Thick-filament strain and interfilament spacing in passive muscle: effect of titin-based passive tension

Discovery of Titin and Its Role in Heart Function and Disease

This review examines the giant elastic protein titin and its critical roles in heart function, both in health and disease, as discovered since its identification nearly 50 years ago. Encoded by the TTN (titin gene), titin has emerged as a major disease locus for cardiac disorders. Functionally, titin acts as a third myofilament type, connecting sarcomeric Z-disks and M-bands, and regulating myocardial passive stiffness and stretch sensing. Its I-band segment, which includes the N2B element and the PEVK (proline, glutamate, valine, and lysine-rich regions), serves as a viscoelastic spring, adjusting sarcomere length and force in response to cardiac stretch. The review details how alternative splicing of titin pre-mRNA produces different isoforms that greatly impact passive tension and cardiac function, under physiological and pathological conditions. Key posttranslational modifications, especially phosphorylation, play crucial roles in adjusting titin's stiffness, allowing for rapid adaptation to changing hemodynamic demands. Abnormal titin modifications and dysregulation of isoforms are linked to cardiac diseases such as heart failure with preserved ejection fraction, where increased stiffness impairs diastolic function. In addition, the review discusses the importance of the A-band region of titin in setting thick filament length and enhancing Ca²+ sensitivity, contributing to the Frank-Starling Mechanism of the heart. TTN truncating variants are frequently associated with dilated cardiomyopathy, and the review outlines potential disease mechanisms, including haploinsufficiency, sarcomere disarray, and altered thick filament regulation. Variants in TTN have also been linked to conditions such as peripartum cardiomyopathy and chemotherapy-induced cardiomyopathy. Therapeutic avenues are explored, including targeting splicing factors such as RBM20 (RNA binding motif protein 20) to adjust isoform ratios or using engineered heart tissues to study disease mechanisms. Advances in genetic engineering, including CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats), offer promise for modifying TTN to treat titin-related cardiomyopathies. This comprehensive review highlights titin's structural, mechanical, and signaling roles in heart function and the impact of TTN mutations on cardiac diseases.

The distinctive mechanical and structural signatures of residual force enhancement in myofibers

In muscle, titin proteins connect myofilaments together and are thought to be critical for contraction, especially during residual force enhancement (RFE) when steady-state force is elevated after an active stretch. We investigated titin's function during contraction using small-angle X-ray diffraction to track structural changes before and after 50% titin cleavage and in the RFE-deficient, mdm titin mutant. We report that the RFE state is structurally distinct from pure isometric contractions, with increased thick filament strain and decreased lattice spacing, most likely caused by elevated titin-based forces. Furthermore, no RFE structural state was detected in mdm muscle. We posit that decreased lattice spacing, increased thick filament stiffness, and increased non-cross-bridge forces are the major contributors to RFE. We conclude that titin directly contributes to RFE.

Mavacamten facilitates myosin head ON-to-OFF transitions and shortens thin filament length in relaxed skeletal muscle

The first-in-its-class cardiac drug mavacamten reduces the proportion of so-called ON-state myosin heads in relaxed sarcomeres, altering contraction performance. However, mavacamten is not completely specific to cardiac myosin and can also affect skeletal muscle myosin, an important consideration since mavacamten is administered orally and so will also be present in skeletal tissue. Here, we studied the effect of mavacamten on skeletal muscle structure using small-angle X-ray diffraction. Mavacamten treatment reduced the proportion of ON myosin heads but did not eliminate the molecular underpinnings of length-dependent activation, demonstrating similar effects to those observed in cardiac muscle. These findings provide valuable insights for the potential use of mavacamten as a tool to study muscle contraction across striated muscle.

Calcium has a direct effect on thick filament activation in porcine myocardium

Sarcomere activation in striated muscle requires both thin filament-based and thick filament-based activation mechanisms. Recent studies have shown that myosin heads on the thick filaments undergo OFF to ON structural transitions in response to calcium (Ca2+) in permeabilized porcine myocardium in the presence of a small molecule inhibitor that eliminated active force. The changes in X-ray diffraction signatures of OFF to ON transitions were interpreted as Ca2+ acting to activate the thick filaments. Alternatively, Ca2+ binding to troponin could initiate a Ca2+-dependent crosstalk from the thin filament to the thick filament via interfilament connections such as the myosin binding protein-C. Here, we exchanged native troponin in permeabilized porcine myocardium for troponin containing the cTnC D65A mutation, which disallows the activation of troponin through Ca2+ binding to determine if Ca2+-dependent thick filament activation persists in the absence of thin filament activation. After the exchange protocol, over 95% of the Ca2+-activated force was eliminated. Equatorial intensity ratio increased significantly in both WT and D65A exchanged myocardium with increasing Ca2+ concentration. The degree of helical ordering of the myosin heads decreased by the same amount in WT and D65A myocardium when Ca2+ concentration increased. These results are consistent with a direct effect of Ca2+ in activating the thick filament rather than an indirect effect due to Ca2+-mediated crosstalk between the thick and thin filaments.

The structural and functional effects of myosin regulatory light chain phosphorylation are amplified by increases in sarcomere length and [Ca2+]

Precise regulation of sarcomeric contraction is essential for normal cardiac function. The heart must generate sufficient force to pump blood throughout the body, but either inadequate or excessive force can lead to dysregulation and disease. Myosin regulatory light chain (RLC) is a thick-filament protein that binds to the neck of the myosin heavy chain. Post-translational phosphorylation of RLC (RLC-P) by myosin light chain kinase is known to influence acto-myosin interactions, thereby increasing force production and Ca2+-sensitivity of contraction. Here, we investigated the role of RLC-P on cardiac structure and function as sarcomere length and [Ca2+] were altered. We found that at low, non-activating levels of Ca2+, RLC-P contributed to myosin head disorder, though there were no effects on isometric stress production and viscoelastic stiffness. With increases in sarcomere length and Ca2+-activation, the structural changes due to RLC-P become greater, which translates into greater force production, greater viscoelastic stiffness, slowed myosin detachment rates and altered nucleotide handling. Altogether, these data suggest that RLC-P may alter thick-filament structure by releasing ordered, off-state myosin. These more disordered myosin heads are available to bind actin, which could result in greater force production as Ca2+ levels increase. However, prolonged cross-bridge attachment duration due to slower ADP release could delay relaxation long enough to enable cross-bridge rebinding. Together, this work further elucidates the effects of RLC-P in regulating muscle function, thereby promoting a better understanding of thick-filament regulatory contributions to cardiac function in health and disease. KEY POINTS: Myosin regulatory light chain (RLC) is a thick-filament protein in the cardiac sarcomere that can be phosphorylated (RLC-P), and changes in RLC-P are associated with cardiac dysfunction and disease. This study assesses how RLC-P alters cardiac muscle structure and function at different sarcomere lengths and calcium concentrations. At low, non-activating levels of Ca2+, RLC-P contributed to myofilament disorder, though there were no effects on isometric stress production and viscoelastic stiffness. With increases in sarcomere length and Ca2+-activation, the structural changes due to RLC-P become greater, which translates into greater force production, greater viscoelastic stiffness, slower myosin detachment rate and altered cross-bridge nucleotide handling rates. This work elucidates the role of RLC-P in regulating muscle function and facilitates understanding of thick-filament regulatory protein contributions to cardiac function in health and disease.

Levosimendan's Effects on Length-Dependent Activation in Murine Fast-Twitch Skeletal Muscle

Levosimendan's calcium sensitizing effects in heart muscle cells are well established; yet, its potential impact on skeletal muscle cells has not been evidently determined. Despite controversial results, levosimendan is still expected to interact with skeletal muscle through off-target sites (further than troponin C). Adding to this debate, we investigated levosimendan's acute impact on fast-twitch skeletal muscle biomechanics in a length-dependent activation study by submersing single muscle fibres in a levosimendan-supplemented solution. We employed our MyoRobot technology to investigate the calcium sensitivity of skinned single muscle fibres alongside their stress-strain response in the presence or absence of levosimendan (100 µM). While control data are in agreement with the theory of length-dependent activation, levosimendan appears to shift the onset of the 'descending limb' of active force generation to longer sarcomere lengths without notably improving myofibrillar calcium sensitivity. Passive stretches in the presence of levosimendan yielded over twice the amount of enlarged restoration stress and Young's modulus in comparison to control single fibres. Both effects have not been described before and may point towards potential off-target sites of levosimendan.

Cooperativity of weak actomyosin interaction

We report the discovery of a new regulatory mechanism of the actomyosin system in muscle. We show that the weak binding of the myosin-nucleotide complex with unregulated F-actin is a cooperative process. Hundreds of myosin heads must work together for efficient force production in muscle, but the precise mechanism by which they coordinate remains elusive. It is known that myosin initially binds actin weakly, then transitions into a strongly bound state to produce force. Using the contiguous cooperative binding model, we interpreted our experimental results in terms of a cooperativity parameter defined as an increased probability for a myosin head to bind to the actin filament next to the already bound head. Considering the geometric organization of a sarcomere, we propose the formation of cross-bridge clusters composed of up to six myosin heads bound consecutively to actin. The cooperativity of weak actomyosin interaction may explain several challenging questions in muscle physiology, such as the role of myosin isoforms in mixed-isoform hybrid muscles, or the yield of supramaximal rate of force production in decorated skinned muscle fibers.

Significance Statement

Force in striated muscle results from myosin interacting with actin. Initially, myosin attaches weakly to the thin filament, transitioning to a strongly bound state, generating force. Our experiments show high cooperativity in myosin's weak interaction with unregulated actin filament. This cooperative behavior may facilitate the formation of cross-bridge clusters and the cooperative steps of myosin heads between clusters. Consequently, the thin- and thick-filament regulation could govern the spacing between cross-bridge clusters and influence the probability of a myosin head stepping along the thin filament during force development in muscle.

Fast myosin binding protein C knockout in skeletal muscle alters length-dependent activation and myofilament structure

In striated muscle, the sarcomeric protein myosin-binding protein-C (MyBP-C) is bound to the myosin thick filament and is predicted to stabilize myosin heads in a docked position against the thick filament, which limits crossbridge formation. Here, we use the homozygous Mybpc2 knockout (C2-/-) mouse line to remove the fast-isoform MyBP-C from fast skeletal muscle and then conduct mechanical functional studies in parallel with small-angle X-ray diffraction to evaluate the myofilament structure. We report that C2-/- fibers present deficits in force production and calcium sensitivity. Structurally, passive C2-/- fibers present altered sarcomere length-independent and -dependent regulation of myosin head conformations, with a shift of myosin heads towards actin. At shorter sarcomere lengths, the thin filament is axially extended in C2-/-, which we hypothesize is due to increased numbers of low-level crossbridges. These findings provide testable mechanisms to explain the etiology of debilitating diseases associated with MyBP-C.

Myosin-binding protein C regulates the sarcomere lattice and stabilizes the OFF states of myosin heads

Muscle contraction is produced via the interaction of myofilaments and is regulated so that muscle performance matches demand. Myosin-binding protein C (MyBP-C) is a long and flexible protein that is tightly bound to the thick filament at its C-terminal end (MyBP-CC8C10), but may be loosely bound at its middle- and N-terminal end (MyBP-CC1C7) to myosin heads and/or the thin filament. MyBP-C is thought to control muscle contraction via the regulation of myosin motors, as mutations lead to debilitating disease. We use a combination of mechanics and small-angle X-ray diffraction to study the immediate and selective removal of the MyBP-CC1C7 domains of fast MyBP-C in permeabilized skeletal muscle. We show that cleavage leads to alterations in crossbridge kinetics and passive structural signatures of myofilaments that are indicative of a shift of myosin heads towards the ON state, highlighting the importance of MyBP-CC1C7 to myofilament force production and regulation.

Fast myosin binding protein C knockout in skeletal muscle alters length-dependent activation and myofilament structure

In striated muscle, some sarcomere proteins regulate crossbridge cycling by varying the propensity of myosin heads to interact with actin. Myosin-binding protein C (MyBP-C) is bound to the myosin thick filament and is predicted to interact and stabilize myosin heads in a docked position against the thick filament and limit crossbridge formation, the so-called OFF state. Via an unknown mechanism, MyBP-C is thought to release heads into the so-called ON state, where they are more likely to form crossbridges. To study this proposed mechanism, we used the C2-/- mouse line to knock down fast-isoform MyBP-C completely and total MyBP-C by ~24%, and conducted mechanical functional studies in parallel with small-angle X-ray diffraction to evaluate the myofilament structure. We report that C2-/- fibers presented deficits in force production and reduced calcium sensitivity. Structurally, passive C2-/- fibers presented altered SL-independent and SL-dependent regulation of myosin head ON/OFF states, with a shift of myosin heads towards the ON state. Unexpectedly, at shorter sarcomere lengths, the thin filament was axially extended in C2-/- vs. non-transgenic controls, which we postulate is due to increased low-level crossbridge formation arising from relatively more ON myosins in the passive muscle that elongates the thin filament. The downstream effect of increasing crossbridge formation in a passive muscle on contraction performance is not known. Such widespread structural changes to sarcomere proteins provide testable mechanisms to explain the etiology of debilitating MyBP-C-associated diseases.

Myosin-binding protein C forms C-links and stabilizes OFF states of myosin

Contraction force in muscle is produced by the interaction of myosin motors in the thick filaments and actin in the thin filaments and is fine-tuned by other proteins such as myosin-binding protein C (MyBP-C). One form of control is through the regulation of myosin heads between an ON and OFF state in passive sarcomeres, which leads to their ability or inability to interact with the thin filaments during contraction, respectively. MyBP-C is a flexible and long protein that is tightly bound to the thick filament at its C-terminal end but may be loosely bound at its middle- and N-terminal end (MyBP-CC1C7). Under considerable debate is whether the MyBP-CC1C7 domains directly regulate myosin head ON/OFF states, and/or link thin filaments ("C-links"). Here, we used a combination of mechanics and small-angle X-ray diffraction to study the immediate and selective removal of the MyBP-CC1C7 domains of fast MyBP-C in permeabilized skeletal muscle. After cleavage, the thin filaments were significantly shorter, a result consistent with direct interactions of MyBP-C with thin filaments thus confirming C-links. Ca2+ sensitivity was reduced at shorter sarcomere lengths, and crossbridge kinetics were increased across sarcomere lengths at submaximal activation levels, demonstrating a role in crossbridge kinetics. Structural signatures of the thick filaments suggest that cleavage also shifted myosin heads towards the ON state - a marker that typically indicates increased Ca2+ sensitivity but that may account for increased crossbridge kinetics at submaximal Ca2+ and/or a change in the force transmission pathway. Taken together, we conclude that MyBP-CC1C7 domains play an important role in contractile performance which helps explain why mutations in these domains often lead to debilitating diseases.

Stretching the story of titin and muscle function

The discovery of the giant protein titin, also known as connectin, dates almost half a century back. In this review, I recapitulate major advances in the discovery of the titin filaments and the recognition of their properties and function until today. I briefly discuss how our understanding of the layout and interactions of titin in muscle sarcomeres has evolved and review key facts about the titin sequence at the gene (TTN) and protein levels. I also touch upon properties of titin important for the stability of the contractile units and the assembly and maintenance of sarcomeric proteins. The greater part of my discussion centers around the mechanical function of titin in skeletal muscle. I cover milestones of research on titin's role in stretch-dependent passive tension development, recollect the reasons behind the enormous elastic diversity of titin, and provide an update on the molecular mechanisms of titin elasticity, details of which are emerging even now. I reflect on current knowledge of how muscle fibers behave mechanically if titin stiffness is removed and how titin stiffness can be dynamically regulated, such as by posttranslational modifications or calcium binding. Finally, I highlight novel and exciting, but still controversially discussed, insight into the role titin plays in active tension development, such as length-dependent activation and contraction from longer muscle lengths.

The history-dependent features of muscle force production: A challenge to the cross-bridge theory and their functional implications

The cross-bridge theory predicts that muscle force is determined by muscle length and the velocity of active muscle length changes. However, before the formulation of the cross-bridge theory, it had been observed that the isometric force at a given muscle length is enhanced or depressed depending on active muscle length changes before that given length is reached. These enhanced and depressed force states are termed residual force enhancement (rFE) and residual force depression (rFD), respectively, and together they are known as the history-dependent features of muscle force production. In this review, we introduce early attempts in explaining rFE and rFD before we discuss more recent research from the past 25 years which has contributed to a better understanding of the mechanisms underpinning rFE and rFD. Specifically, we discuss the increasing number of findings on rFE and rFD which challenge the cross-bridge theory and propose that the elastic element titin plays a role in explaining muscle history-dependence. Accordingly, new three-filament models of force production including titin seem to provide better insight into the mechanism of muscle contraction. Complementary to the mechanisms behind muscle history-dependence, we also show various implications for muscle history-dependence on in-vivo human muscle function such as during stretch-shortening cycles. We conclude that titin function needs to be better understood if a new three-filament muscle model which includes titin, is to be established. From an applied perspective, it remains to be elucidated how muscle history-dependence affects locomotion and motor control, and whether history-dependent features can be changed by training.

The distinctive mechanical and structural signatures of residual force enhancement in myofibers

In muscle, titin proteins connect myofilaments together and are thought to be critical for contraction, especially during residual force enhancement (RFE) when force is elevated after an active stretch. We investigated titin's function during contraction using small-angle X-ray diffraction to track structural changes before and after 50% titin cleavage and in the RFE-deficient, mdm titin mutant. We report that the RFE state is structurally distinct from pure isometric contractions, with increased thick filament strain and decreased lattice spacing, most likely caused by elevated titin-based forces. Furthermore, no RFE structural state was detected in mdm muscle. We posit that decreased lattice spacing, increased thick filament stiffness, and increased non-crossbridge forces are the major contributors to RFE. We conclude that titin directly contributes to RFE.

Rbm20ΔRRM Mice, Expressing a Titin Isoform with Lower Stiffness, Are Protected from Mechanical Ventilation-Induced Diaphragm Weakness

Diaphragm weakness frequently develops in mechanically ventilated critically ill patients and is associated with increased morbidity, including ventilator weaning failure, mortality, and health care costs. The mechanisms underlying diaphragm weakness are incompletely understood but may include the elastic properties of titin, a giant protein whose layout in the muscle's sarcomeres makes it an ideal candidate to sense ventilation-induced diaphragm unloading, resulting in downstream signaling through titin-binding proteins. In the current study, we investigated whether modulating titin stiffness affects the development of diaphragm weakness during mechanical ventilation. To this end, we ventilated genetically engineered mice with reduced titin stiffness (Rbm20^ΔRRM), and robust (Ttn^ΔIAjxn) or severely (Ttn^Δ112-158) increased titin stiffness for 8 h, and assessed diaphragm contractility and protein expression of titin-binding proteins. Mechanical ventilation reduced the maximum active tension of the diaphragm in WT, Ttn^ΔIAjxn and Ttn^Δ112-158 mice. However, in Rbm20^ΔRRM mice maximum active tension was preserved after ventilation. Analyses of titin binding proteins suggest that muscle ankyrin repeat proteins (MARPs) 1 and 2 may play a role in the adaptation of the diaphragm to mechanical ventilation, and the preservation of diaphragm contractility in Rbm20^ΔRRM mice. Thus, Rbm20^ΔRRM mice, expressing titin isoforms with lower stiffness, are protected from mechanical ventilation-induced diaphragm weakness, suggesting that titin elasticity may modulate the diaphragm's response to unloading during mechanical ventilation.

Titin force in muscle cells alters lattice order, thick and thin filament protein formation

Skeletal muscle force production is increased at longer compared to shorter muscle lengths because of length-dependent priming of thick filament proteins in the contractile unit before contraction. Using small-angle X-ray diffraction in combination with a mouse model that specifically cleaves the stretch-sensitive titin protein, we found that titin cleavage diminished the length-dependent priming of the thick filament. Strikingly, a titin-sensitive, length-dependent priming was also present in thin filaments, which seems only possible via bridge proteins between thick and thin filaments in resting muscle, potentially myosin-binding protein C. We further show that these bridges can be forcibly ruptured via high-speed stretches. Our results advance a paradigm shift to the fundamental regulation of length-dependent priming, with titin as the key driver.

Mitochondrial network configuration influences sarcomere and myosin filament structure in striated muscles

Sustained muscle contraction occurs through interactions between actin and myosin filaments within sarcomeres and requires a constant supply of adenosine triphosphate (ATP) from nearby mitochondria. However, it remains unclear how different physical configurations between sarcomeres and mitochondria alter the energetic support for contractile function. Here, we show that sarcomere cross-sectional area (CSA) varies along its length in a cell type-dependent manner where the reduction in Z-disk CSA relative to the sarcomere center is closely coordinated with mitochondrial network configuration in flies, mice, and humans. Further, we find myosin filaments near the sarcomere periphery are curved relative to interior filaments with greater curvature for filaments near mitochondria compared to sarcoplasmic reticulum. Finally, we demonstrate variable myosin filament lattice spacing between filament ends and filament centers in a cell type-dependent manner. These data suggest both sarcomere structure and myofilament interactions are influenced by the location and orientation of mitochondria within muscle cells.

Cross-bridge mechanics estimated from skeletal muscles' work-loop responses to impacts in legged locomotion

Legged locomotion has evolved as the most common form of terrestrial locomotion. When the leg makes contact with a solid surface, muscles absorb some of the shock-wave accelerations (impacts) that propagate through the body. We built a custom-made frame to which we fixated a rat (Rattus norvegicus, Wistar) muscle (m. gastrocnemius medialis and lateralis: GAS) for emulating an impact. We found that the fibre material of the muscle dissipates between 3.5 and [Formula: see text] ranging from fresh, fully active to passive muscle material, respectively. Accordingly, the corresponding dissipated energy in a half-sarcomere ranges between 10.4 and [Formula: see text], respectively. At maximum activity, a single cross-bridge would, thus, dissipate 0.6% of the mechanical work available per ATP split per impact, and up to 16% energy in common, submaximal, activities. We also found the cross-bridge stiffness as low as [Formula: see text], which can be explained by the Coulomb-actuating cross-bridge part dominating the sarcomere stiffness. Results of the study provide a deeper understanding of contractile dynamics during early ground contact in bouncy gait.

Systematic review of skeletal muscle passive mechanics experimental methodology

Understanding passive skeletal muscle mechanics is critical in defining structure-function relationships in skeletal muscle and ultimately understanding pathologically impaired muscle. In this systematic review, we performed an exhaustive literature search using PRISMA guidelines to quantify passive muscle mechanical properties, summarized the methods used to create these data, and make recommendations to standardize future studies. We screened over 7500 papers and found 80 papers that met the inclusion criteria. These papers reported passive muscle mechanics from single muscle fiber to whole muscle across 16 species and 54 distinct muscles. We found a wide range of methodological differences in sample selection, preparation, testing, and analysis. The systematic review revealed that passive muscle mechanics is species and scale dependent-specifically within mammals, the passive mechanics increases non-linearly with scale. However, a detailed understanding of passive mechanics is still unclear because the varied methodologies impede comparisons across studies, scales, species, and muscles. Therefore, we recommend the following: smaller scales may be maintained within storage solution prior to testing, when samples are tested statically use 2-3 min of relaxation time, stress normalization at the whole muscle level be to physiologic cross-sectional area, strain normalization be to sarcomere length when possible, and an exponential equation be used to fit the data. Additional studies using these recommendations will allow exploration of the multiscale relationship of passive force within and across species to provide the fundamental knowledge needed to improve our understanding of passive muscle mechanics.

A mechanism for sarcomere breathing: volume change and advective flow within the myofilament lattice

During muscle contraction, myosin motors anchored to thick filaments bind to and slide actin thin filaments. These motors rely on energy derived from ATP, supplied, in part, by diffusion from the sarcoplasm to the interior of the lattice of actin and myosin filaments. The radial spacing of filaments in this lattice may change or remain constant during contraction. If the lattice is isovolumetric, it must expand when the muscle shortens. If, however, the spacing is constant or has a different pattern of axial and radial motion, then the lattice changes volume during contraction, driving fluid motion and assisting in the transport of molecules between the contractile lattice and the surrounding intracellular space. We first create an advective-diffusive-reaction flow model and show that the flow into and out of the sarcomere lattice would be significant in the absence of lattice expansion. Advective transport coupled to diffusion has the potential to substantially enhance metabolite exchange within the crowded sarcomere. Using time-resolved x-ray diffraction of contracting muscle, we next show that the contractile lattice is neither isovolumetric nor constant in spacing. Instead, lattice spacing is time varying, depends on activation, and can manifest as an effective time-varying Poisson ratio. The resulting fluid flow in the sarcomere lattice of synchronous insect flight muscles is even greater than expected for constant lattice spacing conditions. Lattice spacing depends on a variety of factors that produce radial force, including cross-bridges, titin-like molecules, and other structural proteins. Volume change and advective transport varies with the phase of muscle stimulation during periodic contraction but remains significant at all conditions. Although varying in magnitude, advective transport will occur in all cases in which the sarcomere is not isovolumetric. Akin to "breathing," advective-diffusive transport in sarcomeres is sufficient to promote metabolite exchange and may play a role in the regulation of contraction itself.

Why exercise builds muscles: titin mechanosensing controls skeletal muscle growth under load

Muscles sense internally generated and externally applied forces, responding to these in a coordinated hierarchical manner at different timescales. The center of the basic unit of the muscle, the sarcomeric M-band, is perfectly placed to sense the different types of load to which the muscle is subjected. In particular, the kinase domain of titin (TK) located at the M-band is a known candidate for mechanical signaling. Here, we develop a quantitative mathematical model that describes the kinetics of TK-based mechanosensitive signaling and predicts trophic changes in response to exercise and rehabilitation regimes. First, we build the kinetic model for TK conformational changes under force: opening, phosphorylation, signaling, and autoinhibition. We find that TK opens as a metastable mechanosensitive switch, which naturally produces a much greater signal after high-load resistance exercise than an equally energetically costly endurance effort. Next, for the model to be stable and give coherent predictions, in particular for the lag after the onset of an exercise regime, we have to account for the associated kinetics of phosphate (carried by ATP) and for the nonlinear dependence of protein synthesis rates on muscle fiber size. We suggest that the latter effect may occur via the steric inhibition of ribosome diffusion through the sieve-like myofilament lattice. The full model yields a steady-state solution (homeostasis) for muscle cross-sectional area and tension and, a quantitatively plausible hypertrophic response to training, as well as atrophy after an extended reduction in tension.

Effect of Active Lengthening and Shortening on Small-Angle X-ray Reflections in Skinned Skeletal Muscle Fibres

Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I_1,1/I_1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I_1,1/I_1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.

Mandibular muscle troponin of the Florida carpenter ant Camponotus floridanus: extending our insights into invertebrate Ca2+ regulation

Ants use their mandibles for a variety of functions and behaviors. We investigated mandibular muscle structure and function from major workers of the Florida carpenter ant Camponotus floridanus: force-pCa relation and velocity of unloaded shortening of single, permeabilized fibres, primary sequences of troponin subunits (TnC, TnI and TnT) from a mandibular muscle cDNA library, and muscle fibre ultrastructure. From the mechanical measurements, we found Ca²⁺-sensitivity of isometric force was markedly shifted rightward compared with vertebrate striated muscle. From the troponin sequence results, we identified features that could explain the rightward shift of Ca²⁺-activation: the N-helix of TnC is effectively absent and three of the four EF-hands of TnC (sites I, II and III) do not adhere to canonical sequence rules for divalent cation binding; two alternatively spliced isoforms of TnI were identified with the alternatively spliced exon occurring in the region of the IT-arm α-helical coiled-coil, and the N-terminal extension of TnI may be involved in modulation of regulation, as in mammalian cardiac muscle; and TnT has a Glu-rich C-terminus. In addition, a structural homology model was built of C. floridanus troponin on the thin filament. From analysis of electron micrographs, we found thick filaments are almost as long as the 6.8 μm sarcomeres, have diameter of ~ 16 nm, and typical center-to-center spacing of ~ 46 nm. These results have implications for the mechanisms by which mandibular muscle fibres perform such a variety of functions, and how the structure of the troponin complex aids in these tasks.

The Sliding Filament Theory Since Andrew Huxley: Multiscale and Multidisciplinary Muscle Research

Two groundbreaking papers published in 1954 laid out the theory of the mechanism of muscle contraction based on force-generating interactions between myofilaments in the sarcomere that cause filaments to slide past one another during muscle contraction. The succeeding decades of research in muscle physiology have revealed a unifying interest: to understand the multiscale processes-from atom to organ-that govern muscle function. Such an understanding would have profound consequences for a vast array of applications, from developing new biomimetic technologies to treating heart disease. However, connecting structural and functional properties that are relevant at one spatiotemporal scale to those that are relevant at other scales remains a great challenge. Through a lens of multiscale dynamics, we review in this article current and historical research in muscle physiology sparked by the sliding filament theory.

Molecular Mechanisms of Muscle Tone Impairment under Conditions of Real and Simulated Space Flight

Kozlovskaya et al. [1] and Grigoriev et al. [2] showed that enormous loss of muscle stiffness (atonia) develops in humans under true (space flight) and simulated microgravity conditions as early as after the first days of exposure. This phenomenon is attributed to the inactivation of slow motor units and called reflectory atonia. However, a lot of evidence indicating that even isolated muscle or a single fiber possesses substantial stiffness was published at the end of the 20th century. This intrinsic stiffness is determined by the active component, i.e. the ability to form actin-myosin cross-bridges during muscle stretch and contraction, as well as by cytoskeletal and extracellular matrix proteins, capable of resisting muscle stretch. The main facts on intrinsic muscle stiffness under conditions of gravitational unloading are considered in this review. The data obtained in studies of humans under dry immersion and rodent hindlimb suspension is analyzed. The results and hypotheses regarding reduced probability of cross-bridge formation in an atrophying muscle due to increased interfilament spacing are described. The evidence of cytoskeletal protein (titin, nebulin, etc.) degradation during gravitational unloading is also discussed. The possible mechanisms underlying structural changes in skeletal muscle collagen and its role in reducing intrinsic muscle stiffness are presented. The molecular mechanisms of changes in intrinsic stiffness during space flight and simulated microgravity are reviewed.

The spatial distribution of thin filament activation influences force development and myosin activity in computational models of muscle contraction

Striated muscle contraction is initiated by Ca²⁺ binding to, and activating, thin filament regulatory units (RU) within the sarcomere, which then allows myosin cross-bridges from the opposing thick filament to bind actin and generate force. The amount of overlap between the filaments dictates how many potential cross-bridges are capable of binding, and thus how force is generated by the sarcomere. Myopathies and atrophy can impair muscle function by limiting cross-bridge interactions between the filaments, which can occur when the length of the thin filament is reduced or when RU function is disrupted. To investigate how variations in thin filament length and RU density affect ensemble cross-bridge behavior and force production, we simulated muscle contraction using a spatially explicit computational model of the half-sarcomere. Thin filament RUs were disabled either uniformly from the pointed end of the filament (to model shorter thin filament length) or randomly throughout the length of the half-sarcomere. Both uniform and random RU 'knockout' schemes decreased overall force generation during maximal and submaximal activation. The random knockout scheme also led to decreased calcium sensitivity and cooperativity of the force-pCa relationship. We also found that the rate of force development slowed with the random RU knockout, compared to the uniform RU knockout or conditions of normal RU activation. These findings imply that the relationship between RU density and force production within the sarcomere involves more complex coordination than simply the raw number of RUs available for myosin cross-bridge binding, and that the spatial pattern in which activatable RU are distributed throughout the sarcomere influences the dynamics of force production.

Energy Cost of Force Production After a Stretch-Shortening Cycle in Skinned Muscle Fibers: Does Muscle Efficiency Increase?

Muscle force is enhanced during shortening when shortening is preceded by an active stretch. This phenomenon is known as the stretch-shortening cycle (SSC) effect. For some stretch-shortening conditions this increase in force during shortening is maintained following SSCs when compared to the force following a pure shortening contraction. It has been suggested that the residual force enhancement property of muscles, which comes into play during the stretch phase of SSCs may contribute to the force increase after SSCs. Knowing that residual force enhancement is associated with a substantial reduction in metabolic energy per unit of force, it seems reasonable to assume that the metabolic energy cost per unit of force is also reduced following a SSC. The purpose of this study was to determine the energy cost per unit of force at steady-state following SSCs and compare it to the corresponding energy cost following pure shortening contractions of identical speed and magnitude. We hypothesized that the energy cost per unit of muscle force is reduced following SSCs compared to the pure shortening contractions. For the SSC tests, rabbit psoas fibers (n = 12) were set at an average sarcomere length (SL) of 2.4 μm, activated, actively stretched to a SL of 3.2 μm, and shortened to a SL of 2.6 or 3.0 μm. For the pure shortening contractions, the same fibers were activated at a SL of 3.2 μm and actively shortened to a SL of 2.6 or 3.0 μm. The amount of ATP consumed was measured over a 40 s steady-state total isometric force following either the SSCs or the pure active shortening contractions. Fiber stiffness was determined in an additional set of 12 fibers, at steady-state for both experimental conditions. Total force, ATP consumption, and stiffness were greater following SSCs compared to the pure shortening contractions, but ATP consumption per unit of force was the same between conditions. These results suggest that the increase in total force observed following SSCs was achieved with an increase in the proportion of attached cross-bridges and titin stiffness. We conclude that muscle efficiency is not enhanced at steady-state following SSCs.

Graded titin cleavage progressively reduces tension and uncovers the source of A-band stability in contracting muscle

The giant muscle protein titin is a major contributor to passive force; however, its role in active force generation is unresolved. Here, we use a novel titin-cleavage (TC) mouse model that allows specific and rapid cutting of elastic titin to quantify how titin-based forces define myocyte ultrastructure and mechanics. We show that under mechanical strain, as TC doubles from heterozygous to homozygous TC muscles, Z-disks become increasingly out of register while passive and active forces are reduced. Interactions of elastic titin with sarcomeric actin filaments are revealed. Strikingly, when titin-cleaved muscles contract, myosin-containing A-bands become split and adjacent myosin filaments move in opposite directions while also shedding myosins. This establishes intact titin filaments as critical force-transmission networks, buffering the forces observed by myosin filaments during contraction. To perform this function, elastic titin must change stiffness or extensible length, unveiling its fundamental role as an activation-dependent spring in contracting muscle.

Myosin dynamics during relaxation in mouse soleus muscle and modulation by 2'-deoxy-ATP

KEY POINTS

Skeletal muscle relaxation has been primarily studied by assessing the kinetics of force decay. Little is known about the resultant dynamics of structural changes in myosin heads during relaxation. The naturally occurring nucleotide 2-deoxy-ATP (dATP) is a myosin activator that enhances cross-bridge binding and kinetics. X-ray diffraction data indicate that with elevated dATP, myosin heads were extended closer to actin in relaxed muscle and myosin heads return to an ordered, resting state after contraction more quickly. Molecular dynamics simulations of post-powerstroke myosin suggest that dATP induces structural changes in myosin heads that increase the surface area of the actin-binding regions promoting myosin interaction with actin, which could explain the observed delays in the onset of relaxation. This study of the dATP-induced changes in myosin may be instructive for determining the structural changes desired for other potential myosin-targeted molecular compounds to treat muscle diseases.

ABSTRACT

Here we used time-resolved small-angle X-ray diffraction coupled with force measurements to study the structural changes in FVB mouse skeletal muscle sarcomeres during relaxation after tetanus contraction. To estimate the rate of myosin deactivation, we followed the rate of the intensity recovery of the first-order myosin layer line (MLL1) and restoration of the resting spacing of the third and sixth order of meridional reflection (SM3 and SM6 ) following tetanic contraction. A transgenic mouse model with elevated skeletal muscle 2-deoxy-ATP (dATP) was used to study how myosin activators may affect soleus muscle relaxation. X-ray diffraction evidence indicates that with elevated dATP, myosin heads were extended closer to actin in resting muscle. Following contraction, there is a slight but significant delay in the decay of force relative to WT muscle while the return of myosin heads to an ordered resting state was initially slower, then became more rapid than in WT muscle. Molecular dynamics simulations of post-powerstroke myosin suggest that dATP induces structural changes in myosin that increase the surface area of the actin-binding regions, promoting myosin interaction with actin. With dATP, myosin heads may remain in an activated state near the thin filaments following relaxation, accounting for the delay in force decay and the initial delay in recovery of resting head configuration, and this could facilitate subsequent contractions.

In vivo X-ray diffraction and simultaneous EMG reveal the time course of myofilament lattice dilation and filament stretch

Muscle function within an organism depends on the feedback between molecular and meter-scale processes. Although the motions of muscle's contractile machinery are well described in isolated preparations, only a handful of experiments have documented the kinematics of the lattice occurring when multi-scale interactions are fully intact. We used time-resolved X-ray diffraction to record the kinematics of the myofilament lattice within a normal operating context: the tethered flight of Manduca sexta As the primary flight muscles of M.sexta are synchronous, we used these results to reveal the timing of in vivo cross-bridge recruitment, which occurred 24 ms (s.d. 26) following activation. In addition, the thick filaments stretched an average of 0.75% (s.d. 0.32) and thin filaments stretched 1.11% (s.d. 0.65). In contrast to other in vivo preparations, lattice spacing changed an average of 2.72% (s.d. 1.47). Lattice dilation of this magnitude significantly affects shortening velocity and force generation, and filament stretching tunes force generation. While the kinematics were consistent within individual trials, there was extensive variation between trials. Using a mechanism-free machine learning model we searched for patterns within and across trials. Although lattice kinematics were predictable within trials, the model could not create predictions across trials. This indicates that the variability we see across trials may be explained by latent variables occurring in this naturally functioning system. The diverse kinematic combinations we documented mirror muscle's adaptability and may facilitate its robust function in unpredictable conditions.

Deleting Titin's C-Terminal PEVK Exons Increases Passive Stiffness, Alters Splicing, and Induces Cross-Sectional and Longitudinal Hypertrophy in Skeletal Muscle

The Proline, Glutamate, Valine and Lysine-rich (PEVK) region of titin constitutes an entropic spring that provides passive tension to striated muscle. To study the functional and structural repercussions of a small reduction in the size of the PEVK region, we investigated skeletal muscles of a mouse with the constitutively expressed C-terminal PEVK exons 219-225 deleted, the Ttn^Δ219-225 model (MGI: Ttn^{TM 2.1Mgot} ). Based on this deletion, passive tension in skeletal muscle was predicted to be increased by ∼17% (sarcomere length 3.0 μm). In contrast, measured passive tension (sarcomere length 3.0 μm) in both soleus and EDL muscles was increased 53 ± 11% and 62 ± 4%, respectively. This unexpected increase was due to changes in titin, not to alterations in the extracellular matrix, and is likely caused by co-expression of two titin isoforms in Ttn^Δ219-225 muscles: a larger isoform that represents the Ttn^Δ219-225 N2A titin and a smaller isoform, referred to as N2A2. N2A2 represents a splicing adaption with reduced expression of spring element exons, as determined by titin exon microarray analysis. Maximal tetanic tension was increased in Ttn^Δ219-225 soleus muscle (WT 240 ± 9; Ttn^Δ219-225 276 ± 17 mN/mm²), but was reduced in EDL muscle (WT 315 ± 9; Ttn^Δ219-225 280 ± 14 mN/mm²). The changes in active tension coincided with a switch toward slow fiber types and, unexpectedly, faster kinetics of tension generation and relaxation. Functional overload (FO; ablation) and hindlimb suspension (HS; unloading) experiments were also conducted. Ttn^Δ219-225 mice showed increases in both longitudinal hypertrophy (increased number of sarcomeres in series) and cross-sectional hypertrophy (increased number of sarcomeres in parallel) in response to FO and attenuated cross-sectional atrophy in response to HS. In summary, slow- and fast-twitch muscles in a mouse model devoid of titin's PEVK exons 219-225 have high passive tension, due in part to alterations elsewhere in splicing of titin's spring region, increased kinetics of tension generation and relaxation, and altered trophic responses to both functional overload and unloading. This implicates titin's C-terminal PEVK region in regulating passive and active muscle mechanics and muscle plasticity.

Titin: A Tunable Spring in Active Muscle

Muscle has conventionally been viewed as a motor that converts chemical to kinetic energy in series with a passive spring, but new insights emerge when muscle is viewed as a composite material whose elastic elements are tuned by activation. New evidence demonstrates that calcium-dependent binding of N2A titin to actin increases titin stiffness in active skeletal muscles, which explains many long-standing enigmas of muscle physiology.

Calcium-dependent titin-thin filament interactions in muscle: observations and theory

Gaps in our understanding of muscle mechanics demonstrate that the current model is incomplete. Increasingly, it appears that a role for titin in active muscle contraction might help to fill these gaps. While such a role for titin is increasingly accepted, the underlying molecular mechanisms remain unclear. The goals of this paper are to review recent studies demonstrating Ca²⁺-dependent interactions between N2A titin and actin in vitro, to explore theoretical predictions of muscle behavior based on this interaction, and to review experimental data related to the predictions. In a recent study, we demonstrated that Ca²⁺ increases the association constant between N2A titin and F-actin; that Ca²⁺ increases rupture forces between N2A titin and F-actin; and that Ca²⁺ and N2A titin reduce sliding velocity of F-actin and reconstituted thin filaments in motility assays. Preliminary data support a role for Ig83, but other Ig domains in the N2A region may also be involved. Two mechanical consequences are inescapable if N2A titin binds to thin filaments in active muscle sarcomeres: (1) the length of titin's freely extensible I-band should decrease upon muscle activation; and (2) binding between N2A titin and thin filaments should increase titin stiffness in active muscle. Experimental observations demonstrate that these properties characterize wild type muscles, but not muscles from mdm mice with a small deletion in N2A titin, including part of Ig83. Given the new in vitro evidence for Ca²⁺-dependent binding between N2A titin and actin, it is time for skepticism to give way to further investigation.

Muscle Function from Organisms to Molecules

Gaps in our understanding of muscle contraction at the molecular level limit the ability to predict in vivo muscle forces in humans and animals during natural movements. Because muscles function as motors, springs, brakes, or struts, it is not surprising that uncertainties remain as to how sarcomeres produce these different behaviors. Current theories fail to explain why a single extra stimulus, added shortly after the onset of a train of stimuli, doubles the rate of force development. When stretch and doublet stimulation are combined in a work loop, muscle force doubles and work increases by 50% per cycle, yet no theory explains why this occurs. Current theories also fail to predict persistent increases in force after stretch and decreases in force after shortening. Early studies suggested that all of the instantaneous elasticity of muscle resides in the cross-bridges. Subsequent cross-bridge models explained the increase in force during active stretch, but required ad hoc assumptions that are now thought to be unreasonable. Recent estimates suggest that cross-bridges account for only ∼12% of the energy stored by muscles during active stretch. The inability of cross-bridges to account for the increase in force that persists after active stretching led to development of the sarcomere inhomogeneity theory. Nearly all predictions of this theory fail, yet the theory persists. In stretch-shortening cycles, muscles with similar activation and contractile properties function as motors or brakes. A change in the phase of activation relative to the phase of length changes can convert a muscle from a motor into a spring or brake. Based on these considerations, it is apparent that the current paradigm of muscle mechanics is incomplete. Recent advances in our understanding of giant muscle proteins, including twitchin and titin, allow us to expand our vision beyond cross-bridges to understand how muscles contribute to the biomechanics and control of movement.

Sarcomere length-dependent effects on Ca2+-troponin regulation in myocardium expressing compliant titin

Increases in sarcomere length cause enhanced force generation in cardiomyocytes by an unknown mechanism. Li et al. reveal that titin-based passive tension contributes to length-dependent activation of myofilaments and that tightly bound myosin–actin cross-bridges are associated with this effect.

Cardiac performance is tightly regulated at the cardiomyocyte level by sarcomere length, such that increases in sarcomere length lead to sharply enhanced force generation at the same Ca²⁺ concentration. Length-dependent activation of myofilaments involves dynamic and complex interactions between a multitude of thick- and thin-filament components. Among these components, troponin, myosin, and the giant protein titin are likely to be key players, but the mechanism by which these proteins are functionally linked has been elusive. Here, we investigate this link in the mouse myocardium using in situ FRET techniques. Our objective was to monitor how length-dependent Ca²⁺-induced conformational changes in the N domain of cardiac troponin C (cTnC) are modulated by myosin–actin cross-bridge (XB) interactions and increased titin compliance. We reconstitute FRET donor- and acceptor-modified cTnC(13C/51C)AEDANS-DDPM into chemically skinned myocardial fibers from wild-type and RBM20-deletion mice. The Ca²⁺-induced conformational changes in cTnC are quantified and characterized using time-resolved FRET measurements as XB state and sarcomere length are varied. The RBM20-deficient mouse expresses a more compliant N2BA titin isoform, leading to reduced passive tension in the myocardium. This provides a molecular tool to investigate how altered titin-based passive tension affects Ca²⁺-troponin regulation in response to mechanical stretch. In wild-type myocardium, we observe a direct association of sarcomere length–dependent enhancement of troponin regulation with both Ca²⁺ activation and strongly bound XB states. In comparison, measurements from titin RBM20-deficient animals show blunted sarcomere length–dependent effects. These results suggest that titin-based passive tension contributes to sarcomere length–dependent Ca²⁺-troponin regulation. We also conclude that strong XB binding plays an important role in linking the modulatory effect of titin compliance to Ca²⁺-troponin regulation of the myocardium.

Titin as a force-generating muscle protein under regulatory control

Titin has long been recognized as a mechanical protein in muscle cells that has a main function as a molecular spring in the contractile units, the sarcomeres. Recent work suggests that the titin spring contributes to muscle contraction in a more active manner than previously thought. In this review, we highlight this property, specifically the ability of the immunoglobulin-like (Ig) domains of titin to undergo unfolding-refolding transitions when isolated titin molecules or skeletal myofibrils are held at physiological force levels. Folding of titin Ig domains under force is a hitherto unappreciated, putative source of work production in muscle cells, which could work in synergy with the actomyosin system to maximize the energy delivered by a stretched, actively contracting muscle. This review also focuses on the mechanisms shown to modulate titin-based viscoelastic forces in skeletal muscle cells, including chaperone binding, titin oxidation, phosphorylation, Ca²⁺ binding, and interaction with actin filaments. Along the way, we discuss which of these modulatory mechanisms might contribute to the phenomenon of residual force enhancement relevant for eccentric muscle contractions. Finally, a brief perspective is added on the potential for the alterations in titin-based force to dynamically alter mechano-chemical signaling pathways in the muscle cell. We conclude that titin from skeletal muscle is a determinant of both passive and active tension and a bona fide mechanosensor, whose stiffness is tuned by various independent mechanisms.

Thick-Filament Extensibility in Intact Skeletal Muscle

Myofilament extensibility is a key structural parameter for interpreting myosin cross-bridge kinetics in striated muscle. Previous studies reported much higher thick-filament extensibility at low tension than the better-known and commonly used values at high tension, but in interpreting mechanical studies of muscle, a single value for thick-filament extensibility has usually been assumed. Here, we established the complete thick-filament force-extension curve from actively contracting, intact vertebrate skeletal muscle. To access a wide range of tetanic forces, the myosin inhibitor blebbistatin was used to induce low tetanic forces in addition to the higher tensions obtained from tetanic contractions of the untreated muscle. We show that the force/extensibility curve of the thick filament is nonlinear, so assuming a single value for thick-filament extensibility at all force levels is not justified. We also show that independent of whether tension is generated passively by sarcomere stretch or actively by cross-bridges, the thick-filament extensibility is nonlinear. Myosin head periodicity, however, only changes when active tension is generated under calcium-activated conditions. The nonlinear thick-filament force-extension curve in skeletal muscle, therefore, reflects a purely passive response to either titin-based force or actomyosin-based force, and it does not include a thick-filament activation mechanism. In contrast, the transition of myosin head periodicity to an active configuration appears to only occur in response to increased active force when calcium is present.

Early Deсline in Rat Soleus Passive Tension with Hindlimb Unloading: Inactivation of Cross-bridges or Activation of Calpains?

The study was aimed at testing the hypotheses about the role of cross-bridges and calpains in reduction of rat soleus passive tension under conditions of hindlimb unloading. For this purpose, we used an inhibitor of μ-calpain PD 150606 as well as a blocker of actomyosin interaction (blebbistatin). It was found for the first time that a decrease in passive tension of rat soleus after 3-day hindlimb unloading is associated with the activity of μ-calpain and does not depend on the processes of cross-bridges formation.

Nebulin stiffens the thin filament and augments cross-bridge interaction in skeletal muscle

Nebulin is a giant sarcomeric protein that spans along the actin filament in skeletal muscle, from the Z-disk to near the thin filament pointed end. Mutations in nebulin cause muscle weakness in nemaline myopathy patients, suggesting that nebulin plays important roles in force generation, yet little is known about nebulin's influence on thin filament structure and function. Here, we used small-angle X-ray diffraction and compared intact muscle deficient in nebulin (using a conditional nebulin-knockout, Neb cKO) with control (Ctrl) muscle. When muscles were activated, the spacing of the actin subunit repeat (27 Å) increased in both genotypes; when converted to thin filament stiffness, the obtained value was 30 pN/nm in Ctrl muscle and 10 pN/nm in Neb cKO muscle; that is, the thin filament was approximately threefold stiffer when nebulin was present. In contrast, the thick filament stiffness was not different between the genotypes. A significantly shorter left-handed (59 Å) thin filament helical pitch was found in passive and contracting Neb cKO muscles, as well as impaired tropomyosin and troponin movement. Additionally, a reduced myosin mass transfer toward the thin filament in contracting Neb cKO muscle was found, suggesting reduced cross-bridge interaction. We conclude that nebulin is critically important for physiological force levels, as it greatly stiffens the skeletal muscle thin filament and contributes to thin filament activation and cross-bridge recruitment.

Myosin Head Configurations in Resting and Contracting Murine Skeletal Muscle

Transgenic mouse models have been important tools for studying the relationship of genotype to phenotype for human diseases, including those of skeletal muscle. We show that mouse skeletal muscle can produce high quality X-ray diffraction patterns establishing the mouse intact skeletal muscle X-ray preparation as a potentially powerful tool to test structural hypotheses in health and disease. A notable feature of the mouse model system is the presence of residual myosin layer line intensities in contracting mouse muscle patterns. This provides an additional tool, along with the I1,1/I1,0 intensity ratio, for estimating the proportions of active versus relaxed myosin heads under a given set of conditions that can be used to characterize a given physiological condition or mutant muscle type. We also show that analysis of the myosin layer line intensity distribution, including derivation of the myosin head radius, Rm, may be used to study the role of the super-relaxed state in myosin regulation. When the myosin inhibitor blebbistatin is used to inhibit force production, there is a shift towards a highly quasi-helically ordered configuration that is distinct from the normal resting state, indicating there are more than one helically ordered configuration for resting crossbridges.

Basic science and clinical use of eccentric contractions: History and uncertainties

The peculiar attributes of muscles that are stretched when active have been noted for nearly a century. Understandably, the focus of muscle physiology has been primarily on shortening and isometric contractions, as eloquently revealed by A.V. Hill and subsequently by his students. When the sliding filament theory was introduced by A.F. Huxley and H.E. Huxley, it was a relatively simple task to link Hill's mechanical observations to the actions of the cross bridges during these shortening and isometric contractions. In contrast, lengthening or eccentric contractions have remained somewhat enigmatic. Dismissed as necessarily causing muscle damage, eccentric contractions have been much more difficult to fit into the cross-bridge theory. The relatively recent discovery of the giant elastic sarcomeric filament titin has thrust a previously missing element into any discussion of muscle function, in particular during active stretch. Indeed, the unexpected contribution of giant elastic proteins to muscle contractile function is highlighted by recent discoveries that twitchin-actin interactions are responsible for the "catch" property of invertebrate muscle. In this review, we examine several current theories that have been proposed to account for the properties of muscle during eccentric contraction. We ask how well each of these explains existing data and how an elastic filament can be incorporated into the sliding filament model. Finally, we review the increasing body of evidence for the benefits of including eccentric contractions into a program of muscle rehabilitation and strengthening.

Conservation of cardiac L-type Ca2+ channels and their regulation in Drosophila: A novel genetically-pliable channelopathic model

Dysregulation of L-type Ca2+ channels (LTCCs) underlies numerous cardiac pathologies. Understanding their modulation with high fidelity relies on investigating LTCCs in their native environment with intact interacting proteins. Such studies benefit from genetic manipulation of endogenous channels in cardiomyocytes, which often proves cumbersome in mammalian models. Drosophila melanogaster, however, offers a potentially efficient alternative as it possesses a relatively simple heart, is genetically pliable, and expresses well-conserved genes. Fluorescence in situ hybridization confirmed an abundance of Ca-α1D and Ca-α1T mRNA in fly myocardium, which encode subunits that specify hetero-oligomeric channels homologous to mammalian LTCCs and T-type Ca2+ channels, respectively. Cardiac-specific knockdown of Ca-α1D via interfering RNA abolished cardiac contraction, suggesting Ca-α1D (i.e. A1D) represents the primary functioning Ca2+ channel in Drosophila hearts. Moreover, we successfully isolated viable single cardiomyocytes and recorded Ca2+ currents via patch clamping, a feat never before accomplished with the fly model. The profile of Ca2+ currents recorded in individual cells when Ca2+ channels were hypomorphic, absent, or under selective LTCC blockage by nifedipine, additionally confirmed the predominance of A1D current across all activation voltages. T-type current, activated at more negative voltages, was also detected. Lastly, A1D channels displayed Ca2+-dependent inactivation, a critical negative feedback mechanism of LTCCs, and the current through them was augmented by forskolin, an activator of the protein kinase A pathway. In sum, the Drosophila heart possesses a conserved compendium of Ca2+ channels, suggesting that the fly may serve as a robust and effective platform for studying cardiac channelopathies.

Role of the C-terminus mobile domain of cardiac troponin I in the regulation of thin filament activation in skinned papillary muscle strips

The C-terminus mobile domain of cTnI (cTnI-MD) is a highly conserved region which stabilizes the actin-cTnI interaction during the diastole. Upon Ca²⁺-binding to cTnC, cTnI-MD participates in a regulatory switching that involves cTnI to switch from interacting with actin toward interacting with the Ca²⁺-regulatory domain of cTnC. Despite many studies targeting the cTnI-MD, the role of this region in the length-dependent activation of cardiac contractility is yet to be determined. The present study investigated the functional consequences of losing the entire cTnI-MD in cTnI(1-167) truncation mutant, as it was exchanged for endogenous cTnI in skinned rat papillary muscle fibers. The influence of cTnI-MD truncation on the extent of the N-domain of cTnC hydrophobic cleft opening and the steady-state force as a function of sarcomere length (SL), cross-bridge state, and [Ca²⁺] was assessed using the simultaneous in situ time-resolved FRET and force measurements at short (1.8 μm) and long (2.2 μm) SLs. Our results show the significant role of cTnI-MD in the length dependent thin filament activation and the coupling between thin and thick filament regulations affected by SL. Our results also suggest that cTnI-MD transmits the effects of SL change to the core of troponin complex.

Diaphragm abnormalities in heart failure and aging: mechanisms and integration of cardiovascular and respiratory pathophysiology

Inspiratory function is essential for alveolar ventilation and expulsive behaviors that promote airway clearance (e.g., coughing and sneezing). Current evidence demonstrates that inspiratory dysfunction occurs during healthy aging and is accentuated by chronic heart failure (CHF). This inspiratory dysfunction contributes to key aspects of CHF and aging cardiovascular and pulmonary pathophysiology including: (1) impaired airway clearance and predisposition to pneumonia; (2) inability to sustain ventilation during physical activity; (3) shallow breathing pattern that limits alveolar ventilation and gas exchange; and (4) sympathetic activation that causes cardiac arrhythmias and tissue vasoconstriction. The diaphragm is the primary inspiratory muscle; hence, its neuromuscular integrity is a main determinant of the adequacy of inspiratory function. Mechanistic work within animal and cellular models has revealed specific factors that may be responsible for diaphragm neuromuscular abnormalities in CHF and aging. These include phrenic nerve and neuromuscular junction alterations as well as intrinsic myocyte abnormalities, such as changes in the quantity and quality of contractile proteins, accelerated fiber atrophy, and shifts in fiber type distribution. CHF, aging, or CHF in the presence of aging disturbs the dynamics of circulating factors (e.g., cytokines and angiotensin II) and cell signaling involving sphingolipids, reactive oxygen species, and proteolytic pathways, thus leading to the previously listed abnormalities. Exercise-based rehabilitation combined with pharmacological therapies targeting the pathways reviewed herein hold promise to treat diaphragm abnormalities and inspiratory muscle dysfunction in CHF and aging.

Functional significance of C-terminal mobile domain of cardiac troponin I

Ca²⁺-regulation of cardiac contractility is mediated through the troponin complex, which comprises three subunits: cTnC, cTnI, and cTnT. As intracellular [Ca²⁺] increases, cTnI reduces its binding interactions with actin to primarily interact with cTnC, thereby enabling contraction. A portion of this regulatory switching involves the mobile domain of cTnI (cTnI-MD), the role of which in muscle contractility is still elusive. To study the functional significance of cTnI-MD, we engineered two cTnI constructs in which the MD was truncated to various extents: cTnI(1-167) and cTnI(1-193). These truncations were exchanged for endogenous cTnI in skinned rat papillary muscle fibers, and their influence on Ca²⁺-activated contraction and cross-bridge cycling kinetics was assessed at short (1.9 μm) and long (2.2 μm) sarcomere lengths (SLs). Our results show that the cTnI(1-167) truncation diminished the SL-induced increase in Ca²⁺-sensitivity of contraction, but not the SL-dependent increase in maximal tension, suggesting an uncoupling between the thin and thick filament contributions to length dependent activation. Compared to cTnI(WT), both truncations displayed greater Ca²⁺-sensitivity and faster cross-bridge attachment rates at both SLs. Furthermore, cTnI(1-167) slowed MgADP release rate and enhanced cross-bridge binding. Our findings imply that cTnI-MD truncations affect the blocked-to closed-state transition(s) and destabilize the closed-state position of tropomyosin.

Translocation of molecular chaperones to the titin springs is common in skeletal myopathy patients and affects sarcomere function

Myopathies encompass a wide variety of acquired and hereditary disorders. The pathomechanisms include structural and functional changes affecting, e.g., myofiber metabolism and contractile properties. In this study, we observed increased passive tension (PT) of skinned myofibers from patients with myofibrillar myopathy (MFM) caused by FLNC mutations (MFM-filaminopathy) and limb-girdle muscular dystrophy type-2A due to CAPN3 mutations (LGMD2A), compared to healthy control myofibers. Because the giant protein titin determines myofiber PT, we measured its molecular size and the titin-to-myosin ratio, but found no differences between myopathies and controls. All-titin phosphorylation and site-specific phosphorylation in the PEVK region were reduced in myopathy, which would be predicted to lower PT. Electron microscopy revealed extensive ultrastructural changes in myofibers of various hereditary myopathies and also suggested massive binding of proteins to the sarcomeric I-band region, presumably heat shock proteins (HSPs), which can translocate to elastic titin under stress conditions. Correlative immunofluorescence and immunoelectron microscopy showed that two small HSPs (HSP27 and αB-crystallin) and the ATP-dependent chaperone HSP90 translocated to the titin springs in myopathy. The small HSPs, but not HSP90, were upregulated in myopathic versus control muscles. The titin-binding pattern of chaperones was regularly observed in Duchenne muscular dystrophy (DMD), LGMD2A, MFM-filaminopathy, MFM-myotilinopathy, titinopathy, and inclusion body myopathy due to mutations in valosin-containing protein, but not in acquired sporadic inclusion body myositis. The three HSPs also associated with elastic titin in mouse models of DMD and MFM-filaminopathy. Mechanical measurements on skinned human myofibers incubated with exogenous small HSPs suggested that the elevated PT seen in myopathy is caused, in part, by chaperone-binding to the titin springs. Whereas this interaction may be protective in that it prevents sarcomeric protein aggregation, it also has detrimental effects on sarcomere function. Thus, we identified a novel pathological phenomenon common to many hereditary muscle disorders, which involves sarcomeric alterations.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Muscle and Limb Mechanics

Understanding of the musculoskeletal system has evolved from the collection of individual phenomena in highly selected experimental preparations under highly controlled and often unphysiological conditions. At the systems level, it is now possible to construct complete and reasonably accurate models of the kinetics and energetics of realistic muscles and to combine them to understand the dynamics of complete musculoskeletal systems performing natural behaviors. At the reductionist level, it is possible to relate most of the individual phenomena to the anatomical structures and biochemical processes that account for them. Two large challenges remain. At a systems level, neuroscience must now account for how the nervous system learns to exploit the many complex features that evolution has incorporated into muscle and limb mechanics. At a reductionist level, medicine must now account for the many forms of pathology and disability that arise from the many diseases and injuries to which this highly evolved system is inevitably prone. © 2017 American Physiological Society. Compr Physiol 7:429-462, 2017.