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Biophysical Journal Mar 2020In a contracting muscle, myosin cross-bridges extending from thick filaments pull the interdigitating thin (actin-containing) filaments during cyclical ATP-driven... (Review)
Review
In a contracting muscle, myosin cross-bridges extending from thick filaments pull the interdigitating thin (actin-containing) filaments during cyclical ATP-driven interactions toward the center of the sarcomere, the structural unit of striated muscle. Cross-bridge attachments in the sarcomere have been reported to exhibit a similar stiffness under both positive and negative forces. However, in vitro measurements on filaments with a sparse complement of heads detected a decrease of the cross-bridge stiffness at negative forces attributed to the buckling of the subfragment 2 tail portion. Here, we review some old and new data that confirm that cross-bridge stiffness is nearly linear in the muscle filament lattice. The implications of high myosin stiffness at positive and negative strains are considered in muscle fibers and in nonmuscle intracellular cargo transport.
Topics: Actins; Elasticity; Muscle Contraction; Myosins; Sarcomeres
PubMed: 31968230
DOI: 10.1016/j.bpj.2020.01.002 -
The Journal of General Physiology Mar 2021The March 2021 issue of is a collection of peer-reviewed articles focused on the function and dynamic regulation of contractile systems in muscle and non-muscle cells.
The March 2021 issue of is a collection of peer-reviewed articles focused on the function and dynamic regulation of contractile systems in muscle and non-muscle cells.
Topics: Muscle Proteins; Myofibrils
PubMed: 33620422
DOI: 10.1085/jgp.202112880 -
American Journal of Physiology. Cell... Dec 2022Stretch activation is defined as a delayed increase in force after rapid stretches. Although there is considerable evidence for stretch activation in isolated cardiac...
Stretch activation is defined as a delayed increase in force after rapid stretches. Although there is considerable evidence for stretch activation in isolated cardiac myofibrillar preparations, few studies have measured mechanisms of stretch activation in mammalian skeletal muscle fibers. We measured stretch activation following rapid step stretches [∼1%-4% sarcomere length (SL)] during submaximal Ca activations of rat permeabilized slow-twitch skeletal muscle fibers before and after protein kinase A (PKA), which phosphorylates slow myosin binding protein-C. PKA significantly increased stretch activation during low (∼25%) Ca activation and accelerated rates of delayed force development () during both low and half-maximal Ca activation. Following the step stretches and subsequent force development, fibers were rapidly shortened to original sarcomere length, which often elicited a shortening-induced transient force overshoot. After PKA, step shortening-induced transient force overshoot increased ∼10-fold following an ∼4% SL shortening during low Ca activation levels. following step shortening also increased after PKA during low and half-maximal Ca activations. We next investigated thin filament regulation of stretch activation. We tested the interplay between cardiac troponin I (cTnI) phosphorylation at the canonical PKA and novel tyrosine kinase sites on stretch activation. Native slow-skeletal Tn complexes were exchanged with recombinant human cTn complex with different human cTnI N-terminal pseudo-phosphorylation molecules: ) nonphosphorylated wild type (WT), ) the canonical S22/23D PKA sites, ) the tyrosine kinase Y26E site, and ) the combinatorial S22/23D + Y26E cTnI. All three pseudo-phosphorylated cTnIs elicited greater stretch activation than WT. Following stretch activation, a new, elevated stretch-induced steady-state force was reached with pseudo-phosphorylated cTnI. Combinatorial S22/23D + Y26E pseudo-phosphorylated cTnI increased . These results suggest that slow-skeletal yosin inding rotein- (sMyBP-C) phosphorylation modulates stretch activation by a combination of cross-bridge recruitment and faster cycling kinetics, whereas cTnI phosphorylation regulates stretch activation by both redundant and synergistic mechanisms; and, taken together, these sarcomere phosphoproteins offer precision targets for enhanced contractility.
Topics: Rats; Humans; Animals; Myofibrils; Calcium; Sarcomeres; Cyclic AMP-Dependent Protein Kinases; Troponin I; Phosphorylation; Myosins; Protein-Tyrosine Kinases; Myocardium; Myocardial Contraction; Mammals
PubMed: 36280392
DOI: 10.1152/ajpcell.00101.2022 -
Circulation Sep 2023
Topics: Humans; Sarcomeres; Biomarkers; Phenotype; Cardiomyopathy, Hypertrophic; Mutation
PubMed: 37669360
DOI: 10.1161/CIRCULATIONAHA.123.065789 -
Science (New York, N.Y.) Feb 2022In skeletal muscle, nebulin stabilizes and regulates the length of thin filaments, but the underlying mechanism remains nebulous. In this work, we used cryo-electron...
In skeletal muscle, nebulin stabilizes and regulates the length of thin filaments, but the underlying mechanism remains nebulous. In this work, we used cryo-electron tomography and subtomogram averaging to reveal structures of native nebulin bound to thin filaments within intact sarcomeres. This in situ reconstruction provided high-resolution details of the interaction between nebulin and actin, demonstrating the stabilizing role of nebulin. Myosin bound to the thin filaments exhibited different conformations of the neck domain, highlighting its inherent structural variability in muscle. Unexpectedly, nebulin did not interact with myosin or tropomyosin, but it did interact with a troponin T linker through two potential binding motifs on nebulin, explaining its regulatory role. Our structures support the role of nebulin as a thin filament "molecular ruler" and provide a molecular basis for studying nemaline myopathies.
Topics: Actin Cytoskeleton; Actins; Animals; Electron Microscope Tomography; Humans; Mice; Mice, Inbred BALB C; Models, Molecular; Muscle Proteins; Mutation; Myocardium; Myofibrils; Myopathies, Nemaline; Myosins; Protein Conformation; Protein Structure, Secondary; Psoas Muscles; Sarcomeres
PubMed: 35175800
DOI: 10.1126/science.abn1934 -
Circulation Research Jun 2020
Topics: Estrogens; Heart; Myofibrils
PubMed: 32496915
DOI: 10.1161/CIRCRESAHA.120.317052 -
Circulation Research Jan 2024A healthy heart is able to modify its function and increase relaxation through post-translational modifications of myofilament proteins. While there are known examples...
BACKGROUND
A healthy heart is able to modify its function and increase relaxation through post-translational modifications of myofilament proteins. While there are known examples of serine/threonine kinases directly phosphorylating myofilament proteins to modify heart function, the roles of tyrosine (Y) phosphorylation to directly modify heart function have not been demonstrated. The myofilament protein TnI (troponin I) is the inhibitory subunit of the troponin complex and is a key regulator of cardiac contraction and relaxation. We previously demonstrated that TnI-Y26 phosphorylation decreases calcium-sensitive force development and accelerates calcium dissociation, suggesting a novel role for tyrosine kinase-mediated TnI-Y26 phosphorylation to regulate cardiac relaxation. Therefore, we hypothesize that increasing TnI-Y26 phosphorylation will increase cardiac relaxation in vivo and be beneficial during pathological diastolic dysfunction.
METHODS
The signaling pathway involved in TnI-Y26 phosphorylation was predicted in silico and validated by tyrosine kinase activation and inhibition in primary adult murine cardiomyocytes. To investigate how TnI-Y26 phosphorylation affects cardiac muscle, structure, and function in vivo, we developed a novel TnI-Y26 phosphorylation-mimetic mouse that was subjected to echocardiography, pressure-volume loop hemodynamics, and myofibril mechanical studies. TnI-Y26 phosphorylation-mimetic mice were further subjected to the nephrectomy/DOCA (deoxycorticosterone acetate) model of diastolic dysfunction to investigate the effects of increased TnI-Y26 phosphorylation in disease.
RESULTS
Src tyrosine kinase is sufficient to phosphorylate TnI-Y26 in cardiomyocytes. TnI-Y26 phosphorylation accelerates in vivo relaxation without detrimental structural or systolic impairment. In a mouse model of diastolic dysfunction, TnI-Y26 phosphorylation is beneficial and protects against the development of disease.
CONCLUSIONS
We have demonstrated that tyrosine kinase phosphorylation of TnI is a novel mechanism to directly and beneficially accelerate myocardial relaxation in vivo.
Topics: Mice; Animals; Phosphorylation; Troponin I; Calcium; Protein Processing, Post-Translational; Myocardial Contraction; Myofibrils; Protein-Tyrosine Kinases; Tyrosine
PubMed: 38095088
DOI: 10.1161/CIRCRESAHA.123.323132 -
The Journal of General Physiology Feb 2022Myofilaments and their associated proteins, which together constitute the sarcomeres, provide the molecular-level basis for contractile function in all muscle types. In... (Review)
Review
Myofilaments and their associated proteins, which together constitute the sarcomeres, provide the molecular-level basis for contractile function in all muscle types. In intact muscle, sarcomere-level contraction is strongly coupled to other cellular subsystems, in particular the sarcolemmal membrane. Skinned muscle preparations (where the sarcolemma has been removed or permeabilized) are an experimental system designed to probe contractile mechanisms independently of the sarcolemma. Over the last few decades, experiments performed using permeabilized preparations have been invaluable for clarifying the understanding of contractile mechanisms in both skeletal and cardiac muscle. Today, the technique is increasingly harnessed for preclinical and/or pharmacological studies that seek to understand how interventions will impact intact muscle contraction. In this context, intrinsic functional and structural differences between skinned and intact muscle pose a major interpretational challenge. This review first surveys measurements that highlight these differences in terms of the sarcomere structure, passive and active tension generation, and calcium dependence. We then highlight the main practical challenges and caveats faced by experimentalists seeking to emulate the physiological conditions of intact muscle. Gaining an awareness of these complexities is essential for putting experiments in due perspective.
Topics: Calcium; Muscle Contraction; Myocardial Contraction; Myocardium; Myofibrils; Sarcomeres
PubMed: 35045156
DOI: 10.1085/jgp.202112990 -
Biophysical Journal Dec 2020The smallest contractile unit in striated muscles is the sarcomere. Although some of the classic features of contraction assume a uniform behavior of sarcomeres within... (Review)
Review
The smallest contractile unit in striated muscles is the sarcomere. Although some of the classic features of contraction assume a uniform behavior of sarcomeres within myofibrils, the occurrence of sarcomere length nonuniformities has been well recognized for years, but it is yet not well understood. In the past years, there has been a great advance in experiments using isolated myofibrils and sarcomeres that has allowed scientists to directly evaluate sarcomere length nonuniformity. This review will focus on studies conducted with these preparations to develop the hypotheses that 1) force production in myofibrils is largely altered and regulated by intersarcomere dynamics and that 2) the mechanical work of one sarcomere in a myofibril is transmitted to other sarcomeres in series. We evaluated studies looking into myofibril activation, relaxation, and force changes produced during activation. We conclude that force production in myofibrils is largely regulated by intersarcomere dynamics, which arises from the cooperative work of the contractile and elastic elements within a myofibril.
Topics: Mechanical Phenomena; Muscle Contraction; Muscle, Skeletal; Myofibrils; Sarcomeres
PubMed: 33217382
DOI: 10.1016/j.bpj.2020.11.005 -
Nature Communications May 2021Short-term, systemic expression of the Yamanaka reprogramming factors (Oct-3/4, Sox2, Klf4 and c-Myc [OSKM]) has been shown to rejuvenate aging cells and promote tissue...
Short-term, systemic expression of the Yamanaka reprogramming factors (Oct-3/4, Sox2, Klf4 and c-Myc [OSKM]) has been shown to rejuvenate aging cells and promote tissue regeneration in vivo. However, the mechanisms by which OSKM promotes tissue regeneration are unknown. In this work, we focus on a specific tissue and demonstrate that local expression of OSKM, specifically in myofibers, induces the activation of muscle stem cells or satellite cells (SCs), which accelerates muscle regeneration in young mice. In contrast, expressing OSKM directly in SCs does not improve muscle regeneration. Mechanistically, expressing OSKM in myofibers regulates the expression of genes important for the SC microenvironment, including upregulation of p21, which in turn downregulates Wnt4. This is critical because Wnt4 is secreted by myofibers to maintain SC quiescence. Thus, short-term induction of the Yamanaka factors in myofibers may promote tissue regeneration by modifying the stem cell niche.
Topics: Animals; Cell Differentiation; Cells, Cultured; Cellular Reprogramming; Female; Gene Expression; Kruppel-Like Factor 4; Kruppel-Like Transcription Factors; Mice, Transgenic; Myofibrils; Octamer Transcription Factor-3; Proto-Oncogene Proteins c-myc; Regeneration; SOXB1 Transcription Factors; Satellite Cells, Skeletal Muscle; Stem Cell Niche; Wnt4 Protein
PubMed: 34035273
DOI: 10.1038/s41467-021-23353-z