Contractile effects of the exchange of cardiac troponin for fast skeletal troponin in rabbit psoas single myofibrils

Troponins and Skeletal Muscle Pathologies

Skeletal muscles account for ~30-40% of the total weight of human body and are responsible for its most important functions, including movement, respiration, thermogenesis, and glucose and protein metabolism. Skeletal muscle damage negatively impacts the whole-body functioning, leading to deterioration of the quality of life and, in severe cases, death. Therefore, timely diagnosis and therapy for skeletal muscle dysfunction are important goals of modern medicine. In this review, we focused on the skeletal troponins that are proteins in the thin filaments of muscle fibers. Skeletal troponins play a key role in regulation of muscle contraction. Biochemical properties of these proteins and their use as biomarkers of skeletal muscle damage are described in this review. One of the most convenient and sensitive methods of protein biomarker measurement in biological liquids is immunochemical analysis; hence, we examined the factors that influence immunochemical detection of skeletal troponins and should be taken into account when developing diagnostic test systems. Also, we reviewed the available data on the skeletal troponin mutations that are considered to be associated with pathologies leading to the development of diseases and discussed utilization of troponins as drug targets for treatment of the skeletal muscle disorders.

Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Fundamentally, the heart needs to generate sufficient force and power output to dynamically meet the needs of the body. Cardiomyocytes contain specialized structures referred to as sarcomeres that power and regulate contraction. Disruption of sarcomeric function or regulation impairs contractility and leads to cardiomyopathies and heart failure. Basic, translational, and clinical studies have adapted numerous methods to assess cardiac contraction in a variety of pathophysiological contexts. These tools measure aspects of cardiac contraction at different scales ranging from single molecules to whole organisms. Moreover, these studies have revealed new pathogenic mechanisms of heart disease leading to the development of novel therapies targeting contractility. In this review, the authors explore the breadth of tools available for studying cardiac contractile function across scales, discuss their strengths and limitations, highlight new insights into cardiac physiology and pathophysiology, and describe how these insights can be harnessed for therapeutic candidate development and translational.

The force of the myosin motor sets cooperativity in thin filament activation of skeletal muscles

Contraction of striated muscle is regulated by a dual mechanism involving both thin, actin-containing filament and thick, myosin-containing filament. Thin filament is activated by Ca2+ binding to troponin, leading to tropomyosin displacement that exposes actin sites for interaction with myosin motors, extending from the neighbouring stress-activated thick filaments. Motor attachment to actin contributes to spreading activation along the thin filament, through a cooperative mechanism, still unclear, that determines the slope of the sigmoidal relation between isometric force and pCa (−log[Ca2+]), estimated by Hill coefficient nH. We use sarcomere-level mechanics in demembranated fibres of rabbit skeletal muscle activated by Ca2+ at different temperatures (12–35 °C) to show that nH depends on the motor force at constant number of attached motors. The definition of the role of motor force provides fundamental constraints for modelling the dynamics of thin filament activation and defining the action of small molecules as possible therapeutic tools.

Identification and characterization of troponin genes in Locusta migratoria

Troponin complex comprises three subunits, namely troponin C (TpnC), troponin I (TpnI) and troponin T (TpnT), and regulates the contraction of striated muscle. We found that the locust Locusta migratoria genome has one TpnT gene (LmTpnT), one TpnI gene (LmTpnI) and three TpnC genes (LmTpnC1, LmTpnC2 and LmTpnC3). Through alternative splicing, LmTpnT and LmTpnI potentially encode two and eight isoforms, respectively. The flight muscle and the jump muscle of L. migratoria express an identical LmTpnT isoform, but different LmTpnC isoforms and LmTpnI isoforms. LmTpnC2 and LmTpnC3 both contain highly conserved residues essential for calcium binding in the EF-hand II and IV, thus belonging two-site isoform. LmTpnC1 contains non-conserved substitutions in the EF-hand II and all highly conserved residues for calcium binding in the EF-hand IV. Mutagenesis and tyrosine fluorescence spectroscopic analysis show that both the EF-hand II and IV of LmTpnC1 can serve as calcium-binding site. Therefore, all three LmTpnC isoforms belong to two-site isoform. This is in contrast to the situation in the insect with asynchronous flight muscle, which expresses both one-site isoform and two-site isoform of TpnC. Those results suggest that the origination of insect asynchronous flight muscle is associated with the emergence of one-site isoform of TpnC.

Cycling Cross-Bridges Contribute to Thin Filament Activation in Human Slow-Twitch Fibers

It has been shown that not only calcium but also strong binding myosin heads contribute to thin filament activation in isometrically contracting animal fast-twitch and cardiac muscle preparations. This behavior has not been studied in human muscle fibers or animal slow-twitch fibers. Human slow-twitch fibers are interesting since they contain the same myosin heavy chain isoform as the human heart. To explore myosin-induced activation of the thin filament in isometrically contracting human slow-twitch fibers, the endogenous troponin complex was exchanged for a well-characterized fast-twitch skeletal troponin complex labeled with the fluorescent dye N-((2-(Iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole (fsTn-IANBD). The exchange was ≈70% complete (n = 8). The relative contributions of calcium and strong binding cross-bridges to thin filament activation were dissected by increasing the concentration of calcium from relaxing (pCa 7.5) to saturating levels (pCa 4.5) before and after incubating the exchanged fibers in the myosin inhibitor para-aminoblebbistatin (AmBleb). At pCa 4.5, the relative contributions of calcium and strong binding cross-bridges to thin filament activation were ≈69 and ≈31%, respectively. Additionally, switching from isometric to isotonic contraction at pCa 4.5 revealed that strong binding cross-bridges contributed ≈29% to thin filament activation (i.e., virtually the same magnitude obtained with AmBleb). Thus, we showed through two different approaches that lowering the number of strong binding cross-bridges, at saturating calcium, significantly reduced the activation of the thin filament in human slow-twitch fibers. The contribution of myosin to activation resembled that which was previously reported in rat cardiac and rabbit fast-twitch muscle preparations. This method could be applied to slow-twitch human fibers obtained from the soleus muscle of cardiomyopathy patients. Such studies could lead to a better understanding of the effect of point mutations of the cardiac myosin head on the regulation of muscle contraction and could lead to better management by pharmacological approaches.

The relation between sarcomere energetics and the rate of isometric tension relaxation in healthy and diseased cardiac muscle

Full muscle relaxation happens when [Ca²⁺] falls below the threshold for force activation. Several experimental models, from whole muscle organs and intact muscle down to skinned fibers, have been used to explore the cascade of kinetic events leading to mechanical relaxation. The use of single myofibrils together with fast solution switching techniques, has provided new information about the role of cross-bridge (CB) dissociation in the time course of isometric force decay. Myofibril’s relaxation is biphasic starting with a slow seemingly linear phase, with a rate constant, slow k_REL, followed by a fast mono-exponential phase. Sarcomeres remain isometric during the slow force decay that reflects CB detachment under isometric conditions while the final fast relaxation phase begins with a sudden give of few sarcomeres and is then dominated by intersarcomere dynamics. Based on a simple two-state model of the CB cycle, myofibril slow k_REL represents the apparent forward rate with which CBs leave force generating states (g_app) under isometric conditions and correlates with the energy cost of tension generation (ATPase/tension ratio); in short slow k_REL ~ g_app ~ tension cost. The validation of this relationship is obtained by simultaneously measuring maximal isometric force and ATP consumption in skinned myocardial strips that provide an unambiguous determination of the relation between contractile and energetic properties of the sarcomere. Thus, combining kinetic experiments in isolated myofibrils and mechanical and energetic measurements in multicellular cardiac strips, we are able to provide direct evidence for a positive linear correlation between myofibril isometric relaxation kinetics (slow k_REL) and the energy cost of force production both measured in preparations from the same cardiac sample. This correlation remains true among different types of muscles with different ATPase activities and also when CB kinetics are altered by cardiomyopathy-related mutations. Sarcomeric mutations associated to hypertrophic cardiomyopathy (HCM), a primary cardiac disorder caused by mutations in genes encoding sarcomeric proteins, have been often found to accelerate CB turnover rate and increase the energy cost of myocardial contraction. Here we review data showing that faster CB detachment results in a proportional increase in the energetic cost of tension generation in heart samples from both HCM patients and mouse models of the disease.

The homozygous K280N troponin T mutation alters cross-bridge kinetics and energetics in human HCM

Hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomeric proteins, but the pathogenic mechanism is unclear. Piroddi et al. find impairment of cross-bridge kinetics and energetics in human sarcomeres with a TNNT2 mutation, suggesting that HCM involves inefficient ATP utilization.

Hypertrophic cardiomyopathy (HCM) is a genetic form of left ventricular hypertrophy, primarily caused by mutations in sarcomere proteins. The cardiac remodeling that occurs as the disease develops can mask the pathogenic impact of the mutation. Here, to discriminate between mutation-induced and disease-related changes in myofilament function, we investigate the pathogenic mechanisms underlying HCM in a patient carrying a homozygous mutation (K280N) in the cardiac troponin T gene (TNNT2), which results in 100% mutant cardiac troponin T. We examine sarcomere mechanics and energetics in K280N-isolated myofibrils and demembranated muscle strips, before and after replacement of the endogenous troponin. We also compare these data to those of control preparations from donor hearts, aortic stenosis patients (LVH_ao), and HCM patients negative for sarcomeric protein mutations (HCM_smn). The rate constant of tension generation following maximal Ca²⁺ activation (k_ACT) and the rate constant of isometric relaxation (slow k_REL) are markedly faster in K280N myofibrils than in all control groups. Simultaneous measurements of maximal isometric ATPase activity and Ca²⁺-activated tension in demembranated muscle strips also demonstrate that the energy cost of tension generation is higher in the K280N than in all controls. Replacement of mutant protein by exchange with wild-type troponin in the K280N preparations reduces k_ACT, slow k_REL, and tension cost close to control values. In donor myofibrils and HCM_smn demembranated strips, replacement of endogenous troponin with troponin containing the K280N mutant increases k_ACT, slow k_REL, and tension cost. The K280N TNNT2 mutation directly alters the apparent cross-bridge kinetics and impairs sarcomere energetics. This result supports the hypothesis that inefficient ATP utilization by myofilaments plays a central role in the pathogenesis of the disease.

Kinetic coupling of phosphate release, force generation and rate-limiting steps in the cross-bridge cycle

A basic goal in muscle research is to understand how the cyclic ATPase activity of cross-bridges is converted into mechanical force. A direct approach to study the chemo-mechanical coupling between P_i release and the force-generating step is provided by the kinetics of force response induced by a rapid change in [P_i]. Classical studies on fibres using caged-P_i discovered that rapid increases in [P_i] induce fast force decays dependent on final [P_i] whose kinetics were interpreted to probe a fast force-generating step prior to P_i release. However, this hypothesis was called into question by studies on skeletal and cardiac myofibrils subjected to P_i jumps in both directions (increases and decreases in [P_i]) which revealed that rapid decreases in [P_i] trigger force rises with slow kinetics, similar to those of calcium-induced force development and mechanically-induced force redevelopment at the same [P_i]. A possible explanation for this discrepancy came from imaging of individual sarcomeres in cardiac myofibrils, showing that the fast force decay upon increase in [P_i] results from so-called sarcomere 'give'. The slow force rise upon decrease in [P_i] was found to better reflect overall sarcomeres cross-bridge kinetics and its [P_i] dependence, suggesting that the force generation coupled to P_i release cannot be separated from the rate-limiting transition. The reasons for the different conclusions achieved in fibre and myofibril studies are re-examined as the recent findings on cardiac myofibrils have fundamental consequences for the coupling between P_i release, rate-limiting steps and force generation. The implications from P_i-induced force kinetics of myofibrils are discussed in combination with historical and recent models of the cross-bridge cycle.

The increase in non-cross-bridge forces after stretch of activated striated muscle is related to titin isoforms

Skeletal muscles present a non-cross-bridge increase in sarcomere stiffness and tension on Ca(2+) activation, referred to as static stiffness and static tension, respectively. It has been hypothesized that this increase in tension is caused by Ca(2+)-dependent changes in the properties of titin molecules. To verify this hypothesis, we investigated the static tension in muscles containing different titin isoforms. Permeabilized myofibrils were isolated from the psoas, soleus, and heart ventricle from the rabbit, and tested in pCa 9.0 and pCa 4.5, before and after extraction of troponin C, thin filaments, and treatment with the actomyosin inhibitor blebbistatin. The myofibrils were tested with stretches of different amplitudes in sarcomere lengths varying between 1.93 and 3.37 μm for the psoas, 2.68 and 4.21 μm for the soleus, and 1.51 and 2.86 μm for the ventricle. Using gel electrophoresis, we confirmed that the three muscles tested have different titin isoforms. The static tension was present in psoas and soleus myofibrils, but not in ventricle myofibrils, and higher in psoas myofibrils than in soleus myofibrils. These results suggest that the increase in the static tension is directly associated with Ca(2+)-dependent change in titin properties and not associated with changes in titin-actin interactions.

Impact of tropomyosin isoform composition on fast skeletal muscle thin filament regulation and force development

Tropomyosin (Tm) plays a central role in the regulation of muscle contraction and is present in three main isoforms in skeletal and cardiac muscles. In the present work we studied the functional role of α- and βTm on force development by modifying the isoform composition of rabbit psoas skeletal muscle myofibrils and of regulated thin filaments for in vitro motility measurements. Skeletal myofibril regulatory proteins were extracted (78%) and replaced (98%) with Tm isoforms as homogenous ααTm or ββTm dimers and the functional effects were measured. Maximal Ca(2+) activated force was the same in ααTm versus ββTm myofibrils, but ββTm myofibrils showed a marked slowing of relaxation and an impairment of regulation under resting conditions compared to ααTm and controls. ββTm myofibrils also showed a significantly shorter slack sarcomere length and a marked increase in resting tension. Both these mechanical features were almost completely abolished by 10 mM 2,3-butanedione 2-monoxime, suggesting the presence of a significant degree of Ca(2+)-independent cross-bridge formation in ββTm myofibrils. Finally, in motility assay experiments in the absence of Ca(2+) (pCa 9.0), complete regulation of thin filaments required greater ββTm versus ααTm concentrations, while at full activation (pCa 5.0) no effect was observed on maximal thin filament motility speed. We infer from these observations that high contents of ββTm in skeletal muscle result in partial Ca(2+)-independent activation of thin filaments at rest, and longer-lasting and less complete tension relaxation following Ca(2+) removal.

Altered Tnnt3 characterizes selective weakness of fast fibers in mice overexpressing FSHD region gene 1 (FRG1)

Facioscapulohumeral muscular dystrophy (FSHD), a common hereditary myopathy, is characterized by atrophy and weakness of selective muscle groups. FSHD is considered an autosomal dominant disease with incomplete penetrance and unpredictable variability of clinical expression within families. Mice overexpressing FRG1 (FSHD region gene 1), a candidate gene for this disease, develop a progressive myopathy with features of the human disorder. Here, we show that in FRG1-overexpressing mice, fast muscles, which are the most affected by the dystrophic process, display anomalous fast skeletal troponin T (fTnT) isoform, resulting from the aberrant splicing of the Tnnt3 mRNA that precedes the appearance of dystrophic signs. We determine that muscles of FRG1 mice develop less strength due to impaired contractile properties of fast-twitch fibers associated with an anomalous MyHC-actin ratio and a reduced sensitivity to Ca(2+). We demonstrate that the decrease of Ca(2+) sensitivity of fast-twitch fibers depends on the anomalous troponin complex and can be rescued by the substitution with the wild-type proteins. Finally, we find that the presence of aberrant splicing isoforms of TNNT3 characterizes dystrophic muscles in FSHD patients. Collectively, our results suggest that anomalous TNNT3 profile correlates with the muscle impairment in both humans and mice. On the basis of these results, we propose that aberrant fTnT represents a biological marker of muscle phenotype severity and disease progression.

A re-interpretation of the rate of tension redevelopment (k(TR)) in active muscle

A slackening to zero tension by large length release (~20%) and a restretch of active muscle fibres cause a fall and a redevelopment in tension. According to the model of Brenner (Proc Natl Acad Sci USA 85(9):3265-3269, 1988), the rate constant of tension redevelopment (k TR) is the sum of attachment and detachment rate constants, hence is limited by the fast reaction. Here we propose a model in which, after restretch, cross-bridges cycle many times by stretching series elastic elements, hence k(TR) is limited by a slow reaction. To set up this model, we made an assumption that the stepping rate (v) decreases linearly with tension (F), which is consistent with the Fenn effect. The distance traveled by a cross-bridge stretches series elastic elements with stiffness σ. With these assumptions, we set up a first order differential equation, which results in an exponential time course with the rate constant k(TR) = ση(0)ν(0)(1 - λ)/F(1), where λ = ν(1)/ν(0), η = step size, the subscript 0 indicates unloaded condition, and the subscript 1 indicate isometric condition. We demonstrate that the ATP hydrolysis rate (=[myosin head]/ν(0)) is proportionate to k(TR) as the ambient temperature is changed, and that the published data fit to this relationship well if λ = 0.28. We conclude that k(TR) is limited by the cross-bridge turnover rate; hence it represents the rate constant of the slowest reaction of the cross-bridge cycle, i.e. the ADP isomerization step before ADP is released.

A kinetic model of troponin dissociation in relation to thin filament regulation in striated muscle

The apparent rate of troponin (Tn) dissociation from myofibrils has been used as a method to study thin filament regulation in striated muscle. The rate is dependent upon calcium and strong crossbridges and supports the three-state model for thin filament regulation. The dissociation rate of Tn is extremely low so it is not intuitively clear that such a slow process would probe thin filament regulation. We have investigated this issue by developing a simple kinetic model to explain the Tn dissociation rate measured by labeled Tn exchange in the myofibrils. Tn is composed of three interacting subunits, TnC, TnI and TnT. In our model, TnI's regulatory domain switches from actin-tropomyosin to TnC followed by TnT dissociation from actin-tropomyosin. This TnI regulatory domain switching is linked to the transition of the thin filament from the blocked state to the closed state. It is calcium dependent and several orders of magnitude faster than TnT dissociation from actin-tropomyosin. By integrating the dimensionless rate equations of this model, we have computed the time course of each of the various components. In our numerical simulations, the rate constant for TnI switching from actin-tropomyosin to TnC was varied from 10 s⁻¹ to 1000 s⁻¹ to simulate the low calcium, blocked state to high calcium, closed state. The computed progress curves for labeled Tn exchange into the myofibrils and the derived intensity ratio between the non-overlap and overlap regions well explains the intensity ratio progress curves observed experimentally. These numerical simulations and experimental observations reveal that the apparent rate of Tn dissociation probes the blocked state to closed state equilibrium of the myofibrillar thin filament.

A non-cross-bridge, static tension is present in permeabilized skeletal muscle fibers after active force inhibition or actin extraction

When activated muscle fibers are stretched, there is a long-lasting increase in the force. This phenomenon, referred to as "residual force enhancement," has characteristics similar to those of the "static tension," a long-lasting increase in force observed when muscles are stretched in the presence of Ca(2+) but in the absence of myosin-actin interaction. Independent studies have suggested that these two phenomena have a common mechanism and are caused either by 1) a Ca(2+)-induced stiffening of titin or by 2) promoting titin binding to actin. In this study, we performed two sets of experiments in which activated fibers (pCa(2+) 4.5) treated with the myosin inhibitor blebbistatin were stretched from 2.7 to 2.8 μm at a speed of 40 L(o)/s, first, after partial extraction of TnC, which inhibits myosin-actin interactions, or, second, after treatment with gelsolin, which leads to the depletion of thin (actin) filaments. We observed that the static tension, directly related with the residual force enhancement, was not changed after treatments that inhibit myosin-actin interactions or that deplete fibers from troponin C and actin filaments. The results suggest that the residual force enhancement is caused by a stiffening of titin upon muscle activation but not with titin binding to actin. This finding indicates the existence of a Ca(2+)-regulated, titin-based stiffness in skeletal muscles.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Optical detection system for probing cantilever deflections parallel to a sample surface

To date, commercial atomic force microscopes have been optimized for measurements of forces perpendicular to the sample surface. In many applications, sensitive parallel force measurements are desirable. These can be obtained by positioning the cantilever with its long axis perpendicular to the sample: the so-called pendulum geometry. We present a compact optical beam deflection system which solves the geometrical constraint problems involved in focusing a light beam onto a cantilever in the pendulum geometry. We demonstrate the performance of the system on measurements of forces imparted by a muscle myofibril, which is in-plane to a high-magnification objective of an optical microscope.

Calcium binding kinetics of troponin C strongly modulate cooperative activation and tension kinetics in cardiac muscle

Tension development and relaxation in cardiac muscle are regulated at the thin filament via Ca(2+) binding to cardiac troponin C (cTnC) and strong cross-bridge binding. However, the influence of cTnC Ca(2+)-binding properties on these processes in the organized structure of cardiac sarcomeres is not well-understood and likely differs from skeletal muscle. To study this we generated single amino acid variants of cTnC with altered Ca(2+) dissociation rates (k(off)), as measured in whole troponin (cTn) complex by stopped-flow spectroscopy (I61Q cTn>WT cTn>L48Q cTn), and exchanged them into cardiac myofibrils and demembranated trabeculae. In myofibrils at saturating Ca(2+), L48Q cTnC did not affect maximum tension (T(max)), thin filament activation (k(ACT)) and tension development (k(TR)) rates, or the rates of relaxation, but increased duration of slow phase relaxation. In contrast, I61Q cTnC reduced T(max), k(ACT) and k(TR) by 40-65% with little change in relaxation. Interestingly, k(ACT) was less than k(TR) with I61Q cTnC, and this difference increased with addition of inorganic phosphate, suggesting that reduced cTnC Ca(2+)-affinity can limit thin filament activation kinetics. Trabeculae exchanged with I61Q cTn had reduced T(max), Ca(2+) sensitivity of tension (pCa(50)), and slope (n(H)) of tension-pCa, while L48Q cTn increased pCa(50) and reduced n(H). Increased cross-bridge cycling with 2-deoxy-ATP increased pCa(50) with WT or L48Q cTn, but not I61Q cTn. We discuss the implications of these results for understanding the role of cTn Ca(2+)-binding properties on the magnitude and rate of tension development and relaxation in cardiac muscle.

Extraction and replacement of the tropomyosin-troponin complex in isolated myofibrils

Tropomyosin (Tm) is an essential component in the regulation of striated muscle contraction. Questions about Tm functional role have been difficult to study because sarcomere Tm content is not as easily manipulated as Troponin (Tn). Here we describe the method we recently developed to replace Tm-Tn of skeletal and cardiac myofibrils from animals and humans to generate an experimental model of homogeneous Tm composition and giving the possibility to measure a wide range of mechanical parameters of contraction (e.g. maximal force and kinetics of force generation). The success of the exchange was determined by SDS-PAGE and by mechanical measurements of calcium dependent force activation on the reconstituted myofibrils. In skeletal and cardiac myofibrils, the percentage of Tm replacement was higher than 90%. Maximal isometric tension was 30-35% lower in the reconstituted myofibrils than in control myofibrils but the rate of force activation (k(ACT)) and that of force redevelopment (k(TR)) were not significantly changed. Preliminary results show the effectiveness of Tm replacement in human cardiac myofibrils. This approach can be used to test the functional impact of Tm mutations responsible for human cardiomyopathies.

Differences between cardiac and skeletal troponin interaction with the thin filament probed by troponin exchange in skeletal myofibrils

Troponin (Tn) is the calcium-sensing protein of the thin filament. Although cardiac troponin (cTn) and skeletal troponin (sTn) accomplish the same function, their subunit interactions within Tn and with actin-tropomyosin are different. To further characterize these differences, myofibril ATPase activity as a function of pCa and labeled Tn exchange in rigor myofibrils was used to estimate Tn dissociation rates from the nonoverlap and overlap region as a function of pCa. Measurement of ATPase activity showed that skeletal myofibrils containing >96% cTn had a higher pCa 9 ATPase activity than, but similar pCa 4 activity to, sTn-containing myofibrils. Analysis of the pCa-ATPase activity relation showed that cTn myofibrils were more calcium sensitive but less cooperative (pCa50 = 6.14, nH = 1.46) than sTn myofibrils (pCa50= 5.90, nH = 3.36). The time course of labeled Tn exchange at pCa 9 and 4 were quite different between cTn and sTn. The apparent cTn dissociation rates were approximately 2-10-fold faster than sTn under all the conditions studied. The apparent dissociation rates for cTn were 5 x 10(-3) min(-1), 150 x 10(-3) min(-1), and 260 x 10(-3) min(-1), whereas for sTn they were 0.6 x 10(-3) min(-1), 88 x 10(-3) min(-1), and 68 x 10(-3) min(-1) for the nonoverlap region at pCa 9, nonoverlap region at pCa 4, and overlap region at pCa 4, respectively. Normalization of the apparent dissociation rates gives 1:30:50 for cTn compared with 1:150:110 for sTn (nonoverlap at pCa 9:nonoverlap at pCa 4:overlap at pCa 4) suggesting that calcium has a smaller influence, whereas strong cross-bridges have a larger influence on cTn dissociation compared with sTn. The higher cTn dissociation rate in the nonoverlap region and ATPase activity at pCa 9 suggest that it gives a less off or inactive thin filament. Analysis of the intensity ratio (after a short time of exchange) as a function of pCa showed that cTn had greater calcium sensitivity but lower cooperativity than sTn. In addition, the magnitude of the change in intensity ratio going from pCa 9 to 4 was less for cTn than sTn. These data suggest that the influence of calcium on cTn exchange is less than sTn even though calcium can activate ATPase activity to a similar extent in cTn compared with sTn myofibrils. This may be explained partially by cTn being less off or inactive at pCa 9. Modeling of the intensity profiles obtained after Tn exchange at pCa 5.8 suggest that the profiles are best explained by a model that includes a long-range cross-bridge effect that grades with distance from the rigor cross-bridge for both cTn and sTn.

Mechanical and kinetic effects of shortened tropomyosin reconstituted into myofibrils

The effects of tropomyosin on muscle mechanics and kinetics were examined in skeletal myofibrils using a novel method to remove tropomyosin (Tm) and troponin (Tn) and then replace these proteins with altered versions. Extraction employed a low ionic strength rigor solution, followed by sequential reconstitution at physiological ionic strength with Tm then Tn. SDS-PAGE analysis was consistent with full reconstitution, and fluorescence imaging after reconstitution using Oregon-green-labeled Tm indicated the expected localization. Myofibrils remained mechanically viable: maximum isometric forces of myofibrils after sTm/sTn reconstitution (control) were comparable (~84%) to the forces generated by non-reconstituted preparations, and the reconstitution minimally affected the rate of isometric activation (k_act), calcium sensitivity (pCa₅₀), and cooperativity (n_H). Reconstitutions using various combinations of cardiac and skeletal Tm and Tn indicated that isoforms of both Tm and Tn influence calcium sensitivity of force development in opposite directions, but the isoforms do not otherwise alter cross-bridge kinetics. Myofibrils reconstituted with Δ23Tm, a deletion mutant lacking the second and third of Tm’s seven quasi-repeats, exhibited greatly depressed maximal force, moderately slower k_act rates and reduced n_H. Δ23Tm similarly decreased the cooperativity of calcium binding to the troponin regulatory sites of isolated thin filaments in solution. The mechanisms behind these effects of Δ23Tm also were investigated using P_i and ADP jumps. P_i and ADP kinetics were indistinguishable in Δ23Tm myofibrils compared to controls. The results suggest that the deleted region of tropomyosin is important for cooperative thin filament activation by calcium.

Insights into the kinetics of Ca2+-regulated contraction and relaxation from myofibril studies

Muscle contraction results from force-generating interactions between myosin cross-bridges on the thick filament and actin on the thin filament. The force-generating interactions are regulated by Ca(2+) via specialised proteins of the thin filament. It is controversial how the contractile and regulatory systems dynamically interact to determine the time course of muscle contraction and relaxation. Whereas kinetics of Ca(2+)-induced thin-filament regulation is often investigated with isolated proteins, force kinetics is usually studied in muscle fibres. The gap between studies on isolated proteins and structured fibres is now bridged by recent techniques that analyse the chemical and mechanical kinetics of small components of a muscle fibre, subcellular myofibrils isolated from skeletal and cardiac muscle. Formed of serially arranged repeating units called sarcomeres, myofibrils have a complete fully structured ensemble of contractile and Ca(2+) regulatory proteins. The small diameter of myofibrils (few micrometres) facilitates analysis of the kinetics of sarcomere contraction and relaxation induced by rapid changes of [ATP] or [Ca(2+)]. Among the processes studied on myofibrils are: (1) the Ca(2+)-regulated switch on/off of the troponin complex, (2) the chemical steps in the cross-bridge adenosine triphosphatase cycle, (3) the mechanics of force generation and (4) the length dynamics of individual sarcomeres. These studies give new insights into the kinetics of thin-filament regulation and of cross-bridge turnover, how cross-bridges transform chemical energy into mechanical work, and suggest that the cross-bridge ensembles of each half-sarcomere cooperate with each other across the half-sarcomere borders. Additionally, we now have a better understanding of muscle relaxation and its impairment in certain muscle diseases.

Troponin and titin coordinately regulate length-dependent activation in skinned porcine ventricular muscle

We investigated the molecular mechanism by which troponin (Tn) regulates the Frank-Starling mechanism of the heart. Quasi-complete reconstitution of thin filaments with rabbit fast skeletal Tn (sTn) attenuated length-dependent activation in skinned porcine left ventricular muscle, to a magnitude similar to that observed in rabbit fast skeletal muscle. The rate of force redevelopment increased upon sTn reconstitution at submaximal levels, coupled with an increase in Ca²⁺ sensitivity of force, suggesting the acceleration of cross-bridge formation and, accordingly, a reduction in the fraction of resting cross-bridges that can potentially produce additional active force. An increase in titin-based passive force, induced by manipulating the prehistory of stretch, enhanced length-dependent activation, in both control and sTn-reconstituted muscles. Furthermore, reconstitution of rabbit fast skeletal muscle with porcine left ventricular Tn enhanced length-dependent activation, accompanied by a decrease in Ca²⁺ sensitivity of force. These findings demonstrate that Tn plays an important role in the Frank-Starling mechanism of the heart via on–off switching of the thin filament state, in concert with titin-based regulation.

Thin filament Ca2+ binding properties and regulatory unit interactions alter kinetics of tension development and relaxation in rabbit skeletal muscle

The influence of Ca(2+) binding properties of individual troponin versus cooperative regulatory unit interactions along thin filaments on the rate tension develops and declines was examined in demembranated rabbit psoas fibres and isolated myofibrils. Native skeletal troponin C (sTnC) was replaced with sTnC mutants having altered Ca(2+) dissociation rates (k(off)) or with mixtures of sTnC and D28A, D64A sTnC, that does not bind Ca(2+) at sites I and II (xxsTnC), to reduce near-neighbour regulatory unit (RU) interactions. At saturating Ca(2+), the rate of tension redevelopment (k(TR)) was not altered for fibres containing sTnC mutants with decreased k(off) or mixtures of sTnC:xxsTnC. We examined the influence of k(off) on maximal activation and relaxation in myofibrils because they allow rapid and large changes in [Ca(2+)]. In myofibrils with M80Q sTnC(F27W) (decreased k(off)), maximal tension, activation rate (k(ACT)), k(TR) and rates of relaxation were not altered. With I60Q sTnC(F27W) (increased k(off)), maximal tension, k(ACT) and k(TR) decreased, with no change in relaxation rates. Surprisingly, the duration of the slow phase of relaxation increased or decreased with decreased or increased k(off), respectively. For all sTnC reconstitution conditions, Ca(2+) dependence of k(TR) in fibres showed Ca(2+) sensitivity of k(TR) (pCa(50)) shifted parallel to tension and low-Ca(2+) k(TR) was elevated. Together the data suggest the Ca(2+)-dependent rate of tension development and the duration (but not rate) of relaxation can be greatly influenced by k(off) of sTnC. This influence of sTnC binding kinetics occurs primarily within individual RUs, with only minor contributions of RU interactions at low Ca(2+).

Force transients and minimum cross-bridge models in muscular contraction

Two- and three-state cross-bridge models are considered and examined with respect to their ability to predict three distinct phases of the force transients that occur in response to step change in muscle fiber length. Particular attention is paid to satisfying the Le Châtelier-Brown Principle. This analysis shows that the two-state model can account for phases 1 and 2 of a force transient, but is barely adequate to account for phase 3 (delayed force) unless a stretch results in a sudden increase in the number of cross-bridges in the detached state. The three-state model (A-->B-->C-->A) makes it possible to account for all three phases if we assume that the A-->B transition is fast (corresponding to phase 2), the B-->A transition is of intermediate speed (corresponding to phase 3), and the C-->A transition is slow; in such a scenario, states A and C can support or generate force (high force states) but state B cannot (detached, or low-force state). This model involves at least one ratchet mechanism. In this model, force can be generated by either of two transitions: B-->A or B-->C. To determine which of these is the major force-generating step that consumes ATP and transduces energy, we examine the effects of ATP, ADP, and phosphate (Pi) on force transients. In doing so, we demonstrate that the fast transition (phase 2) is associated with the nucleotide-binding step, and that the intermediate-speed transition (phase 3) is associated with the Pi-release step. To account for all the effects of ligands, it is necessary to expand the three-state model into a six-state model that includes three ligand-bound states. The slowest phase of a force transient (phase 4) cannot be explained by any of the models described unless an additional mechanism is introduced. Here we suggest a role of series compliance to account for this phase, and propose a model that correlates the slowest step of the cross-bridge cycle (transition C-->A) to: phase 4 of step analysis, the rate constant k(tr) of the quick-release and restretch experiment, and the rate constant k(act) for force development time course following Ca(2+) activation.

A novel mutant cardiac troponin C disrupts molecular motions critical for calcium binding affinity and cardiomyocyte contractility

Troponin C (TnC) belongs to the superfamily of EF-hand (helix-loop-helix) Ca(2+)-binding proteins and is an essential component of the regulatory thin filament complex. In a patient diagnosed with idiopathic dilated cardiomyopathy, we identified two novel missense mutations localized in the regulatory Ca(2+)-binding Site II of TnC, TnC((E59D,D75Y)). Expression of recombinant TnC((E59D,D75Y)) in isolated rat cardiomyocytes induced a marked decrease in contractility despite normal intracellular calcium homeostasis in intact cardiomyocytes and resulted in impaired myofilament calcium responsiveness in Triton-permeabilized cardiomyocytes. Expression of the individual mutants in cardiomyocytes showed that TnC(D75Y) was able to recapitulate the TnC((E59D,D75Y)) phenotype, whereas TnC(E59D) was functionally benign. Force-pCa relationships in TnC((E59D,D75Y)) reconstituted rabbit psoas fibers and fluorescence spectroscopy of TnC((E59D,D75Y)) labeled with 2-[(4'-iodoacetamide)-aniline]naphthalene-6-sulfonic acid showed a decrease in myofilament Ca(2+) sensitivity and Ca(2+) binding affinity, respectively. Furthermore, computational analysis of TnC showed the Ca(2+)-binding pocket as an active region of concerted motions, which are decreased markedly by mutation D75Y. We conclude that D75Y interferes with proper concerted motions within the regulatory Ca(2+)-binding pocket of TnC that hinders the relay of the thin filament calcium signal, thereby providing a primary stimulus for impaired cardiomyocyte contractility. This in turn may trigger pathways leading to aberrant ventricular remodeling and ultimately a dilated cardiomyopathy phenotype.

The origin of passive force enhancement in skeletal muscle

The aim of the present study was to test whether titin is a calcium-dependent spring and whether it is the source of the passive force enhancement observed in muscle and single fiber preparations. We measured passive force enhancement in troponin C (TnC)-depleted myofibrils in which active force production was completely eliminated. The TnC-depleted construct allowed for the investigation of the effect of calcium concentration on passive force, without the confounding effects of actin-myosin cross-bridge formation and active force production. Passive forces in TnC-depleted myofibrils ( n = 6) were 35.0 ± 2.9 nN/ μm2when stretched to an average sarcomere length of 3.4 μm in a solution with low calcium concentration (pCa 8.0). Passive forces in the same myofibrils increased by 25% to 30% when stretches were performed in a solution with high calcium concentration (pCa 3.5). Since it is well accepted that titin is the primary source for passive force in rabbit psoas myofibrils and since the increase in passive force in TnC-depleted myofibrils was abolished after trypsin treatment, our results suggest that increasing calcium concentration is associated with increased titin stiffness. However, this calcium-induced titin stiffness accounted for only ∼25% of the passive force enhancement observed in intact myofibrils. Therefore, ∼75% of the normally occurring passive force enhancement remains unexplained. The findings of the present study suggest that passive force enhancement is partly caused by a calcium-induced increase in titin stiffness but also requires cross-bridge formation and/or active force production for full manifestation.

Modulation of the rate of cardiac muscle contraction by troponin C constructs with various calcium binding affinities

We investigated whether changing thin filament Ca2+sensitivity alters the rate of contraction, either during normal cross-bridge cycling or when cross-bridge cycling is increased by inorganic phosphate (Pi). We increased or decreased Ca2+sensitivity of force production by incorporating into rat skinned cardiac trabeculae the troponin C (TnC) mutants V44QTnCF27Wand F20QTnCF27W. The rate of isometric contraction was assessed as the rate of force redevelopment ( ktr) after a rapid release and restretch to the original length of the muscle. Both in the absence of added Piand in the presence of 2.5 mM added Pi1) Ca2+sensitivity of ktrwas increased by V44QTnCF27Wand decreased by F20QTnCF27Wcompared with control TnCF27W; 2) ktrat submaximal Ca2+activation was significantly faster for V44QTnCF27Wand slower for F20QTnCF27Wcompared with control TnCF27W; 3) at maximum Ca2+activation, ktrvalues were similar for control TnCF27W, V44QTnCF27W, and F20QTnCF27W; and 4) ktrexhibited a linear dependence on force that was indistinguishable for all TnCs. In the presence of 2.5 mM Pi, ktrwas faster at all pCa values compared with the values for no added Pifor TnCF27W, V44QTnCF27W, and F20QTnCF27W. This study suggests that TnC Ca2+binding properties modulate the rate of cardiac muscle contraction at submaximal levels of Ca2+activation. This result has physiological relevance considering that, on a beat-to-beat basis, the heart contracts at submaximal Ca2+activation.

Myofilament calcium sensitivity does not affect cross-bridge activation-relaxation kinetics

We employed single myofibril techniques to test whether the presence of slow skeletal troponin-I (ssTnI) is sufficient to induce increased myofilament calcium sensitivity (EC50) and whether modulation of EC50 affects the dynamics of force development. Studies were performed using rabbit psoas myofibrils activated by rapid solution switch and in which Tn was partially replaced for either recombinant cardiac Tn(cTn) or Tn composed of recombinant cTn-T (cTnT) and cTn-C (cTnC), and recombinant ssTnI (ssTnI-chimera Tn). Tn exchange was performed in rigor solution (0.5 mg/ml Tn; 20°C; 2 h) and confirmed by SDS-PAGE. cTnI exchange induced a decrease in EC50; ssTnI-chimera Tn exchange induced a further decrease in EC50 (in μM: endogenous Tn, 1.35 ± 0.08; cTnI, 1.04 ± 0.13; ssTnI-chimera Tn, 0.47 ± 0.03). EC50 was also decreased by application of 100 μM bepridil (control: 2.04 ± 0.03 μM; bepridil 1.35 ± 0.03 μM). Maximum tension was not different between any groups. Despite marked alterations in EC50, none of the dynamic activation-relaxation parameters were affected under any condition. Our results show that 1) incorporation of ssTnI into the fast skeletal sarcomere is sufficient to induce increased myofilament Ca2+ sensitivity, and 2) the dynamics of actin-myosin interaction do not correlate with EC50. This result suggests that intrinsic cross-bridge cycling rate is not altered by the dynamics of thin-filament activation.

Thin-filament regulation of force redevelopment kinetics in rabbit skeletal muscle fibres

Thin-filament regulation of isometric force redevelopment (k(tr)) was examined in rabbit psoas fibres by substituting native TnC with either cardiac TnC (cTnC), a site I-inactive skeletal TnC mutant (xsTnC), or mixtures of native purified skeletal TnC (sTnC) and a site I- and II-inactive skeletal TnC mutant (xxsTnC). Reconstituted maximal Ca(2+)-activated force (rF(max)) decreased as the fraction of sTnC in sTnC: xxsTnC mixtures was reduced, but maximal k(tr) was unaffected until rF(max) was <0.2 of pre-extracted F(max). In contrast, reconstitution with cTnC or xsTnC reduced maximal k(tr) to 0.48 and 0.44 of control (P < 0.01), respectively, with corresponding rF(max) of 0.68 +/- 0.03 and 0.25 +/- 0.02 F(max). The k(tr)-pCa relation of fibres containing sTnC: xxsTnC mixtures (rF(max) > 0.2 F(max)) was little effected, though k(tr) was slightly elevated at low Ca(2+) activation. The magnitude of the Ca(2+)-dependent increase in k(tr) was greatly reduced following cTnC or xsTnC reconstitution because k(tr) at low levels of Ca(2+) was elevated and maximal k(tr) was reduced. Solution Ca(2+) dissociation rates (k(off)) from whole Tn complexes containing sTnC (26 +/- 0.1 s(-1)), cTnC (38 +/- 0.9 s(-1)) and xsTnC (50 +/- 1.2 s(-1)) correlated with k(tr) at low Ca(2+) levels and were inversely related to rF(max). At low Ca(2+) activation, k(tr) was similarly elevated in cTnC-reconstituted fibres with ATP or when cross-bridge cycling rate was increased with 2-deoxy-ATP. Our results and model simulations indicate little or no requirement for cooperative interactions between thin-filament regulatory units in modulating k(tr) at any [Ca(2+)] and suggest Ca(2+) activation properties of individual troponin complexes may influence the apparent rate constant of cross-bridge detachment.

Impaired Diastolic Function After Exchange of Endogenous Troponin I With C-Terminal Truncated Troponin I in Human Cardiac Muscle

The specific and selective proteolysis of cardiac troponin I (cTnI) has been proposed to play a key role in human ischemic myocardial disease, including stunning and acute pressure overload. In this study, the functional implications of cTnI proteolysis were investigated in human cardiac tissue for the first time. The predominant human cTnI degradation product (cTnI 1–192 ) and full-length cTnI were expressed in Escherichia coli , purified, reconstituted with the other cardiac troponin subunits, troponin T and C, and subsequently exchanged into human cardiac myofibrils and permeabilized cardiomyocytes isolated from healthy donor hearts. Maximal isometric force and kinetic parameters were measured in myofibrils, using rapid solution switching, whereas force development was measured in single cardiomyocytes at various calcium concentrations, at sarcomere lengths of 1.9 and 2.2 μm, and after treatment with the catalytic subunit of protein kinase A (PKA) to mimic β-adrenergic stimulation. One-dimensional gel electrophoresis, Western immunoblotting, and 3D imaging revealed that approximately 50% of endogenous cTnI had been homogeneously replaced by cTnI 1–192 in both myofibrils and cardiomyocytes. Maximal tension was not affected, whereas the rates of force activation and redevelopment as well as relaxation kinetics were slowed down. Ca 2+ sensitivity of the contractile apparatus was increased in preparations containing cTnI 1–192 (pCa 50 : 5.73±0.03 versus 5.52±0.03 for cTnI 1–192 and full-length cTnI, respectively). The sarcomere length dependency of force development and the desensitizing effect of PKA were preserved in cTnI 1–192 -exchanged cardiomyocytes. These results indicate that degradation of cTnI in human myocardium may impair diastolic function, whereas systolic function is largely preserved.

Myofibrillar troponin exists in three states and there is signal transduction along skeletal myofibrillar thin filaments

Activation of striated muscle contraction is a highly cooperative signal transduction process converting calcium binding by troponin C (TnC) into interactions between thin and thick filaments. Once calcium is bound, transduction involves changes in protein interactions along the thin filament. The process is thought to involve three different states of actin-tropomyosin (Tm) resulting from changes in troponin's (Tn) interaction with actin-Tm: a blocked (B) state preventing myosin interaction, a closed (C) state allowing weak myosin interactions and favored by calcium binding to Tn, and an open or M state allowing strong myosin interactions. This was tested by measuring the apparent rate of Tn dissociation from rigor skeletal myofibrils using labeled Tn exchange. The location and rate of exchange of Tn or its subunits were measured by high-resolution fluorescence microscopy and image analysis. Three different rates of Tn exchange were observed that were dependent on calcium concentration and strong cross-bridge binding that strongly support the three-state model. The rate of Tn dissociation in the non-overlap region was 200-fold faster at pCa 4 (C-state region) than at pCa 9 (B-state region). When Tn contained engineered TnC mutants with weakened regulatory TnI interactions, the apparent exchange rate at pCa 4 in the non-overlap region increased proportionately with TnI-TnC regulatory affinity. This suggests that the mechanism of calcium enhancement of the rate of Tn dissociation is by favoring a TnI-TnC interaction over a TnI-actin-Tm interaction. At pCa 9, the rate of Tn dissociation in the overlap region (M-state region) was 100-fold faster than the non-overlap region (B-state region) suggesting that strong cross-bridges increase the rate of Tn dissociation. At pCa 4, the rate of Tn dissociation was twofold faster in the non-overlap region (C-state region) than the overlap region (M-state region) that likely involved a strong cross-bridge influence on TnT's interaction with actin-Tm. At sub-maximal calcium (pCa 6.2-5.8), there was a long-range influence of the strong cross-bridge on Tn to enhance its dissociation rate, tens of nanometers from the strong cross-bridge. These observations suggest that the three different states of actin-Tm are associated with three different states of Tn. They also support a model in which strong cross-bridges shift the regulatory equilibrium from a TnI-actin-Tm interaction to a TnC-TnI interaction that likely enhances calcium binding by TnC.

Effects of troponin C isoform on the action of the cardiotonic agent EMD 57033

The effects of the cardiotonic potentiator EMD 57033 on different TnC (troponin C) isoforms were investigated. Endogenous skeletal TnC was extracted from glycerinated, permeabilized rabbit psoas fibres and replaced with either purified native rabbit psoas TnC (fast TnC) or human recombinant cTnC (cardiac TnC) (3 mg/ml in relaxing solution for 30 min). In both conditions, 10 microM EMD 57033 increased maximal calcium-activated force (Pmax) and gave a leftward shift in the pCa-tension curve. With cTnC, the increase in Pmax was much greater (228%) compared with the effect seen for fast TnC (137%), which was the same as that in unextracted control fibres. When the whole troponin was replaced rather than just TnC, the effects of EMD 57033 on fibres replaced with cTn were the same as with the cTnC subunit alone, except that the force at low Ca2+ concentrations was not increased as much. If TnC was only partially extracted, it was found that the degree of extraction did not influence the effect of EMD 57033, except when force was decreased to below 10% of the pre-extraction Pmax. Dynamic stiffness was not altered by EMD 57033 in any of the preparations. The rate of tension recovery following a release-restretch method (ktr) was decreased by EMD 57033. We conclude that EMD 57033 acts by a rate-modulating effect, and that the quantitative response of this effect is dependent on the TnC isoform present.

Different effects of cardiac versus skeletal muscle regulatory proteins on in vitro measures of actin filament speed and force

Mammalian cardiac and skeletal muscle express unique isoforms of the thin filament regulatory proteins, troponin (Tn) and tropomyosin (Tm), and the significance of these different isoforms in thin filament regulation has not been clearly identified. Both in vitro and skinned cellular studies investigating the mechanism of thin filament regulation in striated muscle have often used heterogeneous mixtures of Tn, Tm and myosin isoforms, and variability in reported results might be explained by different combinations of these proteins. Here we used in vitro motility and force (microneedle) assays to investigate the influence of cardiac versus skeletal Tn and Tm isoforms on actin-heavy meromyosin (HMM) mechanics. When interacting with skeletal HMM, thin filaments reconstituted with cardiac Tn/Tm or skeletal Tn/Tm exhibited similar speed-calcium relationships and significantly increased maximum speed and force per filament length (F/l) at pCa 5 (versus unregulated actin filaments). However, augmentation of F/l was greater with skeletal regulatory proteins. Reconstitution of thin filaments with the heterogeneous combination of skeletal Tn and cardiac Tm decreased sliding speeds at all [Ca2+] relative to thin filaments with skeletal Tn/Tm. Finally, for filaments reconstituted with any heterogeneous mix of Tn and Tm isoforms, force was not potentiated over that of unregulated actin filaments. Combined the results suggest (1) that cardiac regulatory proteins limit the allosteric enhancement of force, and (2) that Tn and Tm isoform homogeneity is important when studying Ca2+ regulation of crossbridge binding and kinetics as well as mechanistic differences between cardiac and skeletal muscle.

Cardiac troponin C (TnC) and a site I skeletal TnC mutant alter Ca2+ versus crossbridge contribution to force in rabbit skeletal fibres

We studied the relative contributions of Ca(2+) binding to troponin C (TnC) and myosin binding to actin in activating thin filaments of rabbit psoas fibres. The ability of Ca(2+) to activate thin filaments was reduced by replacing native TnC with cardiac TnC (cTnC) or a site I-inactive skeletal TnC mutant (xsTnC). Acto-myosin (crossbridge) interaction was either inhibited using N-benzyl-p-toluene sulphonamide (BTS) or enhanced by lowering [ATP] from 5.0 to 0.5 mm. Reconstitution with cTnC reduced maximal force (F(max)) by approximately 1/3 and the Ca(2+) sensitivity of force (pCa(50)) by 0.17 unit (P < 0.001), while reconstitution with xsTnC reduced F(max) by approximately 2/3 and pCa(50) by 0.19 unit (P < 0.001). In both cases the apparent cooperativity of activation (n(H)) was greatly decreased. In control fibres 3 mum BTS inhibited force to 57% of F(max) while in fibres reconstituted with cTnC or xsTnC, reconstituted maximal force (rF(max)) was inhibited to 8.8% and 14.3%, respectively. Under control conditions 3 mum BTS significantly decreased the pCa(50), but this effect was considerably reduced in cTnC reconstituted fibres, and eliminated in xsTnC reconstituted fibres. In contrast, when crossbridge cycle kinetics were slowed by lowering [ATP] from 5 to 0.5 mm in xsTnC reconstituted fibres, pCa(50) and n(H) were increased towards control values. Combined, our results demonstrate that when the ability of Ca(2+) binding to activate thin filaments is compromised, the relative contribution of strong crossbridges to maintain thin filament activation is increased. Furthermore, the data suggest that at low levels of Ca(2+), the level of thin filament activation is determined primarily by the direct effects of Ca(2+) on tropomyosin mobility, while at higher levels of Ca(2+) the final level of thin filament activation is primarily determined by strong cycling crossbridges.

Sarcomeric determinants of striated muscle relaxation kinetics

Ca2+ is the primary regulator of force generation by cross-bridges in striated muscle activation and relaxation. Relaxation is as necessary as contraction and, while the kinetics of Ca2+-induced force development have been investigated extensively, those of force relaxation have been both studied and understood less well. Knowledge of the molecular mechanisms underlying relaxation kinetics is of special importance for understanding diastolic function and dysfunction of the heart. A number of experimental models, from whole muscle organs and intact muscle fibres down to single myofibrils, have been used to explore the cascade of kinetic events leading to mechanical relaxation. By using isolated myofibrils and fast solution switching techniques we can distinguish the sarcomeric mechanisms of relaxation from those of myoplasmic Ca2+ removal. There is strong evidence that cross-bridge mechanics and kinetics are major determinants of the time course of striated muscle relaxation whilst thin filament inactivation kinetics and cooperative activation of thin filament by cycling, force-generating cross-bridges do not significantly limit the relaxation rate. Results in myofibrils can be explained well by a simple two-state model of the cross-bridge cycle in which the apparent rate of the force generating transition is modulated by fast, Ca2+-dependent equilibration between off- and on-states of actin. Inter-sarcomere dynamics during the final rapid phase of full force relaxation are responsible for deviations from this simple model.

Contractile effects of the exchange of cardiac troponin for fast skeletal troponin in rabbit psoas single myofibrils

The effects of the removal of fast skeletal troponin C (fsTnC) and its replacement by cardiac troponin C (cTnC) and the exchange of fast skeletal troponin (fsTn) for cardiac troponin (cTn) were measured in rabbit fast skeletal myofibrils. Electrophoretic analysis of myofibril suspensions indicated that replacement of fsTnC or exchange of fsTn with cTnC or cTn was about 90% complete in the protocols used. Mechanical measurements in single myofibrils, which were maximally activated by fast solution switching, showed that replacement of fsTnC with cTnC reduced the isometric tension, the rate of tension rise following a step increase in Ca2+ (kACT), and the rate of tension redevelopment following a quick release and restretch (kTR), but had no effect on the kinetics of the fall in tension when the concentration of inorganic phosphate (Pi) was abruptly increased (kPi(+)). These data suggest that the chimeric protein produced by cTnC replacement in fsTn alters those steps controlling the weak-to-strong crossbridge attachment transition. Inefficient signalling within the chimeric troponin may cause these changes. However, replacement of fsTn by cTn had no effect on maximal isometric tension, kACT or kTR, suggesting that these mechanics are largely determined by the isoform of the myosin molecule. Replacement of fsTn by cTn, on the other hand, shifted the pCa50 of the pCa-tension relationship from 5.70 to 6.44 and reduced the Hill coefficient from 3.3 to 1.4, suggesting that regulatory protein isoforms primarily alter Ca2+ sensitivity and the cooperativity of the force-generating mechanism.