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The Journal of General Physiology Aug 2010
Topics: Computer Simulation; Heart; Humans; Models, Cardiovascular; Sarcomeres
PubMed: 20660659
DOI: 10.1085/jgp.201010497 -
The Journal of General Physiology Apr 2023Sarcomere length (SL) and its variation along the myofibril strongly regulate integrated coordinated myocyte contraction. It is therefore important to obtain individual... (Comparative Study)
Comparative Study
Sarcomere length (SL) and its variation along the myofibril strongly regulate integrated coordinated myocyte contraction. It is therefore important to obtain individual SL properties. Optical imaging by confocal fluorescence (for example, using ANEPPS) or transmitted light microscopy is often used for this purpose. However, this allows for the visualization of structures related to Z-disks only. In contrast, second-harmonic generation (SHG) microscopy visualizes A-band sarcomeric structures directly. Here, we compared averaged SL and its variability in isolated relaxed rat cardiomyocytes by imaging with ANEPPS and SHG. We found that SL variability, evaluated by several absolute and relative measures, is two times smaller using SHG vs. ANEPPS, while both optical methods give the same average (median) SL. We conclude that optical methods with similar optical spatial resolution provide valid estimations of average SL, but the use of SHG microscopy for visualization of sarcomeric A-bands may be the "gold standard" for evaluation of SL variability due to the absence of optical interference between the sarcomere center and non-sarcomeric structures. This contrasts with sarcomere edges where t-tubules may not consistently colocalize to Z-disks. The use of SHG microscopy instead of fluorescent imaging can be a prospective tool to map sarcomere variability both in vitro and in vivo conditions and to reveal its role in the functional behavior of living myocardium.
Topics: Animals; Rats; Myocytes, Cardiac; Myofibrils; Myosins; Sarcomeres; Second Harmonic Generation Microscopy
PubMed: 36695814
DOI: 10.1085/jgp.202213289 -
ELife Aug 2022Human muscle is a hierarchically organised tissue with its contractile cells called myofibers packed into large myofiber bundles. Each myofiber contains periodic...
Human muscle is a hierarchically organised tissue with its contractile cells called myofibers packed into large myofiber bundles. Each myofiber contains periodic myofibrils built by hundreds of contractile sarcomeres that generate large mechanical forces. To better understand the mechanisms that coordinate human muscle morphogenesis from tissue to molecular scales, we adopted a simple in vitro system using induced pluripotent stem cell-derived human myogenic precursors. When grown on an unrestricted two-dimensional substrate, developing myofibers spontaneously align and self-organise into higher-order myofiber bundles, which grow and consolidate to stable sizes. Following a transcriptional boost of sarcomeric components, myofibrils assemble into chains of periodic sarcomeres that emerge across the entire myofiber. More efficient myofiber bundling accelerates the speed of sarcomerogenesis suggesting that tension generated by bundling promotes sarcomerogenesis. We tested this hypothesis by directly probing tension and found that tension build-up precedes sarcomere assembly and increases within each assembling myofibril. Furthermore, we found that myofiber ends stably attach to other myofibers using integrin-based attachments and thus myofiber bundling coincides with stable myofiber bundle attachment in vitro. A failure in stable myofiber attachment results in a collapse of the myofibrils. Overall, our results strongly suggest that mechanical tension across sarcomeric components as well as between differentiating myofibers is key to coordinate the multi-scale self-organisation of muscle morphogenesis.
Topics: Humans; Induced Pluripotent Stem Cells; Muscle Development; Muscle Fibers, Skeletal; Myofibrils; Sarcomeres
PubMed: 35920628
DOI: 10.7554/eLife.76649 -
Biophysical Journal Apr 2023Residual force enhancement (RFE), an increase in isometric force after active stretching of a muscle compared with the purely isometric force at the corresponding...
Residual force enhancement (RFE), an increase in isometric force after active stretching of a muscle compared with the purely isometric force at the corresponding length, has been consistently observed throughout the structural hierarchy of skeletal muscle. Similar to RFE, passive force enhancement (PFE) is also observable in skeletal muscle and is defined as an increase in passive force when a muscle is deactivated after it has been actively stretched compared with the passive force following deactivation of a purely isometric contraction. These history-dependent properties have been investigated abundantly in skeletal muscle, but their presence in cardiac muscle remains unresolved and controversial. The purpose of this study was to investigate whether RFE and PFE exist in cardiac myofibrils and whether the magnitudes of RFE and PFE increase with increasing stretch magnitudes. Cardiac myofibrils were prepared from the left ventricles of New Zealand White rabbits, and the history-dependent properties were tested at three different final average sarcomere lengths (n = 8 for each), 1.8, 2, and 2.2 μm, while the stretch magnitude was kept at 0.2 μm/sarcomere. The same experiment was repeated with a final average sarcomere length of 2.2 μm and a stretching magnitude of 0.4 μm/sarcomere (n = 8). All 32 cardiac myofibrils exhibited increased forces after active stretching compared with the corresponding purely isometric reference conditions (p < 0.05). Furthermore, the magnitude of RFE was greater when myofibrils were stretched by 0.4 compared with 0.2 μm/sarcomere (p < 0.05). We conclude that, like in skeletal muscle, RFE and PFE are properties of cardiac myofibrils and are dependent on stretch magnitude.
Topics: Animals; Rabbits; Myofibrils; Biomechanical Phenomena; Sarcomeres; Muscle, Skeletal; Mechanical Phenomena; Isometric Contraction; Muscle Contraction
PubMed: 36932677
DOI: 10.1016/j.bpj.2023.03.022 -
Arquivos Brasileiros de Cardiologia Apr 2011Titin is a giant sarcomeric protein that extends from the Z-line to the M-line. Due to its location, it represents an important biomechanical sensor, which has a crucial... (Review)
Review
Titin is a giant sarcomeric protein that extends from the Z-line to the M-line. Due to its location, it represents an important biomechanical sensor, which has a crucial role in the maintenance of the sarcomere structural integrity. Titin works as a "bidireactional spring" that regulates the sarcomeric length and performs adequate adjustments of passive tension whenever the length varies. Therefore, it determines not only ventricular rigidity and diastolic function, but also systolic cardiac function, modulating the Frank-Starling mechanism. The myocardium expresses two isoforms of this macromolecule: the N2B, more rigid and the isoform N2BA, more compliant. The alterations in the relative expression of the two titin isoforms or alterations in their state of phosphorylation have been implicated in the pathophysiology of several diseases, such as diastolic heart failure, dilated cardiomyopathy, ischemic cardiomyopathy and aortic stenosis. The aim of this study is to describe, in brief, the structure and location of titin, its association with different cardiomyopathies and understand how alterations in this macromolecule influence the pathophysiology of diastolic heart failure, emphasizing the therapeutic potential of the manipulation of this macromolecule.
Topics: Cardiomyopathies; Connectin; Heart Failure; Humans; Muscle Proteins; Myocardium; Protein Isoforms; Protein Kinases; Sarcomeres
PubMed: 21359482
DOI: 10.1590/s0066-782x2011005000023 -
Archives of Oral Biology Sep 2019Determine sarcomere length (L) operating ranges of the superficial masseter and temporalis in vitro in a macaque model and examine the impact of position-dependent...
The influence of masseter and temporalis sarcomere length operating ranges as determined by laser diffraction on architectural estimates of muscle force and excursion in macaques (Macaca fascicularis and Macaca mulatta).
OBJECTIVE
Determine sarcomere length (L) operating ranges of the superficial masseter and temporalis in vitro in a macaque model and examine the impact of position-dependent variation on L and architectural estimates of muscle function (i.e., fiber length, PCSA) before and after L-normalization.
DESIGN
Heads of adult Macaca fascicularis (n = 4) and M. mulatta (n = 3) were bisected postmortem. One side of the jaw was fixed in occlusion, the other in maximum gape. L was measured bilaterally using laser diffraction and these measurements were used to estimate sarcomere-length operating ranges. Differences in fiber length and PCSA between sides were tested for significance prior to and following L-normalization.
RESULTS
Sarcomere-length operating ranges were widest for the anterior superficial masseter and narrowest for the posterior temporalis. Compared with other mammals, macaque operating ranges were wider and shifted to the right of the descending limb of a representative length-tension curve. Fibers were significantly stretched by as much as 100%, and PCSAs reduced by as much as 43%, on the maximally gaped compared with occluded sides. L-normalization substantially reduced position-dependent variance.
CONCLUSIONS
The superficial masseter ranges between 87-143% and the temporalis between 88-130% of optimal L from maximum gape to occlusion, indicating maximum relative L for these macaque muscles exceeds the upper end range previously reported for the jaw muscles of smaller mammals. The wider macaque operating ranges may be functionally linked to the propensity for facially prognathic primates to engage in agonistic canine display behaviors that require jaw-muscle stretch to facilitate production of wide jaw gapes.
Topics: Animals; Behavior, Animal; Jaw; Lasers; Macaca fascicularis; Macaca mulatta; Masseter Muscle; Sarcomeres
PubMed: 31254839
DOI: 10.1016/j.archoralbio.2019.05.015 -
Pflugers Archiv : European Journal of... Mar 2014Cardiac myosin-binding protein C is a key regulator of cardiac contractility and is capable of both activating the thin filament to initiate actomyosin motion generation... (Review)
Review
Cardiac myosin-binding protein C is a key regulator of cardiac contractility and is capable of both activating the thin filament to initiate actomyosin motion generation and governing maximal sliding velocities. While MyBP-C's C terminus localizes the molecule within the sarcomere, the N terminus appears to confer regulatory function by binding to the myosin motor domain and/or actin. Literature pertaining to how MyBP-C binding to the myosin motor domain and or actin leads to MyBP-C's dual modulatory roles that can impact actomyosin interactions are discussed.
Topics: Actomyosin; Animals; Carrier Proteins; Humans; Myocardial Contraction; Sarcomeres
PubMed: 24407948
DOI: 10.1007/s00424-013-1433-7 -
Journal of Anatomy Oct 2023Ultrastructural analysis of muscular biopsy is based on images of longitudinal sections of the fibers. Sometimes, due to experimental limitations, the resulting sections...
Ultrastructural analysis of muscular biopsy is based on images of longitudinal sections of the fibers. Sometimes, due to experimental limitations, the resulting sections are instead oblique, and no accurate morphological information can be extracted with standard analysis methods. Thus, the biopsy is performed again, but this is too invasive and time-consuming. In this study, we focused our attention on the sarcomere's shape and we investigated which is the structural information that can be obtained from oblique sections. A routine was written in MATLAB to allow the visualization of how a sarcomere's section appears in ultrastructural images obtained by Transmission Electron Microscopy (TEM) at different secant angles. The routine was used also to analyze the intersection between a cylinder and a plane to show how the Z-bands and M-line lengths vary at different secant angles. Moreover, we explored how to calculate sarcomere's radius and length as well as the secant angle from ultrastructural images, based only on geometrical considerations (Pythagorean theorem and trigonometric functions). The equations to calculate these parameters starting from ultrastructural image measurements were found. Noteworthy, to obtain the real sarcomere length in quasi-longitudinal sections, a small correction in the standard procedure is needed and highlighted in the text. In conclusion, even non-longitudinal sections of skeletal muscles can be used to extrapolate morphological information of sarcomeres, which are important parameters for diagnostic purposes.
Topics: Sarcomeres; Muscle, Skeletal
PubMed: 37243921
DOI: 10.1111/joa.13892 -
International Journal of Molecular... Nov 2019Muscle contraction is initiated by the interaction between actin and myosin filaments. The sliding of actin filaments relative to myosin filaments is produced by... (Review)
Review
Muscle contraction is initiated by the interaction between actin and myosin filaments. The sliding of actin filaments relative to myosin filaments is produced by cross-bridge cycling, which is governed by the theoretical framework of the cross-bridge theory. The cross-bridge theory explains well a number of mechanical responses, such as isometric and concentric contractions. However, some experimental observations cannot be explained with the cross-bridge theory; for example, the increased isometric force after eccentric contractions. The steady-state, isometric force after an eccentric contraction is greater than that attained in a purely isometric contraction at the same muscle length and same activation level. This well-acknowledged and universally observed property is referred to as residual force enhancement (rFE). Since rFE cannot be explained by the cross-bridge theory, alternative mechanisms for explaining this force response have been proposed. In this review, we introduce the basic concepts of sarcomere length non-uniformity and titin elasticity, which are the primary candidates that have been used for explaining rFE, and discuss unresolved problems regarding these mechanisms, and how to proceed with future experiments in this exciting area of research.
Topics: Actins; Animals; Connectin; Humans; Muscle Contraction; Myosins; Sarcomeres
PubMed: 31689920
DOI: 10.3390/ijms20215479 -
Biophysical Journal Dec 2017In this study, we measured the stiffness of skeletal muscle myofibrils in rigor. Using a custom-built atomic force microscope, myofibrils were first placed in a rigor...
In this study, we measured the stiffness of skeletal muscle myofibrils in rigor. Using a custom-built atomic force microscope, myofibrils were first placed in a rigor state then stretched and shortened at different displacements (0.1-0.3 μm per sarcomere) and nominal speeds (0.4 and 0.8 μm/s). During stretching, the myofibril stiffness was independent of both displacement and speed (average of 987 nN/μm). During shortening, the myofibril stiffness was independent of displacement, but dependent on speed (1234 nN/μm at 0.4 μm/s; 1106 nN/μm at 0.8 μm/s). Furthermore, the myofibril stiffness during shortening was greater than that during stretching and the difference depended on speed (31% at 0.4 μm/s; 8% at 0.8 μm/s). The results suggest that the myofibrils exhibit nonlinear viscoelastic properties that may be derived from myofibril filaments, similar to what has been observed in muscle fibers.
Topics: Animals; Biomechanical Phenomena; Cytoskeletal Proteins; Female; Mechanical Phenomena; Microscopy, Atomic Force; Rabbits; Sarcomeres
PubMed: 29262369
DOI: 10.1016/j.bpj.2017.10.007