] Open
Abstract
Hypertrophic cardiomyopathy (HCM) is the most common inherited form of heart disease, associated with over 1,000 mutations, many in β-cardiac myosin (MYH7). Molecular studies of myosin with different HCM mutations have revealed a diversity of effects on ATPase and load-sensitive rate of detachment from actin. It has been difficult to predict how such diverse molecular effects combine to influence forces at the cellular level and further influence cellular phenotypes. This study focused on the P710R mutation that dramatically decreased in vitro motility velocity and actin-activated ATPase, in contrast to other MYH7 mutations. Optical trap measurements of single myosin molecules revealed that this mutation reduced the step size of the myosin motor and the load sensitivity of the actin detachment rate. Conversely, this mutation destabilized the super relaxed state in longer, two-headed myosin constructs, freeing more heads to generate force. Micropatterned human induced pluripotent derived stem cell (hiPSC)-cardiomyocytes CRISPR-edited with the P710R mutation produced significantly increased force (measured by traction force microscopy) compared with isogenic control cells. The P710R mutation also caused cardiomyocyte hypertrophy and cytoskeletal remodeling as measured by immunostaining and electron microscopy. Cellular hypertrophy was prevented in the P710R cells by inhibition of ERK or Akt. Finally, we used a computational model that integrated the measured molecular changes to predict the measured traction forces. These results confirm a key role for regulation of the super relaxed state in driving hypercontractility in HCM with the P710R mutation and demonstrate the value of a multiscale approach in revealing key mechanisms of disease.
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Affiliation(s)
- Alison Schroer Vander Roest
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
- Department of Bioengineering, School of Engineering and School of Medicine, Stanford University, Stanford, CA 94305
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Chao Liu
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Makenna M Morck
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Kristina Bezold Kooiker
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- School of Medicine, University of Washington, Seattle, WA 98109
| | - Gwanghyun Jung
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Dan Song
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Aminah Dawood
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Arnav Jhingran
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
| | - Gaspard Pardon
- Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
- Department of Bioengineering, School of Engineering and School of Medicine, Stanford University, Stanford, CA 94305
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Sara Ranjbarvaziri
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Giovanni Fajardo
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Mingming Zhao
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Kenneth S Campbell
- Department of Physiology, University of Kentucky, Lexington, KY 40536
- Division of Cardiovascular Medicine, University of Kentucky, Lexington, KY 40536
| | - Beth L Pruitt
- Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
- Department of Bioengineering, School of Engineering and School of Medicine, Stanford University, Stanford, CA 94305
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Mechanical and Biomolecular Science and Engineering, University of California, Santa Barbara, CA 93106
| | - James A Spudich
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305;
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Kathleen M Ruppel
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Daniel Bernstein
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304;
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
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11
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Vander Roest AS, Liu C, Morck MM, Kooiker KB, Jung G, Song D, Dawood A, Jhingran A, Pardon G, Ranjbarvaziri S, Fajardo G, Zhao M, Campbell KS, Pruitt BL, Spudich JA, Ruppel KM, Bernstein D. Hypertrophic cardiomyopathy β-cardiac myosin mutation (P710R) leads to hypercontractility by disrupting super relaxed state. Proc Natl Acad Sci U S A 2021; 118:e2025030118. [PMID: 34117120 PMCID: PMC8214707 DOI: 10.1073/pnas.2025030118] [Citation(s) in RCA: 46] [Impact Index Per Article: 15.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Hypertrophic cardiomyopathy (HCM) is the most common inherited form of heart disease, associated with over 1,000 mutations, many in β-cardiac myosin (MYH7). Molecular studies of myosin with different HCM mutations have revealed a diversity of effects on ATPase and load-sensitive rate of detachment from actin. It has been difficult to predict how such diverse molecular effects combine to influence forces at the cellular level and further influence cellular phenotypes. This study focused on the P710R mutation that dramatically decreased in vitro motility velocity and actin-activated ATPase, in contrast to other MYH7 mutations. Optical trap measurements of single myosin molecules revealed that this mutation reduced the step size of the myosin motor and the load sensitivity of the actin detachment rate. Conversely, this mutation destabilized the super relaxed state in longer, two-headed myosin constructs, freeing more heads to generate force. Micropatterned human induced pluripotent derived stem cell (hiPSC)-cardiomyocytes CRISPR-edited with the P710R mutation produced significantly increased force (measured by traction force microscopy) compared with isogenic control cells. The P710R mutation also caused cardiomyocyte hypertrophy and cytoskeletal remodeling as measured by immunostaining and electron microscopy. Cellular hypertrophy was prevented in the P710R cells by inhibition of ERK or Akt. Finally, we used a computational model that integrated the measured molecular changes to predict the measured traction forces. These results confirm a key role for regulation of the super relaxed state in driving hypercontractility in HCM with the P710R mutation and demonstrate the value of a multiscale approach in revealing key mechanisms of disease.
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Affiliation(s)
- Alison Schroer Vander Roest
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
- Department of Bioengineering, School of Engineering and School of Medicine, Stanford University, Stanford, CA 94305
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Chao Liu
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Makenna M Morck
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Kristina Bezold Kooiker
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- School of Medicine, University of Washington, Seattle, WA 98109
| | - Gwanghyun Jung
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Dan Song
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Aminah Dawood
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Arnav Jhingran
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
| | - Gaspard Pardon
- Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
- Department of Bioengineering, School of Engineering and School of Medicine, Stanford University, Stanford, CA 94305
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Sara Ranjbarvaziri
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Giovanni Fajardo
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Mingming Zhao
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
| | - Kenneth S Campbell
- Department of Physiology, University of Kentucky, Lexington, KY 40536
- Division of Cardiovascular Medicine, University of Kentucky, Lexington, KY 40536
| | - Beth L Pruitt
- Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
- Department of Bioengineering, School of Engineering and School of Medicine, Stanford University, Stanford, CA 94305
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Mechanical and Biomolecular Science and Engineering, University of California, Santa Barbara, CA 93106
| | - James A Spudich
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305;
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Kathleen M Ruppel
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305
| | - Daniel Bernstein
- Department of Pediatrics (Cardiology), Stanford University School of Medicine, Palo Alto, CA 94304;
- Stanford Cardiovascular Institute, Stanford University School of Medicine, Stanford, CA 94305
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12
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Baldo AP, Tardiff JC, Schwartz SD. A Proposed Mechanism for the Initial Myosin Binding Event on the Cardiac Thin Filament: A Metadynamics Study. J Phys Chem Lett 2021; 12:3509-3513. [PMID: 33793247 PMCID: PMC8080310 DOI: 10.1021/acs.jpclett.1c00223] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
The movement of tropomyosin over filamentous actin regulates the cross-bridge cycle of the thick with thin filament of cardiac muscle by blocking and revealing myosin binding sites. Tropomyosin exists in three, distinct equilibrium states with one state blocking myosin-actin interactions (blocked position) and the remaining two allowing for weak (closed position) and strong myosin binding (open position). However, experimental information illuminating how myosin binds to the thin filament and influences tropomyosin's transition across the actin surface is lacking. Using metadynamics, we mimic the effect of a single myosin head binding by determining the work required to pull small segments of tropomyosin toward the open position in several distinct regions of the thin filament. We find differences in required work due to the influence of cardiac troponin T lead to preferential binding sites and determine the mechanism of further myosin head recruitment.
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Affiliation(s)
- Anthony P Baldo
- Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States
| | - Jil C Tardiff
- Department of Biomedical Engineering, The University of Arizona, Tucson, Arizona 85724, United States
| | - Steven D Schwartz
- Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States
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13
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Risi CM, Pepper I, Belknap B, Landim-Vieira M, White HD, Dryden K, Pinto JR, Chase PB, Galkin VE. The structure of the native cardiac thin filament at systolic Ca 2+ levels. Proc Natl Acad Sci U S A 2021; 118:e2024288118. [PMID: 33753506 PMCID: PMC8020778 DOI: 10.1073/pnas.2024288118] [Citation(s) in RCA: 48] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Every heartbeat relies on cyclical interactions between myosin thick and actin thin filaments orchestrated by rising and falling Ca2+ levels. Thin filaments are comprised of two actin strands, each harboring equally separated troponin complexes, which bind Ca2+ to move tropomyosin cables away from the myosin binding sites and, thus, activate systolic contraction. Recently, structures of thin filaments obtained at low (pCa ∼9) or high (pCa ∼3) Ca2+ levels revealed the transition between the Ca2+-free and Ca2+-bound states. However, in working cardiac muscle, Ca2+ levels fluctuate at intermediate values between pCa ∼6 and pCa ∼7. The structure of the thin filament at physiological Ca2+ levels is unknown. We used cryoelectron microscopy and statistical analysis to reveal the structure of the cardiac thin filament at systolic pCa = 5.8. We show that the two strands of the thin filament consist of a mixture of regulatory units, which are composed of Ca2+-free, Ca2+-bound, or mixed (e.g., Ca2+ free on one side and Ca2+ bound on the other side) troponin complexes. We traced troponin complex conformations along and across individual thin filaments to directly determine the structural composition of the cardiac native thin filament at systolic Ca2+ levels. We demonstrate that the two thin filament strands are activated stochastically with short-range cooperativity evident only on one of the two strands. Our findings suggest a mechanism by which cardiac muscle is regulated by narrow range Ca2+ fluctuations.
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Affiliation(s)
- Cristina M Risi
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Ian Pepper
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Betty Belknap
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Maicon Landim-Vieira
- Department of Biomedical Sciences, Florida State University College of Medicine, Tallahassee, FL 32304
| | - Howard D White
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Kelly Dryden
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22903
| | - Jose R Pinto
- Department of Biomedical Sciences, Florida State University College of Medicine, Tallahassee, FL 32304
| | - P Bryant Chase
- Department of Biological Science, Florida State University, Tallahassee, FL 32306
| | - Vitold E Galkin
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507;
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14
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Tobacman LS. Troponin Revealed: Uncovering the Structure of the Thin Filament On-Off Switch in Striated Muscle. Biophys J 2021; 120:1-9. [PMID: 33221250 PMCID: PMC7820733 DOI: 10.1016/j.bpj.2020.11.014] [Citation(s) in RCA: 31] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/02/2020] [Revised: 11/06/2020] [Accepted: 11/11/2020] [Indexed: 12/25/2022] Open
Abstract
Recently, our understanding of the structural basis of troponin-tropomyosin's Ca2+-triggered regulation of striated muscle contraction has advanced greatly, particularly via cryo-electron microscopy data. Compelling atomic models of troponin-tropomyosin-actin were published for both apo- and Ca2+-saturated states of the cardiac thin filament. Subsequent electron microscopy and computational analyses have supported and further elaborated the findings. Per cryo-electron microscopy, each troponin is highly extended and contacts both tropomyosin strands, which lie on opposite sides of the actin filament. In the apo-state characteristic of relaxed muscle, troponin and tropomyosin hinder strong myosin-actin binding in several different ways, apparently barricading the actin more substantially than does tropomyosin alone. The troponin core domain, the C-terminal third of TnI, and tropomyosin under the influence of a 64-residue helix of TnT located at the overlap of adjacent tropomyosins are all in positions that would hinder strong myosin binding to actin. In the Ca2+-saturated state, the TnI C-terminus dissociates from actin and binds in part to TnC; the core domain pivots significantly; the N-lobe of TnC binds specifically to actin and tropomyosin; and tropomyosin rotates partially away from myosin's binding site on actin. At the overlap domain, Ca2+ causes much less tropomyosin movement, so a more inhibitory orientation persists. In the myosin-saturated state of the thin filament, there is a large additional shift in tropomyosin, with molecular interactions now identified between tropomyosin and both actin and myosin. A new era has arrived for investigation of the thin filament and for functional understandings that increasingly accommodate the recent structural results.
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Affiliation(s)
- Larry S Tobacman
- Departments of Medicine and of Physiology and Biophysics, University of Illinois at Chicago, Chicago, Illinois.
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15
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Pertici I, Bianchi G, Bongini L, Lombardi V, Bianco P. A Myosin II-Based Nanomachine Devised for the Study of Ca 2+-Dependent Mechanisms of Muscle Regulation. Int J Mol Sci 2020; 21:E7372. [PMID: 33036217 PMCID: PMC7583892 DOI: 10.3390/ijms21197372] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2020] [Revised: 10/01/2020] [Accepted: 10/02/2020] [Indexed: 02/03/2023] Open
Abstract
The emergent properties of the array arrangement of the molecular motor myosin II in the sarcomere of the striated muscle, the generation of steady force and shortening, can be studied in vitro with a synthetic nanomachine made of an ensemble of eight heavy-meromyosin (HMM) fragments of myosin from rabbit psoas muscle, carried on a piezoelectric nanopositioner and brought to interact with a properly oriented actin filament attached via gelsolin (a Ca2+-regulated actin binding protein) to a bead trapped by dual laser optical tweezers. However, the application of the original version of the nanomachine to investigate the Ca2+-dependent regulation mechanisms of the other sarcomeric (regulatory or cytoskeleton) proteins, adding them one at a time, was prevented by the impossibility to preserve [Ca2+] as a free parameter. Here, the nanomachine is implemented by assembling the bead-attached actin filament with the Ca2+-insensitive gelsolin fragment TL40. The performance of the nanomachine is determined both in the absence and in the presence of Ca2+ (0.1 mM, the concentration required for actin attachment to the bead with gelsolin). The nanomachine exhibits a maximum power output of 5.4 aW, independently of [Ca2+], opening the possibility for future studies of the Ca2+-dependent function/dysfunction of regulatory and cytoskeletal proteins.
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Affiliation(s)
| | | | | | - Vincenzo Lombardi
- PhysioLab, University of Florence, 50019 Sesto Fiorentino (FI), Italy; (I.P.); (G.B.); (L.B.); (P.B.)
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16
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Comparison of Functional Characteristics of Myosin in Fast and Slow Skeletal Muscles. Bull Exp Biol Med 2020; 169:338-341. [PMID: 32743781 DOI: 10.1007/s10517-020-04882-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2019] [Indexed: 10/23/2022]
Abstract
Myosins of fast and slow skeletal muscles differ by the isoform composition of the heavy and light chains. We compared functional characteristics of myosin from the fast (m. psoas) and slow (m. soleus) muscles of rabbits. The parameters of single actin-myosin interaction were measured in an optical trap, and the characteristics of the Ca2+ regulation of actin-myosin interaction were studied using an in vitro motility assay. The duration of interaction of myosin from the fast muscle with actin was shorter and the filament sliding velocity over this myosin was higher than the corresponding parameters for myosin from the slow muscle. The dependence pCa-velocity for myosin from the fast muscle was less sensitive to Ca2+ than that of slow muscle myosin. Thus, functional properties of myosin determine not only mechanical and kinetic characteristics of muscle contraction, but also the peculiarities of its Ca2+ regulation.
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17
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Janco M, Dedova I, Bryce NS, Hardeman EC, Gunning PW. Visualizing the in vitro assembly of tropomyosin/actin filaments using TIRF microscopy. Biophys Rev 2020; 12:879-885. [PMID: 32638329 PMCID: PMC7429660 DOI: 10.1007/s12551-020-00720-6] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2020] [Accepted: 07/02/2020] [Indexed: 12/23/2022] Open
Abstract
Tropomyosins are elongated alpha-helical proteins that form co-polymers with most actin filaments within a cell and play important roles in the structural and functional diversification of the actin cytoskeleton. How the assembly of tropomyosins along an actin filament is regulated and the kinetics of tropomyosin association with an actin filament is yet to be fully determined. A recent series of publications have used total internal reflection fluorescence (TIRF) microscopy in combination with advanced surface and protein chemistry to visualise the molecular assembly of actin/tropomyosin filaments in vitro. Here, we review the use of the in vitro TIRF assay in the determination of kinetic data on tropomyosin filament assembly. This sophisticated approach has enabled generation of real-time single-molecule data to fill the gap between in vitro bulk assays and in vivo assays of tropomyosin function. The in vitro TIRF assays provide a new foundation for future studies involving multiple actin-binding proteins that will more accurately reflect the physiological protein-protein interactions in cells.
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Affiliation(s)
- Miro Janco
- School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia
| | - Irina Dedova
- School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia
| | - Nicole S Bryce
- School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia
| | - Edna C Hardeman
- School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia
| | - Peter W Gunning
- School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia.
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18
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Cardiomyopathy-associated mutations in tropomyosin differently affect actin–myosin interaction at single-molecule and ensemble levels. J Muscle Res Cell Motil 2019; 40:299-308. [DOI: 10.1007/s10974-019-09560-8] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2019] [Accepted: 10/15/2019] [Indexed: 01/31/2023]
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19
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Skeletal MyBP-C isoforms tune the molecular contractility of divergent skeletal muscle systems. Proc Natl Acad Sci U S A 2019; 116:21882-21892. [PMID: 31591218 DOI: 10.1073/pnas.1910549116] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Skeletal muscle myosin-binding protein C (MyBP-C) is a myosin thick filament-associated protein, localized through its C terminus to distinct regions (C-zones) of the sarcomere. MyBP-C modulates muscle contractility, presumably through its N terminus extending from the thick filament and interacting with either the myosin head region and/or the actin thin filament. Two isoforms of MyBP-C (fast- and slow-type) are expressed in a muscle type-specific manner. Are the expression, localization, and Ca2+-dependent modulatory capacities of these isoforms different in fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles derived from Sprague-Dawley rats? By mass spectrometry, 4 MyBP-C isoforms (1 fast-type MyBP-C and 3 N-terminally spliced slow-type MyBP-C) were expressed in EDL, but only the 3 slow-type MyBP-C isoforms in SOL. Using EDL and SOL native thick filaments in which the MyBP-C stoichiometry and localization are preserved, native thin filament sliding over these thick filaments showed that, only in the C-zone, MyBP-C Ca2+ sensitizes the thin filament and slows thin filament velocity. These modulatory properties depended on MyBP-C's N terminus as N-terminal proteolysis attenuated MyBP-C's functional capacities. To determine each MyBP-C isoform's contribution to thin filament Ca2+ sensitization and slowing in the C-zone, we used a combination of in vitro motility assays using expressed recombinant N-terminal fragments and in silico mechanistic modeling. Our results suggest that each skeletal MyBP-C isoform's N terminus is functionally distinct and has modulatory capacities that depend on the muscle type in which they are expressed, providing the potential for molecular tuning of skeletal muscle performance through differential MyBP-C expression.
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20
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Unger M, Debold EP. Acidosis decreases the Ca 2+ sensitivity of thin filaments by preventing the first actomyosin interaction. Am J Physiol Cell Physiol 2019; 317:C714-C718. [PMID: 31339771 DOI: 10.1152/ajpcell.00196.2019] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Intracellular acidosis is a putative agent of skeletal muscle fatigue, in part, because it depresses the calcium (Ca2+) sensitivity of the myofilaments. However, the molecular mechanism behind this depression in Ca2+ sensitivity is unknown, providing a significant challenge to a complete understanding of the fatigue process. To elucidate this mechanism, we directly determined the effect of acidosis on the ability of a single myosin molecule to bind to a regulated actin filament in a laser trap assay. Decreasing pH from 7.4 to 6.5 significantly (P < 0.05) reduced the frequency of single actomyosin-binding events at submaximal (pCa 8-pCa 6) but not at maximal Ca2+ concentration (pCa 5-pCa 4). To delineate whether this was due to a direct effect on myosin versus an indirect effect on the regulatory proteins troponin (Tn) and tropomyosin (Tm), binding frequency was also quantified in the absence of Tn and Tm. This revealed that acidosis did not significantly alter the frequency of actomyosin binding events in the absence of regulatory proteins (1.4 ± 0.15 vs. 1.4 ± 0.15 events/s for pH 7.4 and 6.5, respectively). Acidosis also did not significantly affect the size of myosin's powerstroke or the duration of binding events in the presence of regulatory proteins, at every [Ca2+]. These data suggest acidosis impedes activation of the thin filament by competitively inhibiting Ca2+ binding to TnC. This slows the rate at which myosin initially attaches to actin; therefore, less cross bridges will be bound and generating force at any given submaximal [Ca2+]. These data provide a molecular explanation for the acidosis-induced decrease in force observed at the submaximal Ca2+ concentrations that might contribute to the loss of force during muscle fatigue.
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Affiliation(s)
- Matthew Unger
- Muscle Biophysics Laboratory, Department of Kinesiology, University of Massachusetts, Amherst, Massachusetts
| | - Edward P Debold
- Muscle Biophysics Laboratory, Department of Kinesiology, University of Massachusetts, Amherst, Massachusetts
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21
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Revealing the mechanism of how cardiac myosin-binding protein C N-terminal fragments sensitize thin filaments for myosin binding. Proc Natl Acad Sci U S A 2019; 116:6828-6835. [PMID: 30877248 PMCID: PMC6452674 DOI: 10.1073/pnas.1816480116] [Citation(s) in RCA: 36] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Diverse demands on cardiac muscle require the fine-tuning of contraction. Cardiac myosin binding protein-C (cMyBP-C) is involved in this regulation; however, its precise molecular mechanism of action remains uncertain. By imaging the interactions of single myosin and cMyBP-C molecules interacting with suspended thin filaments in vitro we observe cMyBP-C N-terminal fragments assist activation and modulate contraction velocity by affecting myosin binding to the thin filament. Fluorescent imaging of Cy3-labeled cMyBP-C revealed that it diffusively scans the thin filament and then strongly binds to displace tropomyosin and activate at low calcium. At high calcium, cMyBP-C decorates the filament more extensively, reducing myosin binding through competing with binding sites. Understanding the mechanism of MyBP-C action has important implications for heart disease. Cardiac muscle contraction is triggered by calcium binding to troponin. The consequent movement of tropomyosin permits myosin binding to actin, generating force. Cardiac myosin-binding protein C (cMyBP-C) plays a modulatory role in this activation process. One potential mechanism for the N-terminal domains of cMyBP-C to achieve this is by binding directly to the actin-thin filament at low calcium levels to enhance the movement of tropomyosin. To determine the molecular mechanisms by which cMyBP-C enhances myosin recruitment to the actin-thin filament, we directly visualized fluorescently labeled cMyBP-C N-terminal fragments and GFP-labeled myosin molecules binding to suspended actin-thin filaments in a fluorescence-based single-molecule microscopy assay. Binding of the C0C3 N-terminal cMyBP-C fragment to the thin filament enhanced myosin association at low calcium levels. However, at high calcium levels, C0C3 bound in clusters, blocking myosin binding. Dynamic imaging of thin filament-bound Cy3-C0C3 molecules demonstrated that these fragments diffuse along the thin filament before statically binding, suggesting a mechanism that involves a weak-binding mode to search for access to the thin filament and a tight-binding mode to sensitize the thin filament to calcium, thus enhancing myosin binding. Although shorter N-terminal fragments (Cy3-C0C1 and Cy3-C0C1f) bound to the thin filaments and displayed modes of motion on the thin filament similar to that of the Cy3-C0C3 fragment, the shorter fragments were unable to sensitize the thin filament. Therefore, the longer N-terminal fragment (C0C3) must possess the requisite domains needed to bind specifically to the thin filament in order for the cMyBP-C N terminus to modulate cardiac contractility.
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22
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Woody MS, Greenberg MJ, Barua B, Winkelmann DA, Goldman YE, Ostap EM. Positive cardiac inotrope omecamtiv mecarbil activates muscle despite suppressing the myosin working stroke. Nat Commun 2018; 9:3838. [PMID: 30242219 PMCID: PMC6155018 DOI: 10.1038/s41467-018-06193-2] [Citation(s) in RCA: 97] [Impact Index Per Article: 16.2] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2018] [Accepted: 08/21/2018] [Indexed: 02/05/2023] Open
Abstract
Omecamtiv mecarbil (OM) is a positive cardiac inotrope in phase-3 clinical trials for treatment of heart failure. Although initially described as a direct myosin activator, subsequent studies are at odds with this description and do not explain OM-mediated increases in cardiac performance. Here we show, via single-molecule, biophysical experiments on cardiac myosin, that OM suppresses myosin's working stroke and prolongs actomyosin attachment 5-fold, which explains inhibitory actions of the drug observed in vitro. OM also causes the actin-detachment rate to become independent of both applied load and ATP concentration. Surprisingly, increased myocardial force output in the presence of OM can be explained by cooperative thin-filament activation by OM-inhibited myosin molecules. Selective suppression of myosin is an unanticipated route to muscle activation that may guide future development of therapeutic drugs.
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Affiliation(s)
- Michael S Woody
- Graduate Group in Biochemistry and Molecular Biophysics, Perelman School of Medicine, University of Pennsylvania, 700A Clinical Research Building, Philadelphia, PA, 19104-6085, USA
| | - Michael J Greenberg
- Pennsylvania Muscle Institute, Perelman School of Medicine, University of Pennsylvania, 700A Clinical Research Building, Philadelphia, PA, 19104-6085, USA.,Department of Biochemistry and Molecular Biophysics, Washington University in Saint Louis, St. Louis, 63110, MO, USA
| | - Bipasha Barua
- Department of Pathology and Laboratory Medicine, Robert Wood Johnson Medical School, Rutgers University, 675 Hoes Lane, Piscataway, NJ, 08854, USA
| | - Donald A Winkelmann
- Department of Pathology and Laboratory Medicine, Robert Wood Johnson Medical School, Rutgers University, 675 Hoes Lane, Piscataway, NJ, 08854, USA
| | - Yale E Goldman
- Pennsylvania Muscle Institute, Perelman School of Medicine, University of Pennsylvania, 700A Clinical Research Building, Philadelphia, PA, 19104-6085, USA.
| | - E Michael Ostap
- Pennsylvania Muscle Institute, Perelman School of Medicine, University of Pennsylvania, 700A Clinical Research Building, Philadelphia, PA, 19104-6085, USA.
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23
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Solís C, Kim GH, Moutsoglou ME, Robinson JM. Ca 2+ and Myosin Cycle States Work as Allosteric Effectors of Troponin Activation. Biophys J 2018; 115:1762-1769. [PMID: 30249400 DOI: 10.1016/j.bpj.2018.08.033] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2017] [Revised: 08/22/2018] [Accepted: 08/24/2018] [Indexed: 10/28/2022] Open
Abstract
In cardiac muscle, troponin (Tn) and tropomyosin inhibit actin and myosin interactions through the steric blocking of myosin binding to F-actin. Ca2+ binding to Tn C modulates this inhibition. Thin filaments become activated upon Ca2+ binding, which enables strong binding of myosin with a concomitant release of ATP hydrolysis products and level arm swinging responsible for force generation. Despite this level of description, the current cross-bridge cycle model does not fully define the structural events that take place within Tn during combinatorial myosin and Ca2+ interventions. Here, we studied conformational changes within Tn bound to F-actin and tropomyosin by fluorescence lifetime imaging combined with Förster resonance energy transfer. Fluorescent dye molecules covalently bound to the Tn C C-lobe and Tn I C-terminal domain report Ca2+- and myosin-induced activation of Tn. Reconstituted thin filaments were deposited on a myosin-coated surface similar to an in vitro motility assay setup without filament sliding involved. Under all the tested conditions, Ca2+ was responsible for the most significant changes in Tn activation. Rigor myosin activated Tn at subsaturated Ca2+ conditions but not to the degree seen in thin filaments with Ca2+. ATP-γ-S did not affect Tn activation significantly; however, blebbistatin induced significant activation at subsaturating Ca2+ levels. The relation between the extent of Tn activation and its conformational flexibility suggests that active/inactive Tn states coexist in different proportions that depend on the combination of effectors. These results satisfy an allosteric activation model of the thin filament as a function of Ca2+ and the myosin catalytic cycle state.
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Affiliation(s)
- Christopher Solís
- Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota.
| | - Giho H Kim
- Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota
| | - Maria E Moutsoglou
- Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota
| | - John M Robinson
- Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota
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24
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Pertici I, Bongini L, Melli L, Bianchi G, Salvi L, Falorsi G, Squarci C, Bozó T, Cojoc D, Kellermayer MSZ, Lombardi V, Bianco P. A myosin II nanomachine mimicking the striated muscle. Nat Commun 2018; 9:3532. [PMID: 30166542 PMCID: PMC6117265 DOI: 10.1038/s41467-018-06073-9] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2018] [Accepted: 08/02/2018] [Indexed: 11/08/2022] Open
Abstract
The contraction of striated muscle (skeletal and cardiac muscle) is generated by ATP-dependent interactions between the molecular motor myosin II and the actin filament. The myosin motors are mechanically coupled along the thick filament in a geometry not achievable by single-molecule experiments. Here we show that a synthetic one-dimensional nanomachine, comprising fewer than ten myosin II dimers purified from rabbit psoas, performs isometric and isotonic contractions at 2 mM ATP, delivering a maximum power of 5 aW. The results are explained with a kinetic model fitted to the performance of mammalian skeletal muscle, showing that the condition for the motor coordination that maximises the efficiency in striated muscle is a minimum of 32 myosin heads sharing a common mechanical ground. The nanomachine offers a powerful tool for investigating muscle contractile-protein physiology, pathology and pharmacology without the potentially disturbing effects of the cytoskeletal-and regulatory-protein environment.
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Affiliation(s)
- Irene Pertici
- PhysioLab, University of Florence, Florence, 50019, Italy
| | | | - Luca Melli
- PhysioLab, University of Florence, Florence, 50019, Italy
- F. Hoffmann-La Roche Ltd, Basel, 4053, Switzerland
| | - Giulio Bianchi
- PhysioLab, University of Florence, Florence, 50019, Italy
| | - Luca Salvi
- PhysioLab, University of Florence, Florence, 50019, Italy
- Department of Biochemistry, University of Munich, Munich, 81377, Germany
| | - Giulia Falorsi
- PhysioLab, University of Florence, Florence, 50019, Italy
| | | | - Tamás Bozó
- Department of Biophysics and Radiation Biology, Semmelweis University, Budapest, H-1094, Hungary
| | | | - Miklós S Z Kellermayer
- Department of Biophysics and Radiation Biology, Semmelweis University, Budapest, H-1094, Hungary
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25
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Do Actomyosin Single-Molecule Mechanics Data Predict Mechanics of Contracting Muscle? Int J Mol Sci 2018; 19:ijms19071863. [PMID: 29941816 PMCID: PMC6073448 DOI: 10.3390/ijms19071863] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2018] [Revised: 06/19/2018] [Accepted: 06/20/2018] [Indexed: 12/15/2022] Open
Abstract
In muscle, but not in single-molecule mechanics studies, actin, myosin and accessory proteins are incorporated into a highly ordered myofilament lattice. In view of this difference we compare results from single-molecule studies and muscle mechanics and analyze to what degree data from the two types of studies agree with each other. There is reasonable correspondence in estimates of the cross-bridge power-stroke distance (7–13 nm), cross-bridge stiffness (~2 pN/nm) and average isometric force per cross-bridge (6–9 pN). Furthermore, models defined on the basis of single-molecule mechanics and solution biochemistry give good fits to experimental data from muscle. This suggests that the ordered myofilament lattice, accessory proteins and emergent effects of the sarcomere organization have only minor modulatory roles. However, such factors may be of greater importance under e.g., disease conditions. We also identify areas where single-molecule and muscle data are conflicting: (1) whether force generation is an Eyring or Kramers process with just one major power-stroke or several sub-strokes; (2) whether the myofilaments and the cross-bridges have Hookean or non-linear elasticity; (3) if individual myosin heads slip between actin sites under certain conditions, e.g., in lengthening; or (4) if the two heads of myosin cooperate.
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Titin-mediated thick filament activation, through a mechanosensing mechanism, introduces sarcomere-length dependencies in mathematical models of rat trabecula and whole ventricle. Sci Rep 2017; 7:5546. [PMID: 28717163 PMCID: PMC5514028 DOI: 10.1038/s41598-017-05999-2] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2017] [Accepted: 06/08/2017] [Indexed: 11/08/2022] Open
Abstract
Recent experimental evidence in skeletal muscle demonstrated the existence of a thick-filament mechanosensing mechanism, acting as a second regulatory system for muscle contraction, in addition to calcium-mediated thin filament regulation. These two systems cooperate to generate force, but the extent to which their interaction is relevant in physiologically contracting muscle was not yet assessed experimentally. Therefore, we included both regulatory mechanisms in a mathematical model of rat trabecula and whole ventricle. No additional regulatory mechanisms were considered in our model. Our simulations suggested that mechanosensing regulation is not limited to the initial phases of contraction but, instead, is crucial during physiological contraction. An important consequence of this finding is that titin mediated thick filament activation can account for several sarcomere length dependencies observed in contracting muscle. Under the hypothesis that a similar mechanism is acting on cardiac muscle, and within the limits of a finite element left ventricle model, we predict that these two regulatory mechanisms are crucial for the molecular basis of the Frank-Starling law of the heart.
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Shchepkin DV, Nabiev SR, Kopylova GV, Matyushenko AM, Levitsky DI, Bershitsky SY, Tsaturyan AK. Cooperativity of myosin interaction with thin filaments is enhanced by stabilizing substitutions in tropomyosin. J Muscle Res Cell Motil 2017; 38:183-191. [PMID: 28540577 DOI: 10.1007/s10974-017-9472-x] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2017] [Accepted: 05/22/2017] [Indexed: 11/24/2022]
Abstract
Muscle contraction is powered by myosin interaction with actin-based thin filaments containing Ca2+-regulatory proteins, tropomyosin and troponin. Coiled-coil tropomyosin molecules form a long helical strand that winds around actin filament and either shields actin from myosin binding or opens it. Non-canonical residues G126 and D137 in the central part of tropomyosin destabilize its coiled-coil structure. Their substitutions for canonical ones, G126R and D137L, increase structural stability and the velocity of sliding of reconstructed thin filaments along myosin coated surface. The effect of these stabilizing mutations on force of the actin-myosin interaction is unknown. It also remains unclear whether the stabilization affects single actin-myosin interactions or it modifies the cooperativity of the binding of myosin molecules to actin. We used an optical trap to measure the effects of the stabilization on step size, unitary force and duration of the interactions at low and high load and compared the results with those obtained in an in vitro motility assay. We found that significant prolongation of lifetime of the actin-myosin complex under high load observed at high extent of tropomyosin stabilization, i.e. with double mutant, G126R/D137L, correlates with higher force in the motility assay. Also, the higher the extent of stabilization of tropomyosin, the fewer myosin molecules are needed to propel the thin filaments. The data suggest that the effects of the stabilizing mutations in tropomyosin on the myosin interaction with regulated thin filaments are mainly realized via cooperative mechanisms by increasing the size of cooperative unit.
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Affiliation(s)
- Daniil V Shchepkin
- Institute of Immunology and Physiology, Russian Academy of Sciences, Yekaterinburg, 620049, Russia
| | - Salavat R Nabiev
- Institute of Immunology and Physiology, Russian Academy of Sciences, Yekaterinburg, 620049, Russia
| | - Galina V Kopylova
- Institute of Immunology and Physiology, Russian Academy of Sciences, Yekaterinburg, 620049, Russia
| | - Alexander M Matyushenko
- A.N. Bach Institute of Biochemistry, Research Centre of Biotechnology, Russian Academy of Sciences, Moscow, 119071, Russia.,Department of Biochemistry, School of Biology, Moscow State University, Moscow, 119234, Russia
| | - Dmitrii I Levitsky
- A.N. Bach Institute of Biochemistry, Research Centre of Biotechnology, Russian Academy of Sciences, Moscow, 119071, Russia.,A.N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow, 119234, Russia
| | - Sergey Y Bershitsky
- Institute of Immunology and Physiology, Russian Academy of Sciences, Yekaterinburg, 620049, Russia
| | - Andrey K Tsaturyan
- Institute of Mechanics, Moscow State University, 1 Mitchurinsky prosp., Moscow, 119192, Russia.
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The molecular basis of thin filament activation: from single molecule to muscle. Sci Rep 2017; 7:1822. [PMID: 28500282 PMCID: PMC5431769 DOI: 10.1038/s41598-017-01604-8] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2017] [Accepted: 03/30/2017] [Indexed: 11/24/2022] Open
Abstract
For muscles to effectively power locomotion, trillions of myosin molecules must rapidly attach and detach from the actin thin filament. This is accomplished by precise regulation of the availability of the myosin binding sites on actin (i.e. activation). Both calcium (Ca++) and myosin binding contribute to activation, but both mechanisms are simultaneously active during contraction, making their relative contributions difficult to determine. Further complicating the process, myosin binding accelerates the attachment rate of neighboring myosin molecules, adding a cooperative element to the activation process. To de-convolve these two effects, we directly determined the effect of Ca++ on the rate of attachment of a single myosin molecule to a single regulated actin thin filament, and separately determined the distance over which myosin binding increases the attachment rate of neighboring molecules. Ca++ alone increases myosin’s attachment rate ~50-fold, while myosin binding accelerates attachment of neighboring molecules 400 nm along the actin thin filament.
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Vandenboom R. Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation. Compr Physiol 2016; 7:171-212. [PMID: 28135003 DOI: 10.1002/cphy.c150044] [Citation(s) in RCA: 45] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.
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Affiliation(s)
- Rene Vandenboom
- Department of Kinesiology, Faculty of Applied Health Sciences, Brock University, Ontario, Canada
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Moore JR, Campbell SG, Lehman W. Structural determinants of muscle thin filament cooperativity. Arch Biochem Biophys 2016; 594:8-17. [PMID: 26891592 DOI: 10.1016/j.abb.2016.02.016] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2015] [Revised: 02/09/2016] [Accepted: 02/10/2016] [Indexed: 11/16/2022]
Abstract
End-to-end connections between adjacent tropomyosin molecules along the muscle thin filament allow long-range conformational rearrangement of the multicomponent filament structure. This process is influenced by Ca(2+) and the troponin regulatory complexes, as well as by myosin crossbridge heads that bind to and activate the filament. Access of myosin crossbridges onto actin is gated by tropomyosin, and in the case of striated muscle filaments, troponin acts as a gatekeeper. The resulting tropomyosin-troponin-myosin on-off switching mechanism that controls muscle contractility is a complex cooperative and dynamic system with highly nonlinear behavior. Here, we review key information that leads us to view tropomyosin as central to the communication pathway that coordinates the multifaceted effectors that modulate and tune striated muscle contraction. We posit that an understanding of this communication pathway provides a framework for more in-depth mechanistic characterization of myopathy-associated mutational perturbations currently under investigation by many research groups.
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Affiliation(s)
- Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts Lowell, One University Avenue, Lowell, MA 018154, USA
| | - Stuart G Campbell
- Department of Biomedical Engineering, Yale University, 55 Prospect Street, New Haven, CT 06511, USA
| | - William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA.
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Wolfenson H, Meacci G, Liu S, Stachowiak MR, Iskratsch T, Ghassemi S, Roca-Cusachs P, O’Shaughnessy B, Hone J, Sheetz MP. Tropomyosin controls sarcomere-like contractions for rigidity sensing and suppressing growth on soft matrices. Nat Cell Biol 2016; 18:33-42. [PMID: 26619148 PMCID: PMC5296190 DOI: 10.1038/ncb3277] [Citation(s) in RCA: 126] [Impact Index Per Article: 15.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2015] [Accepted: 10/26/2015] [Indexed: 12/14/2022]
Abstract
Cells test the rigidity of the extracellular matrix by applying forces to it through integrin adhesions. Recent measurements show that these forces are applied by local micrometre-scale contractions, but how contraction force is regulated by rigidity is unknown. Here we performed high temporal- and spatial-resolution tracking of contractile forces by plating cells on sub-micrometre elastomeric pillars. We found that actomyosin-based sarcomere-like contractile units (CUs) simultaneously moved opposing pillars in net steps of ∼2.5 nm, independent of rigidity. What correlated with rigidity was the number of steps taken to reach a force level that activated recruitment of α-actinin to the CUs. When we removed actomyosin restriction by depleting tropomyosin 2.1, we observed larger steps and higher forces that resulted in aberrant rigidity sensing and growth of non-transformed cells on soft matrices. Thus, we conclude that tropomyosin 2.1 acts as a suppressor of growth on soft matrices by supporting proper rigidity sensing.
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Affiliation(s)
- Haguy Wolfenson
- Department of Biological Sciences, Columbia University, New York, NY 10027
- The Nanomedicine Center for Mechanobiology Directing the Immune Response, Columbia University, New York, NY 10027
| | - Giovanni Meacci
- Department of Biological Sciences, Columbia University, New York, NY 10027
- The Nanomedicine Center for Mechanobiology Directing the Immune Response, Columbia University, New York, NY 10027
- Department of Chemical Engineering, Columbia University, New York, NY 10027
| | - Shuaimin Liu
- The Nanomedicine Center for Mechanobiology Directing the Immune Response, Columbia University, New York, NY 10027
- Department of Mechanical Engineering, Columbia University, New York, NY 10027
| | | | - Thomas Iskratsch
- Department of Biological Sciences, Columbia University, New York, NY 10027
- The Nanomedicine Center for Mechanobiology Directing the Immune Response, Columbia University, New York, NY 10027
| | - Saba Ghassemi
- The Nanomedicine Center for Mechanobiology Directing the Immune Response, Columbia University, New York, NY 10027
- Department of Mechanical Engineering, Columbia University, New York, NY 10027
| | - Pere Roca-Cusachs
- Institute for Bioengineering of Catalonia and University of Barcelona, Barcelona, Spain 08028
| | - Ben O’Shaughnessy
- Department of Chemical Engineering, Columbia University, New York, NY 10027
| | - James Hone
- The Nanomedicine Center for Mechanobiology Directing the Immune Response, Columbia University, New York, NY 10027
- Department of Mechanical Engineering, Columbia University, New York, NY 10027
| | - Michael P. Sheetz
- Department of Biological Sciences, Columbia University, New York, NY 10027
- The Nanomedicine Center for Mechanobiology Directing the Immune Response, Columbia University, New York, NY 10027
- Mechanobiology Institute, National University of Singapore, Singapore 117411
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Walcott S, Kad NM. Direct Measurements of Local Coupling between Myosin Molecules Are Consistent with a Model of Muscle Activation. PLoS Comput Biol 2015; 11:e1004599. [PMID: 26536123 PMCID: PMC4633106 DOI: 10.1371/journal.pcbi.1004599] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2015] [Accepted: 10/13/2015] [Indexed: 01/21/2023] Open
Abstract
Muscle contracts due to ATP-dependent interactions of myosin motors with thin filaments composed of the proteins actin, troponin, and tropomyosin. Contraction is initiated when calcium binds to troponin, which changes conformation and displaces tropomyosin, a filamentous protein that wraps around the actin filament, thereby exposing myosin binding sites on actin. Myosin motors interact with each other indirectly via tropomyosin, since myosin binding to actin locally displaces tropomyosin and thereby facilitates binding of nearby myosin. Defining and modeling this local coupling between myosin motors is an open problem in muscle modeling and, more broadly, a requirement to understanding the connection between muscle contraction at the molecular and macro scale. It is challenging to directly observe this coupling, and such measurements have only recently been made. Analysis of these data suggests that two myosin heads are required to activate the thin filament. This result contrasts with a theoretical model, which reproduces several indirect measurements of coupling between myosin, that assumes a single myosin head can activate the thin filament. To understand this apparent discrepancy, we incorporated the model into stochastic simulations of the experiments, which generated simulated data that were then analyzed identically to the experimental measurements. By varying a single parameter, good agreement between simulation and experiment was established. The conclusion that two myosin molecules are required to activate the thin filament arises from an assumption, made during data analysis, that the intensity of the fluorescent tags attached to myosin varies depending on experimental condition. We provide an alternative explanation that reconciles theory and experiment without assuming that the intensity of the fluorescent tags varies. Despite decades of study, there is no clear connection between muscle contraction at the molecular and the macroscopic scale. For example, we cannot yet predict how a genetic defect in a muscle protein will result in a physiological change in the heart. This multi-scale understanding is difficult, in part, because molecules cooperate during muscle contraction; that is, one molecule’s behavior is influenced by the behavior of its neighbors. It is difficult to make direct measurements from such coupled molecular systems and also difficult to describe them quantitatively. Despite these obstacles, we recently published experimental measurements and theoretical models of this coupling, but there were apparent discrepancies between the two. Here, we use detailed computer simulations of these experiments to show that, in fact, the measurements agree with the model to a remarkable extent. This agreement suggests that the model captures the essential molecular events that underlie the coupling between muscle molecules. This removes a major obstacle to a multi-scale understanding of muscle contraction and, while more work is necessary, suggests that a connection between the molecular and macroscopic scale is within reach.
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Affiliation(s)
- Sam Walcott
- Mathematics, University of California at Davis, Davis, California, United States of America
- * E-mail:
| | - Neil M. Kad
- School of Biosciences, University of Kent, Canterbury, Kent, United Kingdom
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Effects of cardiac Myosin binding protein-C on actin motility are explained with a drag-activation-competition model. Biophys J 2015; 108:10-3. [PMID: 25564844 DOI: 10.1016/j.bpj.2014.11.1852] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2014] [Revised: 10/31/2014] [Accepted: 11/11/2014] [Indexed: 01/26/2023] Open
Abstract
Although mutations in cardiac myosin binding protein-C (cMyBP-C) cause heart disease, its role in muscle contraction is not well understood. A mechanism remains elusive partly because the protein can have multiple effects, such as dual biphasic activation and inhibition observed in actin motility assays. Here we develop a mathematical model for the interaction of cMyBP-C with the contractile proteins actin and myosin and the regulatory protein tropomyosin. We use this model to show that a drag-activation-competition mechanism accurately describes actin motility measurements, while models lacking either drag or competition do not. These results suggest that complex effects can arise simply from cMyBP-C binding to actin.
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34
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Tong CW, Wu X, Liu Y, Rosas PC, Sadayappan S, Hudmon A, Muthuchamy M, Powers PA, Valdivia HH, Moss RL. Phosphoregulation of Cardiac Inotropy via Myosin Binding Protein-C During Increased Pacing Frequency or β1-Adrenergic Stimulation. Circ Heart Fail 2015; 8:595-604. [PMID: 25740838 DOI: 10.1161/circheartfailure.114.001585] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/16/2014] [Accepted: 02/24/2015] [Indexed: 11/16/2022]
Abstract
BACKGROUND Mammalian hearts exhibit positive inotropic responses to β-adrenergic stimulation as a consequence of protein kinase A-mediated phosphorylation or as a result of increased beat frequency (the Bowditch effect). Several membrane and myofibrillar proteins are phosphorylated under these conditions, but the relative contributions of these to increased contractility are not known. Phosphorylation of cardiac myosin-binding protein-C (cMyBP-C) by protein kinase A accelerates the kinetics of force development in permeabilized heart muscle, but its role in vivo is unknown. Such understanding is important because adrenergic responsiveness of the heart and the Bowditch effect are both depressed in heart failure. METHODS AND RESULTS The roles of cMyBP-C phosphorylation were studied using mice in which either WT or nonphosphorylatable forms of cMyBP-C [ser273ala, ser282ala, ser302ala: cMyBP-C(t3SA)] were expressed at similar levels on a cMyBP-C null background. Force and [Ca(2+)]in measurements in isolated papillary muscles showed that the increased force and twitch kinetics because increased pacing or β1-adrenergic stimulation were nearly absent in cMyBP-C(t3SA) myocardium, even though [Ca(2+)]in transients under each condition were similar to WT. Biochemical measurements confirmed that protein kinase A phosphorylated ser273, ser282, and ser302 in WT cMyBP-C. In contrast, CaMKIIδ, which is activated by increased pacing, phosphorylated ser302 principally, ser282 to a lesser degree, and ser273 not at all. CONCLUSIONS Phosphorylation of cMyBP-C increases the force and kinetics of twitches in living cardiac muscle. Further, cMyBP-C is a principal mediator of increased contractility observed with β-adrenergic stimulation or increased pacing because of protein kinase A and CaMKIIδ phosphorylations of cMyB-C.
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Affiliation(s)
- Carl W Tong
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Xin Wu
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Yang Liu
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Paola C Rosas
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Sakthivel Sadayappan
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Andy Hudmon
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Mariappan Muthuchamy
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Patricia A Powers
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Héctor H Valdivia
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.)
| | - Richard L Moss
- From the Department of Cell and Regenerative Biology, University of Wisconsin School of Medicine and Public Health, Madison (C.W.T., P.A.P., R.L.M.); Department of Medical Physiology (C.W.T., Y.L., P.C.R., M.M.) and Neuroscience and Experimental Therapeutics (X.W.), Texas A&M University Health Science Center College of Medicine, Temple; Baylor Scott & White Health, Temple, TX (C.W.T.); Department of Physiology, Loyola University Chicago Stritch School of Medicine, IL (S.S.); Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis (A.H.); and Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor (H.H.V.).
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Desai R, Geeves MA, Kad NM. Using fluorescent myosin to directly visualize cooperative activation of thin filaments. J Biol Chem 2014; 290:1915-25. [PMID: 25429108 PMCID: PMC4303648 DOI: 10.1074/jbc.m114.609743] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
Contraction of striated muscle is tightly regulated by the release and sequestration of calcium within myocytes. At the molecular level, calcium modulates myosin's access to the thin filament. Once bound, myosin is hypothesized to potentiate the binding of further myosins. Here, we directly image single molecules of myosin binding to and activating thin filaments. Using this approach, the cooperative binding of myosin along thin filaments has been quantified. We have found that two myosin heads are required to laterally activate a regulatory unit of thin filament. The regulatory unit is found to be capable of accommodating 11 additional myosins. Three thin filament activation states possessing differential myosin binding capacities are also visible. To describe this system, we have formulated a simple chemical kinetic model of cooperative activation that holds across a wide range of solution conditions. The stochastic nature of activation is strongly highlighted by data obtained in sub-optimal activation conditions where the generation of activation waves and their catastrophic collapse can be observed. This suggests that the thin filament has the potential to be turned fully on or off in a binary fashion.
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Affiliation(s)
- Rama Desai
- From the School of Biosciences, University of Kent, Canterbury CT2 7NH, United Kingdom
| | - Michael A Geeves
- From the School of Biosciences, University of Kent, Canterbury CT2 7NH, United Kingdom
| | - Neil M Kad
- From the School of Biosciences, University of Kent, Canterbury CT2 7NH, United Kingdom
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Walcott S. Muscle activation described with a differential equation model for large ensembles of locally coupled molecular motors. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 90:042717. [PMID: 25375533 DOI: 10.1103/physreve.90.042717] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/27/2014] [Indexed: 06/04/2023]
Abstract
Molecular motors, by turning chemical energy into mechanical work, are responsible for active cellular processes. Often groups of these motors work together to perform their biological role. Motors in an ensemble are coupled and exhibit complex emergent behavior. Although large motor ensembles can be modeled with partial differential equations (PDEs) by assuming that molecules function independently of their neighbors, this assumption is violated when motors are coupled locally. It is therefore unclear how to describe the ensemble behavior of the locally coupled motors responsible for biological processes such as calcium-dependent skeletal muscle activation. Here we develop a theory to describe locally coupled motor ensembles and apply the theory to skeletal muscle activation. The central idea is that a muscle filament can be divided into two phases: an active and an inactive phase. Dynamic changes in the relative size of these phases are described by a set of linear ordinary differential equations (ODEs). As the dynamics of the active phase are described by PDEs, muscle activation is governed by a set of coupled ODEs and PDEs, building on previous PDE models. With comparison to Monte Carlo simulations, we demonstrate that the theory captures the behavior of locally coupled ensembles. The theory also plausibly describes and predicts muscle experiments from molecular to whole muscle scales, suggesting that a micro- to macroscale muscle model is within reach.
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Affiliation(s)
- Sam Walcott
- Department of Mathematics, University of California, Davis, Davis, California 95616, USA
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Olivoto RR, Damiani CEN, Kassouf Silva I, Lofrano-Alves MS, Oliveira MA, Fogaça RTH. Effects of eugenol on resting tension of rat atria. ACTA ACUST UNITED AC 2014; 47:328-33. [PMID: 24676474 PMCID: PMC4075297 DOI: 10.1590/1414-431x20143472] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2013] [Accepted: 02/03/2014] [Indexed: 12/04/2022]
Abstract
In cardiac and skeletal muscle, eugenol (μM range) blocks excitation-contraction
coupling. In skeletal muscle, however, larger doses of eugenol (mM range) induce
calcium release from the sarcoplasmic reticulum. The effects of eugenol are therefore
dependent on its concentration. In this study, we evaluated the effects of eugenol on
the contractility of isolated, quiescent atrial trabeculae from male Wistar rats
(250-300 g; n=131) and measured atrial ATP content. Eugenol (1, 3, 5, 7, and 10 mM)
increased resting tension in a dose-dependent manner. Ryanodine [100 µM; a specific
ryanodine receptor (RyR) blocker] and procaine (30 mM; a nonspecific RyR blocker) did
not block the increased resting tension induced by eugenol regardless of whether
extracellular calcium was present. The myosin-specific inhibitor 2,3-butanedione
monoxime (BDM), however, reversed the increase in resting tension induced by eugenol.
In Triton-skinned atrial trabeculae, in which all membranes were solubilized, eugenol
did not change resting tension, maximum force produced, or the force
vs pCa relationship (pCa=-log [Ca2+]). Given that
eugenol reduced ATP concentration, the increase in resting tension observed in this
study may have resulted from cooperative activation of cardiac thin filaments by
strongly attached cross-bridges (rigor state).
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Affiliation(s)
- R R Olivoto
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - C E N Damiani
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - I Kassouf Silva
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - M S Lofrano-Alves
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - M A Oliveira
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - R T H Fogaça
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
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Batters C, Veigel C, Homsher E, Sellers JR. To understand muscle you must take it apart. Front Physiol 2014; 5:90. [PMID: 24653704 PMCID: PMC3949407 DOI: 10.3389/fphys.2014.00090] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2013] [Accepted: 02/16/2014] [Indexed: 01/08/2023] Open
Abstract
Striated muscle is an elegant system for study at many levels. Much has been learned about the mechanism of contraction from studying the mechanical properties of intact and permeabilized (or skinned) muscle fibers. Structural studies using electron microscopy, X-ray diffraction or spectroscopic probes attached to various contractile proteins were possible because of the highly ordered sarcomeric arrangement of actin and myosin. However, to understand the mechanism of force generation at a molecular level, it is necessary to take the system apart and study the interaction of myosin with actin using in vitro assays. This reductionist approach has lead to many fundamental insights into how myosin powers muscle contraction. In addition, nature has provided scientists with an array of muscles with different mechanical properties and with a superfamily of myosin molecules. Taking advantage of this diversity in myosin structure and function has lead to additional insights into common properties of force generation. This review will highlight the development of the major assays and methods that have allowed this combined reductionist and comparative approach to be so fruitful. This review highlights the history of biochemical and biophysical studies of myosin and demonstrates how a broad comparative approach combined with reductionist studies have led to a detailed understanding of how myosin interacts with actin and uses chemical energy to generate force and movement in muscle contraction and motility in general.
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Affiliation(s)
- Christopher Batters
- Department of Cellular Physiology and Centre for Nanosciences (CeNS), Ludwig-Maximilians-Universität München München, Germany
| | - Claudia Veigel
- Department of Cellular Physiology and Centre for Nanosciences (CeNS), Ludwig-Maximilians-Universität München München, Germany
| | - Earl Homsher
- Physiology Department, University of California Los Angeles Los Angeles, CA, USA
| | - James R Sellers
- Laboratory of Molecular Physiology, National Heart, Lung and Blood Institute, National Institutes of Health Bethesda, MD, USA
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Kaya M, Higuchi H. Stiffness, working stroke, and force of single-myosin molecules in skeletal muscle: elucidation of these mechanical properties via nonlinear elasticity evaluation. Cell Mol Life Sci 2013; 70:4275-92. [PMID: 23685901 PMCID: PMC11113998 DOI: 10.1007/s00018-013-1353-x] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2012] [Revised: 02/27/2013] [Accepted: 04/25/2013] [Indexed: 10/26/2022]
Abstract
In muscles, the arrays of skeletal myosin molecules interact with actin filaments and continuously generate force at various contraction speeds. Therefore, it is crucial for myosin molecules to generate force collectively and minimize the interference between individual myosin molecules. Knowledge of the elasticity of myosin molecules is crucial for understanding the molecular mechanisms of muscle contractions because elasticity directly affects the working and drag (resistance) force generation when myosin molecules are positively or negatively strained. The working stroke distance is also an important mechanical property necessary for elucidation of the thermodynamic efficiency of muscle contractions at the molecular level. In this review, we focus on these mechanical properties obtained from single-fiber and single-molecule studies and discuss recent findings associated with these mechanical properties. We also discuss the potential molecular mechanisms associated with reduction of the drag effect caused by negatively strained myosin molecules.
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Affiliation(s)
- Motoshi Kaya
- Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan,
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40
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Webb M, Jackson DR, Stewart TJ, Dugan SP, Carter MS, Cremo CR, Baker JE. The myosin duty ratio tunes the calcium sensitivity and cooperative activation of the thin filament. Biochemistry 2013; 52:6437-44. [PMID: 23947752 DOI: 10.1021/bi400262h] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
In striated muscle, calcium binding to the thin filament (TF) regulatory complex activates actin-myosin ATPase activity, and actin-myosin kinetics in turn regulates TF activation. However, a quantitative description of the effects of actin-myosin kinetics on the calcium sensitivity (pCa50) and cooperativity (nH) of TF activation is lacking. With the assumption that TF structural transitions and TF-myosin binding transitions are inextricably coupled, we advanced the principles established by Kad et al. [Kad, N., et al. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 16990-16995] and Sich et al. [Sich, N. M., et al. (2011) J. Biol. Chem. 285, 39150-39159] to develop a simple model of TF regulation, which predicts that pCa50 varies linearly with duty ratio and that nH is maximal near physiological duty ratios. Using in vitro motility to determine the calcium sensitivity of TF sliding velocities, we measured pCa50 and nH at different myosin densities and in the presence of ATPase inhibitors. The observed effects of myosin density and actin-myosin duty ratio on pCa50 and nH are consistent with our model predictions. In striated muscle, pCa50 must match cytosolic calcium concentrations and a maximal nH optimizes calcium responsiveness. Our results indicate that pCa50 and nH can be predictably tuned through TF-myosin ATPase kinetics and that drugs and disease states that alter ATPase kinetics can, through their effects on calcium sensitivity, alter the efficiency of muscle contraction.
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Affiliation(s)
- Milad Webb
- Department of Electrical and Biomedical Engineering, University of Nevada, Reno , Reno, Nevada 89557, United States
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41
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Kobayashi M, Debold EP, Turner MA, Kobayashi T. Cardiac muscle activation blunted by a mutation to the regulatory component, troponin T. J Biol Chem 2013; 288:26335-26349. [PMID: 23897817 DOI: 10.1074/jbc.m113.494096] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023] Open
Abstract
The striated muscle thin filament comprises actin, tropomyosin, and troponin. The Tn complex consists of three subunits, troponin C (TnC), troponin I (TnI), and troponin T (TnT). TnT may serve as a bridge between the Ca(2+) sensor (TnC) and the actin filament. In the short helix preceding the IT-arm region, H1(T2), there are known dilated cardiomyopathy-linked mutations (among them R205L). Thus we hypothesized that there is an element in this short helix that plays an important role in regulating the muscle contraction, especially in Ca(2+) activation. We mutated Arg-205 and several other amino acid residues within and near the H1(T2) helix. Utilizing an alanine replacement method to compare the effects of the mutations, the biochemical and mechanical impact on the actomyosin interaction was assessed by solution ATPase activity assay, an in vitro motility assay, and Ca(2+) binding measurements. Ca(2+) activation was markedly impaired by a point mutation of the highly conserved basic residue R205A, residing in the short helix H1(T2) of cTnT, whereas the mutations to nearby residues exhibited little effect on function. Interestingly, rigor activation was unchanged between the wild type and R205A TnT. In addition to the reduction in Ca(2+) sensitivity observed in Ca(2+) binding to the thin filament, myosin S1-ADP binding to the thin filament was significantly affected by the same mutation, which was also supported by a series of S1 concentration-dependent ATPase assays. These suggest that the R205A mutation alters function through reduction in the nature of cooperative binding of S1.
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Affiliation(s)
- Minae Kobayashi
- From the Department of Physiology and Biophysics, Center for Cardiovascular Research, University of Illinois at Chicago, Chicago, Illinois 60612 and.
| | - Edward P Debold
- the Department of Kinesiology, University of Massachusetts, Amherst, Massachusetts 01003
| | - Matthew A Turner
- the Department of Kinesiology, University of Massachusetts, Amherst, Massachusetts 01003
| | - Tomoyoshi Kobayashi
- From the Department of Physiology and Biophysics, Center for Cardiovascular Research, University of Illinois at Chicago, Chicago, Illinois 60612 and
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42
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Abstract
The cause of muscle fatigue has been studied for more than 100 yr, yet its molecular basis remains poorly understood. Prevailing theories suggest that much of the fatigue-induced loss in force and velocity can be attributed to the inhibitory action of metabolites, principally phosphate (Pi) and hydrogen ions (H, i.e., acidosis), on the contractile proteins, but the precise detail of how this inhibition occurs has been difficult to visualize at the molecular level. However, recent technological developments in the areas of biophysics, molecular biology, and structural biology are enabling researchers to directly observe the function and dysfunction of muscle contractile proteins at the level of a single molecule. In fact, the first direct evidence that high levels of H and Pi inhibit the function of muscle's molecular motor, myosin, has recently been observed in a single molecule laser trap assay. Likewise, advances in structural biology are taking our understanding further, providing detail at the atomic level of how some metabolites might alter the internal motions of myosin and thereby inhibit its ability to generate force and motion. Finally, new insights are also being gained into the indirect role that muscle regulatory proteins troponin (Tn) and tropomyosin (Tn) play in the fatigue process. In vitro studies, incorporating TnTm, suggest that a significant portion of the decreased force and motion during fatigue may be mediated through a disruption of the molecular motions of specific regions within Tn and Tm. These recent advances are providing unprecedented molecular insight into the structure and function of the contractile proteins and, in the process, are reshaping our understanding of the process of fatigue.
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43
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Elting MW, Spudich JA. Future challenges in single-molecule fluorescence and laser trap approaches to studies of molecular motors. Dev Cell 2013; 23:1084-91. [PMID: 23237942 DOI: 10.1016/j.devcel.2012.10.002] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Single-molecule analysis is a powerful modern form of biochemistry, in which individual kinetic steps of a catalytic cycle of an enzyme can be explored in exquisite detail. Both single-molecule fluorescence and single-molecule force techniques have been widely used to characterize a number of protein systems. We focus here on molecular motors as a paradigm. We describe two areas where we expect to see exciting developments in the near future: first, characterizing the coupling of force production to chemical and mechanical changes in motors, and second, understanding how multiple motors work together in the environment of the cell.
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44
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Kalganov A, Shalabi N, Zitouni N, Kachmar LH, Lauzon AM, Rassier DE. Forces measured with micro-fabricated cantilevers during actomyosin interactions produced by filaments containing different myosin isoforms and loop 1 structures. Biochim Biophys Acta Gen Subj 2012; 1830:2710-2719. [PMID: 23220701 DOI: 10.1016/j.bbagen.2012.11.022] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2012] [Revised: 11/23/2012] [Accepted: 11/26/2012] [Indexed: 10/27/2022]
Abstract
BACKGROUND There is evidence that the actin-activated ATP kinetics and the mechanical work produced by muscle myosin molecules are regulated by two surface loops, located near the ATP binding pocket (loop 1), and in a region that interfaces with actin (loop 2). These loops regulate force and velocity of contraction, and have been investigated mostly in single molecules. There is a lack of information of the work produced by myosin molecules ordered in filaments and working cooperatively, which is the actual muscle environment. METHODS We use micro-fabricated cantilevers to measure forces produced by myosin filaments isolated from mollusk muscles, skeletal muscles, and smooth muscles containing variations in the structure of loop 1 (tonic and phasic myosins). We complemented the experiments with in-vitro assays to measure the velocity of actin motility. RESULTS Smooth muscle myosin filaments produced more force than skeletal and mollusk myosin filaments when normalized per filament overlap. Skeletal muscle myosin propelled actin filaments in a higher sliding velocity than smooth muscle myosin. The values for force and velocity were consistent with previous studies using myosin molecules, and suggest a close correlation with the myosin isoform and structure of surface loop 1. GENERAL SIGNIFICANCE The technique using micro-fabricated cantilevers to measure force of filaments allows for the investigation of the relation between myosin structure and contractility, allowing experiments to be conducted with an array of different myosin isoforms. Using the technique we observed that the work produced by myosin molecules is regulated by amino-acid sequences aligned in specific loops.
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Affiliation(s)
- Albert Kalganov
- Department of Kinesiology and Physical Education, Faculty of Education, McGill University, Canada
| | - Nabil Shalabi
- Department of Mechanical Engineering, Faculty of Engineering McGill University, Canada
| | - Nedjma Zitouni
- Meakins-Christie Laboratories, McGill University, Canada; Department of Experimental Medicine, Faculty of Medicine McGill University, Canada
| | - Linda Hussein Kachmar
- Meakins-Christie Laboratories, McGill University, Canada; Department of Experimental Medicine, Faculty of Medicine McGill University, Canada
| | - Anne-Marie Lauzon
- Meakins-Christie Laboratories, McGill University, Canada; Department of Experimental Medicine, Faculty of Medicine McGill University, Canada; Departments of Physics, Faculty of Science, McGill University, Canada; Department of Physiology, Faculty of Medicine, McGill University, Canada
| | - Dilson E Rassier
- Department of Kinesiology and Physical Education, Faculty of Education, McGill University, Canada; Meakins-Christie Laboratories, McGill University, Canada; Department of Experimental Medicine, Faculty of Medicine McGill University, Canada; Departments of Physics, Faculty of Science, McGill University, Canada; Department of Physiology, Faculty of Medicine, McGill University, Canada.
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45
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A Differential Equation Model for Tropomyosin-induced Myosin Cooperativity Describes Myosin–Myosin Interactions at Low Calcium. Cell Mol Bioeng 2012. [DOI: 10.1007/s12195-012-0259-2] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
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46
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Debold EP, Longyear TJ, Turner MA. The effects of phosphate and acidosis on regulated thin-filament velocity in an in vitro motility assay. J Appl Physiol (1985) 2012; 113:1413-22. [DOI: 10.1152/japplphysiol.00775.2012] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Muscle fatigue from intense contractile activity is thought to result, in large part, from the accumulation of inorganic phosphate (Pi) and hydrogen ions (H+) acting to directly inhibit the function of the contractile proteins; however, the molecular basis of this process remain unclear. We used an in vitro motility assay and determined the effects of elevated H+ and Pi on the ability of myosin to bind to and translocate regulated actin filaments (RTF) to gain novel insights into the molecular basis of fatigue. At saturating Ca++, acidosis depressed regulated filament velocity ( VRTF) by ∼90% (6.2 ± 0.3 vs. 0.5 ± 0.2 μm/s at pH 7.4 and 6.5, respectively). However, the addition of 30 mM Pi caused VRTF to increase fivefold, from 0.5 ± 0.2 to 2.6 ± 0.3 μm/s at pH 6.5. Similarly, at all subsaturating Ca++ levels, acidosis slowed VRTF, but the addition of Pi significantly attenuated this effect. We also manipulated the [ADP] in addition to the [Pi] to probe which specific step(s) of cross-bridge cycle of myosin is affected by elevated H+. The findings are consistent with acidosis slowing the isomerization step between two actomyosin ADP-bound states. Because the state before this isomerization is most vulnerable to Pi rebinding, and the associated detachment from actin, this finding may also explain the Pi-induced enhancement of VRTF at low pH. These results therefore may provide a molecular basis for a significant portion of the loss of shortening velocity and possibly muscular power during fatigue.
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Affiliation(s)
- Edward P. Debold
- Department of Kinesiology, University of Massachusetts, Amherst, Massachusetts
| | - Thomas J. Longyear
- Department of Kinesiology, University of Massachusetts, Amherst, Massachusetts
| | - Matthew A. Turner
- Department of Kinesiology, University of Massachusetts, Amherst, Massachusetts
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47
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Regulation of actin-myosin interaction by conserved periodic sites of tropomyosin. Proc Natl Acad Sci U S A 2012; 109:18425-30. [PMID: 23091026 DOI: 10.1073/pnas.1212754109] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cooperative activation of actin-myosin interaction by tropomyosin (Tm) is central to regulation of contraction in muscle cells and cellular and intracellular movements in nonmuscle cells. The steric blocking model of muscle regulation proposed 40 y ago has been substantiated at both the kinetic and structural levels. Even with atomic resolution structures of the major players, how Tm binds and is designed for regulatory function has remained a mystery. Here we show that a set of periodically distributed evolutionarily conserved surface residues of Tm is required for cooperative regulation of actomyosin. Based on our results, we propose a model of Tm on a structure of actin-Tm-myosin in the "open" (on) state showing potential electrostatic interactions of the residues with both actin and myosin. The sites alternate with a second set of conserved surface residues that are important for actin binding in the inhibitory state in the absence of myosin. The transition from the closed to open states requires the sites identified here, even when troponin + Ca(2+) is present. The evolutionarily conserved residues are important for actomyosin regulation, a universal function of Tm that has a common structural basis and mechanism.
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48
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Walcott S, Warshaw DM, Debold EP. Mechanical coupling between myosin molecules causes differences between ensemble and single-molecule measurements. Biophys J 2012; 103:501-510. [PMID: 22947866 PMCID: PMC3414898 DOI: 10.1016/j.bpj.2012.06.031] [Citation(s) in RCA: 77] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2012] [Revised: 06/07/2012] [Accepted: 06/21/2012] [Indexed: 11/17/2022] Open
Abstract
In contracting muscle, individual myosin molecules function as part of a large ensemble, hydrolyzing ATP to power the relative sliding of actin filaments. The technological advances that have enabled direct observation and manipulation of single molecules, including recent experiments that have explored myosin's force-dependent properties, provide detailed insight into the kinetics of myosin's mechanochemical interaction with actin. However, it has been difficult to reconcile these single-molecule observations with the behavior of myosin in an ensemble. Here, using a combination of simulations and theory, we show that the kinetic mechanism derived from single-molecule experiments describes ensemble behavior; but the connection between single molecule and ensemble is complex. In particular, even in the absence of external force, internal forces generated between myosin molecules in a large ensemble accelerate ADP release and increase how far actin moves during a single myosin attachment. These myosin-induced changes in strong binding lifetime and attachment distance cause measurable properties, such as actin speed in the motility assay, to vary depending on the number of myosin molecules interacting with an actin filament. This ensemble-size effect challenges the simple detachment limited model of motility, because even when motility speed is limited by ADP release, increasing attachment rate can increase motility speed.
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Affiliation(s)
- Sam Walcott
- Department of Mathematics, University of California, Davis, California.
| | - David M Warshaw
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, Vermont
| | - Edward P Debold
- Department of Kinesiology, University of Massachusetts, Amherst, Massachusetts
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49
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Debold EP. Recent insights into muscle fatigue at the cross-bridge level. Front Physiol 2012; 3:151. [PMID: 22675303 PMCID: PMC3365633 DOI: 10.3389/fphys.2012.00151] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2012] [Accepted: 05/02/2012] [Indexed: 11/22/2022] Open
Abstract
The depression in force and/or velocity associated with muscular fatigue can be the result of a failure at any level, from the initial events in the motor cortex of the brain to the formation of an actomyosin cross-bridge in the muscle cell. Since all the force and motion generated by muscle ultimately derives from the cyclical interaction of actin and myosin, researchers have focused heavily on the impact of the accumulation of intracellular metabolites [e.g., P(i), H(+) and adenosine diphoshphate (ADP)] on the function these contractile proteins. At saturating Ca(++) levels, elevated P(i) appears to be the primary cause for the loss in maximal isometric force, while increased [H(+)] and possibly ADP act to slow unloaded shortening velocity in single muscle fibers, suggesting a causative role in muscular fatigue. However the precise mechanisms through which these metabolites might affect the individual function of the contractile proteins remain unclear because intact muscle is a highly complex structure. To simplify problem isolated actin and myosin have been studied in the in vitro motility assay and more recently the single molecule laser trap assay with the findings showing that both P(i) and H(+) alter single actomyosin function in unique ways. In addition to these new insights, we are also gaining important information about the roles played by the muscle regulatory proteins troponin (Tn) and tropomyosin (Tm) in the fatigue process. In vitro studies, suggest that both the acidosis and elevated levels of P(i) can inhibit velocity and force at sub-saturating levels of Ca(++) in the presence of Tn and Tm and that this inhibition can be greater than that observed in the absence of regulation. To understand the molecular basis of the role of regulatory proteins in the fatigue process researchers are taking advantage of modern molecular biological techniques to manipulate the structure and function of Tn/Tm. These efforts are beginning to reveal the relevant structures and how their functions might be altered during fatigue. Thus, it is a very exciting time to study muscle fatigue because the technological advances occurring in the fields of biophysics and molecular biology are providing researchers with the ability to directly test long held hypotheses and consequently reshaping our understanding of this age-old question.
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Affiliation(s)
- Edward P. Debold
- Department of Kinesiology, University of Massachusetts, AmherstMA, USA
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50
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Tanner BCW, Daniel TL, Regnier M. Filament compliance influences cooperative activation of thin filaments and the dynamics of force production in skeletal muscle. PLoS Comput Biol 2012; 8:e1002506. [PMID: 22589710 PMCID: PMC3349719 DOI: 10.1371/journal.pcbi.1002506] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2011] [Accepted: 03/17/2012] [Indexed: 12/03/2022] Open
Abstract
Striated muscle contraction is a highly cooperative process initiated by Ca2+ binding to the troponin complex, which leads to tropomyosin movement and myosin cross-bridge (XB) formation along thin filaments. Experimental and computational studies suggest skeletal muscle fiber activation is greatly augmented by cooperative interactions between neighboring thin filament regulatory units (RU-RU cooperativity; 1 RU = 7 actin monomers+1 troponin complex+1 tropomyosin molecule). XB binding can also amplify thin filament activation through interactions with RUs (XB-RU cooperativity). Because these interactions occur with a temporal order, they can be considered kinetic forms of cooperativity. Our previous spatially-explicit models illustrated that mechanical forms of cooperativity also exist, arising from XB-induced XB binding (XB-XB cooperativity). These mechanical and kinetic forms of cooperativity are likely coordinated during muscle contraction, but the relative contribution from each of these mechanisms is difficult to separate experimentally. To investigate these contributions we built a multi-filament model of the half sarcomere, allowing RU activation kinetics to vary with the state of neighboring RUs or XBs. Simulations suggest Ca2+ binding to troponin activates a thin filament distance spanning 9 to 11 actins and coupled RU-RU interactions dominate the cooperative force response in skeletal muscle, consistent with measurements from rabbit psoas fibers. XB binding was critical for stabilizing thin filament activation, particularly at submaximal Ca2+ levels, even though XB-RU cooperativity amplified force less than RU-RU cooperativity. Similar to previous studies, XB-XB cooperativity scaled inversely with lattice stiffness, leading to slower rates of force development as stiffness decreased. Including RU-RU and XB-RU cooperativity in this model resulted in the novel prediction that the force-[Ca2+] relationship can vary due to filament and XB compliance. Simulations also suggest kinetic forms of cooperativity occur rapidly and dominate early to get activation, while mechanical forms of cooperativity act more slowly, augmenting XB binding as force continues to develop. In striated muscle myosin binds to actin and converts chemical energy from ATP hydrolysis into force, work, and power. Myosin cross-bridge binding is regulated by Ca2+ and the thin filament proteins troponin and tropomyosin. Cooperative interactions between actin, myosin, troponin, and tropomyosin greatly influence spatial and kinetic properties of thin filament activation, thereby affecting muscle mechanics and contractility. Such cooperative interactions are complex and individual contributions from the different contractile and regulatory proteins are difficult to separate experimentally. However, a few theoretical models have explored interactions between the spatial, kinetic, and mechanical processes that affect cooperative cross-bridge binding to actin. Building on our prior spatially-explicit computational models, we investigated the relative contributions of thin filament regulatory proteins and cross-bridges to cooperatively amplify skeletal muscle force production. We find that Ca2+-dependent contraction in skeletal muscle is dominated by neighboring regulatory protein interactions along the thin filament, while cross-bridge binding is critical for maintaining or stabilizing thin filament activation as force develops. Moreover, we reveal that variations in filament and cross-bridge stiffness can alter Ca2+-sensitivity and cooperativity of skeletal muscle force production. In conclusion, these simulations show that multiple cooperative mechanisms combine to produce physiological force responses measured from muscle cells.
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Affiliation(s)
- Bertrand C W Tanner
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, Vermont, United States of America.
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