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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 -
Circulation Journal : Official Journal... 2015The members of the nebulin protein family, including nebulin, nebulette, LASP-1, LASP-2, and N-RAP, contain various numbers of nebulin repeats and bind to actin, but are... (Review)
Review
The members of the nebulin protein family, including nebulin, nebulette, LASP-1, LASP-2, and N-RAP, contain various numbers of nebulin repeats and bind to actin, but are otherwise heterogeneous with regard to size, expression pattern, and function. This review focuses on the roles of nebulin family members in the heart. Nebulin is the largest member predominantly expressed in skeletal muscle, where it stretches along the thin filament. In heart, nebulin is detectable only at low levels and its absence has no apparent effects. Nebulette is similar in structure to the nebulin C-terminal Z-line region and specifically expressed in heart. Nebulette gene mutations have been identified in dilated cardiomyopathy patients and transgenic mice overexpressing nebulette mutants partially recapitulate the human pathology. In contrast, nebulette knockout mice show no functional phenotype, but exhibit Z-line widening. LASP-2 is an isoform of nebulette expressed in multiple tissues, including the heart. It is present in the Z-line and intercalated disc and able to bind and cross-link filamentous actin. LASP-1 is similar in structure to LASP-2, but expressed only in non-muscle tissue. N-RAP is present in myofibril precursors during myofibrillogenesis and thought to be involved in myofibril assembly, while it is localized at the intercalated disc in adult heart. Additional in vivo models are required to provide further insights into the functions of nebulin family members in the heart.
Topics: Adaptor Proteins, Signal Transducing; Adult; Animals; Cardiomyopathy, Dilated; Carrier Proteins; Cytoskeletal Proteins; Homeodomain Proteins; Humans; LIM Domain Proteins; Mice; Mice, Transgenic; Muscle Proteins; Mutation; Myocardium; Myofibrils
PubMed: 26321576
DOI: 10.1253/circj.CJ-15-0854 -
International Journal of Molecular... Jun 2018In muscle, but not in single-molecule mechanics studies, actin, myosin and accessory proteins are incorporated into a highly ordered myofilament lattice. In view of this... (Review)
Review
In muscle, but not in single-molecule mechanics studies, actin, myosin and accessory proteins are incorporated into a highly ordered myofilament lattice. In view of this difference we compare results from single-molecule studies and muscle mechanics and analyze to what degree data from the two types of studies agree with each other. There is reasonable correspondence in estimates of the cross-bridge power-stroke distance (7⁻13 nm), cross-bridge stiffness (~2 pN/nm) and average isometric force per cross-bridge (6⁻9 pN). Furthermore, models defined on the basis of single-molecule mechanics and solution biochemistry give good fits to experimental data from muscle. This suggests that the ordered myofilament lattice, accessory proteins and emergent effects of the sarcomere organization have only minor modulatory roles. However, such factors may be of greater importance under e.g., disease conditions. We also identify areas where single-molecule and muscle data are conflicting: (1) whether force generation is an Eyring or Kramers process with just one major power-stroke or several sub-strokes; (2) whether the myofilaments and the cross-bridges have Hookean or non-linear elasticity; (3) if individual myosin heads slip between actin sites under certain conditions, e.g., in lengthening; or (4) if the two heads of myosin cooperate.
Topics: Actin Cytoskeleton; Actins; Actomyosin; Animals; Isometric Contraction; Muscle Contraction; Muscle Fibers, Skeletal; Myofibrils; Myosins
PubMed: 29941816
DOI: 10.3390/ijms19071863 -
The Journal of General Physiology Jan 2019Regulation of muscle contraction has been viewed as principally involving Ca binding to regulatory proteins on the thin filament, but while this is an important element...
Regulation of muscle contraction has been viewed as principally involving Ca binding to regulatory proteins on the thin filament, but while this is an important element of regulation, the mechanism does not explain the precise matching of muscle performance to the load it must lift or move. Now, it is increasingly evident that mechanisms instrinsic to the thick filament activate myosin cross-bridges as the force or load on a muscle increases. Both thick and thin filament regulatory mechanisms are featured in this special issue of the .
Topics: Animals; Calcium; Muscle Proteins; Muscles; Myofibrils
PubMed: 30578329
DOI: 10.1085/jgp.201812288 -
The Journal of Physiology Jul 2012Single myofibrils 50–60 μm length and 2–3 μm diameter were isolated from rabbit psoas muscle fibres, and cross-bridge kinetics were studied by small perturbations...
Single myofibrils 50–60 μm length and 2–3 μm diameter were isolated from rabbit psoas muscle fibres, and cross-bridge kinetics were studied by small perturbations of the length (∼0.2%) over a range of 15 frequencies (1–250 Hz). The experiments were performed at 15◦C in the presence of 0.05–10 mM MgATP, 8mM phosphate (Pi), 200 mM ionic strength with KAc (acetate), pCa 4.35–4.65, and pH 7.0. Two exponential processes, B and C, were resolved in tension transients. Their apparent rate constants (2πb and 2πc) increased as the [MgATP] was raised from 0.05 mM to 1mM, and then reached saturation at [MgATP] ≥ 1. Given that these rate constants were similar (c/b ∼1.7) at [Pi] ≥ 4 mM, they were combined to achieve an accurate estimate of the kinetic constants: their sum and product were analysed as functions of [MgATP]. These analyses yielded K1 =2.91 ± 0.31 mM −1, k2 =288 ± 36 s−1, and k−2 =10 ± 21 s−1 (±95% confidence limit, n =13 preparations), based on the cross-bridge model: AM+ATP ↔ (step 1) AM.ATP ↔ (step 2) A+M.ATP, where K1 is the ATP association constant (step 1), k2 is the rate constant of the cross-bridge detachment (step 2), and k−2 is the rate constant of its reversal step. These kinetic constants are respectively comparable to those observed in single fibres from rabbit psoas (K1 =2.35 ± 0.31 mM −1, k2 =243 ± 22 s−1, and k−2 =6 ± 14 s−1; n =8 preparations) when analysed by the same methods and under the same experimental conditions. These values are respectively not significantly different from those obtained in myofibrils, indicating that the same kinetic constants can be deduced from myofibril and muscle fibre studies, in terms of ATP binding and cross-bridge detachments steps. The fact that K1 in myofibrils is 1.2 times that in fibres (P≈0.05) may be explained by a small concentration gradient of ATP, ADP and/or Pi in single fibres.
Topics: Adenosine Triphosphate; Animals; Calcium; Kinetics; Muscle Contraction; Muscle Fibers, Skeletal; Muscle Tonus; Myofibrils; Psoas Muscles; Rabbits
PubMed: 22586213
DOI: 10.1113/jphysiol.2012.228379 -
Biochimica Et Biophysica Acta.... Oct 2020Muscle atrophy is an inevitable sequel of fasting, denervation, aging, exposure to microgravity, and many human diseases including, cancer, type-2 diabetes, and renal... (Review)
Review
Muscle atrophy is an inevitable sequel of fasting, denervation, aging, exposure to microgravity, and many human diseases including, cancer, type-2 diabetes, and renal failure. During atrophy the destruction of the muscle's fundamental contractile machinery, the myofibrils, is accelerated leading to a reduction in muscle mass, weakness, frailty, and physical disability. Recent findings indicate that atrophy can be a major cause of death in affected individuals, and inhibition of muscle wasting is likely to prolong survival. Major advances in our understanding of the mechanisms for myofibril breakdown in atrophy include the discovery of biological pathways and key components that play prominent roles. On fasting or denervation, degradation of myofibrillar proteins requires an initial dissociation of the desmin cytoskeleton, whose integrity is critical for myofibril stability. This loss of desmin filaments involves phosphorylation, ubiquitination, and subsequent depolymerization by calpain-1, and appears to reduce myofibrils integrity and facilitate their destruction. Consequently, depolymerization of desmin filament in atrophy seems to be an early key event for overall proteolysis. A focus of this review is to discuss these new insights and the specific role of calpain-1 in promoting desmin filaments loss, and to highlight important key questions that merit further study.
Topics: Animals; Calpain; Desmin; Humans; Muscular Atrophy; Myofibrils; Polymerization; Ubiquitination
PubMed: 32603758
DOI: 10.1016/j.bbamcr.2020.118788 -
Meat Science Apr 2023The effect of pre-rigor temperature incubation on the activity and distribution in sarcoplasmic and myofibrillar fractions of calpains, and meat quality attributes was...
The effect of pre-rigor temperature incubation on the activity and distribution in sarcoplasmic and myofibrillar fractions of calpains, and meat quality attributes was investigated. Porcine longissimus thoracis muscles were incubated pre-rigor at 14, 22, 30 and 38 °C to 6 h postmortem, followed by another 2 h incubation at 14 °C. Thereafter, muscles were stored at 2 °C for 1 or 4 days. With higher pre-rigor temperature, sarcoplasmic Ca concentration, purge loss and myofibril-bound calpain-1 content increased, while shear force declined. Water-holding capacity of isolated myofibrils was lower after pre-rigor incubation at 38 °C. Desmin and troponin T degradation, and myofibril fragmentation was greater upon incubation of isolated myofibrils with added Ca in the order 800 μM Ca > 40 μM Ca > no Ca, suggesting that calpain-1 and calpain-2 were associated to myofibrils and proteolytically active with sufficient Ca. Activity of myofibril-bound calpain-1 in muscle incubated pre-rigor at 22 and 30 °C were higher than when incubated at 14 and 38 °C. These results indicate that calpains translocate from the sarcoplasm onto myofibrils with higher pre-rigor temperature to 30 °C and the proteolytic potential of myofibril-associated calpains is thereby increased.
Topics: Swine; Animals; Proteolysis; Myofibrils; Calpain; Red Meat; Pork Meat; Temperature; Muscle, Skeletal; Meat
PubMed: 36608417
DOI: 10.1016/j.meatsci.2022.109094 -
Journal of Biomedicine & Biotechnology 2010We review some of the problems in determining how myofibrils may be assembled and just as importantly how this contractile structure may be renewed by sarcomeric... (Review)
Review
We review some of the problems in determining how myofibrils may be assembled and just as importantly how this contractile structure may be renewed by sarcomeric proteins moving between the sarcomere and the cytoplasm. We also address in this personal review the recent evidence that indicates that the assembly and dynamics of myofibrils are conserved whether the cells are analyzed in situ or in tissue culture conditions. We suggest that myofibrillogenesis is a fundamentally conserved process, comparable to protein synthesis, mitosis, or cytokinesis, whether examined in situ or in vitro.
Topics: Animals; Fluorescence Recovery After Photobleaching; Humans; Models, Biological; Muscle Development; Myofibrils
PubMed: 20625425
DOI: 10.1155/2010/858606 -
Life Science Alliance Apr 2022Protein isoform transitions confer muscle fibers with distinct properties and are regulated by differential transcription and alternative splicing. RNA-binding Fox...
Protein isoform transitions confer muscle fibers with distinct properties and are regulated by differential transcription and alternative splicing. RNA-binding Fox protein 1 (Rbfox1) can affect both transcript levels and splicing, and is known to contribute to normal muscle development and physiology in vertebrates, although the detailed mechanisms remain obscure. In this study, we report that Rbfox1 contributes to the generation of adult muscle diversity in Rbfox1 is differentially expressed among muscle fiber types, and RNAi knockdown causes a hypercontraction phenotype that leads to behavioral and eclosion defects. Misregulation of fiber type-specific gene and splice isoform expression, notably loss of an indirect flight muscle-specific isoform of Troponin-I that is critical for regulating myosin activity, leads to structural defects. We further show that Rbfox1 directly binds the 3'-UTR of target transcripts, regulates the expression level of myogenic transcription factors myocyte enhancer factor 2 and Salm, and both modulates expression of and genetically interacts with the CELF family RNA-binding protein Bruno1 (Bru1). Rbfox1 and Bru1 co-regulate fiber type-specific alternative splicing of structural genes, indicating that regulatory interactions between FOX and CELF family RNA-binding proteins are conserved in fly muscle. Rbfox1 thus affects muscle development by regulating fiber type-specific splicing and expression dynamics of identity genes and structural proteins.
Topics: Animals; Drosophila; Drosophila Proteins; Female; Gene Knockdown Techniques; Male; Myofibrils; Protein Isoforms; RNA-Binding Proteins
PubMed: 34996845
DOI: 10.26508/lsa.202101342 -
Cytoskeleton (Hoboken, N.J.) Oct 2021Details of sarcomeric protein assembly during de novo myofibril formation closely resemble myofibrillogenesis in skeletal and cardiac myocytes in birds, rodents, and...
Details of sarcomeric protein assembly during de novo myofibril formation closely resemble myofibrillogenesis in skeletal and cardiac myocytes in birds, rodents, and zebrafish. The arrangement of proteins during myofibrillogenesis follows a three-step process: beginning with premyofibrils, followed by nascent myofibrils, and concluding with mature myofibrils (reviewed in Sanger et al., 2017). Assembly and maintenance of myofibrils in living muscle cells. In: Handbook of experimental pharmacology, 2017 [pp. 39-75]. Our aim is to determine if the same pathway is followed in human cardiomyocytes derived from human inducible pluripotent stem cells. We found that the human cardiomyocytes developed patterns of protein organization identical to the three-step series seen in the model organisms cited above. Further experiments showed that myofibril assembly can be blocked at the nascent myofibril by five different inhibitors of the Ubiquitin Proteasome System (UPS) stage in both avian and human cardiomyocytes. With the exception of Carfilzomib, removal of the UPS inhibitors allows nascent myofibrils to proceed to mature myofibrils. Some proteasomal inhibitors, such as Bortezomib and Carfilzomib, used to treat multiple myeloma patients, have off-target effects of damage to hearts in three to 6 % of these patients. These cardiovascular adverse events may result from prevention of mature myofibril formation in the cardiomyocytes. In summary, our results support a common three-step model for the formation of myofibrils ranging from avian to human cardiomyocytes. The Ubiquitin Proteasome System is required for progression from nascent myofibrils to mature myofibrils. Our experiments suggest a possible explanation for the cardiac and skeletal muscle off-target effects reported in multiple myeloma patients treated with proteasome inhibitors.
Topics: Animals; Cells, Cultured; Chick Embryo; Humans; Multiple Myeloma; Myocytes, Cardiac; Myofibrils; Pluripotent Stem Cells; Proteasome Endopeptidase Complex; Ubiquitin; Zebrafish
PubMed: 35502133
DOI: 10.1002/cm.21697