Calcium regulation of thin filament movement in an in vitro motility assay

Dominant negative mutant actins identified in flightless Drosophila can be classified into three classes

Strongly dominant negative mutant actins, identified by An and Mogami (An, H. S., and Mogami, K. (1996) J. Mol. Biol. 260, 492-505), in the indirect flight muscle of Drosophila impaired its flight, even when three copies of the wild-type gene were present. Understanding how these strongly dominant negative mutant actins disrupt the function of wild-type actin would provide useful information about the molecular mechanism by which actin functions in vivo. Here, we expressed and purified six of these strongly dominant negative mutant actins in Dictyostelium and classified them into three groups based on their biochemical phenotypes. The first group, G156D, G156S, and G268D actins, showed impaired polymerization and a tendency to aggregate under conditions favoring polymerization. G63D actin of the second group was also unable to polymerize but, unlike those in the first group, remained soluble under polymerizing conditions. Kinetic analyses using G63D actin or G63D actin.gelsolin complexes suggested that the pointed end surface is defective, which would alter the polymerization kinetics of wild-type actin when mixed and could affect formation of thin filament structures in indirect flight muscle. The third group, R95C and E226K actins, was normal in terms of polymerization, but their motility on heavy meromyosin surfaces in the presence of tropomyosin-troponin indicated altered sensitivity to Ca(2+). Cofilaments in which R95C or E226K actins were copolymerized with a 3-fold excess of wild-type actin also showed altered Ca(2+) sensitivity in the presence of tropomyosin-troponin.

Motor protein function in skeletal muscle-a multiple scale approach to contractility

We present an approach to skeletal muscle contractility and its regulation over different scales ranging from biomechanical studies in intact muscle fibers down to the motility and interaction of single motor protein molecules. At each scale, shortening velocities as a measure for weak cross-bridge cycling rates are extracted and compared. Experimental approaches include transmitted light microscopy, second harmonic generation imaging of contracting myofibrils, and fluorescence microscopy of single molecule motility. Each method yields image sequences that are analyzed with automated image processing algorithms to extract the contraction velocity. Using this approach, we show how to isolate the contribution of the motor proteins actin and myosin and their modulation by regulatory proteins from the concerted action of electro-mechanical activation on a more complex cellular scale. The advantage of this approach is that averaged contraction velocities can be determined on the different scales ranging from isolated motor proteins to sarcomere levels in myofibrils and myofibril arrays within the cellular architecture. Our results show that maximum shortening velocities during in situ electrical activation of sarcomere contraction in intact single muscle cells can substantially deviate from sliding velocities obtained in oriented in vitro motility assays of isolated motor proteins showing that biophysical contraction kinetics not simply translate linearly between contractility scales. To adequately resolve the very fast initial mechanical activation kinetics of shortening at each scale, it was necessary to implement high-speed imaging techniques. In the case of intact fibers and single molecule motility, we achieved a major increase in temporal resolution up to frame rates of 200-1000 fps using CMOS image sensor technology. The data we obtained at this unprecedented temporal resolution and the parameters extracted can be used to validate results obtained from computational models of motor protein interaction and skeletal muscle contractility in health and muscle disease. Our approach is feasible to explain the possible underlying mechanisms that contribute to different shortening velocities at different scales and complexities.

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.

Impact of cardiac troponin T N-terminal deletion and phosphorylation on myofilament function

Cardiac troponin T (cTnT) is a phosphoprotein that modulates cardiac muscle contraction through its extensive and diverse interactions with neighboring thin filament proteins. Its N-terminal half is the "glue" that anchors the troponin complex to tropomyosin-actin. Until now, studies aimed at investigating the role of the N-terminal tail region have not considered the effects of phosphorylation. To understand better the regulatory role of the N-terminal tail region of phosphorylated cTnT, we investigated the functional effects of N-terminal deletion (amino acids 1-91) and phosphorylation on Ca(2+) dependence of myofilament isometric force production, isometric ATPase rate, and thin filament sliding speed. Chemomechanical profiles were assessed in detergent permeabilized fiber preparations where the native troponin (cTn) was exchanged with recombinant cTn engineered to contain modified cTnT (truncated, phosphorylated) in the presence of wild-type cTnI and cTnC. Removal of the cTnT N-terminal amino acids 1-91 (cTnT-del) enhances myofilament responsiveness to nonsaturating Ca(2+) levels (the physiological range in cardiac myocytes). However, at saturating Ca(2+) levels, there is a reduction in isometric tension and ATPase rate. On one hand, phosphorylation of cTnT-del attenuates the sensitizing effect induced by truncation of the N-terminal tail, "resetting" myofilament Ca(2+) responsiveness back to control levels. On the other hand, it impairs isometric tension development and ATPase rate. Interestingly, phosphorylation of cTnT (cTnT-P) differentially regulates tension cost (an index of cross-bridge cycling rate): increased by cTn-del-P and decreased by intact cTn-wt-P. Like the isometric fiber data, sliding speed of thin filaments regulated by cTn-del is more sensitive to Ca(2+) compared with cTn-wt. Phosphorylation of cTnT (whether cTnT-del or -wt) depresses sliding speed and is associated with Ca(2+) desensitization of thin filament sliding speed.

Study of the interaction between rabbit cardiac contractile and regulatory proteins. An in vitro motility assay

A series of experiments was performed in an in vitro motility assay with reconstructed thin filaments to obtain pCa-force relationships for cardiac isomyosins V1 and V3. Two concentrations of each isomyosin (200 and 300 microg/ml) on the surface of a flow cell were tested. Isometric force was estimated as the amount of actin-binding protein, alpha-actinin, stopping thin filament movement. It was found that the amount of alpha-actinin stopping the movement at saturating calcium concentration for V3 was twice higher than for V1 at both concentrations of isoforms. Hill coefficients of cooperativity (h) were determined for pCa-force relationships. The value of h did not differ significantly for isoforms at 300 microg/ml of protein (h was 1.56 for V1 and 1.54 for V3). However, the Hill coefficient was higher for V3 isoform at 200 microg/ml (h = 2.00 and 1.76 for V3 and V1, respectively). Importantly, the Hill coefficient increased for both isoenzymes when their concentrations were decreased. The connection between Hill coefficient and cooperative interactions between cardiac contractile and regulatory proteins is analyzed in detail.

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.

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.

Cardiac myosin missense mutations cause dilated cardiomyopathy in mouse models and depress molecular motor function

Dilated cardiomyopathy (DCM) leads to heart failure, a leading cause of death in industrialized nations. Approximately 30% of DCM cases are genetic in origin, with some resulting from point mutations in cardiac myosin, the molecular motor of the heart. The effects of these mutations on myosin's molecular mechanics have not been determined. We have engineered two murine models characterizing the physiological, cellular, and molecular effects of DCM-causing missense mutations (S532P and F764L) in the alpha-cardiac myosin heavy chain and compared them with WT mice. Mutant mice developed morphological and functional characteristics of DCM consistent with the human phenotypes. Contractile function of isolated myocytes was depressed and preceded left ventricular dilation and reduced fractional shortening. In an in vitro motility assay, both mutant cardiac myosins exhibited a reduced ability to translocate actin (V(actin)) but had similar force-generating capacities. Actin-activated ATPase activities were also reduced. Single-molecule laser trap experiments revealed that the lower V(actin) in the S532P mutant was due to a reduced ability of the motor to generate a step displacement and an alteration of the kinetics of its chemomechanical cycle. These results suggest that the depressed molecular function in cardiac myosin may initiate the events that cause the heart to remodel and become pathologically dilated.

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

Striated muscle contraction is governed by the thin filament regulatory proteins troponin and tropomyosin. Here, we investigate the molecular mechanisms by which troponin-tropomyosin inhibits myosin's interactions with the thin filament in the absence of calcium by using a laser trap. The displacement events for a single-myosin molecule interacting with a reconstituted thin filament were shorter (step size = 5 nm) and prolonged (69 ms) compared with actin alone (11 nm and 26 ms, respectively). However, these changes alone do not account for the degree of inhibition of thin filament movement observed in an ensemble assay. Our investigations of single- and multiple-myosin molecules with regulated thin filaments suggest the primary basis for this inhibition derives from an approximately 100-fold decrease in the probability of myosin attaching to actin. At higher myosin concentrations, short bursts of motility are observed in a laser trap consistent with the strong binding of a single-myosin crossbridge, resulting in cooperative binding of other cycling crossbridges. We confirmed this cooperativity in the in vitro motility assay by observing thin filament translocation in the absence of calcium but at low [ATP], consistent with rigor activation. We have developed a simple mechanistic model that reproduces and provides insight into both the observed single-myosin molecule and ensemble data in the absence of Ca(2+). These data support the hypothesis that thin filament inhibition in the absence of Ca(2+) is largely achieved by modulating the rate of attachment and/or transition from the weakly to strongly bound state.

Detection of small differences in actomyosin function using actin labeled with different phalloidin conjugates

This study shows that there is only a negligible difference in actomyosin function in the in vitro motility assay among actin filaments labeled with Rhodamine phalloidin (RhPh), Alexa-488 phalloidin (APh), and biotin-XX phalloidin (BPh). Similar results were obtained at varying ionic strengths (0.02-0.13 M), in the presence of imidazole or 3-[N-morpholino]propanesulfonic acid (MOPS) buffer, and at varying MgATP concentrations (0.1-3 mM). If RhPh- and APh-labeled filaments were studied in a given flow cell, there was minimal variability in sliding velocity between the fluorophores (standard deviation of 3% of the absolute sliding velocity). The variability was considerably smaller than that between flow cells, allowing us to use dual labeling of different actin types and then apply analysis of variance to detect minor functional differences between them. Using this method, we could statistically verify a 4% difference (P<0.001) in sliding velocity (3mM Mg ATP) between cardiac and skeletal muscle actin. Suggested improvements of the method would readily allow the detection of even smaller differences. We discuss implications of the results for nanotechnological applications, understanding actomyosin function, and reducing experimental costs and the use of laboratory animals.

Effects of temperature, ionic strength and pH on the function of skeletal muscle myosin from a eurythermal fish, Fundulus heteroclitus

The mummichog, Fundulus heteroclitus, is an intertidal fish that exhibits little change in swimming ability despite large and rapid variations in environmental parameters. We therefore tested the hypothesis that this nearly constant function is due to Fundulus myosin being intrinsically insensitive to changes of temperature, ionic strength and pH. In vitro motility assays were used to quantify the speed of unregulated actin filaments on myosin purified from F. heteroclitus glycolytic skeletal muscle. Filament speed was 2.07+/-0.17 microm s(-1) at 26 degrees C, ionic strength (Gamma/2) of 0.08 M Gamma/2 and pH 7.4. Speed increased as temperature increased over the range of 5-36 degrees C with an activation energy (E (a)) of 94.0+/-7.0 kJ mol(-1)) and an enthalpy (DeltaH (double dagger)) of 91.5+/-7.0 kJ mol(-1) at 20 degrees C. A linear relationship between temperature and ATPase activity was also obtained with actin-activated myosin Mg(2+)-ATPase assays over the temperature range 5-35 degrees C with E (a=)59.9+/-2.4 kJ mol(-1) and DeltaH (double dagger)=57.4+/-2.4 kJ mol(-1) at 20 degrees C. There was little or no effect of ionic strength on filament speed over the range 0.19 M Gamma/2-0.54 M Gamma/2. Speed increased significantly at lower ionic strengths and was 7.9-fold higher at 0.08 M Gamma/2 than at 0.19 M Gamma/2. Speed increased with pH with a 16-fold increase between pH 6.7 and 7.4. These results indicate that changes in physiological parameters that include temperature, pH and ionic strength affect the function of unregulated F. heteroclitus myosin, and thus other factors must be responsible for the mummichog's swimming performance being comparatively insensitive to environmental variation.

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.

Silanized surfaces for in vitro studies of actomyosin function and nanotechnology applications

We have previously shown that selective heavy meromyosin (HMM) adsorption to predefined regions of nanostructured polymer resist surfaces may be used to produce a nanostructured in vitro motility assay. However, actomyosin function was of lower quality than on conventional nitrocellulose films. We have therefore studied actomyosin function on differently derivatized glass surfaces with the aim to find a substitute for the polymer resists. We have found that surfaces derivatized with trimethylchlorosilane (TMCS) were superior to all other surfaces tested, including nitrocellulose. High-quality actin filament motility was observed up to 6 days after incubation with HMM and the fraction of motile actin filaments and the velocity of smooth sliding were generally higher on TMCS than on nitrocellulose. The actomyosin function on TMCS-derivatized glass and nitrocellulose is considered in relation to roughness and hydrophobicity of these surfaces. The results suggest that TMCS is an ideal substitute for polymer resists in the nanostructured in vitro motility assay. Furthermore, TMCS derivatized glass also seems to offer several advantages over nitrocellulose for HMM adsorption in the ordinary in vitro motility assay.

Activation of the calcium-regulated thin filament by myosin strong binding

The current study was undertaken to investigate the relative contribution of calcium and myosin binding to thin filament activation. Using the in vitro motility assay, myosin strong binding to the thin filament was controlled by three mechanisms: 1), varying the myosin concentration of the motility surface, and adding either 2), inorganic phosphate (Pi) or 3), adenosine diphosphate (ADP) to the motility solutions. At saturating myosin conditions, Pi had no effect on thin filament motility. However, at subsaturating myosin concentrations, velocity was reduced at maximal and submaximal calcium in the presence of Pi. Adding ADP to the motility buffers reduced thin filament sliding velocity but increased the pCa(50) of the thin filament. Thus by limiting or increasing myosin strong binding (with the addition of Pi and ADP, respectively), the calcium concentration at which half maximal activation of the thin filament is achieved can be modulated. In experiments without ADP or Pi, the myosin concentration on the motility surface required to reach maximal velocity inversely correlated with the level of calcium activation. Through this approach, we demonstrate that myosin strong binding is essential for thin filament activation at both maximal and submaximal calcium levels, with the relative contribution of myosin strong binding being greatest at submaximal calcium. Furthermore, under conditions in which myosin strong binding is not rate limiting (i.e., saturating myosin conditions), our data suggest that a modulation of myosin cross-bridge kinetics is likely responsible for the graded response to calcium observed in the in vitro motility assay.

In vitro sliding of actin filaments labelled with single quantum dots

We recently refined the in vitro motility assay for studies of actomyosin function to achieve rectified myosin induced sliding of actin filaments. This paves the way, both for detailed functional studies of actomyosin and for nanotechnological applications. In the latter applications it would be desirable to use actin filaments for transportation of cargoes (e.g., enzymes) between different predetermined locations on a chip. We here describe how single quantum dot labelling of isolated actin filaments simultaneously provides handles for cargo attachment and bright and photostable fluorescence labels facilitating cargo detection and filament tracking. Labelling was achieved with preserved actomyosin function using streptavidin-coated CdSe quantum dots (Qdots). These nanocrystals have several unique physical properties and the present work describes their first use for functional studies of isolated proteins outside the cell. The results, in addition to the nanotechnology developments, open for new types of in vitro assays of isolated biomolecules.

C-Terminal Truncation of Cardiac Troponin I Causes Divergent Effects on ATPase and Force

Myocardial stunning is a form of reversible myocardial ischemia/reperfusion injury associated with systolic and diastolic contractile dysfunction. In the isolated rat heart model, myocardial stunning is characterized by specific C-terminal proteolysis of the myofilament protein, troponin I (cTnI) that yields cTnI 1-193 . To determine the effect of this particular C-terminal truncation of cTnI, without the confounding factor of other stunning-induced protein modifications, a series of solution biochemical assays has been undertaken using the human homologue of mouse/rat cTnI 1-193 , cTnI 1-192 . Affinity chromatography and actin sedimentation experiments detected little, or no, difference between the binding of cTnI (cTnI 1-209 ) and cTnI 1-192 to actin-tropomyosin, troponin T, or troponin C. Both cTnI and cTnI 1-192 inhibit the actin-tropomyosin–activated ATPase activity of myosin subfragment 1 (S1), and this inhibition is released by troponin C in the presence of Ca 2+ . However, cTnI 1-192 , when reconstituted as part of the troponin complex (cTn 1-192 ), caused a 54±11% increase in the maximum Ca 2+ -activated actin-tropomyosin-S1 ATPase activity, compared with troponin reconstituted with cTnI (cTn). Furthermore, cTn 1-192 increased Ca 2+ sensitivity of both the actin-tropomyosin-activated S1 ATPase activity and the Ca 2+ -dependent sliding velocity of reconstituted thin filaments, in an in vitro motility assay, compared with cTn. In an in vitro force assay, the actin-tropomyosin filaments bearing cTn 1-192 developed only 76±4% ( P <0.001) of the force obtained with filaments composed of reconstituted cTn. We suggest that cTnI proteolysis may contribute to the pathophysiology of myocardial stunning by altering the Ca 2+ -sensing and chemomechanical properties of the myofilaments.

Cardiomyopathic tropomyosin mutations that increase thin filament Ca2+ sensitivity and tropomyosin N-domain flexibility

The relationship between tropomyosin thermal stability and thin filament activation was explored using two N-domain mutants of alpha-striated muscle tropomyosin, A63V and K70T, each previously implicated in familial hypertrophic cardiomyopathy. Both mutations had prominent effects on tropomyosin thermal stability as monitored by circular dichroism. Wild type tropomyosin unfolded in two transitions, separated by 10 degrees C. The A63V and K70T mutations decreased the melting temperature of the more stable of these transitions by 4 and 10 degrees C, respectively, indicating destabilization of the N-domain in both cases. Global analysis of all three proteins indicated that the tropomyosin N-domain and C-domain fold with a cooperative free energy of 1.0-1.5 kcal/mol. The two mutations increased the apparent affinity of the regulatory Ca2+ binding sites of thin filament in two settings: Ca2+-dependent sliding speed of unloaded thin filaments in vitro (at both pH 7.4 and 6.3), and Ca2+ activation of the thin filament-myosin S1 ATPase rate. Neither mutation had more than small effects on the maximal ATPase rate in the presence of saturating Ca2+ or on the maximal sliding speed. Despite the increased tropomyosin flexibility implied by destabilization of the N-domain, neither the cooperativity of thin filament activation by Ca2+ nor the cooperative binding of myosin S1-ADP to the thin filament was altered by the mutations. The combined results suggest that a more dynamic tropomyosin N-domain influences interactions with actin and/or troponin that modulate Ca2+ sensitivity, but has an unexpectedly small effect on cooperative changes in tropomyosin position on actin.

Ca2+ regulation of rabbit skeletal muscle thin filament sliding: role of cross-bridge number

We investigated how strong cross-bridge number affects sliding speed of regulated Ca(2+)-activated, thin filaments. First, using in vitro motility assays, sliding speed decreased nonlinearly with reduced density of heavy meromyosin (HMM) for regulated (and unregulated) F-actin at maximal Ca(2+). Second, we varied the number of Ca(2+)-activatable troponin complexes at maximal Ca(2+) using mixtures of recombinant rabbit skeletal troponin (WT sTn) and sTn containing sTnC(D27A,D63A), a mutant deficient in Ca(2+) binding at both N-terminal, low affinity Ca(2+)-binding sites (xxsTnC-sTn). Sliding speed decreased nonlinearly as the proportion of WT sTn decreased. Speed of regulated thin filaments varied with pCa when filaments contained WT sTn but filaments containing only xxsTnC-sTn did not move. pCa(50) decreased by 0.12-0.18 when either heavy meromyosin density was reduced to approximately 60% or the fraction of Ca(2+)-activatable regulatory units was reduced to approximately 33%. Third, we exchanged mixtures of sTnC and xxsTnC into single, permeabilized fibers from rabbit psoas. As the proportion of xxsTnC increased, unloaded shortening velocity decreased nonlinearly at maximal Ca(2+). These data are consistent with unloaded filament sliding speed being limited by the number of cycling cross-bridges so that maximal speed is attained with a critical, low level of actomyosin interactions.

Relationship between isometric force and myofibrillar MgATPase at short sarcomere length in skeletal and cardiac muscle and its relevance to the concept of activation heat

1. This paper has been written in recognition of the seminal contributions to cardiac and skeletal muscle energetics made by Professor Colin Gibbs during his distinguished academic career. 2. The paper focuses on what is now known about the relationship between Ca2+-activated isometric force production and myofibrillar MgATPase in intact and skinned (surface membrane rendered permeable) skeletal and cardiac muscle preparations at short sarcomere lengths. 3. The relevance of this relationship to understanding the interactions between the actin and myosin filaments at the cross-bridge level in the region of double actin filament overlap and the cellular basis of 'activation heat' measurements in intact striated muscles is discussed.

Regulatory proteins alter nucleotide binding to acto-myosin of sliding filaments in motility assays

The sliding speed of unregulated thin filaments in motility assays is only about half that of the unloaded shortening velocity of muscle fibers. The addition of regulatory proteins, troponin and tropomyosin, is known to increase the sliding speed of thin filaments in the in vitro motility assay. To learn if this effect is related to the rate of MgADP dissociation from the acto-S1 cross-bridge head, the effects of regulatory proteins on nucleotide binding and release in motility assays were measured in the presence and absence of regulatory proteins. The apparent affinity of acto-heavy meromyosin (acto-HMM) for MgATP was reduced by the presence of regulatory proteins. Similarly, the regulatory proteins increase the concentration of MgADP required to inhibit sliding. These results suggest that regulatory proteins either accelerate the rate of MgADP release from acto-HMM-MgADP or slow its binding to acto-HMM. The reduction of temperature also altered the relationship between thin filament sliding speed and the regulatory proteins. At lower temperatures, the regulatory proteins lost their ability to increase thin filament sliding speed above that of unregulated thin filaments. It is hypothesized that structural changes in the actin portion of the acto-myosin interface are induced by regulatory protein binding to actin.

Familial hypertrophic cardiomyopathy mutations in troponin I (K183D, G203S, K206Q) enhance filament sliding

A major cause of familial hypertrophic cardiomyopathy (FHC) is dominant mutations in cardiac sarcomeric genes. Linkage studies identified FHC-related mutations in the COOH terminus of cardiac troponin I (cTnI), a region with unknown function in Ca(2+) regulation of the heart. Using in vitro assays with recombinant rat troponin subunits, we tested the hypothesis that mutations K183Delta, G203S, and K206Q in cTnI affect Ca(2+) regulation. All three mutants enhanced Ca(2+) sensitivity and maximum speed (s(max)) of filament sliding of in vitro motility assays. Enhanced s(max) (pCa 5) was observed with rabbit skeletal and rat cardiac (alpha-MHC or beta-MHC) heavy meromyosin (HMM). We developed a passive exchange method for replacing endogenous cTn in permeabilized rat cardiac trabeculae. Ca(2+) sensitivity and maximum isometric force did not differ between preparations exchanged with cTn(cTnI,K206Q) or wild-type cTn. In both trabeculae and motility assays, there was no loss of inhibition at pCa 9. These results are consistent with COOH terminus of TnI modulating actomyosin kinetics during unloaded sliding, but not during isometric force generation, and implicate enhanced cross-bridge cycling in the cTnI-related pathway(s) to hypertrophy.

Thin filament activation and unloaded shortening velocity of rabbit skinned muscle fibres

The unloaded shortening velocity of skinned rabbit psoas muscle fibres is sensitive to [Ca2+]. To determine whether Ca2+ affects the unloaded shortening velocity via regulation of crossbridge kinetics or crossbridge number, the shortening velocity was measured following changes in either [Ca2+] or the number of active thin filament regulatory units. The native troponin C (TnC) was extracted and replaced with either cardiac TnC (cTnC) or a mixture of cTnC and an inactive mutant cardiac TnC (CBMII TnC). The unloaded shortening velocity of the cTnC-replaced fibres was determined at various values of [Ca2+] and compared with different cTnC:CBMII TnC ratios at a saturating [Ca2+]. If Ca2+ regulates the unloaded shortening velocity via kinetic modulation, differences in the velocity-tension relationship between the cTnC fibres and the cTnC:CBMII TnC fibres would be apparent. Alternatively, Ca2+ control of the number of active crossbridges would yield similar velocity-tension relationships when comparing the cTnC and cTnC:CBMII TnC fibres. The results show a decline in the unloaded shortening velocity that is determined by the relative tension, defined as the level of thin filament activation, rather than the [Ca2+]. Furthermore, at lower levels of relative tension, the reduction in unloaded shortening is not the result of changes in any cooperative effects of myosin on Ca2+ binding to the thin filament. Rather, it may be related to a decrease in crossbridge-induced activation of the thin filament at the level of the individual regulatory unit. In summary, the results suggest that Ca2+ regulates the unloaded shortening velocity in skinned fibres by reducing the number of crossbridges able to productively bind to the thin filament without affecting any inherent property of the myosin.

Phosphorylation or glutamic acid substitution at protein kinase C sites on cardiac troponin I differentially depress myofilament tension and shortening velocity

There is evidence that multi-site phosphorylation of cardiac troponin I (cTnI) by protein kinase C is important in both long- and short-term regulation of cardiac function. To determine the specific functional effects of these phosphorylation sites (Ser-43, Ser-45, and Thr-144), we measured tension and sliding speed of thin filaments in reconstituted preparations in which endogenous cTnI was replaced with cTnI phosphorylated by protein kinase C-epsilon or mutated to cTnI-S43E/S45E/T144E, cTnI-S43E/S45E, or cTnI-T144E. We used detergent-skinned mouse cardiac fiber bundles to measure changes in Ca(2+)-dependence of force. Compared with controls, fibers reconstituted with phosphorylated cTnI, cTnI-S43E/S45E/T144E, or cTnI-S43E/S45E were desensitized to Ca(2+), and maximum tension was as much as 27% lower, whereas fibers reconstituted with cTnI-T144E showed no change. In the in vitro motility assay actin filaments regulated by troponin complexes containing phosphorylated cTnI or cTnI-S43E/S45E/T144E showed both a decrease in Ca(2+) sensitivity and maximum sliding speed compared with controls, whereas filaments regulated by cTnI-S43E/S45E showed only decreased maximum sliding speed and filaments regulated by cTnI-T144E demonstrated only desensitization to Ca(2+). Our results demonstrate novel site specificity of effects of PKC phosphorylation on cTnI function and emphasize the complexity of modulation of the actin-myosin interaction by specific changes in the thin filament.

Multivariate statistics in analysis of data from the in vitro motility assay

A novel approach is described for classification of filaments as stationary or moving and for extraction of velocity data for smooth actin filament sliding in vitro. Moving and stationary filaments were effectively classified using four discriminating variables in a multivariate statistical analysis. The variables were (1) two different measures of the average filament distance from its starting point, (2) a measure of the variability in sliding direction, and (3) the coefficient of variation (CV) of the frame-to-frame sliding velocity (v(mean)). On the basis of this multivariate analysis we obtained correct classification of 98% of the stationary filaments and 94% of the moving filaments in a cross-validation data set. The same classification functions were useful throughout despite a 10-fold variation in the average sliding velocity in the cross-validation data. Further analysis of motile filaments suggested that the velocity of smooth sliding should, ideally, be obtained from the intercept on the velocity axis of a plot of v(mean) against CV. The velocity, so obtained, was between 10 and 30% (mean 20+/-3%; n=7; p<0.001) higher than if average sliding velocity was obtained for all moving filaments with CV<0.5.

Actomyosin motility on nanostructured surfaces

We have here, for the first time, used nanofabrication techniques to reproduce aspects of the ordered actomyosin arrangement in a muscle cell. The adsorption of functional heavy meromyosin (HMM) to five different resist polymers was first assessed. One group of resists (MRL-6000.1XP and ZEP-520) consistently exhibited high quality motility of actin filaments after incubation with HMM. A second group (PMMA-200, PMMA-950, and MRI-9030) generally gave low quality of motility with only few smoothly moving filaments. Based on these findings electron beam lithography was applied to a bi-layer resist system with PMMA-950 on top of MRL-6000.1XP. Grooves (100-200nm wide) in the PMMA layer were created to expose the MRL-6000.1XP surface for adsorption of HMM and guidance of actin filament motility. This guidance was quite efficient allowing no U-turns of the filaments and approximately 20 times higher density of moving filaments in the grooves than on the surrounding PMMA.

Random walks with thin filaments: application of in vitro motility assay to the study of actomyosin regulation

The in vitro motility devised by Kron and Spudich (Kron and Spudich, 1986; Kron et al., 1991) has proved a very valuable technique for studying the motor properties of myosin of all kinds but it is equally useful for the study of the thin filaments of muscle and their regulation. The movement of a population of thin filaments over immobilised myosin appears to be random but it does in fact yield a large amount of information about contractility and its regulation. The key to extracting useful information from in vitro motility assay experiments is the logical and comprehensive analysis of filament movements.

Thin filament regulation and ionic interactions between the N-terminal region in actin and troponin

The N-terminal region in actin has been shown to interact with both myosin and troponin (Tn) during the cross-bridge cycle and in regulation. To study the role of this region in regulation, we used yeast actin mutants with increased and decreased numbers of acidic residues. The mutants included D24A/D25A, with Asp(24) and Asp(25) replaced with alanines; DNEQ, with the substitution of Asp(2) and Glu(4) with their amide analogs; and 4Ac, with Glu(3) and Asp(4) inserted in lieu of Ser(3). In the in vitro motility assay, using reconstituted regulated thin filaments, the sliding speeds of DNEQ, D24A/D25A, and 4Ac were similar at all pCa values. Thus, Ca(2+)-sensitivity of the thin filaments and the inhibitory function of TnI appear to be insensitive to changes in charge (+/-2) at the N-terminus of actin, suggesting little, if any, role of that actin region in regulation. A Ca(2+)-independent conformational change in that region was detected upon troponin binding to actin-Tm via an increase in the fluorescence of a pyrene probe attached to another yeast actin mutant that we used (Cys(1)).

Characterization of three regulatory states of the striated muscle thin filament

The troponin-tropomyosin-linked regulation of striated muscle contraction occurs through allosteric control by both Ca(2+) and myosin. The thin filament fluctuates between two extreme states: the inactive "off" state and the active "on" state. Intermediate states have been proposed from structural studies and transient kinetic measurements. However, in contrast to the well-characterised, on and off states, the mechanochemical properties of the intermediate states are much less well understood because of the instability of those states. In the present study, we have characterized a myosin-induced intermediate that is stabilized by cross-linking myosin motor domains (S1) to actin filaments (with a maximum of one S1 molecule for 50 actin monomers). A single S1 molecule is known to interact with two adjacent actin monomers. A detailed analysis revealed that thin filaments containing S1 molecules cross-linked to just one actin monomer (actin(1)-S1 complexes) are regulated with a 79% inhibition of the ATPase in the absence of Ca(2+). In contrast, filaments containing S1 molecules cross-linked at two positions, to two adjacent actin monomers (actin(2)-S1 complexes) totally lose their regulation in a highly cooperative manner. This loss of regulation was due both to an enhancement of the ATPase activity without calcium and an inhibition of the ATPase with calcium. Filaments containing actin(2)-S1 complexes, with significant ATPase activity in the absence of calcium (about 50%), did not move on a myosin-coated surface unless calcium was present. This partial uncoupling between the ATPase activity and in vitro motility in the absence of calcium demonstrates that the mechanical steps require actin-myosin contacts, which take place only in the on state and not in the off or intermediate states. These data provide new insights concerning the difference in cooperativity of Ca(2+) regulation that exists between the biochemical and mechanical cycles of the actin-myosin motor.

The troponin tail domain promotes a conformational state of the thin filament that suppresses myosin activity

In cardiac and skeletal muscles tropomyosin binds to the actin outer domain in the absence of Ca(2+), and in this position tropomyosin inhibits muscle contraction by interfering sterically with myosin-actin binding. The globular domain of troponin is believed to produce this B-state of the thin filament (Lehman, W., Hatch, V., Korman, V. L., Rosol, M., Thomas, L. T., Maytum, R., Geeves, M. A., Van Eyk, J. E., Tobacman, L. S., and Craig, R. (2000) J. Mol. Biol. 302, 593-606) via troponin I-actin interactions that constrain the tropomyosin. The present study shows that the B-state can be promoted independently by the elongated tail region of troponin (the NH(2) terminus (TnT-(1-153)) of cardiac troponin T). In the absence of the troponin globular domain, TnT-(1-153) markedly inhibited both myosin S1-actin-tropomyosin MgATPase activity and (at low S1 concentrations) myosin S1-ADP binding to the thin filament. Similarly, TnT-(1-153) increased the concentration of heavy meromyosin required to support in vitro sliding of thin filaments. Electron microscopy and three-dimensional reconstruction of thin filaments containing TnT-(1-153) and either cardiac or skeletal muscle tropomyosin showed that tropomyosin was in the B-state in the complete absence of troponin I. All of these results indicate that portions of the troponin tail domain, and not only troponin I, contribute to the positioning of tropomyosin on the actin outer domain, thereby inhibiting muscle contraction in the absence of Ca(2+).

Cardiac troponin T isoforms demonstrate similar effects on mechanical performance in a regulated contractile system

Alteration of troponin T (TnT) isoform expression has been reported in human and animal models of myocardial failure. The two adult beef cardiac TnT isoforms (TnT(3) and TnT(4)) were isolated for comparative functional analysis. Thin filaments were reconstituted containing pure populations of the isoforms. The in vitro motility assay was used to directly compare the effect of the two TnT isoforms on force and unloaded shortening as a function of free calcium. We found no significant differences between the two isoforms in terms of calcium sensitivity, cooperativity, or maximal activation (velocity and force) as assessed in a fully calcium-regulated system. Activation by myosin strong binding was similar for thin filaments containing either of the two TnT isoforms. Whereas maximally activated velocity and cooperativity was depressed at pH 6.5, no difference between thin filaments containing the two isoforms was detected. From the small magnitude of the TnT isoform shifts detected in myocardial failure and the lack of significant mechanical effect detected in the motility assay, variable TnT isoform expression is unlikely to be any functional significance in heart failure.

Effects of phosphorylation and mutation R145G on human cardiac troponin I function

We have studied functional consequences of the mutations R145G, S22A, and S23A of human cardiac troponin I (cTnI) and of phosphorylation of two adjacent N-terminal serine residues in the wild-type cTnI and the mutated proteins. The mutation R145G has been linked to the development of familial hypertrophic cardiomyopathy. Cardiac troponin was reconstituted from recombinant human subunits including either wild-type or mutant cTnI and was used for reconstitution of thin filaments with skeletal muscle actin and tropomyosin. The Ca(2+)-dependent thin filament-activated myosin subfragment 1 ATPase (actoS1-ATPase) activity and the in vitro motility of these filaments driven by myosin were measured as a function of the cTnI phosphorylation state. Bisphosphorylation of wild-type cTnI decreases the Ca(2+) sensitivity of the actoS1-ATPase activity and the in vitro thin filament motility by about 0.15-0.21 pCa unit. The nonconservative replacement R145G in cTnI enhances the Ca(2+) sensitivity of the actoS1-ATPase activity by about 0.6 pCa unit independent of the phosphorylation state of cTnI. Furthermore, it mimics a strong suppressing effect on both the maximum actoS1-ATPase activity and the maximum in vitro filament sliding velocity which has been observed upon bisphosphorylation of wild-type cTnI. Bisphosphorylation of the mutant cTnI-R145G itself had no such suppressing effects anymore. Differential analysis of the effect of phosphorylation of each of the two serines, Ser23 in cTnI-S22A and Ser22 in cTnI-S23A, indicates that phosphorylation of Ser23 may already be sufficient for causing the reduction of maximum actoS1-ATPase activity and thin filament sliding velocity seen upon phosphorylation of both of these serines.

Modulation of myosin function by isoform-specific properties of Saccharomyces cerevisiae and muscle tropomyosins

Tropomyosin is an extended coiled-coil protein that influences actin function by binding longitudinally along thin filaments. The present work compares cardiac tropomyosin and the two tropomyosins from Saccharomyces cerevisiae, TPM1 and TPM2, that are much shorter than vertebrate tropomyosins. Unlike cardiac tropomyosin, the phase of the coiled-coil-forming heptad repeat of TPM2 is discontinuous; it is interrupted by a 4-residue deletion. TPM1 has two such deletions, which flank the 38-residue partial gene duplication that causes TPM1 to span five actins instead of the four of TPM2. Each of the three tropomyosin isoforms modulates actin-myosin interactions, with isoform-specific effects on cooperativity and strength of myosin binding. These different properties can be explained by a model that combines opposite effects, steric hindrance between myosin and tropomyosin when the latter is bound to a subset of its sites on actin, and also indirect, favorable interactions between tropomyosin and myosin, mediated by mutually promoted changes in actin. Both of these effects are influenced by which tropomyosin isoform is present. Finally, the tropomyosins have isoform-specific effects on in vitro sliding speed and on the myosin concentration dependence of this movement, suggesting that non-muscle tropomyosin isoforms exist, at least in part, to modulate myosin function.

Strong binding of myosin increases shortening velocity of rabbit skinned skeletal muscle fibres at low levels of Ca(2+)

1. At low levels of activation, unloaded shortening of skinned skeletal muscle fibres takes place in two phases: an initial phase of high-velocity shortening followed by a phase of low-velocity shortening. The basis for Ca(2+) dependence of unloaded shortening velocity (V(o)) in the low-velocity phase was investigated by varying the level of thin filament activation with Ca(2+) and N-ethyl-maleimide myosin subfragment-1 (NEM-S1), a non-tension-generating, strong binding derivative of subfragment-1. V(o) was measured with the slack-test method. 2. Treatment of skinned fibres with 5 microM NEM-S1 eliminated the low-velocity phase of shortening but had no effect on the high-velocity phase of shortening during submaximal activation with Ca(2+), or on V(o) during maximal activation with Ca(2+). 3. Extensive washout of NEM-S1 from the treated fibres restored the low-velocity phase of shortening and returned low-velocity V(o) to pre-treatment values. 4. The effect of NEM-S1 to increase low-velocity V(o) can be explained in terms of a model in which strong binding myosin cross-bridges activate the thin filament to a state in which the rate of ADP release from the actin-myosin-ADP complex and the rate of cross-bridge detachment from actin are accelerated during unloaded shortening.

Hypertrophic cardiomyopathy caused by a novel alpha-tropomyosin mutation (V95A) is associated with mild cardiac phenotype, abnormal calcium binding to troponin, abnormal myosin cycling, and poor prognosis

BACKGROUND

We report hypertrophic cardiomyopathy (HCM) in a Spanish-American family caused by a novel alpha-tropomyosin (TPM1) mutation and examine the pathogenesis of the clinical disease by characterizing functional defects in the purified mutant protein.

METHODS AND RESULTS

HCM was linked to the TPM1 gene (logarithm of the odds [LOD] score 3.17). Sequencing and restriction digestion analysis demonstrated a TPM1 mutation V95A that cosegregated with HCM. The mutation has been associated with 13 deaths in 26 affected members (11 sudden deaths and 2 related to heart failure), with a cumulative survival rate of 73+/-10% at the age of 40 years. Left ventricular wall thickness (mean 16+/-6 mm) and disease penetrance (53%) were similar to those for the ss-myosin mutations L908V and G256E previously associated with a benign prognosis. Left ventricular hypertrophy was milder than with the ss-myosin mutation R403Q, but the prognosis was similarly poor. With the use of recombinant tropomyosins, we identified several functional alterations at the protein level. The mutation caused a 40% to 50% increase in calcium affinity in regulated thin filament-myosin subfragment-1 (S1) MgATPase assays, a 20% decrease in MgATPase rates in the presence of saturating calcium, a 5% decrease in unloaded shortening velocity in in vitro motility assays, and no change in cooperative myosin S1 binding to regulated thin filaments.

CONCLUSIONS

In contrast to other reported TPM1 mutations, V95A-associated HCM exhibits unusual features of mild phenotype but poor prognosis. Both myosin cycling and calcium binding to troponin are abnormal in the presence of the mutant tropomyosin. The genetic diagnosis afforded by this mutation will be valuable in the management of HCM.

A simple method for measuring the relative force exerted by myosin on actin filaments in the in vitro motility assay: evidence that tropomyosin and troponin increase force in single thin filaments

We have studied the effect of an internal load on the movement of actin filaments over a bed of heavy meromyosin (HMM) in the in vitro motility assay. Immobilized alpha-actinin can bind to actin filaments reversibly and ultimately stop the filaments from moving. Above a critical concentration of alpha-actinin, thin filament velocity rapidly diminished to zero. The fraction of thin motile filaments decreased linearly to zero with increasing alpha-actinin concentration. The concentration of alpha-actinin needed to stop all filaments from moving (0.8 microg/ml with actin) was very consistent both within and between experiments. In the present study we have defined the 'index of retardation' as the concentration of alpha-actinin needed to stop all filament movement, and we propose that this index is a measure of the isometric force exerted by HMM on actin filaments. When we measured the effect of immobilized alpha-actinin on motility in the presence of 10 mM P(i) we found that the index of retardation was 0.62+/-0.07 (n=3) times that in the absence of P(i). This observation is in agreement with the reduction of isometric tension in chemically-skinned muscle due to P(i). In a series of comparative experiments we observed that tropomyosin and troponin increase the index of retardation and that the degree of increase depends upon the tropomyosin isoform studied. The index of retardation of actin is increased 1.8-fold by skeletal-muscle tropomyosin, and 3-fold by both cardiac-muscle and smooth-muscle tropomyosin. In the presence of troponin the index of retardation is 2.9-3.4-fold greater than that of actin with all tropomyosin isoforms.

Effect of hypertrophic cardiomyopathy mutations in human cardiac muscle alpha -tropomyosin (Asp175Asn and Glu180Gly) on the regulatory properties of human cardiac troponin determined by in vitro motility assay

The properties of mutant contractile proteins that cause hypertrophic cardiomyopathy (HCM) have been investigated in expression studies and in mouse models. There is growing evidence that the precise isoforms of both the mutated protein and its interacting partners can qualitatively influence the effects of the mutation. We therefore investigated the functional effects of two HCM mutations in alpha -tropomyosin, Asp175Asn and Glu180Gly, in the in vitro motility assay using recombinant human alpha -tropomyosin, expressed with an N-terminal alanine-serine extension (AStm) to mimic acetylation in vivo, and purified native human cardiac troponin. The expected switching off of reconstituted filament movement at pCa9, and switching on at pCa5, was observed with no difference in fraction of filaments motile or filament velocity, between wild-type and mutant filaments. However, we observed increased Ca(2+)sensitivity of fraction of filaments motile using the mutant tropomyosin compared to wild-type (DeltaEC(50)+0.082+/-0. 019 pCa units for Asp175Asn and +0.115+/-0.021 for Glu180Gly). Indirect measurements using immobilized alpha -actinin to retard filament movement showed that filaments reconstituted with mutant AStm produced the same force as wild-type filaments. The results using human cardiac regulatory proteins reveal different effects of the HCM mutations in tropomyosin compared to studies using heterologous systems. By performing parallel experiments using either human cardiac or rabbit skeletal troponin we show that the cardiac-specific phenotype of HCM mutations in alpha -tropomyosin is not the result of more marked functional changes when interacting with cardiac troponin.

2-deoxy-ATP enhances contractility of rat cardiac muscle

To investigate the kinetic parameters of the crossbridge cycle that regulate force and shortening in cardiac muscle, we compared the mechanical properties of cardiac trabeculae with either ATP or 2-deoxy-ATP (dATP) as the substrate for contraction. Comparisons were made in trabeculae from untreated rats (predominantly V1 myosin) and those treated with propylthiouracil (PTU; V3 myosin). Steady-state hydrolytic activity of cardiac heavy meromyosin (HMM) showed that PTU treatment resulted in >40% reduction of ATPase activity. dATPase activity was >50% elevated above ATPase activity in HMM from both untreated and PTU-treated rats. V(max) of actin-activated hydrolytic activity was also >50% greater with dATP, whereas the K(m) for dATP was similar to that for ATP. This indicates that dATP increased the rate of crossbridge cycling in cardiac muscle. Increases in hydrolytic activity were paralleled by increases of 30% to 80% in isometric force (F(max)), rate of tension redevelopment (k(tr)), and unloaded shortening velocity (V(u)) in trabeculae from both untreated and PTU-treated rats (at maximal Ca(2+) activation), and F-actin sliding speed in an in vitro motility assay (V(f)). These results contrast with the effect of dATP in rabbit psoas and soleus fibers, where F(max) is unchanged even though k(tr), V(u), and V(f) are increased. The substantial enhancement of mechanical performance with dATP in cardiac muscle suggests that it may be a better substrate for contractility than ATP and warrants exploration of ribonucleotide reductase as a target for therapy in heart failure.

Viscosity and solute dependence of F-actin translocation by rabbit skeletal heavy meromyosin

We tested the hypothesis that solvent viscosity affects translocation of rhodamine phalloidin-labeled F-actin by rabbit skeletal heavy meromyosin (HMM). When viscosity was increased using either glycerol, fructose, sucrose, or dextran (1.5, 6.0, or 15-20 kDa mol mass), there was little or no effect on the fraction of moving filaments, whereas sliding speed decreased in inverse proportion to viscosity. The results could be explained neither by an effect of osmotic pressure at high solute concentrations nor by altered solvent drag on the actin filament. Elevated viscosity inhibited HMM ATPase activity in solution, but only at much higher viscosities than were needed to reduce sliding speed. Polyethylene glycols (300, 1,000, or 3,000 mol wt) also inhibited speed via elevated viscosity but secondarily inhibited by enhancing electrostatic interactions. These results demonstrate that a diffusion-controlled process intrinsic to cross-bridge cycling can be limiting to actomyosin function.

Influence of ADP on cross-bridge-dependent activation of myofibrillar thin filaments

Contraction of skeletal muscle is regulated by calcium at the level of the thin filament via troponin and tropomyosin. Studies have indicated that strong cross-bridge binding is also involved in activation of the thin filament. To further test this, myofibrils were incubated with a wide range of fluorescent myosin subfragment 1(fS1) at pCa 9 or pCa 4 with or without ADP. Sarcomere fluorescence intensity and the fluorescence intensity ratio (non-overlap region/overlap region) were measured to determine the amount and location of bound fS1 in the myofibril. There was lower sarcomere fluorescence intensity with ADP compared to without ADP for both calcium levels. Similar data were obtained from biochemical measures of bound fS1, validating the fluorescence microscopy measurements. The intensity ratio, which is related to activation of the thin filament, increased with increasing [fS1] with or without ADP. At pCa 9, the fluorescence intensity ratio was constant until 80-160 nM fS1 without ADP conditions, then it went up dramatically and finally attained saturation. The dramatic shift of the ratio demonstrated the cooperative character of strong cross-bridge binding, and this was not observed at high calcium. A similar pattern was observed with ADP in that the ratio was right-shifted with respect to total [fS1]. Saturation was obtained with both the fluorescence intensity and ratio data. Plots of intensity ratio as a function of normalized sarcomere intensity (bound fS1) showed little difference between with and without ADP. This suggests that the amount of strongly bound fS1, not fS1 state (with or without ADP) is related to activation of the thin filament.

Regulation of force and unloaded sliding speed in single thin filaments: effects of regulatory proteins and calcium

1. Measurements of the unloaded sliding speed of and isometric force exerted on single thin filaments in in vitro motility assays were made to evaluate the role of regulatory proteins in the control of unloaded thin filament sliding speed and isometric force production. 2. Regulated actin filaments were reconstituted from rabbit F-actin, native bovine cardiac tropomyosin (nTm), and either native bovine cardiac troponin (nTn), troponin containing a TnC mutant, CBMII, in which the sole regulatory site in cardiac TnC (site II) is inactivated (CBMII-Tn), or troponin containing a point mutation in TnT (I79N, where isoleucine at position 79 is replaced with asparagine) associated with familial hypertrophic cardiomyopathy (FHC). 3. Addition of regulatory proteins to the thin filament increases both the unloaded sliding speed and the isometric force exerted by myosin heads on the thin filaments. 4. Variation of thin filament activation by varying [Ca2+] or the fraction of CBMII/TnC bound to the thin filament at pCa 5, had little effect on the unloaded filament sliding speed until the fraction of the thin filament containing calcium bound to TnC was less than 0.15. These results suggest that [Ca2+] primarily affects the number of attached and cycling crossbridges. 5. The presence of the FHC TnT mutant increased the thin filament sliding speed but reduced the isometric force that heavy meromyosin exerted on regulated thin filaments. These latter results, together with the increased sliding speed and isometric force seen in the presence of regulatory proteins, suggest that thin filament regulatory proteins exert significant allosteric effects on the interaction of crossbridges with the thin filament.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Functional consequences of troponin T mutations found in hypertrophic cardiomyopathy

Missense mutations in the cardiac thin filament protein troponin T (TnT) are a cause of familial hypertrophic cardiomyopathy (FHC). To understand how these mutations produce dysfunction, five TnTs were produced and purified containing FHC mutations found in several regions of TnT. Functional defects were diverse. Mutations F110I, E244D, and COOH-terminal truncation weakened the affinity of troponin for the thin filament. Mutation DeltaE160 resulted in thin filaments with increased calcium affinity at the regulatory site of troponin C. Mutations R92Q and F110I resulted in impaired troponin solubility, suggesting abnormal protein folding. Depending upon the mutation, the in vitro unloaded actin-myosin sliding speed showed small increases, showed small decreases, or was unchanged. COOH-terminal truncation mutation resulted in a decreased thin filament-myosin subfragment 1 MgATPase rate. The results indicate that the mutations cause diverse immediate effects, despite similarities in disease manifestations. Separable but repeatedly observed abnormalities resulting from FHC TnT mutations include increased unloaded sliding speed, increased or decreased Ca(2+) affinity, impairment of folding or sarcomeric integrity, and decreased force. Enhancement as well as impairment of contractile protein function is observed, suggesting that TnT, including the troponin tail region, modulates the regulation of cardiac contraction.

Effects of tropomyosin internal deletions on thin filament function

Striated muscle tropomyosin spans seven actin monomers and contains seven quasi-repeating regions with loose sequence similarity. Each region contains a hypothesized actin binding motif. To examine the functions of these regions, full-length tropomyosin was compared with tropomyosin internal deletion mutants spanning either five or four actins. Actin-troponin-tropomyosin filaments lacking tropomyosin regions 2-3 exhibited calcium-sensitive regulation in in vitro motility and myosin S1 ATP hydrolysis experiments, similar to filaments with full-length tropomyosin. In contrast, filaments lacking tropomyosin regions 3-4 were inhibitory to these myosin functions. Deletion of regions 2-4, 3-5, or 4-6 had little effect on tropomyosin binding to actin in the presence of troponin or troponin-Ca(2+), or in the absence of troponin. However, all of these mutants inhibited myosin cycling. Deletion of the quasi-repeating regions diminished the prominent effect of myosin S1 on tropomyosin-actin binding. Interruption of this cooperative, myosin-tropomyosin interaction was least severe for the mutant lacking regions 2-3 and therefore correlated with inhibition of myosin cycling. Regions 3, 4, and 5 each contributed about 1.5 kcal/mol to this process, whereas regions 2 and 6 contributed much less. We suggest that a myosin-induced conformational change in actin facilitates the azimuthal repositioning of tropomyosin which is an essential part of regulation.