1
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Ragusa R, Caselli C. Focus on cardiac troponin complex: From gene expression to cardiomyopathy. Genes Dis 2024; 11:101263. [PMID: 39211905 PMCID: PMC11357864 DOI: 10.1016/j.gendis.2024.101263] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2023] [Revised: 01/26/2024] [Accepted: 02/21/2024] [Indexed: 09/04/2024] Open
Abstract
The cardiac troponin complex (cTn) is a regulatory component of sarcomere. cTn consists of three subunits: cardiac troponin C (cTnC), which confers Ca2+ sensitivity to muscle; cTnI, which inhibits the interaction of cross-bridge of myosin with thin filament during diastole; and cTnT, which has multiple roles in sarcomere, such as promoting the link between the cTnI-cTnC complex and tropomyosin within the thin filament and influencing Ca2+ sensitivity of cTn and force development during contraction. Conditions that interfere with interactions within cTn and/or other thin filament proteins can be key factors in the regulation of cardiac contraction. These conditions include alterations in myofilament Ca2+ sensitivity, direct changes in cTn function, and triggering downstream events that lead to adverse cardiac remodeling and impairment of heart function. This review describes gene expression and post-translational modifications of cTn as well as the conditions that can adversely affect the delicate balance among the components of cTn, thereby promoting contractile dysfunction.
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Affiliation(s)
- Rosetta Ragusa
- Institute of Clinical Physiology, CNR, via Moruzzi 1, Pisa 56124, Italy
| | - Chiara Caselli
- Institute of Clinical Physiology, CNR, via Moruzzi 1, Pisa 56124, Italy
- Fondazione Toscana Gabriele Monasterio, via Moruzzi 1, Pisa 56124, Italy
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2
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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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3
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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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4
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Halder SS, Rynkiewicz MJ, Creso JG, Sewanan LR, Howland L, Moore JR, Lehman W, Campbell SG. Mechanisms of pathogenicity in the hypertrophic cardiomyopathy-associated TPM1 variant S215L. PNAS NEXUS 2023; 2:pgad011. [PMID: 36896133 PMCID: PMC9991458 DOI: 10.1093/pnasnexus/pgad011] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/10/2022] [Revised: 07/12/2022] [Accepted: 01/09/2023] [Indexed: 01/22/2023]
Abstract
Hypertrophic cardiomyopathy (HCM) is an inherited disorder often caused by mutations to sarcomeric genes. Many different HCM-associated TPM1 mutations have been identified but they vary in their degrees of severity, prevalence, and rate of disease progression. The pathogenicity of many TPM1 variants detected in the clinical population remains unknown. Our objective was to employ a computational modeling pipeline to assess pathogenicity of one such variant of unknown significance, TPM1 S215L, and validate predictions using experimental methods. Molecular dynamic simulations of tropomyosin on actin suggest that the S215L significantly destabilizes the blocked regulatory state while increasing flexibility of the tropomyosin chain. These changes were quantitatively represented in a Markov model of thin-filament activation to infer the impacts of S215L on myofilament function. Simulations of in vitro motility and isometric twitch force predicted that the mutation would increase Ca2+ sensitivity and twitch force while slowing twitch relaxation. In vitro motility experiments with thin filaments containing TPM1 S215L revealed higher Ca2+ sensitivity compared with wild type. Three-dimensional genetically engineered heart tissues expressing TPM1 S215L exhibited hypercontractility, upregulation of hypertrophic gene markers, and diastolic dysfunction. These data form a mechanistic description of TPM1 S215L pathogenicity that starts with disruption of the mechanical and regulatory properties of tropomyosin, leading thereafter to hypercontractility and finally induction of a hypertrophic phenotype. These simulations and experiments support the classification of S215L as a pathogenic mutation and support the hypothesis that an inability to adequately inhibit actomyosin interactions is the mechanism whereby thin-filament mutations cause HCM.
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Affiliation(s)
- Saiti S Halder
- Department of Biomedical Engineering, Yale University, New Haven, CT 06511
| | | | - Jenette G Creso
- Department of Biomedical Engineering, Yale University, New Haven, CT 06511
| | - Lorenzo R Sewanan
- Department of Biomedical Engineering, Yale University, New Haven, CT 06511
- Department of Internal Medicine, Columbia University, New York, NY 10032
| | - Lindsey Howland
- Department of Biological Sciences, University of Massachusetts Lowell, MA 01854
| | - Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts Lowell, MA 01854
| | - William Lehman
- Department of Physiology/Biophysics, Boston University, Boston, MA 02215
| | - Stuart G Campbell
- Department of Biomedical Engineering, Yale University, New Haven, CT 06511
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5
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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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6
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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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7
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Agarwal R, Paulo JA, Toepfer CN, Ewoldt JK, Sundaram S, Chopra A, Zhang Q, Gorham J, DePalma SR, Chen CS, Gygi SP, Seidman CE, Seidman JG. Filamin C Cardiomyopathy Variants Cause Protein and Lysosome Accumulation. Circ Res 2021; 129:751-766. [PMID: 34405687 PMCID: PMC9053646 DOI: 10.1161/circresaha.120.317076] [Citation(s) in RCA: 21] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/03/2020] [Accepted: 08/17/2021] [Indexed: 01/02/2023]
Abstract
[Figure: see text].
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Affiliation(s)
- Radhika Agarwal
- Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
| | - Joao A. Paulo
- Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA
| | - Christopher N. Toepfer
- Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
- Radcliffe Department of Medicine, University of Oxford, OX3 9DU, UK
- Wellcome Centre for Human Genetics, University of Oxford, OX3 7BN, UK
| | - Jourdan K. Ewoldt
- Department of Biomedical Engineering, Boston University, Boston, MA 02115, USA
| | - Subramanian Sundaram
- Department of Biomedical Engineering, Boston University, Boston, MA 02115, USA
- Wyss Institute for Biologically Inspired Engineering, Harvard University, Boston, MA 02115, USA
| | - Anant Chopra
- Department of Biomedical Engineering, Boston University, Boston, MA 02115, USA
- Wyss Institute for Biologically Inspired Engineering, Harvard University, Boston, MA 02115, USA
| | - Qi Zhang
- Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
| | - Joshua Gorham
- Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
| | - Steven R. DePalma
- Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
| | - Christopher S. Chen
- Department of Biomedical Engineering, Boston University, Boston, MA 02115, USA
- Wyss Institute for Biologically Inspired Engineering, Harvard University, Boston, MA 02115, USA
| | - Steven P. Gygi
- Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA
| | - Christine E. Seidman
- Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
- Howard Hughes Medical Institute, Chevy Chase, MD 20815, USA
- Division of Genetics, Department of Medicine, Brigham and Women’s Hospital, Boston, MA 02115, USA
| | - J. G. Seidman
- Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
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8
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Baldo AP, Tardiff JC, Schwartz SD. A Proposed Mechanism for the Initial Myosin Binding Event on the Cardiac Thin Filament: A Metadynamics Study. J Phys Chem Lett 2021; 12:3509-3513. [PMID: 33793247 PMCID: PMC8080310 DOI: 10.1021/acs.jpclett.1c00223] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
The movement of tropomyosin over filamentous actin regulates the cross-bridge cycle of the thick with thin filament of cardiac muscle by blocking and revealing myosin binding sites. Tropomyosin exists in three, distinct equilibrium states with one state blocking myosin-actin interactions (blocked position) and the remaining two allowing for weak (closed position) and strong myosin binding (open position). However, experimental information illuminating how myosin binds to the thin filament and influences tropomyosin's transition across the actin surface is lacking. Using metadynamics, we mimic the effect of a single myosin head binding by determining the work required to pull small segments of tropomyosin toward the open position in several distinct regions of the thin filament. We find differences in required work due to the influence of cardiac troponin T lead to preferential binding sites and determine the mechanism of further myosin head recruitment.
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Affiliation(s)
- Anthony P Baldo
- Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States
| | - Jil C Tardiff
- Department of Biomedical Engineering, The University of Arizona, Tucson, Arizona 85724, United States
| | - Steven D Schwartz
- Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States
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9
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Viswanathan MC, Schmidt W, Franz P, Rynkiewicz MJ, Newhard CS, Madan A, Lehman W, Swank DM, Preller M, Cammarato A. A role for actin flexibility in thin filament-mediated contractile regulation and myopathy. Nat Commun 2020; 11:2417. [PMID: 32415060 PMCID: PMC7229152 DOI: 10.1038/s41467-020-15922-5] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/04/2019] [Accepted: 03/30/2020] [Indexed: 12/20/2022] Open
Abstract
Striated muscle contraction is regulated by the translocation of troponin-tropomyosin strands over the thin filament surface. Relaxation relies partly on highly-favorable, conformation-dependent electrostatic contacts between actin and tropomyosin, which position tropomyosin such that it impedes actomyosin associations. Impaired relaxation and hypercontractile properties are hallmarks of various muscle disorders. The α-cardiac actin M305L hypertrophic cardiomyopathy-causing mutation lies near residues that help confine tropomyosin to an inhibitory position along thin filaments. Here, we investigate M305L actin in vivo, in vitro, and in silico to resolve emergent pathological properties and disease mechanisms. Our data suggest the mutation reduces actin flexibility and distorts the actin-tropomyosin electrostatic energy landscape that, in muscle, result in aberrant contractile inhibition and excessive force. Thus, actin flexibility may be required to establish and maintain interfacial contacts with tropomyosin as well as facilitate its movement over distinct actin surface features and is, therefore, likely necessary for proper regulation of contraction. The α-cardiac actin M305L hypertrophic cardiomyopathy-causing mutation is located near residues that help confine tropomyosin to an inhibitory position along thin filaments. Here the authors assessed M305L actin in vivo, in vitro, and in silico to characterize emergent pathological properties and define the mechanistic basis of disease.
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Affiliation(s)
- Meera C Viswanathan
- Department of Medicine, Division of Cardiology, Johns Hopkins University, 720 Rutland Avenue, Baltimore, MD, 21205, USA
| | - William Schmidt
- Department of Medicine, Division of Cardiology, Johns Hopkins University, 720 Rutland Avenue, Baltimore, MD, 21205, USA
| | - Peter Franz
- Institute for Biophysical Chemistry, Hannover Medical School, Carl-Neuberg-Straße 1, 30625, Hannover, Germany
| | - Michael J Rynkiewicz
- Department of Physiology and Biophysics, Boston University School of Medicine, 700 Albany Street St, Boston, MA, 02118, USA
| | - Christopher S Newhard
- Department of Biological Sciences and Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY, 12180-3590, USA
| | - Aditi Madan
- Department of Medicine, Division of Cardiology, Johns Hopkins University, 720 Rutland Avenue, Baltimore, MD, 21205, USA
| | - William Lehman
- Department of Physiology and Biophysics, Boston University School of Medicine, 700 Albany Street St, Boston, MA, 02118, USA
| | - Douglas M Swank
- Department of Biological Sciences and Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY, 12180-3590, USA
| | - Matthias Preller
- Institute for Biophysical Chemistry, Hannover Medical School, Carl-Neuberg-Straße 1, 30625, Hannover, Germany.
| | - Anthony Cammarato
- Department of Medicine, Division of Cardiology, Johns Hopkins University, 720 Rutland Avenue, Baltimore, MD, 21205, USA. .,Department of Physiology, Johns Hopkins University School of Medicine, 720 Rutland Avenue, Baltimore, MD, 21205, USA.
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10
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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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11
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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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12
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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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13
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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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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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15
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Abstract
Regulation of muscle contraction has been viewed as principally involving Ca2+ binding to regulatory proteins on the thin filament, but while this is an important element of regulation, the mechanism does not explain the precise matching of muscle performance to the load it must lift or move. Now, it is increasingly evident that mechanisms instrinsic to the thick filament activate myosin cross-bridges as the force or load on a muscle increases. Both thick and thin filament regulatory mechanisms are featured in this special issue of the Journal of General Physiology .
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Affiliation(s)
- Richard L Moss
- Cardiovascular Research Center, School of Medicine and Public Health, University of Wisconsin, Madison, Madison, WI
| | - Pieter P de Tombe
- Department of Physiology and Biophysics, College of Medicine, University of Illinois at Chicago, Chicago, IL
| | - R John Solaro
- Center for Cardiovascular Research, Department of Physiology and Biophysics, College of Medicine, University of Illinois at Chicago, Chicago, IL
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