Cooperativity and switching within the three-state model of muscle regulation

Myosin-1C differentially displaces tropomyosin isoforms altering their inhibition of motility

Force generation and motility by actomyosin in nonmuscle cells are spatially regulated by ∼40 tropomyosin (Tpm) isoforms. The means by which Tpms are targeted to specific cellular regions and the mechanisms that result in differential activity of myosin paralogs are unknown. We show that Tpm3.1 and Tpm1.7 inhibit Myosin-IC (Myo1C), with Tpm1.7 more effectively reducing the number of gliding filaments than Tpm3.1. Strikingly, cosedimentation and fluorescence microscopy assays revealed that Tpm3.1 is displaced from actin by Myo1C and not by myosin-II. In contrast, Tpm1.7 is only weakly displaced by Myo1C. Unlike other characterized myosins, Myo1C motility is inhibited by Tpm when the Tpm-actin filament is activated by myosin-II. These results point to a mechanism for the exclusion of myosin-I paralogs from cellular Tpm-decorated actin filaments that are activated by other myosins. Additionally, our results suggest a potential mechanism for myosin-induced Tpm sorting in cells.

High-resolution cryo-EM structure of the junction region of the native cardiac thin filament in relaxed state

Cardiac contraction depends on molecular interactions among sarcomeric proteins coordinated by the rising and falling intracellular Ca²⁺ levels. Cardiac thin filament (cTF) consists of two strands composed of actin, tropomyosin (Tm), and equally spaced troponin (Tn) complexes forming regulatory units. Tn binds Ca²⁺ to move Tm strand away from myosin-binding sites on actin to enable actomyosin cross-bridges required for force generation. The Tn complex has three subunits-Ca²⁺-binding TnC, inhibitory TnI, and Tm-binding TnT. Tm strand is comprised of adjacent Tm molecules that overlap "head-to-tail" along the actin filament. The N-terminus of TnT (e.g., TnT1) binds to the Tm overlap region to form the cTF junction region-the region that connects adjacent regulatory units and confers to cTF internal cooperativity. Numerous studies have predicted interactions among actin, Tm, and TnT1 within the junction region, although a direct structural description of the cTF junction region awaited completion. Here, we report a 3.8 Å resolution cryo-EM structure of the native cTF junction region at relaxing (pCa 8) Ca²⁺ conditions. We provide novel insights into the "head-to-tail" interactions between adjacent Tm molecules and interactions between the Tm junction with F-actin. We demonstrate how TnT1 stabilizes the Tm overlap region via its interactions with the Tm C- and N-termini and actin. Our data show that TnT1 works as a joint that anchors the Tm overlap region to actin, which stabilizes the relaxed state of the cTF. Our structure provides insight into the molecular basis of cardiac diseases caused by missense mutations in TnT1.

Low expression of the K280N TNNT2 mutation is sufficient to increase basal myofilament activation in human hypertrophy cardiomyopathy

Background

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant genetic disorder with patients typically showing heterozygous inheritance of a pathogenic variant in a gene encoding a contractile protein. Here, we study the contractile effects of a rare homozygous mutation using explanted tissue and human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to gain insight into how the balance between mutant and WT protein expression affects cardiomyocyte function.

Methods

Force measurements were performed in cardiomyocytes isolated from a HCM patient carrying a homozygous troponin T mutation (cTnT-K280N) and healthy donors. To discriminate between mutation-mediated and phosphorylation-related effects on Ca2+-sensitivity, cardiomyocytes were treated with alkaline phosphatase (AP) or protein kinase A (PKA). Troponin exchange experiments characterized the relation between mutant levels and myofilament function. To define mutation-mediated effects on Ca2+-dynamics we used CRISPR/Cas9 to generate hiPSC-CMs harbouring heterozygous and homozygous TnT-K280N mutations. Ca2+-transient and cell shortening experiments compared these lines against isogenic controls.

Results

Myofilament Ca2+-sensitivity was higher in homozygous cTnT-K280N cardiomyocytes and was not corrected by AP- and PKA-treatment. In cTnT-K280N cells exchanged with cTnT-WT, a low level (14%) of cTnT-K280N mutation elevated Ca2+-sensitivity. Similarly, exchange of donor cells with 45 ± 2% cTnT-K280N increased Ca2+-sensitivity and was not corrected by PKA. cTnT-K280N hiPSC-CMs show elevated diastolic Ca2+ and increases in cell shortening. Impaired cardiomyocyte relaxation was only evident in homozygous cTnT-K280N hiPSC-CMs.

Conclusions

The cTnT-K280N mutation increases myofilament Ca2+-sensitivity, elevates diastolic Ca2+, enhances contractility and impairs cellular relaxation. A low level (14%) of the cTnT-K280N sensitizes myofilaments to Ca2+, a universal finding of human HCM.

Functional Characterization of Cardiac Actin Mutants Causing Hypertrophic (p.A295S) and Dilated Cardiomyopathy (p.R312H and p.E361G)

Human wild type (wt) cardiac α-actin and its mutants p.A295S or p.R312H and p.E361G correlated with hypertrophic or dilated cardiomyopathy, respectively, were expressed by using the baculovirus/Sf21 insect cell system. The c-actin variants inhibited DNase I, indicating maintenance of their native state. Electron microscopy showed the formation of normal appearing actin filaments though they showed mutant specific differences in length and straightness correlating with their polymerization rates. TRITC-phalloidin staining showed that p.A295S and p.R312H exhibited reduced and the p.E361G mutant increased lengths of their formed filaments. Decoration of c-actins with cardiac tropomyosin (cTm) and troponin (cTn) conveyed Ca²⁺-sensitivity of the myosin-S1 ATPase stimulation, which was higher for the HCM p.A295S mutant and lower for the DCM p.R312H and p.E361G mutants than for wt c-actin. The lower Ca²⁺-sensitivity of myosin-S1 stimulation by both DCM actin mutants was corrected by the addition of levosimendan. Ca²⁺-dependency of the movement of pyrene-labeled cTm along polymerized c-actin variants decorated with cTn corresponded to the relations observed for the myosin-S1 ATPase stimulation though shifted to lower Ca²⁺-concentrations. The N-terminal C0C2 domain of cardiac myosin-binding protein-C increased the Ca²⁺-sensitivity of the pyrene-cTM movement of bovine, recombinant wt, p.A295S, and p.E361G c-actins, but not of the p.R312H mutant, suggesting decreased affinity to cTm.

Single-molecule imaging reveals the concerted release of myosin from regulated thin filaments

Regulated thin filaments (RTFs) tightly control striated muscle contraction through calcium binding to troponin, which enables tropomyosin to expose myosin-binding sites on actin. Myosin binding holds tropomyosin in an open position, exposing more myosin-binding sites on actin, leading to cooperative activation. At lower calcium levels, troponin and tropomyosin turn off the thin filament; however, this is antagonised by the high local concentration of myosin, questioning how the thin filament relaxes. To provide molecular details of deactivation, we used single-molecule imaging of green fluorescent protein (GFP)-tagged myosin-S1 (S1-GFP) to follow the activation of RTF tightropes. In sub-maximal activation conditions, RTFs are not fully active, enabling direct observation of deactivation in real time. We observed that myosin binding occurs in a stochastic step-wise fashion; however, an unexpectedly large probability of multiple contemporaneous detachments is observed. This suggests that deactivation of the thin filament is a coordinated active process.

A troponin T variant linked with pediatric dilated cardiomyopathy reduces the coupling of thin filament activation to myosin and calcium binding

Dilated cardiomyopathy (DCM) is a significant cause of pediatric heart failure. Mutations in proteins that regulate cardiac muscle contraction can cause DCM; however, the mechanisms by which molecular-level mutations contribute to cellular dysfunction are not well understood. Better understanding of these mechanisms might enable the development of targeted therapeutics that benefit patient subpopulations with mutations that cause common biophysical defects. We examined the molecular- and cellular-level impacts of a troponin T variant associated with pediatric-onset DCM, R134G. The R134G variant decreased calcium sensitivity in an in vitro motility assay. Using stopped-flow and steady-state fluorescence measurements, we determined the molecular mechanism of the altered calcium sensitivity: R134G decouples calcium binding by troponin from the closed-to-open transition of the thin filament and decreases the cooperativity of myosin binding to regulated thin filaments. Consistent with the prediction that these effects would cause reduced force per sarcomere, cardiomyocytes carrying the R134G mutation are hypocontractile. They also show hallmarks of DCM that lie downstream of the initial insult, including disorganized sarcomeres and cellular hypertrophy. These results reinforce the importance of multiscale studies to fully understand mechanisms underlying human disease and highlight the value of mechanism-based precision medicine approaches for DCM.

The structure of the native cardiac thin filament at systolic Ca2+ levels

Every heartbeat relies on cyclical interactions between myosin thick and actin thin filaments orchestrated by rising and falling Ca2+ levels. Thin filaments are comprised of two actin strands, each harboring equally separated troponin complexes, which bind Ca2+ to move tropomyosin cables away from the myosin binding sites and, thus, activate systolic contraction. Recently, structures of thin filaments obtained at low (pCa ∼9) or high (pCa ∼3) Ca2+ levels revealed the transition between the Ca2+-free and Ca2+-bound states. However, in working cardiac muscle, Ca2+ levels fluctuate at intermediate values between pCa ∼6 and pCa ∼7. The structure of the thin filament at physiological Ca2+ levels is unknown. We used cryoelectron microscopy and statistical analysis to reveal the structure of the cardiac thin filament at systolic pCa = 5.8. We show that the two strands of the thin filament consist of a mixture of regulatory units, which are composed of Ca2+-free, Ca2+-bound, or mixed (e.g., Ca2+ free on one side and Ca2+ bound on the other side) troponin complexes. We traced troponin complex conformations along and across individual thin filaments to directly determine the structural composition of the cardiac native thin filament at systolic Ca2+ levels. We demonstrate that the two thin filament strands are activated stochastically with short-range cooperativity evident only on one of the two strands. Our findings suggest a mechanism by which cardiac muscle is regulated by narrow range Ca2+ fluctuations.

Crossbridge Recruitment Capacity of Wild-Type and Hypertrophic Cardiomyopathy-Related Mutant Troponin-T Evaluated by X-ray Diffraction and Mechanical Study of Cardiac Skinned Fibers

X-ray diffraction and tension measurement experiments were conducted on rat left ventricular skinned fibers with or without “troponin-T treatment,” which exchanges the endogenous troponin T/I/C complex with exogenous troponin-T. These experiments were performed to observe the structural changes in troponin-T within a fiber elicited by contractile crossbridge formation and investigate the abnormality of hypertrophic cardiomyopathy-related troponin-T mutants. The intensity of the troponin reflection at 1/38.5 nm⁻¹ was decreased significantly by ATP addition after treatment with wild-type or mutant troponin-T, indicating that crossbridge formation affected the conformation of troponin-T. In experiments on cardiac fibers treated with the hypertrophic cardiomyopathy-related mutants E244D- and K247R-troponin-T, treatment with K247R-troponin-T did not recruit contracting actomyosin to a greater extent than wild-type-troponin-T, although a similar drop in the intensity of the troponin reflection occurred. Therefore, the conformational change in K247R-troponin-T was suggested to be unable to fully recruit actomyosin interaction, which may be the cause of cardiomyopathy.

A new twist on tropomyosin binding to actin filaments: perspectives on thin filament function, assembly and biomechanics

Tropomyosin, best known for its role in the steric regulation of muscle contraction, polymerizes head-to-tail to form cables localized along the length of both muscle and non-muscle actin-based thin filaments. In skeletal and cardiac muscles, tropomyosin, under the control of troponin and myosin, moves in a cooperative manner between blocked, closed and open positions on filaments, thereby masking and exposing actin-binding sites necessary for myosin crossbridge head interactions. While the coiled-coil signature of tropomyosin appears to be simple, closer inspection reveals surprising structural complexity required to perform its role in steric regulation. For example, component α-helices of coiled coils are typically zippered together along a continuous core hydrophobic stripe. Tropomyosin, however, contains a number of anomalous, functionally controversial, core amino acid residues. We argue that the atypical residues at this interface, including clusters of alanines and a charged aspartate, are required for preshaping tropomyosin to readily fit to the surface of the actin filament, but do so without compromising tropomyosin rigidity once the filament is assembled. Indeed, persistence length measurements of tropomyosin are characteristic of a semi-rigid cable, in this case conducive to cooperative movement on thin filaments. In addition, we also maintain that tropomyosin displays largely unrecognized and residue-specific torsional variance, which is involved in optimizing contacts between actin and tropomyosin on the assembled thin filament. Corresponding twist-induced stiffness may also enhance cooperative translocation of tropomyosin across actin filaments. We conclude that anomalous core residues of tropomyosin facilitate thin filament regulatory behavior in a multifaceted way.

Myosin and Other Energy-Transducing ATPases: Structural Dynamics Studied by Electron Paramagnetic Resonance

The objective of this article was to document the energy-transducing and regulatory interactions in supramolecular complexes such as motor, pump, and clock ATPases. The dynamics and structural features were characterized by motion and distance measurements using spin-labeling electron paramagnetic resonance (EPR) spectroscopy. In particular, we focused on myosin ATPase with actin-troponin-tropomyosin, neural kinesin ATPase with microtubule, P-type ion-motive ATPase, and cyanobacterial clock ATPase. Finally, we have described the relationships or common principles among the molecular mechanisms of various energy-transducing systems and how the large-scale thermal structural transition of flexible elements from one state to the other precedes the subsequent irreversible chemical reactions.

Omecamtiv Mecarbil Slows Myosin Kinetics in Skinned Rat Myocardium at Physiological Temperature

Heart failure is a life-threatening condition that occurs when the heart muscle becomes weakened and cannot adequately circulate blood and nutrients around the body. Omecamtiv mecarbil (OM) is a compound that has been developed to treat systolic heart failure via targeting the cardiac myosin heavy chain to increase myocardial contractility. Biophysical and biochemical studies have found that OM increases calcium (Ca²⁺) sensitivity of contraction by prolonging the myosin working stroke and increasing the actin-myosin cross-bridge duty ratio. Most in vitro studies probing the effects of OM on cross-bridge kinetics and muscle force production have been conducted at subphysiological temperature, even though temperature plays a critical role in enzyme activity and cross-bridge function. Herein, we used skinned, ventricular papillary muscle strips from rats to investigate the effects of [OM] on Ca²⁺-activated force production, cross-bridge kinetics, and myocardial viscoelasticity at physiological temperature (37°C). We find that OM only increases myocardial contractility at submaximal Ca²⁺ activation levels and not maximal Ca²⁺ activation levels. As [OM] increased, the kinetic rate constants for cross-bridge recruitment and detachment slowed for both submaximal and maximal Ca²⁺-activated conditions. These findings support a mechanism by which OM increases cardiac contractility at physiological temperature via increasing cross-bridge contributions to thin-filament activation as cross-bridge kinetics slow and the duration of cross-bridge attachment increases. Thus, force only increases at submaximal Ca²⁺ activation due to cooperative recruitment of neighboring cross-bridges, because thin-filament activation is not already saturated. In contrast, OM does not increase myocardial force production for maximal Ca²⁺-activated conditions at physiological temperature because cooperative activation of thin filaments may already be saturated.

Nebulin and titin modulate cross-bridge cycling and length-dependent calcium sensitivity

Various mutations in the structural proteins nebulin and titin that are present in human disease are known to affect the contractility of striated muscle. Loss of nebulin is associated with reduced actin filament length and impairment of myosin binding to actin, whereas titin is thought to regulate muscle passive elasticity and is likely involved in length-dependent activation. Here, we sought to assess the modulation of muscle function by these sarcomeric proteins by using the computational platform muscle simulation code (MUSICO) to quantitatively separate the effects of structural changes, kinetics of cross-bridge cycling, and calcium sensitivity of the thin filaments. The simulations show that variation in thin filament length cannot by itself account for experimental observations of the contractility in nebulin-deficient muscle, but instead must be accompanied by a decreased myosin binding rate. Additionally, to match the observed calcium sensitivity, the rate of TnI detachment from actin needed to be increased. Simulations for cardiac muscle provided quantitative estimates of the effects of different titin-based passive elasticities on muscle force and activation in response to changes in sarcomere length and interfilament lattice spacing. Predicted force-pCa relations showed a decrease in both active tension and sensitivity to calcium with a decrease in passive tension and sarcomere length. We conclude that this behavior is caused by partial redistribution of the muscle load between active muscle force and titin-dependent passive force, and also by redistribution of stretch along the thin filament, which together modulate the release of TnI from actin. These data help advance understanding of how nebulin and titin mutations affect muscle function.

Revealing the mechanism of how cardiac myosin-binding protein C N-terminal fragments sensitize thin filaments for myosin binding

Diverse demands on cardiac muscle require the fine-tuning of contraction. Cardiac myosin binding protein-C (cMyBP-C) is involved in this regulation; however, its precise molecular mechanism of action remains uncertain. By imaging the interactions of single myosin and cMyBP-C molecules interacting with suspended thin filaments in vitro we observe cMyBP-C N-terminal fragments assist activation and modulate contraction velocity by affecting myosin binding to the thin filament. Fluorescent imaging of Cy3-labeled cMyBP-C revealed that it diffusively scans the thin filament and then strongly binds to displace tropomyosin and activate at low calcium. At high calcium, cMyBP-C decorates the filament more extensively, reducing myosin binding through competing with binding sites. Understanding the mechanism of MyBP-C action has important implications for heart disease.

Cardiac muscle contraction is triggered by calcium binding to troponin. The consequent movement of tropomyosin permits myosin binding to actin, generating force. Cardiac myosin-binding protein C (cMyBP-C) plays a modulatory role in this activation process. One potential mechanism for the N-terminal domains of cMyBP-C to achieve this is by binding directly to the actin-thin filament at low calcium levels to enhance the movement of tropomyosin. To determine the molecular mechanisms by which cMyBP-C enhances myosin recruitment to the actin-thin filament, we directly visualized fluorescently labeled cMyBP-C N-terminal fragments and GFP-labeled myosin molecules binding to suspended actin-thin filaments in a fluorescence-based single-molecule microscopy assay. Binding of the C0C3 N-terminal cMyBP-C fragment to the thin filament enhanced myosin association at low calcium levels. However, at high calcium levels, C0C3 bound in clusters, blocking myosin binding. Dynamic imaging of thin filament-bound Cy3-C0C3 molecules demonstrated that these fragments diffuse along the thin filament before statically binding, suggesting a mechanism that involves a weak-binding mode to search for access to the thin filament and a tight-binding mode to sensitize the thin filament to calcium, thus enhancing myosin binding. Although shorter N-terminal fragments (Cy3-C0C1 and Cy3-C0C1f) bound to the thin filaments and displayed modes of motion on the thin filament similar to that of the Cy3-C0C3 fragment, the shorter fragments were unable to sensitize the thin filament. Therefore, the longer N-terminal fragment (C0C3) must possess the requisite domains needed to bind specifically to the thin filament in order for the cMyBP-C N terminus to modulate cardiac contractility.

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.

Deviations in conformational rearrangements of thin filaments and myosin caused by the Ala155Thr substitution in hydrophobic core of tropomyosin

Effects of the Ala155Thr substitution in hydrophobic core of tropomyosin Tpm1.1 on conformational rearrangements of the components of the contractile system (Tpm1.1, actin and myosin heads) were studied by polarized fluorimetry technique at different stages of the actomyosin ATPase cycle. The proteins were labelled by fluorescent probes and incorporated into ghost muscle fibres. The substitution violated the blocked and closed states of thin filaments stimulating abnormal displacement of tropomyosin to the inner domains of actin, switching actin on and increasing the relative number of the myosin heads in strong-binding state. Furthermore, the mutant tropomyosin disrupted the major function of troponin to alter the distribution of the different functional states of thin filaments. At low Ca²⁺ troponin did not effectively switch thin filament off and the myosin head lost the ability to drive the spatial arrangement of the mutant tropomyosin. The information about tropomyosin flexibility obtained from the fluorescent probes at Cys190 indicates that this tropomyosin is generally more rigid, that obviously prevents tropomyosin to bend and adopt the appropriate conformation required for proper regulation.

The Frank-Starling Law: a jigsaw of titin proportions

The Frank-Starling Law dictates that the heart is able to match ejection to the dynamic changes occurring during cardiac filling, hence efficiently regulating isovolumetric contraction and shortening. In the last four decades, efforts have been made to identify a common fundamental basis for the Frank-Starling heart that can explain the direct relationship between muscle lengthening and its increased sensitization to Ca²⁺. The term 'myofilament length-dependent activation' describes the length-dependent properties of the myofilaments, but what is(are) the underlying molecular mechanism(s) is a matter of ongoing debate. Length-dependent activation increases formation of thick-filament strongly-bound cross-bridges on actin and imposes structural-mechanical alterations on the thin-filament with greater than normal bound Ca²⁺. Stretch-induced effects, rather than changes in filament spacing, appear to be primarily involved in the regulation of length-dependent activation. Here, evidence is provided to support the notion that stretch-mediated effects induced by titin govern alterations of thick-filament force-producing cross-bridges and thin-filament Ca²⁺-cooperative responses.

Switching Muscles On and Off in Steps: The McKillop-Geeves Three-State Model of Muscle Regulation

BJ Classic highlighting the article "Regulation of the interaction between actin and myosin subfragment 1: evidence for three states of the thin filament."

Molecular mechanisms of deregulation of the thin filament associated with the R167H and K168E substitutions in tropomyosin Tpm1.1

Point mutations R167H and K168E in tropomyosin Tpm1.1 (TM) disturb Ca²⁺-dependent regulation of the actomyosin ATPase. To understand mechanisms of this defect we studied multistep changes in mobility and spatial arrangement of tropomyosin, actin and myosin heads during the ATPase cycle in reconstituted ghost fibres using the polarized fluorescence microscopy. It was found that both mutations disturbed the mode of troponin operation in the fibres. At high Ca²⁺, troponin increased the fraction of actin monomers that were in the "switched on" state, but both mutant tropomyosins were shifted toward the outer actin domains, which decreased the fraction of strongly bound myosin heads throughout the ATPase cycle. At low Ca²⁺, the R167H-TM was located close to the outer actin domains, which reduced the number of strongly-bound myosin heads. However, under these conditions troponin increased the number of actin monomers that were switched on. The K168E-TM was displaced far to the outer actin domains and troponin binding decreased the fraction of switched on actin monomers, but the proportion of the strongly bound myosin heads was abnormally high. Thus, the mutations differently disturbed transmission of conformational changes between troponin, tropomyosin and actin, which is essential for the Са²⁺-dependent regulation of the thin filament.

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.

The Importance of Intrinsically Disordered Segments of Cardiac Troponin in Modulating Function by Phosphorylation and Disease-Causing Mutations

Troponin plays a central role in regulation of muscle contraction. It is the Ca²⁺ switch of striated muscles including the heart and in the cardiac muscle it is physiologically modulated by PKA-dependent phosphorylation at Ser22 and 23. Many cardiomyopathy-related mutations affect Ca²⁺ regulation and/or disrupt the relationship between Ca²⁺ binding and phosphorylation. Unlike the mechanism of heart activation, the modulation of Ca²⁺-sensitivity by phosphorylation of the cardiac specific N-terminal segment of TnI (1–30) is structurally subtle and has proven hard to investigate. The crystal structure of cardiac troponin describes only the relatively stable core of the molecule and the crucial mobile parts of the molecule are missing including TnI C-terminal region, TnI (1–30), TnI (134–149) (“inhibitory” peptide) and the C-terminal 28 amino acids of TnT that are intrinsically disordered. Recent studies have been performed to answer this matter by building structural models of cardiac troponin in phosphorylated and dephosphorylated states based on peptide NMR studies. Now these have been updated by more recent concepts derived from molecular dynamic simulations treating troponin as a dynamic structure. The emerging model confirms the stable core structure of troponin and the mobile structure of the intrinsically disordered segments. We will discuss how we can describe these segments in terms of dynamic transitions between a small number of states, with the probability distributions being altered by phosphorylation and by HCM or DCM-related mutations that can explain how Ca²⁺-sensitivity is modulated by phosphorylation and the effects of mutations.

Abnormal movement of tropomyosin and response of myosin heads and actin during the ATPase cycle caused by the Arg167His, Arg167Gly and Lys168Glu mutations in TPM1 gene

Amino acid substitutions: Arg167His, Arg167Gly and Lys168Glu, located in a consensus actin-binding site of the striated muscle tropomyosin Tpm1.1 (TM), were used to investigate mechanisms of the thin filament regulation. The azimuthal movement of TM strands on the actin filament and the responses of the myosin heads and actin subunits during the ATPase cycle were studied using fluorescence polarization of muscle fibres. The recombinant wild-type and mutant TMs labelled with 5-IAF, 1,5-IAEDANS-labelled S1and FITC-phalloidin F-actin were incorporated into the ghost muscle fibres to acquire information on the orientation of the probes relative to the fibre axis. The substitutions Arg167Gly and Lys168Glu shifted TM strands into the actin filament centre, whereas Arg167His moved TM towards the periphery of the filament. In the presence of Arg167Gly-TM and Lys168Glu-TM the fraction of actin monomers that were switched on and the number of the myosin heads strongly bound to F-actin were abnormally high even under conditions close to relaxation. In contrast, Arg167His-TM decreased the fraction of switched on actin and reduced the formation of strongly bound myosin heads throughout the ATPase cycle. We concluded that the altered TM-actin contacts destabilized the thin filament and affected the actin-myosin interactions.

Thin Filament Structure and the Steric Blocking Model

By interacting with the troponin-tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling "steric model" began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin-binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X-ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three-dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low-Ca2+, tropomyosin was shown to occupy a "blocked-state" position on the filament, and switched-on in a two-step process, involving first a movement of tropomyosin away from the majority of the myosin-binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+-regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model.

Structural determinants of muscle thin filament cooperativity

End-to-end connections between adjacent tropomyosin molecules along the muscle thin filament allow long-range conformational rearrangement of the multicomponent filament structure. This process is influenced by Ca(2+) and the troponin regulatory complexes, as well as by myosin crossbridge heads that bind to and activate the filament. Access of myosin crossbridges onto actin is gated by tropomyosin, and in the case of striated muscle filaments, troponin acts as a gatekeeper. The resulting tropomyosin-troponin-myosin on-off switching mechanism that controls muscle contractility is a complex cooperative and dynamic system with highly nonlinear behavior. Here, we review key information that leads us to view tropomyosin as central to the communication pathway that coordinates the multifaceted effectors that modulate and tune striated muscle contraction. We posit that an understanding of this communication pathway provides a framework for more in-depth mechanistic characterization of myopathy-associated mutational perturbations currently under investigation by many research groups.

Historical perspective on heart function: the Frank-Starling Law

More than a century of research on the Frank-Starling Law has significantly advanced our knowledge about the working heart. The Frank-Starling Law mandates that the heart is able to match cardiac ejection to the dynamic changes occurring in ventricular filling and thereby regulates ventricular contraction and ejection. Significant efforts have been attempted to identify a common fundamental basis for the Frank-Starling heart and, although a unifying idea has still to come forth, there is mounting evidence of a direct relationship between length changes in individual constituents (cardiomyocytes) and their sensitivity to Ca²⁺ ions. As the Frank-Starling Law is a vital event for the healthy heart, it is of utmost importance to understand its mechanical basis in order to optimize and organize therapeutic strategies to rescue the failing human heart. The present review is a historic perspective on cardiac muscle function. We "revive" a century of scientific research on the heart's fundamental protein constituents (contractile proteins), to their assemblies in the muscle (the sarcomeres), culminating in a thorough overview of the several synergistically events that compose the Frank-Starling mechanism. It is the authors' personal beliefs that much can be gained by understanding the Frank-Starling relationship at the cellular and whole organ level, so that we can finally, in this century, tackle the pathophysiologic mechanisms underlying heart failure.

Tropomyosin as a Regulator of Actin Dynamics

Tropomyosin is a major regulatory protein of contractile systems and cytoskeleton, an actin-binding protein that positions laterally along actin filaments and modulates actin-myosin interaction. About 40 tropomyosin isoforms have been found in a variety of cytoskeleton systems, not necessarily connected with actin-myosin interaction and contraction. Involvement of specific tropomyosin isoforms in the regulation of key cell processes was shown, and specific features of tropomyosin genes and protein structure have been investigated with molecular biology and genetics approaches. However, the mechanisms underlying the effects of tropomyosin on cytoskeleton dynamics are still unclear. As tropomyosin is primarily an F-actin-binding protein, it is important to understand how it interacts both with actin and actin-binding proteins functioning in muscles and cytoskeleton to regulate actin dynamics. This review focuses on biochemical data on the effects of tropomyosin on actin assembly and dynamics, as well as on the modulation of these effects by actin-binding proteins. The data indicate that tropomyosin can efficiently regulate actin dynamics via allosteric conformational changes within actin filaments.

The force dependence of isometric and concentric potentiation in mouse muscle with and without skeletal myosin light chain kinase

The isometric potentiation associated with myosin phosphorylation is force dependent. The purpose of this study was to assess the influence of a pre-existing period of isometric force on the concentric force potentiation displayed by mouse muscles with and without the ability to phosphorylate myosin. We tested isometric (ISO) and concentric (CON) potentiation, as well as concentric potentiation after isometric force (ISO-CON), in muscles from wild-type (WT) and skeletal myosin light chain kinase-deficient (skMLCK−/−) mice. A conditioning stimulus increased (i.e., potentiated) mean concentric force in the ISO-CON and CON conditions to 1.31 ± 0.02 and 1.35 ± 0.02 (WT) and to 1.19 ± 0.02 and 1.21 ± 0.01 (skMLCK−/−) of prestimulus levels, respectively (data n = 6–8, p < 0.05). No potentiation of mean isometric force was observed in either genotype. The potentiation of mean concentric force was inversely related to relative tetanic force level (P/Po) in both genotypes. Moreover, concentric potentiation varied greatly within each contraction type and was negatively correlated with unpotentiated force in both genotypes. Thus, although no effect of pre-existing force was observed, strong and inverse relationships between concentric force potentiation and unpotentiated concentric force may suggest an influence of attached and force-generating crossbridges on potentiation magnitude in both WT and skMLCK−/− muscles.

Mechanism of the calcium-regulation of muscle contraction--in pursuit of its structural basis

The author reviewed the research that led to establish the structural basis for the mechanism of the calcium-regulation of the contraction of striated muscles. The target of calcium ions is troponin on the thin filaments, of which the main component is the double-stranded helix of actin. A model of thin filament was generated by adding tropomyosin and troponin. During the process to provide the structural evidence for the model, the troponin arm was found to protrude from the calcium-depleted troponin and binds to the carboxyl-terminal region of actin. As a result, the carboxyl-terminal region of tropomyosin shifts and covers the myosin-binding sites of actin to block the binding of myosin. At higher calcium concentrations, the troponin arm changes its partner from actin to the main body of calcium-loaded troponin. Then, tropomyosin shifts back to the position near the grooves of actin double helix, and the myosin-binding sites of actin becomes available to myosin resulting in force generation through actin-myosin interactions.

Structure of the F-actin-tropomyosin complex

Filamentous actin (F-actin) is the major protein of muscle thin filaments, and actin microfilaments are the main component of the eukaryotic cytoskeleton. Mutations in different actin isoforms lead to early-onset autosomal dominant non-syndromic hearing loss, familial thoracic aortic aneurysms and dissections, and multiple variations of myopathies. In striated muscle fibres, the binding of myosin motors to actin filaments is mainly regulated by tropomyosin and troponin. Tropomyosin also binds to F-actin in smooth muscle and in non-muscle cells and stabilizes and regulates the filaments there in the absence of troponin. Although crystal structures for monomeric actin (G-actin) are available, a high-resolution structure of F-actin is still missing, hampering our understanding of how disease-causing mutations affect the function of thin muscle filaments and microfilaments. Here we report the three-dimensional structure of F-actin at a resolution of 3.7 Å in complex with tropomyosin at a resolution of 6.5 Å, determined by electron cryomicroscopy. The structure reveals that the D-loop is ordered and acts as a central region for hydrophobic and electrostatic interactions that stabilize the F-actin filament. We clearly identify map density corresponding to ADP and Mg(2+) and explain the possible effect of prominent disease-causing mutants. A comparison of F-actin with G-actin reveals the conformational changes during filament formation and identifies the D-loop as their key mediator. We also confirm that negatively charged tropomyosin interacts with a positively charged groove on F-actin. Comparison of the position of tropomyosin in F-actin-tropomyosin with its position in our previously determined F-actin-tropomyosin-myosin structure reveals a myosin-induced transition of tropomyosin. Our results allow us to understand the role of individual mutations in the genesis of actin- and tropomyosin-related diseases and will serve as a strong foundation for the targeted development of drugs.

Using fluorescent myosin to directly visualize cooperative activation of thin filaments

Contraction of striated muscle is tightly regulated by the release and sequestration of calcium within myocytes. At the molecular level, calcium modulates myosin's access to the thin filament. Once bound, myosin is hypothesized to potentiate the binding of further myosins. Here, we directly image single molecules of myosin binding to and activating thin filaments. Using this approach, the cooperative binding of myosin along thin filaments has been quantified. We have found that two myosin heads are required to laterally activate a regulatory unit of thin filament. The regulatory unit is found to be capable of accommodating 11 additional myosins. Three thin filament activation states possessing differential myosin binding capacities are also visible. To describe this system, we have formulated a simple chemical kinetic model of cooperative activation that holds across a wide range of solution conditions. The stochastic nature of activation is strongly highlighted by data obtained in sub-optimal activation conditions where the generation of activation waves and their catastrophic collapse can be observed. This suggests that the thin filament has the potential to be turned fully on or off in a binary fashion.

Calcium-dependent interaction sites of tropomyosin on reconstituted muscle thin filaments with bound Myosin heads as studied by site-directed spin-labeling

To identify the interaction sites of Tm, we measured the rotational motion of a spin-label covalently bound to the side chain of a cysteine that was genetically incorporated into rabbit skeletal muscle tropomyosin (Tm) at positions 13, 36, 146, 160, 174, 190, 209, 230, 271, or 279. Most of the Tm residues were immobilized on actin filaments with myosin-S1 bound to them. The residues in the mid-portion of Tm, namely, 146, 174, 190, 209, and 230, were mobilized when the troponin (Tn) complex bound to the actin-Tm-S1 filaments. The addition of Ca(2+) ions partially reversed the Tn-induced mobilization. In contrast, residues at the joint region of Tm, 13, 36, 271, and 279 were unchanged or oppositely changed. All of these changes were detected using a maleimide spin label and less obviously using a methanesulfonate label. These results indicated that Tm was fixed on thin filaments with myosin bound to them, although a small change in the flexibility of the side chains of Tm residues, presumably interfaced with Tn, actin and myosin, was induced by the binding of Tn and Ca(2+). These findings suggest that even in the myosin-bound (open) state, Ca(2+) may regulate actomyosin contractile properties via Tm.

Instability in the central region of tropomyosin modulates the function of its overlapping ends

The causal link between disparate tropomyosin (Tm) functions and the structural instability in Tm is unknown. To test the hypothesis that the structural instability in the central region of Tm modulates the function of the overlapping ends of contiguous Tm dimers, we used transgenic mice (Tm(DM)) that expressed a mutant α-Tm in the heart; S229E and H276N substitutions induce structural instability in the central region and the overlapping ends of Tm, respectively. In addition, two mouse cardiac troponin T mutants (TnT(1-44Δ) and TnT(45-74Δ)) that have a divergent effect on the overlapping ends of Tm were employed. The S229E-induced instability in the central region of Tm(DM) altered the overlapping ends of Tm(DM), thereby it negated the attenuating effect of H276N on Ca(2+)-activated maximal tension. The rate of cross-bridge detachment (g) decreased in Tm(DM)+TnT(WT) and Tm(H276N)+TnT(WT) fibers but increased in Tm(DM)+TnT(45-74Δ) fibers; however, TnT(45-74Δ) did not alter g, demonstrating that S229E in Tm(DM) had divergent effects on g. The S229E substitution in Tm(DM) ablated the H276N-induced desensitization of myofilament Ca(2+) sensitivity in Tm(DM)+TnT(1-44Δ) fibers. To our knowledge, novel findings from this study show that the structural instability in the central region of Tm modifies cardiac contractile function via its effect on the overlapping ends of contiguous Tm.

Tropomyosin dynamics

Tropomyosin is a two chained α-helical coiled coil protein that binds actin filaments and interacts with various actin binding proteins. Tropomyosin function depends on its ability to move to distinct locations on the surface of actin in response to the binding of different thin filament effectors. Tropomyosin dynamics plays an important role in these fluctuating interactions with actin and is thought to be fundamental to many of its biological activities. For example tropomyosin concerted movement on the surface of actin triggered by Ca(2+) binding to troponin or myosin head binding to actin has been argued to be key to the cooperative allosteric regulation of muscle contraction. These large-scale motions are affected by tropomyosin internal dynamics and mechanical properties. Tropomyosin internal dynamics corresponding to smaller and more localised structural fluctuations are increasingly recognised to play an important role in its function. A thorough understanding of the coupling between local and global structural fluctuations in tropomyosin is required to understand how time dependent structural fluctuations in tropomyosin contribute to the overall thin filament dynamics and dictate their various biological activities.

Tropomyosin movement on F-actin during muscle activation explained by energy landscapes

Muscle contraction is regulated by tropomyosin movement across the thin filament surface, which exposes or blocks myosin-binding sites on actin. Recent atomic structures of F-actin-tropomyosin have yielded the positions of tropomyosin on myosin-free and myosin-decorated actin. Here, the repositioning of α-tropomyosin between these locations on F-actin was systematically examined by optimizing the energy of the complex for a wide range of tropomyosin positions on F-actin. The resulting energy landscape provides a full-map of the F-actin surface preferred by tropomyosin, revealing a broad energy basin associated with the tropomyosin position that blocks myosin-binding. This is consistent with previously proposed low-energy oscillations of semi-rigid tropomyosin, necessary for shifting of tropomyosin following troponin-binding. In contrast, the landscape shows much less favorable energies when tropomyosin locates near its myosin-induced "open-state" position. This indicates that spontaneous movement of tropomyosin away from its energetic "ground-state" to the open-state is unlikely in absence of myosin. Instead, myosin-binding must drive tropomyosin toward the open-state to activate the thin filament. Additional energy landscapes were computed for disease-causing actin mutants that distort the topology of the actin-tropomyosin energy landscape, explaining their phenotypes. Thus, the computation of such energy landscapes offers a sensitive way to estimate the impact of mutations.

Cryo-EM structures of the actin:tropomyosin filament reveal the mechanism for the transition from C- to M-state

Tropomyosin (Tm) is a key factor in the molecular mechanisms that regulate the binding of myosin motors to actin filaments (F-Actins) in most eukaryotic cells. This regulation is achieved by the azimuthal repositioning of Tm along the actin (Ac):Tm:troponin (Tn) thin filament to block or expose myosin binding sites on Ac. In striated muscle, including involuntary cardiac muscle, Tm regulates muscle contraction by coupling Ca(2+) binding to Tn with myosin binding to the thin filament. In smooth muscle, the switch is the posttranslational modification of the myosin. Depending on the activation state of Tn and the binding state of myosin, Tm can occupy the blocked, closed, or open position on Ac. Using native cryogenic 3DEM (three-dimensional electron microscopy), we have directly resolved and visualized cardiac and gizzard muscle Tm on filamentous Ac in the position that corresponds to the closed state. From the 8-Å-resolution structure of the reconstituted Ac:Tm filament formed with gizzard-derived Tm, we discuss two possible mechanisms for the transition from closed to open state and describe the role Tm plays in blocking myosin tight binding in the closed-state position.

The nemaline myopathy-causing E117K mutation in β-tropomyosin reduces thin filament activation

The effect of the nemaline myopathy-causing E117K mutation in β-tropomyosin (TM) on the structure and function of this regulatory protein was studied. The E117K mutant was found to have indistinguishable actin affinity compared with wild-type (WT) and similar secondary structure as measured by circular dichroism. However the E117K mutation significantly lowered maximum activation of actomyosin ATPase. To explain the molecular mechanism of impaired ATPase activation, WT and E117K TMs were covalently labeled at Cys-36 with 5-iodoacetimido-fluorescein and incorporated into ghost muscle fibers. The changes in the position and flexibility of tropomyosin strands on the thin filaments were observed at simulation of weak and strong binding states of actomyosin at high or low Ca(2+) by polarized fluorescence techniques. The E117K mutation was found to shift the tropomyosin strands towards the closed position and restrict the tropomyosin displacement during the transformation of actomyosin from weak to strong binding state thus leading to a reduction in thin filament activation.

Structural and functional consequences of cardiac troponin C L57Q and I61Q Ca(2+)-desensitizing variants

Two cTnC variants, L57Q and I61Q, both of which are located on helix C within the N domain of cTnC, were originally reported in the skeletal muscle system [Tikunova, Davis, J. Biol. Chem. 279 (2004) 35341-35352], as the analogous L58Q and I62Q sTnC, and demonstrated a decreased Ca(2+) binding affinity. Here, we provide detailed characterization of structure-function relationships for these two cTnC variants, to determine if they behave differently in the cardiac system and as a framework for determining similarities and differences with other cTnC mutations that have been associated with DCM. We have used an integrative approach to study the structure and function of these cTnC variants both in solution and in silico, to understand how the L57Q and I61Q mutations influence Ca(2+) binding at site II, the subsequent effects on the interaction with cTnI, and the structural changes which are associated with these changes. Steady-state and stopped flow fluorescence spectroscopy confirmed that a decrease in Ca(2+) affinity for recombinant cTnC and cTn complexes containing the L57Q or I61Q variants. The L57Q variant was intermediate between WT and I61Q cTnC and also did not significantly alter cTnC-cTnI interaction in the absence of Ca(2+), but did decrease the interaction in the presence of Ca(2+). In contrast, I61Q decreased the cTnC-cTnI interaction in both the absence and presence of Ca(2+). This difference in the absence of Ca(2+) suggests a greater structural change in cNTnC may occur with the I61Q mutation than the L57Q mutation. MD simulations revealed that the decreased Ca(2+) binding induced by I61Q may result from destabilization of the Ca(2+) binding site through interruption of intra-molecular interactions when residue 61 forms new hydrogen bonds with G70 on the Ca(2+) binding loop. The experimentally observed interruption of the cTnC-cTnI interaction caused by L57Q or I61Q is due to the disruption of key hydrophobic interactions between helices B and C in cNTnC. This study provides a molecular basis of how single mutations in the C helix of cTnC can reduce Ca(2+) binding affinity and cTnC-cTnI interaction, which may provide useful insights for a better understanding of cardiomyopathies and future gene-based therapies.

Potentiation in mouse lumbrical muscle without myosin light chain phosphorylation: is resting calcium responsible?

The increase in isometric twitch force observed in fast-twitch rodent muscles during or after activity, known universally as potentiation, is normally associated with myosin regulatory light chain (RLC) phosphorylation. Interestingly, fast muscles from mice devoid of detectable skeletal myosin light chain kinase (skMLCK) retain a reduced ability to potentiate twitch force, indicating the presence of a secondary origin for this characteristic feature of the fast muscle phenotype. The purpose of this study was to assess changes in intracellular cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) after a potentiating stimulus in mouse lumbrical muscle (37°C). Lumbricals were loaded with the Ca²⁺-sensitive fluorescent indicators fura-2 or furaptra to detect changes in resting and peak, respectively, intracellular Ca²⁺ levels caused by 2.5 s of 20-Hz stimulation. Although this protocol produced an immediate increase in twitch force of 17 ± 3% (all data are n = 10) (P < 0.01), this potentiation dissipated quickly and was absent 30 s afterward. Fura-2 fluorescence signals at rest were increased by 11.1 ± 1.3% (P < 0.01) during potentiation, indicating a significant increase in resting [Ca²⁺]_i. Interestingly, furaptra signals showed no change to either the amplitude or the duration of the intracellular Ca²⁺ transients (ICTs) that triggered potentiated twitches during this time (P < 0.50). Immunofluorescence work showed that 77% of lumbrical fibers expressed myosin heavy chain isoform IIx and/or IIb, but with low expression of skMLCK and high expression of myosin phosphatase targeting subunit 2. As a result, lumbrical muscles displayed no detectable RLC phosphorylation either at rest or after stimulation. We conclude that stimulation-induced elevations in resting [Ca²⁺]_i, in the absence of change in the ICT, are responsible for a small-magnitude, short-lived potentiation of isometric twitch force. If operative in other fast-twitch muscles, this mechanism may complement the potentiating influence of myosin RLC phosphorylation.

Kinetic mechanism of Ca²⁺-controlled changes of skeletal troponin I in psoas myofibrils

Conformational changes in the skeletal troponin complex (sTn) induced by rapidly increasing or decreasing the [Ca(2+)] were probed by 5-iodoacetamidofluorescein covalently bound to Cys-133 of skeletal troponin I (sTnI). Kinetics of conformational changes was determined for the isolated complex and after incorporating the complex into rabbit psoas myofibrils. Isolated and incorporated sTn exhibited biphasic Ca(2+)-activation kinetics. Whereas the fast phase (k(obs)∼1000 s(-1)) is only observed in this study, where kinetics were induced by Ca(2+), the slower phase resembles the monophasic kinetics of sTnI switching observed in another study (Brenner and Chalovich. 1999. Biophys. J. 77:2692-2708) that investigated the sTnI switching induced by releasing the feedback of force-generating cross-bridges on thin filament activation. Therefore, the slower conformational change likely reflects the sTnI switch that regulates force development. Modeling reveals that the fast conformational change can occur after the first Ca(2+) ion binds to skeletal troponin C (sTnC), whereas the slower change requires Ca(2+) binding to both regulatory sites of sTnC. Incorporating sTn into myofibrils increased the off-rate and lowered the Ca(2+) sensitivity of sTnI switching. Comparison of switch-off kinetics with myofibril force relaxation kinetics measured in a mechanical setup indicates that sTnI switching might limit the rate of fast skeletal muscle relaxation.

Slowed Dynamics of Thin Filament Regulatory Units Reduces Ca2+-Sensitivity of Cardiac Biomechanical Function

Actomyosin kinetics in both skinned skeletal muscle fibers at maximum Ca²⁺-activation and unregulated in vitro motility assays are modulated by solvent microviscosity in a manner consistent with a diffusion limited process. Viscosity might also influence cardiac thin filament Ca²⁺-regulatory protein dynamics. In vitro motility assays were conducted using thin filaments reconstituted with recombinant human cardiac troponin and tropomyosin; solvent microviscosity was varied by addition of sucrose or glucose. At saturating Ca²⁺, filament sliding speed (s) was inversely proportional to viscosity. Ca²⁺-sensitivity (pCa₅₀ ) of s decreased markedly with elevated viscosity (η/η₀ ≥ ~1.3). For comparison with unloaded motility assays, steady-state isometric force (F) and kinetics of isometric tension redevelopment (k_TR ) were measured in single, permeabilized porcine cardiomyocytes when viscosity surrounding the myofilaments was altered. Maximum Ca²⁺-activated F changed little for sucrose ≤ 0.3 M (η/η₀ ~1.4) or glucose ≤ 0.875 M (η/η₀ ~1.66), but decreased at higher concentrations. Sucrose (0.3 M) or glucose (0.875 M) decreased pCa₅₀ for F. k_TR at saturating Ca²⁺ decreased steeply and monotonically with increased viscosity but there was little effect on k_TR at sub-maximum Ca²⁺. Modeling indicates that increased solutes affect dynamics of cardiac muscle Ca²⁺-regulatory proteins to a much greater extent than actomyosin cross-bridge cycling.

α-Tropomyosin with a D175N or E180G mutation in only one chain differs from tropomyosin with mutations in both chains

α-Tropomyosin (Tm) carrying hypertrophic cardiomyopathy mutation D175N or E180G was expressed in Escherichia coli. We have assembled dimers of two polypeptide chains in vitro that carry one (αα*) or two (α*α*) copies of the mutation. We found that the presence of the mutation has little effect on dimer assembly, thereby predicting that individuals heterozygous for the Tm mutations are likely to express both αα* and α*α* Tm. Depending on the expression level, the heterodimer may be the predominant form in individuals carrying the mutation. Thus, it is important to define differences in the properties of Tm molecules carrying one or two copies of the mutation. We examined the Tm homo- and heterodimer properties: actin affinity, thermal stability, calcium regulation of myosin subfragment 1 binding, and calcium regulation of myofibril force. We report that the properties of the heterodimer may be similar to those of the wild-type homodimer (actin affinity, thermal stability, D175N αα*), similar to those of the mutant homodimer (calcium sensitivity, D175N αα*), intermediate between the two (actin affinity, E180G αα*), or different from both (thermal stability, E180G αα*). Thus, the properties of the homodimer are not a completely reliable guide to the properties of the heterodimer.

Significance of troponin dynamics for Ca2+-mediated regulation of contraction and inherited cardiomyopathy

Ca(2+) dissociation from troponin causes cessation of muscle contraction by incompletely understood structural mechanisms. To investigate this process, regulatory site Ca(2+) binding in the NH(2)-lobe of subunit troponin C (TnC) was abolished by mutagenesis, and effects on cardiac troponin dynamics were mapped by hydrogen-deuterium exchange (HDX)-MS. The findings demonstrate the interrelationships among troponin's detailed dynamics, troponin's regulatory actions, and the pathogenesis of cardiomyopathy linked to troponin mutations. Ca(2+) slowed HDX up to 2 orders of magnitude within the NH(2)-lobe and the NH(2)-lobe-associated TnI switch helix, implying that Ca(2+) greatly stabilizes this troponin regulatory region. HDX of the TnI COOH terminus indicated that its known role in regulation involves a partially folded rather than unfolded structure in the absence of Ca(2+) and actin. Ca(2+)-triggered stabilization extended beyond the known direct regulatory regions: to the start of the nearby TnI helix 1 and to the COOH terminus of the TnT-TnI coiled-coil. Ca(2+) destabilized rather than stabilized specific TnI segments within the coiled-coil and destabilized a region not previously implicated in Ca(2+)-mediated regulation: the coiled-coil's NH(2)-terminal base plus the preceding TnI loop with which the base interacts. Cardiomyopathy-linked mutations clustered almost entirely within influentially dynamic regions of troponin, and many sites were Ca(2+)-sensitive. Overall, the findings demonstrate highly selective effects of regulatory site Ca(2+), including opposite changes in protein dynamics at opposite ends of the troponin core domain. Ca(2+) release triggers an intramolecular switching mechanism that propagates extensively within the extended troponin structure, suggests specific movements of the TnI inhibitory regions, and prominently involves troponin's dynamic features.

Cooperative regulation of myosin-S1 binding to actin filaments by a continuous flexible Tm-Tn chain

The regulation of striated muscle contraction involves cooperative interactions between actin filaments, myosin-S1 (S1), tropomyosin (Tm), troponin (Tn), and calcium. These interactions are modeled by treating overlapping tropomyosins as a continuous flexible chain (CFC), weakly confined by electrostatic interactions with actin. The CFC is displaced locally in opposite directions on the actin surface by the binding of either S1 or Troponin I (TnI) to actin. The apparent rate constants for myosin and TnI binding to and detachment from actin are then intrinsically coupled via the CFC model to the presence of neighboring bound S1s and TnIs. Monte Carlo simulations at prescribed values of the CFC stiffness, the CFC’s degree of azimuthal confinement, and the angular displacements caused by the bound proteins were able to predict the stopped-flow transients of S1 binding to regulated F-actin. The transients collected over a large range of calcium concentrations could be well described by adjusting a single calcium-dependent parameter, the rate constant of TnI detachment from actin, k_−I. The resulting equilibrium constant \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ K_{\text{B}} \equiv 1/K_{\text{I}} $$\end{document} varied sigmoidally with the free calcium, increasing from 0.12 at low calcium (pCa >7) to 12 at high calcium (pCa <5.5) with a Hill coefficient of ~2.15. The similarity of the curves for excess-actin and excess-myosin data confirms their allosteric relationship. The spatially explicit calculations confirmed variable sizes for the cooperative units and clustering of bound myosins at low calcium concentrations. Moreover, inclusion of negative cooperativity between myosin units predicted the observed slowing of myosin binding at excess-myosin concentrations.

Tropomyosin: double helix from the protein world

This review concerns the structure and functions of tropomyosin (TM), an actin-binding protein that plays a key role in the regulation of muscle contraction. The TM molecule is a dimer of α-helices, which form a coiled-coil. Recent views on the TM structure are analyzed, and special attention is concentrated on those structural traits of the TM molecule that distinguish it from the other coiled-coil proteins. Modern data are presented on TM functional properties, such as its interaction with actin and ability to move on the surface of actin filaments, which underlies the regulation of the actin-myosin interaction upon contraction of skeletal and cardiac muscles. Also, part of the review is devoted to analysis of the effects of mutations in TM genes associated with muscle diseases (myopathies) on the structure and functions of TM.

Filament compliance influences cooperative activation of thin filaments and the dynamics of force production in skeletal muscle

Striated muscle contraction is a highly cooperative process initiated by Ca²⁺ binding to the troponin complex, which leads to tropomyosin movement and myosin cross-bridge (XB) formation along thin filaments. Experimental and computational studies suggest skeletal muscle fiber activation is greatly augmented by cooperative interactions between neighboring thin filament regulatory units (RU-RU cooperativity; 1 RU = 7 actin monomers+1 troponin complex+1 tropomyosin molecule). XB binding can also amplify thin filament activation through interactions with RUs (XB-RU cooperativity). Because these interactions occur with a temporal order, they can be considered kinetic forms of cooperativity. Our previous spatially-explicit models illustrated that mechanical forms of cooperativity also exist, arising from XB-induced XB binding (XB-XB cooperativity). These mechanical and kinetic forms of cooperativity are likely coordinated during muscle contraction, but the relative contribution from each of these mechanisms is difficult to separate experimentally. To investigate these contributions we built a multi-filament model of the half sarcomere, allowing RU activation kinetics to vary with the state of neighboring RUs or XBs. Simulations suggest Ca²⁺ binding to troponin activates a thin filament distance spanning 9 to 11 actins and coupled RU-RU interactions dominate the cooperative force response in skeletal muscle, consistent with measurements from rabbit psoas fibers. XB binding was critical for stabilizing thin filament activation, particularly at submaximal Ca²⁺ levels, even though XB-RU cooperativity amplified force less than RU-RU cooperativity. Similar to previous studies, XB-XB cooperativity scaled inversely with lattice stiffness, leading to slower rates of force development as stiffness decreased. Including RU-RU and XB-RU cooperativity in this model resulted in the novel prediction that the force-[Ca²⁺] relationship can vary due to filament and XB compliance. Simulations also suggest kinetic forms of cooperativity occur rapidly and dominate early to get activation, while mechanical forms of cooperativity act more slowly, augmenting XB binding as force continues to develop.

In striated muscle myosin binds to actin and converts chemical energy from ATP hydrolysis into force, work, and power. Myosin cross-bridge binding is regulated by Ca²⁺ and the thin filament proteins troponin and tropomyosin. Cooperative interactions between actin, myosin, troponin, and tropomyosin greatly influence spatial and kinetic properties of thin filament activation, thereby affecting muscle mechanics and contractility. Such cooperative interactions are complex and individual contributions from the different contractile and regulatory proteins are difficult to separate experimentally. However, a few theoretical models have explored interactions between the spatial, kinetic, and mechanical processes that affect cooperative cross-bridge binding to actin. Building on our prior spatially-explicit computational models, we investigated the relative contributions of thin filament regulatory proteins and cross-bridges to cooperatively amplify skeletal muscle force production. We find that Ca²⁺-dependent contraction in skeletal muscle is dominated by neighboring regulatory protein interactions along the thin filament, while cross-bridge binding is critical for maintaining or stabilizing thin filament activation as force develops. Moreover, we reveal that variations in filament and cross-bridge stiffness can alter Ca²⁺-sensitivity and cooperativity of skeletal muscle force production. In conclusion, these simulations show that multiple cooperative mechanisms combine to produce physiological force responses measured from muscle cells.

Tropomyosin flexural rigidity and single ca(2+) regulatory unit dynamics: implications for cooperative regulation of cardiac muscle contraction and cardiomyocyte hypertrophy

Striated muscle contraction is regulated by dynamic and cooperative interactions among Ca²⁺, troponin, and tropomyosin on the thin filament. While Ca²⁺ regulation has been extensively studied, little is known about the dynamics of individual regulatory units and structural changes of individual tropomyosin molecules in relation to their mechanical properties, and how these factors are altered by cardiomyopathy mutations in the Ca²⁺ regulatory proteins. In this hypothesis paper, we explore how various experimental and analytical approaches could broaden our understanding of the cooperative regulation of cardiac contraction in health and disease.

The Hill model for binding myosin S1 to regulated actin is not equivalent to the McKillop-Geeves model

The Hill two-state cooperativity model and the McKillop-Geeves (McK-G) three-state model predict very similar binding traces of myosin subfragment 1 (S1) binding to regulated actin filaments in the presence and absence of calcium, and both fit the experimental data reasonably well [Chen et al., Biophys. J., 80, 2338-2349]. Here, we compared the Hill model and the McK-G model for binding myosin S1 to regulated actin against three sets of experimental data: the titration of regulated actin with S1 and the kinetics of S1 binding of regulated actin with either excess S1 to actin or excess actin to S1. Each data set was collected for a wide range of specified calcium concentrations. Both models were able to generate reasonable fits to the time course data and to titration data. The McK-G model can fit all three data sets with the same calcium-concentration-sensitive parameters. Only K(B) and K(T) show significant calcium dependence, and the parameters have a classic pCa curve. A unique set of the Hill model parameters was extremely difficult to estimate from the best fits of multiple sets of data. In summary, the McK-G cooperativity model more uniquely resolves parameters estimated from kinetic and titration data than the Hill model, predicts a sigmoidal dependence of key parameters with calcium concentration, and is simpler and more suitable for practical use.

Structural dynamics of C-domain of cardiac troponin I protein in reconstituted thin filament

The regulatory function of cardiac troponin I (cTnI) involves three important contiguous regions within its C-domain: the inhibitory region (IR), the regulatory region (RR), and the mobile domain (MD). Within these regions, the dynamics of regional structure and kinetics of transitions in dynamic state are believed to facilitate regulatory signaling. This study was designed to use fluorescence anisotropy techniques to acquire steady-state and kinetic information on the dynamic state of the C-domain of cTnI in the reconstituted thin filament. A series of single cysteine cTnI mutants was generated, labeled with the fluorophore tetramethylrhodamine, and subjected to various anisotropy experiments at the thin filament level. The structure of the IR was found to be less dynamic than that of the RR and the MD, and Ca(2+) binding induced minimal changes in IR dynamics: the flexibility of the RR decreased, whereas the MD became more flexible. Anisotropy stopped-flow experiments showed that the kinetics describing the transition of the MD and RR from the Ca(2+)-bound to the Ca(2+)-free dynamic states were significantly faster (53.2-116.8 s(-1)) than that of the IR (14.1 s(-1)). Our results support the fly casting mechanism, implying that an unstructured MD with rapid dynamics and kinetics plays a critical role to initiate relaxation upon Ca(2+) dissociation by rapidly interacting with actin to promote the dissociation of the RR from the N-domain of cTnC. In contrast, the IR responds to Ca(2+) signals with slow structural dynamics and transition kinetics. The collective findings suggested a fourth state of activation.

The effect of the Asp175Asn and Glu180Gly TPM1 mutations on actin-myosin interaction during the ATPase cycle

Hypertrophic cardiomyopathy (HCM), characterized by cardiac hypertrophy and contractile dysfunction, is a major cause of heart failure. HCM can result from mutations in the gene encoding cardiac α-tropomyosin (TM). To understand how the HCM-causing Asp175Asn and Glu180Gly mutations in α-tropomyosin affect on actin-myosin interaction during the ATPase cycle, we labeled the SH1 helix of myosin subfragment-1 and the actin subdomain-1 with the fluorescent probe N-iodoacetyl-N'-(5-sulfo-1-naphtylo)ethylenediamine. These proteins were incorporated into ghost muscle fibers and their conformational states were monitored during the ATPase cycle by measuring polarized fluorescence. For the first time, the effect of these α-tropomyosins on the mobility and rotation of subdomain-1 of actin and the SH1 helix of myosin subfragment-1 during the ATP hydrolysis cycle have been demonstrated directly by polarized fluorimetry. Wild-type α-tropomyosin increases the amplitude of the SH1 helix and subdomain-1 movements during the ATPase cycle, indicating the enhancement of the efficiency of the work of cross-bridges. Both mutant TMs increase the proportion of the strong-binding sub-states, with the effect of the Glu180Gly mutation being greater than that of Asp175Asn. It is suggested that the alteration in the concerted conformational changes of actomyosin is likely to provide the structural basis for the altered cardiac muscle contraction.