The effect of genetically expressed cardiac titin fragments on in vitro actin motility

Titin: A Tunable Spring in Active Muscle

Muscle has conventionally been viewed as a motor that converts chemical to kinetic energy in series with a passive spring, but new insights emerge when muscle is viewed as a composite material whose elastic elements are tuned by activation. New evidence demonstrates that calcium-dependent binding of N2A titin to actin increases titin stiffness in active skeletal muscles, which explains many long-standing enigmas of muscle physiology.

Calcium-dependent titin-thin filament interactions in muscle: observations and theory

Gaps in our understanding of muscle mechanics demonstrate that the current model is incomplete. Increasingly, it appears that a role for titin in active muscle contraction might help to fill these gaps. While such a role for titin is increasingly accepted, the underlying molecular mechanisms remain unclear. The goals of this paper are to review recent studies demonstrating Ca²⁺-dependent interactions between N2A titin and actin in vitro, to explore theoretical predictions of muscle behavior based on this interaction, and to review experimental data related to the predictions. In a recent study, we demonstrated that Ca²⁺ increases the association constant between N2A titin and F-actin; that Ca²⁺ increases rupture forces between N2A titin and F-actin; and that Ca²⁺ and N2A titin reduce sliding velocity of F-actin and reconstituted thin filaments in motility assays. Preliminary data support a role for Ig83, but other Ig domains in the N2A region may also be involved. Two mechanical consequences are inescapable if N2A titin binds to thin filaments in active muscle sarcomeres: (1) the length of titin's freely extensible I-band should decrease upon muscle activation; and (2) binding between N2A titin and thin filaments should increase titin stiffness in active muscle. Experimental observations demonstrate that these properties characterize wild type muscles, but not muscles from mdm mice with a small deletion in N2A titin, including part of Ig83. Given the new in vitro evidence for Ca²⁺-dependent binding between N2A titin and actin, it is time for skepticism to give way to further investigation.

Passive force enhancement in striated muscle

Passive force enhancement is defined as the increase in passive, steady-state, isometric force of an actively stretched muscle compared with the same muscle stretched passively to that same length. Passive force enhancement is long lasting, increases with increasing muscle length and increasing stretch magnitudes, contributes to the residual force enhancement in skeletal and cardiac muscle, and is typically only observed at muscle lengths at which passive forces occur naturally. Passive force enhancement is typically equal to or smaller than the total residual force enhancement, it persists when a muscle is deactivated and reactivated, but can be abolished instantaneously when a muscle is shortened quickly from its stretched length. There is strong evidence that the passive force enhancement is caused by the filamentous sarcomeric protein titin, although the detailed molecular mechanisms underlying passive force enhancement remain unknown. Here I propose a tentative mechanism based on experimental evidence that associates passive force enhancement with the shortening of titin's free spring length in the I-band region of sarcomeres. I suggest that this shortening is accomplished by titin binding to actin and that the trigger for titin-actin interactions is associated with the formation of strongly bound cross bridges between actin and myosin that exposes actin attachment sites for titin through movement of the regulatory proteins troponin and tropomyosin.

The multiple roles of titin in muscle contraction and force production

Titin is a filamentous protein spanning the half-sarcomere, with spring-like properties in the I-band region. Various structural, signaling, and mechanical functions have been associated with titin, but not all of these are fully elucidated and accepted in the scientific community. Here, I discuss the primary mechanical functions of titin, including its accepted role in passive force production, stabilization of half-sarcomeres and sarcomeres, and its controversial contribution to residual force enhancement, passive force enhancement, energetics, and work production in shortening muscle. Finally, I provide evidence that titin is a molecular spring whose stiffness changes with muscle activation and actin-myosin-based force production, suggesting a novel model of force production that, aside from actin and myosin, includes titin as a "third contractile" filament. Using this three-filament model of sarcomeres, the stability of (half-) sarcomeres, passive force enhancement, residual force enhancement, and the decrease in metabolic energy during and following eccentric contractions can be explained readily.

Differences in titin segmental elongation between passive and active stretch in skeletal muscle

Since the 1950s, muscle contraction has been explained using a two-filament system in which actin and myosin exclusively dictate active force in muscle sarcomeres. Decades later, a third filament called titin was discovered. This titin filament has recently been identified as an important regulator of active force, but has yet to be incorporated into contemporary theories of muscle contraction. When sarcomeres are actively stretched, a substantial and rapid increase in force occurs, which has been suggested to arise in part from titin-actin binding that is absent in passively stretched sarcomeres. However, there is currently no direct evidence for such binding within muscle sarcomeres. Therefore, we aimed to determine whether titin binds to actin in actively but not in passively stretched sarcomeres by observing length changes of proximal and distal titin segments in the presence and absence of calcium. We labeled I-band titin with fluorescent F146 antibody in rabbit psoas myofibrils and tracked segmental elongations during passive (no calcium) and active (high calcium) stretch. Without calcium, proximal and distal segments of titin elongated as expected based on their free spring properties. In contrast, active stretch differed statistically from passive stretch, demonstrating that calcium activation increases titin segment stiffness, but not in an actin-dependent manner. The consistent elongation of the proximal segment was contrary to what was expected if titin's proximal segment was attached to actin. This rapid calcium-dependent change in titin stiffness likely contributes to active muscle force regulation in addition to actin and myosin.

Residual Force Enhancement Following Eccentric Contractions: A New Mechanism Involving Titin

Eccentric muscle properties are not well characterized by the current paradigm of the molecular mechanism of contraction: the cross-bridge theory. Findings of force contributions by passive structural elements a decade ago paved the way for a new theory. Here, we present experimental evidence and theoretical support for the idea that the structural protein titin contributes to active force production, thereby explaining many of the unresolved properties of eccentric muscle contraction.

A novel three-filament model of force generation in eccentric contraction of skeletal muscles

We propose and examine a three filament model of skeletal muscle force generation, thereby extending classical cross-bridge models by involving titin-actin interaction upon active force production. In regions with optimal actin-myosin overlap, the model does not alter energy and force predictions of cross-bridge models for isometric contractions. However, in contrast to cross-bridge models, the three filament model accurately predicts history-dependent force generation in half sarcomeres for eccentric and concentric contractions, and predicts the activation-dependent forces for stretches beyond actin-myosin filament overlap.

Mechanisms of enhanced force production in lengthening (eccentric) muscle contractions

In contrast to isometric and shortening contractions, many observations made on actively lengthening muscles cannot be readily explained with the sliding filament and cross-bridge theory. Specifically, residual force enhancement, the persistent increase in force following active muscle lengthening, beyond what one would expect based on muscle length, has not been explained satisfactorily. Here, we summarize the experimental evidence on residual force enhancement, critically evaluate proposed mechanisms for the residual force enhancement, and propose a mechanism for residual force enhancement that explains all currently agreed upon experimental observations. The proposed mechanism is based on the engagement of the structural protein titin upon muscle activation and an increase in titin's resistance to active compared with passive stretching. This change in resistance from the passive to the active state is suggested to be based on 1) calcium binding by titin upon activation, 2) binding of titin to actin upon activation, and 3) as a consequence of titin-actin binding—a shift toward stiffer titin segments that are used in active compared with passive muscle elongation. Although there is some experimental evidence for the proposed mechanism, it must be stressed that much of the details proposed here remain unclear and should provide ample research opportunities for scientists in the future. Nevertheless, the proposed mechanism for residual force enhancement explains all basic findings in this area of research.

Force enhancement following stretch in a single sarcomere

It has been accepted for half a century that, for a given level of activation, the steady-state isometric force of a muscle sarcomere depends exclusively on the amount of overlap between the contractile filaments actin and myosin, or equivalently sarcomere length (Gordon AM et al., J Physiol 184: 170–192, 1966). Moreover, according to the generally accepted paradigm of muscle contraction, the cross-bridge theory (Huxley AF, Prog Biophys Biophys Chem 7: 255–318, 1957), this steady-state isometric sarcomere force is independent of the muscle's contractile history (Huxley AF, Prog Biophys Biophys Chem 7: 255–318, 1957; Walcott S and Herzog W, Math Biosci 216: 172–186, 2008); i.e., it is independent of whether a muscle is held at a constant length before and during the contraction or whether the muscle is shortened or lengthened to the same constant length. This, however, is not the case, as muscles and single fibers that are stretched show greatly increased steady-state isometric forces compared with preparations that are held at a constant length (Abbott BC and Aubert XM, J Physiol 117: 77–86, 1952; De Ruiter CJ et al., J Physiol 526.3: 671–681, 2000; Edman KAP et al., J Physiol 281: 139–155, 1978; Edman KAP et al., J Gen Physiol 80: 769–784, 1982; Edman KAP and Tsuchiya T, J Physiol 490.1: 191–205, 1996). This so-called “residual force enhancement” (Edman KAP et al., J Gen Physiol 80: 769–784, 1982) offers a perplexing puzzle for muscle physiologists. Many theories have been advanced to address the discrepancy between prediction and observation with the most popular and accepted being the sarcomere length nonuniformity theory (Morgan DL, Biophys J 57: 209–221, 1990), which explains the residual force enhancement with the development of large nonuniformities in sarcomere lengths during muscle stretching. Here, we performed experiments in mechanically isolated sarcomeres and observed that the residual force enhancement following active stretching is preserved. Since our preparation utilizes a single sarcomere, a redistribution of the length of neighboring sarcomeres to produce the higher force following stretch is, by design, precluded. Furthermore, the enhanced forces in the single sarcomeres always exceed the isometric forces on the plateau of the force-length relationship, thereby eliminating the possibility that our result might have been obtained because of a redistribution of half-sarcomere lengths. Since force enhancement in single myofibrils has been associated with actin-titin interactions (Kulke M et al., Circ Res 89: 874–881, 2001; Li Q et al., Biophys J 69: 1508–1518, 1995) and calcium binding to titin (Joumaa V et al., Am J Physiol Cell Physiol 294: C74–C78, 2008; Labeit D et al., Proc Natl Acad Sci USA 100: 13716–13721, 2003), titin may regulate the sarcomeric force enhancement observed here.

Is a parallel elastic element responsible for the enhancement of steady-state muscle force following active stretch?

For over 50 years, it has been recognised that muscles from many different species of animals are able to generate a higher steady-state isometric force after active stretch than during a purely isometric contraction at the same length. This is known as `residual force enhancement' (rFE). The mechanism underlying this phenomenon remains controversial. One proposal is that an elastic element parallel to the cross-bridges becomes stiffer, or is engaged,when the muscle is activated and generates force when stretched. If this is indeed the sole mechanism, then rFE should be eliminated by subsequently shortening the muscle by a distance equal to or greater than the initial stretch. We tested this hypothesis using six intact single fibres from frog lumbrical muscle. The fibres were activated and stretched to generate rFE and then rapidly shortened by between 25% and 700% of the initial stretch distance. In contrast to previous reports, we found that rapid shortening induced a depression of subsequent isometric force. We used two methods to account for this force depression when calculating rFE, thereby obtaining upper and lower bounds for the true rFE. With both methods of calculation, rFE was significantly greater than zero when shortening distance was equal to stretch distance (P=0.0004 and P=0.03, respectively). Therefore, our hypothesis was not supported. We conclude that rFE is unlikely to be generated solely by a parallel elastic element.

Structure-function relations of the giant elastic protein titin in striated and smooth muscle cells

The striated muscle sarcomere contains, in addition to thin and thick filaments, a third myofilament comprised of titin. The extensible region of titin spans the I-band region of the sarcomere and develops passive force in stretched sarcomeres. This force positions the A-bands in the middle of the sarcomere, maintains sarcomere length homogeneity and, importantly, is responsible for myocardial passive tension that determines diastolic filling. Recent work suggests that smooth muscle expresses a truncated titin isoform with a short extensible region that is predicted to develop high passive force levels. Several mechanisms for tuning the titin-based passive tension have been discovered that involve alternative splicing as well as posttranslational modification, mechanisms that are at play both during normal muscle function as well as during disease.

Effects of the N-terminal domains of myosin binding protein-C in an in vitro motility assay: Evidence for long-lived cross-bridges

Myosin binding protein-C (MyBP-C) is a thick-filament protein whose precise function within the sarcomere is not known. However, recent evidence from cMyBP-C knock-out mice that lack MyBP-C in the heart suggest that cMyBP-C normally slows cross-bridge cycling rates and reduces myocyte power output. To investigate possible mechanisms by which cMyBP-C limits cross-bridge cycling kinetics we assessed effects of recombinant N-terminal domains of MyBP-C on the ability of heavy meromyosin (HMM) to support movement of actin filaments using in vitro motility assays. Here we show that N-terminal domains of cMyBP-C containing the MyBP-C "motif," a sequence of approximately 110 amino acids, which is conserved across all MyBP-C isoforms, reduced actin filament velocity under conditions where filaments are maximally activated (i.e. either in the absence of thin filament regulatory proteins or in the presence of troponin and tropomyosin and high [Ca2+]). By contrast, under conditions where thin filament sliding speed is submaximal (i.e. in the presence of troponin and tropomyosin and low [Ca2+]), proteins containing the motif increased filament speed. Recombinant N-terminal proteins also bound to F-actin and inhibited acto-HMM ATPase rates in solution. The results suggest that N-terminal domains of MyBP-C slow cross-bridge cycling kinetics by reducing rates of cross-bridge detachment.

Calcium transients regulate titin organization during myofibrillogenesis

Titin has a Ca2+-dependent kinase domain and may act as a molecular template for myofibrillogenesis. Therefore, we examined the relationship between endogenous Ca2+ transients and titin organization in embryonic myocytes. When transients were blocked during sarcomere assembly, titin organization was disrupted. Titin was distributed in punctate aggregates on an otherwise diffuse background, resulting in a 66% decrease in organization. Myosin, as reported previously, was also disrupted in a similar manner (75% decrease). In titin-actin-myosin triple-labeling experiments, myosin and titin were highly colocalized, although titin aggregates without significant myosin accumulation were also observed. This suggests that myosin-titin association is not dependent on Ca2+ transients, although terminal aspects of titin-myosin organization require transients. We also examined whether titin organization is dependent on actin filament dynamics. The data indicate that (1) the normal sarcomeric arrangement of titin depends on Ca2+ transients, (2) titin-myosin association does not require Ca2+ transients, and (3) titin filament organization does not depend on barbed-end actin dynamics.

Regulation of the actin-myosin interaction by titin

Titin is known to interact with actin thin filaments within the I-band region of striated muscle sarcomeres. In this study, we have used a titin fragment of 800 kDa (T800) purified from striated skeletal muscle to measure the effect of this interaction on the functional properties of the actin-myosin complex. MALDI-TOF MS revealed that T800 contains the entire titin PEVK (Pro, Glu, Val, Lys-rich) domain. In the presence of tropomyosin-troponin, T800 increased the sliding velocity (both average and maximum values) of actin filaments on heavy-meromyosin (HMM)-coated surfaces and dramatically decreased the number of stationary filaments. These results were correlated with a 30% reduction in actin-activated HMM ATPase activity and with an inhibition of HMM binding to actin N-terminal residues as shown by chemical cross-linking. At the same time, T800 did not affect the efficiency of the Ca(2+)-controlled on/off switch, nor did it alter the overall binding energetics of HMM to actin, as revealed by cosedimentation experiments. These data are consistent with a competitive effect of PEVK domain-containing T800 on the electrostatic contacts at the actin-HMM interface. They also suggest that titin may participate in the regulation of the active tension generated by the actin-myosin complex.

Sarcomeric visco-elasticity of chemically skinned skeletal muscle fibres of the rabbit at rest

The giant muscle protein titin (connectin), contained in the gap filament that connect a thick filament to the Z-line in a sarcomere, is generally considered to be responsible for the passive force (tension) and visco-elasticity in resting striated muscle. However, whether it can account for all the features of the resting tension response remains unclear. In this paper, we examine the basic features of the 'sarcomeric visco-elasticity' in a single resting mammalian muscle fibre and attempt to account for various tension components on the basis of known structural features of a sarcomere. At sarcomere length of approximately 2.6 microm, the force response to a ramp stretch of 2-5% is complex but can be resolved into four functionally different components. The behaviour displayed by the components ranges from pure viscous type (directly proportional to stretch velocity, ranging from 0.1 to 30 lengths s(-1)) to predominantly elastic type (insensitive to stretch velocity at 1-2 s time scale); simulations show two components of visco-elasticity with characteristically different relaxation times. The velocity-sensitive components (only) are enhanced by filament lattice compression (dextran - 500 kD) and by increased medium viscosity (dextran - 12 kD); also, the relaxation time of visco-elasticity is longer with increased medium viscosity. Amplitude of all the components and the relaxation time of visco-elasticity are increased at longer sarcomere length (range approximately 2.5 - 3.0 microm). The study, and quantitative analyses, extend our previous work on intact muscle fibres and suggest that the velocity-sensitive tension components in intact sarcomere arise from interactions between sarcomeric filaments, filament segments and inter-filamentary medium; the two components of visco-elasticity arise from distinct regions of titin (connectin) molecules.

Calcium binding to an elastic portion of connectin/titin filaments

Alpha-connectin/titin-1 exists as an elastic filament that links a thick filament with the Z-disk, keeping thick filaments centered within the sarcomere during force generation. We have shown that the connectin filament has an affinity for calcium ions and its binding site(s) is restricted to the beta-connectin/titin-2 portion. We now report the localization and the characterization of calcium-binding sites on beta-connectin. Purified beta-connectin was digested by trypsin into 1700- and 400-kDa fragments. which were then subjected to fluorescence calcium-binding assays. The 400-kDa fragment possesses calcium-binding activity; the binding constant was 1.0 x 10(7) M(-1) and the molar ratio of bound calcium ions to the 400-kDa fragment reached a maximum of 12 at a free calcium ion concentration of approximately 1.0 microM. Antibodies against the 400-kDa fragment formed a sharp dense stripe at the boundary of the A and the I bands, indicating that the calcium-binding domain constitutes the N-terminal region of beta-connectin, that is, the elastic portion of connectin filaments. Furthermore, we estimated the N-terminal location of beta-connectin of various origins (n = 26). Myofibrils were treated with a solution containing 0.1 mM CaCl2 and 70 microM leupeptin to split connectin filaments into beta-connectin and a subfragment, and chain weights of these polypeptides were estimated according to their mobility in 2% polyacrylamide slab gels. The subfragment exhibited a similar chain weight of 1200+/-33 kDa (mean+/-SD), while alpha- and beta-connectins were variable in size according to their origin. These results suggest that the apparent length of the 1200-kDa subfragment portion is almost constant in all instances, about 0.34 microm at the slack condition, therefore that the C-terminus of the 1200-kDa subfragment, that is, the N-terminus of the calcium-binding domain, is at the N2 line region of parent filaments in situ. Because the secondary structure of the 400-kDa fragment was changed by the binding of calcium ions, connectin filaments could be expected to alter their elasticity during the contraction-relaxation cycle of skeletal muscle.

PEVK domain of titin: an entropic spring with actin-binding properties

The PEVK domain of the giant muscle protein titin is a proline-rich sequence with unknown secondary/tertiary structure. Here we compared the force-extension behavior of cloned cardiac PEVK titin measured by single-molecule atomic force spectroscopy with the extensibility of the PEVK domain measured in intact cardiac muscle sarcomeres. The analysis revealed that cardiac PEVK titin acts as an entropic spring with the properties of a random coil exhibiting mechanical conformations of different flexibility. Since in situ, titin is in close proximity to the thin filaments, we also studied whether the PEVK domain of cardiac or skeletal titin may interact with actin filaments. Interaction was indeed found in the in vitro motility assay, in which recombinant PEVK titin constructs slowed down the sliding velocity of actin filaments over myosin. Skeletal PEVK titin affected the actin sliding to a lesser degree than cardiac PEVK titin. The cardiac PEVK effect was partially suppressed by physiological Ca(2+) concentrations, whereas the skeletal PEVK effect was independent of [Ca(2+)]. Cosedimentation assays confirmed the Ca(2+)-modulated actin-binding propensity of cardiac PEVK titin, but did not detect interaction between actin and skeletal PEVK titin. In myofibrils, the relatively weak actin-PEVK interaction gives rise to a viscous force component opposing filament sliding. Thus, the PEVK domain contributes not only to the extensibility of the sarcomere, but also affects contractile properties.

Interaction between PEVK-titin and actin filaments: origin of a viscous force component in cardiac myofibrils

The giant muscle protein titin contains a unique sequence, the PEVK domain, the elastic properties of which contribute to the mechanical behavior of relaxed cardiomyocytes. Here, human N2-B-cardiac PEVK was expressed in Escherichia coli and tested-along with recombinant cardiac titin constructs containing immunoglobulin-like or fibronectin-like domains-for a possible interaction with actin filaments. In the actomyosin in vitro motility assay, only the PEVK construct inhibited actin filament sliding over myosin. The slowdown occurred in a concentration-dependent manner and was accompanied by an increase in the number of stationary actin filaments. High [Ca(2+)] reversed the PEVK effect. PEVK concentrations >/=10 microgram/mL caused actin bundling. Actin-PEVK association was found also in actin fluorescence binding assays without myosin at physiological ionic strength. In cosedimentation assays, PEVK-titin interacted weakly with actin at 0 degrees C, but more strongly at 30 degrees C, suggesting involvement of hydrophobic interactions. To probe the interaction in a more physiological environment, nonactivated cardiac myofibrils were stretched quickly, and force was measured during the subsequent hold period. The observed force decline could be fit with a three-order exponential-decay function, which revealed an initial rapid-decay component (time constant, 4 to 5 ms) making up 30% to 50% of the whole decay amplitude. The rapid, viscous decay component, but not the slower decay components, decreased greatly and immediately on actin extraction with Ca(2+)-independent gelsolin fragment, both at physiological sarcomere lengths and beyond actin-myosin overlap. Steady-state passive force dropped only after longer exposure to gelsolin. We conclude that interaction between PEVK-titin and actin occurs in the sarcomere and may cause viscous drag during diastolic stretch of cardiac myofibrils. The interaction could also oppose shortening during contraction.

Structural and functional studies of titin's fn3 modules reveal conserved surface patterns and binding to myosin S1--a possible role in the Frank-Starling mechanism of the heart

The A-band part of titin, a striated-muscle specific protein spanning from the Z-line to the M-line, mainly consists of a well-ordered super-repeat array of immunoglobulin-like and fibronectin-type III (fn3)-like domains. Since it has been suspected that the fn3 domains might represent titin's binding sites to myosin, we have developed structural models for all of titin's 132 fn3-like domains. A subset of eight experimentally determined fn3 structures from a range of proteins, including titin itself, was used as homology templates. After grouping the models according to their position within the super-repeat segment of the central A-band titin region, we analyzed the models with respect to side-chain conservation. This showed that conserved residues form an extensive surface pattern predominantly at one side of the domains, whereas domains outside the central C-zone super-repeat region show generally less conserved surfaces. Since the conserved surface residues may function as protein-binding sites, we experimentally studied the binding properties of expressed multi-domain fn3 fragments. This revealed that fn3 fragments specifically bind to the sub-fragment 1 of myosin. We also measured the effect of fn3 fragments on the contractile properties of single cardiac myocytes. At sub-maximal Ca(2+) concentrations, fn3 fragments significantly enhance active tension. This effect is most pronounced at short sarcomere length, and as a result the length-dependence of Ca(2+) activation is reduced. A model of how titin's fn3-like domains may influence actomyosin interaction is proposed.

Titin-actin interaction in mouse myocardium: passive tension modulation and its regulation by calcium/S100A1

Passive tension in striated muscles derives primarily from the extension of the giant protein titin. However, several studies have suggested that, in cardiac muscle, interactions between titin and actin might also contribute to passive tension. We expressed recombinant fragments representing the subdomains of the extensible region of cardiac N2B titin (tandem-Ig segments, the N2B splice element, and the PEVK domain), and assayed them for binding to F-actin. The PEVK fragment bound F-actin, but no binding was detected for the other fragments. Comparison with a skeletal muscle PEVK fragment revealed that only the cardiac PEVK binds actin at physiological ionic strengths. The significance of PEVK-actin interaction was investigated using in vitro motility and single-myocyte mechanics. As F-actin slid relative to titin in the motility assay, a dynamic interaction between the PEVK domain and F-actin retarded filament sliding. Myocyte results suggest that a similar interaction makes a significant contribution to the passive tension. We also investigated the effect of calcium on PEVK-actin interaction. Although calcium alone had no effect, S100A1, a soluble calcium-binding protein found at high concentrations in the myocardium, inhibited PEVK-actin interaction in a calcium-dependent manner. Gel overlay analysis revealed that S100A1 bound the PEVK region in vitro in a calcium-dependent manner, and S100A1 binding was observed at several sites along titin's extensible region in situ, including the PEVK domain. In vitro motility results indicate that S100A1-PEVK interaction reduces the force that arises as F-actin slides relative to the PEVK domain, and we speculate that S100A1 may provide a mechanism to free the thin filament from titin and reduce titin-based tension before active contraction.

Titin-thin filament interaction and potential role in muscle function

Titin (connectin) is a giant polypeptide that forms a single-molecule filamental structure extending from the M-line to the Z-line in the sarcomere of striated muscle. The primary structure of titin consists mainly of repeats of two types of approximately 100-amino acid modules (fibronectin type III and immunoglobulin-like motifs, respectively) and a Pro rich segment named the PEVK domain. The I-band region of titin shows an elasticity important to the passive properties of the myofibril. To investigate the biological function of titin, we cloned cDNA segments encoding single or linked structural modules of titin into expression vectors to produce non-fusion titin fragments in E. coli. High level expression of titin fragments was achieved and effective purification procedures were developed. We also developed specific monoclonal antibodies against the titin fragments and solid-phase protein-binding assays to investigate the interaction of the titin structural modules and other sarcomeric proteins. The results show that the immunoglobin-like module that are enriched in the I-band titin binds to F-actin. In contrast to the rigid association of A-band titin with the thick filament, the relatively weak titin-actin binding suggests that the I-band titin may reversibly interact with the thin filament during muscle contraction. This hypothesis is supported by an epitope similarity between the actin-binding site of caldesmon and the immunoglobulin-like module of titin, which suggests analogous functions of caldesmon and titin in organizing the contractile proteins. Together with data from mechanical studies demonstrating that the titin-actin interaction may contribute to the passive property of cardiac muscle in a Ca(2+)-dependent manner, we speculate that the Ca(2+)-mediated thin filament regulation may coordinate the function of titin during muscle contraction and relaxation.

Ca(2+)-dependence of diastolic properties of cardiac sarcomeres: involvement of titin

The stiffness of the sarcomeres was studied during the diastolic interval of 18 stimulated (0.5 Hz) cardiac trabeculae of rat (pH 7.4; temperature = 25 degrees C). Sarcomere length (SL) and force (F) were measured using, respectively, laser diffraction techniques (resolution: 4 nm) and a silicon strain gauge (resolution: 0.63 microN). Sinusoidal perturbations (frequency = 500 Hz) were imposed to the length of the preparation. The stiffness was evaluated from the corresponding F and SL sinusoids by analysis of both signals together either in the time domain or in the frequency domain. A short burst (duration = 30 ms) of sinusoidal perturbations was repeated at 5 predetermined times during diastole providing 5 measurements of stiffness during the time interval separating two twitches. These measurements revealed that stiffness increases by approximately 30% during diastole, while a simultaneous expansion of the sarcomeres (amplitude = 10-60 nm) was detected. Measurements of the fluorescence of fura-2 under the same conditions revealed a continuous exponential decline of [Ca2+]i from 210 to 90 nM (constant of time approximately 300 ms) during diastole. In order to test the possibility that the increase of sarcomere stiffness and the decline of [Ca2+]i were coupled during diastole of intact trabeculae, we studied the effect of different free Ca(2+)-concentrations ([Ca2+]) between 1 and 430 nM on sarcomere stiffness in rat cardiac trabeculae skinned by saponin (n = 17). Stiffness was studied using 500 Hz sinusoidal perturbations of muscle length (ML). We found that, below 70 nM, the stiffness was independent of [Ca2+]; between 70 and 200 nM, the stiffness declined with increase of [Ca2+]; above 200 nM, the stiffness increased steeply with [Ca2+]. The data fitted accurately to the sum of two sigmoids (Hill functions): (1) at [Ca2+] < 200 nM the stiffness decreased with [Ca2+] (EC50 = 160 +/- 13 nM; n = -2.6 +/- 0.7) and (2) at [Ca2+] > 200 nM, stiffness increased with [Ca2+] (EC50 = 3.4 +/- 0.3 microM; n = 2.1 +/- 0.2) due to attachment of cross-bridges. From these results, it was possible to reproduce accurately the time course of diastolic stiffness observed in intact trabeculae and to predict the effect on stiffness of a spontaneous elevation of the diastolic [Ca2+]. Identical stiffness measurements were performed in 4 skinned preparations exposed to a cloned fragment of titin (Ti I-II) which has been shown to exhibit a strong interaction with F-actin in vitro. It was anticipated that Ti I-II would compete with endogenous titin for the same binding site on actin in the I-band. Below 200 nM, Ti I-II (2 microM) eliminated the Ca(2+)-dependence of stiffness. These results are consistent with the hypothesis that the Ca(2+)-sensitivity of the sarcomeres at [Ca2+] < 200 nM, i.e. where the myocytes in intact muscle operate during diastole, involves an association between titin molecules and the thin filament.

Regulation of catch muscle by twitchin phosphorylation: effects on force, ATPase, and shortening

Recent experiments on permeabilized anterior byssus retractor muscle (ABRM) of Mytilus edulis have shown that phosphorylation of twitchin releases catch force at pCa > 8 and decreases force at suprabasal but submaximum [Ca2+]. Twitchin phosphorylation decreases force with no detectable change in ATPase activity, and thus increases the energy cost of force maintenance at subsaturating [Ca2+]. Similarly, twitchin phosphorylation causes no change in unloaded shortening velocity (Vo) at any [Ca2+], but when compared at equal submaximum forces, there is a higher Vo when twitchin is phosphorylated. During calcium activation, the force-maintaining structure controlled by twitchin phosphorylation adjusts to a 30% Lo release to maintain force at the shorter length. The data suggest that during both catch and calcium-mediated submaximum contractions, twitchin phosphorylation removes a structure that maintains force with a very low ATPase, but which can slowly cycle during submaximum calcium activation. A quantitative cross-bridge model of catch is presented that is based on modifications of the Hai and Murphy (1988. Am. J. Physiol. 254:C99-C106) latch bridge model for regulation of mammalian smooth muscle.

PEVK extension of human soleus muscle titin revealed by immunolabeling with the anti-titin antibody 9D10

Titin is a giant protein that spans half of the striated muscle sarcomere. The I-band portion of titin extends as the sarcomere is stretched, developing what is known as passive force. This portion of the molecule is composed mainly of tandem immunoglobulin (Ig) segments, consisting of serially linked Ig-like domains, and a recently discovered unique sequence termed the PEVK segment. The tandem Ig and PEVK segments have been suggested to extend sequentially when sarcomeres are stretched, with PEVK extension dominating at moderate to high degrees of sarcomere stretch (M. Gautel and D. Goulding, 1996, FEBS Lett. 385, 11-14; W. Linke et al., 1996, J. Mol. Biol. 261, 62-71; K. Trombitás et al., 1998). Previously we observed that the anti-titin antibody 9D10 labels a region in the I-band that increases in width as sarcomeres are stretched. Here we tested whether 9D10 labels the PEVK segment. The 9D10-labeled region of human soleus fibers was followed by immunoelectron microscopy as sarcomeres were stretched. It was found that 9D10 labeled a region in the I-band that was approximately 100 nm wide at a sarcomere length of 2.4 micrometer and approximately 550 nm wide at a sarcomere length of 4.0 micrometer. Results were compared with those obtained with sequence-specific antibodies that were used to mark the boundaries of the PEVK segment. Findings indicate that 9D10 labels the PEVK segment from close to its N-terminal end to its C-terminal end. Consistent with this conclusion are the results on cardiac myocytes that express a much shorter PEVK segment than skeletal muscle and where 9D10 labels a region that is much less wide than in skeletal muscle. The anti-titin antibody 9D10 is a useful tool for investigating the extensible behavior of the PEVK segment in both skeletal and cardiac muscles.

Titin extensibility in situ: entropic elasticity of permanently folded and permanently unfolded molecular segments

Titin (also known as connectin) is a giant protein that spans half of the striated muscle sarcomere. In the I-band titin extends as the sarcomere is stretched, developing what is known as passive force. The I-band region of titin contains tandem Ig segments (consisting of serially linked immunoglobulin-like domains) with the unique PEVK segment in between (Labeit, S., and B. Kolmerer. 1995. Science. 270:293-296). Although the tandem Ig and PEVK segments have been proposed to behave as stiff and compliant springs, respectively, precise experimental testing of the hypothesis is still needed. Here, sequence-specific antibodies were used to mark the ends of the tandem Ig and PEVK segments. By following the extension of the segments as a function of sarcomere length (SL), their respective contributions to titin's elastic behavior were established. In slack sarcomeres (approximately 2.0 micron) the tandem Ig and PEVK segments were contracted. Upon stretching sarcomeres from approximately 2.0 to 2.7 micron, the "contracted" tandem Ig segments straightened while their individual Ig domains remained folded. When sarcomeres were stretched beyond approximately 2.7 micron, the tandem Ig segments did not further extend, instead PEVK extension was now dominant. Modeling tandem Ig and PEVK segments as entropic springs with different bending rigidities (Kellermayer, M., S. Smith, H. Granzier, and C. Bustamante. 1997. Science. 276:1112-1116) indicated that in the physiological SL range (a) the Ig-like domains of the tandem Ig segments remain folded and (b) the PEVK segment behaves as a permanently unfolded polypeptide. Our model provides a molecular basis for the sequential extension of titin's different segments. Initially, the tandem Ig segments extend at low forces due to their high bending rigidity. Subsequently, extension of the PEVK segment occurs only upon reaching sufficiently high external forces due to its low bending rigidity. The serial linking of tandem Ig and PEVK segments with different bending rigidities provides a unique passive force-SL relation that is not achievable with a single elastic segment.

Titin elasticity and mechanism of passive force development in rat cardiac myocytes probed by thin-filament extraction

Titin (also known as connectin) is a giant filamentous protein whose elastic properties greatly contribute to the passive force in muscle. In the sarcomere, the elastic I-band segment of titin may interact with the thin filaments, possibly affecting the molecule's elastic behavior. Indeed, several studies have indicated that interactions between titin and actin occur in vitro and may occur in the sarcomere as well. To explore the properties of titin alone, one must first eliminate the modulating effect of the thin filaments by selectively removing them. In the present work, thin filaments were selectively removed from the cardiac myocyte by using a gelsolin fragment. Partial extraction left behind approximately 100-nm-long thin filaments protruding from the Z-line, whereas the rest of the I-band became devoid of thin filaments, exposing titin. By applying a much more extensive gelsolin treatment, we also removed the remaining short thin filaments near the Z-line. After extraction, the extensibility of titin was studied by using immunoelectron microscopy, and the passive force-sarcomere length relation was determined by using mechanical techniques. Titin's regional extensibility was not detectably affected by partial thin-filament extraction. Passive force, on the other hand, was reduced at sarcomere lengths longer than approximately 2.1 microm, with a 33 +/- 9% reduction at 2.6 microm. After a complete extraction, the slack sarcomere length was reduced to approximately 1.7 microm. The segment of titin near the Z-line, which is otherwise inextensible, collapsed toward the Z-line in sarcomeres shorter than approximately 2.0 microm, but it was extended in sarcomeres longer than approximately 2.3 microm. Passive force became elevated at sarcomere lengths between approximately 1.7 and approximately 2.1 microm, but was reduced at sarcomere lengths of >2.3 microm. These changes can be accounted for by modeling titin as two wormlike chains in series, one of which increases its contour length by recruitment of the titin segment near the Z-line into the elastic pool.

Actin removal from cardiac myocytes shows that near Z line titin attaches to actin while under tension

The I band of cardiac sarcomeres contains both actin and titin/connectin filaments. Earlier work has suggested that titin binds to actin in situ. This interaction must be weak in the region of the I band where titin behaves elastically. On the other hand, titin may bind strongly to actin in the approximately 100-nm-wide region adjoining the Z line, where titin has been found to be inelastic. To study the putative interaction between titin and actin, techniques for selective removal of actin from different regions of the I band are needed. Here we report studies with a gelsolin fragment (FX-45) and extract actin from rat cardiac myocytes. Actin extraction was biphasic: the majority of actin was extracted in approximately 10 min, whereas actin near the Z line (where titin is inelastic) required a approximately 10-fold longer extraction time. Thus, by controlling the extraction time, we could remove either the full actin filament outside the Z line or just the segment of the actin filament that extends beyond the inelastic region of titin that adjoins the Z line. The actin filament-free I band contained titin filaments, typically with one filament extending from each thick filament. In addition, we observed a dark transverse line (junction line), the location of which in the sarcomere varied linearly with sarcomere length. The position in the sarcomere of the junction line coincided with the binding site of the anti-titin antibody 9D10. Actin removal significantly affected the slack sarcomere length. Slack sarcomere length was 1.85 +/- 0.04 microns in control cells and decreased to 1.71 +/- 0.05 microns after actin near the Z line was extracted. This length reduction may be caused by contraction of the titin segment that becomes exposed after actin removal near the Z line, indicating that titin is not only attached to the actin filament but is also under tension.

Actin-titin interaction in cardiac myofibrils: probing a physiological role

The high stiffness of relaxed cardiac myofibrils is explainable mainly by the expression of a short-length titin (connectin), the giant elastic protein of the vertebrate myofibrillar cytoskeleton. However, additional molecular features could account for this high stiffness, such as interaction between titin and actin, which has previously been reported in vitro. To probe this finding for a possible physiological significance, isolated myofibrils from rat heart were subjected to selective removal of actin filaments by a calcium-independent gelsolin fragment, and the "passive" stiffness of the specimens was recorded. Upon actin extraction, stiffness decreased by nearly 60%, and to a similar degree after high-salt extraction of thick filaments. Thus actin-titin association indeed contributes to the stiffness of resting cardiac muscle. To identify possible sites of association, we employed a combination of different techniques. Immunofluorescence microscopy revealed that actin extraction increased the extensibility of the previously stiff Z-disc-flanking titin region. Actin-titin interaction within this region was confirmed in in vitro cosedimentation assays, in which multimodule recombinant titin fragments were tested for their ability to interact with F-actin. By contrast, such assays showed no actin-titin-binding propensity for sarcomeric regions outside the Z-disc comb. Accordingly, the results of mechanical measurements demonstrated that competition with native titin by recombinant titin fragments from Z-disc-remote, I-band or A-band regions did not affect passive myofibril stiffness. These results indicate that it is actin-titin association near the Z-disc, but not along the remainder of the sarcomere, that helps to anchor the titin molecule at its N-terminus and maintain a high stiffness of the relaxed cardiac myofibril.

Dynamics of viscoelastic properties of rat cardiac sarcomeres during the diastolic interval: involvement of Ca2+

1. Cardiac sarcomere stiffness was investigated during diastole in eighteen trabeculae dissected from the right ventricle of rat heart. The trabeculae were stimulated at 0.5 Hz, in a modified Krebs-Henseleit solution (pH, 7.4; 25 degrees C). Sarcomere length (SL) was measured using high resolution (+/-2 nm) laser diffraction techniques. Force (F) was measured with a silicon strain gauge. 2. SL increased exponentially (amplitude, 25 +/- 9 nm; n = 15) throughout diastole. This increase occurred even at slack SL, showing that this phenomenon was due to an internal expansion. The majority of the muscles showed discrete spontaneous fluctuations of SL (amplitude < 20 nm) starting approximately 1 s after the end of the twitch. 3. The intracellular free Ca2+ concentration ([Ca2+]i) was measured from the fluorescence of microinjected fura-2 salt in seven trabeculae under the same experimental conditions. [Ca2+]i continuously declined (from 240 to 90 nM) during diastole following a monoexponential time course (time constant, 210-325 ms). 4. The stiffness of the sarcomere was evaluated at 10, 30, 50, 70 and 90% of diastole using bursts (30 ms) of 500 Hz sinusoidal perturbations of muscle length (amplitude of SL oscillations < 30 nm). At 1 nM external Ca2+ concentration ([Ca2+]o), the average stiffness modulus (Mod) increased from 9.3 +/- 0.6 to 12 +/- 0.6 nN mm-2 micron-1 (n = 18; P < 0.05), while the average phase shift (phi) between F and SL signals decreased from 84 +/- 3 to 73 +/- 4 deg (n = 18; P < 0.05) between 10 and 90% during diastole. The increase in Mod and the decrease in phi reversed when spontaneous activity occurred. When [Ca2+]o was raised to 2 mM, the stiffness time course reversed approximately 450 ms earlier, simultaneously with the occurrence of spontaneous activity. 5. Our results show that diastole is only an apparent steady state and suggest that the structural system responsible for the viscoelastic properties of the sarcomere is regulated by [Ca2+]i in the submicromolar range. Different possible origins of the dynamic changes in viscoelasticity during diastole are discussed.

Interaction between titin and thin filaments in intact cardiac muscle

A 'freeze break' technique and immunoelectron microscopy were used to study the elastic properties of cardiac titin filaments. Small bundles consisting of a few fibres from freshly prepared dog papillary muscle were quickly frozen and broken under liquid nitrogen to fracture sarcomeres in planes perpendicular to the filament axes. Breaks occurred at each of several regions along the sarcomeres. The still-frozen specimens were thawed during fixation to allow elastic filaments to retract. The broken muscle segments were then treated with monoclonal titin antibody 9D10 which labelled a unique epitope in the I-band. In sarcomeres broken at the A-I junction, the titin filaments reacted toward the Z-line, independently of the thin filaments. The retracted epitopes did not reach the Z-line; retraction stopped at the N1-line level. In sarcomeres broken near the Z-line, the titin filaments retracted in the opposite direction, to the tip of the thick filaments. When the break occurred in the A-band, by contrast, the titin-epitope position was unaffected. On the basis of these results, and despite the reported interaction of titin and actin in vitro, it appears that cardiac titin molecules form elastic filaments that are functionally independent of the thin filaments. Near the Z-line, however, the titin filaments seem to associate firmly with the thin filaments.

Calcium-dependent inhibition of in vitro thin-filament motility by native titin

Titin ( also known as connectin) is a giant filamentous protein that spans the distance between the Z- and M-lines of the vertebrate muscle sarcomere and plays a fundamental role in the generation of passive tension. Titin has been shown to bind strongly to myosin, making it tightly associated to the thick filament in the sarcomere. Recent observations have suggested the possibility that titin also interacts with actin, implying further functions of titin in muscle contraction. We show -- using in vitro motility and binding assays -- that native titin interacts with both filamentous actin and reconstituted thin filaments. The interaction results in the inhibition of the filaments' in vitro motility. Furthermore, the titin-thin filament interaction occurs in a calcium-dependent manner: increased calcium results in enhanced binding of thin filaments to titin and greater suppression of in vitro motility.