Calcium regulation of skeletal muscle thin filament motility in vitro

Mechanical interaction of myosin and native thin filament in the disused rat soleus muscle

The disuse of skeletal limb muscles occurs in a variety of conditions, yet our comprehension of the molecular mechanisms involved in adaptation to disuse remains incomplete. We studied the mechanical characteristics of actin-myosin interaction using an in vitro motility assay and isoform composition of myosin heavy and light chains by dint of SDS-PAGE in soleus muscle of both control and hindlimb-unloaded rats. 14 days of hindlimb unloading led to the increased maximum sliding velocity of actin, reconstituted, and native thin filaments over rat soleus muscle myosin by 24 %, 19 %, and 20 %, respectively. The calcium sensitivity of the "pCa-velocity" relationship decreased. There was a 26 % increase in fast myosin heavy chain IIa (MHC IIa), a 22 % increase in fast myosin light chain 2 (MLC 2f), and a 13 % increase in fast MLC 1f content. The content of MLC 1s/v, typical for slow skeletal muscles and cardiac ventricles did not change. At the same time, MLC 1s, typical only for slow skeletal muscles, disappeared. The maximum velocity of soleus muscle native thin filaments was 24 % higher compared to control ones sliding over the same rabbit myosin. Therefore, both myosin and native thin filament kinetics could influence the mechanical characteristics of the soleus muscle. Additionally, the MLC 1s and MLC 1s/v ratio may contribute to the mechanical characteristics of slow skeletal muscle, along with MHC, MLC 2, and MLC 1 slow/fast isoforms ratio.

Glutamate 139 of tropomyosin is critical for cardiac thin filament blocked-state stabilization

The cardiac thin filament proteins troponin and tropomyosin control actomyosin formation and thus cardiac contractility. Calcium binding to troponin changes tropomyosin position along the thin filament, allowing myosin head binding to actin required for heart muscle contraction. The thin filament regulatory proteins are hot spots for genetic mutations causing heart muscle dysfunction. While much of the thin filament structure has been characterized, critical regions of troponin and tropomyosin involved in triggering conformational changes remain unresolved. A poorly resolved region, helix-4 (H4) of troponin I, is thought to stabilize tropomyosin in a position on actin that blocks actomyosin interactions at low calcium concentrations during muscle relaxation. We have proposed that contact between glutamate 139 on tropomyosin and positively charged residues on H4 leads to blocking-state stabilization. In this study, we attempted to disrupt these interactions by replacing E139 with lysine (E139K) to define the importance of this residue in thin filament regulation. Comparison of mutant and wild-type tropomyosin was carried out using in-vitro motility assays, actin co-sedimentation, and molecular dynamics simulations to determine perturbations in troponin-tropomyosin function caused by the tropomyosin mutation. Motility assays revealed that mutant thin filaments moved at higher velocity at low calcium with increased calcium sensitivity demonstrating that tropomyosin residue 139 is vital for proper tropomyosin-mediated inhibition during relaxation. Similarly, molecular dynamic simulations revealed a mutation-induced decrease in interaction energy between tropomyosin-E139K and troponin I (R170 and K174). These results suggest that salt-bridge stabilization of tropomyosin position by troponin IH4 is essential to prevent actomyosin interactions during cardiac muscle relaxation.

Motility Assay to Probe the Calcium Sensitivity of Myosin and Regulated Thin Filaments

Calcium-dependent activation of the thin filament mediated by the troponin-tropomyosin complex is key in the regulation of actin-myosin based muscle contraction. Perturbations to this system, either physiological (e.g., phosphorylation of myosin light chains) or pathological (e.g., mutations that cause familial cardiomyopathies), can alter calcium sensitivity and thus have important implications in human health and disease. The in vitro motility assay provides a quantitative and precise method to study the calcium sensitivity of the reconstituted myosin-thin filament motile system. Here we present a simple and robust protocol to perform calcium-dependent motility of β-cardiac myosin and regulated thin filaments. The experiment is done on a multichannel microfluidic slide requiring minimal amounts of proteins. A complete velocity vs. calcium concentration curve is produced from one experiment in under 1 h.

Myosin and tropomyosin-troponin complementarily regulate thermal activation of muscles

Contraction of striated muscles is initiated by an increase in cytosolic Ca2+ concentration, which is regulated by tropomyosin and troponin acting on actin filaments at the sarcomere level. Namely, Ca2+-binding to troponin C shifts the "on-off" equilibrium of the thin filament state toward the "on" state, promoting actomyosin interaction; likewise, an increase in temperature to within the body temperature range shifts the equilibrium to the on state, even in the absence of Ca2+. Here, we investigated the temperature dependence of sarcomere shortening along isolated fast skeletal myofibrils using optical heating microscopy. Rapid heating (25 to 41.5°C) within 2 s induced reversible sarcomere shortening in relaxing solution. Further, we investigated the temperature-dependence of the sliding velocity of reconstituted fast skeletal or cardiac thin filaments on fast skeletal or β-cardiac myosin in an in vitro motility assay within the body temperature range. We found that (a) with fast skeletal thin filaments on fast skeletal myosin, the temperature dependence was comparable to that obtained for sarcomere shortening in fast skeletal myofibrils (Q10 ∼8), (b) both types of thin filaments started to slide at lower temperatures on fast skeletal myosin than on β-cardiac myosin, and (c) cardiac thin filaments slid at lower temperatures compared with fast skeletal thin filaments on either type of myosin. Therefore, the mammalian striated muscle may be fine-tuned to contract efficiently via complementary regulation of myosin and tropomyosin-troponin within the body temperature range, depending on the physiological demands of various circumstances.

Molecular mechanisms of mechanical function changes of the rat myocardium under subchronic lead exposure

A moderate degree of lead intoxication was observed in male rats after repeated intraperitoneal injections with two doses of lead acetate three times a week during 5 (12.5 mg of Pb per kg body mass) and 6 (6.01 mg of Pb per kg body mass) weeks. Using an in vitro motility assay, we investigated the impact of this intoxication on the characteristics of actin-myosin interaction and its regulation in the atria, right, and left ventricles. Both lead doses exposure decreased the maximum sliding velocity of reconstituted thin filaments over myosin and fraction of motile filaments in all heart chambers, caused the myosin isoforms shift towards slower β-myosin heavy chains in ventricles and decreased regulatory light chain phosphorylation in atria. No statistically significant difference was found in force and calcium regulation of actin-myosin interaction. A dose-dependent effect of lead on myosin functional characteristics was found in all heart chambers, but the degree of this effect varied depending on the heart chamber.

Structure and Dynamics of the Flexible Cardiac Troponin T Linker Domain in a Fully Reconstituted Thin Filament

The structural analysis of large protein complexes has been greatly enhanced through the application of electron microscopy techniques. One such multiprotein complex, the cardiac thin filament (cTF), has cyclic interactions with thick filament proteins to drive contraction of the heart that has recently been the subject of such studies. As important as these studies are, they provide limited or no information on highly flexible regions that in isolation would be characterized as inherently disordered. One such region is the extended cardiac troponin T (cTnT) linker between the regions of cTnT which have been labeled TNT1 and TNT2. It comprises a hinge region (residues 158-166) and a highly flexible region (residues 167-203). Critically, this region modulates the troponin/tropomyosin complex's position across the actin filament. Thus, the cTnT linker structure and dynamics are central to the regulation of the function of cardiac muscles, but up to now, it was ill-understood. To establish the cTnT linker structure, we coupled an atomistic computational cTF model with time-resolved fluorescence resonance energy transfer measurements in both ±Ca²⁺ conditions utilizing fully reconstituted cTFs. We mapped the cTnT linker's positioning across the actin filament, and by coupling the experimental results to computation, we found mean structures and ranges of motion of this part of the complex. With this new insight, we can now address cTnT linker structural dynamics in both myofilament activation and disease.

Significance of Cardiac Troponins as an Identification Tool in COVID-19 Patients Using Biosensors: An Update

Coronavirus disease 2019 (COVID-19) has rapidly developed as a global health emergency. Respiratory diseases are significant causes of morbidity and mortality in these patients with a spectrum of different diseases, from asymptomatic subclinical infection to the progression of severe pneumonia and subsequent acute respiratory distress syndrome. Individuals with cardiovascular disease are more likely to become infected with SARS-CoV-2 and develop severe symptoms. Hence, patients with underlying cardiovascular disease mortality rate are over three times. Furthermore, note that patients with a history of cardiovascular disease are more likely to have higher cardiac biomarkers, especially cardiac troponins, than infected patients, especially those with severe disease, making these patients more susceptible to cardiac damage caused by SARS-2-CoV. Biomarkers are important in decision-making to facilitate the efficient allocation of resources. Viral replication in the heart muscle can lead to a cascade of inflammatory processes that lead to fibrosis and, ultimately, cardiac necrosis. Elevated troponin may indicate damage to the heart muscle and may predict death. After the first Chinese analysis, increased cardiac troponin value was observed in a significant proportion of patients, suggesting that myocardial damage is a possible pathogenic mechanism leading to severe disease and death. However, the prognostic performance of troponin and whether its value is affected by different comorbidities present in COVID-19 patients are not known. This review aimed to assess the diagnostic value of troponin to offer insight into pathophysiological mechanisms and reported new assessment methods, including new biosensors for troponin in patients with COVID-19.

Force Measurements From Myofibril to Filament

Contractility, the generation of force and movement by molecular motors, is the hallmark of all muscles, including striated muscle. Contractility can be studied at every level of organization from a whole animal to single molecules. Measurements at sub-cellular level are particularly useful since, in the absence of the excitation-contraction coupling system, the properties of the contractile proteins can be directly investigated; revealing mechanistic details not accessible in intact muscle. Moreover, the conditions can be manipulated with ease, for instance changes in activator Ca²⁺, small molecule effector concentration or phosphorylation levels and introducing mutations. Subcellular methods can be successfully applied to frozen materials and generally require the smallest amount of tissue, thus greatly increasing the range of possible experiments compared with the study of intact muscle and cells. Whilst measurement of movement at the subcellular level is relatively simple, measurement of force is more challenging. This mini review will describe current methods for measuring force production at the subcellular level including single myofibril and single myofilament techniques.

Mechanical contribution to muscle thin filament activation

Vertebrate striated muscle thin filaments are thought to be thermodynamically activated in response to an increase in Ca²⁺ concentration. We tested this hypothesis by measuring time intervals for gliding runs and pauses of individual skeletal muscle thin filaments in cycling myosin motility assays. A classic thermodynamic mechanism predicts that if chemical potential is constant, transitions between runs and pauses of gliding thin filaments will occur at constant rate as given by a Poisson distribution. In this scenario, rate is given by the odds of a pause, and hence, run times between pauses fit an exponential distribution that slopes negatively for all observable run times. However, we determined that relative density of observed run times fits an exponential only at low Ca²⁺ levels that activate filament gliding. Further titration with Ca²⁺, or adding excess regulatory proteins tropomyosin and troponin, shifted the relative density of short run times to fit the positive slope of a gamma distribution, which derives from waiting times between Poisson events. Events that arise during a run and prevent the chance of ending a run for a random interval of time account for the observed run time distributions, suggesting that the events originate with cycling myosin. We propose that regulatory proteins of the thin filament require the mechanical force of cycling myosin to achieve the transition state for activation. During activation, combinations of cycling myosin that contribute insufficient activation energy delay deactivation.

Microscopic heat pulses activate cardiac thin filaments

During the excitation-contraction coupling of the heart, sarcomeres are activated via thin filament structural changes (i.e., from the "off" state to the "on" state) in response to a release of Ca2+ from the sarcoplasmic reticulum. This process involves chemical reactions that are highly dependent on ambient temperature; for example, catalytic activity of the actomyosin ATPase rises with increasing temperature. Here, we investigate the effects of rapid heating by focused infrared (IR) laser irradiation on the sliding of thin filaments reconstituted with human α-tropomyosin and bovine ventricular troponin in an in vitro motility assay. We perform high-precision analyses measuring temperature by the fluorescence intensity of rhodamine-phalloidin-labeled F-actin coupled with a fluorescent thermosensor sheet containing the temperature-sensitive dye Europium (III) thenoyltrifluoroacetonate trihydrate. This approach enables a shift in temperature from 25°C to ∼46°C within 0.2 s. We find that in the absence of Ca2+ and presence of ATP, IR laser irradiation elicits sliding movements of reconstituted thin filaments with a sliding velocity that increases as a function of temperature. The heating-induced acceleration of thin filament sliding likewise occurs in the presence of Ca2+ and ATP; however, the temperature dependence is more than twofold less pronounced. These findings could indicate that in the mammalian heart, the on-off equilibrium of the cardiac thin filament state is partially shifted toward the on state in diastole at physiological body temperature, enabling rapid and efficient myocardial dynamics in systole.

Troponin structure and function: a view of recent progress

The molecular mechanism by which Ca²⁺ binding and phosphorylation regulate muscle contraction through Troponin is not yet fully understood. Revealing the differences between the relaxed and active structure of cTn, as well as the conformational changes that follow phosphorylation has remained a challenge for structural biologists over the years. Here we review the current understanding of how Ca²⁺, phosphorylation and disease-causing mutations affect the structure and dynamics of troponin to regulate the thin filament based on electron microscopy, X-ray diffraction, NMR and molecular dynamics methodologies.

Determination of the Maximum Velocity of Filaments in the in vitro Motility Assay

The in vitro motility assay (IVMA) is a powerful tool commonly used in basic muscle research and for drug screenings with the aim to find treatment options for neuromuscular disorders. In brief, the sliding movement of fluorescence-labeled actin filaments on myosin motor proteins is recorded, and the sliding velocity is analyzed via image analysis methods. Due to low signal-to-noise ratios and large variability in the velocity signal, accurate determination of the maximum sliding velocity is challenging. We introduce a new method and software program named Actin Phase Velocity (ActiPHV). The method extracts the maximum velocity from filament tracking data. Based on simulated and real reference data we show that our method yields a higher accuracy compared to previous methods. As a result, our method enables enhancing the sensitivity of the IVMA to better exploit its full potential.

Functional significance of HCM mutants of tropomyosin, V95A and D175N, studied with in vitro motility assays

The majority of hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomere proteins. We examined tropomyosin (Tpm)’s HCM mutants in humans, V95A and D175N, with in vitro motility assay using optical tweezers to evaluate the effects of the Tpm mutations on the actomyosin interaction at the single molecular level. Thin filaments were reconstituted using these Tpm mutants, and their sliding velocity and force were measured at varying Ca²⁺ concentrations. Our results indicate that the sliding velocity at pCa ≥8.0 was significantly increased in mutants, which is expected to cause a diastolic problem. The velocity that can be activated by Ca²⁺ decreased significantly in mutants causing a systolic problem. With sliding force, Ca²⁺ activatable force decreased in V95A and increased in D175N, which may cause a systolic problem. Our results further demonstrate that the duty ratio determined at the steady state of force generation in saturating [Ca²⁺] decreased in V95A and increased in D175N. The Ca²⁺ sensitivity and cooperativity were not significantly affected by the mutations. These results suggest that the two mutants modulate molecular processes of the actomyosin interaction differently, but to result in the same pathology known as HCM.

Further analysis of rat myocardium contractility changes associated with a subchronic lead intoxication

A moderate subchronic lead intoxication was observed in male rats after repeated intraperitoneal injections of lead acetate. Right ventricular trabeculae and papillary muscles were isolated for in vitro studying of the contraction-relaxation cycle under isotonic and physiological loading. The contractile function of the myocardium was also assessed by measuring the velocity of thin filament movement over myosin. Lead intoxication led in papillary muscles to a decrease in the maximal rate of isotonic shortening for all afterloads and a decrease in the thin filament sliding velocity. Papillary muscles from lead-exposed rats displayed marked changes in most of the main characteristics of afterload contraction-relaxation cycles, but in trabeculae these changes were less pronounced. The reported changes were attenuated to some extent in rats treated with a Ca-containing bioprotector. The amount of work produced by both types of heart muscle preparations was not changed by lead. Only in papillary muscles the load-dependent relaxation index was significantly increased in the lead-treated groups. Thus subchronic lead intoxication affects the peak rate of force development and relaxation properties of cardiac muscle contracting in isotonic/physiological regimes rather than the total amount of mechanical work, which may reflect adaptive changes in the myocardial function under decreased contractility.

Spatiotemporal magnetic fields enhance cytosolic Ca2+ levels and induce actin polymerization via activation of voltage-gated sodium channels in skeletal muscle cells

Cellular function is modulated by the electric membrane potential controlling intracellular physiology and signal propagation from a motor neuron to a muscle fiber resulting in muscle contraction. Unlike electric fields, magnetic fields are not attenuated by biological materials and penetrate deep into the tissue. We used complex spatiotemporal magnetic fields (17-70 mT) to control intracellular signaling in skeletal muscle cells. By changing different parameters of the alternating magnetic field (amplitude, inversion time, rotation frequency), we induced transient depolarization of cellular membranes leading to i) Na⁺ influx through voltage-gated sodium channels (VGSC), ii) cytosolic calcium increase, and iii) VGSC- and ryanodine receptor-dependent increase of actin polymerization. The ion fluxes occurred only, when the field was applied and returned to baseline after the field was turned off. The 30-s-activation-cycle could be repeated without any loss of signal intensity. By contrast, static magnetic fields of the same strength exhibited no effect on myotube Ca²⁺ levels. Mathematical modeling suggested a role for the alternating magnetic field-induced eddy current, which mediates a local change in the membrane potential triggering the activation of VGSC. These findings might pave the way for the use of complex magnetic fields to improve function of skeletal muscles in myopathies.

Estimation of actomyosin active force maintained by tropomyosin and troponin complex under vertical forces in the in vitro motility assay system

The interaction between actin filaments and myosin molecular motors is a power source of a variety of cellular functions including cell division, cell motility, and muscular contraction. In vitro motility assay examines actin filaments interacting with myosin molecules that are adhered to a substrate (e.g., glass surface). This assay has been the standard method of studying the molecular mechanisms of contraction under an optical microscope. While the force generation has been measured through an optically trapped bead to which an actin filament is attached, a force vector vertical to the glass surface has been largely ignored with the in vitro motility assay. The vertical vector is created by the gap (distance) between the trapped bead and the glass surface. In this report, we propose a method to estimate the angle between the actin filament and the glass surface by optically determining the gap size. This determination requires a motorized stage in a standard epi-fluorescence microscope equipped with optical tweezers. This facile method is applied to force measurements using both pure actin filaments, and thin filaments reconstituted from actin, tropomyosin and troponin. We find that the angle-corrected force per unit filament length in the active condition (pCa = 5.0) decreases as the angle between the filament and the glass surface increases; i.e. as the force in the vertical direction increases. At the same time, we demonstrate that the force on reconstituted thin filaments is approximately 1.5 times larger than that on pure actin filaments. The range of angles we tested was between 11° and 36° with the estimated measurement error less than 6°. These results suggest the ability of cytoplasmic tropomyosin isoforms maintaining actomyosin active force to stabilize cytoskeletal architecture.

Stretch activation properties of Drosophila and Lethocerus indirect flight muscle suggest similar calcium-dependent mechanisms

Muscle stretch activation (SA) is critical for optimal cardiac and insect indirect flight muscle (IFM) power generation. The SA mechanism has been investigated for decades with many theories proposed, but none proven. One reason for the slow progress could be that multiple SA mechanisms may have evolved in multiple species or muscle types. Laboratories studying IFM SA in the same or different species have reported differing SA functional properties which would, if true, suggest divergent mechanisms. However, these conflicting results might be due to different experimental methodologies. Thus, we directly compared SA characteristics of IFMs from two SA model systems, Drosophila and Lethocerus, using two different fiber bathing solutions. Compared with Drosophila IFM, Lethocerus IFM isometric tension is 10- or 17-fold higher and SA tension was 5- or 10-fold higher, depending on the bathing solution. However, the rate of SA tension generation was 9-fold faster for Drosophila IFM. The inverse differences between rate and tension in the two species causes maximum power output to be similar, where Drosophila power is optimized in the bathing solution that favors faster muscle kinetics and Lethocerus in the solution that favors greater tension generation. We found that isometric tension and SA tension increased with calcium concentration for both species in both solutions, reaching a maximum plateau around pCa 5.0. Our results favor a similar mechanism for both species, perhaps involving a troponin complex that does not fully calcium activate the thin filament thus leaving room for further tension generation by SA.

Clinically Divergent Mutation Effects on the Structure and Function of the Human Cardiac Tropomyosin Overlap

The progression of genetically inherited cardiomyopathies from an altered protein structure to clinical presentation of disease is not well understood. One of the main roadblocks to mechanistic insight remains a lack of high-resolution structural information about multiprotein complexes within the cardiac sarcomere. One example is the tropomyosin (Tm) overlap region of the thin filament that is crucial for the function of the cardiac sarcomere. To address this central question, we devised coupled experimental and computational modalities to characterize the baseline function and structure of the Tm overlap, as well as the effects of mutations causing divergent patterns of ventricular remodeling on both structure and function. Because the Tm overlap contributes to the cooperativity of myofilament activation, we hypothesized that mutations that enhance the interactions between overlap proteins result in more cooperativity, and conversely, those that weaken interaction between these elements lower cooperativity. Our results suggest that the Tm overlap region is affected differentially by dilated cardiomyopathy-associated Tm D230N and hypertrophic cardiomyopathy-associated human cardiac troponin T (cTnT) R92L. The Tm D230N mutation compacts the Tm overlap region, increasing the cooperativity of the Tm filament, contributing to a dilated cardiomyopathy phenotype. The cTnT R92L mutation causes weakened interactions closer to the N-terminal end of the overlap, resulting in decreased cooperativity. These studies demonstrate that mutations with differential phenotypes exert opposite effects on the Tm-Tn overlap, and that these effects can be directly correlated to a molecular level understanding of the structure and dynamics of the component proteins.

The isoforms of α-actin and myosin affect the Ca2+ regulation of the actin-myosin interaction in the heart

Myocardium of mammals contains a wide range of isoforms of proteins that provides contractile function of the heart. These are two isoforms of ventricular and two of atrial myosin, α- and β-tropomyosin, and two isoforms of α-actin: cardiac and skeletal. We believe that the difference in the amino acid sequence of α-actin can affect the calcium regulation of the actin-myosin interaction. To test this hypothesis, we investigated effects of the isoforms of α-actin, cardiac and skeletal, and the isoforms of cardiac myosin on the calcium regulation of the actin-myosin interaction in an in vitro motility assay using reconstructed regulated thin filaments. The results show that isoforms of α-actin and the ratio of α/β-chains of Tpm differently affect the calcium regulation of the actin-myosin interaction in myocardium in dependence on cardiac myosin isoforms.

Cooperativity of myosin interaction with thin filaments is enhanced by stabilizing substitutions in tropomyosin

Muscle contraction is powered by myosin interaction with actin-based thin filaments containing Ca²⁺-regulatory proteins, tropomyosin and troponin. Coiled-coil tropomyosin molecules form a long helical strand that winds around actin filament and either shields actin from myosin binding or opens it. Non-canonical residues G126 and D137 in the central part of tropomyosin destabilize its coiled-coil structure. Their substitutions for canonical ones, G126R and D137L, increase structural stability and the velocity of sliding of reconstructed thin filaments along myosin coated surface. The effect of these stabilizing mutations on force of the actin-myosin interaction is unknown. It also remains unclear whether the stabilization affects single actin-myosin interactions or it modifies the cooperativity of the binding of myosin molecules to actin. We used an optical trap to measure the effects of the stabilization on step size, unitary force and duration of the interactions at low and high load and compared the results with those obtained in an in vitro motility assay. We found that significant prolongation of lifetime of the actin-myosin complex under high load observed at high extent of tropomyosin stabilization, i.e. with double mutant, G126R/D137L, correlates with higher force in the motility assay. Also, the higher the extent of stabilization of tropomyosin, the fewer myosin molecules are needed to propel the thin filaments. The data suggest that the effects of the stabilizing mutations in tropomyosin on the myosin interaction with regulated thin filaments are mainly realized via cooperative mechanisms by increasing the size of cooperative unit.

Molecular mechanisms underlying deoxy-ADP.Pi activation of pre-powerstroke myosin

Myosin activation is a viable approach to treat systolic heart failure. We previously demonstrated that striated muscle myosin is a promiscuous ATPase that can use most nucleoside triphosphates as energy substrates for contraction. When 2-deoxy ATP (dATP) is used, it acts as a myosin activator, enhancing cross-bridge binding and cycling. In vivo, we have demonstrated that elevated dATP levels increase basal cardiac function and rescues function of infarcted rodent and pig hearts. Here we investigate the molecular mechanism underlying this physiological effect. We show with molecular dynamics simulations that the binding of dADP.Pi (dATP hydrolysis products) to myosin alters the structure and dynamics of the nucleotide binding pocket, myosin cleft conformation, and actin binding sites, which collectively yield a myosin conformation that we predict favors weak, electrostatic binding to actin. In vitro motility assays at high ionic strength were conducted to test this prediction and we found that dATP increased motility. These results highlight alterations to myosin that enhance cross-bridge formation and reveal a potential mechanism that may underlie dATP-induced improvements in cardiac function.

Effect of Cardiac Myosin-Binding Protein C on Tropomyosin Regulation of Actin-Myosin Interaction Using In Vitro Motility Assay

We studied the modulating role of cardiac myosin-binding protein C (cMyBP-C) in tropomyosin regulation of the actin-myosin interaction. The effect of cMyBP-C on the velocity of actin-tropomyosin filament sliding over cardiac and slow skeletal myosins was evaluated using in vitro motility assay. The effect of cMyBP-C on the actin-tropomyosin filaments sliding depended on the type of myosin. The regulatory effect of cMyBP-C differs for cardiac and slow skeletal myosin because of the presence of specific essential light chain (LC1sa) in slow skeletal myosin isoform.

The interchain disulfide cross-linking of tropomyosin alters its regulatory properties and interaction with actin filament

Tropomyosin (Tpm) is an α-helical coiled-coil actin-binding protein that plays a key role in the Ca²⁺-regulated contraction of striated muscles. Two chains of Tpm can be cross-linked by formation of a disulfide bond between Cys-190 residues. Normally, the SH-groups of these residues in cardiac muscle are in reduced state but in heart pathologies the interchain cross-linking of Tpm was shown to occur. Previous studies have shown that this cross-linking increases the thermal stability of the C-terminal part of the Tpm molecule. However it was unclear how this affects its functional properties. In the current work, we studied functional features of cross-linked Tpm at the level of isolated proteins. The results have shown that the cross-linking greatly decreases affinity of Tpm for F-actin and stability of the Tpm-F-actin complex. It also increases sliding velocity of regulated thin filaments in an in vitro motility assay. This last effect was mostly pronounced when cardiac isoforms of myosin and troponin were used instead of skeletal ones. The results indicate that cross-linking significantly affects properties of Tpm and actin-myosin interaction and can explain, at least partly, the role of the interchain disulfide cross-linking of cardiac Tpm in human heart diseases.

Investigations of Molecular Mechanisms of Actin-Myosin Interactions in Cardiac Muscle

The functional characteristics of cardiac muscle depend on the composition of protein isoforms in the cardiomyocyte contractile machinery. In the ventricular myocardium of mammals, several isoforms of contractile and regulatory proteins are expressed - two isoforms of myosin (V1 and V3) and three isoforms of tropomyosin chains (α, β, and κ). Expression of protein isoforms depends on the animal species, its age and hormonal status, and this can change with pathologies of the myocardium. Mutations in these proteins can lead to cardiomyopathies. The functional significance of the protein isoform composition has been studied mainly on intact hearts or on isolated preparations of myocardium, which could not provide a clear comprehension of the role of each particular isoform. Present-day experimental techniques such as an optical trap and in vitro motility assay make it possible to investigate the phenomena of interactions of contractile and regulatory proteins on the molecular level, thus avoiding effects associated with properties of a whole muscle or muscle tissue. These methods enable free combining of the isoforms to test the molecular mechanisms of their participation in the actin-myosin interaction. Using the optical trap and the in vitro motility assay, we have studied functional characteristics of the cardiac myosin isoforms, molecular mechanisms of the calcium-dependent regulation of actin-myosin interaction, and the role of myosin and tropomyosin isoforms in the cooperativity mechanisms in myocardium. The knowledge of molecular mechanisms underlying myocardial contractility and its regulation is necessary for comprehension of cardiac muscle functioning, its disorders in pathologies, and for development of approaches for their correction.

Role of cardiac troponin I carboxy terminal mobile domain and linker sequence in regulating cardiac contraction

Inhibition of striated muscle contraction at resting Ca(2+) depends on the C-terminal half of troponin I (TnI) in thin filaments. Much focus has been on a short inhibitory peptide (Ip) sequence within TnI, but structural studies and identification of disease-associated mutations broadened emphasis to include a larger mobile domain (Md) sequence at the C-terminus of TnI. For Md to function effectively in muscle relaxation, tight mechanical coupling to troponin's core-and thus tropomyosin-is presumably needed. We generated recombinant, human cardiac troponins containing one of two TnI constructs: either an 8-amino acid linker between Md and the rest of troponin (cTnILink8), or an Md deletion (cTnI1-163). Motility assays revealed that Ca(2+)-sensitivity of reconstituted thin filament sliding was markedly increased with cTnILink8 (∼0.9 pCa unit leftward shift of speed-pCa relation compared to WT), and increased further when Md was missing entirely (∼1.4 pCa unit shift). Cardiac Tn's ability to turn off filament sliding at diastolic Ca(2+) was mostly (61%), but not completely eliminated with cTnI1-163. TnI's Md is required for full inhibition of unloaded filament sliding, although other portions of troponin-presumably including Ip-are also necessary. We also confirm that TnI's Md is not responsible for superactivation of actomyosin cycling by troponin.

The functional significance of the last 5 residues of the C-terminus of cardiac troponin I

The C-terminal region of cardiac troponin I (cTnI) is known to be important in cardiac function, as removal of the last 17 C-terminal residues of human cTnI has been associated with myocardial stunning. To investigate the C-terminal region of cTnI, three C-terminal deletion mutations in human cTnI were generated: Δ1 (deletion of residue 210), Δ3 (deletion of residues 208-210), and Δ5 (deletion of residues 206-210). Mammalian two-hybrid studies showed that the interactions between cTnI mutants and cardiac troponin C (cTnC) or cardiac troponin T (cTnT) were impaired in Δ3 and Δ5 mutants when compared to wild-type cTnI. Troponin complexes containing 2-[4'-(iodoacetamido) anilino] naphthalene-6-sulfonic acid (IAANS) labeled cTnC showed that the troponin complex containing cTnI Δ5 had a small increase in Ca(2+) affinity (P < 0.05); while the cTnI Δ1- and Δ3 troponin complexes showed no difference in Ca(2+) affinity when compared to wild-type troponin. In vitro motility assays showed that all truncation mutants had increased Ca(2+) dependent motility relative to wild-type cTnI. These results suggest that the last 5 C-terminal residues of cTnI influence the binding of cTnI with cTnC and cTnT and affect the Ca(2+) dependence of filament sliding, and demonstrate the importance of this region of cTnI.

Contractile properties of developing human fetal cardiac muscle

KEY POINTS

The contractile properties of human fetal cardiac muscle have not been previously studied. Small-scale approaches such as isolated myofibril and isolated contractile protein biomechanical assays allow study of activation and relaxation kinetics of human fetal cardiac muscle under well-controlled conditions. We have examined the contractile properties of human fetal cardiac myofibrils and myosin across gestational age 59-134 days. Human fetal cardiac myofibrils have low force and slow kinetics of activation and relaxation that increase during the time period studied, and kinetic changes may result from structural maturation and changes in protein isoform expression. Understanding the time course of human fetal cardiac muscle structure and contractile maturation can provide a framework to study development of contractile dysfunction with disease and evaluate the maturation state of cultured stem cell-derived cardiomyocytes.

ABSTRACT

Little is known about the contractile properties of human fetal cardiac muscle during development. Understanding these contractile properties, and how they change throughout development, can provide valuable insight into human heart development, and provide a framework to study the early stages of cardiac diseases that develop in utero. We characterized the contractile properties of isolated human fetal cardiac myofibrils across 8-19 weeks of gestation. Mechanical measurements revealed that in early stages of gestation there is low specific force and slow rates of force development and relaxation, with increases in force and the rates of activation and relaxation as gestation progresses. The duration and slope of the initial, slow phase of relaxation, related to myosin detachment and thin filament deactivation rates, decreased with gestation age. F-actin sliding on human fetal cardiac myosin-coated surfaces slowed significantly from 108 to 130 days of gestation. Electron micrographs showed human fetal muscle myofibrils elongate and widen with age, but features such as the M-line and Z-band are apparent even as early as day 52. Protein isoform analysis revealed that β-myosin is predominantly expressed even at the earliest time point studied, but there is a progressive increase in expression of cardiac troponin I (TnI), with a concurrent decrease in slow skeletal TnI. Together, our results suggest that cardiac myofibril force production and kinetics of activation and relaxation change significantly with gestation age and are influenced by the structural maturation of the sarcomere and changes in contractile filament protein isoforms.

2-Deoxy adenosine triphosphate improves contraction in human end-stage heart failure

We are developing a novel treatment for heart failure by increasing myocardial 2 deoxy-ATP (dATP). Our studies in rodent models have shown that substitution of dATP for adenosine triphosphate (ATP) as the energy substrate in vitro or elevation of dATP in vivo increases myocardial contraction and that small increases in the native dATP pool of heart muscle are sufficient to improve cardiac function. Here we report, for the first time, the effect of dATP on human adult cardiac muscle contraction. We measured the contractile properties of chemically-demembranated multicellular ventricular wall preparations and isolated myofibrils from human subjects with end-stage heart failure. Isometric force was increased at both saturating and physiologic Ca(2+) concentrations with dATP compared to ATP. This resulted in an increase in the Ca(2+) sensitivity of force (pCa50) by 0.06 pCa units. The rate of force redevelopment (ktr) in demembranated wall muscle was also increased, as was the rate of contractile activation (kACT) in isolated myofibrils, indicating increased cross-bridge binding and cycling compared with ATP in failing human myocardium. These data suggest that dATP could increase dP/dT and end systolic pressure in failing human myocardium. Importantly, even though the magnitude and rate of force development were increased, there was no increase in the time to 50% and 90% myofibril relaxation. These data, along with our previous studies in rodent models, show the promise of elevating myocardial dATP to enhance contraction and restore cardiac pump function. These data also support further pre-clinical evaluation of this new approach for treating heart failure.

Myosin-binding protein C corrects an intrinsic inhomogeneity in cardiac excitation-contraction coupling

The beating heart exhibits remarkable contractile fidelity over a lifetime, which reflects the tight coupling of electrical, chemical, and mechanical elements within the sarcomere, the elementary contractile unit. On a beat-to-beat basis, calcium is released from the ends of the sarcomere and must diffuse toward the sarcomere center to fully activate the myosin- and actin-based contractile proteins. The resultant spatial and temporal gradient in free calcium across the sarcomere should lead to nonuniform and inefficient activation of contraction. We show that myosin-binding protein C (MyBP-C), through its positioning on the myosin thick filaments, corrects this nonuniformity in calcium activation by exquisitely sensitizing the contractile apparatus to calcium in a manner that precisely counterbalances the calcium gradient. Thus, the presence and correct localization of MyBP-C within the sarcomere is critically important for normal cardiac function, and any disturbance of MyBP-C localization or function will contribute to the consequent cardiac pathologies.

Ca(2+)-regulatory function of the inhibitory peptide region of cardiac troponin I is aided by the C-terminus of cardiac troponin T: Effects of familial hypertrophic cardiomyopathy mutations cTnI R145G and cTnT R278C, alone and in combination, on filament sliding

Investigations of cardiomyopathy mutations in Ca(2+) regulatory proteins troponin and tropomyosin provide crucial information about cardiac disease mechanisms, and also provide insights into functional domains in the affected polypeptides. Hypertrophic cardiomyopathy-associated mutations TnI R145G, located within the inhibitory peptide (Ip) of human cardiac troponin I (hcTnI), and TnT R278C, located immediately C-terminal to the IT arm in human cardiac troponin T (hcTnT), share some remarkable features: structurally, biochemically, and pathologically. Using bioinformatics, we find compelling evidence that TnI and TnT, and more specifically the affected regions of hcTnI and hcTnT, may be related not just structurally but also evolutionarily. To test for functional interactions of these mutations on Ca(2+)-regulation, we generated and characterized Tn complexes containing either mutation alone, or both mutations simultaneously. The most important results from in vitro motility assays (varying [Ca(2+)], temperature or HMM density) show that the TnT mutant "rescued" some deleterious effects of the TnI mutant at high Ca(2+), but exacerbated the loss of function, i.e., switching off the actomyosin interaction, at low Ca(2+). Taken together, our experimental results suggest that the C-terminus of cTnT aids Ca(2+)-regulatory function of cTnI Ip within the troponin complex.

Allosteric effects of cardiac troponin TNT1 mutations on actomyosin binding: a novel pathogenic mechanism for hypertrophic cardiomyopathy

The majority of hypertrophic cardiomyopathy mutations in (cTnT) occur within the alpha-helical tropomyosin binding TNT1 domain. A highly charged region at the C-terminal end of TNT1 unwinds to create a flexible "hinge". While this region has not been structurally resolved, it likely acts as an extended linker between the two cTnT functional domains. Mutations in this region cause phenotypically diverse and often severe forms of HCM. Mechanistic insight, however, has been limited by the lack of structural information. To overcome this limitation, we evaluated the effects of cTnT 160-163 mutations using regulated in vitro motility (R-IVM) assays and transgenic mouse models. R-IVM revealed that cTnT mutations Δ160E, E163R and E163K disrupted weak electrostatic actomyosin binding. Reducing the ionic strength or decreasing Brownian motion rescued function. This is the first observation of HCM-linked mutations in cTnT disrupting weak interactions between the thin filament and myosin. To evaluate the in vivo effects of altering weak actomyosin binding we generated transgenic mice expressing Δ160E and E163R mutant cTnT and observed severe cardiac remodeling and profound myofilament disarray. The functional changes observed in vitro may contribute to the structural impairment seen in vivo by destabilizing myofilament structure and acting as a constant pathophysiologic stress.

Cardiac muscle activation blunted by a mutation to the regulatory component, troponin T

The striated muscle thin filament comprises actin, tropomyosin, and troponin. The Tn complex consists of three subunits, troponin C (TnC), troponin I (TnI), and troponin T (TnT). TnT may serve as a bridge between the Ca(2+) sensor (TnC) and the actin filament. In the short helix preceding the IT-arm region, H1(T2), there are known dilated cardiomyopathy-linked mutations (among them R205L). Thus we hypothesized that there is an element in this short helix that plays an important role in regulating the muscle contraction, especially in Ca(2+) activation. We mutated Arg-205 and several other amino acid residues within and near the H1(T2) helix. Utilizing an alanine replacement method to compare the effects of the mutations, the biochemical and mechanical impact on the actomyosin interaction was assessed by solution ATPase activity assay, an in vitro motility assay, and Ca(2+) binding measurements. Ca(2+) activation was markedly impaired by a point mutation of the highly conserved basic residue R205A, residing in the short helix H1(T2) of cTnT, whereas the mutations to nearby residues exhibited little effect on function. Interestingly, rigor activation was unchanged between the wild type and R205A TnT. In addition to the reduction in Ca(2+) sensitivity observed in Ca(2+) binding to the thin filament, myosin S1-ADP binding to the thin filament was significantly affected by the same mutation, which was also supported by a series of S1 concentration-dependent ATPase assays. These suggest that the R205A mutation alters function through reduction in the nature of cooperative binding of S1.

Contractility and kinetics of human fetal and human adult skeletal muscle

Little is known about the contraction and relaxation properties of fetal skeletal muscle, and measurements thus far have been made with non-human mammalian muscle. Data on human fetal skeletal muscle contraction are lacking, and there are no published reports on the kinetics of either fetal or adult human skeletal muscle myofibrils. Understanding the contractile properties of human fetal muscle would be valuable in understanding muscle development and a variety of muscle diseases that are associated with mutations in fetal muscle sarcomere proteins. Therefore, we characterised the contractile properties of developing human fetal skeletal muscle and compared them to adult human skeletal muscle and rabbit psoas muscle. Electron micrographs showed human fetal muscle sarcomeres are not fully formed but myofibril formation is visible. Isolated myofibril mechanical measurements revealed much lower specific force, and slower rates of isometric force development, slow phase relaxation, and fast phase relaxation in human fetal when compared to human adult skeletal muscle. The duration of slow phase relaxation was also significantly longer compared to both adult groups, but was similarly affected by elevated ADP. F-actin sliding on human fetal skeletal myosin coated surfaces in in vitro motility (IVM) assays was much slower compared with adult rabbit skeletal myosin, though the Km(app) (apparent (fitted) Michaelis-Menten constant) of F-actin speed with ATP titration suggests a greater affinity of human fetal myosin for nucleotide binding. Replacing ATP with 2 deoxy-ATP (dATP) increased F-actin speed for both groups by a similar amount. Titrations of ADP into IVM assays produced a similar inhibitory affect for both groups, suggesting ADP binding may be similar, at least under low load. Together, our results suggest slower but similar mechanisms of myosin chemomechanical transduction for human fetal muscle that may also be limited by immature myofilament structure.

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.

Quasiperiodic distribution of rigor cross-bridges along a reconstituted thin filament in a skeletal myofibril

Electron microscopy has shown that cross-bridges (CBs) are formed at the target zone that is periodically distributed on the thin filament in striated muscle. Here, by manipulating a single bead-tailed actin filament with optical tweezers, we measured the unbinding events of rigor CBs one by one on the surface of the A-band in rabbit skeletal myofibrils. We found that the spacings between adjacent CBs were not always the same, and instead were 36, 72, or 108 nm. Tropomyosin and troponin did not affect the CB spacing except for a relative increase in the appearance of longer spacing in the presence of Ca(2+). In addition, in an in vitro assay where myosin molecules were randomly distributed, were obtained the same spacing, i.e., a multiple of 36 nm. These results indicate that the one-dimensional distribution of CBs matches with the 36-nm half pitch of a long helical structure of actin filaments. A stereospecific model composed of three actin protomers per target zone was shown to explain the experimental results. Additionally, the unbinding force (i.e., the binding affinity) of CBs for the reconstituted thin filaments was found to be larger and smaller relative to that for actin filaments with and without Ca(2+), respectively.

Micromechanical thermal assays of Ca2+-regulated thin-filament function and modulation by hypertrophic cardiomyopathy mutants of human cardiac troponin

Microfabricated thermoelectric controllers can be employed to investigate mechanisms underlying myosin-driven sliding of Ca(2+)-regulated actin and disease-associated mutations in myofilament proteins. Specifically, we examined actin filament sliding-with or without human cardiac troponin (Tn) and α-tropomyosin (Tm)-propelled by rabbit skeletal heavy meromyosin, when temperature was varied continuously over a wide range (~20-63°C). At the upper end of this temperature range, reversible dysregulation of thin filaments occurred at pCa 9 and 5; actomyosin function was unaffected. Tn-Tm enhanced sliding speed at pCa 5 and increased a transition temperature (T(t)) between a high activation energy (E(a)) but low temperature regime and a low E(a) but high temperature regime. This was modulated by factors that alter cross-bridge number and kinetics. Three familial hypertrophic cardiomyopathy (FHC) mutations, cTnI R145G, cTnI K206Q, and cTnT R278C, cause dysregulation at temperatures ~5-8°C lower; the latter two increased speed at pCa 5 at all temperatures.

Correlation of molecular and functional effects of mutations in cardiac troponin T linked to familial hypertrophic cardiomyopathy: an integrative in silico/in vitro approach

Nearly 70% of all of the known cTnT mutations that cause familial hypertrophic cardiomyopathy fall within the TNT1 region that is critical to cTn-Tm binding. The high resolution structure of this domain has not been determined, and this lack of information has hindered structure-function analysis. In the current study, a coupled computational experimental approach was employed to correlate changes in cTnT dynamics to basic function using the regulated in vitro motility assay (R-IVM). An in silico approach to calculate forces in terms of a bending coordinate was used to precisely identify decreases in bending forces at residues 105 and 106 within the proposed cTnT "hinge" region. Significant functional changes were observed in multiple functional properties, including a decrease in the cooperativity of calcium activation, the calcium sensitivity of sliding speed, and maximum sliding speed. Correlation of the computational and experimental findings revealed an association between TNT1 flexibility and the cooperativity of thin filament calcium activation where an increase in flexibility led to a decrease in cooperativity. Further analysis of the primary sequence of the TNT1 region revealed a unique pattern of conserved charged TNT1 residues altered by the R92W and R92L mutations and may represent the underlying "structure" modulating this central functional domain. These data provide a framework for further integrated in silico/in vitro approaches that may be extended into a high-throughput predictive screen to overcome the current structural limitations in linking molecular phenotype to genotype in thin filament cardiomyopathies.

Facilitated cross-bridge interactions with thin filaments by familial hypertrophic cardiomyopathy mutations in α-tropomyosin

Familial hypertrophic cardiomyopathy (FHC) is a disease of cardiac sarcomeres. To identify molecular mechanisms underlying FHC pathology, functional and structural differences in three FHC-related mutations in recombinant α-Tm (V95A, D175N, and E180G) were characterized using both conventional and modified in vitro motility assays and circular dichroism spectroscopy. Mutant Tm's exhibited reduced α-helical structure and increased unordered structure. When thin filaments were fully occupied by regulatory proteins, little or no motion was detected at pCa 9, and maximum speed (pCa 5) was similar for all tropomyosins. Ca²⁺-responsiveness of filament sliding speed was increased either by increased pCa₅₀ (V95A), reduced cooperativity n (D175N), or both (E180G). When temperature was increased, thin filaments with E180G exhibited dysregulation at temperatures ~10°C lower, and much closer to body temperature, than WT. When HMM density was reduced, thin filaments with D175N required fewer motors to initiate sliding or achieve maximum sliding speed.

Thin filament-reconstituted skinned muscle fibers for the study of muscle physiology

We review the use of thin filament-reconstituted muscle fibers in the study of muscle physiology. Thin filament extraction and reconstitution protocol is a powerful technique to study the role of each component of the thin filament. It is also useful for studying the properties of genetically modified molecules such as actin and tropomyosin. We also review the combination of this protocol with sinusoidal analysis, which will provide a solid technique for determining the effect of regulatory proteins on actomyosin interaction and concomitant cross-bridge kinetics. We suggest that thin filament-reconstituted muscle fibers are an ideal system for studying muscle physiology especially when gene modifications of actin or tropomyosin are involved.

Interaction between troponin and myosin enhances contractile activity of myosin in cardiac muscle

Ca(2+) signaling in striated muscle cells is critically dependent upon thin filament proteins tropomyosin (Tm) and troponin (Tn) to regulate mechanical output. Using in vitro measurements of contractility, we demonstrate that even in the absence of actin and Tm, human cardiac Tn (cTn) enhances heavy meromyosin MgATPase activity by up to 2.5-fold in solution. In addition, cTn without Tm significantly increases, or superactivates sliding speed of filamentous actin (F-actin) in skeletal motility assays by at least 12%, depending upon [cTn]. cTn alone enhances skeletal heavy meromyosin's MgATPase in a concentration-dependent manner and with sub-micromolar affinity. cTn-mediated increases in myosin ATPase may be the cause of superactivation of maximum Ca(2+)-activated regulated thin filament sliding speed in motility assays relative to unregulated skeletal F-actin. To specifically relate this classical superactivation to cardiac muscle, we demonstrate the same response using motility assays where only cardiac proteins were used, where regulated cardiac thin filament sliding speeds with cardiac myosin are >50% faster than unregulated cardiac F-actin. We additionally demonstrate that the COOH-terminal mobile domain of cTnI is not required for this interaction or functional enhancement of myosin activity. Our results provide strong evidence that the interaction between cTn and myosin is responsible for enhancement of cross-bridge kinetics when myosin binds in the vicinity of Tn on thin filaments. These data imply a novel and functionally significant molecular interaction that may provide new insights into Ca(2+) activation in cardiac muscle cells.

Myosin binding protein C interaction with actin: characterization and mapping of the binding site

Myosin binding protein C (MyBPC) is a multidomain protein associated with the thick filaments of striated muscle. Although both structural and regulatory roles have been proposed for MyBPC, its interactions with other sarcomeric proteins remain obscure. The current study was designed to examine the actin-binding properties of MyBPC and to define MyBPC domain regions involved in actin interaction. Here, we have expressed full-length mouse cardiac MyBPC (cMyBPC) in a baculovirus system and shown that purified cMyBPC binds actin filaments with an affinity of 4.3 ± 1.1 μM and a 1:1 molar ratio with regard to an actin protomer. The actin binding by cMyBPC is independent of protein phosphorylation status and is not significantly affected by the presence of tropomyosin and troponin on the actin filament. In addition, cMyBPC-actin interaction is not modulated by calmodulin. To determine the region of cMyBPC that is responsible for its interaction with actin, we have expressed and characterized five recombinant proteins encoding fragments of the cMyBPC sequence. Recombinant N-terminal fragments such as C0-C1, C0-C4, and C0-C5 cosediment with actin in a linear, nonsaturable manner. At the same time, MyBPC fragments lacking either the C0-C1 or C0-C4 region bind F-actin with essentially the same properties as full-length protein. Together, our results indicate that cMyBPC interacts with actin via a single, moderate affinity site localized to the C-terminal region of the protein. In contrast, certain basic regions of the N-terminal domains of MyBPC may act as small polycations and therefore bind actin via nonspecific electrostatic interactions.

Effects of cardiac myosin binding protein-C on the regulation of interaction of cardiac myosin with thin filament in an in vitro motility assay

Modulatory role of whole cardiac myosin binding protein-C (сMyBP-C) in regulation of cardiac muscle contractility was studied in the in vitro motility assay with rabbit cardiac myosin as a motor protein. The effects of cMyBP-C on the interaction of cardiac myosin with regulated thin filament were tested in both in vitro motility and ATPase assays. We demonstrate that the addition of cMyBP-C increases calcium regulated Mg-ATPase activity of cardiac myosin at submaximal calcium. The Hill coefficient for 'pCa-velocity' relation in the in vitro motility assay decreased and the calcium sensitivity increased when сMyBP-C was added. Results of our experiments testifies in favor of the hypothesis that сMyBP-C slows down cross-bridge kinetics when binding to actin.

Contractile properties of muscle fibers from the deep and superficial digital flexors of horses

Equine digital flexor muscles have independent tendons but a nearly identical mechanical relationship to the main joint they act upon. Yet these muscles have remarkable diversity in architecture, ranging from long, unipennate fibers ("short" compartment of DDF) to very short, multipennate fibers (SDF). To investigate the functional relevance of the form of the digital flexor muscles, fiber contractile properties were analyzed in the context of architecture differences and in vivo function during locomotion. Myosin heavy chain (MHC) isoform fiber type was studied, and in vitro motility assays were used to measure actin filament sliding velocity (V(f)). Skinned fiber contractile properties [isometric tension (P(0)/CSA), velocity of unloaded shortening (V(US)), and force-Ca(2+) relationships] at both 10 and 30°C were characterized. Contractile properties were correlated with MHC isoform and their respective V(f). The DDF contained a higher percentage of MHC-2A fibers with myosin (heavy meromyosin) and V(f) that was twofold faster than SDF. At 30°C, P(0)/CSA was higher for DDF (103.5 ± 8.75 mN/mm(2)) than SDF fibers (81.8 ± 7.71 mN/mm(2)). Similarly, V(US) (pCa 5, 30°C) was faster for DDF (2.43 ± 0.53 FL/s) than SDF fibers (1.20 ± 0.22 FL/s). Active isometric tension increased with increasing Ca(2+) concentration, with maximal Ca(2+) activation at pCa 5 at each temperature in fibers from each muscle. In general, the collective properties of DDF and SDF were consistent with fiber MHC isoform composition, muscle architecture, and the respective functional roles of the two muscles in locomotion.

Effect of Ca2+ binding properties of troponin C on rate of skeletal muscle force redevelopment

To investigate effects of altering troponin (Tn)C Ca(2+) binding properties on rate of skeletal muscle contraction, we generated three mutant TnCs with increased or decreased Ca(2+) sensitivities. Ca(2+) binding properties of the regulatory domain of TnC within the Tn complex were characterized by following the fluorescence of an IAANS probe attached onto the endogenous Cys(99) residue of TnC. Compared with IAANS-labeled wild-type Tn complex, V43QTnC, T70DTnC, and I60QTnC exhibited ∼1.9-fold higher, ∼5.0-fold lower, and ∼52-fold lower Ca(2+) sensitivity, respectively, and ∼3.6-fold slower, ∼5.7-fold faster, and ∼21-fold faster Ca(2+) dissociation rate (k(off)), respectively. On the basis of K(d) and k(off), these results suggest that the Ca(2+) association rate to the Tn complex decreased ∼2-fold for I60QTnC and V43QTnC. Constructs were reconstituted into single-skinned rabbit psoas fibers to assess Ca(2+) dependence of force development and rate of force redevelopment (k(tr)) at 15°C, resulting in sensitization of both force and k(tr) to Ca(2+) for V43QTnC, whereas T70DTnC and I60QTnC desensitized force and k(tr) to Ca(2+), I60QTnC causing a greater desensitization. In addition, T70DTnC and I60QTnC depressed both maximal force (F(max)) and maximal k(tr). Although V43QTnC and I60QTnC had drastically different effects on Ca(2+) binding properties of TnC, they both exhibited decreases in cooperativity of force production and elevated k(tr) at force levels <30%F(max) vs. wild-type TnC. However, at matched force levels >30%F(max) k(tr) was similar for all TnC constructs. These results suggest that the TnC mutants primarily affected k(tr) through modulating the level of thin filament activation and not by altering intrinsic cross-bridge cycling properties. To corroborate this, NEM-S1, a non-force-generating cross-bridge analog that activates the thin filament, fully recovered maximal k(tr) for I60QTnC at low Ca(2+) concentration. Thus TnC mutants with altered Ca(2+) binding properties can control the rate of contraction by modulating thin filament activation without directly affecting intrinsic cross-bridge cycling rates.