Single-myosin crossbridge interactions with actin filaments regulated by troponin-tropomyosin

The effect of work cycle frequency on the potentiation of dynamic force in mouse fast twitch skeletal muscle

The purpose of this study was to test the hypothesis that the potentiation of concentric twitch force during work cycles is dependent upon both the speed and direction of length change. Concentric and eccentric forces were elicited by stimulating muscles during the shortening and lengthening phases, respectively, of work cycles. Work cycle frequency was varied in order to vary the speed of muscle shortening and/or lengthening; all forces were measured as the muscle passed though optimal length (Lo). Both concentric and eccentric force were assessed before (unpotentiated control) and after (potentiated) the application of a tetanic conditioning protocol known to potentiate twitch force output. The influence of the conditioning protocol on relative concentric force was speed dependent, with forces increased to 1.19±0.01, 1.25±0.01 and 1.30±0.01 of controls at 1.5, 3.3 and 6.9 Hz, respectively (all data N=9–10 with P<0.05). In contrast, the conditioning protocol had only a limited effect on eccentric force at these frequencies (range: 1.06±0.01 to 0.96±0.03). The effect of the conditioning protocol on concentric work (force × distance) was also speed dependent, being decreased at 1.5 Hz (0.84±0.01) and increased at 3.3 and 6.9 Hz (1.05±0.01 and 1.39±0.01, respectively). In contrast, eccentric work was not increased at any frequency (range: 0.88±0.02 to 0.99±0.01). Thus, our results reveal a hysteresis-like influence of activity-dependent potentiation such that concentric force and/or work were increased but eccentric force and/or work were not. These outcomes may have implications for skeletal muscle locomotor function in vivo.

A technique for simultaneous measurement of force and overlap between single muscle filaments of myosin and actin

In this study, we show a method for direct measurements of force and simultaneous visualization of isolated muscle filaments. Single actin filaments isolated from chicken skeletal muscle and single thick filaments isolated from Mussels were imaged using fluorescence and dark field microscopy, respectively. Force generated by the filaments was measured using micro-fabricated cantilevers. Force values were in the range observed previously with myosin filaments and molecules. The results suggest that the technique can be used to investigate many issues of interest and debate in the field of muscle biophysics.

Effects of actin-myosin kinetics on the calcium sensitivity of regulated thin filaments

Activation of thin filaments in striated muscle occurs when tropomyosin exposes myosin binding sites on actin either through calcium-troponin (Ca-Tn) binding or by actin-myosin (A-M) strong binding. However, the extent to which these binding events contributes to thin filament activation remains unclear. Here we propose a simple analytical model in which strong A-M binding and Ca-Tn binding independently activates the rate of A-M weak-to-strong binding. The model predicts how the level of activation varies with pCa as well as A-M attachment, N·k(att), and detachment, k(det), kinetics. To test the model, we use an in vitro motility assay to measure the myosin-based sliding velocities of thin filaments at different pCa, N·k(att), and k(det) values. We observe that the combined effects of varying pCa, N·k(att), and k(det) are accurately fit by the analytical model. The model and supporting data imply that changes in attachment and detachment kinetics predictably affect the calcium sensitivity of striated muscle mechanics, providing a novel A-M kinetic-based interpretation for perturbations (e.g. disease-related mutations) that alter calcium sensitivity.

Human actin mutations associated with hypertrophic and dilated cardiomyopathies demonstrate distinct thin filament regulatory properties in vitro

Two cardiomyopathic mutations were expressed in human cardiac actin, using a Baculovirus/insect cell system; E99K is associated with hypertrophic cardiomyopathy whereas R312H is associated with dilated cardiomyopathy. The hypothesis that the divergent phenotypes of these two cardiomyopathies are associated with fundamental differences in the molecular mechanics and thin filament regulation of the underlying actin mutation was tested using the in vitro motility and laser trap assays. In the presence of troponin (Tn) and tropomyosin (Tm), beta-cardiac myosin moved both E99K and R312H thin filaments at significantly (p<0.05) slower velocities than wild type (WT) at maximal Ca(++). At submaximal Ca(++), R312H thin filaments demonstrated significantly increased Ca(++) sensitivity (pCa(50)) when compared to WT. Velocity as a function of ATP concentration revealed similar ATP binding rates but slowed ADP release rates for the two actin mutants compared to WT. Single molecule laser trap experiments performed using both unregulated (i.e. actin) and regulated thin filaments in the absence of Ca(++) revealed that neither actin mutation significantly affected the myosin's unitary step size (d) or duration of strong actin binding (t(on)) at 20 microM ATP. However, the frequency of individual strong-binding events in the presence of Tn and Tm, was significantly lower for E99K than WT at comparable myosin surface concentrations. The cooperativity of a second myosin head binding to the thin filament was also impaired by E99K. In conclusion, E99K inhibits the activation of the thin filament by myosin strong-binding whereas R312H demonstrates enhanced calcium activation.

Phosphorylation of tropomyosin extends cooperative binding of myosin beyond a single regulatory unit

Tropomyosin (Tm) is one of the major phosphoproteins comprising the thin filament of muscle. However, the specific role of Tm phosphorylation in modulating the mechanics of actomyosin interaction has not been determined. Here we show that Tm phosphorylation is necessary for long-range cooperative activation of myosin binding. We used a novel optical trapping assay to measure the isometric stall force of an ensemble of myosin molecules moving actin filaments reconstituted with either natively phosphorylated or dephosphorylated Tm. The data show that the thin filament is cooperatively activated by myosin across regulatory units when Tm is phosphorylated. When Tm is dephosphorylated, this "long-range" cooperative activation is lost and the filament behaves identically to bare actin filaments. However, these effects are not due to dissociation of dephosphorylated Tm from the reconstituted thin filament. The data suggest that end-to-end interactions of adjacent Tm molecules are strengthened when Tm is phosphorylated, and that phosphorylation is thus essential for long range cooperative activation along the thin filament.

Expression of the fast twitch troponin complex, fTnT, fTnI and fTnC, in vascular smooth muscle

It is generally believed that proteins of the troponin complex are not expressed in smooth muscle. We have directly assayed for expression of troponin transcripts in mouse vascular smooth muscle and found that troponin sequences normally associated with fast twitch skeletal muscle (fTnT, fTnI, fTnC) were present at significant levels in the thoracic aorta. In situ hybridization experiments demonstrated that fTnT, fTnI and fTnC transcripts were expressed in the smooth muscle layer of mouse blood vessels of all sizes. Protein blot analysis using rat tissue showed that at least two members of the troponin complex, Troponin T and Troponin I, were translated in vascular smooth muscle of the aorta. Finally, immuno-fluorescence microscopy of rat aortic smooth muscle revealed that TnT and TnI are localized in a unique pattern, coincident with the distribution of tropomyosin. It seems likely therefore, that a complete troponin complex is expressed in vascular smooth muscle and is associated with the contractile machinery of the cell. These observations raise the possibility that troponins play a role in regulation of smooth muscle function.

Cardiac myosin binding protein-C modulates actomyosin binding and kinetics in the in vitro motility assay

The modulatory role of whole cardiac myosin binding protein-C (cMyBP-C) on myosin force and motion generation was assessed in an in vitro motility assay. The presence of cMyBP-C at an approximate molar ratio of cMyBP-C to whole myosin of 1:2, resulted in a 25% reduction in thin filament velocity (P<0.002) with no effect on relative isometric force under maximally activated conditions (pCa 5). Cardiac MyBP-C was capable of inhibiting actin filament velocity in a concentration-dependent manner using either whole myosin, HMM or S1, indicating that the cMyBP-C does not have to bind to myosin LMM or S2 subdomains to exert its effect. The reduction in velocity by cMyBP-C was independent of changes in ionic strength or excess inorganic phosphate. Co-sedimentation experiments demonstrated S1 binding to actin is reduced as a function of cMyBP-C concentration in the presence of ATP. In contrast, S1 avidly bound to actin in the absence of ATP and limited cMyBP-C binding, indicating that cMyBP-C and S1 compete for actin binding in an ATP-dependent fashion. However, based on the relationship between thin filament velocity and filament length, the cMyBP-C induced reduction in velocity was independent of the number of cross-bridges interacting with the thin filament. In conclusion, the effects of cMyBP-C on velocity and force at both maximal and submaximal activation demonstrate that cMyBP-C does not solely act as a tether between the myosin S2 and LMM subdomains but likely affects both the kinetics and recruitment of myosin cross-bridges through its direct interaction with actin and/or myosin head.

Effect of low pH on single skeletal muscle myosin mechanics and kinetics

Acidosis (low pH) is the oldest putative agent of muscular fatigue, but the molecular mechanism underlying its depressive effect on muscular performance remains unresolved. Therefore, the effect of low pH on the molecular mechanics and kinetics of chicken skeletal muscle myosin was studied using in vitro motility (IVM) and single molecule laser trap assays. Decreasing pH from 7.4 to 6.4 at saturating ATP slowed actin filament velocity (V(actin)) in the IVM by 36%. Single molecule experiments, at 1 microM ATP, decreased the average unitary step size of myosin (d) from 10 +/- 2 nm (pH 7.4) to 2 +/- 1 nm (pH 6.4). Individual binding events at low pH were consistent with the presence of a population of both productive (average d = 10 nm) and nonproductive (average d = 0 nm) actomyosin interactions. Raising the ATP concentration from 1 microM to 1 mM at pH 6.4 restored d (9 +/- 3 nm), suggesting that the lifetime of the nonproductive interactions is solely dependent on the [ATP]. V(actin), however, was not restored by raising the [ATP] (1-10 mM) in the IVM assay, suggesting that low pH also prolongs actin strong binding (t(on)). Measurement of t(on) as a function of the [ATP] in the single molecule assay suggested that acidosis prolongs t(on) by slowing the rate of ADP release. Thus, in a detachment limited model of motility (i.e., V(actin) approximately d/t(on)), a slowed rate of ADP release and the presence of nonproductive actomyosin interactions could account for the acidosis-induced decrease in V(actin), suggesting a molecular explanation for this component of muscular fatigue.

Nonlinear force-length relationship in the ADP-induced contraction of skeletal myofibrils

The regulatory mechanism of sarcomeric activity has not been fully clarified yet because of its complex and cooperative nature, which involves both Ca(2+) and cross-bridge binding to the thin filament. To reveal the mechanism of regulation mediated by the cross-bridges, separately from the effect of Ca(2+), we investigated the force-sarcomere length (SL) relationship in rabbit skeletal myofibrils (a single myofibril or a thin bundle) at SL > 2.2 microm in the absence of Ca(2+) at various levels of activation by exogenous MgADP (4-20 mM) in the presence of 1 mM MgATP. The individual SLs were measured by phase-contrast microscopy to confirm the homogeneity of the striation pattern of sarcomeres during activation. We found that at partial activation with 4-8 mM MgADP, the developed force nonlinearly depended on the length of overlap between the thick and the thin filaments; that is, contrary to the maximal activation, the maximal active force was generated at shorter overlap. Besides, the active force became larger, whereas this nonlinearity tended to weaken, with either an increase in [MgADP] or the lateral osmotic compression of the myofilament lattice induced by the addition of a macromolecular compound, dextran T-500. The model analysis, which takes into account the [MgADP]- and the lattice-spacing-dependent probability of cross-bridge formation, was successfully applied to account for the force-SL relationship observed at partial activation. These results strongly suggest that the cross-bridge works as a cooperative activator, the function of which is highly sensitive to as little as <or=1 nm changes in the lattice spacing.