Influence of a strong-binding myosin analogue on calcium-sensitive mechanical properties of skinned skeletal muscle fibers

Cooperative mechanisms underlie differences in myocardial contractile dynamics between large and small mammals

Ca2+ binding to troponin C (TnC) and myosin cross-bridge binding to actin act in a synergistic cooperative manner to modulate myocardial contraction and relaxation. The responsiveness of the myocardial thin filament to the activating effects of Ca2+ and myosin cross-bridge binding has been well-characterized in small mammals (e.g., mice). Given the nearly 10-fold difference in resting heart rates and twitch kinetics between small and large mammals, it is unlikely that the cooperative mechanisms underlying thin filament activation are identical in these two species. To test this idea, we measured the Ca2+ dependencies of steady-state force and the rate constant of force redevelopment (ktr) in murine and porcine permeabilized ventricular myocardium. While murine myocardium exhibited a steep activation-dependence of ktr, the activation-dependent profile of ktr was significantly reduced in porcine ventricular myocardium. Further insight was attained by examining force-pCa and ktr-pCa relationships. In the murine myocardium, the pCa50 for ktr was right-shifted compared with the pCa50 for force, meaning that increases in steady-state force occurred well before increases in the rate of force redevelopment were observed. In the porcine myocardium, we observed a tighter coupling of the force-pCa and ktr-pCa relationships, as evidenced by near-maximal rates of force redevelopment at low levels of Ca2+ activation. These results demonstrate that the molecular mechanisms underlying the cooperative activation of force are a dynamic property of the mammalian heart, involving, at least in part, the species- and tissue-specific expression of cardiac myosin heavy chain isoforms.

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.

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

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

Force variability is mostly not motor noise: Theoretical implications for motor control

Variability in muscle force is a hallmark of healthy and pathological human behavior. Predominant theories of sensorimotor control assume 'motor noise' leads to force variability and its 'signal dependence' (variability in muscle force whose amplitude increases with intensity of neural drive). Here, we demonstrate that the two proposed mechanisms for motor noise (i.e. the stochastic nature of motor unit discharge and unfused tetanic contraction) cannot account for the majority of force variability nor for its signal dependence. We do so by considering three previously underappreciated but physiologically important features of a population of motor units: 1) fusion of motor unit twitches, 2) coupling among motoneuron discharge rate, cross-bridge dynamics, and muscle mechanics, and 3) a series-elastic element to account for the aponeurosis and tendon. These results argue strongly against the idea that force variability and the resulting kinematic variability are generated primarily by 'motor noise.' Rather, they underscore the importance of variability arising from properties of control strategies embodied through distributed sensorimotor systems. As such, our study provides a critical path toward developing theories and models of sensorimotor control that provide a physiologically valid and clinically useful understanding of healthy and pathologic force variability.

Regulation of myofilament force and loaded shortening by skeletal myosin binding protein C

Myosin binding protein C (MyBP-C) is thought to regulate the contraction of skeletal muscle. Robinett et al. show that phosphorylation of slow skeletal MyBP-C modulates contraction by recruiting cross-bridges, modifying cross-bridge kinetics, and altering internal drag forces in the C-zone.

Myosin binding protein C (MyBP-C) is a 125–140-kD protein located in the C-zone of each half-thick filament. It is thought to be an important regulator of contraction, but its precise role is unclear. Here we investigate mechanisms by which skeletal MyBP-C regulates myofilament function using rat permeabilized skeletal muscle fibers. We mount either slow-twitch or fast-twitch skeletal muscle fibers between a force transducer and motor, use Ca²⁺ to activate a range of forces, and measure contractile properties including transient force overshoot, rate of force development, and loaded sarcomere shortening. The transient force overshoot is greater in slow-twitch than fast-twitch fibers at all Ca²⁺ activation levels. In slow-twitch fibers, protein kinase A (PKA) treatment (a) augments phosphorylation of slow skeletal MyBP-C (sMyBP-C), (b) doubles the magnitude of the relative transient force overshoot at low Ca²⁺ activation levels, and (c) increases force development rates at all Ca²⁺ activation levels. We also investigate the role that phosphorylated and dephosphorylated sMyBP-C plays in loaded sarcomere shortening. We test the hypothesis that MyBP-C acts as a brake to filament sliding within the myofilament lattice by measuring sarcomere shortening as thin filaments traverse into the C-zone during lightly loaded slow-twitch fiber contractions. Before PKA treatment, shortening velocity decelerates as sarcomeres traverse from ∼3.10 to ∼3.00 µm. After PKA treatment, sarcomeres shorten a greater distance and exhibit less deceleration during similar force clamps. After sMyBP-C dephosphorylation, sarcomere length traces display a brief recoil (i.e., “bump”) that initiates at ∼3.06 µm during loaded shortening. Interestingly, the timing of the bump shifts with changes in load but manifests at the same sarcomere length. Our results suggest that sMyBP-C and its phosphorylation state regulate sarcomere contraction by a combination of cross-bridge recruitment, modification of cross-bridge cycling kinetics, and alteration of drag forces that originate in the C-zone.

Interplay of actin, ADP and Mg2+ interactions with striated muscle myosin: Implications of their roles in ATPase

The effects of Mg²⁺ on the interaction between ADP, a product of the ATPase reaction, and striated muscle myosin-subfragment 1 (S1) were investigated with both functional and spectroscopic methods. Mg²⁺ inhibited striated muscle myosin ATPase in the presence of F-actin. Significant effects of Mg²⁺ were observed in both rate constants of NOE build-up and maximal intensities in WaterLOGSY NMR experiments as F-actin concentration increased. In the absence of F-actin, myosin S1 with Mg²⁺ bound to a fluorescent ADP analog about five-times tighter than without Mg²⁺. In the presence of F-actin, the affinity of myosin S1 toward the ADP analog significantly decreased both with and without Mg²⁺. The equilibrium titration of myosin-S1 into F-actin revealed that in the presence of ADP the apparent dissociation constant (K_d) without Mg²⁺ was more than five-fold smaller than with Mg²⁺. Further, we examined effects of F-actin, ADP and Mg²⁺ binding to myosin on the tertiary structure of myosin-S1 using near UV circular dichroism (CD) spectroscopy. Both in the presence and absence of ADP, there was a Mg²⁺-dependent difference in the near UV CD spectra of actomyosin. Our results show that Mg²⁺ affects myosin-ADP and actin-myosin interactions which may be reflected in myosin ATPase activity.

Point mutations in the tri-helix bundle of the M-domain of cardiac myosin binding protein-C influence systolic duration and delay cardiac relaxation

Cardiac myosin binding protein-C (cMyBP-C) is an essential regulatory protein required for proper systolic contraction and diastolic relaxation. We previously showed that N'-terminal domains of cMyBP-C stimulate contraction by binding to actin and activating the thin filament in vitro. In principle, thin filament activating effects of cMyBP-C could influence contraction and relaxation rates, or augment force amplitude in vivo. cMyBP-C binding to actin could also contribute to an internal load that slows muscle shortening velocity as previously hypothesized. However, the functional significance of cMyBP-C binding to actin has not yet been established in vivo. We previously identified an actin binding site in the regulatory M-domain of cMyBP-C and described two missense mutations that either increased (L348P) or decreased (E330K) binding affinity of recombinant cMyBP-C N'-terminal domains for actin in vitro. Here we created transgenic mice with either the L348P or E330K mutations to determine the functional significance of cMyBP-C binding to actin in vivo. Results showed that enhanced binding of cMyBP-C to actin in L348P-Tg mice prolonged the time to end-systole and slowed relaxation rates. Reduced interactions between cMyBP-C and actin in E330K-Tg mice had the opposite effect and significantly shortened the duration of ejection. Neither mouse model displayed overt systolic dysfunction, but L348P-Tg mice showed diastolic dysfunction presumably resulting from delayed relaxation. We conclude that cMyBP-C binding to actin contributes to sustained thin filament activation at the end of systole and during isovolumetric relaxation. These results provide the first functional evidence that cMyBP-C interactions with actin influence cardiac function in vivo.

Ca2+-induced movement of tropomyosin on native cardiac thin filaments revealed by cryoelectron microscopy

Muscle contraction relies on the interaction of myosin motors with F-actin, which is regulated through a translocation of tropomyosin by the troponin complex in response to Ca²⁺ The current model of muscle regulation holds that at relaxing (low-Ca²⁺) conditions tropomyosin blocks myosin binding sites on F-actin, whereas at activating (high-Ca²⁺) conditions tropomyosin translocation only partially exposes myosin binding sites on F-actin so that binding of rigor myosin is required to fully activate the thin filament (TF). Here we used a single-particle approach to helical reconstruction of frozen hydrated native cardiac TFs under relaxing and activating conditions to reveal the azimuthal movement of the tropomyosin on the surface of the native cardiac TF upon Ca²⁺ activation. We demonstrate that at either relaxing or activating conditions tropomyosin is not constrained in one structural state, but rather is distributed between three structural positions on the surface of the TF. We show that two of these tropomyosin positions restrain actomyosin interactions, whereas in the third position, which is significantly enhanced at high Ca²⁺, tropomyosin does not block myosin binding sites on F-actin. Our data provide a structural framework for the enhanced activation of the cardiac TF over the skeletal TF by Ca²⁺ and lead to a mechanistic model for the regulation of the cardiac TF.

Myosin-Induced Gliding Patterns at Varied [MgATP] Unveil a Dynamic Actin Filament

Actin filaments have key roles in cell motility but are generally claimed to be passive interaction partners in actin-myosin-based motion generation. Here, we present evidence against this static view based on an altered myosin-induced actin filament gliding pattern in an in vitro motility assay at varied [MgATP]. The statistics that characterize the degree of meandering of the actin filament paths suggest that for [MgATP] ≥ 0.25 mM, the flexural rigidity of heavy meromyosin (HMM)-propelled actin filaments is similar (without phalloidin) or slightly lower (with phalloidin) than that of HMM-free filaments observed in solution without surface tethering. When [MgATP] was reduced to ≤0.1 mM, the actin filament paths in the in vitro motility assay became appreciably more winding in both the presence and absence of phalloidin. This effect of lowered [MgATP] was qualitatively different from that seen when HMM was mixed with ATP-insensitive, N-ethylmaleimide-treated HMM (NEM-HMM; 25-30%). In particular, the addition of NEM-HMM increased a non-Gaussian tail in the path curvature distribution as well as the number of events in which different parts of an actin filament followed different paths. These effects were the opposite of those observed with reduced [MgATP]. Theoretical modeling suggests a 30-40% lowered flexural rigidity of the actin filaments at [MgATP] ≤ 0.1 mM and local bending of the filament front upon each myosin head attachment. Overall, the results fit with appreciable structural changes in the actin filament during actomyosin-based motion generation, and modulation of the actin filament mechanical properties by the dominating chemomechanical actomyosin state.

Different actions of salt and pyrophosphate on protein extraction from myofibrils reveal the mechanism controlling myosin dissociation

BACKGROUND

Myosin is the major functional protein in muscle foods for water retention, protein binding/gelation and fat holding/emulsification. To maximize its functionality, myosin needs to be released from thick filaments. Understanding of the mechanism controlling myosin extraction will help improve quality traits of meat products.

RESULTS

The data obtained show that actomyosin binding is the rate-limiting constraint for myosin release in rigor condition. Magnesium pyrophosphate (MgPPi) increased myosin extraction by weakening actomyosin interaction and maximized myosin extraction at 0.4 mol L(-1) NaCl, which was not attained at 1.0 mol L(-1) NaCl in the absence of PPi. Interaction between myosin rod domains is another critical constraint for myosin extraction, which is, rather than PPi, salt dependent. Further, our data suggest that MyBP-C (myosin binding protein C) and M-line might not be of significance in the process of NaCl-induced myosin extraction, though further study was needed.

CONCLUSION

Our study provides new insight into the mechanism that controls myosin extraction from intact sarcomere, which could be applied to maximize myosin function and to improve meat quality in practice.

Structural determinants of muscle thin filament cooperativity

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

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

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

Muscle activation described with a differential equation model for large ensembles of locally coupled molecular motors

Molecular motors, by turning chemical energy into mechanical work, are responsible for active cellular processes. Often groups of these motors work together to perform their biological role. Motors in an ensemble are coupled and exhibit complex emergent behavior. Although large motor ensembles can be modeled with partial differential equations (PDEs) by assuming that molecules function independently of their neighbors, this assumption is violated when motors are coupled locally. It is therefore unclear how to describe the ensemble behavior of the locally coupled motors responsible for biological processes such as calcium-dependent skeletal muscle activation. Here we develop a theory to describe locally coupled motor ensembles and apply the theory to skeletal muscle activation. The central idea is that a muscle filament can be divided into two phases: an active and an inactive phase. Dynamic changes in the relative size of these phases are described by a set of linear ordinary differential equations (ODEs). As the dynamics of the active phase are described by PDEs, muscle activation is governed by a set of coupled ODEs and PDEs, building on previous PDE models. With comparison to Monte Carlo simulations, we demonstrate that the theory captures the behavior of locally coupled ensembles. The theory also plausibly describes and predicts muscle experiments from molecular to whole muscle scales, suggesting that a micro- to macroscale muscle model is within reach.

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

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

Titin-mediated control of cardiac myofibrillar function

According to the Frank-Starling relationship, ventricular pressure or stroke volume increases with end-diastolic volume. This is regulated, in large part, by the sarcomere length (SL) dependent changes in cardiac myofibrillar force, loaded shortening, and power. Consistent with this, both cardiac myofibrillar force and absolute power fall at shorter SL. However, when Ca(2+) activated force levels are matched between short and long SL (by increasing the activator [Ca(2+)]), short SL actually yields faster loaded shortening and greater peak normalized power output (PNPO). A potential mechanism for faster loaded shortening at short SL is that, at short SL, titin becomes less taut, which increases the flexibility of the cross-bridges, a process that may be mediated by titin's interactions with thick filament proteins. We propose a more slackened titin yields greater myosin head radial and azimuthal mobility and these flexible cross-bridges are more likely to maintain thin filament activation, which would allow more force-generating cross-bridges to work against a fixed load resulting in faster loaded shortening. We tested this idea by measuring SL-dependence of power at matched forces in rat skinned cardiac myocytes containing either N2B titin or a longer, more compliant N2BA titin. We predicted that, in N2BA titin containing cardiac myocytes, power-load curves would not be shifted upward at short SL compared to long SL (when force is matched). Consistent with this, peak normalized power was actually less at short SL versus long SL (at matched force) in N2BA-containing myocytes (N2BA titin: ΔPNPO (Short SL peak power minus long SL peak power)=-0.057±0.049 (n=5) versus N2B titin: ΔPNPO=+0.012±0.012 (n=5). These findings support a model whereby SL per se controls mechanical properties of cross-bridges and this process is mediated by titin. This myofibrillar mechanism may help sustain ventricular power during periods of low preloads, and perhaps a breakdown of this mechanism is involved in impaired function of failing hearts.

Ca++-sensitizing mutations in troponin, P(i), and 2-deoxyATP alter the depressive effect of acidosis on regulated thin-filament velocity

Repeated, intense contractile activity compromises the ability of skeletal muscle to generate force and velocity, resulting in fatigue. The decrease in velocity is thought to be due, in part, to the intracellular build-up of acidosis inhibiting the function of the contractile proteins myosin and troponin; however, the underlying molecular basis of this process remains poorly understood. We sought to gain novel insight into the decrease in velocity by determining whether the depressive effect of acidosis could be altered by 1) introducing Ca(++)-sensitizing mutations into troponin (Tn) or 2) by agents that directly affect myosin function, including inorganic phosphate (Pi) and 2-deoxy-ATP (dATP) in an in vitro motility assay. Acidosis reduced regulated thin-filament velocity (VRTF) at both maximal and submaximal Ca(++) levels in a pH-dependent manner. A truncated construct of the inhibitory subunit of Tn (TnI) and a Ca(++)-sensitizing mutation in the Ca(++)-binding subunit of Tn (TnC) increased VRTF at submaximal Ca(++) under acidic conditions but had no effect on VRTF at maximal Ca(++) levels. In contrast, both Pi and replacement of ATP with dATP reversed much of the acidosis-induced depression of VRTF at saturating Ca(++). Interestingly, despite producing similar magnitude increases in VRTF, the combined effects of Pi and dATP were additive, suggesting different underlying mechanisms of action. These findings suggest that acidosis depresses velocity by slowing the detachment rate from actin but also by possibly slowing the attachment rate.

Molecular modulation of actomyosin function by cardiac myosin-binding protein C

Cardiac myosin-binding protein C is a key regulator of cardiac contractility and is capable of both activating the thin filament to initiate actomyosin motion generation and governing maximal sliding velocities. While MyBP-C's C terminus localizes the molecule within the sarcomere, the N terminus appears to confer regulatory function by binding to the myosin motor domain and/or actin. Literature pertaining to how MyBP-C binding to the myosin motor domain and or actin leads to MyBP-C's dual modulatory roles that can impact actomyosin interactions are discussed.

Dynamic coupling of regulated binding sites and cycling myosin heads in striated muscle

Myosim, a computer model of muscle contraction, includes molecular-level effects and incorporates dynamic coupling of myosin heads and binding sites.

In an activated muscle, binding sites on the thin filament and myosin heads switch frequently between different states. Because the status of the binding sites influences the status of the heads, and vice versa, the binding sites and myosin heads are dynamically coupled. The functional consequences of this coupling were investigated using MyoSim, a new computer model of muscle. MyoSim extends existing models based on Huxley-type distribution techniques by incorporating Ca²⁺ activation and cooperative effects. It can also simulate arbitrary cross-bridge schemes set by the researcher. Initial calculations investigated the effects of altering the relative speeds of binding-site and cross-bridge kinetics, and of manipulating cooperative processes. Subsequent tests fitted simulated force records to experimental data recorded using permeabilized myocardial preparations. These calculations suggest that the rate of force development at maximum activation is limited by myosin cycling kinetics, whereas the rate at lower levels of activation is limited by how quickly binding sites become available. Additional tests investigated the behavior of transiently activated cells by driving simulations with experimentally recorded Ca²⁺ signals. The unloaded shortening profile of a twitching myocyte could be reproduced using a model with two myosin states, cooperative activation, and strain-dependent kinetics. Collectively, these results demonstrate that dynamic coupling of binding sites and myosin heads is important for contractile function.

Effects of low cell pH and elevated inorganic phosphate on the pCa-force relationship in single muscle fibers at near-physiological temperatures

Intense muscle contraction induces high rates of ATP hydrolysis with resulting increases in Pi, H(+), and ADP, factors thought to induce fatigue by interfering with steps in the cross-bridge cycle. Force inhibition is less at physiological temperatures; thus the role of low pH in fatigue has been questioned. Effects of pH 6.2 and collective effects with 30 mM Pi on the pCa-force relationship were assessed in skinned fast and slow rat skeletal muscle fibers at 15 and 30°C. At 30°C, pH 6.2 + 30 mM Pi significantly depressed peak force in all fiber types, with the greatest effect in type IIx fibers. Across fiber types, Ca(2+) sensitivity was depressed by low pH and low pH + high Pi, with the greater effect at 30°C. For type IIx fibers at 30°C, half-maximal activation (pCa50) was 5.36 at pH 6.2 (no added Pi) and 4.98 at pH 6.2 + 30 mM Pi compared with 6.58 in the control condition (pH 7, no added Pi). At 30°C, n2, reflective of thick filament cooperativity, was unchanged by low cell pH but was depressed from 5.02 to 2.46 in type IIx fibers with pH 6.2 + 30 mM Pi. With acidosis, activation thresholds of all fiber types required higher free Ca(2+) at 15 and 30°C. With the exception of type IIx fibers, the Ca(2+) required to reach activation threshold increased further with added Pi. In conclusion, it is clear that fatigue-inducing effects of low cell pH and elevated Pi at near-physiological temperatures are substantial.

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.

Length dependence of striated muscle force generation is controlled by phosphorylation of cTnI at serines 23/24

  According to the Frank-Starling relationship, greater end-diastolic volume increases ventricular output. The Frank-Starling relationship is based, in part, on the length-tension relationship in cardiac myocytes. Recently, we identified a dichotomy in the steepness of length-tension relationships in mammalian cardiac myocytes that was dependent upon protein kinase A (PKA)-induced myofibrillar phosphorylation. Because PKA has multiple myofibrillar substrates including titin, myosin-binding protein-C and cardiac troponin I (cTnI), we sought to define if phosphorylation of one of these molecules could control length-tension relationships. We focused on cTnI as troponin can be exchanged in permeabilized striated muscle cell preparations, and tested the hypothesis that phosphorylation of cTnI modulates length dependence of force generation. For these experiments, we exchanged unphosphorylated recombinant cTn into either a rat cardiac myocyte preparation or a skinned slow-twitch skeletal muscle fibre. In all cases unphosphorylated cTn yielded a shallow length-tension relationship, which was shifted to a steep relationship after PKA treatment. Furthermore, exchange with cTn having cTnI serines 23/24 mutated to aspartic acids to mimic phosphorylation always shifted a shallow length-tension relationship to a steep relationship. Overall, these results indicate that phosphorylation of cTnI serines 23/24 is a key regulator of length dependence of force generation in striated muscle.

The myosin duty ratio tunes the calcium sensitivity and cooperative activation of the thin filament

In striated muscle, calcium binding to the thin filament (TF) regulatory complex activates actin-myosin ATPase activity, and actin-myosin kinetics in turn regulates TF activation. However, a quantitative description of the effects of actin-myosin kinetics on the calcium sensitivity (pCa50) and cooperativity (nH) of TF activation is lacking. With the assumption that TF structural transitions and TF-myosin binding transitions are inextricably coupled, we advanced the principles established by Kad et al. [Kad, N., et al. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 16990-16995] and Sich et al. [Sich, N. M., et al. (2011) J. Biol. Chem. 285, 39150-39159] to develop a simple model of TF regulation, which predicts that pCa50 varies linearly with duty ratio and that nH is maximal near physiological duty ratios. Using in vitro motility to determine the calcium sensitivity of TF sliding velocities, we measured pCa50 and nH at different myosin densities and in the presence of ATPase inhibitors. The observed effects of myosin density and actin-myosin duty ratio on pCa50 and nH are consistent with our model predictions. In striated muscle, pCa50 must match cytosolic calcium concentrations and a maximal nH optimizes calcium responsiveness. Our results indicate that pCa50 and nH can be predictably tuned through TF-myosin ATPase kinetics and that drugs and disease states that alter ATPase kinetics can, through their effects on calcium sensitivity, alter the efficiency of muscle contraction.

The effects of phosphate and acidosis on regulated thin-filament velocity in an in vitro motility assay

Muscle fatigue from intense contractile activity is thought to result, in large part, from the accumulation of inorganic phosphate (Pi) and hydrogen ions (H+) acting to directly inhibit the function of the contractile proteins; however, the molecular basis of this process remain unclear. We used an in vitro motility assay and determined the effects of elevated H+ and Pi on the ability of myosin to bind to and translocate regulated actin filaments (RTF) to gain novel insights into the molecular basis of fatigue. At saturating Ca++, acidosis depressed regulated filament velocity ( VRTF) by ∼90% (6.2 ± 0.3 vs. 0.5 ± 0.2 μm/s at pH 7.4 and 6.5, respectively). However, the addition of 30 mM Pi caused VRTF to increase fivefold, from 0.5 ± 0.2 to 2.6 ± 0.3 μm/s at pH 6.5. Similarly, at all subsaturating Ca++ levels, acidosis slowed VRTF, but the addition of Pi significantly attenuated this effect. We also manipulated the [ADP] in addition to the [Pi] to probe which specific step(s) of cross-bridge cycle of myosin is affected by elevated H+. The findings are consistent with acidosis slowing the isomerization step between two actomyosin ADP-bound states. Because the state before this isomerization is most vulnerable to Pi rebinding, and the associated detachment from actin, this finding may also explain the Pi-induced enhancement of VRTF at low pH. These results therefore may provide a molecular basis for a significant portion of the loss of shortening velocity and possibly muscular power during fatigue.

A mutation in TNNC1-encoded cardiac troponin C, TNNC1-A31S, predisposes to hypertrophic cardiomyopathy and ventricular fibrillation

Defined as clinically unexplained hypertrophy of the left ventricle, hypertrophic cardiomyopathy (HCM) is traditionally understood as a disease of the cardiac sarcomere. Mutations in TNNC1-encoded cardiac troponin C (cTnC) are a relatively rare cause of HCM. Here, we report clinical and functional characterization of a novel TNNC1 mutation, A31S, identified in a pediatric HCM proband with multiple episodes of ventricular fibrillation and aborted sudden cardiac death. Diagnosed at age 5, the proband is family history-negative for HCM or sudden cardiac death, suggesting a de novo mutation. TnC-extracted cardiac skinned fibers were reconstituted with the cTnC-A31S mutant, which increased Ca(2+) sensitivity with no effect on the maximal contractile force generation. Reconstituted actomyosin ATPase assays with 50% cTnC-A31S:50% cTnC-WT demonstrated Ca(2+) sensitivity that was intermediate between 100% cTnC-A31S and 100% cTnC-WT, whereas the mutant increased the activation of the actomyosin ATPase without affecting the inhibitory qualities of the ATPase. The secondary structure of the cTnC mutant was evaluated by circular dichroism, which did not indicate global changes in structure. Fluorescence studies demonstrated increased Ca(2+) affinity in isolated cTnC, the troponin complex, thin filament, and to a lesser degree, thin filament with myosin subfragment 1. These results suggest that this mutation has a direct effect on the Ca(2+) sensitivity of the myofilament, which may alter Ca(2+) handling and contribute to the arrhythmogenesis observed in the proband. In summary, we report a novel mutation in the TNNC1 gene that is associated with HCM pathogenesis and may predispose to the pathogenesis of a fatal arrhythmogenic subtype of HCM.

Congenital myopathy-causing tropomyosin mutations induce thin filament dysfunction via distinct physiological mechanisms

In humans, congenital myopathy-linked tropomyosin mutations lead to skeletal muscle dysfunction, but the cellular and molecular mechanisms underlying such dysfunction remain obscure. Recent studies have suggested a unifying mechanism by which tropomyosin mutations partially inhibit thin filament activation and prevent proper formation and cycling of myosin cross-bridges, inducing force deficits at the fiber and whole-muscle levels. Here, we aimed to verify this mechanism using single membrane-permeabilized fibers from patients with three tropomyosin mutations (TPM2-null, TPM3-R167H and TPM2-E181K) and measuring a broad range of parameters. Interestingly, we identified two divergent, mutation-specific pathophysiological mechanisms. (i) The TPM2-null and TPM3-R167H mutations both decreased cooperative thin filament activation in combination with reductions in the myosin cross-bridge number and force production. The TPM3-R167H mutation also induced a concomitant reduction in thin filament length. (ii) In contrast, the TPM2-E181K mutation increased thin filament activation, cross-bridge binding and force generation. In the former mechanism, modulating thin filament activation by administering troponin activators (CK-1909178 and EMD 57033) to single membrane-permeabilized fibers carrying tropomyosin mutations rescued the thin filament activation defect associated with the pathophysiology. Therefore, administration of troponin activators may constitute a promising therapeutic approach in the future.

Mouse and computational models link Mlc2v dephosphorylation to altered myosin kinetics in early cardiac disease

Actin-myosin interactions provide the driving force underlying each heartbeat. The current view is that actin-bound regulatory proteins play a dominant role in the activation of calcium-dependent cardiac muscle contraction. In contrast, the relevance and nature of regulation by myosin regulatory proteins (for example, myosin light chain-2 [MLC2]) in cardiac muscle remain poorly understood. By integrating gene-targeted mouse and computational models, we have identified an indispensable role for ventricular Mlc2 (Mlc2v) phosphorylation in regulating cardiac muscle contraction. Cardiac myosin cycling kinetics, which directly control actin-myosin interactions, were directly affected, but surprisingly, Mlc2v phosphorylation also fed back to cooperatively influence calcium-dependent activation of the thin filament. Loss of these mechanisms produced early defects in the rate of cardiac muscle twitch relaxation and ventricular torsion. Strikingly, these defects preceded the left ventricular dysfunction of heart disease and failure in a mouse model with nonphosphorylatable Mlc2v. Thus, there is a direct and early role for Mlc2 phosphorylation in regulating actin-myosin interactions in striated muscle contraction, and dephosphorylation of Mlc2 or loss of these mechanisms can play a critical role in heart failure.

Sarcomere length-dependent Ca2+ activation in skinned rabbit psoas muscle fibers: coordinated regulation of thin filament cooperative activation and passive force

In skeletal muscle, active force production varies as a function of sarcomere length (SL). It has been considered that this SL dependence results simply from a change in the overlap length between the thick and thin filaments. The purpose of this study was to provide a systematic understanding of the SL-dependent increase in Ca²⁺ sensitivity in skeletal muscle, by investigating how thin filament “on–off” switching and passive force are involved in the regulation. Rabbit psoas muscles were skinned, and active force measurements were taken at various Ca²⁺ concentrations with single fibers, in the short (2.0 and 2.4 μm) and long (2.4 and 2.8 μm) SL ranges. Despite the same magnitude of SL elongation, the SL-dependent increase in Ca²⁺ sensitivity was more pronounced in the long SL range. MgADP (3 mM) increased the rate of rise of active force and attenuated SL-dependent Ca²⁺ activation in both SL ranges. Conversely, inorganic phosphate (Pi, 20 mM) decreased the rate of rise of active force and enhanced SL-dependent Ca²⁺ activation in both SL ranges. Our analyses revealed that, in the absence and presence of MgADP or Pi, the magnitude of SL-dependent Ca²⁺ activation was (1) inversely correlated with the rate of rise of active force, and (2) in proportion to passive force. These findings suggest that the SL dependence of active force in skeletal muscle is regulated via thin filament “on–off” switching and titin (connectin)-based interfilament lattice spacing modulation in a coordinated fashion, in addition to the regulation via the filament overlap.

Impact of myocyte strain on cardiac myofilament activation

When cardiac myocytes are stretched by a longitudinal strain, they develop proportionally more active force at a given sub-maximal Ca(2+) concentration than they did at the shorter length. This is known as length-dependent activation. It is one of the most important contributors to the Frank-Starling relationship, a critical part of normal cardiovascular function. Despite intense research efforts, the mechanistic basis of the Frank-Starling relationship remains unclear. Potential mechanisms involving myofibrillar lattice spacing, titin-based effects, and cooperative activation have all been proposed. This review summarizes some of these mechanisms and discusses two additional potential theories that reflect the effects of localized strains that occur within and between half-sarcomeres. The main conclusion is that the Frank-Starling relationship is probably the integrated result of many interacting molecular mechanisms. Multiscale computational modeling may therefore provide the best way of determining the key processes that underlie length-dependent activation and their relative strengths.

The Ca2+/Mg2+ sites of troponin C modulate crossbridge-mediated thin filament activation in cardiac myofibrils

The Ca(2+)/Mg(2+) sites (III and IV) located in the C-terminal domain of cardiac troponin C (cTnC) have been generally considered to play a purely structural role in keeping the cTnC bound to the thin filament. However, several lines of evidence, including the discovery of cardiomyopathy-associated mutations in the C-domain, have raised the possibility that these sites may have a more complex role in contractile regulation. To explore this possibility, the ATPase activity of rat cardiac myofibrils was assayed under conditions in which no Ca(2+) was bound to the N-terminal regulatory Ca(2+)-binding site (site II). Myosin-S1 was treated with N-ethylmaleimide to create strong-binding myosin heads (NEM-S1), which could activate the cardiac thin filament in the absence of Ca(2+). NEM-S1 activation was assayed at pCa 8.0 to 6.5 and in the presence of either 1mM or 30 μM free Mg(2+). ATPase activity was maximal when sites III and IV were occupied by Mg(2+) and it steadily declined as Ca(2+) displaced Mg(2+). The data suggest that in the absence of Ca(2+) at site II strong-binding myosin crossbridges cause the opening of more active sites on the thin filament if the C-domain is occupied by Mg(2+) rather than Ca(2+). This finding could be relevant to the contraction-relaxation kinetics of cardiac muscle. As Ca(2+) dissociates from site II of cTnC during the early relaxing phase of the cardiac cycle, residual Ca(2+) bound at sites III and IV might facilitate the switching off of the thin filament and the detachment of crossbridges from actin.

Calculation of muscle maximal shortening velocity by extrapolation of the force-velocity relationship: afterloaded versus isotonic release contractions

The maximal shortening velocity of a muscle (V(max)) provides a link between its macroscopic properties and the underlying biochemical reactions and is altered in some diseases. Two methods that are widely used for determining V(max) are afterloaded and isotonic release contractions. To determine whether these two methods give equivalent results, we calculated V(max) in 9 intact single fibres from the lumbrical muscles of the frog Xenopus laevis (9.5-15.5 °C, stimulation frequency 35-70 Hz). The data were modelled using a 3-state cross-bridge model in which the states were inactive, detached, and attached. Afterloaded contractions gave lower predictions of Vmax than did isotonic release contractions in all 9 fibres (3.20 ± 0.84 versus 4.11 ± 1.08 lengths per second, respectively; means ± SD, p = 0.001) and underestimated unloaded shortening velocity measured with the slack test by an average of 29% (p = 0.001, n = 6). Excellent model predictions could be obtained by assuming that activation is inhibited by shortening. We conclude that under the experimental conditions used in this study, afterloaded and isotonic release contractions do not give equivalent results. When a change in the V(max) measured with afterloaded contractions is observed in diseased muscle, it is important to consider that this may reflect differences in either activation kinetics or cross-bridge cycling rates.

The role of thin filament cooperativity in cardiac length-dependent calcium activation

Length-dependent activation (LDA) is a prominent feature of cardiac muscle characterized by decreases in the Ca(2+) levels required to generate force (i.e., increases in Ca(2+) sensitivity) when muscle is stretched. Previous studies have concluded that LDA originates from the increased ability of (strong) cross-bridges to attach when muscle is lengthened, which in turn enhances Ca(2+) binding to the troponin C (TnC) subunit of the troponin complex. However, our results demonstrate that inhibition of strong cross-bridge attachment with blebbistatin had no effect on the length-dependent modulation of Ca(2+) sensitivity (i.e., EC(50)) or Ca(2+) cooperativity, suggesting that LDA originates upstream of cross-bridge attachment. To test whether LDA arises from length dependence of thin-filament activation, we replaced native cTnC with a mutant cTnC (DM-TnC) that is incapable of binding Ca(2+). Although progressive replacement of native cTnC with DM-TnC caused an expected monotonic decrease in the maximal force (F(max)), DM-TnC incorporation induced much larger increases in EC(50) and decreases in Ca(2+) cooperativity at short lengths than at long lengths. These findings support the conclusion that LDA arises primarily from the influence of length on the modulation of the Ca(2+) cooperativity arising from interaction between adjacent troponin-tropomyosin complexes on the thin filament.

Regulatory mechanism of length-dependent activation in skinned porcine ventricular muscle: role of thin filament cooperative activation in the Frank-Starling relation

Cardiac sarcomeres produce greater active force in response to stretch, forming the basis of the Frank-Starling mechanism of the heart. The purpose of this study was to provide the systematic understanding of length-dependent activation by investigating experimentally and mathematically how the thin filament "on-off" switching mechanism is involved in its regulation. Porcine left ventricular muscles were skinned, and force measurements were performed at short (1.9 µm) and long (2.3 µm) sarcomere lengths. We found that 3 mM MgADP increased Ca(2+) sensitivity of force and the rate of rise of active force, consistent with the increase in thin filament cooperative activation. MgADP attenuated length-dependent activation with and without thin filament reconstitution with the fast skeletal troponin complex (sTn). Conversely, 20 mM of inorganic phosphate (Pi) decreased Ca(2+) sensitivity of force and the rate of rise of active force, consistent with the decrease in thin filament cooperative activation. Pi enhanced length-dependent activation with and without sTn reconstitution. Linear regression analysis revealed that the magnitude of length-dependent activation was inversely correlated with the rate of rise of active force. These results were quantitatively simulated by a model that incorporates the Ca(2+)-dependent on-off switching of the thin filament state and interfilament lattice spacing modulation. Our model analysis revealed that the cooperativity of the thin filament on-off switching, but not the Ca(2+)-binding ability, determines the magnitude of the Frank-Starling effect. These findings demonstrate that the Frank-Starling relation is strongly influenced by thin filament cooperative activation.

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.

Influence of salt and pyrophosphate on bovine fast and slow myosin S1 dissociation from actin

The kinetics of myosin dissociation from actin was investigated and also the impact of salt, MgPPi, and myosin heavy chain isoform on myosin subfragment 1 (S1) dissociation from actin using purified proteins and fluorescence spectroscopy. Both NaCl and MgPPi increased myosin S1 dissociation rate. When salt concentrations increased from 0.1 to 1.0 M, the dissociation rate of S1 from bovine masseter (slow) and cutaneous trunci (fast) muscle increased 38 and 78 fold, respectively. MgPPi had an even greater effect on S1 dissociation from actin. With the addition of MgPPi to the mixture of pyrene actin and S1, the fluorescence increased about 85% within the dead time of the mixing approach.. Unlike salt, MgPPi had no apparent difference in its ability to dissociate slow or fast S1 isoforms from actin. The results reveal that salt and MgPPi increase myosin extraction and functionality in meat by weakening the actomyosin interaction and that some of the difference in the functionality of red and white muscle may be related to actomyosin dissociation.

Length dependence of force generation exhibit similarities between rat cardiac myocytes and skeletal muscle fibres

According to the Frank-Starling relationship, increased ventricular volume increases cardiac output, which helps match cardiac output to peripheral circulatory demand. The cellular basis for this relationship is in large part the myofilament length-tension relationship. Length-tension relationships in maximally calcium activated preparations are relatively shallow and similar between cardiac myocytes and skeletal muscle fibres. During twitch activations length-tension relationships become steeper in both cardiac and skeletal muscle; however, it remains unclear whether length dependence of tension differs between striated muscle cell types during submaximal activations. The purpose of this study was to compare sarcomere length-tension relationships and the sarcomere length dependence of force development between rat skinned left ventricular cardiac myocytes and fast-twitch and slow-twitch skeletal muscle fibres. Muscle cell preparations were calcium activated to yield 50% maximal force, after which isometric force and rate constants (k(tr)) of force development were measured over a range of sarcomere lengths. Myofilament length-tension relationships were considerably steeper in fast-twitch fibres compared to slow-twitch fibres. Interestingly, cardiac myocyte preparations exhibited two populations of length-tension relationships, one steeper than fast-twitch fibres and the other similar to slow-twitch fibres. Moreover, myocytes with shallow length-tension relationships were converted to steeper length-tension relationships by protein kinase A (PKA)-induced myofilament phosphorylation. Sarcomere length-k(tr) relationships were distinct between all three cell types and exhibited patterns markedly different from Ca(2+) activation-dependent k(tr) relationships. Overall, these findings indicate cardiac myocytes exhibit varied length-tension relationships and sarcomere length appears a dominant modulator of force development rates. Importantly, cardiac myocyte length-tension relationships appear able to switch between slow-twitch-like and fast-twitch-like by PKA-mediated myofibrillar phosphorylation, which implicates a novel means for controlling Frank-Starling relationships.

Tripolyphosphate hydrolysis by bovine fast and slow myosin subfragment 1 isoforms

Polyphosphates are used in the meat industry to increase the water holding capacity of meat products. Tripolyphosphate (TPP) is a commonly used polyphosphate and it is metabolized into pyrophosphate and monophosphate in meat. The enzymes responsible for its metabolism have not been fully characterized. The motor domain of myosin (subfragment 1 or S1) is a likely candidate. The objectives of this study were to determine if bovine S1 hydrolyzes TPP, to characterize the TPPase activity of the fast (cutaneous trunci) and slow (masseter) isoforms, and to determine the influence of pH on S1 TPPase activity. S1 hydrolyzed TPP and in comparison with ATP as substrate, it hydrolyzed TPP 16-32% more slowly. Fast S1 hydrolyzed both substrates faster compared to slow S1 and the difference between the isoforms was greater with TPP as the substrate. The V(max) was 0.94 and 5.0 nmol Pi/mg S1 protein/min while the K(m) was 0.38 and 0.90 mM TPP for slow and fast S1, respectively. Pyrophosphate was a strong inhibitor of TPPase activity with a K(i) of 88 and 8.3 microM PPi for fast and slow S1 isoforms, respectively. Both ATPase and TPPase activities were influenced by pH with the activity being higher at low pH for both fast and slow S1 isoforms. The activity at pH 5.4 was 1.5 to 4-fold higher than that at pH 7.6 for the different isoforms and substrates. These data show that myosin S1 readily hydrolyzes TPP and suggest that it is a major TPPase in meat.

Molecular and functional characterization of a novel cardiac-specific human tropomyosin isoform

BACKGROUND

Tropomyosin (TM), an essential actin-binding protein, is central to the control of calcium-regulated striated muscle contraction. Although TPM1alpha (also called alpha-TM) is the predominant TM isoform in human hearts, the precise TM isoform composition remains unclear.

METHODS AND RESULTS

In this study, we quantified for the first time the levels of striated muscle TM isoforms in human heart, including a novel isoform called TPM1kappa. By developing a TPM1kappa-specific antibody, we found that the TPM1kappa protein is expressed and incorporated into organized myofibrils in hearts and that its level is increased in human dilated cardiomyopathy and heart failure. To investigate the role of TPM1kappa in sarcomeric function, we generated transgenic mice overexpressing cardiac-specific TPM1kappa. Incorporation of increased levels of TPM1kappa protein in myofilaments leads to dilated cardiomyopathy. Physiological alterations include decreased fractional shortening, systolic and diastolic dysfunction, and decreased myofilament calcium sensitivity with no change in maximum developed tension. Additional biophysical studies demonstrate less structural stability and weaker actin-binding affinity of TPM1kappa compared with TPM1alpha.

CONCLUSIONS

This functional analysis of TPM1kappa provides a possible mechanism for the consequences of the TM isoform switch observed in dilated cardiomyopathy and heart failure patients.

Myofilament length dependent activation

The Frank-Starling law of the heart describes the interrelationship between end-diastolic volume and cardiac ejection volume, a regulatory system that operates on a beat-to-beat basis. The main cellular mechanism that underlies this phenomenon is an increase in the responsiveness of cardiac myofilaments to activating Ca(2+) ions at a longer sarcomere length, commonly referred to as myofilament length-dependent activation. This review focuses on what molecular mechanisms may underlie myofilament length dependency. Specifically, the roles of inter-filament spacing, thick and thin filament based regulation, as well as sarcomeric regulatory proteins are discussed. Although the "Frank-Starling law of the heart" constitutes a fundamental cardiac property that has been appreciated for well over a century, it is still not known in muscle how the contractile apparatus transduces the information concerning sarcomere length to modulate ventricular pressure development.

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.

Characterization of secophalloidin-induced force loss in cardiac myofibrils

Secophalloidin (SPH) is known to cause in cardiac myofibrils force without Ca(2+) (half-maximal effect approximately 2 mM) followed by irreversible loss of Ca(2+)-activated force. At maximal Ca(2+) activation, SPH increases force (half-maximal effect < 0.1 mM). We found that SPH at low concentration (0.5 mM) did not cause either force activation or force loss at pCa 8.7, but both of these effects did occur when force was activated by Ca(2+). The force loss was prevented when SPH was applied during rigor or in the presence of 2,3-butanedione monoxime (85 mM). Furthermore, studying muscle in which the force was previously reduced by SPH (up to 50%) did not reveal significant changes in Ca(2+) sensitivity and cooperativity of Ca(2+) activation or qualitative alterations in SPH-induced changes in Ca(2+)-activated contraction. Data suggest that the force loss is mediated by cycling cross-bridges, and might reflect a reduction in force generated by individual cross-bridges.

Ala scanning of the inhibitory region of cardiac troponin I

In skeletal and cardiac muscles, troponin (Tn), which resides on the thin filament, senses a change in intracellular Ca(2+) concentration. Tn is composed of TnC, TnI, and TnT. Ca(2+) binding to the regulatory domain of TnC removes the inhibitory effect by TnI on the contraction. The inhibitory region of cardiac TnI spans from residue 138 to 149. Upon Ca(2+) activation, the inhibitory region is believed to be released from actin, thus triggering actin-activation of myosin ATPase. In this study, we created a series of Ala-substitution mutants of cTnI to delineate the functional contribution of each amino acid in the inhibitory region to myofilament regulation. We found that most of the point mutations in the inhibitory region reduced the ATPase activity in the presence of Ca(2+), which suggests the same region also acts as an activator of the ATPase. The thin filaments can also be activated by strong myosin head (S1)-actin interactions. The binding of N-ethylmaleimide-treated myosin subfragment 1 (NEM-S1) to actin filaments mimics such strong interactions. Interestingly, in the absence of Ca(2+) NEM-S1-induced activation of S1 ATPase was significantly less with the thin filaments containing TnI(T144A) than that with the wild-type TnI. However, in the presence of Ca(2+), there was little difference in the activation of ATPase activity between these preparations.