Myosin-18B Regulates Higher-Order Organization of the Cardiac Sarcomere through Thin Filament Cross-Linking and Thick Filament Dynamics

Patterns of intrinsic birefringence were revealed in formalin-fixed, glycerinated myofibrils from rabbit striated muscle, by perfusing them with solvents of refractive index near to that of protein, about 1.570. The patterns differ substantially from those obtained in physiological salt solutions, due to the elimination of edge- and form birefringence. Analysis of myofibrils at various stages of shortening has produced results fully consistent with the sliding filament theory of contraction. On a weight basis, the intrinsic birefringence of thick-filament protein is about 2.4 times that of thin-filament protein. Nonadditivity of thick- and thin-filament birefringence in the overlap regions of A bands may indicate an alteration of macromolecular structure due to interaction between the two types of filaments.

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Specific Myosin Heavy Chain Mutations Suppress Troponin I Defects in Drosophila Muscles

The Journal of Cell Biology ◽

10.1083/jcb.144.5.989 ◽

1999 ◽

Vol 144 (5) ◽

pp. 989-1000 ◽

Cited By ~ 28

Author(s):

William A. Kronert ◽

Angel Acebes ◽

Alberto Ferrús ◽

Sanford I. Bernstein

Keyword(s):

Myosin Heavy Chain ◽

Heavy Chain ◽

Thin Filament ◽

Troponin I ◽

Point Mutations ◽

Thick Filament ◽

Actin Binding ◽

Filament Protein ◽

Phenotypic Rescue

We show that specific mutations in the head of the thick filament molecule myosin heavy chain prevent a degenerative muscle syndrome resulting from the hdp2 mutation in the thin filament protein troponin I. One mutation deletes eight residues from the actin binding loop of myosin, while a second affects a residue at the base of this loop. Two other mutations affect amino acids near the site of nucleotide entry and exit in the motor domain. We document the degree of phenotypic rescue each suppressor permits and show that other point mutations in myosin, as well as null mutations, fail to suppress the hdp2 phenotype. We discuss mechanisms by which the hdp2 phenotypes are suppressed and conclude that the specific residues we identified in myosin are important in regulating thick and thin filament interactions. This in vivo approach to dissecting the contractile cycle defines novel molecular processes that may be difficult to uncover by biochemical and structural analysis. Our study illustrates how expression of genetic defects are dependent upon genetic background, and therefore could have implications for understanding gene interactions in human disease.

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I-Band Titin in Cardiac Muscle Is a Three-Element Molecular Spring and Is Critical for Maintaining Thin Filament Structure

The Journal of Cell Biology ◽

10.1083/jcb.146.3.631 ◽

1999 ◽

Vol 146 (3) ◽

pp. 631-644 ◽

Cited By ~ 162

Author(s):

Wolfgang A. Linke ◽

Diane E. Rudy ◽

Thomas Centner ◽

Mathias Gautel ◽

Christian Witt ◽

...

Keyword(s):

Cardiac Muscle ◽

Thin Filament ◽

Thick Filament ◽

Cardiac Cells ◽

Elastic Behavior ◽

Unique Region ◽

Cardiac Titin ◽

Filament Structure ◽

Pevk Domain ◽

Cardiac Sarcomeres

In cardiac muscle, the giant protein titin exists in different length isoforms expressed in the molecule's I-band region. Both isoforms, termed N2-A and N2-B, comprise stretches of Ig-like modules separated by the PEVK domain. Central I-band titin also contains isoform-specific Ig-motifs and nonmodular sequences, notably a longer insertion in N2-B. We investigated the elastic behavior of the I-band isoforms by using single-myofibril mechanics, immunofluorescence microscopy, and immunoelectron microscopy of rabbit cardiac sarcomeres stained with sequence-assigned antibodies. Moreover, we overexpressed constructs from the N2-B region in chick cardiac cells to search for possible structural properties of this cardiac-specific segment. We found that cardiac titin contains three distinct elastic elements: poly-Ig regions, the PEVK domain, and the N2-B sequence insertion, which extends ∼60 nm at high physiological stretch. Recruitment of all three elements allows cardiac titin to extend fully reversibly at physiological sarcomere lengths, without the need to unfold Ig domains. Overexpressing the entire N2-B region or its NH2 terminus in cardiac myocytes greatly disrupted thin filament, but not thick filament structure. Our results strongly suggest that the NH2-terminal N2-B domains are necessary to stabilize thin filament integrity. N2-B–titin emerges as a unique region critical for both reversible extensibility and structural maintenance of cardiac myofibrils.

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A Dominant Role of Cardiac Molecular Motors in the Intrinsic Regulation of Ventricular Ejection and Relaxation

Physiology ◽

10.1152/physiol.00043.2006 ◽

2007 ◽

Vol 22 (2) ◽

pp. 73-80 ◽

Cited By ~ 49

Author(s):

Aaron C. Hinken ◽

R. John Solaro

Keyword(s):

Molecular Motors ◽

Thin Filament ◽

Dominant Role ◽

Thick Filament ◽

Ejection Time ◽

Filament Activation ◽

Intrinsic Regulation ◽

Myosin Interaction ◽

Isovolumic Relaxation

Molecular motors housed in myosins of the thick filament react with thin-filament actins and promote force and shortening in the sarcomeres. However, other actions of these motors sustain sarcomeric activation by cooperative feedback mechanisms in which the actin-myosin interaction promotes thin-filament activation. Mechanical feedback also affects the actin-myosin interaction. We discuss current concepts of how these relatively under-appreciated actions of molecular motors are responsible for modulation of the ejection time and isovolumic relaxation in the beating heart.

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Ubiquitylation by Trim32 causes coupled loss of desmin, Z-bands, and thin filaments in muscle atrophy

The Journal of Cell Biology ◽

10.1083/jcb.201110067 ◽

2012 ◽

Vol 198 (4) ◽

pp. 575-589 ◽

Cited By ~ 105

Author(s):

Shenhav Cohen ◽

Bo Zhai ◽

Steven P. Gygi ◽

Alfred L. Goldberg

Keyword(s):

Muscle Atrophy ◽

Ubiquitin Ligase ◽

Thin Filament ◽

Thick Filament ◽

Myofibrillar Proteins ◽

Rapid Loss ◽

Thin Filaments ◽

Thin Filament Proteins ◽

Rapid Destruction ◽

Fiber Atrophy

During muscle atrophy, myofibrillar proteins are degraded in an ordered process in which MuRF1 catalyzes ubiquitylation of thick filament components (Cohen et al. 2009. J. Cell Biol. http://dx.doi.org/10.1083/jcb.200901052). Here, we show that another ubiquitin ligase, Trim32, ubiquitylates thin filament (actin, tropomyosin, troponins) and Z-band (α-actinin) components and promotes their degradation. Down-regulation of Trim32 during fasting reduced fiber atrophy and the rapid loss of thin filaments. Desmin filaments were proposed to maintain the integrity of thin filaments. Accordingly, we find that the rapid destruction of thin filament proteins upon fasting was accompanied by increased phosphorylation of desmin filaments, which promoted desmin ubiquitylation by Trim32 and degradation. Reducing Trim32 levels prevented the loss of both desmin and thin filament proteins. Furthermore, overexpression of an inhibitor of desmin polymerization induced disassembly of desmin filaments and destruction of thin filament components. Thus, during fasting, desmin phosphorylation increases and enhances Trim32-mediated degradation of the desmin cytoskeleton, which appears to facilitate the breakdown of Z-bands and thin filaments.

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Invited Review: Pathophysiology of cardiac muscle contraction and relaxation as a result of alterations in thin filament regulation

Journal of Applied Physiology ◽

10.1152/jappl.2001.90.3.1125 ◽

2001 ◽

Vol 90 (3) ◽

pp. 1125-1136 ◽

Cited By ~ 54

Author(s):

Olga M. Hernandez ◽

Philippe R. Housmans ◽

James D. Potter

Keyword(s):

Muscle Contraction ◽

Cardiac Muscle ◽

Thin Filament ◽

Thick Filament ◽

Familial Hypertrophic Cardiomyopathy ◽

Cardiac Muscle Contraction ◽

Sensitivity Changes ◽

Contraction And Relaxation

Cardiac muscle contraction depends on the tightly regulated interactions of thin and thick filament proteins of the contractile apparatus. Mutations of thin filament proteins (actin, tropomyosin, and troponin), causing familial hypertrophic cardiomyopathy (FHC), occur predominantly in evolutionarily conserved regions and induce various functional defects that impair the normal contractile mechanism. Dysfunctional properties observed with the FHC mutants include altered Ca2+ sensitivity, changes in ATPase activity, changes in the force and velocity of contraction, and destabilization of the contractile complex. One apparent tendency observed in these thin filament mutations is an increase in the Ca2+ sensitivity of force development. This trend in Ca2+ sensitivity is probably induced by altering the cross-bridge kinetics and the Ca2+ affinity of troponin C. These in vitro defects lead to a wide variety of in vivo cardiac abnormalities and phenotypes, some more severe than others and some resulting in sudden cardiac death.

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Abstract 448: Strain Rate Dependent Myosin Head Position Changes During Relaxation

Circulation Research ◽

10.1161/res.127.suppl_1.448 ◽

2020 ◽

Vol 127 (Suppl_1) ◽

Author(s):

Weikang Ma ◽

Melissa J Bukowski ◽

Alexandra R Matus ◽

Thomas C Irving ◽

Charles S Chung

Keyword(s):

Strain Rate ◽

Thin Filament ◽

Myosin Head ◽

Thick Filament ◽

Calcium Sensitivity ◽

Head Position ◽

Mechanical Control ◽

Myocardial Relaxation ◽

Rate Dependent ◽

Head Positioning

Impaired cardiac relaxation is present in nearly all cases of heart failure and possibly in up to 25% of the asymptomatic population. Myocardial relaxation is known to be biochemically modified by the calcium reuptake rate, thin filament calcium sensitivity, and crossbridge kinetics. Mechanical regulation of relaxation was thought to be regulated via afterload, but we have recently shown that a lengthening strain was sufficient to modify relaxation. Further, the relaxation rate is actually dependent on the strain rate, a relationship that we termed Mechanical Control of Relaxation. Computational modeling suggests that myosin detachment is a key mechanism underlying Mechanical Control of Regulation, but to date, no experimental evidence for this was available. The objective of this study was to determine if myosin head position changed in response to lengthening strains during relaxation. Intact cardiac trabeculae were mounted within the beamline of the Biophysical Collaborative Access Team (BioCAT) beamline at the Advanced Photon Source at Argonne National Laboratories. The trabeculae were paced and load-clamps were performed during time-resolved imaging of the equatorial axis, which primarily reflects myosin head positioning. Activation (pacing) caused the myosin head localization to shift from the thick filament to near the thin filament (increased I 1,1 /I 1,0 ratio). During stretch, there was a transient decline of the I 1,1 /I 1,0 ratio which recovered until relaxation was complete, when the ratio again reduced indicating myosin returned to the thick filament. These preliminary data suggest that Mechanical Control of Relaxation is caused by perturbations in myosin, but the late-diastolic kinetics suggests that the strain-rate dependent detachment does not lead to immediate deactivation of myosin heads. Modifications of myosin ATPase properties may reveal more specific regulatory targets, which may provide new insight and targets for treating impaired myocardial relaxation.

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