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Elkrief D, Matusovsky O, Cheng YS, Rassier DE. From amino-acid to disease: the effects of oxidation on actin-myosin interactions in muscle. J Muscle Res Cell Motil 2023; 44:225-254. [PMID: 37805961 DOI: 10.1007/s10974-023-09658-0] [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: 06/07/2023] [Accepted: 08/15/2023] [Indexed: 10/10/2023]
Abstract
Actin-myosin interactions form the basis of the force-producing contraction cycle within the sarcomere, serving as the primary mechanism for muscle contraction. Post-translational modifications, such as oxidation, have a considerable impact on the mechanics of these interactions. Considering their widespread occurrence, the explicit contributions of these modifications to muscle function remain an active field of research. In this review, we aim to provide a comprehensive overview of the basic mechanics of the actin-myosin complex and elucidate the extent to which oxidation influences the contractile cycle and various mechanical characteristics of this complex at the single-molecule, myofibrillar and whole-muscle levels. We place particular focus on amino acids shown to be vulnerable to oxidation in actin, myosin, and some of their binding partners. Additionally, we highlight the differences between in vitro environments, where oxidation is controlled and limited to actin and myosin and myofibrillar or whole muscle environments, to foster a better understanding of oxidative modification in muscle. Thus, this review seeks to encompass a broad range of studies, aiming to lay out the multi layered effects of oxidation in in vitro and in vivo environments, with brief mention of clinical muscular disorders associated with oxidative stress.
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Affiliation(s)
- Daren Elkrief
- Department of Physiology, McGill University, Montreal, QC, Canada
| | - Oleg Matusovsky
- Department of Kinesiology and Physical Education, McGill University, Montreal, QC, Canada
| | - Yu-Shu Cheng
- Department of Kinesiology and Physical Education, McGill University, Montreal, QC, Canada
| | - Dilson E Rassier
- Department of Physiology, McGill University, Montreal, QC, Canada.
- Department of Kinesiology and Physical Education, McGill University, Montreal, QC, Canada.
- Simon Fraser University, Burnaby, BC, Canada.
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2
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Awinda PO, Watanabe M, Bishaw Y, Huckabee AM, Agonias KB, Kazmierczak K, Szczesna-Cordary D, Tanner BCW. Mavacamten decreases maximal force and Ca 2+ sensitivity in the N47K-myosin regulatory light chain mouse model of hypertrophic cardiomyopathy. Am J Physiol Heart Circ Physiol 2021; 320:H881-H890. [PMID: 33337957 PMCID: PMC8082789 DOI: 10.1152/ajpheart.00345.2020] [Citation(s) in RCA: 25] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/12/2020] [Revised: 12/10/2020] [Accepted: 12/10/2020] [Indexed: 01/12/2023]
Abstract
Morbidity and mortality associated with heart disease is a growing threat to the global population, and novel therapies are needed. Mavacamten (formerly called MYK-461) is a small molecule that binds to cardiac myosin and inhibits myosin ATPase. Mavacamten is currently in clinical trials for the treatment of obstructive hypertrophic cardiomyopathy (HCM), and it may provide benefits for treating other forms of heart disease. We investigated the effect of mavacamten on cardiac muscle contraction in two transgenic mouse lines expressing the human isoform of cardiac myosin regulatory light chain (RLC) in their hearts. Control mice expressed wild-type RLC (WT-RLC), and HCM mice expressed the N47K RLC mutation. In the absence of mavacamten, skinned papillary muscle strips from WT-RLC mice produced greater isometric force than strips from N47K mice. Adding 0.3 µM mavacamten decreased maximal isometric force and reduced Ca2+ sensitivity of contraction for both genotypes, but this reduction in pCa50 was nearly twice as large for WT-RLC versus N47K. We also used stochastic length-perturbation analysis to characterize cross-bridge kinetics. The cross-bridge detachment rate was measured as a function of [MgATP] to determine the effect of mavacamten on myosin nucleotide handling rates. Mavacamten increased the MgADP release and MgATP binding rates for both genotypes, thereby contributing to faster cross-bridge detachment, which could speed up myocardial relaxation during diastole. Our data suggest that mavacamten reduces isometric tension and Ca2+ sensitivity of contraction via decreased strong cross-bridge binding. Mavacamten may become a useful therapy for patients with heart disease, including some forms of HCM.NEW & NOTEWORTHY Mavacamten is a pharmaceutical that binds to myosin, and it is under investigation as a therapy for some forms of heart disease. We show that mavacamten reduces isometric tension and Ca2+ sensitivity of contraction in skinned myocardial strips from a mouse model of hypertrophic cardiomyopathy that expresses the N47K mutation in cardiac myosin regulatory light chain. Mavacamten reduces contractility by decreasing strong cross-bridge binding, partially due to faster cross-bridge nucleotide handling rates that speed up myosin detachment.
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Affiliation(s)
- Peter O Awinda
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
| | - Marissa Watanabe
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
| | - Yemeserach Bishaw
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
| | - Anna M Huckabee
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
| | - Keinan B Agonias
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
| | - Katarzyna Kazmierczak
- Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, Florida
| | - Danuta Szczesna-Cordary
- Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, Florida
| | - Bertrand C W Tanner
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
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3
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Sarcomere integrated biosensor detects myofilament-activating ligands in real time during twitch contractions in live cardiac muscle. J Mol Cell Cardiol 2020; 147:49-61. [PMID: 32791214 DOI: 10.1016/j.yjmcc.2020.07.012] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/18/2020] [Revised: 07/14/2020] [Accepted: 07/30/2020] [Indexed: 11/24/2022]
Abstract
The sarcomere is the functional unit of cardiac muscle, essential for normal heart function. To date, it has not been possible to study, in real time, thin filament-based activation dynamics in live cardiac muscle. We report here results from a cardiac troponin C (TnC) FRET-based biosensor integrated into the cardiac sarcomere via stoichiometric replacement of endogenous TnC. The TnC biosensor provides, for the first time, evidence of multiple thin filament activating ligands, including troponin I interfacing with TnC and cycling myosin, during a cardiac twitch. Results show that the TnC FRET biosensor transient significantly precedes that of peak twitch force. Using small molecules and genetic modifiers known to alter sarcomere activation, independently of the intracellular Ca2+ transient, the data show that the TnC biosensor detects significant effects of the troponin I switch domain as a sarcomere-activating ligand. Interestingly, the TnC biosensor also detected the effects of load-dependent altered myosin cycling, as shown by a significant delay in TnC biosensor transient inactivation during the isometric twitch. In addition, the TnC biosensor detected the effects of myosin as an activating ligand during the twitch by using a small molecule that directly alters cross-bridge cycling, independently of the intracellular Ca2+ transient. Collectively, these results aid in illuminating the basis of cardiac muscle contractile activation with implications for gene, protein, and small molecule-based strategies designed to target the sarcomere in regulating beat-to-beat heart performance in health and disease.
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4
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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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5
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Phung LA, Karvinen SM, Colson BA, Thomas DD, Lowe DA. Age affects myosin relaxation states in skeletal muscle fibers of female but not male mice. PLoS One 2018; 13:e0199062. [PMID: 30226869 PMCID: PMC6143227 DOI: 10.1371/journal.pone.0199062] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2018] [Accepted: 09/03/2018] [Indexed: 12/16/2022] Open
Abstract
The recent discovery that myosin has two distinct states in relaxed muscle–disordered relaxed (DRX) and super-relaxed (SRX)–provides another factor to consider in our fundamental understanding of the aging mechanism in skeletal muscle, since myosin is thought to be a potential contributor to dynapenia (age-associated loss of muscle strength independent of atrophy). The primary goal of this study was to determine the effects of age on DRX and SRX states and to examine their sex specificity. We have used quantitative fluorescence microscopy of the fluorescent nucleotide analog 2′/3′-O-(N-methylanthraniloyl) ATP (mantATP) to measure single-nucleotide turnover kinetics of myosin in skinned skeletal muscle fibers under relaxing conditions. We examined changes in DRX and SRX in response to the natural aging process by measuring the turnover of mantATP in skinned fibers isolated from psoas muscle of adult young (3–4 months old) and aged (26–28 months old) C57BL/6 female and male mice. Fluorescence decays were fitted to a multi-exponential decay function to determine both the time constants and mole fractions of fast and slow turnover populations, and significance was analyzed by a t-test. We found that in females, both the DRX and SRX lifetimes of myosin ATP turnover at steady state were shorter in aged muscle fibers compared to young muscle fibers (p ≤ 0.033). However, there was no significant difference in relaxation lifetime of either DRX (p = 0.202) or SRX (p = 0.804) between young and aged male mice. No significant effects were measured on the mole fractions (populations) of these states, as a function of sex or age (females, p = 0.100; males, p = 0.929). The effect of age on the order of myosin heads at rest and their ATPase function is sex specific, affecting only females. These findings provide new insight into the molecular factors and mechanisms that contribute to aging muscle dysfunction in a sex-specific manner.
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Affiliation(s)
- Lien A. Phung
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota, United States of America
| | - Sira M. Karvinen
- Department of Rehabilitation Medicine, University of Minnesota, Minneapolis, Minnesota, United States of America
| | - Brett A. Colson
- Department of Cellular and Molecular Medicine, University of Arizona, Tucson, Arizona, United States of America
| | - David D. Thomas
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota, United States of America
- * E-mail: (DDT); (DAL)
| | - Dawn A. Lowe
- Department of Rehabilitation Medicine, University of Minnesota, Minneapolis, Minnesota, United States of America
- * E-mail: (DDT); (DAL)
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6
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Gresham KS, Stelzer JE. The contributions of cardiac myosin binding protein C and troponin I phosphorylation to β-adrenergic enhancement of in vivo cardiac function. J Physiol 2016; 594:669-86. [PMID: 26635197 DOI: 10.1113/jp270959] [Citation(s) in RCA: 35] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2015] [Accepted: 11/30/2015] [Indexed: 01/12/2023] Open
Abstract
KEY POINTS β-adrenergic stimulation increases cardiac myosin binding protein C (MyBP-C) and troponin I phosphorylation to accelerate pressure development and relaxation in vivo, although their relative contributions remain unknown. Using a novel mouse model lacking protein kinase A-phosphorylatable troponin I (TnI) and MyBP-C, we examined in vivo haemodynamic function before and after infusion of the β-agonist dobutamine. Mice expressing phospho-ablated MyBP-C displayed cardiac hypertrophy and prevented full acceleration of pressure development and relaxation in response to dobutamine, whereas expression of phosphor-ablated TnI alone had little effect on the acceleration of contractile function in response to dobutamine. Our data demonstrate that MyBP-C phosphorylation is the principal mediator of the contractile response to increased β-agonist stimulation in vivo. These results help us understand why MyBP-C dephosphorylation in the failing heart contributes to contractile dysfunction and decreased adrenergic reserve in response to acute stress. β-adrenergic stimulation plays a critical role in accelerating ventricular contraction and speeding relaxation to match cardiac output to changing circulatory demands. Two key myofilaments proteins, troponin I (TnI) and myosin binding protein-C (MyBP-C), are phosphorylated following β-adrenergic stimulation; however, their relative contributions to the enhancement of in vivo cardiac contractility are unknown. To examine the roles of TnI and MyBP-C phosphorylation in β-adrenergic-mediated enhancement of cardiac function, transgenic (TG) mice expressing non-phosphorylatable TnI protein kinase A (PKA) residues (i.e. serine to alanine substitution at Ser23/24; TnI(PKA-)) were bred with mice expressing non-phosphorylatable MyBP-C PKA residues (i.e. serine to alanine substitution at Ser273, Ser282 and Ser302; MyBPC(PKA-)) to generate a novel mouse model expressing non-phosphorylatable PKA residues in TnI and MyBP-C (DBL(PKA-)). MyBP-C dephosphorylation produced cardiac hypertrophy and increased wall thickness in MyBPC(PKA-) and DBL(PKA-) mice, and in vivo echocardiography and pressure-volume catheterization studies revealed impaired systolic function and prolonged diastolic relaxation compared to wild-type and TnI(PKA-) mice. Infusion of the β-agonist dobutamine resulted in accelerated rates of pressure development and relaxation in all mice; however, MyBPC(PKA-) and DBL(PKA-) mice displayed a blunted contractile response compared to wild-type and TnI(PKA-) mice. Furthermore, unanaesthesized MyBPC(PKA-) and DBL(PKA-) mice displayed depressed maximum systolic pressure in response to dobutamine as measured using implantable telemetry devices. Taken together, our data show that MyBP-C phosphorylation is a critical modulator of the in vivo acceleration of pressure development and relaxation as a result of enhanced β-adrenergic stimulation, and reduced MyBP-C phosphorylation may underlie depressed adrenergic reserve in heart failure.
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Affiliation(s)
- Kenneth S Gresham
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, USA
| | - Julian E Stelzer
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, USA
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7
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Pulcastro HC, Awinda PO, Methawasin M, Granzier H, Dong W, Tanner BCW. Increased Titin Compliance Reduced Length-Dependent Contraction and Slowed Cross-Bridge Kinetics in Skinned Myocardial Strips from Rbm (20ΔRRM) Mice. Front Physiol 2016; 7:322. [PMID: 27524973 PMCID: PMC4966298 DOI: 10.3389/fphys.2016.00322] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2016] [Accepted: 07/14/2016] [Indexed: 01/12/2023] Open
Abstract
Titin is a giant protein spanning from the Z-disk to the M-band of the cardiac sarcomere. In the I-band titin acts as a molecular spring, contributing to passive mechanical characteristics of the myocardium throughout a heartbeat. RNA Binding Motif Protein 20 (RBM20) is required for normal titin splicing, and its absence or altered function leads to greater expression of a very large, more compliant N2BA titin isoform in Rbm20 homozygous mice (Rbm20ΔRRM) compared to wild-type mice (WT) that almost exclusively express the stiffer N2B titin isoform. Prior studies using Rbm20ΔRRM animals have shown that increased titin compliance compromises muscle ultrastructure and attenuates the Frank-Starling relationship. Although previous computational simulations of muscle contraction suggested that increasing compliance of the sarcomere slows the rate of tension development and prolongs cross-bridge attachment, none of the reported effects of Rbm20ΔRRM on myocardial function have been attributed to changes in cross-bridge cycling kinetics. To test the relationship between increased sarcomere compliance and cross-bridge kinetics, we used stochastic length-perturbation analysis in Ca2+-activated, skinned papillary muscle strips from Rbm20ΔRRM and WT mice. We found increasing titin compliance depressed maximal tension, decreased Ca2+-sensitivity of the tension-pCa relationship, and slowed myosin detachment rate in myocardium from Rbm20ΔRRM vs. WT mice. As sarcomere length increased from 1.9 to 2.2 μm, length-dependent activation of contraction was eliminated in the Rbm20ΔRRM myocardium, even though myosin MgADP release rate decreased ~20% to prolong strong cross-bridge binding at longer sarcomere length. These data suggest that increasing N2BA expression may alter cardiac performance in a length-dependent manner, showing greater deficits in tension production and slower cross-bridge kinetics at longer sarcomere length. This study also supports the idea that passive mechanical characteristics of the myocardium influence ensemble cross-bridge behavior and maintenance of tension generation throughout the sarcomere.
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Affiliation(s)
- Hannah C Pulcastro
- Department of Integrative Physiology and Neuroscience, Washington State University Pullman, WA, USA
| | - Peter O Awinda
- Department of Integrative Physiology and Neuroscience, Washington State University Pullman, WA, USA
| | - Mei Methawasin
- Department of Cellular and Molecular Medicine, University of Arizona Tucson, AZ, USA
| | - Henk Granzier
- Department of Cellular and Molecular Medicine, University of Arizona Tucson, AZ, USA
| | - Wenji Dong
- Department of Integrative Physiology and Neuroscience, Washington State UniversityPullman, WA, USA; Voiland School of Chemical Engineering and Bioengineering, Washington State UniversityPullman, WA, USA
| | - Bertrand C W Tanner
- Department of Integrative Physiology and Neuroscience, Washington State University Pullman, WA, USA
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8
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Moore JR, Campbell SG, Lehman W. Structural determinants of muscle thin filament cooperativity. Arch Biochem Biophys 2016; 594:8-17. [PMID: 26891592 DOI: 10.1016/j.abb.2016.02.016] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2015] [Revised: 02/09/2016] [Accepted: 02/10/2016] [Indexed: 11/16/2022]
Abstract
End-to-end connections between adjacent tropomyosin molecules along the muscle thin filament allow long-range conformational rearrangement of the multicomponent filament structure. This process is influenced by Ca(2+) and the troponin regulatory complexes, as well as by myosin crossbridge heads that bind to and activate the filament. Access of myosin crossbridges onto actin is gated by tropomyosin, and in the case of striated muscle filaments, troponin acts as a gatekeeper. The resulting tropomyosin-troponin-myosin on-off switching mechanism that controls muscle contractility is a complex cooperative and dynamic system with highly nonlinear behavior. Here, we review key information that leads us to view tropomyosin as central to the communication pathway that coordinates the multifaceted effectors that modulate and tune striated muscle contraction. We posit that an understanding of this communication pathway provides a framework for more in-depth mechanistic characterization of myopathy-associated mutational perturbations currently under investigation by many research groups.
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Affiliation(s)
- Jeffrey R Moore
- Department of Biological Sciences, University of Massachusetts Lowell, One University Avenue, Lowell, MA 018154, USA
| | - Stuart G Campbell
- Department of Biomedical Engineering, Yale University, 55 Prospect Street, New Haven, CT 06511, USA
| | - William Lehman
- Department of Physiology & Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA.
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9
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Tanner BCW, Breithaupt JJ, Awinda PO. Myosin MgADP release rate decreases at longer sarcomere length to prolong myosin attachment time in skinned rat myocardium. Am J Physiol Heart Circ Physiol 2015; 309:H2087-97. [PMID: 26475586 DOI: 10.1152/ajpheart.00555.2015] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/14/2015] [Accepted: 10/05/2015] [Indexed: 11/22/2022]
Abstract
Cardiac contractility increases as sarcomere length increases, suggesting that intrinsic molecular mechanisms underlie the Frank-Starling relationship to confer increased cardiac output with greater ventricular filling. The capacity of myosin to bind with actin and generate force in a muscle cell is Ca(2+) regulated by thin-filament proteins and spatially regulated by sarcomere length as thick-to-thin filament overlap varies. One mechanism underlying greater cardiac contractility as sarcomere length increases could involve longer myosin attachment time (ton) due to slowed myosin kinetics at longer sarcomere length. To test this idea, we used stochastic length-perturbation analysis in skinned rat papillary muscle strips to measure ton as [MgATP] varied (0.05-5 mM) at 1.9 and 2.2 μm sarcomere lengths. From this ton-MgATP relationship, we calculated cross-bridge MgADP release rate and MgATP binding rates. As MgATP increased, ton decreased for both sarcomere lengths, but ton was roughly 70% longer for 2.2 vs. 1.9 μm sarcomere length at maximally activated conditions. These ton differences were driven by a slower MgADP release rate at 2.2 μm sarcomere length (41 ± 3 vs. 74 ± 7 s(-1)), since MgATP binding rate was not different between the two sarcomere lengths. At submaximal activation levels near the pCa50 value of the tension-pCa relationship for each sarcomere length, length-dependent increases in ton were roughly 15% longer for 2.2 vs. 1.9 μm sarcomere length. These changes in cross-bridge kinetics could amplify cooperative cross-bridge contributions to force production and thin-filament activation at longer sarcomere length and suggest that length-dependent changes in myosin MgADP release rate may contribute to the Frank-Starling relationship in the heart.
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Affiliation(s)
- Bertrand C W Tanner
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
| | - Jason J Breithaupt
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
| | - Peter O Awinda
- Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
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10
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Land S, Niederer SA. A Spatially Detailed Model of Isometric Contraction Based on Competitive Binding of Troponin I Explains Cooperative Interactions between Tropomyosin and Crossbridges. PLoS Comput Biol 2015; 11:e1004376. [PMID: 26262582 PMCID: PMC4532474 DOI: 10.1371/journal.pcbi.1004376] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2015] [Accepted: 06/03/2015] [Indexed: 01/20/2023] Open
Abstract
Biophysical models of cardiac tension development provide a succinct representation of our understanding of force generation in the heart. The link between protein kinetics and interactions that gives rise to high cooperativity is not yet fully explained from experiments or previous biophysical models. We propose a biophysical ODE-based representation of cross-bridge (XB), tropomyosin and troponin within a contractile regulatory unit (RU) to investigate the mechanisms behind cooperative activation, as well as the role of cooperativity in dynamic tension generation across different species. The model includes cooperative interactions between regulatory units (RU-RU), between crossbridges (XB-XB), as well more complex interactions between crossbridges and regulatory units (XB-RU interactions). For the steady-state force-calcium relationship, our framework predicts that: (1) XB-RU effects are key in shifting the half-activation value of the force-calcium relationship towards lower [Ca2+], but have only small effects on cooperativity. (2) XB-XB effects approximately double the duty ratio of myosin, but do not significantly affect cooperativity. (3) RU-RU effects derived from the long-range action of tropomyosin are a major factor in cooperative activation, with each additional unblocked RU increasing the rate of additional RU’s unblocking. (4) Myosin affinity for short (1–4 RU) unblocked stretches of actin of is very low, and the resulting suppression of force at low [Ca2+] is a major contributor in the biphasic force-calcium relationship. We also reproduce isometric tension development across mouse, rat and human at physiological temperature and pacing rate, and conclude that species differences require only changes in myosin affinity and troponin I/troponin C affinity. Furthermore, we show that the calcium dependence of the rate of tension redevelopment ktr is explained by transient blocking of RU’s by a temporary decrease in XB-RU effects. Force generation in cardiac muscle cells is driven by changes in calcium concentration. Relatively small changes in the calcium concentration over the course of a heart beat lead to the large changes in force required to fully contract and relax the heart. This is known as ‘cooperative activation’, and involves a complex interaction of several proteins involved in contraction. Current computer models which reproduce force generation often do not represent these processes explicitly, and stochastic approaches that do tend to require large amounts of computational power to solve, which limit the range of investigations in which they can be used. We have created an new computational model that captures the underlying physiological processes in more detail, and is more efficient than stochastic approaches, while still being able to run a large range of simulations. The model is able to explain the biological processes leading to the cooperative activation of muscle. In addition, the model reproduces how this cooperative activation translates to normal muscle function to generate force from changes in calcium across three different species.
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Affiliation(s)
- Sander Land
- Department of Biomedical Engineering, King’s College London, United Kingdom
- * E-mail:
| | - Steven A. Niederer
- Department of Biomedical Engineering, King’s College London, United Kingdom
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11
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Ohira T, Kawano F, Ohira T, Goto K, Ohira Y. Responses of skeletal muscles to gravitational unloading and/or reloading. J Physiol Sci 2015; 65:293-310. [PMID: 25850921 PMCID: PMC10717835 DOI: 10.1007/s12576-015-0375-6] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2014] [Accepted: 03/17/2015] [Indexed: 02/04/2023]
Abstract
Adaptation of morphological, metabolic, and contractile properties of skeletal muscles to inhibition of antigravity activities by exposure to a microgravity environment or by simulation models, such as chronic bedrest in humans or hindlimb suspension in rodents, has been well reported. Such physiological adaptations are generally detrimental in daily life on earth. Since the development of suitable countermeasure(s) is essential to prevent or inhibit these adaptations, effects of neural, mechanical, and metabolic factors on these properties in both humans and animals were reviewed. Special attention was paid to the roles of the motoneurons (both efferent and afferent neurograms) and electromyogram activities as the neural factors, force development, and/or length of sarcomeres as the mechanical factors and mitochondrial bioenergetics as the metabolic factors.
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Affiliation(s)
- Takashi Ohira
- Space Biomedical Research Office, Japan Aerospace Exploration Agency, Tsukuba, Ibaraki 305-8505 Japan
| | - Fuminori Kawano
- Graduate School of Medicine, Osaka University, Toyonaka, Osaka 560-0043 Japan
| | - Tomotaka Ohira
- Graduate School of Health Sciences, Toyohashi SOZO University, Toyohashi, Aichi 440-8511 Japan
| | - Katsumasa Goto
- Graduate School of Health Sciences, Toyohashi SOZO University, Toyohashi, Aichi 440-8511 Japan
| | - Yoshinobu Ohira
- Graduate School of Health and Sports Science, Doshisha University, Miyakodani 1-3, Tatara, Kyotanabe, Kyoto 610-0394 Japan
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12
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Olivoto RR, Damiani CEN, Kassouf Silva I, Lofrano-Alves MS, Oliveira MA, Fogaça RTH. Effects of eugenol on resting tension of rat atria. ACTA ACUST UNITED AC 2014; 47:328-33. [PMID: 24676474 PMCID: PMC4075297 DOI: 10.1590/1414-431x20143472] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2013] [Accepted: 02/03/2014] [Indexed: 12/04/2022]
Abstract
In cardiac and skeletal muscle, eugenol (μM range) blocks excitation-contraction
coupling. In skeletal muscle, however, larger doses of eugenol (mM range) induce
calcium release from the sarcoplasmic reticulum. The effects of eugenol are therefore
dependent on its concentration. In this study, we evaluated the effects of eugenol on
the contractility of isolated, quiescent atrial trabeculae from male Wistar rats
(250-300 g; n=131) and measured atrial ATP content. Eugenol (1, 3, 5, 7, and 10 mM)
increased resting tension in a dose-dependent manner. Ryanodine [100 µM; a specific
ryanodine receptor (RyR) blocker] and procaine (30 mM; a nonspecific RyR blocker) did
not block the increased resting tension induced by eugenol regardless of whether
extracellular calcium was present. The myosin-specific inhibitor 2,3-butanedione
monoxime (BDM), however, reversed the increase in resting tension induced by eugenol.
In Triton-skinned atrial trabeculae, in which all membranes were solubilized, eugenol
did not change resting tension, maximum force produced, or the force
vs pCa relationship (pCa=-log [Ca2+]). Given that
eugenol reduced ATP concentration, the increase in resting tension observed in this
study may have resulted from cooperative activation of cardiac thin filaments by
strongly attached cross-bridges (rigor state).
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Affiliation(s)
- R R Olivoto
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - C E N Damiani
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - I Kassouf Silva
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - M S Lofrano-Alves
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - M A Oliveira
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
| | - R T H Fogaça
- Departamento de Fisiologia, Universidade Federal do Paraná, Curitiba, PR, Brasil
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13
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Structural basis for the in situ Ca(2+) sensitization of cardiac troponin C by positive feedback from force-generating myosin cross-bridges. Arch Biochem Biophys 2013; 537:198-209. [PMID: 23896515 DOI: 10.1016/j.abb.2013.07.013] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2013] [Revised: 07/10/2013] [Accepted: 07/11/2013] [Indexed: 11/21/2022]
Abstract
The in situ structural coupling between the cardiac troponin (cTn) Ca(2+)-sensitive regulatory switch (CRS) and strong myosin cross-bridges was investigated using Förster resonance energy transfer (FRET). The double cysteine mutant cTnC(T13C/N51C) was fluorescently labeled with the FRET pair 5-(iodoacetamidoethyl)aminonaphthelene-1-sulfonic acid (IAEDENS) and N-(4-dimethylamino-3,5-dinitrophenyl)maleimide (DDPM) and then incorporated into detergent skinned left ventricular papillary fiber bundles. Ca(2+) titrations of cTnC(T13C/N51C)AEDENS/DDPM-reconstituted fibers showed that the Ca(2+)-dependence of the opening of the N-domain of cTnC (N-cTnC) statistically matched the force-Ca(2+) relationship. N-cTnC opening still occurred steeply during Ca(2+) titrations in the presence of 1mM vanadate, but the maximal extent of ensemble-averaged N-cTnC opening and the Ca(2+)-sensitivity of the CRS were significantly reduced. At nanomolar, resting Ca(2+) levels, treatment with ADP·Mg in the absence of ATP caused a partial opening of N-cTnC. During subsequent Ca(2+) titrations in the presence of ADP·Mg and absence of ATP, further N-cTnC opening was stimulated as the CRS responded to Ca(2+) with increased Ca(2+)-sensitivity and reduced steepness. These findings supported our hypothesis here that strong cross-bridge interactions with the cardiac thin filament exert a Ca(2+)-sensitizing effect on the CRS by stabilizing the interaction between the exposed hydrophobic patch of N-cTnC and the switch region of cTnI.
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14
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Wang W, Barnabei MS, Asp ML, Heinis FI, Arden E, Davis J, Braunlin E, Li Q, Davis JP, Potter JD, Metzger JM. Noncanonical EF-hand motif strategically delays Ca2+ buffering to enhance cardiac performance. Nat Med 2013; 19:305-12. [PMID: 23396207 DOI: 10.1038/nm.3079] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2012] [Accepted: 12/21/2012] [Indexed: 12/26/2022]
Abstract
EF-hand proteins are ubiquitous in cell signaling. Parvalbumin (Parv), the archetypal EF-hand protein, is a high-affinity Ca(2+) buffer in many biological systems. Given the centrality of Ca(2+) signaling in health and disease, EF-hand motifs designed to have new biological activities may have widespread utility. Here, an EF-hand motif substitution that had been presumed to destroy EF-hand function, that of glutamine for glutamate at position 12 of the second cation binding loop domain of Parv (ParvE101Q), markedly inverted relative cation affinities: Mg(2+) affinity increased, whereas Ca(2+) affinity decreased, forming a new ultra-delayed Ca(2+) buffer with favorable properties for promoting cardiac relaxation. In therapeutic testing, expression of ParvE101Q fully reversed the severe myocyte intrinsic contractile defect inherent to expression of native Parv and corrected abnormal myocardial relaxation in diastolic dysfunction disease models in vitro and in vivo. Strategic design of new EF-hand motif domains to modulate intracellular Ca(2+) signaling could benefit many biological systems with abnormal Ca(2+) handling, including the diseased heart.
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Affiliation(s)
- Wang Wang
- Department of Integrative Biology and Physiology, University of Minnesota Medical School, Minneapolis, Minnesota, USA
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15
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Michael JJ, Gollapudi SK, Ford SJ, Kazmierczak K, Szczesna-Cordary D, Chandra M. Deletion of 1-43 amino acids in cardiac myosin essential light chain blunts length dependency of Ca(2+) sensitivity and cross-bridge detachment kinetics. Am J Physiol Heart Circ Physiol 2013; 304:H253-9. [PMID: 23144314 PMCID: PMC3543674 DOI: 10.1152/ajpheart.00572.2012] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/27/2012] [Accepted: 11/01/2012] [Indexed: 11/22/2022]
Abstract
The role of cardiac myosin essential light chain (ELC) in the sarcomere length (SL) dependency of myofilament contractility is unknown. Therefore, mechanical and dynamic contractile properties were measured at SL 1.9 and 2.2 μm in cardiac muscle fibers from two groups of transgenic (Tg) mice: 1) Tg-wild-type (WT) mice that expressed WT human ventricular ELC and 2) Tg-Δ43 mice that expressed a mutant ELC lacking 1-43 amino acids. In agreement with previous studies, Ca(2+)-activated maximal tension decreased significantly in Tg-Δ43 fibers. pCa(50) (-log(10) [Ca(2+)](free) required for half maximal activation) values at SL of 1.9 μm were 5.64 ± 0.02 and 5.70 ± 0.02 in Tg-WT and Tg-Δ43 fibers, respectively. pCa(50) values at SL of 2.2 μm were 5.70 ± 0.01 and 5.71 ± 0.01 in Tg-WT and Tg-Δ43 fibers, respectively. The SL-mediated increase in the pCa(50) value was statistically significant only in Tg-WT fibers (P < 0.01), indicating that the SL dependency of myofilament Ca(2+) sensitivity was blunted in Tg-Δ43 fibers. The SL dependency of cross-bridge (XB) detachment kinetics was also blunted in Tg-Δ43 fibers because the decrease in XB detachment kinetics was significant (P < 0.001) only at SL 1.9 μm. Thus the increased XB dwell time at the short SL augments Ca(2+) sensitivity at short SL and thus blunts SL-mediated increase in myofilament Ca(2+) sensitivity. Our data suggest that the NH(2)-terminal extension of cardiac ELC not only augments the amplitude of force generation, but it also may play a role in mediating the SL dependency of XB detachment kinetics and myofilament Ca(2+) sensitivity.
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Affiliation(s)
- John Jeshurun Michael
- Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology, Washington State University, Pullman, WA 99164, USA
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16
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Mijailovich SM, Kayser-Herold O, Li X, Griffiths H, Geeves MA. Cooperative regulation of myosin-S1 binding to actin filaments by a continuous flexible Tm-Tn chain. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2012; 41:1015-32. [PMID: 23052974 PMCID: PMC3509328 DOI: 10.1007/s00249-012-0859-8] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/04/2012] [Revised: 08/30/2012] [Accepted: 09/06/2012] [Indexed: 12/20/2022]
Abstract
The regulation of striated muscle contraction involves cooperative interactions between actin filaments, myosin-S1 (S1), tropomyosin (Tm), troponin (Tn), and calcium. These interactions are modeled by treating overlapping tropomyosins as a continuous flexible chain (CFC), weakly confined by electrostatic interactions with actin. The CFC is displaced locally in opposite directions on the actin surface by the binding of either S1 or Troponin I (TnI) to actin. The apparent rate constants for myosin and TnI binding to and detachment from actin are then intrinsically coupled via the CFC model to the presence of neighboring bound S1s and TnIs. Monte Carlo simulations at prescribed values of the CFC stiffness, the CFC’s degree of azimuthal confinement, and the angular displacements caused by the bound proteins were able to predict the stopped-flow transients of S1 binding to regulated F-actin. The transients collected over a large range of calcium concentrations could be well described by adjusting a single calcium-dependent parameter, the rate constant of TnI detachment from actin, k−I. The resulting equilibrium constant \documentclass[12pt]{minimal}
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\begin{document}$$ K_{\text{B}} \equiv 1/K_{\text{I}} $$\end{document} varied sigmoidally with the free calcium, increasing from 0.12 at low calcium (pCa >7) to 12 at high calcium (pCa <5.5) with a Hill coefficient of ~2.15. The similarity of the curves for excess-actin and excess-myosin data confirms their allosteric relationship. The spatially explicit calculations confirmed variable sizes for the cooperative units and clustering of bound myosins at low calcium concentrations. Moreover, inclusion of negative cooperativity between myosin units predicted the observed slowing of myosin binding at excess-myosin concentrations.
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Affiliation(s)
- Srboljub M Mijailovich
- Department of Environmental Health, Harvard School of Public Health, Boston, MA 02115, USA.
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17
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Tanner BCW, Daniel TL, Regnier M. Filament compliance influences cooperative activation of thin filaments and the dynamics of force production in skeletal muscle. PLoS Comput Biol 2012; 8:e1002506. [PMID: 22589710 PMCID: PMC3349719 DOI: 10.1371/journal.pcbi.1002506] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2011] [Accepted: 03/17/2012] [Indexed: 12/03/2022] Open
Abstract
Striated muscle contraction is a highly cooperative process initiated by Ca2+ binding to the troponin complex, which leads to tropomyosin movement and myosin cross-bridge (XB) formation along thin filaments. Experimental and computational studies suggest skeletal muscle fiber activation is greatly augmented by cooperative interactions between neighboring thin filament regulatory units (RU-RU cooperativity; 1 RU = 7 actin monomers+1 troponin complex+1 tropomyosin molecule). XB binding can also amplify thin filament activation through interactions with RUs (XB-RU cooperativity). Because these interactions occur with a temporal order, they can be considered kinetic forms of cooperativity. Our previous spatially-explicit models illustrated that mechanical forms of cooperativity also exist, arising from XB-induced XB binding (XB-XB cooperativity). These mechanical and kinetic forms of cooperativity are likely coordinated during muscle contraction, but the relative contribution from each of these mechanisms is difficult to separate experimentally. To investigate these contributions we built a multi-filament model of the half sarcomere, allowing RU activation kinetics to vary with the state of neighboring RUs or XBs. Simulations suggest Ca2+ binding to troponin activates a thin filament distance spanning 9 to 11 actins and coupled RU-RU interactions dominate the cooperative force response in skeletal muscle, consistent with measurements from rabbit psoas fibers. XB binding was critical for stabilizing thin filament activation, particularly at submaximal Ca2+ levels, even though XB-RU cooperativity amplified force less than RU-RU cooperativity. Similar to previous studies, XB-XB cooperativity scaled inversely with lattice stiffness, leading to slower rates of force development as stiffness decreased. Including RU-RU and XB-RU cooperativity in this model resulted in the novel prediction that the force-[Ca2+] relationship can vary due to filament and XB compliance. Simulations also suggest kinetic forms of cooperativity occur rapidly and dominate early to get activation, while mechanical forms of cooperativity act more slowly, augmenting XB binding as force continues to develop. In striated muscle myosin binds to actin and converts chemical energy from ATP hydrolysis into force, work, and power. Myosin cross-bridge binding is regulated by Ca2+ and the thin filament proteins troponin and tropomyosin. Cooperative interactions between actin, myosin, troponin, and tropomyosin greatly influence spatial and kinetic properties of thin filament activation, thereby affecting muscle mechanics and contractility. Such cooperative interactions are complex and individual contributions from the different contractile and regulatory proteins are difficult to separate experimentally. However, a few theoretical models have explored interactions between the spatial, kinetic, and mechanical processes that affect cooperative cross-bridge binding to actin. Building on our prior spatially-explicit computational models, we investigated the relative contributions of thin filament regulatory proteins and cross-bridges to cooperatively amplify skeletal muscle force production. We find that Ca2+-dependent contraction in skeletal muscle is dominated by neighboring regulatory protein interactions along the thin filament, while cross-bridge binding is critical for maintaining or stabilizing thin filament activation as force develops. Moreover, we reveal that variations in filament and cross-bridge stiffness can alter Ca2+-sensitivity and cooperativity of skeletal muscle force production. In conclusion, these simulations show that multiple cooperative mechanisms combine to produce physiological force responses measured from muscle cells.
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Affiliation(s)
- Bertrand C W Tanner
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, Vermont, United States of America.
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18
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Loong CKP, Badr MA, Chase PB. Tropomyosin flexural rigidity and single ca(2+) regulatory unit dynamics: implications for cooperative regulation of cardiac muscle contraction and cardiomyocyte hypertrophy. Front Physiol 2012; 3:80. [PMID: 22493584 PMCID: PMC3318232 DOI: 10.3389/fphys.2012.00080] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2011] [Accepted: 03/18/2012] [Indexed: 01/04/2023] Open
Abstract
Striated muscle contraction is regulated by dynamic and cooperative interactions among Ca2+, troponin, and tropomyosin on the thin filament. While Ca2+ regulation has been extensively studied, little is known about the dynamics of individual regulatory units and structural changes of individual tropomyosin molecules in relation to their mechanical properties, and how these factors are altered by cardiomyopathy mutations in the Ca2+ regulatory proteins. In this hypothesis paper, we explore how various experimental and analytical approaches could broaden our understanding of the cooperative regulation of cardiac contraction in health and disease.
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Affiliation(s)
- Campion K P Loong
- Department of Biological Science, The Florida State University Tallahassee, FL, USA
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19
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Vandenboom R, Herron T, Favre E, Albayya FP, Metzger JM. Gene transfer, expression, and sarcomeric incorporation of a headless myosin molecule in cardiac myocytes: evidence for a reserve in myofilament motor function. Am J Physiol Heart Circ Physiol 2010; 300:H574-82. [PMID: 21112946 DOI: 10.1152/ajpheart.00786.2009] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The purpose of this study was to implement a living myocyte in vitro model system to test whether a motor domain-deleted headless myosin construct could be incorporated into the sarcomere and affect contractility. To this end we used gene transfer to express a "headless" myosin heavy chain (headless-MHC) in complement with the native full-length myosin motors in the cardiac sarcomere. An NH2-terminal Flag epitope was used for unique detection of the motor domain-deleted headless-MHC. Total MHC content (i.e., headless-MHC+endogenous MHC) remained constant, while expression of the headless-MHC in transduced myocytes increased from 24 to 72 h after gene transfer until values leveled off at 96 h after gene transfer, at which time the headless-MHC comprised ∼20% of total MHC. Moreover, immunofluorescence labeling and confocal imaging confirmed expression and demonstrated incorporation of the headless-MHC in the A band of the cardiac sarcomere. Functional measurements in intact myocytes showed that headless-MHC modestly reduced amplitude of dynamic twitch contractions compared with controls (P<0.05). In chemically permeabilized myocytes, maximum steady-state isometric force and the tension-pCa relationship were unaltered by the headless-MHC. These data suggest that headless-MHC can express to 20% of total myosin and incorporate into the sarcomere yet have modest to no effects on dynamic and steady-state contractile function. This would indicate a degree of functional tolerance in the sarcomere for nonfunctional myosin molecules.
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Affiliation(s)
- Rene Vandenboom
- Integrative Biology and Physiology, University of Minnesota School of Medicine, 6-125 Jackson Hall, 321 Church St. E, Minneapolis, MN 55455, USA
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20
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Ramsey KA, Bakker AJ, Pinniger GJ. Fiber-type dependence of stretch-induced force enhancement in rat skeletal muscle. Muscle Nerve 2010; 42:769-77. [PMID: 20976780 DOI: 10.1002/mus.21744] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
When an active muscle is stretched, the force increases due to strain of contractile and noncontractile proteins. We examined this force enhancement in rat extensor digitorum longus (EDL) and soleus muscles, which differ in their composition of these proteins, and their susceptibility to damage. Small stretches were applied at different velocities during isometric contractions from which we quantified the velocity-dependent contractile and velocity-independent noncontractile contributions to force enhancement. Whereas the contractile contribution was significantly greater in soleus than EDL, the noncontractile force enhancement was significantly greater in EDL than soleus, and increased ≈6-fold after damaging eccentric contractions. The increased contractile stiffness may be functionally beneficial in slow muscle, as resistance to lengthening is fundamental to maintaining posture. Following stretch-induced muscle damage this capacity is compromised, leading to increased strain of noncontractile proteins that may facilitate the activation of signaling pathways involved in muscle adaptation to injury.
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Affiliation(s)
- Kathryn A Ramsey
- School of Biomedical, Biomolecular and Chemical Sciences, University of Western Australia, Crawley, Western Australia, Australia
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21
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Davis J, Metzger JM. Combinatorial effects of double cardiomyopathy mutant alleles in rodent myocytes: a predictive cellular model of myofilament dysregulation in disease. PLoS One 2010; 5:e9140. [PMID: 20161772 PMCID: PMC2818843 DOI: 10.1371/journal.pone.0009140] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2009] [Accepted: 01/19/2010] [Indexed: 12/05/2022] Open
Abstract
Inherited cardiomyopathy (CM) represents a diverse group of cardiac muscle diseases that present with a broad spectrum of symptoms ranging from benign to highly malignant. Contributing to this genetic complexity and clinical heterogeneity is the emergence of a cohort of patients that are double or compound heterozygotes who have inherited two different CM mutant alleles in the same or different sarcomeric gene. These patients typically have early disease onset with worse clinical outcomes. Little experimental attention has been directed towards elucidating the physiologic basis of double CM mutations at the cellular-molecular level. Here, dual gene transfer to isolated adult rat cardiac myocytes was used to determine the primary effects of co-expressing two different CM-linked mutant proteins on intact cardiac myocyte contractile physiology. Dual expression of two CM mutants, that alone moderately increase myofilament activation, tropomyosin mutant A63V and cardiac troponin mutant R146G, were shown to additively slow myocyte relaxation beyond either mutant studied in isolation. These results were qualitatively similar to a combination of moderate and strong activating CM mutant alleles alphaTmA63V and cTnI R193H, which approached a functional threshold. Interestingly, a combination of a CM myofilament deactivating mutant, troponin C G159D, together with an activating mutant, cTnIR193H, produced a hybrid phenotype that blunted the strong activating phenotype of cTnIR193H alone. This is evidence of neutralizing effects of activating/deactivating mutant alleles in combination. Taken together, this combinatorial mutant allele functional analysis lends molecular insight into disease severity and forms the foundation for a predictive model to deconstruct the myriad of possible CM double mutations in presenting patients.
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Affiliation(s)
- Jennifer Davis
- Department of Integrative Biology and Physiology, University of Minnesota, Minneapolis, Minnesota, United States of America
| | - Joseph M. Metzger
- Department of Integrative Biology and Physiology, University of Minnesota, Minneapolis, Minnesota, United States of America
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22
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Kawano F, Goto K, Wang XD, Terada M, Ohira T, Nakai N, Yoshioka T, Ohira Y. Role(s) of gravitational loading during developing period on the growth of rat soleus muscle fibers. J Appl Physiol (1985) 2010; 108:676-85. [PMID: 20056853 DOI: 10.1152/japplphysiol.00478.2009] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Effects of gravitational loading or unloading on the gain of the characteristics in soleus muscle fibers were studied in rats. The tail suspension was performed in newborn rats from postnatal day 4 to month 3, and the reloading was allowed for 3 mo in some rats. Single expression of type I myosin heavy chain (MHC) was observed in approximately 82% of fibers in 3-mo-old controls, but the fibers expressing multiple MHC isoforms were noted in the unloaded rats. Although 97% of fibers in 3-mo-old controls had a single neuromuscular junction at the central region of fiber, fibers with multiple nerve endplates were seen in the unloaded group. Faster contraction speed and lower maximal tension development, even after normalization with fiber size, were observed in the unloaded pure type I MHC fibers. These parameters generally returned to the age-matched control levels after reloading. It was suggested that antigravity-related tonic activity plays an important role in the gain of single neural innervation and of slow contractile properties and phenotype in soleus muscle fibers.
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Affiliation(s)
- Fuminori Kawano
- Graduate School of Medicine, Osaka University, Toyonaka City, Osaka 560-0043, Japan
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23
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Herron TJ, Devaney E, Mundada L, Arden E, Day S, Guerrero-Serna G, Turner I, Westfall M, Metzger JM. Ca2+-independent positive molecular inotropy for failing rabbit and human cardiac muscle by alpha-myosin motor gene transfer. FASEB J 2009; 24:415-24. [PMID: 19801488 DOI: 10.1096/fj.09-140566] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Current inotropic therapies used to increase cardiac contractility of the failing heart center on increasing the amount of calcium available for contraction, but their long-term use is associated with increased mortality due to fatal arrhythmias. Thus, there is a need to develop and explore novel inotropic therapies that can act via calcium-independent mechanisms. The purpose of this study was to determine whether fast alpha-myosin molecular motor gene transfer can confer calcium-independent positive inotropy in slow beta-myosin-dominant rabbit and human failing ventricular myocytes. To this end, we generated a recombinant adenovirus (AdMYH6) to deliver the full-length human alpha-myosin gene to adult rabbit and human cardiac myocytes in vitro. Fast alpha-myosin motor expression was determined by Western blotting and immunocytochemical analysis and confocal imaging. In experiments using electrically stimulated myocytes from ischemic failing hearts, AdMYH6 increased the contractile amplitude of failing human [23.9+/-7.8 nm (n=10) vs. AdMYH6 amplitude 78.4+/-16.5 nm (n=6)] and rabbit myocytes. The intracellular calcium transient amplitude was not altered. Control experiments included the use of a green fluorescent protein or a beta-myosin heavy chain adenovirus. Our data provide evidence for a novel form of calcium-independent positive inotropy in failing cardiac myocytes by fast alpha-myosin motor protein gene transfer.
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Affiliation(s)
- Todd J Herron
- Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor, Michigan, USA
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24
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Characterization of secophalloidin-induced force loss in cardiac myofibrils. J Muscle Res Cell Motil 2009; 30:209-16. [PMID: 19763850 DOI: 10.1007/s10974-009-9188-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2009] [Accepted: 08/26/2009] [Indexed: 10/20/2022]
Abstract
Secophalloidin (SPH) is known to cause in cardiac myofibrils force without Ca(2+) (half-maximal effect approximately 2 mM) followed by irreversible loss of Ca(2+)-activated force. At maximal Ca(2+) activation, SPH increases force (half-maximal effect < 0.1 mM). We found that SPH at low concentration (0.5 mM) did not cause either force activation or force loss at pCa 8.7, but both of these effects did occur when force was activated by Ca(2+). The force loss was prevented when SPH was applied during rigor or in the presence of 2,3-butanedione monoxime (85 mM). Furthermore, studying muscle in which the force was previously reduced by SPH (up to 50%) did not reveal significant changes in Ca(2+) sensitivity and cooperativity of Ca(2+) activation or qualitative alterations in SPH-induced changes in Ca(2+)-activated contraction. Data suggest that the force loss is mediated by cycling cross-bridges, and might reflect a reduction in force generated by individual cross-bridges.
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25
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Ottenheijm CAC, Hidalgo C, Rost K, Gotthardt M, Granzier H. Altered contractility of skeletal muscle in mice deficient in titin's M-band region. J Mol Biol 2009; 393:10-26. [PMID: 19683008 DOI: 10.1016/j.jmb.2009.08.009] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2009] [Revised: 08/06/2009] [Accepted: 08/07/2009] [Indexed: 12/19/2022]
Abstract
We investigated the contractile phenotype of skeletal muscle deficient in exons MEx1 and MEx2 (KO) of the titin M-band by using the cre-lox recombination system and a multidisciplinary physiological approach to study skeletal muscle contractile performance. At a maximal tetanic stimulation frequency, intact KO extensor digitorum longus muscle was able to produce wild-type levels of force. However, at submaximal stimulation frequency, force was reduced in KO mice, giving rise to a rightward shift of the force-frequency curve. This rightward shift of the force-frequency curve could not be explained by altered sarcoplasmic reticulum Ca(2+) handling, as indicated by analysis of Ca(2+) transients in intact myofibers and expression of Ca(2)(+)-handling proteins, but can be explained by the reduced myofilament Ca(2+) sensitivity of force generation that we found. Western blotting experiments suggested that the excision of titin exons MEx1 and MEx2 did not result in major changes in expression of titin M-band binding proteins or phosphorylation level of the thin-filament regulatory proteins, but rather in a shift toward expression of slow isoforms of the thick-filament-associated protein, myosin binding protein-C. Extraction of myosin binding protein-C from skinned muscle normalized myofilament Ca(2+) sensitivity of the KO extensor digitorum longus muscle. Thus, our data suggest that the M-band region of titin affects the expression of genes involved in the regulation of skeletal muscle contraction.
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Affiliation(s)
- Coen A C Ottenheijm
- Department of Physiology, Sarver Molecular Cardiovascular Research Program, University of Arizona, Tucson, AZ 85724, USA
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26
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Rao VS, Marongelli EN, Guilford WH. Phosphorylation of tropomyosin extends cooperative binding of myosin beyond a single regulatory unit. ACTA ACUST UNITED AC 2009; 66:10-23. [PMID: 18985725 DOI: 10.1002/cm.20321] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Tropomyosin (Tm) is one of the major phosphoproteins comprising the thin filament of muscle. However, the specific role of Tm phosphorylation in modulating the mechanics of actomyosin interaction has not been determined. Here we show that Tm phosphorylation is necessary for long-range cooperative activation of myosin binding. We used a novel optical trapping assay to measure the isometric stall force of an ensemble of myosin molecules moving actin filaments reconstituted with either natively phosphorylated or dephosphorylated Tm. The data show that the thin filament is cooperatively activated by myosin across regulatory units when Tm is phosphorylated. When Tm is dephosphorylated, this "long-range" cooperative activation is lost and the filament behaves identically to bare actin filaments. However, these effects are not due to dissociation of dephosphorylated Tm from the reconstituted thin filament. The data suggest that end-to-end interactions of adjacent Tm molecules are strengthened when Tm is phosphorylated, and that phosphorylation is thus essential for long range cooperative activation along the thin filament.
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Affiliation(s)
- Vijay S Rao
- Department of Biomedical Engineering, University of Virginia, Charlottesville, VA 22908, USA
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27
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Palpant NJ, D'Alecy LG, Metzger JM. Single histidine button in cardiac troponin I sustains heart performance in response to severe hypercapnic respiratory acidosis in vivo. FASEB J 2009; 23:1529-40. [PMID: 19141534 DOI: 10.1096/fj.08-121996] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Intracellular acidosis is a profound negative regulator of myocardial performance. We hypothesized that titrating myofilament calcium sensitivity by a single histidine substituted cardiac troponin I (A164H) would protect the whole animal physiological response to acidosis in vivo. To experimentally induce severe hypercapnic acidosis, mice were exposed to a 40% CO(2) challenge. By echocardiography, it was found that systolic function and ventricular geometry were maintained in cTnI A164H transgenic (Tg) mice. By contrast, non-Tg (Ntg) littermates experienced rapid and marked cardiac decompensation during this same challenge. For detailed hemodymanic assessment, Millar pressure-conductance catheterization was performed while animals were treated with a beta-blocker, esmolol, during a severe hypercapnic acidosis challenge. Survival and load-independent measures of contractility were significantly greater in Tg vs. Ntg mice. This assay showed that Ntg mice had 100% mortality within 5 min of acidosis. By contrast, systolic and diastolic function were protected in Tg mice during acidosis, and they had 100% survival. This study shows that, independent of any beta-adrenergic compensation, myofilament-based molecular manipulation of inotropy by histidine-modified troponin I maintains cardiac inotropic and lusitropic performance and markedly improves survival during severe acidosis in vivo.
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Affiliation(s)
- Nathan J Palpant
- Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor, Michigan, USA
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28
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Davis J, Wen H, Edwards T, Metzger JM. Allele and species dependent contractile defects by restrictive and hypertrophic cardiomyopathy-linked troponin I mutants. J Mol Cell Cardiol 2008; 44:891-904. [PMID: 18423659 PMCID: PMC2443058 DOI: 10.1016/j.yjmcc.2008.02.274] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/12/2007] [Revised: 02/13/2008] [Accepted: 02/16/2008] [Indexed: 11/15/2022]
Abstract
Restrictive cardiomyopathy (RCM) is a debilitating disease characterized by impaired ventricular filling, reduced ventricular volumes, and severe diastolic dysfunction. Hypertrophic cardiomyopathy (HCM) is characterized by ventricular hypertrophy and heightened risk of premature sudden cardiac death. These cardiomyopathies can result from mutations in the same gene that encodes for cardiac troponin I (cTnI). Acute genetic engineering of adult rat cardiac myocytes was used to ascertain whether primary physiologic outcomes could distinguish between RCM and HCM alleles at the cellular level. Co-transduction of cardiac myocytes with wild-type (WT) cTnI and RCM/HCM linked mutants in cTnI's inhibitory region (IR) demonstrated that WT cTnI preferentially incorporated into the sarcomere over IR mutants. The cTnI IR mutants exhibited minor effects in single myocyte Ca(2+)-activated tension assays yet prolonged relaxation and Ca(2+) decay. In comparison RCM cTnI mutants in the helix-4/C-terminal region demonstrated a) hyper-sensitivity to Ca(2+) under loaded conditions, b) slowed myocyte mechanical relaxation and Ca(2+) transient decay, c) frequency-dependent Ca(2+)-independent diastolic tone, d) heightened myofilament incorporation and e) irreversible cellular contractile defects with acute diltiazem administration. For species comparison, a subset of cTnI mutants were tested in isolated adult rabbit cardiac myocytes. Here, RCM and HCM mutant cTnIs exerted similar effects of slowed myocyte relaxation and Ca(2+) transient decay but did not show variable phenotypes by cTnI region. This study highlights cellular contractile defects by cardiomyopathy mutant cTnIs that are allele and species dependent. The species dependent results in particular raise important issues toward elucidating a unifying mechanistic pathway underlying the inherited cardiomyopathies.
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Affiliation(s)
- Jennifer Davis
- Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, Michigan, USA
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29
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Hanft LM, Korte FS, McDonald KS. Cardiac function and modulation of sarcomeric function by length. Cardiovasc Res 2007; 77:627-36. [PMID: 18079105 DOI: 10.1093/cvr/cvm099] [Citation(s) in RCA: 74] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
The Frank-Starling relationship provides beat-to-beat regulation of ventricular function by matching ventricular input and output. This review addresses the subcellular mechanisms by which the ventricle adjusts its output (i.e. stroke volume) by changes in end-diastolic volume. The subcellular processes are placed in the context of the four phases of the cardiac cycle with emphasis on the sarcomeric properties that mediate the number of force-generating cross-bridges recruited during pressure development. Additional mechanistic insight is provided regarding the factors that regulate myocyte loaded shortening speeds, which are paramount for dictating ejection volume. Emphasis is placed on the interplay between cross-bridge-induced cooperative activation of the thin filament and cooperative deactivation of the thin filament induced by muscle shortening. The balance of these two properties seems to determine systolic haemodynamics, and how this balance is modulated by sarcomere length, in part, underlies the Frank-Starling relationship.
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Affiliation(s)
- Laurin M Hanft
- Department of Medical Pharmacology & Physiology, MA 415, Medical Sciences Building, School of Medicine, University of Missouri, Columbia, MO 65212, USA
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30
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Prochniewicz E, Lowe DA, Spakowicz DJ, Higgins L, O'Conor K, Thompson LV, Ferrington DA, Thomas DD. Functional, structural, and chemical changes in myosin associated with hydrogen peroxide treatment of skeletal muscle fibers. Am J Physiol Cell Physiol 2007; 294:C613-26. [PMID: 18003749 DOI: 10.1152/ajpcell.00232.2007] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
To understand the molecular mechanism of oxidation-induced inhibition of muscle contractility, we have studied the effects of hydrogen peroxide on permeabilized rabbit psoas muscle fibers, focusing on changes in myosin purified from these fibers. Oxidation by 5 mM peroxide decreased fiber contractility (isometric force and shortening velocity) without significant changes in the enzymatic activity of myofibrils and isolated myosin. The inhibitory effects were reversed by treating fibers with dithiothreitol. Oxidation by 50 mM peroxide had a more pronounced and irreversible inhibitory effect on fiber contractility and also affected enzymatic activity of myofibrils, myosin, and actomyosin. Peroxide treatment also affected regulation of contractility, resulting in fiber activation in the absence of calcium. Electron paramagnetic resonance of spin-labeled myosin in muscle fibers showed that oxidation increased the fraction of myosin heads in the strong-binding structural state under relaxing conditions (low calcium) but had no effect under activating conditions (high calcium). This change in the distribution of structural states of myosin provides a plausible explanation for the observed changes in both contractile and regulatory functions. Mass spectroscopy analysis showed that 50 mM but not 5 mM peroxide induced oxidative modifications in both isoforms of the essential light chains and in the heavy chain of myosin subfragment 1 by targeting multiple methionine residues. We conclude that 1) inhibition of muscle fiber contractility via oxidation of myosin occurs at high but not low concentrations of peroxide and 2) the inhibitory effects of oxidation suggest a critical and previously unknown role of methionines in myosin function.
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Affiliation(s)
- Ewa Prochniewicz
- Department of Biochemistry, University of Minnesota, Jackson Hall 6-155, 321 Church St., Minneapolis, MN 55455, USA.
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31
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Yasuda SI, Coutu P, Sadayappan S, Robbins J, Metzger JM. Cardiac transgenic and gene transfer strategies converge to support an important role for troponin I in regulating relaxation in cardiac myocytes. Circ Res 2007; 101:377-86. [PMID: 17615373 DOI: 10.1161/circresaha.106.145557] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Elucidating the relative roles of cardiac troponin I (cTnI) and phospholamban (PLN) in beta-adrenergic-mediated hastening of cardiac relaxation has been challenging and controversial. To test the hypothesis that beta-adrenergic phosphorylation of cTnI has a prominent role in accelerating cardiac myocyte relaxation performance we used transgenic (Tg) mice bearing near complete replacement of native cTnI with a beta-adrenergic phospho-mimetic of cTnI whereby tandem serine codons 23/24 were converted to aspartic acids (cTnI S23/24D). Adult cardiac myocytes were isolated and contractility determined at physiological temperature under unloaded and loaded conditions using micro-carbon fibers. At baseline, cTnI S23/24D myocytes had significantly faster relaxation times relative to controls, and isoproterenol stimulation (Iso) had only a small effect to further speed relaxation in cTnI S23/24D myocytes (delta Iso: 7.2 ms) relative to the maximum Iso effect (31.2 ms) in control. The Ca(2+) transient decay rate was similarly accelerated by Iso in Tg and nontransgenic (Ntg) myocytes. Gene transfer of cTnI S23/24D to myocytes in primary culture showed comparable findings. Gene transfer of cTnI with both serines 23/24 converted to alanines (cTnI S23/24A), or gene transfer of slow skeletal TnI, both of which lack PKA phosphorylation sites, significantly blunted Iso-mediated enhanced relaxation compared with controls. Gene transfer of wild-type cTnI had no effect on relaxation. These findings support a key role of cTnI in myocyte relaxation and highlight a direct contribution of the myofilaments in modulating the dynamics of myocardial performance.
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Affiliation(s)
- So-ichiro Yasuda
- Department of Molecular and Integrative Physiology, 7730 Medical Science II, University of Michigan, Ann Arbor, MI 48109-0622, USA
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32
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Davis J, Wen H, Edwards T, Metzger JM. Thin Filament Disinhibition by Restrictive Cardiomyopathy Mutant R193H Troponin I Induces Ca
2+
-Independent Mechanical Tone and Acute Myocyte Remodeling. Circ Res 2007; 100:1494-502. [PMID: 17463320 DOI: 10.1161/01.res.0000268412.34364.50] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Inherited restrictive cardiomyopathy (RCM) is a debilitating disease characterized by a stiff heart with impaired ventricular relaxation. Mutations in cardiac troponin I (cTnI) were identified as causal for RCM. Acute genetic engineering of adult cardiac myocytes was used to identify primary structure/function effects of mutant cTnI. Studies focused on R193H cTnI owing to the poor prognosis of this allele. Compared with wild-type cTnI, R193H mutant cTnI more effectively incorporated into the sarcomere, where it exerted dose-dependent effects on basal and dynamic contractile function. Under loaded conditions, permeabilized myocyte Ca
2+
sensitivity of tension was increased, whereas the passive tension–extension relationship was not altered by R193H cTnI. Normal rod-shaped myocyte morphology acutely transitioned to a “short-squat” phenotype in concert with progressive stoichiometric incorporation of R193H in the absence of altered diastolic Ca
2+
. The specific myosin inhibitor blebbistatin fully blocked this transition. Heightened Ca
2+
buffering by the R193H myofilaments, and not alterations in Ca
2+
handling by the sarcoplasmic reticulum, slowed the decay rate of the Ca
2+
transient. Incomplete mechanical relaxation conferred by R193H was exacerbated at increasing pacing frequencies independent of elevated diastolic Ca
2+
. R193H cTnI–dependent mechanical tone caused acute remodeling to a quasicontracted state not elicited by other Ca
2+
-sensitizing proteins and is a direct correlate of the stiff heart characteristic of RCM in vivo. These results point toward targets downstream of Ca
2+
handling, notably thin filament regulation and actin–myosin interaction, in designing therapeutic strategies to redress the primary cell morphological and mechanical underpinnings of RCM.
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Affiliation(s)
- Jennifer Davis
- Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI, USA
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33
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Herron TJ, Vandenboom R, Fomicheva E, Mundada L, Edwards T, Metzger JM. Calcium-independent negative inotropy by beta-myosin heavy chain gene transfer in cardiac myocytes. Circ Res 2007; 100:1182-90. [PMID: 17363698 DOI: 10.1161/01.res.0000264102.00706.4e] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Increased relative expression of the slow molecular motor of the heart (beta-myosin heavy chain [MyHC]) is well known to occur in many rodent models of cardiovascular disease and in human heart failure. The direct effect of increased relative beta-MyHC expression on intact cardiac myocyte contractility, however, is unclear. To determine the direct effects of increased relative beta-MyHC expression on cardiac contractility, we used acute genetic engineering with a recombinant adenoviral vector (AdMYH7) to genetically titrate beta-MyHC protein expression in isolated rodent ventricular cardiac myocytes that predominantly expressed alpha-MyHC (fast molecular motor). AdMYH7-directed beta-MyHC protein expression and sarcomeric incorporation was observed as soon as 1 day after gene transfer. Effects of beta-MyHC expression on myocyte contractility were determined in electrically paced single myocytes (0.2 Hz, 37 degrees C) by measuring sarcomere shortening and intracellular calcium cycling. Gene transfer-based replacement of alpha-MyHC with beta-MyHC attenuated contractility in a dose-dependent manner, whereas calcium transients were unaffected. For example, when beta-MyHC expression accounted for approximately 18% of the total sarcomeric myosin, the amplitude of sarcomere-length shortening (nanometers, nm) was depressed by 42% (151.0+/-10.7 [control] versus 87.0+/-5.4 nm [AdMYH7 transduced]); and genetic titration of beta-MyHC, leading to 38% beta-MyHC content, attenuated shortening by 57% (138.9+/-13.0 versus 59.7+/-7.1 nm). Maximal isometric cross-bridge cycling rate was also slower in AdMYH7-transduced myocytes. Results indicate that small increases of beta-MyHC expression (18%) have Ca2+ transient-independent physiologically relevant effects to decrease intact cardiac myocyte function. We conclude that beta-MyHC is a negative inotrope among the cardiac myofilament proteins.
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Affiliation(s)
- Todd J Herron
- Department of Molecular & Integrative Physiology, University of Michigan, Ann Arbor, MI 48109-0622, USA.
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34
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Vandenboom R, Weihe EK, Hannon JD. Dynamics of crossbridge-mediated activation in the heart. J Muscle Res Cell Motil 2005; 26:247-57. [PMID: 16322913 DOI: 10.1007/s10974-005-9042-5] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2005] [Accepted: 10/11/2005] [Indexed: 10/25/2022]
Abstract
Both intracellular calcium and strongly bound crossbridges contribute to thin filament activation in the heart, but the magnitude and the duration of the effects due to crossbridges are not well characterized. In this study, crossbridge attachment was altered in tetanized ferret papillary muscles and changes in the rate constant for the recovery of force (k (TR)) and unloaded shortening velocity (V (U)) were measured to track thin filament activation. k (TR) decreased as the time the muscles spent at low levels of crossbridge attachment (shortening deactivation) increased (0.02 s=17.9+/-2.3 s(-1), 0.32 s=3.3+/-0.4 s(-1); half-time=0.052 s; P<0.05). Furthermore, the deactivation was reversible and k (TR) recovered when muscles were allowed to regenerate force isometrically during the same tetanus. V (U) also decreased when the preceding load was lower (isometric load, V (U)=1.93+/-0.26 muscle lengths/s (ML/s); zero load, V (U)=0.93+/-0.14 ML/s, P<0.05) and as the length of time the muscle spent unloaded increased (>60% decline after 0.3 s). In addition, V (U) recovered when the muscle was allowed to regenerate force isometrically. These results indicate that crossbridge attachment increases thin filament activation as reflected in measurements of V (U) and k (TR). This 'extra' activation by crossbridges appears to be a dynamic process that decays during unloaded shortening and redevelops during isometric contraction.
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Affiliation(s)
- Rene Vandenboom
- Department of Anesthesiology, Mayo Clinic and Foundation, Rochester, MN 55905, USA
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35
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Bell MG, Lankford EB, Gonye GE, Ellis-Davies GCR, Martyn DA, Regnier M, Barsotti RJ. Kinetics of cardiac thin-filament activation probed by fluorescence polarization of rhodamine-labeled troponin C in skinned guinea pig trabeculae. Biophys J 2005; 90:531-43. [PMID: 16258047 PMCID: PMC1367058 DOI: 10.1529/biophysj.105.072769] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
A genetically engineered cardiac TnC mutant labeled at Cys-84 with tetramethylrhodamine-5-iodoacetamide dihydroiodide was passively exchanged for the endogenous form in skinned guinea pig trabeculae. The extent of exchange averaged nearly 70%, quantified by protein microarray of individual trabeculae. The uniformity of its distribution was verified by confocal microscopy. Fluorescence polarization, giving probe angle and its dispersion relative to the fiber long axis, was monitored simultaneously with isometric tension. Probe angle reflects underlying cTnC orientation. In steady-state experiments, rigor cross-bridges and Ca2+ with vanadate to inhibit cross-bridge formation produce a similar change in probe orientation as that observed with cycling cross-bridges (no Vi). Changes in probe angle were found at [Ca2+] well below those required to generate tension. Cross-bridges increased the Ca2+ dependence of angle change (cooperativity). Strong cross-bridge formation enhanced Ca2+ sensitivity and was required for full change in probe position. At submaximal [Ca2+], the thin filament regulatory system may act in a coordinated fashion, with the probe orientation of Ca2+-bound cTnC significantly affected by Ca2+ binding at neighboring regulatory units. The time course of the probe angle change and tension after photolytic release [Ca2+] by laser photolysis of NP-EGTA was Ca2+ sensitive and biphasic: a rapid component approximately 10 times faster than that of tension and a slower rate similar to that of tension. The fast component likely represents steps closely associated with Ca2+ binding to site II of cTnC, whereas the slow component may arise from cross-bridge feedback. These results suggest that the thin filament activation rate does not limit the tension time course in cardiac muscle.
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Affiliation(s)
- Marcus G Bell
- Department of Biomedical Sciences, Philadelphia College of Osteopathic Medicine, Philadelphia, Pennsylvania 19131, USA
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36
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Fuchs F, Martyn DA. Length-dependent Ca2+ activation in cardiac muscle: some remaining questions. J Muscle Res Cell Motil 2005; 26:199-212. [PMID: 16205841 DOI: 10.1007/s10974-005-9011-z] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2005] [Accepted: 08/20/2005] [Indexed: 10/25/2022]
Abstract
The steep relationship between systolic force and end diastolic volume in cardiac muscle (Frank-Starling relation) is, to a large extent, based on length-dependent changes in myofilament Ca(2+) sensitivity. How sarcomere length modulates Ca(2+) sensitivity is still a topic of active investigation. Two general themes have emerged in recent years. On the one hand, there is a large body of evidence indicating that length-dependent changes in lattice spacing determine changes in Ca(2+) sensitivity for a given set of conditions. A model has been put forward in which the number of strong-binding cross-bridges that are formed is directly related to the proximity of the myosin heads to binding sites on actin. On the other hand, there is also a body of evidence suggesting that lattice spacing and Ca(2+) sensitivity are not tightly linked and that there is a length-sensing element in the sarcomere, which can modulate actin-myosin interactions independent of changes in lattice spacing. In this review, we examine the evidence that has been cited in support of these viewpoints. Much recent progress has been based on the combination of mechanical measurements with X-ray diffraction analysis of lattice spacing and cross-bridge interaction with actin. Compelling evidence indicates that the relationship between sarcomere length and lattice spacing is influenced by the elastic properties of titin and that changes in lattice spacing directly modulate cross-bridge interactions with thin filaments. However, there is also evidence that the precise relationship between Ca(2+) sensitivity and lattice spacing can be altered by changes in protein isoform expression, protein phosphorylation, modifiers of cross-bridge kinetics, and changes in titin compliance. Hence although there is no unique relationship between Ca(2+) sensitivity and lattice spacing the evidence strongly suggests that under any given set of physiological circumstances variation in lattice spacing is the major determinant of length-dependent changes in Ca(2+) sensitivity.
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Affiliation(s)
- Franklin Fuchs
- Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA
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37
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Clemmens EW, Entezari M, Martyn DA, Regnier M. Different effects of cardiac versus skeletal muscle regulatory proteins on in vitro measures of actin filament speed and force. J Physiol 2005; 566:737-46. [PMID: 15905219 PMCID: PMC1464789 DOI: 10.1113/jphysiol.2005.084194] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
Mammalian cardiac and skeletal muscle express unique isoforms of the thin filament regulatory proteins, troponin (Tn) and tropomyosin (Tm), and the significance of these different isoforms in thin filament regulation has not been clearly identified. Both in vitro and skinned cellular studies investigating the mechanism of thin filament regulation in striated muscle have often used heterogeneous mixtures of Tn, Tm and myosin isoforms, and variability in reported results might be explained by different combinations of these proteins. Here we used in vitro motility and force (microneedle) assays to investigate the influence of cardiac versus skeletal Tn and Tm isoforms on actin-heavy meromyosin (HMM) mechanics. When interacting with skeletal HMM, thin filaments reconstituted with cardiac Tn/Tm or skeletal Tn/Tm exhibited similar speed-calcium relationships and significantly increased maximum speed and force per filament length (F/l) at pCa 5 (versus unregulated actin filaments). However, augmentation of F/l was greater with skeletal regulatory proteins. Reconstitution of thin filaments with the heterogeneous combination of skeletal Tn and cardiac Tm decreased sliding speeds at all [Ca2+] relative to thin filaments with skeletal Tn/Tm. Finally, for filaments reconstituted with any heterogeneous mix of Tn and Tm isoforms, force was not potentiated over that of unregulated actin filaments. Combined the results suggest (1) that cardiac regulatory proteins limit the allosteric enhancement of force, and (2) that Tn and Tm isoform homogeneity is important when studying Ca2+ regulation of crossbridge binding and kinetics as well as mechanistic differences between cardiac and skeletal muscle.
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38
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Adhikari BB, Regnier M, Rivera AJ, Kreutziger KL, Martyn DA. Cardiac length dependence of force and force redevelopment kinetics with altered cross-bridge cycling. Biophys J 2005; 87:1784-94. [PMID: 15345557 PMCID: PMC1304583 DOI: 10.1529/biophysj.103.039131] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We examined the influence of cross-bridge cycling kinetics on the length dependence of steady-state force and the rate of force redevelopment (k(tr)) during Ca(2+)-activation at sarcomere lengths (SL) of 2.0 and 2.3 microm in skinned rat cardiac trabeculae. Cross-bridge kinetics were altered by either replacing ATP with 2-deoxy-ATP (dATP) or by reducing [ATP]. At each SL dATP increased maximal force (F(max)) and Ca(2+)-sensitivity of force (pCa(50)) and reduced the cooperativity (n(H)) of force-pCa relations, whereas reducing [ATP] to 0.5 mM (low ATP) increased pCa(50) and n(H) without changing F(max). The difference in pCa(50) between SL 2.0 and 2.3 microm (Delta pCa(50)) was comparable between ATP and dATP, but reduced with low ATP. Maximal k(tr) was elevated by dATP and reduced by low ATP. Ca(2+)-sensitivity of k(tr) increased with both dATP and low ATP and was unaffected by altered SL under all conditions. Significantly, at equivalent levels of submaximal force k(tr) was faster at short SL or increased lattice spacing. These data demonstrate that the SL dependence of force depends on cross-bridge kinetics and that the increase of force upon SL extension occurs without increasing the rate of transitions between nonforce and force-generating cross-bridge states, suggesting SL or lattice spacing may modulate preforce cross-bridge transitions.
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Affiliation(s)
- Bishow B Adhikari
- Department of Bioengineering, University of Washington, Seattle Washington 98195, USA.
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39
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Moreno-Gonzalez A, Fredlund J, Regnier M. Cardiac troponin C (TnC) and a site I skeletal TnC mutant alter Ca2+ versus crossbridge contribution to force in rabbit skeletal fibres. J Physiol 2004; 562:873-84. [PMID: 15611027 PMCID: PMC1665546 DOI: 10.1113/jphysiol.2004.077891] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
We studied the relative contributions of Ca(2+) binding to troponin C (TnC) and myosin binding to actin in activating thin filaments of rabbit psoas fibres. The ability of Ca(2+) to activate thin filaments was reduced by replacing native TnC with cardiac TnC (cTnC) or a site I-inactive skeletal TnC mutant (xsTnC). Acto-myosin (crossbridge) interaction was either inhibited using N-benzyl-p-toluene sulphonamide (BTS) or enhanced by lowering [ATP] from 5.0 to 0.5 mm. Reconstitution with cTnC reduced maximal force (F(max)) by approximately 1/3 and the Ca(2+) sensitivity of force (pCa(50)) by 0.17 unit (P < 0.001), while reconstitution with xsTnC reduced F(max) by approximately 2/3 and pCa(50) by 0.19 unit (P < 0.001). In both cases the apparent cooperativity of activation (n(H)) was greatly decreased. In control fibres 3 mum BTS inhibited force to 57% of F(max) while in fibres reconstituted with cTnC or xsTnC, reconstituted maximal force (rF(max)) was inhibited to 8.8% and 14.3%, respectively. Under control conditions 3 mum BTS significantly decreased the pCa(50), but this effect was considerably reduced in cTnC reconstituted fibres, and eliminated in xsTnC reconstituted fibres. In contrast, when crossbridge cycle kinetics were slowed by lowering [ATP] from 5 to 0.5 mm in xsTnC reconstituted fibres, pCa(50) and n(H) were increased towards control values. Combined, our results demonstrate that when the ability of Ca(2+) binding to activate thin filaments is compromised, the relative contribution of strong crossbridges to maintain thin filament activation is increased. Furthermore, the data suggest that at low levels of Ca(2+), the level of thin filament activation is determined primarily by the direct effects of Ca(2+) on tropomyosin mobility, while at higher levels of Ca(2+) the final level of thin filament activation is primarily determined by strong cycling crossbridges.
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Affiliation(s)
- Alicia Moreno-Gonzalez
- Department of Bioengineering, University of Washington, Box 357962, Seattle, WA 98195-7962, USA
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40
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Metzger JM, Michele DE, Rust EM, Borton AR, Westfall MV. Sarcomere thin filament regulatory isoforms. Evidence of a dominant effect of slow skeletal troponin I on cardiac contraction. J Biol Chem 2003; 278:13118-23. [PMID: 12551900 DOI: 10.1074/jbc.m212601200] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Thin filament proteins tropomyosin (Tm), troponin T (TnT), and troponin I (TnI) form an allosteric regulatory complex that is required for normal cardiac contraction. Multiple isoforms of TnT, Tm, and TnI are differentially expressed in both cardiac development and disease, but concurrent TnI, Tm, and TnT isoform switching has hindered assignment of cellular function to these transitions. We systematically incorporated into the adult sarcomere the embryonic/fetal isoforms of Tm, TnT, and TnI by using gene transfer. In separate experiments, greater than 90% of native TnI and 40-50% of native Tm or TnT were specifically replaced. The Ca(2+) sensitivity of tension development was markedly enhanced by TnI replacement but not by TnT or Tm isoform replacement. Titration of TnI replacement from >90% to <30% revealed a dominant functional effect of slow skeletal TnI to modulate regulation. Over this range of isoform replacement, TnI, but not Tm or TnT embryonic isoforms, influenced calcium regulation of contraction, and this identifies TnI as a potential target to modify contractile performance in normal and diseased myocardium.
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Affiliation(s)
- Joseph M Metzger
- Department of Physiology, School of Medicine, University of Michigan, Ann Arbor, Michigan 48109-0622, USA.
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41
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Westfall MV, Borton AR, Albayya FP, Metzger JM. Myofilament calcium sensitivity and cardiac disease: insights from troponin I isoforms and mutants. Circ Res 2002; 91:525-31. [PMID: 12242271 DOI: 10.1161/01.res.0000034710.46739.c0] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The heightened Ca2+ sensitivity of force found with hypertrophic cardiomyopathy (HCM)-associated mutant cardiac troponin I (cTnIR145G; R146G in rodents) has been postulated to be an underlying cause of hypertrophic growth and premature sudden death in humans and in animal models of the disease. Expression of slow skeletal TnI (ssTnI), a TnI isoform naturally expressed in developing heart, also increases myofilament Ca2+ sensitivity, yet its expression in transgenic mouse hearts is not associated with overt cardiac disease. Gene transfer of TnI isoforms or mutants into adult cardiac myocytes is used here to ascertain if expression levels or functional differences between HCM TnI and ssTnI could help explain these divergent organ-level effects. Results showed significantly reduced myofilament incorporation of cTnIR146G compared with ssTnI or wild-type cTnI. Despite differences in myofilament incorporation, ssTnI and cTnIR146G expression each resulted in enhanced myofilament tension in response to submaximal Ca2+ under physiological ionic conditions. Myofilament expression of an analogous HCM mutation in ssTnI (ssTnIR115G) did not further increase myofilament Ca2+ sensitivity of tension compared with ssTnI. In contrast, there was a divergent response under acidic pH conditions, a condition associated with the myocardial ischemia that often accompanies hypertrophic cardiomyopathy. The acidic pH-induced decrease in myofilament Ca2+ sensitivity was significantly greater in myocytes expressing cTnIR146G and ssTnIR115G compared with ssTnI. These results suggest that differences in pH sensitivities between wild-type ssTnI and mutant TnI proteins may be one factor in helping explain the divergent organ and organismal outcomes in TnI HCM- and ssTnI-expressing mice.
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Affiliation(s)
- Margaret V Westfall
- Department of Surgery, School of Medicine, University of Michigan, Ann Arbor, Mich 48109-0686, USA
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42
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Wang Y, Fuchs F. Interfilament spacing, Ca2+ sensitivity, and Ca2+ binding in skinned bovine cardiac muscle. J Muscle Res Cell Motil 2002; 22:251-7. [PMID: 11763197 DOI: 10.1023/a:1012298921684] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The length-dependence of myofilament Ca2+ sensitivity in cardiac muscle appears to be a function of length-dependent variation in the lateral separation of actin and myosin filaments. The goal of this study was to determine how force, Ca2+ sensitivity, and Ca2+ binding to troponin C are correlated in skinned bovine ventricular muscle bundles set at sarcomere length 1.9 microm and subjected to varying degrees of osmotic compression with Dextran T-500. With 5, 10, and 15% Dextran T-500 the muscle diameter was reduced by 13, 21, and 25%, respectively. Addition of 5% Dextran T-500 caused increases in developed force, Ca2+ sensitivity, and in the affinity of Ca2+ for the regulatory binding site on troponin C. All of these parameters were reversed back toward control levels with 10% Dextran T-500. With 15% Dextran T-500 all parameters were decreased to below control levels. These data indicate that (1) there is an optimal filament separation at which both Ca2+ sensitivity and Ca2+ binding are maximized, and (2) Ca2+-troponin C affinity is linked to changes in Ca2+ sensitivity rather than to changes in interfilament spacing.
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Affiliation(s)
- Y Wang
- Department of Cell Biology and Physiology, University of Pittsburgh, School of Medicine, PA 15261, USA
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43
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Fuchs F. The Frank -Starling Relationship: Cellular and Molecular Mechanisms. MOLECULAR CONTROL MECHANISMS IN STRIATED MUSCLE CONTRACTION 2002. [DOI: 10.1007/978-94-015-9926-9_11] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
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Yamashita-Goto K, Okuyama R, Honda M, Kawasaki K, Fujita K, Yamada T, Nonaka I, Ohira Y, Yoshioka T. Maximal and submaximal forces of slow fibers in human soleus after bed rest. J Appl Physiol (1985) 2001; 91:417-24. [PMID: 11408459 DOI: 10.1152/jappl.2001.91.1.417] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The effects of 2 and 4 mo of bed rest, with or without exercise countermeasures, on the contractile properties of slow fibers in the human soleus muscle were examined. Mean fiber diameters were 8 and 36% smaller after 2 and 4 mo of bed rest, respectively, than the pre-bed rest level. Maximum tetanic force (P(o)), maximum activated force (F(max)) per cross-sectional area (CSA), and the common-logarithm value of free Ca(2+) concentration required for half-maximal activation (pCa(50)) also decreased after 2 and 4 mo of bed rest. In contrast, maximum unloaded shortening velocity (V(o)) was increased after 2 and 4 mo of bed rest. After 1 mo of recovery, fiber diameters, P(o), F(max) per CSA (P > 0.05), and pCa(50) were increased and V(o) decreased toward pre-bed rest levels. Effects of knee extension/flexion exercise by wearing an anti-G Penguin suit for 10 h daily, and the effects of loading or unloading of the plantar flexors with (Penguin-1) or without (Penguin-2) placing the elastic loading elements of the suit, respectively, were investigated during ~2 mo of bed rest. In the Penguin-1 group, mean fiber diameter, P(o), F(max) per CSA, V(o), and pCa(50) were similar before and after bed rest. However, the responses of fiber size and contractile properties to bed rest were not prevented in the Penguin-2 group, although the degree of the changes was less than those induced by bed rest without any countermeasure. These results indicate that long-term bed rest results in reductions of fiber size, force-generation capacity, and Ca(2+) sensitivity, and enhancement of shortening velocity in slow fibers of the soleus. The data indicate that continuous mechanical loading on muscle, such as stretching of muscle, is an effective countermeasure for the prevention of muscular adaptations to gravitational unloading.
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Affiliation(s)
- K Yamashita-Goto
- Department of Physiology, St. Marianna University School of Medicine, Kawasaki City, Kanagawa 216-8511, Japan
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45
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Cazorla O, Wu Y, Irving TC, Granzier H. Titin-based modulation of calcium sensitivity of active tension in mouse skinned cardiac myocytes. Circ Res 2001; 88:1028-35. [PMID: 11375272 DOI: 10.1161/hh1001.090876] [Citation(s) in RCA: 177] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
We studied the effect of titin-based passive force on the length dependence of activation of cardiac myocytes to explore whether titin may play a role in the generation of systolic force. Force-pCa relations were measured at sarcomere lengths (SLs) of 2.0 and 2.3 microm. Passive tension at 2.3 microm SL was varied from approximately 1 to approximately 10 mN/mm(2) by adjusting the characteristics of the stretch imposed on the passive cell before activation. Relative to 2.0 microm SL, the force-pCa curve at 2.3 microm SL and low passive tension showed a leftward shift (pCa(50) [change in pCa at half-maximal activation]) of 0.09+/-0.02 pCa units while at 2.3 microm SL and high passive tension the shift was increased to 0.25+/-0.03 pCa units. Passive tension also increased pCa(50) at reduced interfilament lattice spacing achieved with dextran. We tested whether titin-based passive tension influences the interfilament lattice spacing by measuring the width of the myocyte and by using small-angle x-ray diffraction of mouse left ventricular wall muscle. Cell width and interfilament lattice spacing varied inversely with passive tension, in the presence and absence of dextran. The passive tension effect on length-dependent activation may therefore result from a radial titin-based force that modulates the interfilament lattice spacing.
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Affiliation(s)
- O Cazorla
- Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, WA 88164, USA
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46
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Westfall MV, Turner I, Albayya FP, Metzger JM. Troponin I chimera analysis of the cardiac myofilament tension response to protein kinase A. Am J Physiol Cell Physiol 2001; 280:C324-32. [PMID: 11208528 DOI: 10.1152/ajpcell.2001.280.2.c324] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Viral-mediated gene transfer of troponin I (TnI) isoforms and chimeras into adult rat cardiac myocytes was used to investigate the role TnI domains play in the myofilament tension response to protein kinase A (PKA). In myocytes expressing endogenous cardiac TnI (cTnI), PKA phosphorylated TnI and myosin-binding protein C and decreased the Ca2+ sensitivity of myofilament tension. In marked contrast, PKA did not influence Ca2+-activated tension in myocytes expressing the slow skeletal isoform of TnI or a chimera (N-slow/card-C TnI), which lack the unique phosphorylatable amino terminal extension found in cTnI. PKA-mediated phosphorylation of a second TnI chimera, N-card/slow-C TnI, which has the amino terminal region of cTnI, caused a decrease in the Ca2+ sensitivity of tension comparable in magnitude to control myocytes. Based on these results, we propose the amino terminal region shared by cTnI and N-card/slow-C TnI plays a central role in determining the magnitude of the PKA-mediated shift in myofilament Ca2+ sensitivity, independent of the isoform-specific functional domains previously defined within the carboxyl terminal backbone of TnI. Interestingly, exposure of permeabilized myocytes to acidic pH after PKA-mediated phosphorylation of cTnI resulted in an additive decrease in myofilament Ca2+ sensitivity. The isoform-specific, pH-sensitive region within TnI lies in the carboxyl terminus of TnI, and the additive response provides further evidence for the presence of a separate domain that directly transduces the PKA phosphorylation signal.
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Affiliation(s)
- M V Westfall
- Department of Physiology, School of Medicine, University of Michigan, Ann Arbor, Michigan 48109-0622, USA.
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47
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Martyn DA, Regnier M, Xu D, Gordon AM. Ca2+ - and cross-bridge-dependent changes in N- and C-terminal structure of troponin C in rat cardiac muscle. Biophys J 2001; 80:360-70. [PMID: 11159408 PMCID: PMC1301239 DOI: 10.1016/s0006-3495(01)76020-1] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Linear dichroism of 5'-tetramethylrhodamine (5'ATR)-labeled cardiac troponin C (cTnC) was measured to monitor cTnC structure during Ca2+-activation of force in rat skinned myocardium. Mono-cysteine mutants allowed labeling at Cys-84 (cTnC(C84), near the D/E helix linker); Cys-35 (cTnC(C35), at nonfunctional site I); or near the C-terminus with a cysteine inserted at site 98 (cTnC-C35S,C84S,S98C, cTnC(C98)). With 5'ATR-labeled cTnC(C84) and cTnC(C98) dichroism increased with increasing [Ca2+], while rigor cross-bridges caused dichroism to increase more with 5'ATR-labeled cTnC(C84) than cTnC(C98). The pCa50 values and n(H) from Hill analysis of the Ca2+-dependence of force and dichroism were 6.4 (+/-0.02) and 1.08 (+/-0.04) for force and 6.3 (+/-0.04) and 1.02 (+/-0.09) (n = 5) for dichroism in cTnC(C84) reconstituted trabeculae. Corresponding data from cTnC(C98) reconstituted trabeculae were 5.53 (+/-0.03) and 3.1 (+/-0.17) for force, and 5.39 (+/-0.03) and 1.87 (+/-0.17) (n = 5) for dichroism. The contribution of active cycling cross-bridges to changes in cTnC structure was determined by inhibition of force to 6% of pCa 4.0 controls with 1.0 mM sodium vanadate (Vi). With 5'ATR-labeled cTnC(C84) Vi caused both the pCa50)of dichroism and the maximum value at pCa 4.0 to decrease, while with 5'ATR-labeled cTnC(C98) the pCa50 of dichroism decreased with no change of dichroism at pCa 4.0. The dichroism of 5'ATR-labeled cTnC(C35) was insensitive to either Ca2+ or strong cross-bridges. These data suggest that both Ca2+ and cycling cross-bridges perturb the N-terminal structure of cTnC at Cys-84, while C-terminal structure is altered by site II Ca2+-binding, but not cross-bridges.
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Affiliation(s)
- D A Martyn
- Department of Bioengineering, University of Washington, Seattle, Washington 98195, USA.
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48
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Abstract
Length-dependent Ca(2+)activation of the thin filament plays a critical role in the steep force-length relationship of cardiac muscle (Frank-Starling relation). Recent evidence indicates that the increase in myofilament Ca(2+)sensitivity and Ca(2+)-troponin C affinity that occurs with increase in sarcomere length results from a cooperative activation of the thin filament by attached cross-bridges. At short sarcomere length the Ca(2+)sensitivity is lower because the access of cross-bridges for actin is reduced. The aim of this study was to determine the length-dependence of myosin-mediated thin filament activation in skinned bovine ventricular muscle, as assayed by the generation of force with progressive reduction of MgATP concentration in the absence of Ca(2+). If the interaction between myosin and actin is weaker at short sarcomere length there should be a lower MgATP concentration needed to maintain the relaxed state. Contrary to expectation, the force-pMgATP relationship was not significantly influenced by change in sarcomere length. However this relationship became length-sensitive in the presence of phosphate analogs which stabilize weak-binding cross-bridges. We suggest that sarcomere length modulates Ca(2+)sensitivity by controlling the size of the population of thin filament regulatory units in the weakly-bound state.
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Affiliation(s)
- S H Smith
- Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA
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49
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Abstract
Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)
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Affiliation(s)
- A M Gordon
- Department of Physiology and Biophysics, University of Washington, Seattle, Washington 98195-7290, USA.
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50
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Westfall MV, Albayya FP, Turner II, Metzger JM. Chimera analysis of troponin I domains that influence Ca(2+)-activated myofilament tension in adult cardiac myocytes. Circ Res 2000; 86:470-7. [PMID: 10700453 DOI: 10.1161/01.res.86.4.470] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The goal of this study was to investigate isoform-specific functional domains of the inhibitory troponin subunit, troponin I (TnI), as it functions within the intact myofilaments of adult cardiac myocytes. Adenovirus-mediated gene transfer was used to deliver and express a TnI chimera composed of the amino terminus of cardiac TnI (cTnI) and the carboxy terminus of slow skeletal TnI (ssTnI) in adult rat cardiac myocytes. The TnI chimera, designated N-card/slow-C TnI, was expressed and incorporated into myofilaments after gene transfer, without detectable changes in contractile protein stoichiometry or sarcomere architecture. Interestingly, force at submaximal Ca(2+) levels was markedly elevated in single permeabilized myocytes expressing the N-card/slow-C TnI chimera relative to force generated in adult myocytes expressing ssTnI or cTnI. Based on these results, a hierarchy of myofilament Ca(2+) sensitivity is emerging by use of TnI chimera analysis, with the order of sensitivity being N-card/slow-C TnI>>ssTnI>>cTnI. These results also strongly suggest that independent isoform-specific domains in both the amino and carboxy portions of TnI influence myofilament Ca(2+) sensitivity. In additional studies carried out under pathophysiological ionic conditions (pH 6.2), the dramatic acidosis-induced decrease in myofilament Ca(2+) sensitivity observed in myocytes expressing cTnI was blunted in myocytes expressing N-card/slow-C TnI in a manner similar to that in ssTnI-expressing myocytes. These results demonstrate that there is a pH-sensitive domain residing in the carboxy-terminal portion of TnI. The dissection of isoform-specific functional domains under physiological and acidic pH conditions demonstrates the utility of TnI chimeras for analysis of TnI function and provides important insights into the overall function of TnI within the intact myofilament of adult cardiac myocytes.
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Affiliation(s)
- M V Westfall
- Department of Physiology, School of Medicine, University of Michigan, Ann Arbor, MI 48109-0622, USA.
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