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Caremani M, Fusi L, Reconditi M, Piazzesi G, Narayanan T, Irving M, Lombardi V, Linari M, Brunello E. Dependence of myosin filament structure on intracellular calcium concentration in skeletal muscle. J Gen Physiol 2023; 155:e202313393. [PMID: 37756601 PMCID: PMC10533363 DOI: 10.1085/jgp.202313393] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2023] [Revised: 07/15/2023] [Accepted: 09/05/2023] [Indexed: 09/29/2023] Open
Abstract
Contraction of skeletal muscle is triggered by an increase in intracellular calcium concentration that relieves the structural block on actin-binding sites in resting muscle, potentially allowing myosin motors to bind and generate force. However, most myosin motors are not available for actin binding because they are stabilized in folded helical tracks on the surface of myosin-containing thick filaments. High-force contraction depends on the release of the folded motors, which can be triggered by stress in the thick filament backbone, but additional mechanisms may link the activation of the thick filaments to that of the thin filaments or to intracellular calcium concentration. Here, we used x-ray diffraction in combination with temperature-jump activation to determine the steady-state calcium dependence of thick filament structure and myosin motor conformation in near-physiological conditions. We found that x-ray signals associated with the perpendicular motors characteristic of isometric force generation had almost the same calcium sensitivity as force, but x-ray signals associated with perturbations in the folded myosin helix had a much higher calcium sensitivity. Moreover, a new population of myosin motors with a longer axial periodicity became prominent at low levels of calcium activation and may represent an intermediate regulatory state of the myosin motors in the physiological pathway of filament activation.
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Affiliation(s)
| | - Luca Fusi
- Randall Centre for Cell and Molecular Biophysics and British Heart Foundation Centre of Research Excellence, King’s College London, London, UK
- Centre for Human and Applied Physiological Sciences, King’s College London, London, UK
| | - Massimo Reconditi
- PhysioLab, University of Florence, Florence, Italy
- Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Florence, Italy
| | | | | | - Malcolm Irving
- Randall Centre for Cell and Molecular Biophysics and British Heart Foundation Centre of Research Excellence, King’s College London, London, UK
| | | | - Marco Linari
- PhysioLab, University of Florence, Florence, Italy
- Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Florence, Italy
| | - Elisabetta Brunello
- Randall Centre for Cell and Molecular Biophysics and British Heart Foundation Centre of Research Excellence, King’s College London, London, UK
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2
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Hill C, Brunello E, Fusi L, Ovejero JG, Irving M. Activation of the myosin motors in fast-twitch muscle of the mouse is controlled by mechano-sensing in the myosin filaments. J Physiol 2022; 600:3983-4000. [PMID: 35912434 PMCID: PMC9544795 DOI: 10.1113/jp283048] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2022] [Accepted: 07/27/2022] [Indexed: 11/08/2022] Open
Abstract
Myosin motors in resting muscle are inactivated by folding against the backbone of the myosin filament in an ordered helical array and must be released from that conformation to engage in force generation. Time-resolved X-ray diffraction from single fibres of amphibian muscle showed that myosin filament activation could be inhibited by imposing unloaded shortening at the start of stimulation, suggesting that filaments were activated by mechanical stress. Here we improved the signal-to-noise ratio of that approach using whole extensor digitorum longus muscles of the mouse contracting tetanically at 28°C. Changes in X-ray signals associated with myosin filament activation, including the decrease in the first-order myosin layer line associated with the helical motor array, increase in the spacing of a myosin-based reflection associated with packing of myosin tails in the filament backbone, and increase in the ratio of the 1,1 and 1,0 equatorial reflections associated with movement of motors away from the backbone, were delayed by imposing 10-ms unloaded shortening at the start of stimulation. These results show that myosin filaments are predominantly activated by filament stress, as in amphibian muscle. However, a small component of filament activation at zero load was detected, implying an independent mechanism of partial filament activation. X-ray interference measurements indicated a switch-like change in myosin motor conformation at the start of force development, accompanied by transient disordering of motors in the regions of the myosin filament near its midpoint, suggesting that filament zonal dynamics also play a role in its activation. KEY POINTS: Activation of myosin filaments in extensor digitorum longus muscles of the mouse is delayed by imposing rapid shortening from the start of stimulation. Stress is the major mechanism of myosin filament activation in these muscles, but there is a small component of filament activation during electrical stimulation at zero stress. Myosin motors switch rapidly from the folded inhibited conformation to the actin-attached force-generating conformation early in force development.
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Affiliation(s)
- Cameron Hill
- Randall Centre for Cell & Molecular Biophysics, King's College London, London, UK
| | - Elisabetta Brunello
- Randall Centre for Cell & Molecular Biophysics, King's College London, London, UK
| | - Luca Fusi
- Randall Centre for Cell & Molecular Biophysics, King's College London, London, UK.,Centre for Human & Applied Physiological Sciences, King's College London, London, UK
| | - Jesús Garcia Ovejero
- Randall Centre for Cell & Molecular Biophysics, King's College London, London, UK
| | - Malcolm Irving
- Randall Centre for Cell & Molecular Biophysics, King's College London, London, UK
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3
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Rockenfeller R, Günther M, Hooper SL. Muscle active force-length curve explained by an electrophysical model of interfilament spacing. Biophys J 2022; 121:1823-1855. [PMID: 35450825 PMCID: PMC9199101 DOI: 10.1016/j.bpj.2022.04.019] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2021] [Revised: 11/02/2021] [Accepted: 04/14/2022] [Indexed: 11/20/2022] Open
Abstract
The active isometric force-length relation (FLR) of striated muscle sarcomeres is central to understanding and modeling muscle function. The mechanistic basis of the descending arm of the FLR is well explained by the decreasing thin:thick filament overlap that occurs at long sarcomere lengths. The mechanistic basis of the ascending arm of the FLR (the decrease in force that occurs at short sarcomere lengths), alternatively, has never been well explained. Because muscle is a constant-volume system, interfilament lattice distances must increase as sarcomere length shortens. This increase would decrease thin and thick-filament electrostatic interactions independently of thin:thick filament overlap. To examine this effect, we present here a fundamental, physics-based model of the sarcomere that includes filament molecular properties, calcium binding, sarcomere geometry including both thin:thick filament overlap and interfilament radial distance, and electrostatics. The model gives extremely good fits to existing FLR data from a large number of different muscles across their entire range of measured activity levels, with the optimized parameter values in all cases lying within anatomically and physically reasonable ranges. A local first-order sensitivity analysis (varying individual parameters while holding the values of all others constant) shows that model output is most sensitive to a subset of model parameters, most of which are related to sarcomere geometry, with model output being most sensitive to interfilament radial distance. This conclusion is supported by re-running the fits with only this parameter subset being allowed to vary, which increases fit errors only moderately. These results show that the model well reproduces existing experimental data, and indicate that changes in interfilament spacing play as central a role as changes in filament overlap in determining the FLR, particularly on its ascending arm.
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Affiliation(s)
| | - Michael Günther
- Biomechanics and Biorobotics, Stuttgart Center for Simulation Sciences (SC SimTech), Universität Stuttgart, Stuttgart, Germany; Friedrich-Schiller-Universität, Jena, Germany
| | - Scott L Hooper
- Neuroscience Program, Department of Biological Sciences, Ohio University, Athens, Ohio
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4
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Ma W, Irving TC. Small Angle X-ray Diffraction as a Tool for Structural Characterization of Muscle Disease. Int J Mol Sci 2022; 23:3052. [PMID: 35328477 PMCID: PMC8949570 DOI: 10.3390/ijms23063052] [Citation(s) in RCA: 32] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2022] [Revised: 03/01/2022] [Accepted: 03/02/2022] [Indexed: 02/01/2023] Open
Abstract
Small angle X-ray fiber diffraction is the method of choice for obtaining molecular level structural information from striated muscle fibers under hydrated physiological conditions. For many decades this technique had been used primarily for investigating basic biophysical questions regarding muscle contraction and regulation and its use confined to a relatively small group of expert practitioners. Over the last 20 years, however, X-ray diffraction has emerged as an important tool for investigating the structural consequences of cardiac and skeletal myopathies. In this review we show how simple and straightforward measurements, accessible to non-experts, can be used to extract biophysical parameters that can help explain and characterize the physiology and pathology of a given experimental system. We provide a comprehensive guide to the range of the kinds of measurements that can be made and illustrate how they have been used to provide insights into the structural basis of pathology in a comprehensive review of the literature. We also show how these kinds of measurements can inform current controversies and indicate some future directions.
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Affiliation(s)
- Weikang Ma
- The Biophysics Collaborative Access Team (BioCAT), Center for Synchrotron Radiation Research and Instrumentation (CSSRI), Illinois Institute of Technology, Chicago, IL 60616, USA;
- Department of Biology, Illinois Institute of Technology, Chicago, IL 60616, USA
| | - Thomas C. Irving
- The Biophysics Collaborative Access Team (BioCAT), Center for Synchrotron Radiation Research and Instrumentation (CSSRI), Illinois Institute of Technology, Chicago, IL 60616, USA;
- Department of Biology, Illinois Institute of Technology, Chicago, IL 60616, USA
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5
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Reconditi M, Brunello E, Fusi L, Linari M, Lombardi V, Irving M, Piazzesi G. Myosin motors that cannot bind actin leave their folded OFF state on activation of skeletal muscle. J Gen Physiol 2021; 153:212712. [PMID: 34668926 PMCID: PMC8532561 DOI: 10.1085/jgp.202112896] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
The myosin motors in resting skeletal muscle are folded back against their tails in the thick filament in a conformation that makes them unavailable for binding to actin. When muscles are activated, calcium binding to troponin leads to a rapid change in the structure of the actin-containing thin filaments that uncovers the myosin binding sites on actin. Almost as quickly, myosin motors leave the folded state and move away from the surface of the thick filament. To test whether motor unfolding is triggered by the availability of nearby actin binding sites, we measured changes in the x-ray reflections that report motor conformation when muscles are activated at longer sarcomere length, so that part of the thick filaments no longer overlaps with thin filaments. We found that the intensity of the M3 reflection from the axial repeat of the motors along the thick filaments declines almost linearly with increasing sarcomere length up to 2.8 µm, as expected if motors in the nonoverlap zone had left the folded state and become relatively disordered. In a recent article in JGP, Squire and Knupp challenged this interpretation of the data. We show here that their analysis is based on an incorrect assumption about how the interference subpeaks of the M3 reflection were reported in our previous paper. We extend previous models of mass distribution along the filaments to show that the sarcomere length dependence of the M3 reflection is consistent with <10% of no-overlap motors remaining in the folded conformation during active contraction, confirming our previous conclusion that unfolding of myosin motors on muscle activation is not due to the availability of local actin binding sites.
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Affiliation(s)
- Massimo Reconditi
- PhysioLab, Università di Firenze, Sesto Fiorentino, Italy.,Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Unità di Ricerca Università di Firenze, Florence, Italy
| | - Elisabetta Brunello
- Randall Centre for Cell and Molecular Biophysics, King's College London, London, UK
| | - Luca Fusi
- Randall Centre for Cell and Molecular Biophysics, King's College London, London, UK
| | - Marco Linari
- PhysioLab, Università di Firenze, Sesto Fiorentino, Italy
| | | | - Malcolm Irving
- Randall Centre for Cell and Molecular Biophysics, King's College London, London, UK
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6
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Effect of Active Lengthening and Shortening on Small-Angle X-ray Reflections in Skinned Skeletal Muscle Fibres. Int J Mol Sci 2021; 22:ijms22168526. [PMID: 34445232 PMCID: PMC8395229 DOI: 10.3390/ijms22168526] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2021] [Revised: 07/30/2021] [Accepted: 08/03/2021] [Indexed: 01/04/2023] Open
Abstract
Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I1,1/I1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I1,1/I1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.
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7
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Squire JM, Knupp C. Analysis methods and quality criteria for investigating muscle physiology using x-ray diffraction. J Gen Physiol 2021; 153:212538. [PMID: 34351359 PMCID: PMC8348228 DOI: 10.1085/jgp.202012778] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2020] [Revised: 12/23/2020] [Accepted: 07/12/2021] [Indexed: 12/20/2022] Open
Abstract
X-ray diffraction studies of muscle have been tremendously powerful in providing fundamental insights into the structures of, for example, the myosin and actin filaments in a variety of muscles and the physiology of the cross-bridge mechanism during the contractile cycle. However, interpretation of x-ray diffraction patterns is far from trivial, and if modeling of the observed diffraction intensities is required it needs to be performed carefully with full knowledge of the possible pitfalls. Here, we discuss (1) how x-ray diffraction can be used as a tool to monitor various specific muscle properties and (2) how to get the most out of the rest of the observed muscle x-ray diffraction patterns by modeling where the reliability of the modeling conclusions can be objectively tested. In other x-ray diffraction methods, such as protein crystallography, the reliability of every step of the process is estimated and quoted in published papers. In this way, the quality of the structure determination can be properly assessed. To be honest with ourselves in the muscle field, we need to do as near to the same as we can, within the limitations of the techniques that we are using. We discuss how this can be done. We also use test cases to reveal the dos and don’ts of using x-ray diffraction to study muscle physiology.
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Affiliation(s)
- John M Squire
- Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, UK.,Faculty of Medicine, Imperial College, London, UK
| | - Carlo Knupp
- School of Optometry and Vision Sciences, Cardiff University, Cardiff, UK
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8
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Hill C, Brunello E, Fusi L, Ovejero JG, Irving M. Myosin-based regulation of twitch and tetanic contractions in mammalian skeletal muscle. eLife 2021; 10:e68211. [PMID: 34121660 PMCID: PMC8275128 DOI: 10.7554/elife.68211] [Citation(s) in RCA: 21] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2021] [Accepted: 06/11/2021] [Indexed: 01/16/2023] Open
Abstract
Time-resolved X-ray diffraction of isolated fast-twitch muscles of mice was used to show how structural changes in the myosin-containing thick filaments contribute to the regulation of muscle contraction, extending the previous focus on regulation by the actin-containing thin filaments. This study shows that muscle activation involves the following sequence of structural changes: thin filament activation, disruption of the helical array of myosin motors characteristic of resting muscle, release of myosin motor domains from the folded conformation on the filament backbone, and actin attachment. Physiological force generation in the 'twitch' response of skeletal muscle to single action potential stimulation is limited by incomplete activation of the thick filament and the rapid inactivation of both filaments. Muscle relaxation after repetitive stimulation is accompanied by a complete recovery of the folded motor conformation on the filament backbone but by incomplete reformation of the helical array, revealing a structural basis for post-tetanic potentiation in isolated muscles.
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Affiliation(s)
- Cameron Hill
- Randall Centre for Cell & Molecular Biophysics, New Hunt’s House, Guy’s Campus, King’s College LondonLondonUnited Kingdom
| | - Elisabetta Brunello
- Randall Centre for Cell & Molecular Biophysics, New Hunt’s House, Guy’s Campus, King’s College LondonLondonUnited Kingdom
| | - Luca Fusi
- Randall Centre for Cell & Molecular Biophysics, New Hunt’s House, Guy’s Campus, King’s College LondonLondonUnited Kingdom
| | - Jesús G Ovejero
- Randall Centre for Cell & Molecular Biophysics, New Hunt’s House, Guy’s Campus, King’s College LondonLondonUnited Kingdom
| | - Malcolm Irving
- Randall Centre for Cell & Molecular Biophysics, New Hunt’s House, Guy’s Campus, King’s College LondonLondonUnited Kingdom
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9
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Caremani M, Brunello E, Linari M, Fusi L, Irving TC, Gore D, Piazzesi G, Irving M, Lombardi V, Reconditi M. Low temperature traps myosin motors of mammalian muscle in a refractory state that prevents activation. J Gen Physiol 2019; 151:1272-1286. [PMID: 31554652 PMCID: PMC6829559 DOI: 10.1085/jgp.201912424] [Citation(s) in RCA: 39] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2019] [Accepted: 08/29/2019] [Indexed: 12/12/2022] Open
Abstract
The active force of mammalian skeletal muscle is reduced at low temperatures. Caremani et al. reveal that this is due to the rise of a population of myosin motors captured in a refractory state insensitive to muscle activation. Myosin motors in the thick filament of resting striated (skeletal and cardiac) muscle are trapped in an OFF state, in which the motors are packed in helical tracks on the filament surface, inhibiting their interactions with actin and utilization of ATP. To investigate the structural changes induced in the thick filament of mammalian skeletal muscle by changes in temperature, we collected x-ray diffraction patterns from the fast skeletal muscle extensor digitorum longus of the mouse in the temperature range from near physiological (35°C) to 10°C, in which the maximal isometric force (T0) shows a threefold decrease. In resting muscle, x-ray reflections signaling the OFF state of the thick filament indicate that cooling produces a progressive disruption of the OFF state with motors moving away from the ordered helical tracks on the surface of the thick filament. We find that the number of myosin motors in the OFF state at 10°C is half of that at 35°C. At T0, changes in the x-ray signals that report the fraction and conformation of actin-attached motors can be explained if the threefold decrease in force associated with lowering temperature is due not only to a decrease in the force-generating transition in the actin-attached motors but also to a twofold decrease in the number of such motors. Thus, lowering the temperature reduces to the same extent the fraction of motors in the OFF state at rest and the fraction of motors attached to actin at T0, suggesting that motors that leave the OFF state accumulate in a disordered refractory state that makes them unavailable for interaction with actin upon stimulation. This regulatory effect of temperature on the thick filament of mammalian skeletal muscle could represent an energetically convenient mechanism for hibernating animals.
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Affiliation(s)
| | | | - Marco Linari
- PhysioLab, University of Florence, Florence, Italy.,Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Firenze, Italy
| | - Luca Fusi
- Randall Centre for Cell and Molecular Biophysics, King's College London, London, UK
| | - Thomas C Irving
- Center for Synchrotron Radiation Research and Instrumentation and Department of Biological Sciences, Illinois Institute of Technology, Chicago, IL
| | - David Gore
- Center for Synchrotron Radiation Research and Instrumentation and Department of Biological Sciences, Illinois Institute of Technology, Chicago, IL
| | | | - Malcolm Irving
- Randall Centre for Cell and Molecular Biophysics, King's College London, London, UK
| | | | - Massimo Reconditi
- PhysioLab, University of Florence, Florence, Italy.,Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Firenze, Italy
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10
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Eakins F, Knupp C, Squire JM. Monitoring the myosin crossbridge cycle in contracting muscle: steps towards 'Muscle-the Movie'. J Muscle Res Cell Motil 2019; 40:77-91. [PMID: 31327096 PMCID: PMC6726672 DOI: 10.1007/s10974-019-09543-9] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2019] [Accepted: 07/10/2019] [Indexed: 11/20/2022]
Abstract
Some vertebrate muscles (e.g. those in bony fish) have a simple lattice A-band which is so well ordered that low-angle X-ray diffraction patterns are sampled in a simple way amenable to crystallographic techniques. Time-resolved X-ray diffraction through the contractile cycle should provide a movie of the molecular movements involved in muscle contraction. Generation of 'Muscle-The Movie' was suggested in the 1990s and since then efforts have been made to work out how to achieve it. Here we discuss how a movie can be generated, we discuss the problems and opportunities, and present some new observations. Low angle X-ray diffraction patterns from bony fish muscles show myosin layer lines that are well sampled on row-lines expected from the simple hexagonal A-band lattice. The 1st, 2nd and 3rd myosin layer lines at d-spacings of around 42.9 nm, 21.5 nm and 14.3 nm respectively, get weaker in patterns from active muscle, but there is a well-sampled intensity remnant along the layer lines. We show here that the pattern from the tetanus plateau is not a residual resting pattern from fibres that have not been fully activated, but is a different well-sampled pattern showing the presence of a second, myosin-centred, arrangement of crossbridges within the active crossbridge population. We also show that the meridional M3 peak from active muscle has two components of different radial widths consistent with (i) active myosin-centred (probably weak-binding) heads giving a narrow peak and (ii) heads on actin in strong states giving a broad peak.
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Affiliation(s)
- Felicity Eakins
- Faculty of Medicine, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
| | - Carlo Knupp
- School of Optometry and Vision Sciences, Cardiff University, Cardiff, CF10 3NB, UK
| | - John M Squire
- Faculty of Medicine, Imperial College London, Exhibition Road, London, SW7 2AZ, UK.
- Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, BS8 1TD, UK.
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11
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The basic mechanical structure of the skeletal muscle machinery: One model for linking microscopic and macroscopic scales. J Theor Biol 2018; 456:137-167. [DOI: 10.1016/j.jtbi.2018.07.023] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2017] [Revised: 06/18/2018] [Accepted: 07/19/2018] [Indexed: 11/19/2022]
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12
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Percario V, Boncompagni S, Protasi F, Pertici I, Pinzauti F, Caremani M. Mechanical parameters of the molecular motor myosin II determined in permeabilised fibres from slow and fast skeletal muscles of the rabbit. J Physiol 2018; 596:1243-1257. [PMID: 29148051 DOI: 10.1113/jp275404] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2017] [Accepted: 11/10/2017] [Indexed: 12/31/2022] Open
Abstract
KEY POINTS The different performance of slow and fast muscles is mainly attributed to diversity of the myosin heavy chain (MHC) isoform expressed within them. In this study fast sarcomere-level mechanics has been applied to Ca2+ -activated single permeabilised fibres isolated from soleus (containing the slow myosin isoform) and psoas (containing the fast myosin isoform) muscles of rabbit for a comparative definition of the mechano-kinetics of force generation by slow and fast myosin isoforms in situ. The stiffness and the force of the slow myosin isoform are three times smaller than those of the fast isoform, suggesting that the stiffness of the myosin motor is a determinant of the isoform-dependent functional diversity between skeletal muscles. These results open the question of the mechanism that can reconcile the reduced performance of the slow MHC with the higher efficiency of the slow muscle. ABSTRACT The skeletal muscle exhibits large functional differences depending on the myosin heavy chain (MHC) isoform expressed in its molecular motor, myosin II. The differences in the mechanical features of force generation by myosin isoforms were investigated in situ by using fast sarcomere-level mechanical methods in permeabilised fibres (sarcomere length 2.4 μm, temperature 12°C, 4% dextran T-500) from slow (soleus, containing the MHC-1 isoform) and fast (psoas, containing the MHC-2X isoform) skeletal muscle of the rabbit. The stiffness of the half-sarcomere was determined at the plateau of Ca2+ -activated isometric contractions and in rigor and analysed with a model that accounted for the filament compliance to estimate the stiffness of the myosin motor (ε). ε was 0.56 ± 0.04 and 1.70 ± 0.37 pN nm-1 for the slow and fast isoform, respectively, while the average strain per attached motor (s0 ) was similar (∼3.3 nm) in both isoforms. Consequently the force per motor (F0 = εs0 ) was three times smaller in the slow isoform than in the fast isoform (1.89 ± 0.43 versus 5.35 ± 1.51 pN). The fraction of actin-attached motors responsible for maximum isometric force at saturating Ca2+ (T0,4.5 ) was 0.47 ± 0.09 in soleus fibres, 70% larger than that in psoas fibres (0.29 ± 0.08), so that F0 in slow fibres was decreased by only 53%. The lower stiffness and force of the slow myosin isoform open the question of the molecular basis of the higher efficiency of slow muscle with respect to fast muscle.
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Affiliation(s)
- Valentina Percario
- PhysioLab, Department of Biology, University of Florence, Florence, Italy
| | - Simona Boncompagni
- CeSI-Met - Centre for Research on Ageing and Translational Medicine, University G. d'Annunzio, I-66100, Chieti, Italy.,DNICS - Department of Neuroscience, Imaging and Clinical Sciences, University G. d'Annunzio, I-66100, Chieti, Italy
| | - Feliciano Protasi
- CeSI-Met - Centre for Research on Ageing and Translational Medicine, University G. d'Annunzio, I-66100, Chieti, Italy.,DMSI - Department of Medicine and Aging Science, University G. d'Annunzio, I-66100, Chieti, Italy
| | - Irene Pertici
- PhysioLab, Department of Biology, University of Florence, Florence, Italy
| | - Francesca Pinzauti
- PhysioLab, Department of Biology, University of Florence, Florence, Italy
| | - Marco Caremani
- PhysioLab, Department of Biology, University of Florence, Florence, Italy
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13
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Irving M. Regulation of Contraction by the Thick Filaments in Skeletal Muscle. Biophys J 2017; 113:2579-2594. [PMID: 29262355 PMCID: PMC5770512 DOI: 10.1016/j.bpj.2017.09.037] [Citation(s) in RCA: 113] [Impact Index Per Article: 16.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2017] [Revised: 09/27/2017] [Accepted: 09/29/2017] [Indexed: 11/23/2022] Open
Abstract
Contraction of skeletal muscle cells is initiated by a well-known signaling pathway. An action potential in a motor nerve triggers an action potential in a muscle cell membrane, a transient increase of intracellular calcium concentration, binding of calcium to troponin in the actin-containing thin filaments, and a structural change in the thin filaments that allows myosin motors from the thick filaments to bind to actin and generate force. This calcium/thin filament mediated pathway provides the "START" signal for contraction, but it is argued that the functional response of the muscle cell, including the speed of its contraction and relaxation, adaptation to the external load, and the metabolic cost of contraction is largely determined by additional mechanisms. This review considers the role of the thick filaments in those mechanisms, and puts forward a paradigm for the control of contraction in skeletal muscle in which both the thick and thin filaments have a regulatory function. The OFF state of the thick filament is characterized by helical packing of most of the myosin head or motor domains on the thick filament surface in a conformation that makes them unavailable for actin binding or ATP hydrolysis, although a small fraction of the myosin heads are constitutively ON. The availability of the majority fraction of the myosin heads for contraction is controlled in part by the external load on the muscle, so that these heads only attach to actin and hydrolyze ATP when they are required. This phenomenon seems to be the major determinant of the well-known force-velocity relationship of muscle, and controls the metabolic cost of contraction. The regulatory state of the thick filament also seems to control the dynamics of both muscle activation and relaxation.
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Affiliation(s)
- Malcolm Irving
- Randall Centre for Cell and Molecular Biophysics and BHF Centre of Research Excellence, King's College London, London, United Kingdom.
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14
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Eakins F, Pinali C, Gleeson A, Knupp C, Squire JM. X-ray Diffraction Evidence for Low Force Actin-Attached and Rigor-Like Cross-Bridges in the Contractile Cycle. BIOLOGY 2016; 5:E41. [PMID: 27792170 PMCID: PMC5192421 DOI: 10.3390/biology5040041] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 08/26/2016] [Revised: 10/01/2016] [Accepted: 10/10/2016] [Indexed: 11/17/2022]
Abstract
Defining the structural changes involved in the myosin cross-bridge cycle on actin in active muscle by X-ray diffraction will involve recording of the whole two dimensional (2D) X-ray diffraction pattern from active muscle in a time-resolved manner. Bony fish muscle is the most highly ordered vertebrate striated muscle to study. With partial sarcomere length (SL) control we show that changes in the fish muscle equatorial A-band (10) and (11) reflections, along with (10)/(11) intensity ratio and the tension, are much more rapid than without such control. Times to 50% change with SL control were 19.5 (±2.0) ms, 17.0 (±1.1) ms, 13.9 (±0.4) ms and 22.5 (±0.8) ms, respectively, compared to 25.0 (±3.4) ms, 20.5 (±2.6) ms, 15.4 (±0.6) ms and 33.8 (±0.6) ms without control. The (11) intensity and the (10)/(11) intensity ratio both still change ahead of tension, supporting the likelihood of the presence of a head population close to or on actin, but producing little or no force, in the early stages of the contractile cycle. Higher order equatorials (e.g., (30), (31), and (32)), more sensitive to crossbridge conformation and distribution, also change very rapidly and overshoot their tension plateau values by a factor of around two, well before the tension plateau has been reached, once again indicating an early low-force cross-bridge state in the contractile cycle. Modelling of these intensity changes suggests the presence of probably two different actin-attached myosin head structural states (mainly low-force attached and rigor-like). No more than two main attached structural states are necessary and sufficient to explain the observations. We find that 48% of the heads are off actin giving a resting diffraction pattern, 20% of heads are in the weak binding conformation and 32% of the heads are in the strong (rigor-like) state. The strong states account for 96% of the tension at the tetanus plateau.
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Affiliation(s)
- Felicity Eakins
- Faculty of Medicine, Imperial College London, Exhibition Road, London SW7 2AZ, UK.
| | - Christian Pinali
- Biophysics Group, Optometry & Vision Sciences, University of Cardiff, Cardiff CF10 3XQ, UK.
| | | | - Carlo Knupp
- Faculty of Medicine, Imperial College London, Exhibition Road, London SW7 2AZ, UK.
- Biophysics Group, Optometry & Vision Sciences, University of Cardiff, Cardiff CF10 3XQ, UK.
| | - John M Squire
- Faculty of Medicine, Imperial College London, Exhibition Road, London SW7 2AZ, UK.
- Muscle Contraction Group, School of Physiology, Pharmacology & Neuroscience, Faculty of Biomedical Sciences, University of Bristol, Bristol BS8 1TH, UK.
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15
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Caruel M, Truskinovsky L. Statistical mechanics of the Huxley-Simmons model. Phys Rev E 2016; 93:062407. [PMID: 27415298 DOI: 10.1103/physreve.93.062407] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2015] [Indexed: 06/06/2023]
Abstract
The chemomechanical model of Huxley and Simmons (HS) [A. F. Huxley and R. M. Simmons, Nature 233, 533 (1971)NATUAS0028-083610.1038/233533a0] provides a paradigmatic description of mechanically induced collective conformational changes relevant in a variety of biological contexts, from muscles power stroke and hair cell gating to integrin binding and hairpin unzipping. We develop a statistical mechanical perspective on the HS model by exploiting a formal analogy with a paramagnetic Ising model. We first study the equilibrium HS model with a finite number of elements and compute explicitly its mechanical and thermal properties. To model kinetics, we derive a master equation and solve it for several loading protocols. The developed formalism is applicable to a broad range of allosteric systems with mean-field interactions.
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Affiliation(s)
- M Caruel
- MSME, CNRS-UMR 8208, 61 Avenue du Général de Gaulle, 94010 Créteil, France
| | - L Truskinovsky
- LMS, CNRS-UMR 7649, Ecole Polytechnique, 91128 Palaiseau Cedex, France
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16
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Yu H, Chakravorty S, Song W, Ferenczi MA. Phosphorylation of the regulatory light chain of myosin in striated muscle: methodological perspectives. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2016; 45:779-805. [PMID: 27084718 PMCID: PMC5101276 DOI: 10.1007/s00249-016-1128-z] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/18/2015] [Revised: 03/10/2016] [Accepted: 03/23/2016] [Indexed: 12/18/2022]
Abstract
Phosphorylation of the regulatory light chain (RLC) of myosin modulates cellular functions such as muscle contraction, mitosis, and cytokinesis. Phosphorylation defects are implicated in a number of diseases. Here we focus on striated muscle where changes in RLC phosphorylation relate to diseases such as hypertrophic cardiomyopathy and muscular dystrophy, or age-related changes. RLC phosphorylation in smooth muscle and non-muscle cells are covered briefly where relevant. There is much scientific interest in controlling the phosphorylation levels of RLC in vivo and in vitro in order to understand its physiological function in striated muscles. A summary of available and emerging in vivo and in vitro methods is presented. The physiological role of RLC phosphorylation and novel pathways are discussed to highlight the differences between muscle types and to gain insights into disease processes.
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Affiliation(s)
- Haiyang Yu
- Lee Kong Chian School of Medicine, Nanyang Technological University, Experimental Medicine Building, Level 3, 59 Nanyang Drive, Singapore, 636921, Singapore
| | - Samya Chakravorty
- Lee Kong Chian School of Medicine, Nanyang Technological University, Experimental Medicine Building, Level 3, 59 Nanyang Drive, Singapore, 636921, Singapore
| | - Weihua Song
- Lee Kong Chian School of Medicine, Nanyang Technological University, Experimental Medicine Building, Level 3, 59 Nanyang Drive, Singapore, 636921, Singapore
| | - Michael A Ferenczi
- Lee Kong Chian School of Medicine, Nanyang Technological University, Experimental Medicine Building, Level 3, 59 Nanyang Drive, Singapore, 636921, Singapore.
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17
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Contraction induced muscle injury: towards personalized training and recovery programs. Ann Biomed Eng 2014; 43:388-403. [PMID: 25352440 DOI: 10.1007/s10439-014-1173-7] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2014] [Accepted: 10/20/2014] [Indexed: 12/25/2022]
Abstract
Skeletal muscles can be injured by their own contractions. Such contraction-induced injury, often accompanied by delayed onset of muscle soreness, is a leading cause of the loss of mobility in the rapidly increasing population of elderly people. Unlike other types of muscle injuries which hurt almost exclusively those who are subjected to intensive exercise such as professional athletes and soldiers in training, contraction induced injury is a phenomenon which may be experienced by people of all ages while performing a variety of daily-life activities. Subjects that experience contraction induced injury report on soreness that usually increases in intensity in the first 24 h after the activity, peaks from 24 to 72 h, and then subsides and disappears in a few days. Despite their clinical importance and wide influence, there are almost no studies, clinical, experimental or computational, that quantitatively relate between the extent of contraction induced injury and activity factors, such as number of repetitions, their frequency and magnitude. The lack of such quantitative information is even more emphasized by the fact that contraction induced injury can be used, if moderate and controlled, to improve muscle performance in the long term. Thus, if properly understood and carefully implemented, contraction induced injury can be used for the purpose of personalized training and recovery programs. In this paper, we review experimental, clinical, and theoretical works, attempting towards drawing a more quantitative description of contraction induced injury and related phenomena.
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Brunello E, Caremani M, Melli L, Linari M, Fernandez-Martinez M, Narayanan T, Irving M, Piazzesi G, Lombardi V, Reconditi M. The contributions of filaments and cross-bridges to sarcomere compliance in skeletal muscle. J Physiol 2014; 592:3881-99. [PMID: 25015916 DOI: 10.1113/jphysiol.2014.276196] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Force generation in the muscle sarcomere is driven by the head domain of the myosin molecule extending from the thick filament to form cross-bridges with the actin-containing thin filament. Following attachment, a structural working stroke in the head pulls the thin filament towards the centre of the sarcomere, producing, under unloaded conditions, a filament sliding of ∼ 11 nm. The mechanism of force generation by the myosin head depends on the relationship between cross-bridge force and movement, which is determined by compliances of the cross-bridge (C(cb)) and filaments. By measuring the force dependence of the spacing of the high-order myosin- and actin-based X-ray reflections from sartorius muscles of Rana esculenta we find a combined filament compliance (Cf) of 13.1 ± 1.2 nm MPa(-1), close to recent estimates from single fibre mechanics (12.8 ± 0.5 nm MPa(-1)). C(cb) calculated using these estimates is 0.37 ± 0.12 nm pN(-1), a value fully accounted for by the compliance of the myosin head domain, 0.38 ± 0.06 nm pN(-1), obtained from the intensity changes of the 14.5 nm myosin-based X-ray reflection in response to 3 kHz oscillations imposed on single muscle fibres in rigor. Thus, a significant contribution to C(cb) from the myosin tail that joins the head to the thick filament is excluded. The low C(cb) value indicates that the myosin head generates isometric force by a small sub-step of the 11 nm stroke that drives filament sliding at low load. The implications of these results for the mechanism of force generation by myosins have general relevance for cardiac and non-muscle myosins as well as for skeletal muscle.
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Affiliation(s)
- Elisabetta Brunello
- Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
| | - Marco Caremani
- Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
| | - Luca Melli
- Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
| | - Marco Linari
- Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
| | | | | | - Malcolm Irving
- Randall Division, King's College London, London, SE1 1UL, UK
| | - Gabriella Piazzesi
- Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
| | - Vincenzo Lombardi
- Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
| | - Massimo Reconditi
- Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, UdR Firenze, Italy
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19
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Reconditi M, Brunello E, Fusi L, Linari M, Martinez MF, Lombardi V, Irving M, Piazzesi G. Sarcomere-length dependence of myosin filament structure in skeletal muscle fibres of the frog. J Physiol 2013; 592:1119-37. [PMID: 24344169 DOI: 10.1113/jphysiol.2013.267849] [Citation(s) in RCA: 59] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
X-ray diffraction patterns were recorded at beamline ID02 of the European Synchrotron Radiation Facility from small bundles of skeletal muscle fibres from Rana esculenta at sarcomere lengths between 2.1 and 3.5 μm at 4°C. The intensities of the X-ray reflections from resting fibres associated with the quasi-helical order of the myosin heads and myosin binding protein C (MyBP-C) decreased in the sarcomere length range 2.6-3.0 μm but were constant outside it, suggesting that an OFF conformation of the thick filament is maintained by an interaction between MyBP-C and the thin filaments. During active isometric contraction the intensity of the M3 reflection from the regular repeat of the myosin heads along the filaments decreased in proportion to the overlap between thick and thin filaments, with no change in its interference fine structure. Thus, myosin heads in the regions of the thick filaments that do not overlap with thin filaments are highly disordered during isometric contraction, in contrast to their quasi-helical order at rest. Heads in the overlap region that belong to two-headed myosin molecules that are fully detached from actin are also highly disordered, in contrast to the detached partners of actin-attached heads. These results provide strong support for the concept of a regulatory structural transition in the thick filament involving changes in both the organisation of the myosin heads on its surface and the axial periodicity of the myosin tails in its backbone, mediated by an interaction between MyBP-C and the thin filaments.
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Affiliation(s)
- Massimo Reconditi
- Randall Division of Cell and Molecular Biophysics, King's College London, London SE1 1UL, UK.
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20
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Behrmann E, Müller M, Penczek PA, Mannherz HG, Manstein DJ, Raunser S. Structure of the rigor actin-tropomyosin-myosin complex. Cell 2012; 150:327-38. [PMID: 22817895 DOI: 10.1016/j.cell.2012.05.037] [Citation(s) in RCA: 267] [Impact Index Per Article: 22.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2012] [Revised: 03/13/2012] [Accepted: 05/20/2012] [Indexed: 01/13/2023]
Abstract
Regulation of myosin and filamentous actin interaction by tropomyosin is a central feature of contractile events in muscle and nonmuscle cells. However, little is known about molecular interactions within the complex and the trajectory of tropomyosin movement between its "open" and "closed" positions on the actin filament. Here, we report the 8 Å resolution structure of the rigor (nucleotide-free) actin-tropomyosin-myosin complex determined by cryo-electron microscopy. The pseudoatomic model of the complex, obtained from fitting crystal structures into the map, defines the large interface involving two adjacent actin monomers and one tropomyosin pseudorepeat per myosin contact. Severe forms of hereditary myopathies are linked to mutations that critically perturb this interface. Myosin binding results in a 23 Å shift of tropomyosin along actin. Complex domain motions occur in myosin, but not in actin. Based on our results, we propose a structural model for the tropomyosin-dependent modulation of myosin binding to actin.
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Affiliation(s)
- Elmar Behrmann
- Department of Physical Biochemistry, Max Planck Institute of Molecular Physiology, 44227 Dortmund, Germany
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21
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Wu S, Liu J, Reedy MC, Perz-Edwards RJ, Tregear RT, Winkler H, Franzini-Armstrong C, Sasaki H, Lucaveche C, Goldman YE, Reedy MK, Taylor KA. Structural changes in isometrically contracting insect flight muscle trapped following a mechanical perturbation. PLoS One 2012; 7:e39422. [PMID: 22761792 PMCID: PMC3382574 DOI: 10.1371/journal.pone.0039422] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2012] [Accepted: 05/18/2012] [Indexed: 11/18/2022] Open
Abstract
The application of rapidly applied length steps to actively contracting muscle is a classic method for synchronizing the response of myosin cross-bridges so that the average response of the ensemble can be measured. Alternatively, electron tomography (ET) is a technique that can report the structure of the individual members of the ensemble. We probed the structure of active myosin motors (cross-bridges) by applying 0.5% changes in length (either a stretch or a release) within 2 ms to isometrically contracting insect flight muscle (IFM) fibers followed after 5–6 ms by rapid freezing against a liquid helium cooled copper mirror. ET of freeze-substituted fibers, embedded and thin-sectioned, provides 3-D cross-bridge images, sorted by multivariate data analysis into ∼40 classes, distinct in average structure, population size and lattice distribution. Individual actin subunits are resolved facilitating quasi-atomic modeling of each class average to determine its binding strength (weak or strong) to actin. ∼98% of strong-binding acto-myosin attachments present after a length perturbation are confined to “target zones” of only two actin subunits located exactly midway between successive troponin complexes along each long-pitch helical repeat of actin. Significant changes in the types, distribution and structure of actin-myosin attachments occurred in a manner consistent with the mechanical transients. Most dramatic is near disappearance, after either length perturbation, of a class of weak-binding cross-bridges, attached within the target zone, that are highly likely to be precursors of strong-binding cross-bridges. These weak-binding cross-bridges were originally observed in isometrically contracting IFM. Their disappearance following a quick stretch or release can be explained by a recent kinetic model for muscle contraction, as behaviour consistent with their identification as precursors of strong-binding cross-bridges. The results provide a detailed model for contraction in IFM that may be applicable to contraction in other types of muscle.
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Affiliation(s)
- Shenping Wu
- Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America
| | - Jun Liu
- Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America
| | - Mary C. Reedy
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, United States of America
| | - Robert J. Perz-Edwards
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, United States of America
| | - Richard T. Tregear
- Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom
| | - Hanspeter Winkler
- Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America
| | - Clara Franzini-Armstrong
- Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Hiroyuki Sasaki
- Institute of DNA Medicine, Jikei University School of Medicine, Tokyo, Japan
| | - Carmen Lucaveche
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, United States of America
| | - Yale E. Goldman
- Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Michael K. Reedy
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, United States of America
| | - Kenneth A. Taylor
- Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America
- * E-mail:
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Koubassova NA, Tsaturyan AK. Molecular mechanism of actin-myosin motor in muscle. BIOCHEMISTRY (MOSCOW) 2012; 76:1484-506. [PMID: 22339600 DOI: 10.1134/s0006297911130086] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
The interaction of actin and myosin powers striated and smooth muscles and some other types of cell motility. Due to its highly ordered structure, skeletal muscle is a very convenient object for studying the general mechanism of the actin-myosin molecular motor. The history of investigation of the actin-myosin motor is briefly described. Modern concepts and data obtained with different techniques including protein crystallography, electron microscopy, biochemistry, and protein engineering are reviewed. Particular attention is given to X-ray diffraction studies of intact muscles and single muscle fibers with permeabilized membrane as they give insight into structural changes that underlie force generation and work production by the motor. Time-resolved low-angle X-ray diffraction on contracting muscle fibers using modern synchrotron radiation sources is used to follow movement of myosin heads with unique time and spatial resolution under near physiological conditions.
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Affiliation(s)
- N A Koubassova
- Institute of Mechanics, Lomonosov Moscow State University, Moscow, Russia.
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23
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Motion of myosin head domains during activation and force development in skeletal muscle. Proc Natl Acad Sci U S A 2011; 108:7236-40. [PMID: 21482782 DOI: 10.1073/pnas.1018330108] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Muscle contraction is driven by a change in the structure of the head domain of myosin, the "working stroke" that pulls the actin filaments toward the midpoint of the myosin filaments. This movement of the myosin heads can be measured very precisely in intact muscle cells by X-ray interference, but until now this technique has not been applied to physiological activation and force generation following electrical stimulation of muscle cells. By using this approach, we show that the long axes of the myosin head domains are roughly parallel to the filaments in resting muscle, with their center of mass offset by approximately 7 nm from the C terminus of the head domain. The observed mass distribution matches that seen in electron micrographs of isolated myosin filaments in which the heads are folded back toward the filament midpoint. Following electrical stimulation, the heads move by approximately 10 nm away from the filament midpoint, in the opposite direction to the working stroke. The time course of this motion matches that of force generation, but is slower than the other structural changes in the myosin filaments on activation, including the loss of helical and axial order of the myosin heads and the change in periodicity of the filament backbone. The rate of force development is limited by that of attachment of myosin heads to actin in a conformation that is the same as that during steady-state isometric contraction; force generation in the actin-attached head is fast compared with the attachment step.
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24
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Skubiszak L. Geometrical conditions indispensable for muscle contraction. Int J Mol Sci 2011; 12:2138-57. [PMID: 21731432 PMCID: PMC3127108 DOI: 10.3390/ijms12042138] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2011] [Revised: 03/10/2011] [Accepted: 03/18/2011] [Indexed: 11/16/2022] Open
Abstract
Computer simulation has uncovered the geometrical conditions under which the vertebrate striated muscle sarcomere can contract. First, all thick filaments should have identical structure, namely: three myosin cross-bridges, building a crown, should be aligned at angles of 0°, 120°, 180°, and the successive crowns and the two filament halves should be turned around 120°. Second, all thick filaments should act simultaneously. Third, coordination in action of the myosin cross-bridges should exist, namely: the three cross-bridges of a crown should act simultaneously and the cross-bridge crowns axially 43 and 14.333 nm apart should act, respectively, simultaneously and with a phase shift. Fifth, six thin filaments surrounding the thick filament should be turned around 180° to each other in each sarcomere half. Sixth, thin filaments should be oppositely oriented in relation to the sarcomere middle. Finally, the structure of each of the thin filaments should change in consequence of strong interaction with myosin heads, namely: the axial distance and the angular alignment between neighboring actin monomers should be, respectively, 2.867 nm and 168° instead of 2.75 nm and 166.15°. These conditions ensure the stereo-specific interaction between actin and myosin and good agreement with the data gathered by electron microscopy and X-ray diffraction methods. The results suggest that the force is generated not only by the myosin cross-bridges but also by the thin filaments; the former acts by cyclical unwrapping and wrapping the thick filament backbone, and the latter byelongation.
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Affiliation(s)
- Ludmila Skubiszak
- Nałęcz Institute of Biocybernetics and Biomedical Engineering of the Polish Academy of Sciences, Ks. Trojdena 4, 02-109 Warszawa, Poland; E-Mail: ; Tel.: +48-22-6599143
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25
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Hallen MA, Liang ZY, Endow SA. Two-state displacement by the kinesin-14 Ncd stalk. Biophys Chem 2011; 154:56-65. [PMID: 21288629 DOI: 10.1016/j.bpc.2011.01.001] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2010] [Revised: 01/03/2011] [Accepted: 01/03/2011] [Indexed: 11/25/2022]
Abstract
The nonprocessive kinesin-14 Ncd motor binds to microtubules and hydrolyzes ATP, undergoing a single displacement before releasing the microtubule. A lever-like rotation of the coiled-coil stalk is thought to drive Ncd displacements or steps along microtubules. Crystal structures and cryoelectron microscopy reconstructions imply that stalk rotation is correlated with ADP release and microtubule binding by the motor. Here we report FRET assays showing that the end of the stalk is more than ~9nm from the microtubule when wild-type Ncd binds microtubules without added nucleotide, but the stalk is within ~6nm of the microtubule surface when the microtubule-bound motor binds an ATP analogue, matching the rotated state observed in crystal structures. We propose that the stalk rotation is initiated when the motor binds to microtubules and releases ADP, and is completed when ATP binds.
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Affiliation(s)
- Mark A Hallen
- Department of Cell Biology, Structural Biology & Biophysics Program, Duke University Medical Center, Durham, NC 27710, USA.
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26
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Wu S, Liu J, Reedy MC, Tregear RT, Winkler H, Franzini-Armstrong C, Sasaki H, Lucaveche C, Goldman YE, Reedy MK, Taylor KA. Electron tomography of cryofixed, isometrically contracting insect flight muscle reveals novel actin-myosin interactions. PLoS One 2010; 5. [PMID: 20844746 PMCID: PMC2936580 DOI: 10.1371/journal.pone.0012643] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2010] [Accepted: 07/29/2010] [Indexed: 11/18/2022] Open
Abstract
Background Isometric muscle contraction, where force is generated without muscle shortening, is a molecular traffic jam in which the number of actin-attached motors is maximized and all states of motor action are trapped with consequently high heterogeneity. This heterogeneity is a major limitation to deciphering myosin conformational changes in situ. Methodology We used multivariate data analysis to group repeat segments in electron tomograms of isometrically contracting insect flight muscle, mechanically monitored, rapidly frozen, freeze substituted, and thin sectioned. Improved resolution reveals the helical arrangement of F-actin subunits in the thin filament enabling an atomic model to be built into the thin filament density independent of the myosin. Actin-myosin attachments can now be assigned as weak or strong by their motor domain orientation relative to actin. Myosin attachments were quantified everywhere along the thin filament including troponin. Strong binding myosin attachments are found on only four F-actin subunits, the “target zone”, situated exactly midway between successive troponin complexes. They show an axial lever arm range of 77°/12.9 nm. The lever arm azimuthal range of strong binding attachments has a highly skewed, 127° range compared with X-ray crystallographic structures. Two types of weak actin attachments are described. One type, found exclusively in the target zone, appears to represent pre-working-stroke intermediates. The other, which contacts tropomyosin rather than actin, is positioned M-ward of the target zone, i.e. the position toward which thin filaments slide during shortening. Conclusion We present a model for the weak to strong transition in the myosin ATPase cycle that incorporates azimuthal movements of the motor domain on actin. Stress/strain in the S2 domain may explain azimuthal lever arm changes in the strong binding attachments. The results support previous conclusions that the weak attachments preceding force generation are very different from strong binding attachments.
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Affiliation(s)
- Shenping Wu
- Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America
| | - Jun Liu
- Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America
| | - Mary C. Reedy
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, United States of America
| | - Richard T. Tregear
- Medical Research Council Laboratory of Molecular Biology, Cambridge, England
| | - Hanspeter Winkler
- Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America
| | - Clara Franzini-Armstrong
- Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Hiroyuki Sasaki
- Division of Fine Morphology, Core Research Facilities, Jikei University School of Medicine, Tokyo, Japan
| | - Carmen Lucaveche
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, United States of America
| | - Yale E. Goldman
- Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Michael K. Reedy
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, United States of America
| | - Kenneth A. Taylor
- Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America
- * E-mail:
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Givli S. Towards multi-scale modeling of muscle fibers with sarcomere non-uniformities. J Theor Biol 2010; 264:882-92. [DOI: 10.1016/j.jtbi.2010.02.048] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2009] [Revised: 02/26/2010] [Accepted: 02/27/2010] [Indexed: 10/19/2022]
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Probing myosin structural conformation in vivo by second-harmonic generation microscopy. Proc Natl Acad Sci U S A 2010; 107:7763-8. [PMID: 20385845 DOI: 10.1073/pnas.0914782107] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Understanding of complex biological processes requires knowledge of molecular structures and measurement of their dynamics in vivo. The collective chemomechanical action of myosin molecules (the molecular motors) in the muscle sarcomere represents a paradigmatic example in this respect. Here, we describe a label-free imaging method sensitive to protein conformation in vivo. We employed the order-based contrast enhancement by second-harmonic generation (SHG) for the functional imaging of muscle cells. We found that SHG polarization anisotropy (SPA) measurements report on the structural state of the actomyosin motors, with significant sensitivity to the conformation of myosin. In fact, each physiological/biochemical state we probed (relaxed, rigor, isometric contraction) produced a distinct value of polarization anisotropy. Employing a full reconstruction of the contributing elementary SHG emitters in the actomyosin motor array at atomic scale, we provide a molecular interpretation of the SPA measurements in terms of myosin conformations. We applied this method to the discrimination between attached and detached myosin heads in an isometrically contracting intact fiber. Our observations indicate that isometrically contracting muscle sustains its tetanic force by steady-state commitment of 30% of myosin heads. Applying SPA and molecular structure modeling to the imaging of unstained living tissues provides the basis for a generation of imaging and diagnostic tools capable of probing molecular structures and dynamics in vivo.
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29
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Colombini B, Nocella M, Benelli G, Cecchi G, Griffiths PJ, Bagni MA. Reversal of the myosin power stroke induced by fast stretching of intact skeletal muscle fibers. Biophys J 2010; 97:2922-9. [PMID: 19948121 DOI: 10.1016/j.bpj.2009.09.018] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2009] [Revised: 09/10/2009] [Accepted: 09/11/2009] [Indexed: 10/20/2022] Open
Abstract
Force generation and movement in skeletal muscle result from a cyclical interaction of overlapping myosin and actin filaments that permits the free energy of ATP hydrolysis to be converted into mechanical work. The rapid force recovery that occurs after a step release imposed on a muscle is thought to result from a synchronized tilting of myosin lever arms toward a position of lower free energy (the power stroke). We investigated the power stroke mechanism in intact muscle fibers of Rana esculenta using a fast stretch to detach forcibly cross-bridges. Stretches were applied either with or without a conditioning step release. Cross-bridge rupture tension was not significantly influenced by the release, whereas sarcomere elongation at the rupture point increased immediately after the release and returned to the prerelease condition within 15-20 ms, following a slower time course compared to the recovery of tension. These observations suggest that the rupture force of a bridge is unaltered by a conditioning release, but rupture must first be preceded by a power stroke reversal, which restores the prepower stroke state. The sarcomere extension at the rupture point indicates both the extent of this power stroke reversal and the time course of strained bridge replenishment.
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Affiliation(s)
- Barbara Colombini
- Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, Firenze, Italy
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30
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Cardiomyopathy mutations reveal variable region of myosin converter as major element of cross-bridge compliance. Biophys J 2009; 97:806-24. [PMID: 19651039 DOI: 10.1016/j.bpj.2009.05.023] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2008] [Revised: 05/05/2009] [Accepted: 05/08/2009] [Indexed: 11/23/2022] Open
Abstract
The ability of myosin to generate motile forces is based on elastic distortion of a structural element of the actomyosin complex (cross-bridge) that allows strain to develop before filament sliding. Addressing the question, which part of the actomyosin complex experiences main elastic distortion, we suggested previously that the converter domain might be the most compliant region of the myosin head domain. Here we test this proposal by studying functional effects of naturally occurring missense mutations in the beta-myosin heavy chain, 723Arg --> Gly (R723G) and 736Ile --> Thr (I736T), in comparison to 719Arg --> Trp (R719W). All three mutations are associated with hypertrophic cardiomyopathy and are located in the converter region of the myosin head domain. We determined several mechanical parameters of single skinned slow fibers isolated from Musculus soleus biopsies of hypertrophic cardiomyopathy patients and healthy controls. Major findings of this study for mutation R723G were i), a >40% increase in fiber stiffness in rigor with a 2.9-fold increase in stiffness per myosin head (S( *)(rigor R723G) = 0.84 pN/nm S( *)(rigor WT) = 0.29 pN/nm); and ii), a significant increase in force per head (F( *)(10 degrees C), 1.99 pN vs. 1.49 pN = 1.3-fold increase; F( *)(20 degrees C), 2.56 pN vs. 1.92 pN = 1.3-fold increase) as well as stiffness per head during isometric steady-state contraction (S( *)(active10 degrees C), 0.52 pN/nm vs. 0.28 pN/nm = 1.9-fold increase). Similar changes were found for mutation R719W (2.6-fold increase in S( *)(rigor); 1.8-fold increase in F( *)(10 degrees C), 1.6-fold in F( *)(20 degrees C); twofold increase in S( *)(active10 degrees C)). Changes in active cross-bridge cycling kinetics could not account for the increase in force and active stiffness. For the above estimates the previously determined fraction of mutated myosin in the biopsies was taken into account. Data for wild-type myosin of slow soleus muscle fibers support previous findings that for the slow myosin isoform S( *) and F( *) are significantly lower than for fast myosin e.g., of rabbit psoas muscle. The data indicate that two mutations, R723G and R719W, are associated with an increase in resistance to elastic distortion of the individual mutated myosin heads whereas mutation I736T has essentially no effect. The data strongly support the notion that major elastic distortion occurs within the converter itself. Apparently, the compliance depends on specific residues, e.g., R719 and R723, presumably located at strategic positions near the long alpha-helix of the light chain binding domain. Because amino acids 719 and 723 are nonconserved residues, cross-bridge stiffness may well be specifically tuned for different myosins.
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31
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Koubassova NA, Bershitsky SY, Tsaturyan AK. A mathematical model of mechanical responses of contracting muscle fibers to temperature jumps. Biophysics (Nagoya-shi) 2009. [DOI: 10.1134/s0006350909040204] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
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32
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Koubassova NA, Bershitsky SY, Ferenczi MA, Panine P, Narayanan T, Tsaturyan AK. X-ray interferometry of the axial movement of myosin heads during muscle force generation initiated by T-Jump. Mol Biol 2009. [DOI: 10.1134/s0026893309040165] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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33
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Pant K, Watt J, Greenberg M, Jones M, Szczesna-Cordary D, Moore JR. Removal of the cardiac myosin regulatory light chain increases isometric force production. FASEB J 2009; 23:3571-80. [PMID: 19470801 DOI: 10.1096/fj.08-126672] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
The myosin neck, which is supported by the interactions between light chains and the underlying alpha-helical heavy chain, is thought to act as a lever arm to amplify movements originating in the globular motor domain. Here, we studied the role of the cardiac myosin regulatory light chains (RLCs) in the capacity of myosin to produce force using a novel optical-trap-based isometric force in vitro motility assay. We measured the isometric force and actin filament velocity for native porcine cardiac (PC) myosin, RLC-depleted PC (PC(depl)) myosin, and PC myosin reconstituted with recombinant bacterially expressed human cardiac RLC (PC(recon)). RLC depletion reduced unloaded actin filament velocity by 58% and enhanced the myosin-based isometric force approximately 2-fold. No significant change between PC and PC(depl) preparations was observed in the maximal rate of actin-activated myosin ATPase activity. Reconstitution of PC(depl) myosin with human RLC partially restored the velocity and force levels to near untreated values. The reduction in unloaded velocity after RLC extraction is consistent with the myosin neck acting as a lever, while the enhancement in isometric force can be directly related to enhancement of unitary force. The force data are consistent with a model in which the neck region behaves as a cantilevered beam.
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Affiliation(s)
- Kiran Pant
- Boston University School of Medicine, Department of Physiology and Biophysics, 72 E. Concord St., Boston, MA 02118, USA
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34
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Probing muscle myosin motor action: x-ray (m3 and m6) interference measurements report motor domain not lever arm movement. J Mol Biol 2009; 390:168-81. [PMID: 19394345 DOI: 10.1016/j.jmb.2009.04.047] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2008] [Revised: 04/10/2009] [Accepted: 04/16/2009] [Indexed: 11/22/2022]
Abstract
The key question in understanding how force and movement are produced in muscle concerns the nature of the cyclic interaction of myosin molecules with actin filaments. The lever arm of the globular head of each myosin molecule is thought in some way to swing axially on the actin-attached motor domain, thus propelling the actin filament past the myosin filament. Recent X-ray diffraction studies of vertebrate muscle, especially those involving the analysis of interference effects between myosin head arrays in the two halves of the thick filaments, have been claimed to prove that the lever arm moves at the same time as the sliding of actin and myosin filaments in response to muscle length or force steps. It was suggested that the sliding of myosin and actin filaments, the level of force produced and the lever arm angle are all directly coupled and that other models of lever arm movement will not fit the X-ray data. Here, we show that, in addition to interference across the A-band, which must be occurring, the observed meridional M3 and M6 X-ray intensity changes can all be explained very well by the changing diffraction effects during filament sliding caused by heads stereospecifically attached to actin moving axially relative to a population of detached or non-stereospecifically attached heads that remain fixed in position relative to the myosin filament backbone. Crucially, and contrary to previous interpretations, the X-ray interference results provide little direct information about the position of the myosin head lever arm; they are, in fact, reporting relative motor domain movements. The implications of the new interpretation are briefly assessed.
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35
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Koubassova NA, Bershitsky SY, Ferenczi MA, Tsaturyan AK. Direct modeling of X-ray diffraction pattern from contracting skeletal muscle. Biophys J 2008; 95:2880-94. [PMID: 18539638 PMCID: PMC2527261 DOI: 10.1529/biophysj.107.120832] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2007] [Accepted: 05/12/2008] [Indexed: 11/18/2022] Open
Abstract
A direct modeling approach was used to quantitatively interpret the two-dimensional x-ray diffraction patterns obtained from contracting mammalian skeletal muscle. The dependence of the calculated layer line intensities on the number of myosin heads bound to the thin filaments, on the conformation of these heads and on their mode of attachment to actin, was studied systematically. Results of modeling are compared to experimental data collected from permeabilized fibers from rabbit skeletal muscle contracting at 5 degrees C and 30 degrees C and developing low and high isometric tension, respectively. The results of the modeling show that: i), the intensity of the first actin layer line is independent of the tilt of the light chain domains of myosin heads and can be used as a measure of the fraction of myosin heads stereospecifically attached to actin; ii), during isometric contraction at near physiological temperature, the fraction of these heads is approximately 40% and the light chain domains of the majority of them are more perpendicular to the filament axis than in rigor; and iii), at low temperature, when isometric tension is low, a majority of the attached myosin heads are bound to actin nonstereospecifically whereas at high temperature and tension they are bound stereospecifically.
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Affiliation(s)
- Natalia A Koubassova
- Institute of Mechanics, Lomonosov Moscow State University, Moscow 119992, Russia.
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36
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Piazzesi G, Reconditi M, Linari M, Lucii L, Bianco P, Brunello E, Decostre V, Stewart A, Gore DB, Irving TC, Irving M, Lombardi V. Skeletal muscle performance determined by modulation of number of myosin motors rather than motor force or stroke size. Cell 2008; 131:784-95. [PMID: 18022371 DOI: 10.1016/j.cell.2007.09.045] [Citation(s) in RCA: 220] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2007] [Revised: 06/08/2007] [Accepted: 09/27/2007] [Indexed: 10/22/2022]
Abstract
Skeletal muscle can bear a high load at constant length, or shorten rapidly when the load is low. This force-velocity relationship is the primary determinant of muscle performance in vivo. Here we exploited the quasi-crystalline order of myosin II motors in muscle filaments to determine the molecular basis of this relationship by X-ray interference and mechanical measurements on intact single cells. We found that, during muscle shortening at a wide range of velocities, individual myosin motors maintain a force of about 6 pN while pulling an actin filament through a 6 nm stroke, then quickly detach when the motor reaches a critical conformation. Thus we show that the force-velocity relationship is primarily a result of a reduction in the number of motors attached to actin in each filament in proportion to the filament load. These results explain muscle performance and efficiency in terms of the molecular mechanism of the myosin motor.
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Affiliation(s)
- Gabriella Piazzesi
- Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
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37
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Skeletal muscle resists stretch by rapid binding of the second motor domain of myosin to actin. Proc Natl Acad Sci U S A 2007; 104:20114-9. [PMID: 18077437 DOI: 10.1073/pnas.0707626104] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
A shortening muscle is a machine that converts metabolic energy into mechanical work, but, when a muscle is stretched, it acts as a brake, generating a high resistive force at low metabolic cost. The braking action of muscle can be activated with remarkable speed, as when the leg extensor muscles rapidly decelerate the body at the end of a jump. Here we used time-resolved x-ray and mechanical measurements on isolated muscle cells to elucidate the molecular basis of muscle braking and its rapid control. We show that a stretch of only 5 nm between each overlapping set of myosin and actin filaments in a muscle sarcomere is sufficient to double the number of myosin motors attached to actin within a few milliseconds. Each myosin molecule has two motor domains, only one of which is attached to actin during shortening or activation at constant length. A stretch strains the attached motor domain, and we propose that combined steric and mechanical coupling between the two domains promotes attachment of the second motor domain. This mechanism allows skeletal muscle to resist external stretch without increasing the force per motor and provides an answer to the longstanding question of the functional role of the dimeric structure of muscle myosin.
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38
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Sugiura K, Ohkawa K, Hirai T, Fujii T. ATPase-coupled release control from polyion complex capsules encapsulating muscle proteins. Macromol Biosci 2007; 7:508-16. [PMID: 17429832 DOI: 10.1002/mabi.200600269] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
In the present study, a muscle contractile protein complex, actomyosin, has been successfully encapsulated into gellan-chitosan polyion complex (PIC) capsules. The recovery of the myosin-ATPase activity is approximately 50% and the Mg2+-ATPase activity is stimulated by the presence of F-actin, which implies the formation of the actomyosin complex inside the capsule. Furthermore, encapsulation could protect the myosin, F-actin, and actomyosin inside from hydrolysis by proteases. Two small proteins, myoglobin and cytochrome c, have been used in the release tests. The release of myoglobin is not affected by the ionic strength of the external solution, while the release of cytochrome c increases with increasing ionic strength. The maximal releases are found in the external pH solution close to the isoelectric points of each protein. The Mg2+-ATP complex itself reduces the release percentages of the small proteins from the PIC capsule. The release amounts further decrease when coexisting with Mg2+-ATP and the encapsulated actomyosin, which indicates the release regulation by actomyosin. The present study suggests that the ATPase-coupled sliding motion of the myosin-F-actin filaments modifies the pore size of the polymer networks in the PIC capsule membranes.
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Affiliation(s)
- Kousuke Sugiura
- Department of Kansei Engineering, Faculty of Textile Science and Technology, Shinshu University, Japan
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39
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Squire JM, Knupp C, Roessle M, Al-Khayat HA, Irving TC, Eakins F, Mok NS, Harford JJ, Reedy MK. X-ray diffraction studies of striated muscles. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2007; 565:45-60; discussion 359-69. [PMID: 16106966 DOI: 10.1007/0-387-24990-7_5] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- John M Squire
- Biological Structure and Function Section, Biomedical Sciences Division, Faculty of Medicine, Imperial College London, Exhibition Road, London SW7 2AZ, UK
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40
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Oshima K, Takezawa Y, Sugimoto Y, Kobayashi T, Irving TC, Wakabayashi K. Axial dispositions and conformations of myosin crossbridges along thick filaments in relaxed and contracting states of vertebrate striated muscles by X-ray fiber diffraction. J Mol Biol 2006; 367:275-301. [PMID: 17239393 DOI: 10.1016/j.jmb.2006.12.036] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2006] [Revised: 12/09/2006] [Accepted: 12/12/2006] [Indexed: 10/23/2022]
Abstract
X-ray diffraction patterns from live vertebrate striated muscles were analyzed to elucidate the detailed structural models of the myosin crown arrangement and the axial disposition of two-headed myosin crossbridges along the thick filaments in the relaxed and contracting states. The modeling studies were based upon the previous notion that individual myosin filaments had a mixed structure with two regions, a "regular" and a "perturbed". In the relaxed state the distributions and sizes of the regular and perturbed regions on myosin filaments, each having its own axial periodicity for the arrangement of crossbridge crowns within the basic period, were similar to those reported previously. A new finding was that in the contracting state, this mixed structure was maintained but the length of each region, the periodicities of the crowns and the axial disposition of two heads of a crossbridge were altered. The perturbed regions of the crossbridge repeat shifted towards the Z-bands in the sarcomere without changing the lengths found in the relaxed state, but in which the intervals between three successive crowns within the basic period became closer to the regular 14.5-nm repeat in the contracting state. In high resolution modeling for a myosin head, the two heads of a crossbridge were axially tilted in opposite directions along the three-fold helical tracks of myosin filaments and their axial orientations were different from each other in perturbed and regular regions in both states. Under relaxing conditions, one head of a double-headed crossbridge pair appeared to be in close proximity to another head in a pair at the adjacent crown level in the axial direction in the regular region. In the perturbed region this contact between heads occurred only on the narrower inter-crown levels. During contraction, one head of a crossbridge oriented more perpendicular to the fiber axis and the partner head flared axially. Several factors that significantly influence the intensities of the myosin based-meridional reflections and their relative contributions are discussed.
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Affiliation(s)
- Kanji Oshima
- Division of Biophysical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
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41
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Yagi N, Iwamoto H, Inoue K. Structural changes of cross-bridges on transition from isometric to shortening state in frog skeletal muscle. Biophys J 2006; 91:4110-20. [PMID: 16980365 PMCID: PMC1635662 DOI: 10.1529/biophysj.106.087502] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2006] [Accepted: 08/23/2006] [Indexed: 11/18/2022] Open
Abstract
Structural changes in the myosin cross-bridges were studied by small-angle x-ray diffraction at a time resolution of 0.53 ms. A frog sartorius muscle, which was electrically stimulated to induce isometric contraction, was released by approximately 1% in 1 ms, and then its length was decreased to allow steady shortening with tension of approximately 30% of the isometric level. Intensity of all reflections reached a constant level in 5-8 ms. Intensity of the 7.2-nm meridional reflection and the (1,0) sampling spot of the 14.5-nm layer line increased after the initial release but returned to the isometric level during steady shortening. The 21.5-nm meridional reflection showed fast and slow components of intensity increase. The intensity of the 10.3-nm layer line, which arises from myosin heads attached to actin, decreased to a steady level in 2 ms, whereas other reflections took longer, 5-20 ms. The results show that myosin heads adapt quickly to an altered level of tension, and that there is a distinct structural state just after a quick release.
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Affiliation(s)
- Naoto Yagi
- SPring-8/JASRI, 1-1-1 Kouto, Sayo, Hyogo, Japan.
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Huxley H, Reconditi M, Stewart A, Irving T. X-ray Interference Studies of Crossbridge Action in Muscle Contraction: Evidence from Quick Releases. J Mol Biol 2006; 363:743-61. [PMID: 17007871 DOI: 10.1016/j.jmb.2006.08.075] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2006] [Revised: 08/21/2006] [Accepted: 08/22/2006] [Indexed: 11/18/2022]
Abstract
We have used a high-resolution small angle X-ray scattering system, together with a high-performance CCD camera, on the BioCAT beamline at the APS synchrotron radiation facility at the Argonne National Laboratory, to study X-ray interference effects in the meridional reflections generated by the arrays of myosin crossbridges in contracting muscle. These give information about axial movements of the myosin heads during contraction with sub-nanometer resolution. Using whole intact muscle preparations (frog sartorius) we have been able to record the detailed behavior of M3 (the first order meridional reflection from the myosin crossbridges, at 14.56 nm) at each of a number of quick releases of increasing magnitude, on the same specimen, and at the same time make similar measurements on higher order myosin meridional reflections, particularly M6. The latter provides information about the dispersion of lever arm angles of the actin-attached myosin heads. The observations show that in isometric contraction the lever arm angles are dispersed through +/- 20-25 degrees on either side of a mean orientation that is about 60 degrees away from their orientation at the end of the working stroke: and that they move towards that orientation in synchronized fashion, with constant dispersion, during quick releases. The relationship between the shift in the interference fringes (which measures the shift of the myosin heads scattering mass towards the center of the sarcomere, and the changes in the total intensity of the reflections, which measures the changes in the axial profile of the heads, is consistent with the tilting lever arm mechanism of muscle contraction. Significant fixed contributions to the meridional reflections come from unattached myosin heads and from backbone components of the myosin filaments, and the interaction of these with the contributions from actin-attached myosin heads determines the behavior of these reflections.
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Affiliation(s)
- Hugh Huxley
- Rosenstiel Center, Brandeis University, Waltham, MA 02454, USA.
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43
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Reconditi M. RECENT IMPROVEMENTS IN SMALL ANGLE X-RAY DIFFRACTION FOR THE STUDY OF MUSCLE PHYSIOLOGY. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2006; 69:2709-2759. [PMID: 19946470 PMCID: PMC2783642 DOI: 10.1088/0034-4885/69/10/r01] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
The molecular mechanism of muscle contraction is one of the most important unresolved problems in Biology and Biophysics. Notwithstanding the great advances of recent years, it is not yet known in detail how the molecular motor in muscle, the class II myosin, converts the free energy of ATP hydrolysis into work by interacting with its track, the actin filament, neither it is understood how the high efficiency in energy conversion depends on the cooperative action of myosin motors working in parallel along the actin filament. Researches in muscle contraction imply the combination of mechanical, biochemical and structural methods in studies that span from tissue to single molecule. Therefore, more than for any other research field, progresses in the comprehension of muscle contraction at molecular level are related to, and in turn contribute to, the advancement of methods in Biophysics.This review will focus on the progresses achieved by time resolved small angle X-ray scattering (SAXS) from muscle, an approach made possible by the highly ordered arrangement of both the contractile proteins myosin and actin in the ca 2 mum long structural unit the sarcomere that repeats along the whole length of the muscle cell. Among the time resolved structural techniques, SAXS has proved to be the most powerful method of investigation, as it allows the molecular motor to be studied in situ, in intact single muscle cells, where it is possible to combine the structural study with fast mechanical methods that synchronize the action of the molecular motors. The latest development of this technique allows Angstrom-scale measurements of the axial movement of the motors that pull the actin filament toward the centre of the sarcomere, by exploiting the X-ray interference between the two arrays of myosin motors in the two halves of the sarcomere.
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Affiliation(s)
- Massimo Reconditi
- Università di Firenze, Lab di Fisiologia - DBAG, c/o Dip. di Fisica, via Sansone 1, I-50019 Sesto Fiorentino, ITALY
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Brunello E, Bianco P, Piazzesi G, Linari M, Reconditi M, Panine P, Narayanan T, Helsby WI, Irving M, Lombardi V. Structural changes in the myosin filament and cross-bridges during active force development in single intact frog muscle fibres: stiffness and X-ray diffraction measurements. J Physiol 2006; 577:971-84. [PMID: 16990403 PMCID: PMC1890380 DOI: 10.1113/jphysiol.2006.115394] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Structural and mechanical changes occurring in the myosin filament and myosin head domains during the development of the isometric tetanus have been investigated in intact frog muscle fibres at 4 degrees C and 2.15 microm sarcomere length, using sarcomere level mechanics and X-ray diffraction at beamline ID2 of the European Synchrotron Radiation Facility (Grenoble, France). The time courses of changes in both the M3 and M6 myosin-based reflections were recorded with 5 ms frames using the gas-filled RAPID detector (MicroGap Technology). Following the end of the latent period (11 ms after the start of stimulation), force increases to the tetanus plateau value (T(0)) with a half-time of 40 ms, and the spacings of the M3 and M6 reflections (S(M3) and S(M6)) increase by 1.5% from their resting values, with time courses that lead that of force by approximately 10 and approximately 20 ms, respectively. These temporal relations are maintained when the increase of force is delayed by approximately 10 ms by imposing, from 5 ms after the first stimulus, 50 nm (half-sarcomere)(-1) shortening at the velocity (V(0)) that maintains zero force. Shortening at V(0) transiently reduces S(M3) following the latent period and delays the subsequent increase in S(M3), but only delays the S(M6) increase without a transient decrease. Shortening at V(0) imposed at the tetanus plateau causes an abrupt reduction of the intensity of the M3 reflection (I(M3)), whereas the intensity of the M6 reflection (I(M6)) is only slightly reduced. The changes in half-sarcomere stiffness indicate that the isometric force at each time point is proportional to the number of myosin heads bound to actin. The different sensitivities of the intensity and spacing of the M3 and M6 reflections to the mechanical responses support the view that the M3 reflection in active muscle originates mainly from the myosin heads attached to the actin filament and the M6 reflection originates mainly from a fixed structure in the myosin filament signalling myosin filament length changes during the tetanus rise.
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Affiliation(s)
- E Brunello
- Laboratory of Physiology, Department of Animal Biology and Genetics, University of Florence, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
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Affiliation(s)
- John M Squire
- Biological Structure & Function Section, Biomedical Sciences Division, Imperial College Faculty of Medicine, London SW7 2AZ London, United Kingdom
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Reconditi M, Linari M, Lucii L, Stewart A, Sun YB, Narayanan T, Irving T, Piazzesi G, Irving M, Lombardi V. Structure-function relation of the myosin motor in striated muscle. Ann N Y Acad Sci 2005; 1047:232-47. [PMID: 16093500 DOI: 10.1196/annals.1341.021] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Force and shortening in striated muscle are driven by a structural working stroke in the globular portion of the myosin molecules-the myosin head-that cross-links the myosin-containing filaments and the actin-containing filaments. We use time-resolved X-ray diffraction in single fibers from frog skeletal muscle to link the conformational changes in the myosin head determined at atomic resolution in crystallographic studies with the kinetic and mechanical features of the molecular motor in the preserved sarcomeric structure. Our approach exploits the improved brightness and collimation of the X-ray beams of the third generation synchrotrons by using X-ray interference between the two arrays of myosin heads in each bipolar myosin filament to measure with A sensitivity the axial motions of myosin heads in situ during the synchronous execution of the working stroke elicited by rapid decreases in length or load imposed during an active isometric contraction. Changes in the intensity and interference-fine structure of the axial X-ray reflections following the mechanical perturbation allowed to establish the average conformation of the myosin heads during the active isometric contraction and the extent of tilt during the elastic response and during the subsequent working stroke. The myosin working stroke is 12 nm at low loads, which is consistent with crystallographic studies, while it is smaller and slower at higher loads. The load dependence of the size and speed of the myosin working stroke is the molecular determinant of the macroscopic performance and efficiency of muscle.
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Affiliation(s)
- Massimo Reconditi
- Laboratorio di Fisiologia, DBAG, Università di Firenze, 50019 Sesto Fiorentino, Italy
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Griffiths PJ, Bagni MA, Colombini B, Amenitsch H, Bernstorff S, Ashley CC, Cecchi G. Myosin lever disposition during length oscillations when power stroke tilting is reduced. Am J Physiol Cell Physiol 2005; 289:C177-86. [PMID: 15743885 DOI: 10.1152/ajpcell.00020.2005] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
M3 reflection intensity (IM3) from tetanized, intact skeletal muscle fiber bundles was measured during sinusoidal length oscillations at 2.8 kHz, a frequency at which the myosin motor’s power stroke is greatly reduced. IM3 signals were approximately sinusoidal, but showed a “double peak” distortion previously observed only at lower oscillation frequencies. A tilting lever arm model simulated this distortion, where IM3 was calculated from the molecular structure of myosin subfragment 1 (S1). Simulations showed an isometric lever arm disposition close to normal to the filament axis at isometric tension, similar to that found using lower oscillation frequencies, where the power stroke contributes more toward total S1 movement. Inclusion of a second detached S1 in each actin-bound myosin dimer increased simulated IM3 signal amplitude and improved agreement with the experimental data. The best agreement was obtained when detached heads have a fixed orientation, insensitive to length changes, and similar to that of attached heads at tetanus plateau. This configuration also accounts for the variations in relative intensity of the two main peaks of the M3 reflection substructure after a length change. This evidence of an IM3 signal distortion when power stroke tilting is suppressed, provided that a large enough amplitude of length oscillation is used, is consistent with the tilting lever arm model of the power stroke.
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Affiliation(s)
- P J Griffiths
- University Laboratory of Physiology, Oxford University, United Kingdom
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Linari M, Brunello E, Reconditi M, Sun YB, Panine P, Narayanan T, Piazzesi G, Lombardi V, Irving M. The structural basis of the increase in isometric force production with temperature in frog skeletal muscle. J Physiol 2005; 567:459-69. [PMID: 15961426 PMCID: PMC1474186 DOI: 10.1113/jphysiol.2005.089672] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
X-ray diffraction patterns were recorded from isolated single fibres of frog skeletal muscle during isometric contraction at temperatures between 0 and 17 degrees C. Isometric force was 43 +/- 2% (mean +/- S.E.M., n = 10) higher at 17 degrees C than 0 degrees C. The intensity of the first actin layer line increased by 57 +/- 18% (n = 5), and the ratio of the intensities of the equatorial 1,1 and 1,0 reflections by 20 +/- 7% (n = 10), signalling radial or azimuthal motions of the myosin head domains. The M3 X-ray reflection from the axial repeat of the heads along the filaments was 27 +/- 4% more intense at 17 degrees C, suggesting that the heads became more perpendicular to the filaments. The ratio of the intensities of the higher and lower angle peaks of the M3 reflection (R(M3)) was 0.93 +/- 0.02 (n = 5) at 0 degrees C and 0.77 +/- 0.02 at 17 degrees C. These peaks are due to interference between the two halves of each myosin filament, and the R(M3) decrease shows that heads move towards the midpoint of the myosin filament at the higher temperature. Calculations based on a crystallographic model of the heads indicated that the observed R(M3) change corresponds to tilting of their light-chain domains by 9 deg, producing an axial displacement of 1.4 nm, which is equal to that required to strain the actin and myosin filaments under the increased force. We conclude that the higher force generated by skeletal muscle at higher temperature can be accounted for by axial tilting of the myosin heads.
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MESH Headings
- Actins/chemistry
- Actins/physiology
- Actins/ultrastructure
- Animals
- Cells, Cultured
- Isometric Contraction/physiology
- Models, Biological
- Models, Chemical
- Models, Molecular
- Molecular Motor Proteins/chemistry
- Molecular Motor Proteins/physiology
- Muscle Fibers, Skeletal/physiology
- Muscle Fibers, Skeletal/ultrastructure
- Muscle, Skeletal/chemistry
- Muscle, Skeletal/physiology
- Muscle, Skeletal/ultrastructure
- Myosins/chemistry
- Myosins/physiology
- Myosins/ultrastructure
- Rana temporaria
- Sarcomeres/chemistry
- Sarcomeres/physiology
- Sarcomeres/ultrastructure
- Stress, Mechanical
- Structure-Activity Relationship
- Temperature
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Affiliation(s)
- M Linari
- Laboratorio di Fisiologia, DBAG, Università di Firenze, Sesto Fiorentino, Italy
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Kinbara K, Aida T. Toward intelligent molecular machines: directed motions of biological and artificial molecules and assemblies. Chem Rev 2005; 105:1377-400. [PMID: 15826015 DOI: 10.1021/cr030071r] [Citation(s) in RCA: 673] [Impact Index Per Article: 35.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Kazushi Kinbara
- Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
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Yagi N, Iwamoto H, Wakayama J, Inoue K. Structural changes of actin-bound myosin heads after a quick length change in frog skeletal muscle. Biophys J 2005; 89:1150-64. [PMID: 15894638 PMCID: PMC1366600 DOI: 10.1529/biophysj.105.059089] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Changes in the x-ray diffraction pattern from a frog skeletal muscle were recorded after a quick release or stretch, which was completed within one millisecond, at a time resolution of 0.53 ms using the high-flux beamline at the SPring-8 third-generation synchrotron radiation facility. Reversibility of the effects of the length changes was checked by quickly restoring the muscle length. Intensities of seven reflections were measured. A large, instantaneous intensity drop of a layer line at an axial spacing of 1/10.3 nm(-1) after a quick release and stretch, and its partial recovery by reversal of the length change, indicate a conformational change of myosin heads that are attached to actin. Intensity changes on the 14.5-nm myosin layer line suggest that the attached heads alter their radial mass distribution upon filament sliding. Intensity changes of the myosin reflections at 1/21.5 and 1/7.2 nm(-1) are not readily explained by a simple axial swing of cross-bridges. Intensity changes of the actin-based layer lines at 1/36 and 1/5.9 nm(-1) are not explained by it either, suggesting a structural change in actin molecules.
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Affiliation(s)
- Naoto Yagi
- Japan Synchrotron Radiation Research Institute, SPring-8, Kouto, Sayo-gun, Hyogo.
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