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Rynkiewicz MJ, Childers MC, Karpicheva OE, Regnier M, Geeves MA, Lehman W. Myosin's powerstroke transitions define atomic scale movement of cardiac thin filament tropomyosin. J Gen Physiol 2024; 156:e202413538. [PMID: 38607351 PMCID: PMC11010328 DOI: 10.1085/jgp.202413538] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2024] [Revised: 02/27/2024] [Accepted: 03/28/2024] [Indexed: 04/13/2024] Open
Abstract
Dynamic interactions between the myosin motor head on thick filaments and the actin molecular track on thin filaments drive the myosin-crossbridge cycle that powers muscle contraction. The process is initiated by Ca2+ and the opening of troponin-tropomyosin-blocked myosin-binding sites on actin. The ensuing recruitment of myosin heads and their transformation from pre-powerstroke to post-powerstroke conformation on actin produce the force required for contraction. Cryo-EM-based atomic models confirm that during this process, tropomyosin occupies three different average positions on actin. Tropomyosin pivoting on actin away from a TnI-imposed myosin-blocking position accounts for part of the Ca2+ activation observed. However, the structure of tropomyosin on thin filaments that follows pre-powerstroke myosin binding and its translocation during myosin's pre-powerstroke to post-powerstroke transition remains unresolved. Here, we approach this transition computationally in silico. We used the myosin helix-loop-helix motif as an anchor to dock models of pre-powerstroke cardiac myosin to the cleft between neighboring actin subunits along cardiac thin filaments. We then performed targeted molecular dynamics simulations of the transition between pre- and post-powerstroke conformations on actin in the presence of cardiac troponin-tropomyosin. These simulations show Arg 369 and Glu 370 on the tip of myosin Loop-4 encountering identically charged residues on tropomyosin. The charge repulsion between residues causes tropomyosin translocation across actin, thus accounting for the final regulatory step in the activation of the thin filament, and, in turn, facilitating myosin movement along the filament. We suggest that during muscle activity, myosin-induced tropomyosin movement is likely to result in unencumbered myosin head interactions on actin at low-energy cost.
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Affiliation(s)
- Michael J. Rynkiewicz
- Department of Pharmacology, Physiology and Biophysics, Boston University Chobanian and Avedisian School of Medicine, Boston, MA, USA
| | | | - Olga E. Karpicheva
- Department of Pharmacology, Physiology and Biophysics, Boston University Chobanian and Avedisian School of Medicine, Boston, MA, USA
| | - Michael Regnier
- Department of Bioengineering, University of Washington, Seattle, WA, USA
| | | | - William Lehman
- Department of Pharmacology, Physiology and Biophysics, Boston University Chobanian and Avedisian School of Medicine, Boston, MA, USA
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2
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Abstract
Force generation in striated muscle is primarily controlled by structural changes in the actin-containing thin filaments triggered by an increase in intracellular calcium concentration. However, recent studies have elucidated a new class of regulatory mechanisms, based on the myosin-containing thick filament, that control the strength and speed of contraction by modulating the availability of myosin motors for the interaction with actin. This review summarizes the mechanisms of thin and thick filament activation that regulate the contractility of skeletal and cardiac muscle. A novel dual-filament paradigm of muscle regulation is emerging, in which the dynamics of force generation depends on the coordinated activation of thin and thick filaments. We highlight the interfilament signaling pathways based on titin and myosin-binding protein-C that couple thin and thick filament regulatory mechanisms. This dual-filament regulation mediates the length-dependent activation of cardiac muscle that underlies the control of the cardiac output in each heartbeat.
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Affiliation(s)
- Elisabetta Brunello
- Randall Centre for Cell and Molecular Biophysics, School of Basic and Medical Biosciences and British Heart Foundation Centre of Research Excellence, King's College London, London, United Kingdom; ,
| | - Luca Fusi
- Randall Centre for Cell and Molecular Biophysics, School of Basic and Medical Biosciences and British Heart Foundation Centre of Research Excellence, King's College London, London, United Kingdom; ,
- Centre for Human and Applied Physiological Sciences, School of Basic and Medical Biosciences, King's College London, London, United Kingdom
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3
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Robaszkiewicz K, Wróbel J, Moraczewska J. Troponin and a Myopathy-Linked Mutation in TPM3 Inhibit Cofilin-2-Induced Thin Filament Depolymerization. Int J Mol Sci 2023; 24:16457. [PMID: 38003645 PMCID: PMC10671271 DOI: 10.3390/ijms242216457] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2023] [Revised: 11/10/2023] [Accepted: 11/13/2023] [Indexed: 11/26/2023] Open
Abstract
Uniform actin filament length is required for synchronized contraction of skeletal muscle. In myopathies linked to mutations in tropomyosin (Tpm) genes, irregular thin filaments are a common feature, which may result from defects in length maintenance mechanisms. The current work investigated the effects of the myopathy-causing p.R91C variant in Tpm3.12, a tropomyosin isoform expressed in slow-twitch muscle fibers, on the regulation of actin severing and depolymerization by cofilin-2. The affinity of cofilin-2 for F-actin was not significantly changed by either Tpm3.12 or Tpm3.12-R91C, though it increased two-fold in the presence of troponin (without Ca2+). Saturation of the filament with cofilin-2 removed both Tpm variants from the filament, although Tpm3.12-R91C was more resistant. In the presence of troponin (±Ca2+), Tpm remained on the filament, even at high cofilin-2 concentrations. Both Tpm3.12 variants inhibited filament severing and depolymerization by cofilin-2. However, the inhibition was more efficient in the presence of Tpm3.12-R91C, indicating that the pathogenic variant impaired cofilin-2-dependent actin filament turnover. Troponin (±Ca2+) further inhibited but did not completely stop cofilin-2-dependent actin severing and depolymerization.
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Affiliation(s)
| | | | - Joanna Moraczewska
- Department of Biochemistry and Cell Biology, Faculty of Biological Sciences, Kazimierz Wielki University in Bydgoszcz, 85-671 Bydgoszcz, Poland; (K.R.); (J.W.)
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4
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Parry DAD. 50 Years of the steric-blocking mechanism in vertebrate skeletal muscle: a retrospective. J Muscle Res Cell Motil 2023; 44:133-141. [PMID: 35789471 PMCID: PMC10542282 DOI: 10.1007/s10974-022-09619-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2022] [Accepted: 05/12/2022] [Indexed: 11/27/2022]
Abstract
Fifty years have now passed since Parry and Squire proposed a detailed structural model that explained how tropomyosin, mediated by troponin, played a steric-blocking role in the regulation of vertebrate skeletal muscle. In this Special Issue dedicated to the memory of John Squire it is an opportune time to look back on this research and to appreciate John's key contributions. A review is also presented of a selection of the developments and insights into muscle regulation that have occurred in the years since this proposal was formulated.
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Affiliation(s)
- David A D Parry
- School of Natural Sciences, Massey University, Private Bag 11-222, Palmerston North, 4442, New Zealand.
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5
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Lehman W, Rynkiewicz MJ. Troponin-I-induced tropomyosin pivoting defines thin-filament function in relaxed and active muscle. J Gen Physiol 2023; 155:e202313387. [PMID: 37249525 PMCID: PMC10227645 DOI: 10.1085/jgp.202313387] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2023] [Revised: 04/25/2023] [Accepted: 05/15/2023] [Indexed: 05/31/2023] Open
Abstract
Regulation of the crossbridge cycle that drives muscle contraction involves a reconfiguration of the troponin-tropomyosin complex on actin filaments. By comparing atomic models of troponin-tropomyosin fitted to cryo-EM structures of inhibited and Ca2+-activated thin filaments, we find that tropomyosin pivots rather than rolls or slides across actin as generally thought. We propose that pivoting can account for the Ca2+ activation that initiates muscle contraction and then relaxation influenced by troponin-I (TnI). Tropomyosin is well-known to occupy either of three meta-stable configurations on actin, regulating access of myosin motorheads to their actin-binding sites and thus the crossbridge cycle. At low Ca2+ concentrations, tropomyosin is trapped by TnI in an inhibitory B-state that sterically blocks myosin binding to actin, leading to muscle relaxation. Ca2+ binding to TnC draws TnI away from tropomyosin, while tropomyosin moves to a C-state location over actin. This partially relieves the steric inhibition and allows weak binding of myosin heads to actin, which then transition to strong actin-bound configurations, fully activating the thin filament. Nevertheless, the reconfiguration that accompanies the initial Ca2+-sensitive B-state/C-state shift in troponin-tropomyosin on actin remains uncertain and at best is described by moderate-resolution cryo-EM reconstructions. Our recent computational studies indicate that intermolecular residue-to-residue salt-bridge linkage between actin and tropomyosin is indistinguishable in B- and C-state thin filament configurations. We show here that tropomyosin can pivot about relatively fixed points on actin to accompany B-state/C-state structural transitions. We argue that at low Ca2+ concentrations C-terminal TnI domains attract tropomyosin, causing it to bend and then pivot toward the TnI, thus blocking myosin binding and contraction.
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Affiliation(s)
- William Lehman
- Department of Pharmacology, Physiology and Biophysics, Boston University Chobanian and Avedisian School of Medicine, Boston, MA, USA
| | - Michael J. Rynkiewicz
- Department of Pharmacology, Physiology and Biophysics, Boston University Chobanian and Avedisian School of Medicine, Boston, MA, USA
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6
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Prodanovic M, Wang Y, Mijailovich SM, Irving T. Using Multiscale Simulations as a Tool to Interpret Equatorial X-ray Fiber Diffraction Patterns from Skeletal Muscle. Int J Mol Sci 2023; 24:8474. [PMID: 37239821 PMCID: PMC10218096 DOI: 10.3390/ijms24108474] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2023] [Revised: 05/04/2023] [Accepted: 05/05/2023] [Indexed: 05/28/2023] Open
Abstract
Synchrotron small-angle X-ray diffraction is the method of choice for nm-scale structural studies of striated muscle under physiological conditions and on millisecond time scales. The lack of generally applicable computational tools for modeling X-ray diffraction patterns from intact muscles has been a significant barrier to exploiting the full potential of this technique. Here, we report a novel "forward problem" approach using the spatially explicit computational simulation platform MUSICO to predict equatorial small-angle X-ray diffraction patterns and the force output simultaneously from resting and isometrically contracting rat skeletal muscle that can be compared to experimental data. The simulation generates families of thick-thin filament repeating units, each with their individually predicted occupancies of different populations of active and inactive myosin heads that can be used to generate 2D-projected electron density models based on known Protein Data Bank structures. We show how, by adjusting only a few selected parameters, we can achieve a good correspondence between experimental and predicted X-ray intensities. The developments presented here demonstrate the feasibility of combining X-ray diffraction and spatially explicit modeling to form a powerful hypothesis-generating tool that can be used to motivate experiments that can reveal emergent properties of muscle.
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Affiliation(s)
- Momcilo Prodanovic
- Institute for Information Technologies, University of Kragujevac, 34000 Kragujevac, Serbia;
- FilamenTech, Inc., Newton, MA 02458, USA;
| | - Yiwei Wang
- Department of Applied Mathematics, Illinois Institute of Technology, Chicago, IL 60616, USA;
- Department of Biology, Illinois Institute of Technology, Chicago, IL 60616, USA
- Department of Mathematics, University of California, Riverside, CA 92521, USA
| | | | - Thomas Irving
- Department of Biology, Illinois Institute of Technology, Chicago, IL 60616, USA
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7
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Issahaku AR, Ibrahim MAA, Mukelabai N, Soliman MES. Intermolecular And Dynamic Investigation of The Mechanism of Action of Reldesemtiv on Fast Skeletal Muscle Troponin Complex Toward the Treatment of Impaired Muscle Function. Protein J 2023:10.1007/s10930-023-10091-y. [PMID: 36959428 DOI: 10.1007/s10930-023-10091-y] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 01/07/2023] [Indexed: 03/25/2023]
Abstract
Muscle weakness as a secondary feature of attenuated neuronal input often leads to disability and sometimes death in patients with neurogenic neuromuscular diseases. These impaired muscle function has been observed in several diseases including amyotrophic lateral sclerosis, Charcot-Marie-Tooth, spinal muscular atrophy and Myasthenia gravis. This has spurred the search for small molecules which could activate fast skeletal muscle troponin complex as a means to increase muscle strength. Discovered small molecules have however been punctuated by off-target and side effects leading to the development of the second-generation small molecule, Reldesemtiv. In this study, we investigated the impact of Reldesemtiv binding to the fast skeletal troponin complex and the molecular determinants that condition the therapeutic prowess of Redesemtiv through computational techniques. It was revealed that Reldesemtiv binding possibly potentiates troponin C compacting characterized by reduced exposure to solvent molecules which could favor the slow release of calcium ions and the resultant sensitization of the subunit to calcium. These conformational changes were underscored by conventional and carbon hydrogen bonds, pi-alkyl, pi-sulfur and halogen interactions between Reldesemtiv the binding site residues. Arg113 (-3.96 kcal/mol), Met116 (-2.23 kcal/mol), Val114 (-1.28 kcal/mol) and Met121 (-0.63 kcal/mol) of the switch region of the inhibitory subunit were among the residues that contributed the most to the total free binding energy of Reldesemtiv highlighting their importance. These findings present useful insights which could lay the foundation for the development of fast skeletal muscle small molecule activators with high specificity and potency.
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Affiliation(s)
- Abdul Rashid Issahaku
- Molecular Bio-Computation and Drug Design Laboratory, School of Health Sciences, University of KwaZulu-Natal, Westville Campus, Durban, 4001, South Africa
- West African Centre for Computational Research and Innovation, Accra, Ghana
| | - Mahmoud A A Ibrahim
- CompChem Research Group, Chemistry Department, Faculty of Science, Minia University, Minia, 61519, Egypt
| | - Namutula Mukelabai
- Department of Physiotherapy, School of Health Sciences, University of KwaZulu- Natal, Westville Campus, Durban, 4001, South Africa
| | - Mahmoud E S Soliman
- Molecular Bio-Computation and Drug Design Laboratory, School of Health Sciences, University of KwaZulu-Natal, Westville Campus, Durban, 4001, South Africa.
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8
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Molecular Dynamics Assessment of Mechanical Properties of the Thin Filaments in Cardiac Muscle. Int J Mol Sci 2023; 24:ijms24054792. [PMID: 36902223 PMCID: PMC10003134 DOI: 10.3390/ijms24054792] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2023] [Revised: 02/19/2023] [Accepted: 02/27/2023] [Indexed: 03/06/2023] Open
Abstract
Contraction of cardiac muscle is regulated by Ca2+ ions via regulatory proteins, troponin (Tn), and tropomyosin (Tpm) associated with the thin (actin) filaments in myocardial sarcomeres. The binding of Ca2+ to a Tn subunit causes mechanical and structural changes in the multiprotein regulatory complex. Recent cryo-electron microscopy (cryo-EM) models of the complex allow one to study the dynamic and mechanical properties of the complex using molecular dynamics (MD). Here we describe two refined models of the thin filament in the calcium-free state that include protein fragments unresolved by cryo-EM and reconstructed using structure prediction software. The parameters of the actin helix and the bending, longitudinal, and torsional stiffness of the filaments estimated from the MD simulations performed with these models were close to those found experimentally. However, problems revealed from the MD simulation suggest that the models require further refinement by improving the protein-protein interaction in some regions of the complex. The use of relatively long refined models of the regulatory complex of the thin filament allows one to perform MD simulation of the molecular mechanism of Ca2+ regulation of contraction without additional constraints and study the effects of cardiomyopathy-associated mutation of the thin filament proteins of cardiac muscle.
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9
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Doran MH, Rynkiewicz MJ, Pavadai E, Bodt SM, Rasicci D, Moore JR, Yengo CM, Bullitt E, Lehman W. Myosin loop-4 is critical for optimal tropomyosin repositioning on actin during muscle activation and relaxation. J Gen Physiol 2023; 155:e202213274. [PMID: 36459134 PMCID: PMC9723511 DOI: 10.1085/jgp.202213274] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/02/2022] [Revised: 10/28/2022] [Accepted: 11/11/2022] [Indexed: 12/04/2022] Open
Abstract
During force-generating steps of the muscle crossbridge cycle, the tip of the myosin motor, specifically loop-4, contacts the tropomyosin cable of actin filaments. In the current study, we determined the corresponding effect of myosin loop-4 on the regulatory positioning of tropomyosin on actin. To accomplish this, we compared high-resolution cryo-EM structures of myosin S1-decorated thin filaments containing either wild-type or a loop-4 mutant construct, where the seven-residue portion of myosin loop-4 that contacts tropomyosin was replaced by glycine residues, thus removing polar side chains from residues 366-372. Cryo-EM analysis of fully decorated actin-tropomyosin filaments with wild-type and mutant S1, yielded 3.4-3.6 Å resolution reconstructions, with even higher definition at the actin-myosin interface. Loop-4 densities both in wild-type and mutant S1 were clearly identified, and side chains were resolved in the wild-type structure. Aside from loop-4, actin and myosin structural domains were indistinguishable from each other when filaments were decorated with either mutant or wild-type S1. In marked contrast, the position of tropomyosin on actin in the two reconstructions differed by 3 to 4 Å. In maps of filaments containing the mutant, tropomyosin was located closer to the myosin-head and thus moved in the direction of the C-state conformation adopted by myosin-free thin filaments. Complementary interaction energy measurements showed that tropomyosin in the mutant thin filaments sits on actin in a local energy minimum, whereas tropomyosin is positioned by wild-type S1 in an energetically unfavorable location. We propose that the high potential energy associated with tropomyosin positioning in wild-type filaments favors an effective transition to B- and C-states following release of myosin from the thin filaments during relaxation.
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Affiliation(s)
- Matthew H. Doran
- Department of Physiology & Biophysics, Boston University Chobanian & Avedisian School of Medicine, Boston, MA
| | - Michael J. Rynkiewicz
- Department of Physiology & Biophysics, Boston University Chobanian & Avedisian School of Medicine, Boston, MA
| | - Elumalai Pavadai
- Department of Physiology & Biophysics, Boston University Chobanian & Avedisian School of Medicine, Boston, MA
| | - Skylar M.L. Bodt
- Department of Cellular and Molecular Physiology, Pennsylvania State College of Medicine, Hershey, PA
| | - David Rasicci
- Department of Cellular and Molecular Physiology, Pennsylvania State College of Medicine, Hershey, PA
| | - Jeffrey R. Moore
- Department of Biological Science, University of Massachusetts Lowell, Lowell, MA
| | - Christopher M. Yengo
- Department of Cellular and Molecular Physiology, Pennsylvania State College of Medicine, Hershey, PA
| | - Esther Bullitt
- Department of Physiology & Biophysics, Boston University Chobanian & Avedisian School of Medicine, Boston, MA
| | - William Lehman
- Department of Physiology & Biophysics, Boston University Chobanian & Avedisian School of Medicine, Boston, MA
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10
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Pavadai E, Rynkiewicz MJ, Yang Z, Gould IR, Marston SB, Lehman W. Modulation of cardiac thin filament structure by phosphorylated troponin-I analyzed by protein-protein docking and molecular dynamics simulation. Arch Biochem Biophys 2022; 725:109282. [PMID: 35577070 PMCID: PMC10680062 DOI: 10.1016/j.abb.2022.109282] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2022] [Revised: 04/14/2022] [Accepted: 05/04/2022] [Indexed: 12/16/2022]
Abstract
Tropomyosin, controlled by troponin-linked Ca2+-binding, regulates muscle contraction by a macromolecular scale steric-mechanism that governs myosin-crossbridge-actin interactions. At low-Ca2+, C-terminal domains of troponin-I (TnI) trap tropomyosin in a position on thin filaments that interferes with myosin-binding, thus causing muscle relaxation. Steric inhibition is reversed at high-Ca2+ when TnI releases from F-actin-tropomyosin as Ca2+ and the TnI switch-peptide bind to the N-lobe of troponin-C (TnC). The opposite end of cardiac TnI contains a phosphorylation-sensitive ∼30 residue-long N-terminal peptide that is absent in skeletal muscle, and likely modifies these interactions in hearts. Here, PKA-dependent phosphorylation of serine 23 and 24 modulates Ca2+ and possibly switch-peptide binding to TnC, causing faster relaxation during the cardiac-cycle (lusitropy). The cardiac-specific N-terminal TnI domain is not captured in crystal structures of troponin or in cryo-EM reconstructions of thin filaments; thus, its global impact on thin filament structure and function is uncertain. Here, we used protein-protein docking and molecular dynamics simulation-based protocols to build a troponin model that was guided by and hence consistent with the recent seminal Yamada structure of Ca2+-activated thin filaments. We find that when present on thin filaments, phosphorylated Ser23/24 along with adjacent polar TnI residues interact closely with both tropomyosin and the N-lobe of TnC during our simulations. These interactions would likely bias tropomyosin to an off-state positioning on actin. In situ, such enhanced relaxation kinetics would promote cardiac lusitropy.
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Affiliation(s)
- Elumalai Pavadai
- Department of Physiology & Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, MA, 02118, USA
| | - Michael J Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, MA, 02118, USA
| | - Zeyu Yang
- Department of Chemistry and Institute of Chemical Biology, Imperial College London, Shepard's Bush, London, W12 0BZ, UK
| | - Ian R Gould
- Department of Chemistry and Institute of Chemical Biology, Imperial College London, Shepard's Bush, London, W12 0BZ, UK
| | - Steven B Marston
- National Heart & Lung Institute, Imperial College London, Dovehouse Street, W12 0NN, UK
| | - William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, MA, 02118, USA.
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11
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Chalovich JM, Zhu L, Johnson D. Hypertrophic Cardiomyopathy Mutations of Troponin Reveal Details of Striated Muscle Regulation. Front Physiol 2022; 13:902079. [PMID: 35694406 PMCID: PMC9178916 DOI: 10.3389/fphys.2022.902079] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2022] [Accepted: 05/05/2022] [Indexed: 11/13/2022] Open
Abstract
Striated muscle contraction is inhibited by the actin associated proteins tropomyosin, troponin T, troponin I and troponin C. Binding of Ca2+ to troponin C relieves this inhibition by changing contacts among the regulatory components and ultimately repositioning tropomyosin on the actin filament creating a state that is permissive for contraction. Several lines of evidence suggest that there are three possible positions of tropomyosin on actin commonly called Blocked, Closed/Calcium and Open or Myosin states. These states are thought to correlate with different functional states of the contractile system: inactive-Ca2+-free, inactive-Ca2+-bound and active. The inactive-Ca2+-free state is highly occupied at low free Ca2+ levels. However, saturating Ca2+ produces a mixture of inactive and active states making study of the individual states difficult. Disease causing mutations of troponin, as well as phosphomimetic mutations change the stabilities of the states of the regulatory complex thus providing tools for studying individual states. Mutants of troponin are available to stabilize each of three structural states. Particular attention is given to the hypertrophic cardiomyopathy causing mutation, Δ14 of TnT, that is missing the last 14 C-terminal residues of cardiac troponin T. Removal of the basic residues in this region eliminates the inactive-Ca2+-free state. The major state occupied with Δ14 TnT at inactivating Ca2+ levels resembles the inactive-Ca2+-bound state in function and in displacement of TnI from actin-tropomyosin. Addition of Ca2+, with Δ14TnT, shifts the equilibrium between the inactive-Ca2+-bound and the active state to favor that latter state. These mutants suggest a unique role for the C-terminal region of Troponin T as a brake to limit Ca2+ activation.
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12
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Zhu L, Johnson D, Chalovich JM. C-Terminal Basic Region of Troponin T Alters the Ca 2+-Dependent Changes in Troponin I Interactions. Biochemistry 2022; 61:1103-1112. [PMID: 35522994 DOI: 10.1021/acs.biochem.2c00090] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The C-terminal 14-16 residues of human troponin T are required for full inactivation, and they prevent full activation at saturating Ca2+. Basic residues within that C-terminal region of TnT are essential for its function, but the mechanism of action is unknown. That region of TnT is natively disordered and does not appear in reconstructions of the troponin structure. We used Förster resonance energy transfer to determine if the C-terminal basic region of TnT alters transitions of TnI or if it operates independently. We also examined Ca2+-dependent changes in the C-terminal region of TnT itself. Probes on TnI-143 (inhibitory region) and TnI-159 (switch region) moved away from sites on actin and tropomyosin and toward TnC-84 at high Ca2+. Ca2+ also displaced C-terminal TnT from actin-tropomyosin but without movement toward TnC. Deletion of C-terminal TnT produced changes in TnI-143 like those effected by Ca2+, but effects on TnI-159 were muted; there was no effect on the distance of the switch region to TnC-84. Substituting Ala for basic residues within C-terminal TnT displaced C-terminal TnT from actin-tropomyosin. The results suggest that C-terminal TnT stabilizes tropomyosin in the inactive position on actin. Removal of basic residues from C-terminal TnT produced a Ca2+-like state except that the switch region of TnI was not bound to TnC. Addition of Ca2+ caused more extreme displacement from actin-tropomyosin as the active state became more fully occupied as in the case of wild-type TnT in the presence of both Ca2+ and bound rigor myosin S1.
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Affiliation(s)
- Li Zhu
- Department of Biochemistry & Molecular Biology, Brody School of Medicine at East Carolina University, 600 Moye Boulevard, Greenville, North Carolina 27834, United States
| | - Dylan Johnson
- Department of Biochemistry & Molecular Biology, Brody School of Medicine at East Carolina University, 600 Moye Boulevard, Greenville, North Carolina 27834, United States
| | - Joseph M Chalovich
- Department of Biochemistry & Molecular Biology, Brody School of Medicine at East Carolina University, 600 Moye Boulevard, Greenville, North Carolina 27834, United States
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13
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Small Angle X-ray Diffraction as a Tool for Structural Characterization of Muscle Disease. Int J Mol Sci 2022; 23:ijms23063052. [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] [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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14
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Wang Z, Grange M, Pospich S, Wagner T, Kho AL, Gautel M, Raunser S. Structures from intact myofibrils reveal mechanism of thin filament regulation through nebulin. Science 2022; 375:eabn1934. [PMID: 35175800 DOI: 10.1126/science.abn1934] [Citation(s) in RCA: 54] [Impact Index Per Article: 27.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
In skeletal muscle, nebulin stabilizes and regulates the length of thin filaments, but the underlying mechanism remains nebulous. In this work, we used cryo-electron tomography and subtomogram averaging to reveal structures of native nebulin bound to thin filaments within intact sarcomeres. This in situ reconstruction provided high-resolution details of the interaction between nebulin and actin, demonstrating the stabilizing role of nebulin. Myosin bound to the thin filaments exhibited different conformations of the neck domain, highlighting its inherent structural variability in muscle. Unexpectedly, nebulin did not interact with myosin or tropomyosin, but it did interact with a troponin T linker through two potential binding motifs on nebulin, explaining its regulatory role. Our structures support the role of nebulin as a thin filament "molecular ruler" and provide a molecular basis for studying nemaline myopathies.
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Affiliation(s)
- Zhexin Wang
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, 44227 Dortmund, Germany
| | - Michael Grange
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, 44227 Dortmund, Germany
| | - Sabrina Pospich
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, 44227 Dortmund, Germany
| | - Thorsten Wagner
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, 44227 Dortmund, Germany
| | - Ay Lin Kho
- Randall Centre for Cell and Molecular Biophysics, School of Basic and Medical Biosciences, Kings College London BHF Centre of Research Excellence, Guy's Campus, London SE1 1UL, UK
| | - Mathias Gautel
- Randall Centre for Cell and Molecular Biophysics, School of Basic and Medical Biosciences, Kings College London BHF Centre of Research Excellence, Guy's Campus, London SE1 1UL, UK
| | - Stefan Raunser
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, 44227 Dortmund, Germany
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15
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Prodanovic M, Geeves MA, Poggesi C, Regnier M, Mijailovich SM. Effect of Myosin Isoforms on Cardiac Muscle Twitch of Mice, Rats and Humans. Int J Mol Sci 2022; 23:1135. [PMID: 35163054 PMCID: PMC8835009 DOI: 10.3390/ijms23031135] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2021] [Revised: 01/16/2022] [Accepted: 01/18/2022] [Indexed: 02/04/2023] Open
Abstract
To understand how pathology-induced changes in contractile protein isoforms modulate cardiac muscle function, it is necessary to quantify the temporal-mechanical properties of contractions that occur under various conditions. Pathological responses are much easier to study in animal model systems than in humans, but extrapolation between species presents numerous challenges. Employing computational approaches can help elucidate relationships that are difficult to test experimentally by translating the observations from rats and mice, as model organisms, to the human heart. Here, we use the spatially explicit MUSICO platform to model twitch contractions from rodent and human trabeculae collected in a single laboratory. This approach allowed us to identify the variations in kinetic characteristics of α- and β-myosin isoforms across species and to quantify their effect on cardiac muscle contractile responses. The simulations showed how the twitch transient varied with the ratio of the two myosin isoforms. Particularly, the rate of tension rise was proportional to the fraction of α-myosin present, while the β-isoform dominated the rate of relaxation unless α-myosin was >50%. Moreover, both the myosin isoform and the Ca2+ transient contributed to the twitch tension transient, allowing two levels of regulation of twitch contraction.
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Affiliation(s)
- Momcilo Prodanovic
- Institute for Information Technologies, University of Kragujevac, 34000 Kragujevac, Serbia;
- Bioengineering Research and Development Center (BioIRC), 34000 Kragujevac, Serbia
- FilamenTech, Inc., Newtown, MA 02458, USA
| | - Michael A. Geeves
- Department of Biosciences, University of Kent, Canterbury CT2 7NJ, Kent, UK;
| | - Corrado Poggesi
- Department of Experimental & Clinical Medicine, University of Florence, 20134 Florence, Italy;
| | - Michael Regnier
- Department of Bioengineering, University of Washington, Seattle, WA 98105, USA;
| | - Srboljub M. Mijailovich
- FilamenTech, Inc., Newtown, MA 02458, USA
- Department of Biology, Illinois Institute of Technology, Chicago, IL 60616, USA
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16
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Smith QM, Inchingolo AV, Mihailescu MD, Dai H, Kad NM. Single-molecule imaging reveals the concerted release of myosin from regulated thin filaments. eLife 2021; 10:69184. [PMID: 34569933 PMCID: PMC8476120 DOI: 10.7554/elife.69184] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2021] [Accepted: 09/11/2021] [Indexed: 11/13/2022] Open
Abstract
Regulated thin filaments (RTFs) tightly control striated muscle contraction through calcium binding to troponin, which enables tropomyosin to expose myosin-binding sites on actin. Myosin binding holds tropomyosin in an open position, exposing more myosin-binding sites on actin, leading to cooperative activation. At lower calcium levels, troponin and tropomyosin turn off the thin filament; however, this is antagonised by the high local concentration of myosin, questioning how the thin filament relaxes. To provide molecular details of deactivation, we used single-molecule imaging of green fluorescent protein (GFP)-tagged myosin-S1 (S1-GFP) to follow the activation of RTF tightropes. In sub-maximal activation conditions, RTFs are not fully active, enabling direct observation of deactivation in real time. We observed that myosin binding occurs in a stochastic step-wise fashion; however, an unexpectedly large probability of multiple contemporaneous detachments is observed. This suggests that deactivation of the thin filament is a coordinated active process.
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Affiliation(s)
- Quentin M Smith
- School of Biosciences, University of Kent, Canterbury, United Kingdom
| | | | | | - Hongsheng Dai
- Department of Mathematical Sciences, University of Essex, Colchester, United Kingdom
| | - Neil M Kad
- School of Biosciences, University of Kent, Canterbury, United Kingdom
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17
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Wang Z, Grange M, Wagner T, Kho AL, Gautel M, Raunser S. The molecular basis for sarcomere organization in vertebrate skeletal muscle. Cell 2021; 184:2135-2150.e13. [PMID: 33765442 PMCID: PMC8054911 DOI: 10.1016/j.cell.2021.02.047] [Citation(s) in RCA: 80] [Impact Index Per Article: 26.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2020] [Revised: 01/27/2021] [Accepted: 02/22/2021] [Indexed: 12/20/2022]
Abstract
Sarcomeres are force-generating and load-bearing devices of muscles. A precise molecular picture of how sarcomeres are built underpins understanding their role in health and disease. Here, we determine the molecular architecture of native vertebrate skeletal sarcomeres by electron cryo-tomography. Our reconstruction reveals molecular details of the three-dimensional organization and interaction of actin and myosin in the A-band, I-band, and Z-disc and demonstrates that α-actinin cross-links antiparallel actin filaments by forming doublets with 6-nm spacing. Structures of myosin, tropomyosin, and actin at ~10 Å further reveal two conformations of the "double-head" myosin, where the flexible orientation of the lever arm and light chains enable myosin not only to interact with the same actin filament, but also to split between two actin filaments. Our results provide unexpected insights into the fundamental organization of vertebrate skeletal muscle and serve as a strong foundation for future investigations of muscle diseases.
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Affiliation(s)
- Zhexin Wang
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany
| | - Michael Grange
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany
| | - Thorsten Wagner
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany
| | - Ay Lin Kho
- The Randall Centre for Cell and Molecular Biophysics, School of Basic and Medical Biosciences, Kings College London BHF Excellence Centre, New Hunt's House, Guy's Campus, London SE1 1UL, UK
| | - Mathias Gautel
- The Randall Centre for Cell and Molecular Biophysics, School of Basic and Medical Biosciences, Kings College London BHF Excellence Centre, New Hunt's House, Guy's Campus, London SE1 1UL, UK
| | - Stefan Raunser
- Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany.
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18
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Risi CM, Pepper I, Belknap B, Landim-Vieira M, White HD, Dryden K, Pinto JR, Chase PB, Galkin VE. The structure of the native cardiac thin filament at systolic Ca 2+ levels. Proc Natl Acad Sci U S A 2021; 118:e2024288118. [PMID: 33753506 PMCID: PMC8020778 DOI: 10.1073/pnas.2024288118] [Citation(s) in RCA: 48] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Every heartbeat relies on cyclical interactions between myosin thick and actin thin filaments orchestrated by rising and falling Ca2+ levels. Thin filaments are comprised of two actin strands, each harboring equally separated troponin complexes, which bind Ca2+ to move tropomyosin cables away from the myosin binding sites and, thus, activate systolic contraction. Recently, structures of thin filaments obtained at low (pCa ∼9) or high (pCa ∼3) Ca2+ levels revealed the transition between the Ca2+-free and Ca2+-bound states. However, in working cardiac muscle, Ca2+ levels fluctuate at intermediate values between pCa ∼6 and pCa ∼7. The structure of the thin filament at physiological Ca2+ levels is unknown. We used cryoelectron microscopy and statistical analysis to reveal the structure of the cardiac thin filament at systolic pCa = 5.8. We show that the two strands of the thin filament consist of a mixture of regulatory units, which are composed of Ca2+-free, Ca2+-bound, or mixed (e.g., Ca2+ free on one side and Ca2+ bound on the other side) troponin complexes. We traced troponin complex conformations along and across individual thin filaments to directly determine the structural composition of the cardiac native thin filament at systolic Ca2+ levels. We demonstrate that the two thin filament strands are activated stochastically with short-range cooperativity evident only on one of the two strands. Our findings suggest a mechanism by which cardiac muscle is regulated by narrow range Ca2+ fluctuations.
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Affiliation(s)
- Cristina M Risi
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Ian Pepper
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Betty Belknap
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Maicon Landim-Vieira
- Department of Biomedical Sciences, Florida State University College of Medicine, Tallahassee, FL 32304
| | - Howard D White
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Kelly Dryden
- Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22903
| | - Jose R Pinto
- Department of Biomedical Sciences, Florida State University College of Medicine, Tallahassee, FL 32304
| | - P Bryant Chase
- Department of Biological Science, Florida State University, Tallahassee, FL 32306
| | - Vitold E Galkin
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507;
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19
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Lehman W, Pavadai E, Rynkiewicz MJ. C-terminal troponin-I residues trap tropomyosin in the muscle thin filament blocked-state. Biochem Biophys Res Commun 2021; 551:27-32. [PMID: 33714756 DOI: 10.1016/j.bbrc.2021.03.010] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2021] [Accepted: 03/02/2021] [Indexed: 12/16/2022]
Abstract
Tropomyosin and troponin regulate muscle contraction by participating in a macromolecular scale steric-mechanism to control myosin-crossbridge - actin interactions and consequently contraction. At low-Ca2+, the C-terminal 30% of troponin subunit-I (TnI) is proposed to trap tropomyosin in a position on thin filaments that sterically interferes with myosin-binding, thus causing muscle relaxation. In contrast, at high-Ca2+, inhibition is released after the C-terminal domains dissociate from F-actin-tropomyosin as its component switch-peptide domain binds to the N-lobe of troponin-C (TnC). Recent, paradigm-shifting, cryo-EM reconstructions by the Namba group have revealed density attributed to TnI along cardiac muscle thin filaments at both low- and high-Ca2+ concentration. Modeling the reconstructions showed expected high-Ca2+ hydrophobic interactions of the TnI switch-peptide and TnC. However, under low-Ca2+ conditions, sparse interactions of TnI and tropomyosin, and in particular juxtaposition of non-polar switch-peptide residues and charged tropomyosin amino acids in the published model seem difficult to reconcile with an expected steric-blocking conformation. This anomaly is likely due to inaccurate fitting of tropomyosin into the cryo-EM volume. In the current study, the low-Ca2+ cryo-EM volume was fitted with a more accurate tropomyosin model and representation of cardiac TnI. Our results show that at low-Ca2+ a cluster of hydrophobic residues at the TnI switch-peptide and adjacent H4 helix (Ala149, Ala151, Met 154, Leu159, Gly160, Ala161, Ala163, Leu167, Leu169, Ala171, Leu173) draw-in tropomyosin surface residues (Ile143, Ile146, Ala151, Ile154), presumably attracting the entire tropomyosin cable to its myosin-blocking position on actin. The modeling confirms that neighboring TnI "inhibitory domain" residues (Arg145, Arg148) bind to thin filaments at actin residue Asp25, as previously suggested. ClusPro docking of TnI residues 137-184 to actin-tropomyosin, including the TnI inhibitory-domain, switch-peptide and Helix H4, verified the modeled configuration. Our residue-to-residue contact-mapping of the TnI-tropomyosin association lends itself to experimental validation and functional localization of disease-bearing mutations.
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Affiliation(s)
- William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, MA, 02118, USA.
| | - Elumalai Pavadai
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, MA, 02118, USA.
| | - Michael J Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, MA, 02118, USA.
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20
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Mijailovich SM, Prodanovic M, Poggesi C, Geeves MA, Regnier M. Multiscale modeling of twitch contractions in cardiac trabeculae. J Gen Physiol 2021; 153:e202012604. [PMID: 33512405 PMCID: PMC7852458 DOI: 10.1085/jgp.202012604] [Citation(s) in RCA: 23] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2020] [Revised: 08/31/2020] [Accepted: 11/20/2020] [Indexed: 12/23/2022] Open
Abstract
Understanding the dynamics of a cardiac muscle twitch contraction is complex because it requires a detailed understanding of the kinetic processes of the Ca2+ transient, thin-filament activation, and the myosin-actin cross-bridge chemomechanical cycle. Each of these steps has been well defined individually, but understanding how all three of the processes operate in combination is a far more complex problem. Computational modeling has the potential to provide detailed insight into each of these processes, how the dynamics of each process affect the complexity of contractile behavior, and how perturbations such as mutations in sarcomere proteins affect the complex interactions of all of these processes. The mechanisms involved in relaxation of tension during a cardiac twitch have been particularly difficult to discern due to nonhomogeneous sarcomere lengthening during relaxation. Here we use the multiscale MUSICO platform to model rat trabecular twitches. Validation of computational models is dependent on being able to simulate different experimental datasets, but there has been a paucity of data that can provide all of the required parameters in a single experiment, such as simultaneous measurements of force, intracellular Ca2+ transients, and sarcomere length dynamics. In this study, we used data from different studies collected under similar experimental conditions to provide information for all the required parameters. Our simulations established that twitches either in an isometric sarcomere or in fixed-length, multiple-sarcomere trabeculae replicate the experimental observations if models incorporate a length-tension relationship for the nonlinear series elasticity of muscle preparations and a scheme for thick-filament regulation. The thick-filament regulation assumes an off state in which myosin heads are parked onto the thick-filament backbone and are unable to interact with actin, a state analogous to the super-relaxed state. Including these two mechanisms provided simulations that accurately predict twitch contractions over a range of different conditions.
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Affiliation(s)
| | - Momcilo Prodanovic
- Bioengineering Research and Development Center, Kragujevac, Serbia
- Faculty of Engineering, University of Kragujevac, Kragujevac, Serbia
| | - Corrado Poggesi
- Department of Experimental & Clinical Medicine, University of Florence, Florence, Italy
| | | | - Michael Regnier
- Department of Bioengineering, University of Washington, Seattle, WA
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21
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Mijailovich SM, Prodanovic M, Poggesi C, Powers JD, Davis J, Geeves MA, Regnier M. The effect of variable troponin C mutation thin filament incorporation on cardiac muscle twitch contractions. J Mol Cell Cardiol 2021; 155:112-124. [PMID: 33636222 DOI: 10.1016/j.yjmcc.2021.02.009] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/11/2020] [Revised: 02/17/2021] [Accepted: 02/19/2021] [Indexed: 11/19/2022]
Abstract
One of the complexities of understanding the pathology of familial forms of cardiac diseases is the level of mutation incorporation in sarcomeres. Computational models of the sarcomere that are spatially explicit offer an approach to study aspects of mutational incorporation into myofilaments that are more challenging to get at experimentally. We studied two well characterized mutations of cardiac TnC, L48Q and I61Q, that decrease or increase the release rate of Ca2+ from cTnC, k-Ca, resulting in HCM and DCM respectively [1]. Expression of these mutations in transgenic mice was used to provide experimental data for incorporation of 30 and 50% (respectively) into sarcomeres. Here we demonstrate that fixed length twitch contractions of trabeculae from mice containing mutant differ from WT; L48Q trabeculae have slower relaxation while I61Q trabeculae have markedly reduced peak tension. Using our multiscale modelling approach [2] we were able to describe the tension transients of WT mouse myocardium. Tension transients for the mutant cTnCs were simulated with changes in k-Ca, measured experimentally for each cTnC mutant in whole troponin complex, a change in the affinity of cTnC for cTnI, and a reduction in the number of detached crossbridges available for binding. A major advantage of the multiscale explicit 3-D model is that it predicts the effects of variable mutation incorporation, and the effects of variations in mutation distribution within thin filaments in sarcomeres. Such effects are currently impossible to explore experimentally. We explored random and clustered distributions of mutant cTnCs in thin filaments, as well as distributions of individual thin filaments with only WT or mutant cTnCs present. The effects of variable amounts of incorporation and non-random distribution of mutant cTnCs are more marked for I61Q than L48Q cTnC. We conclude that this approach can be effective for study on mutations in multiple proteins of the sarcomere. SUMMARY: A challenge in experimental studies of diseases is accounting for the effect of variable mutation incorporation into myofilaments. Here we use a spatially explicit computational approach, informed by experimental data from transgenic mice expressing one of two mutations in cardiac Troponin C that increase or decrease calcium sensitivity. We demonstrate that the model can accurately describe twitch contractions for the data and go on to explore the effect of variable mutant incorporation and localization on simulated cardiac muscle twitches.
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Affiliation(s)
| | - Momcilo Prodanovic
- Bioengineering Research and Development Center (BioIRC), Kragujevac 34000, Serbia; Faculty of Engineering, University of Kragujevac, Kragujevac 34000, Serbia
| | - Corrado Poggesi
- Department of Experimental & Clinical Medicine, University of Florence, Florence 50134, Italy
| | - Joseph D Powers
- Department of Bioengineering, University of Washington, Seattle, WA 98105, USA; Dept. of Bioengineering, University of California, San Diego, CA 92093, USA
| | - Jennifer Davis
- Department of Bioengineering, University of Washington, Seattle, WA 98105, USA
| | - Michael A Geeves
- Dept. of Biosciences, University of Kent, Canterbury, Kent CT2 7NJ, UK
| | - Michael Regnier
- Department of Bioengineering, University of Washington, Seattle, WA 98105, USA
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22
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A comprehensive guide to genetic variants and post-translational modifications of cardiac troponin C. J Muscle Res Cell Motil 2020; 42:323-342. [PMID: 33179204 DOI: 10.1007/s10974-020-09592-5] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2020] [Accepted: 10/24/2020] [Indexed: 02/07/2023]
Abstract
Familial cardiomyopathy is an inherited disease that affects the structure and function of heart muscle and has an extreme range of phenotypes. Among the millions of affected individuals, patients with hypertrophic (HCM), dilated (DCM), or left ventricular non-compaction (LVNC) cardiomyopathy can experience morphologic changes of the heart which lead to sudden death in the most detrimental cases. TNNC1, the gene that codes for cardiac troponin C (cTnC), is a sarcomere gene associated with cardiomyopathies in which probands exhibit young age of presentation and high death, transplant or ventricular fibrillation events relative to TNNT2 and TNNI3 probands. Using GnomAD, ClinVar, UniProt and PhosphoSitePlus databases and published literature, an extensive list to date of identified genetic variants in TNNC1 and post-translational modifications (PTMs) in cTnC was compiled. Additionally, a recent cryo-EM structure of the cardiac thin filament regulatory unit was used to localize each functionally studied amino acid variant and each PTM (acetylation, glycation, s-nitrosylation, phosphorylation) in the structure of cTnC. TNNC1 has a large number of variants (> 100) relative to other genes of the same transcript size. Surprisingly, the mapped variant amino acids and PTMs are distributed throughout the cTnC structure. While many cardiomyopathy-associated variants are localized in α-helical regions of cTnC, this was not statistically significant χ2 (p = 0.72). Exploring the variants in TNNC1 and PTMs of cTnC in the contexts of cardiomyopathy association, physiological modulation and potential non-canonical roles provides insights into the normal function of cTnC along with the many facets of TNNC1 as a cardiomyopathic gene.
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23
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Doran MH, Pavadai E, Rynkiewicz MJ, Walklate J, Bullitt E, Moore JR, Regnier M, Geeves MA, Lehman W. Cryo-EM and Molecular Docking Shows Myosin Loop 4 Contacts Actin and Tropomyosin on Thin Filaments. Biophys J 2020; 119:821-830. [PMID: 32730789 DOI: 10.1016/j.bpj.2020.07.006] [Citation(s) in RCA: 35] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2020] [Accepted: 07/07/2020] [Indexed: 12/29/2022] Open
Abstract
The motor protein myosin drives muscle and nonmuscle motility by binding to and moving along actin of thin filaments. Myosin binding to actin also modulates interactions of the regulatory protein, tropomyosin, on thin filaments, and conversely tropomyosin affects myosin binding to actin. Insight into this reciprocity will facilitate a molecular level elucidation of tropomyosin regulation of myosin interaction with actin in muscle contraction, and in turn, promote better understanding of nonmuscle cell motility. Indeed, experimental approaches such as fiber diffraction, cryoelectron microscopy, and three-dimensional reconstruction have long been used to define regulatory interaction of tropomyosin and myosin on actin at a structural level. However, their limited resolution has not proven sufficient to determine tropomyosin and myosin contacts at an atomic-level and thus to fully substantiate possible functional contributions. To overcome this deficiency, we have followed a hybrid approach by performing new cryogenic electron microscopy reconstruction of myosin-S1-decorated F-actin-tropomyosin together with atomic scale protein-protein docking of tropomyosin to the EM models. Here, cryo-EM data were derived from filaments reconstituted with α1-actin, cardiac αα-tropomyosin, and masseter muscle β-myosin complexes; masseter myosin, which shares sequence identity with β-cardiac myosin-heavy chain, was used because of its stability in vitro. The data were used to build an atomic model of the tropomyosin cable that fits onto the actin filament between the tip of the myosin head and a cleft on the innermost edge of actin subunits. The docking and atomic scale fitting showed multiple discrete interactions of myosin loop 4 and acidic residues on successive 39-42 residue-long tropomyosin pseudorepeats. The contacts between S1 and tropomyosin on actin appear to compete with and displace ones normally found between actin and tropomyosin on myosin-free thin filaments in relaxed muscle, thus restructuring the filament during myosin-induced activation.
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Affiliation(s)
- Matthew H Doran
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts
| | - Elumalai Pavadai
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts
| | - Michael J Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts
| | - Jonathan Walklate
- School of Biosciences, University of Kent, Canterbury, Kent, United Kingdom
| | - Esther Bullitt
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts
| | - Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts-Lowell, Lowell, Massachusetts
| | - Michael Regnier
- Department of Bioengineering, University of Washington, Seattle, Washington
| | - Michael A Geeves
- School of Biosciences, University of Kent, Canterbury, Kent, United Kingdom
| | - William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts.
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24
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Pavadai E, Lehman W, Rynkiewicz MJ. Protein-Protein Docking Reveals Dynamic Interactions of Tropomyosin on Actin Filaments. Biophys J 2020; 119:75-86. [PMID: 32521240 DOI: 10.1016/j.bpj.2020.05.017] [Citation(s) in RCA: 24] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2020] [Revised: 05/01/2020] [Accepted: 05/18/2020] [Indexed: 01/01/2023] Open
Abstract
Experimental approaches such as fiber diffraction and cryo-electron microscopy reconstruction have defined regulatory positions of tropomyosin on actin but have not, as yet, succeeded at determining key atomic-level contacts between these proteins or fully substantiated the dynamics of their interactions at a structural level. To overcome this deficiency, we have previously employed computational approaches to deduce global dynamics of thin filament components by energy landscape determination and molecular dynamics simulations. Still, these approaches remain computationally challenging for any complex and large macromolecular assembly like the thin filament. For example, tropomyosin cable wrapping around actin of thin filaments features both head-to-tail polymeric interactions and local twisting, both of which depart from strict superhelical symmetry. This produces a complex energy surface that is difficult to model and thus to evaluate globally. Therefore, at this stage of our understanding, assessing global molecular dynamics can prove to be inherently impractical. As an alternative, we adopted a "divide and conquer" protocol to investigate actin-tropomyosin interactions at an atomistic level. Here, we first employed unbiased protein-protein docking tools to identify binding specificity of individual tropomyosin pseudorepeat segments over the actin surface. Accordingly, tropomyosin "ligand" segments were rotated and translated over potential "target" binding sites on F-actin where the corresponding interaction energetics of billions of conformational poses were ranked by the programs PIPER and ClusPro. These data were used to assess favorable interactions and then to rebuild models of seamless and continuous tropomyosin cables over the F-actin substrate, which were optimized further by flexible fitting routines and molecular dynamics. The models generated azimuthally distinct regulatory positions for tropomyosin cables along thin filaments on actin dominated by stereo-specific head-to-tail overlap linkage. The outcomes are in good agreement with current cryo-electron microscopy topology and consistent with long-thought residue-to-residue interactions between actin and tropomyosin.
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Affiliation(s)
- Elumalai Pavadai
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts
| | - William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts.
| | - Michael J Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts
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25
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Spinks GM. Advanced Actuator Materials Powered by Biomimetic Helical Fiber Topologies. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2020; 32:e1904093. [PMID: 31793710 DOI: 10.1002/adma.201904093] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/28/2019] [Revised: 09/18/2019] [Indexed: 06/10/2023]
Abstract
Helical constructs are ubiquitous in nature at all size domains, from molecular to macroscopic. The helical topology confers unique mechanical functions that activate certain phenomena, such as twining vines and vital cellular functions like the folding and packing of DNA into chromosomes. The understanding of active mechanical processes in plants, certain musculature in animals, and some biochemical processes in cells provides insight into the versatility of the helix. Most of these natural systems consist of helically oriented filaments embedded in a compliant matrix. In some cases, the matrix can change volume and in others the filaments can contract and the matrix is passive. In both cases, the helically arranged fibers determine the overall shape change with a great variety of responses involving length contraction/elongation, twisting, bending, and coiling. Synthetic actuator materials and systems that employ helical topologies have been described recently and demonstrate many fascinating and complex shape changes. However, significant new opportunities exist to mimic some of the most remarkable actions in nature, including the Vorticella's coiling stalk and DNA's supercoils, in the quest for superior artificial muscles.
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Affiliation(s)
- Geoffrey M Spinks
- Australian Institute for Innovative Materials, University of Wollongong, Wollongong, NSW, 2522, Australia
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Kalda M, Vendelin M. Cardiac muscle regulatory units are predicted to interact stronger than neighboring cross-bridges. Sci Rep 2020; 10:5530. [PMID: 32218497 PMCID: PMC7099078 DOI: 10.1038/s41598-020-62452-7] [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] [Received: 05/31/2019] [Accepted: 03/14/2020] [Indexed: 11/09/2022] Open
Abstract
Strong interactions between cross-bridges (XB) and regulatory units (RU) lead to a steep response of cardiac muscle to an increase in intracellular calcium. We developed a model to quantitatively assess the influence of different types of interactions within the sarcomere on the properties of cardiac muscle. In the model, the ensembles consisting of cross-bridge groups connected by elastic tropomyosin are introduced, and their dynamics is described by a set of partial differential equations. Through large scans in the free energy landscape, we demonstrate the different influence of RU-RU, XB-XB, and XB-RU interactions on the cooperativity coefficient of calcium binding, developed maximal force, and calcium sensitivity. The model solution was fitted to reproduce experimental data on force development during isometric contraction, shortening in physiological contraction, and ATP consumption by acto-myosin. On the basis of the fits, we quantified the free energy change introduced through RU-RU and XB-XB interactions and showed that RU-RU interaction leads to ~ 5 times larger change in the free energy profile of the reaction than XB-XB interaction. Due to the deterministic description of muscle contraction and its thermodynamic consistency, we envision that the developed model can be used to study heart muscle biophysics on tissue and organ levels.
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Affiliation(s)
- Mari Kalda
- Laboratory of Systems Biology, Department of Cybernetics, School of Science, Tallinn University of Technology, 12618, Tallinn, Estonia
| | - Marko Vendelin
- Laboratory of Systems Biology, Department of Cybernetics, School of Science, Tallinn University of Technology, 12618, Tallinn, Estonia.
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Lehman W, Rynkiewicz MJ, Moore JR. A new twist on tropomyosin binding to actin filaments: perspectives on thin filament function, assembly and biomechanics. J Muscle Res Cell Motil 2020; 41:23-38. [PMID: 30771202 PMCID: PMC6697252 DOI: 10.1007/s10974-019-09501-5] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2018] [Accepted: 02/07/2019] [Indexed: 02/07/2023]
Abstract
Tropomyosin, best known for its role in the steric regulation of muscle contraction, polymerizes head-to-tail to form cables localized along the length of both muscle and non-muscle actin-based thin filaments. In skeletal and cardiac muscles, tropomyosin, under the control of troponin and myosin, moves in a cooperative manner between blocked, closed and open positions on filaments, thereby masking and exposing actin-binding sites necessary for myosin crossbridge head interactions. While the coiled-coil signature of tropomyosin appears to be simple, closer inspection reveals surprising structural complexity required to perform its role in steric regulation. For example, component α-helices of coiled coils are typically zippered together along a continuous core hydrophobic stripe. Tropomyosin, however, contains a number of anomalous, functionally controversial, core amino acid residues. We argue that the atypical residues at this interface, including clusters of alanines and a charged aspartate, are required for preshaping tropomyosin to readily fit to the surface of the actin filament, but do so without compromising tropomyosin rigidity once the filament is assembled. Indeed, persistence length measurements of tropomyosin are characteristic of a semi-rigid cable, in this case conducive to cooperative movement on thin filaments. In addition, we also maintain that tropomyosin displays largely unrecognized and residue-specific torsional variance, which is involved in optimizing contacts between actin and tropomyosin on the assembled thin filament. Corresponding twist-induced stiffness may also enhance cooperative translocation of tropomyosin across actin filaments. We conclude that anomalous core residues of tropomyosin facilitate thin filament regulatory behavior in a multifaceted way.
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Affiliation(s)
- William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts, U.S.A
| | - Michael J. Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, Massachusetts, U.S.A
| | - Jeffrey R. Moore
- Department of Biological Sciences, University of Massachusetts-Lowell, Lowell, Massachusetts, U.S.A
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Arata T. Myosin and Other Energy-Transducing ATPases: Structural Dynamics Studied by Electron Paramagnetic Resonance. Int J Mol Sci 2020; 21:E672. [PMID: 31968570 PMCID: PMC7014194 DOI: 10.3390/ijms21020672] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2019] [Revised: 01/05/2020] [Accepted: 01/06/2020] [Indexed: 02/07/2023] Open
Abstract
The objective of this article was to document the energy-transducing and regulatory interactions in supramolecular complexes such as motor, pump, and clock ATPases. The dynamics and structural features were characterized by motion and distance measurements using spin-labeling electron paramagnetic resonance (EPR) spectroscopy. In particular, we focused on myosin ATPase with actin-troponin-tropomyosin, neural kinesin ATPase with microtubule, P-type ion-motive ATPase, and cyanobacterial clock ATPase. Finally, we have described the relationships or common principles among the molecular mechanisms of various energy-transducing systems and how the large-scale thermal structural transition of flexible elements from one state to the other precedes the subsequent irreversible chemical reactions.
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Affiliation(s)
- Toshiaki Arata
- Department of Biology, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan
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Yamada Y, Namba K, Fujii T. Cardiac muscle thin filament structures reveal calcium regulatory mechanism. Nat Commun 2020; 11:153. [PMID: 31919429 PMCID: PMC6952405 DOI: 10.1038/s41467-019-14008-1] [Citation(s) in RCA: 138] [Impact Index Per Article: 34.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2019] [Accepted: 12/12/2019] [Indexed: 01/04/2023] Open
Abstract
Contraction of striated muscles is driven by cyclic interactions of myosin head projecting from the thick filament with actin filament and is regulated by Ca2+ released from sarcoplasmic reticulum. Muscle thin filament consists of actin, tropomyosin and troponin, and Ca2+ binding to troponin triggers conformational changes of troponin and tropomyosin to allow actin-myosin interactions. However, the structural changes involved in this regulatory mechanism remain unknown. Here we report the structures of human cardiac muscle thin filament in the absence and presence of Ca2+ by electron cryomicroscopy. Molecular models in the two states built based on available crystal structures reveal the structures of a C-terminal region of troponin I and an N-terminal region of troponin T in complex with the head-to-tail junction of tropomyosin together with the troponin core on actin filament. Structural changes of the thin filament upon Ca2+ binding now reveal the mechanism of Ca2+ regulation of muscle contraction.
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Affiliation(s)
- Yurika Yamada
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka, 565-0871, Japan
| | - Keiichi Namba
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka, 565-0871, Japan.
- RIKEN Center for Biosystems Dynamics Research and SPring-8 Center, 1-3 Yamadaoka, Suita, Osaka, 565-0871, Japan.
- JEOL YOKOGUSHI Research Alliance Laboratories, Osaka University, 1-3 Yamadaoka, Suita, Osaka, 565-0871, Japan.
| | - Takashi Fujii
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka, 565-0871, Japan.
- JEOL YOKOGUSHI Research Alliance Laboratories, Osaka University, 1-3 Yamadaoka, Suita, Osaka, 565-0871, Japan.
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30
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Aboelkassem Y, McCabe KJ, Huber GA, Regnier M, McCammon JA, McCulloch AD. A Stochastic Multiscale Model of Cardiac Thin Filament Activation Using Brownian-Langevin Dynamics. Biophys J 2019; 117:2255-2272. [PMID: 31547973 PMCID: PMC6990154 DOI: 10.1016/j.bpj.2019.08.003] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2019] [Revised: 07/31/2019] [Accepted: 08/02/2019] [Indexed: 11/16/2022] Open
Abstract
We use Brownian-Langevin dynamics principles to derive a coarse-graining multiscale myofilament model that can describe the thin-filament activation process during contraction. The model links atomistic molecular simulations of protein-protein interactions in the thin-filament regulatory unit to sarcomere-level activation dynamics. We first calculate the molecular interaction energy between tropomyosin and actin surface using Brownian dynamics simulations. This energy profile is then generalized to account for the observed tropomyosin transitions between its regulatory stable states. The generalized energy landscape then served as a basis for developing a filament-scale model using Langevin dynamics. This integrated analysis, spanning molecular to thin-filament scales, is capable of tracking the events of the tropomyosin conformational changes as it moves over the actin surface. The tropomyosin coil with flexible overlap regions between adjacent tropomyosins is represented in the model as a system of coupled stochastic ordinary differential equations. The proposed multiscale approach provides a more detailed molecular connection between tropomyosin dynamics, the trompomyosin-actin interaction-energy landscape, and the generated force by the sarcomere.
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Affiliation(s)
- Yasser Aboelkassem
- Department of Bioengineering, University of California San Diego, La Jolla, California.
| | - Kimberly J McCabe
- Department of Bioengineering, University of California San Diego, La Jolla, California
| | - Gary A Huber
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California
| | - Michael Regnier
- Department of Bioengineering, University of Washington, Seattle, Washington
| | - J Andrew McCammon
- Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California
| | - Andrew D McCulloch
- Department of Bioengineering, University of California San Diego, La Jolla, California
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31
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Geeves MA, Lehrer SS, Lehman W. The mechanism of thin filament regulation: Models in conflict? J Gen Physiol 2019; 151:1265-1271. [PMID: 31570503 PMCID: PMC6829557 DOI: 10.1085/jgp.201912446] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2019] [Accepted: 09/05/2019] [Indexed: 01/29/2023] Open
Abstract
Evidence on two- and three-state models of the calcium regulation models of muscle contractions remain in favor of three-state models. In a recent JGP article, Heeley et al. (2019. J. Gen. Physiol. https://doi.org/10.1085/jgp.201812198) reopened the debate about two- versus three-state models of thin filament regulation. The authors review their work, which measures the rate constant of Pi release from myosin.ADP.Pi activated by actin or thin filaments under a variety of conditions. They conclude that their data can be described by a two-state model and raise doubts about the generally accepted three-state model as originally formulated by McKillop and Geeves (1993. Biophys. J.https://doi.org/10.1016/S0006-3495(93)81110-X). However, in the following article, we follow Plato’s dictum that “twice and thrice over, as they say, good it is to repeat and review what is good.” We have therefore reviewed the evidence for the three- and two-state models and present our view that the evidence is overwhelmingly in favor of three structural states of the thin filament, which regulate access of myosin to its binding sites on actin and, hence, muscle contractility.
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Affiliation(s)
| | | | - William Lehman
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA
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32
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Abdullah S, Lynn ML, McConnell MT, Klass MM, Baldo AP, Schwartz SD, Tardiff JC. FRET-based analysis of the cardiac troponin T linker region reveals the structural basis of the hypertrophic cardiomyopathy-causing Δ160E mutation. J Biol Chem 2019; 294:14634-14647. [PMID: 31387947 DOI: 10.1074/jbc.ra118.005098] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2018] [Revised: 07/12/2019] [Indexed: 11/06/2022] Open
Abstract
Mutations in the cardiac thin filament (TF) have highly variable effects on the regulatory function of the cardiac sarcomere. Understanding the molecular-level dysfunction elicited by TF mutations is crucial to elucidate cardiac disease mechanisms. The hypertrophic cardiomyopathy-causing cardiac troponin T (cTnT) mutation Δ160Glu (Δ160E) is located in a putative "hinge" adjacent to an unstructured linker connecting domains TNT1 and TNT2. Currently, no high-resolution structure exists for this region, limiting significantly our ability to understand its role in myofilament activation and the molecular mechanism of mutation-induced dysfunction. Previous regulated in vitro motility data have indicated mutation-induced impairment of weak actomyosin interactions. We hypothesized that cTnT-Δ160E repositions the flexible linker, altering weak actomyosin electrostatic binding and acting as a biophysical trigger for impaired contractility and the observed remodeling. Using time-resolved FRET and an all-atom TF model, here we first defined the WT structure of the cTnT-linker region and then identified Δ160E mutation-induced positional changes. Our results suggest that the WT linker runs alongside the C terminus of tropomyosin. The Δ160E-induced structural changes moved the linker closer to the tropomyosin C terminus, an effect that was more pronounced in the presence of myosin subfragment (S1) heads, supporting previous findings. Our in silico model fully supported this result, indicating a mutation-induced decrease in linker flexibility. Our findings provide a framework for understanding basic pathogenic mechanisms that drive severe clinical hypertrophic cardiomyopathy phenotypes and for identifying structural targets for intervention that can be tested in silico and in vitro.
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Affiliation(s)
- Salwa Abdullah
- Department of Cellular and Molecular Medicine, University of Arizona, Tucson, Arizona, 85721
| | - Melissa L Lynn
- Department of Biomedical Engineering, University of Arizona, Tucson, Arizona, 85721
| | - Mark T McConnell
- Department of Biomedical Engineering, University of Arizona, Tucson, Arizona, 85721
| | - Matthew M Klass
- Department of Physiological Sciences, University of Arizona, Tucson, Arizona, 85721
| | - Anthony P Baldo
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona, 85721
| | - Steven D Schwartz
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona, 85721
| | - Jil C Tardiff
- Department of Cellular and Molecular Medicine, University of Arizona, Tucson, Arizona, 85721 .,Department of Biomedical Engineering, University of Arizona, Tucson, Arizona, 85721.,Department of Physiological Sciences, University of Arizona, Tucson, Arizona, 85721.,Department of Medicine, University of Arizona, Tucson, Arizona 85721
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33
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Thin filament dysfunctions caused by mutations in tropomyosin Tpm3.12 and Tpm1.1. J Muscle Res Cell Motil 2019; 41:39-53. [PMID: 31270709 PMCID: PMC7109180 DOI: 10.1007/s10974-019-09532-y] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2019] [Accepted: 06/26/2019] [Indexed: 12/14/2022]
Abstract
Tropomyosin is the major regulator of the thin filament. In striated muscle its function is to bind troponin complex and control the access of myosin heads to actin in a Ca2+-dependent manner. It also participates in the maintenance of thin filament length by regulation of tropomodulin and leiomodin, the pointed end-binding proteins. Because the size of the overlap between actin and myosin filaments affects the number of myosin heads which interact with actin, the filament length is one of the determinants of force development. Numerous point mutations in genes encoding tropomyosin lead to single amino acid substitutions along the entire length of the coiled coil that are associated with various types of cardiomyopathy and skeletal muscle disease. Specific regions of tropomyosin interact with different binding partners; therefore, the mutations affect diverse tropomyosin functions. In this review, results of studies on mutations in the genes TPM1 and TPM3, encoding Tpm1.1 and Tpm3.12, are described. The paper is particularly focused on mutation-dependent alterations in the mechanisms of actin-myosin interactions and dynamics of the thin filament at the pointed end.
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34
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Lehman W, Moore JR, Campbell SG, Rynkiewicz MJ. The Effect of Tropomyosin Mutations on Actin-Tropomyosin Binding: In Search of Lost Time. Biophys J 2019; 116:2275-2284. [PMID: 31130236 PMCID: PMC6588729 DOI: 10.1016/j.bpj.2019.05.009] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2019] [Revised: 05/06/2019] [Accepted: 05/09/2019] [Indexed: 12/14/2022] Open
Abstract
The initial binding of tropomyosin onto actin filaments and then its polymerization into continuous cables on the filament surface must be precisely tuned to overall thin-filament structure, function, and performance. Low-affinity interaction of tropomyosin with actin has to be sufficiently strong to localize the tropomyosin on actin, yet not so tight that regulatory movement on filaments is curtailed. Likewise, head-to-tail association of tropomyosin molecules must be favorable enough to promote tropomyosin cable formation but not so tenacious that polymerization precedes filament binding. Arguably, little molecular detail on early tropomyosin binding steps has been revealed since Wegner's seminal studies on filament assembly almost 40 years ago. Thus, interpretation of mutation-based actin-tropomyosin binding anomalies leading to cardiomyopathies cannot be described fully. In vitro, tropomyosin binding is masked by explosive tropomyosin polymerization once cable formation is initiated on actin filaments. In contrast, in silico analysis, characterizing molecular dynamics simulations of single wild-type and mutant tropomyosin molecules on F-actin, is not complicated by tropomyosin polymerization at all. In fact, molecular dynamics performed here demonstrates that a midpiece tropomyosin domain is essential for normal actin-tropomyosin interaction and that this interaction is strictly conserved in a number of tropomyosin mutant species. Elsewhere along these mutant molecules, twisting and bending corrupts the tropomyosin superhelices as they "lose their grip" on F-actin. We propose that residual interactions displayed by these mutant tropomyosin structures with actin mimic ones that occur in early stages of thin-filament generation, as if the mutants are recapitulating the assembly process but in reverse. We conclude therefore that an initial binding step in tropomyosin assembly onto actin involves interaction of the essential centrally located domain.
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Affiliation(s)
- William Lehman
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts.
| | - Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts-Lowell, Lowell, Massachusetts
| | - Stuart G Campbell
- Departments of Biomedical Engineering and Cellular and Molecular Physiology, Yale University, New Haven, Connecticut
| | - Michael J Rynkiewicz
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts
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35
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Zheng W, Wen H. Molecular dynamics simulation of tropomyosin bound to actins/myosin in the closed and open states. Proteins 2019; 87:805-814. [PMID: 31090107 DOI: 10.1002/prot.25707] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2018] [Revised: 04/22/2019] [Accepted: 05/05/2019] [Indexed: 11/09/2022]
Abstract
Tropomyosin (Tpm) is a dimeric coiled-coil protein that binds to filamentous actin, and regulates actin-myosin interaction by moving between three positions corresponding to the blocked, closed, and open states. To elucidate how Tpm undergoes transitions between these functional states, we have built structural models and conducted extensive molecular dynamics simulations of the Tpm-actins/myosin complex in the closed and open states (total simulation time >1.4 μs). Based on the simulation trajectories, we have analyzed the dynamics and energetics of a truncated Tpm interacting with actins/myosin under the physiological conditions. Our simulations have shown distinct dynamics of four Tpm periods (P3-P6), featuring pronounced biased fluctuations of P4 and P5 toward the open position in the closed state, which is consistent with a conformational selection mechanism for Tpm-regulated myosin binding. Additionally, we have identified key residues of Tpm specifically binding to actins/myosin in the closed and open state. Some of them were validated as functionally important in comparison with past functional/clinical studies, and the rest will make promising targets for future mutational experiments.
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Affiliation(s)
- Wenjun Zheng
- Department of Physics, University at Buffalo, Buffalo, New York
| | - Han Wen
- Department of Physics, University at Buffalo, Buffalo, New York
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36
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Marston S, Zamora JE. Troponin structure and function: a view of recent progress. J Muscle Res Cell Motil 2019; 41:71-89. [PMID: 31030382 PMCID: PMC7109197 DOI: 10.1007/s10974-019-09513-1] [Citation(s) in RCA: 55] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/09/2019] [Accepted: 04/12/2019] [Indexed: 12/15/2022]
Abstract
The molecular mechanism by which Ca2+ binding and phosphorylation regulate muscle contraction through Troponin is not yet fully understood. Revealing the differences between the relaxed and active structure of cTn, as well as the conformational changes that follow phosphorylation has remained a challenge for structural biologists over the years. Here we review the current understanding of how Ca2+, phosphorylation and disease-causing mutations affect the structure and dynamics of troponin to regulate the thin filament based on electron microscopy, X-ray diffraction, NMR and molecular dynamics methodologies.
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Affiliation(s)
- Steven Marston
- NHLI and Chemistry Departments, Imperial College London, W12 0NN, London, UK.
| | - Juan Eiros Zamora
- NHLI and Chemistry Departments, Imperial College London, W12 0NN, London, UK
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37
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Mijailovich SM, Stojanovic B, Nedic D, Svicevic M, Geeves MA, Irving TC, Granzier HL. Nebulin and titin modulate cross-bridge cycling and length-dependent calcium sensitivity. J Gen Physiol 2019; 151:680-704. [PMID: 30948421 PMCID: PMC6504291 DOI: 10.1085/jgp.201812165] [Citation(s) in RCA: 25] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2018] [Revised: 01/15/2019] [Accepted: 03/03/2019] [Indexed: 12/13/2022] Open
Abstract
Various mutations in the structural proteins nebulin and titin that are present in human disease are known to affect the contractility of striated muscle. Loss of nebulin is associated with reduced actin filament length and impairment of myosin binding to actin, whereas titin is thought to regulate muscle passive elasticity and is likely involved in length-dependent activation. Here, we sought to assess the modulation of muscle function by these sarcomeric proteins by using the computational platform muscle simulation code (MUSICO) to quantitatively separate the effects of structural changes, kinetics of cross-bridge cycling, and calcium sensitivity of the thin filaments. The simulations show that variation in thin filament length cannot by itself account for experimental observations of the contractility in nebulin-deficient muscle, but instead must be accompanied by a decreased myosin binding rate. Additionally, to match the observed calcium sensitivity, the rate of TnI detachment from actin needed to be increased. Simulations for cardiac muscle provided quantitative estimates of the effects of different titin-based passive elasticities on muscle force and activation in response to changes in sarcomere length and interfilament lattice spacing. Predicted force-pCa relations showed a decrease in both active tension and sensitivity to calcium with a decrease in passive tension and sarcomere length. We conclude that this behavior is caused by partial redistribution of the muscle load between active muscle force and titin-dependent passive force, and also by redistribution of stretch along the thin filament, which together modulate the release of TnI from actin. These data help advance understanding of how nebulin and titin mutations affect muscle function.
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Affiliation(s)
- Srboljub M Mijailovich
- Department of Chemistry and Chemical Biology, Northeastern University, Boston, MA .,Department of Biological Sciences, Illinois Institute of Technology, Chicago, IL
| | - Boban Stojanovic
- University of Kragujevac, Faculty of Science, Kragujevac, Serbia
| | - Djordje Nedic
- University of Kragujevac, Faculty of Science, Kragujevac, Serbia
| | - Marina Svicevic
- University of Kragujevac, Faculty of Science, Kragujevac, Serbia
| | - Michael A Geeves
- Department of Biosciences, University of Kent, Canterbury, Kent, UK
| | - Thomas C Irving
- Department of Biological Sciences, Illinois Institute of Technology, Chicago, IL
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Revealing the mechanism of how cardiac myosin-binding protein C N-terminal fragments sensitize thin filaments for myosin binding. Proc Natl Acad Sci U S A 2019; 116:6828-6835. [PMID: 30877248 PMCID: PMC6452674 DOI: 10.1073/pnas.1816480116] [Citation(s) in RCA: 36] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Diverse demands on cardiac muscle require the fine-tuning of contraction. Cardiac myosin binding protein-C (cMyBP-C) is involved in this regulation; however, its precise molecular mechanism of action remains uncertain. By imaging the interactions of single myosin and cMyBP-C molecules interacting with suspended thin filaments in vitro we observe cMyBP-C N-terminal fragments assist activation and modulate contraction velocity by affecting myosin binding to the thin filament. Fluorescent imaging of Cy3-labeled cMyBP-C revealed that it diffusively scans the thin filament and then strongly binds to displace tropomyosin and activate at low calcium. At high calcium, cMyBP-C decorates the filament more extensively, reducing myosin binding through competing with binding sites. Understanding the mechanism of MyBP-C action has important implications for heart disease. Cardiac muscle contraction is triggered by calcium binding to troponin. The consequent movement of tropomyosin permits myosin binding to actin, generating force. Cardiac myosin-binding protein C (cMyBP-C) plays a modulatory role in this activation process. One potential mechanism for the N-terminal domains of cMyBP-C to achieve this is by binding directly to the actin-thin filament at low calcium levels to enhance the movement of tropomyosin. To determine the molecular mechanisms by which cMyBP-C enhances myosin recruitment to the actin-thin filament, we directly visualized fluorescently labeled cMyBP-C N-terminal fragments and GFP-labeled myosin molecules binding to suspended actin-thin filaments in a fluorescence-based single-molecule microscopy assay. Binding of the C0C3 N-terminal cMyBP-C fragment to the thin filament enhanced myosin association at low calcium levels. However, at high calcium levels, C0C3 bound in clusters, blocking myosin binding. Dynamic imaging of thin filament-bound Cy3-C0C3 molecules demonstrated that these fragments diffuse along the thin filament before statically binding, suggesting a mechanism that involves a weak-binding mode to search for access to the thin filament and a tight-binding mode to sensitize the thin filament to calcium, thus enhancing myosin binding. Although shorter N-terminal fragments (Cy3-C0C1 and Cy3-C0C1f) bound to the thin filaments and displayed modes of motion on the thin filament similar to that of the Cy3-C0C3 fragment, the shorter fragments were unable to sensitize the thin filament. Therefore, the longer N-terminal fragment (C0C3) must possess the requisite domains needed to bind specifically to the thin filament in order for the cMyBP-C N terminus to modulate cardiac contractility.
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Matsuo T, Kono F, Fujiwara S. Effects of the cardiomyopathy-causing E244D mutation of troponin T on the structures of cardiac thin filaments studied by small-angle X-ray scattering. J Struct Biol 2018; 205:196-205. [PMID: 30599212 DOI: 10.1016/j.jsb.2018.12.005] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2018] [Revised: 12/07/2018] [Accepted: 12/10/2018] [Indexed: 02/04/2023]
Abstract
Small-angle X-ray scattering experiments were carried out to investigate the structural changes of cardiac thin filaments induced by the cardiomyopathy-causing E244D mutation in troponin T (TnT). We examined native thin filaments (NTF) from a bovine heart, reconstituted thin filaments containing human cardiac wild-type Tn (WTF), and filaments containing the E244D mutant of Tn (DTF), in the absence and presence of Ca2+. Analysis by model calculation showed that upon Ca2+-activation, tropomyosin (Tm) and Tn in the WTF and NTF moved together in a direction to expose myosin-binding sites on actin. On the other hand, Tm and Tn of the DTF moved in the opposite directions to each other upon Ca2+-activation. These movements caused Tm to expose more myosin-binding sites on actin than the WTF, suggesting that the affinity of myosin for actin is higher for the DTF. Thus, the mutation-induced structural changes in thin filaments would increase the number of myosin molecules bound to actin compared with the WTF, resulting in the force enhancement observed for the E244D mutation.
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Affiliation(s)
- Tatsuhito Matsuo
- Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology, 2-4 Shirakata, Tokai, Naka-Gun, Ibaraki 319-1106, Japan
| | - Fumiaki Kono
- Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology, 2-4 Shirakata, Tokai, Naka-Gun, Ibaraki 319-1106, Japan
| | - Satoru Fujiwara
- Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology, 2-4 Shirakata, Tokai, Naka-Gun, Ibaraki 319-1106, Japan.
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40
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Ly T, Pappas CT, Johnson D, Schlecht W, Colpan M, Galkin VE, Gregorio CC, Dong WJ, Kostyukova AS. Effects of cardiomyopathy-linked mutations K15N and R21H in tropomyosin on thin-filament regulation and pointed-end dynamics. Mol Biol Cell 2018; 30:268-281. [PMID: 30462572 PMCID: PMC6589558 DOI: 10.1091/mbc.e18-06-0406] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Missense mutations K15N and R21H in striated muscle tropomyosin are linked to dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM), respectively. Tropomyosin, together with the troponin complex, regulates muscle contraction and, along with tropomodulin and leiomodin, controls the uniform thin-filament lengths crucial for normal sarcomere structure and function. We used Förster resonance energy transfer to study effects of the tropomyosin mutations on the structure and kinetics of the cardiac troponin core domain associated with the Ca2+-dependent regulation of cardiac thin filaments. We found that the K15N mutation desensitizes thin filaments to Ca2+ and slows the kinetics of structural changes in troponin induced by Ca2+ dissociation from troponin, while the R21H mutation has almost no effect on these parameters. Expression of the K15N mutant in cardiomyocytes decreases leiomodin’s thin-filament pointed-end assembly but does not affect tropomodulin’s assembly at the pointed end. Our in vitro assays show that the R21H mutation causes a twofold decrease in tropomyosin’s affinity for F-actin and affects leiomodin’s function. We suggest that the K15N mutation causes DCM by altering Ca2+-dependent thin-filament regulation and that one of the possible HCM-causing mechanisms by the R21H mutation is through alteration of leiomodin’s function.
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Affiliation(s)
- Thu Ly
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164
| | - Christopher T Pappas
- Department of Cellular and Molecular Medicine and Sarver Molecular Cardiovascular Research Program, University of Arizona, Tucson, AZ 85721
| | - Dylan Johnson
- Department of Biochemistry and Molecular Biology, Brody School of Medicine at East Carolina University, Greenville, NC 27834
| | - William Schlecht
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164.,Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, WA 99164
| | - Mert Colpan
- Department of Cellular and Molecular Medicine and Sarver Molecular Cardiovascular Research Program, University of Arizona, Tucson, AZ 85721
| | - Vitold E Galkin
- Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507
| | - Carol C Gregorio
- Department of Cellular and Molecular Medicine and Sarver Molecular Cardiovascular Research Program, University of Arizona, Tucson, AZ 85721
| | - Wen-Ji Dong
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164.,Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, WA 99164
| | - Alla S Kostyukova
- Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164
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41
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Kiani FA, Lehman W, Fischer S, Rynkiewicz MJ. Spontaneous transitions of actin-bound tropomyosin toward blocked and closed states. J Gen Physiol 2018; 151:4-8. [PMID: 30442774 PMCID: PMC6314389 DOI: 10.1085/jgp.201812188] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2018] [Accepted: 10/30/2018] [Indexed: 11/20/2022] Open
Abstract
The detachment of myosin from actin is associated with tropomyosin adopting a blocked or closed state, but the mechanism is unclear. Using MD simulations, Kiani et al. show that tropomyosin undergoes spontaneous transitions on the F-actin surface toward blocked or closed positions. After muscle contraction, myosin cross-bridge heads detach from thin actin filaments during relaxation. Structural and kinetic data of cross-bridge–thin filament interactions have shown that tropomyosin’s position on F-actin is biased toward the blocked or closed states when myosin detaches. It is not clear if structural linkages between tropomyosin and myosin cross-bridge heads, or tropomyosin and Ca2+-free troponin, drive the process or whether tropomyosin movement is energetically independent of myosin and troponin influence. Here we provide in silico data about tropomyosin dynamics on troponin/myosin-free F-actin indicating that tropomyosin moves from the open state toward blocked- or closed-state positions on actin. To follow transitions inherent to tropomyosin itself on F-actin, we performed MD simulations initiated from the blocked-, open-, and intermediate-state models and followed tropomyosin over the surface of F-actin in the absence of myosin and troponin. These MD simulations maintain tropomyosin in a cable-like conformation, including the tropomyosin overlap domain, while allowing tropomyosin to retain most of its motional freedom. Tropomyosin shows considerable azimuthal movement away from the open state toward the surrounds of a more energetically favorable blocked B-state position over F-actin. In contrast, little movement away from the B-state location is observed. Our results are consistent with previous predictions based on electrostatic interaction energy landscapes determined by rigid-body translocation of tropomyosin. They support the view that in the absence of myosin, i.e., when myosin cross-bridges detach from actin, the blocked- or closed-state positions of tropomyosin are energetically favored, while the open state is not.
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Affiliation(s)
- Farooq A Kiani
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, MA
| | - William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, MA
| | - Stefan Fischer
- Interdisciplinary Center for Scientific Computing, University of Heidelberg, Heidelberg, Germany
| | - Michael J Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, Boston, MA
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42
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Ma W, Gong H, Irving T. Myosin Head Configurations in Resting and Contracting Murine Skeletal Muscle. Int J Mol Sci 2018; 19:E2643. [PMID: 30200618 PMCID: PMC6165214 DOI: 10.3390/ijms19092643] [Citation(s) in RCA: 49] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2018] [Revised: 08/29/2018] [Accepted: 09/03/2018] [Indexed: 12/30/2022] Open
Abstract
Transgenic mouse models have been important tools for studying the relationship of genotype to phenotype for human diseases, including those of skeletal muscle. We show that mouse skeletal muscle can produce high quality X-ray diffraction patterns establishing the mouse intact skeletal muscle X-ray preparation as a potentially powerful tool to test structural hypotheses in health and disease. A notable feature of the mouse model system is the presence of residual myosin layer line intensities in contracting mouse muscle patterns. This provides an additional tool, along with the I1,1/I1,0 intensity ratio, for estimating the proportions of active versus relaxed myosin heads under a given set of conditions that can be used to characterize a given physiological condition or mutant muscle type. We also show that analysis of the myosin layer line intensity distribution, including derivation of the myosin head radius, Rm, may be used to study the role of the super-relaxed state in myosin regulation. When the myosin inhibitor blebbistatin is used to inhibit force production, there is a shift towards a highly quasi-helically ordered configuration that is distinct from the normal resting state, indicating there are more than one helically ordered configuration for resting crossbridges.
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Affiliation(s)
- Weikang Ma
- BioCAT, Department of Biological Sciences, Illinois Institute of Technology, Chicago, IL 60616, USA.
| | - Henry Gong
- BioCAT, Department of Biological Sciences, Illinois Institute of Technology, Chicago, IL 60616, USA.
| | - Thomas Irving
- BioCAT, Department of Biological Sciences, Illinois Institute of Technology, Chicago, IL 60616, USA.
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43
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Williams MR, Tardiff JC, Schwartz SD. Mechanism of Cardiac Tropomyosin Transitions on Filamentous Actin As Revealed by All-Atom Steered Molecular Dynamics Simulations. J Phys Chem Lett 2018; 9:3301-3306. [PMID: 29863359 PMCID: PMC6019281 DOI: 10.1021/acs.jpclett.8b00958] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The three-state model of tropomyosin (Tm) positioning along filamentous actin allows for Tm to act as a gate for myosin head binding with actin. The blocked state of Tm prevents myosin binding, while the open state allows for strong binding. Intermediate to this transition is the closed state. The details of the transition from the blocked to the closed state and then finally to the open state by Tm have not been fully accessible to experiment. Utilizing steered molecular dynamics, we investigate the work required to move the Tm strand through the extant set of proposed transitions. We find that an azimuthal motion around the actin filament by Tm is most probable in spite of increased initial energy barrier from the topographical landscape of actin.
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Affiliation(s)
- Michael R. Williams
- Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States
| | - Jil C. Tardiff
- Department of Medicine, The University of Arizona, Tucson, Arizona 85724, United States
- Department of Cellular and Molecular Medicine, The University of Arizona, Tucson, Arizona 85724, United States
| | - Steven D. Schwartz
- Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States
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44
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Farman GP, Rynkiewicz MJ, Orzechowski M, Lehman W, Moore JR. HCM and DCM cardiomyopathy-linked α-tropomyosin mutations influence off-state stability and crossbridge interaction on thin filaments. Arch Biochem Biophys 2018; 647:84-92. [PMID: 29626422 DOI: 10.1016/j.abb.2018.04.002] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2017] [Revised: 04/02/2018] [Accepted: 04/03/2018] [Indexed: 12/19/2022]
Abstract
Calcium regulation of cardiac muscle contraction is controlled by the thin-filament proteins troponin and tropomyosin bound to actin. In the absence of calcium, troponin-tropomyosin inhibits myosin-interactions on actin and induces muscle relaxation, whereas the addition of calcium relieves the inhibitory constraint to initiate contraction. Many mutations in thin filament proteins linked to cardiomyopathy appear to disrupt this regulatory switching. Here, we tested perturbations caused by mutant tropomyosins (E40K, DCM; and E62Q, HCM) on intra-filament interactions affecting acto-myosin interactions including those induced further by myosin association. Comparison of wild-type and mutant human α-tropomyosin (Tpm1.1) behavior was carried out using in vitro motility assays and molecular dynamics simulations. Our results show that E62Q tropomyosin destabilizes thin filament off-state function by increasing calcium-sensitivity, but without apparent affect on global tropomyosin structure by modifying coiled-coil rigidity. In contrast, the E40K mutant tropomyosin appears to stabilize the off-state, demonstrates increased tropomyosin flexibility, while also decreasing calcium-sensitivity. In addition, the E40K mutation reduces thin filament velocity at low myosin concentration while the E62Q mutant tropomyosin increases velocity. Corresponding molecular dynamics simulations indicate specific residue interactions that are likely to redefine underlying molecular regulatory mechanisms, which we propose explain the altered contractility evoked by the disease-causing mutations.
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Affiliation(s)
- Gerrie P Farman
- Department of Biological Sciences, University of Massachusetts-Lowell, One University Avenue, Lowell, MA 01854, USA; Department of Physiology & Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, MA 02118, USA.
| | - Michael J Rynkiewicz
- Department of Physiology & Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, MA 02118, USA.
| | - Marek Orzechowski
- Department of Physiology & Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, MA 02118, USA.
| | - William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, 700 Albany Street, Boston, MA 02118, USA.
| | - Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts-Lowell, One University Avenue, Lowell, MA 01854, USA.
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45
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Lin BL, Li A, Mun JY, Previs MJ, Previs SB, Campbell SG, Dos Remedios CG, Tombe PDP, Craig R, Warshaw DM, Sadayappan S. Skeletal myosin binding protein-C isoforms regulate thin filament activity in a Ca 2+-dependent manner. Sci Rep 2018; 8:2604. [PMID: 29422607 PMCID: PMC5805719 DOI: 10.1038/s41598-018-21053-1] [Citation(s) in RCA: 25] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2017] [Accepted: 01/29/2018] [Indexed: 01/17/2023] Open
Abstract
Muscle contraction, which is initiated by Ca2+, results in precise sliding of myosin-based thick and actin-based thin filament contractile proteins. The interactions between myosin and actin are finely tuned by three isoforms of myosin binding protein-C (MyBP-C): slow-skeletal, fast-skeletal, and cardiac (ssMyBP-C, fsMyBP-C and cMyBP-C, respectively), each with distinct N-terminal regulatory regions. The skeletal MyBP-C isoforms are conditionally coexpressed in cardiac muscle, but little is known about their function. Therefore, to characterize the functional differences and regulatory mechanisms among these three isoforms, we expressed recombinant N-terminal fragments and examined their effect on contractile properties in biophysical assays. Addition of the fragments to in vitro motility assays demonstrated that ssMyBP-C and cMyBP-C activate thin filament sliding at low Ca2+. Corresponding 3D electron microscopy reconstructions of native thin filaments suggest that graded shifts of tropomyosin on actin are responsible for this activation (cardiac > slow-skeletal > fast-skeletal). Conversely, at higher Ca2+, addition of fsMyBP-C and cMyBP-C fragments reduced sliding velocities in the in vitro motility assays and increased force production in cardiac muscle fibers. We conclude that due to the high frequency of Ca2+ cycling in cardiac muscle, cardiac MyBP-C may play dual roles at both low and high Ca2+. However, skeletal MyBP-C isoforms may be tuned to meet the needs of specific skeletal muscles.
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Affiliation(s)
- Brian Leei Lin
- Department of Cell and Molecular Physiology, Health Sciences Division, Loyola University Chicago, Maywood, IL, 60153, USA
| | - Amy Li
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT, 05405, USA
- Bosch Institute, Discipline of Anatomy and Histology, University of Sydney, Sydney, 2006, Australia
| | - Ji Young Mun
- Department of Cell and Developmental Biology, University of Massachusetts Medical School, Worcester, MA, 01655, USA
- Department of Structure and Function of Neural Network, Korea Brain Research Institute, Dong-gu, Daegu, Korea
| | - Michael J Previs
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT, 05405, USA
| | - Samantha Beck Previs
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT, 05405, USA
| | - Stuart G Campbell
- Departments of Biomedical Engineering and Cellular and Molecular Physiology, Yale University, New Haven, CT, 06520, USA
| | - Cristobal G Dos Remedios
- Bosch Institute, Discipline of Anatomy and Histology, University of Sydney, Sydney, 2006, Australia
| | - Pieter de P Tombe
- Department of Cell and Molecular Physiology, Health Sciences Division, Loyola University Chicago, Maywood, IL, 60153, USA
| | - Roger Craig
- Department of Cell and Developmental Biology, University of Massachusetts Medical School, Worcester, MA, 01655, USA
| | - David M Warshaw
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT, 05405, USA
| | - Sakthivel Sadayappan
- Department of Cell and Molecular Physiology, Health Sciences Division, Loyola University Chicago, Maywood, IL, 60153, USA.
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46
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Ca 2+-induced movement of tropomyosin on native cardiac thin filaments revealed by cryoelectron microscopy. Proc Natl Acad Sci U S A 2017; 114:6782-6787. [PMID: 28607071 DOI: 10.1073/pnas.1700868114] [Citation(s) in RCA: 54] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
Muscle contraction relies on the interaction of myosin motors with F-actin, which is regulated through a translocation of tropomyosin by the troponin complex in response to Ca2+ The current model of muscle regulation holds that at relaxing (low-Ca2+) conditions tropomyosin blocks myosin binding sites on F-actin, whereas at activating (high-Ca2+) conditions tropomyosin translocation only partially exposes myosin binding sites on F-actin so that binding of rigor myosin is required to fully activate the thin filament (TF). Here we used a single-particle approach to helical reconstruction of frozen hydrated native cardiac TFs under relaxing and activating conditions to reveal the azimuthal movement of the tropomyosin on the surface of the native cardiac TF upon Ca2+ activation. We demonstrate that at either relaxing or activating conditions tropomyosin is not constrained in one structural state, but rather is distributed between three structural positions on the surface of the TF. We show that two of these tropomyosin positions restrain actomyosin interactions, whereas in the third position, which is significantly enhanced at high Ca2+, tropomyosin does not block myosin binding sites on F-actin. Our data provide a structural framework for the enhanced activation of the cardiac TF over the skeletal TF by Ca2+ and lead to a mechanistic model for the regulation of the cardiac TF.
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47
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Lehman W. Switching Muscles On and Off in Steps: The McKillop-Geeves Three-State Model of Muscle Regulation. Biophys J 2017; 112:2459-2466. [PMID: 28552313 DOI: 10.1016/j.bpj.2017.04.053] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2017] [Revised: 04/14/2017] [Accepted: 04/21/2017] [Indexed: 01/12/2023] Open
Abstract
BJ Classic highlighting the article "Regulation of the interaction between actin and myosin subfragment 1: evidence for three states of the thin filament."
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Affiliation(s)
- William Lehman
- Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts.
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48
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Bershitsky SY, Koubassova NA, Ferenczi MA, Kopylova GV, Narayanan T, Tsaturyan AK. The Closed State of the Thin Filament Is Not Occupied in Fully Activated Skeletal Muscle. Biophys J 2017; 112:1455-1461. [PMID: 28402887 DOI: 10.1016/j.bpj.2017.02.017] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2016] [Revised: 01/25/2017] [Accepted: 02/16/2017] [Indexed: 11/29/2022] Open
Abstract
Muscle contraction is powered by actin-myosin interaction controlled by Ca2+ via the regulatory proteins troponin (Tn) and tropomyosin (Tpm), which are associated with actin filaments. Tpm forms coiled-coil dimers, which assemble into a helical strand that runs along the whole ∼1 μm length of a thin filament. In the absence of Ca2+, Tn that is tightly bound to Tpm binds actin and holds the Tpm strand in the blocked, or B, state, where Tpm shields actin from the binding of myosin heads. Ca2+ binding to Tn releases the Tpm from actin so that it moves azimuthally around the filament axis to a closed, or C, state, where actin is partially available for weak binding of myosin heads. Upon transition of the weak actin-myosin bond into a strong, stereo-specific complex, the myosin heads push Tpm strand to the open, or O, state allowing myosin binding sites on several neighboring actin monomers to become open for myosin binding. We used low-angle x-ray diffraction at the European Synchrotron Radiation Facility to check whether the O- to C-state transition in fully activated fibers of fast skeletal muscle of the rabbit occurs during transition from isometric contraction to shortening under low load. No decrease in the intensity of the second actin layer line at reciprocal radii in the range of 0.15-0.275 nm-1 was observed during shortening suggesting that an azimuthal Tpm movement from the O- to C-state does not occur, although during shortening muscle stiffness is reduced compared to the isometric state, and the intensities of other actin layer lines demonstrate a ∼2-fold decrease in the fraction of myosin heads strongly bound to actin. The data show that a small fraction of actin-bound myosin heads is sufficient for supporting the O-state and, therefore the C-state is not occupied in fully activated skeletal muscle that produces mechanical work at low load.
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Affiliation(s)
- Sergey Y Bershitsky
- Institute of Immunology and Physiology, Russian Academy of Sciences, Laboratory of Biological Motility, Yekaterinburg, Russia.
| | | | - Michael A Ferenczi
- Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, Singapore
| | - Galina V Kopylova
- Institute of Immunology and Physiology, Russian Academy of Sciences, Laboratory of Biological Motility, Yekaterinburg, Russia
| | | | - Andrey K Tsaturyan
- Institute of Mechanics, M.V. Lomonosov Moscow University, Moscow, Russia
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49
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Abstract
Electron microscopy and X-ray diffraction have together played a key role in our understanding of the molecular structure and mechanism of contraction of muscle. This review highlights the role of electron microscopy, from early insights into thick and thin filament structure by negative staining, to studies of single myosin molecule structure, and finally to recent high-resolution structures by cryo-electron microscopy. Muscle filaments are designed for movement. Their labile structures thus present challenges to obtaining near-atomic detail, which are also discussed.
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50
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Abstract
In the last decade, improvements in electron microscopy and image processing have permitted significantly higher resolutions to be achieved (sometimes <1 nm) when studying isolated actin and myosin filaments. In the case of actin filaments the changing structure when troponin binds calcium ions can be followed using electron microscopy and single particle analysis to reveal what happens on each of the seven non-equivalent pseudo-repeats of the tropomyosin α-helical coiled-coil. In the case of the known family of myosin filaments not only are the myosin head arrangements under relaxing conditions being defined, but the latest analysis, also using single particle methods, is starting to reveal the way that the α-helical coiled-coil myosin rods are packed to give the filament backbones.
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Affiliation(s)
- John M Squire
- Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, BS8 1TD, UK.
| | - Danielle M Paul
- Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, BS8 1TD, UK
| | - Edward P Morris
- Division of Structural Biology, The Institute of Cancer Research, London, SW3 6JB, UK
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