Localization and elasticity of connectin (titin) filaments in skinned frog muscle fibres subjected to partial depolymerization of thick filaments

Stretching the story of titin and muscle function

The discovery of the giant protein titin, also known as connectin, dates almost half a century back. In this review, I recapitulate major advances in the discovery of the titin filaments and the recognition of their properties and function until today. I briefly discuss how our understanding of the layout and interactions of titin in muscle sarcomeres has evolved and review key facts about the titin sequence at the gene (TTN) and protein levels. I also touch upon properties of titin important for the stability of the contractile units and the assembly and maintenance of sarcomeric proteins. The greater part of my discussion centers around the mechanical function of titin in skeletal muscle. I cover milestones of research on titin's role in stretch-dependent passive tension development, recollect the reasons behind the enormous elastic diversity of titin, and provide an update on the molecular mechanisms of titin elasticity, details of which are emerging even now. I reflect on current knowledge of how muscle fibers behave mechanically if titin stiffness is removed and how titin stiffness can be dynamically regulated, such as by posttranslational modifications or calcium binding. Finally, I highlight novel and exciting, but still controversially discussed, insight into the role titin plays in active tension development, such as length-dependent activation and contraction from longer muscle lengths.

Restoring force development by titin/connectin and assessment of Ig domain unfolding

Titin/connectin is the main determinant of physiological levels of passive muscle force. This force is generated by the extensible I-band region of the molecule, which is composed of serially-linked immunoglobulin (Ig)-like domains and several unique sequence elements. Here we address the role of titin/connectin in sarcomeres shortened to below the slack length (length attained by an un-activated cell in absence of external forces). Such shortened cells develop so-called restoring forces that re-extend the cells upon relaxation. The experiments that we present are based on a high throughput method with a rapid solution switching system which allows unattached single cardiac myocytes to be activated (resulting in shortening below the slack length) and then to be rapidly relaxed while their maximal re-lengthening velocity is measured at the sarcomere level (dSL/dtmax), with high-resolution imaging techniques. Experiments were carried out on myocytes that express different isoforms of titin/connectin. We measured the relation between dSL/dtmax and the minimal SL during contraction (SLmin) and determined the slope of this relation as a measure of 'restoring stiffness.' We found that the restoring stiffness correlates with the isoform expression profile with myocytes that express high levels of the stiff isoform (N2B) having the highest restoring stiffness. These results support the notion that titin/connectin is a bi-directional spring that develops passive force when stretched above the slack length and restoring force when shortened to below this length. We also discuss in detail the mechanisms that underlie titin/connectin's restoring force development and focus on whether or not unfolding of Ig domains plays a role.

Profiles of connectin (titin) in atrophied soleus muscle induced by unloading of rats

Responses of the properties of connectin molecules in the slow-twitch soleus (Sol) and fast-twitch extensor digitorum longus muscles of rats to 3 days of unloading with or without 3-day reloading were investigated. The wet weight (relative to body wt) of Sol, not of extensor digitorum longus, in the unloaded group was significantly less than in the age-matched control (P < 0.05). Immunoelectron microscopic analyses showed that a monoclonal antibody against connectin (SM1) bound to the I-band region close to the edge of the A band at resting length and moved reversibly away from the Z line as the muscle fibers were stretched. In Sol, the displacement of the SM1-bound dense spots in response to stretching decreased after hindlimb suspension. There were no changes in the molecular weights and the percent distributions of alpha- and beta-connectin in both muscles after hindlimb suspension. A significant increment of percent beta-connectin in Sol was observed after 3 days of reloading after hindlimb suspension (P < 0.05). It is suggested that the elasticity of connectin filaments in the I-band region of the atrophied Sol fibers was reduced relative to that of the control fibers. The lack of the elasticity in atrophied muscle fibers may cause a decrease in contractile function.

Mechanical manipulation of single titin molecules with laser tweezers

Titin (also known as connectin) is a giant filamentous polypeptide of multi-domain construction spanning between the Z- and M-lines of the vertebrate muscle sarcomere. The molecule is significant in maintaining sarcomeric structural integrity and generating passive muscle force via its elastic properties. Here we summarize our efforts to characterize titin's elastic properties by manipulating single molecules with force-measuring laser tweezers. The titin molecules can be described as an entropic spring in which domain unfolding occurs at high forces during stretch and refolding at low forces during release. Statistical analysis of a large number (> 500) of stretch-release experiments and comparison of experimental data with the predictions of the wormlike chain theory permit the estimation of unfolded titin's mean persistence length as 16.86 A (+/- 0.11 SD). The slow rates of unfolding and refolding compared with the rates of stretch and release, respectively, result in a state of non-equilibrium and the display of force hysteresis. Folding kinetics as the source of non-equilibrium is directly demonstrated here by the abolishment of force hysteresis in the presence of chemical denaturant. Experimental observations were well simulated by superimposing a simple domain folding kinetics model on the wormlike chain behavior of titin and considering the characteristics of the compliant laser trap. The original video presentation of this paper may be viewed on the web at http:¿www.pote.hu/mm/prezentacio/mkpres/++ +mkpres.htm.

Complete unfolding of the titin molecule under external force

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. Several earlier studies have implicated titin as playing a fundamental role in maintaining sarcomeric structural integrity and generating the passive force of muscle. The elastic properties of titin were characterized in recent single-molecule mechanical works that described the molecule as an entropic spring in which partial unfolding may take place at high forces during stretch and refolding at low forces during release. In the present work titin molecules were stretched using a laser tweezer with forces above 400 pN. The high external forces resulted in complete mechanical unfolding of the molecule, characterized by the disappearance of force hysteresis at high forces. Titin refolded following complete denaturation, as the hysteresis at low forces reappeared in subsequent stretch-release cycles. The broad force range throughout which unfolding occurred indicates that the various globular domains in titin require different unfolding forces due to differences in the activation energies for their unfolding.

Evidence that the tandem Ig domains near the end of the muscle thick filament form an inelastic part of the I-band titin

In vertebrate striated muscle the titin/connectin molecule spans half a sarcomere, and in the I-band forms and elastic filament connecting the Z-line with the end of the thick filaments. The only part of the elastic filament that has been described in intact rest length muscle is a short extension to the thick filament observed in freeze-fractured cardiac muscle which has similarities to the end-filament of negatively stained isolated thick filaments. We report here further observations made in sections of rabbit psoas muscle. In very thin longitudinal sections thin extensions to the thick filaments some 0.11 micron long and 5-6 nm in diameter are seen. Transverse sections show that each thick filament has such an extension. Nothing similar is seen further into the I-band or at the Z-line. The common features of this structure in both cardiac and skeletal muscle suggest that it corresponds to a common sequence in their titins. Such a sequence is to be found in the 22 tandem Ig domains near the A/I junction. Taken together with other information about the arrangement of domains in this part of the sarcomere, this leads to a calculated length for the end extension of 104 nm. The length of the extension does not vary with sarcomere length between 2.2 and 3.0 microns and therefore it corresponds to an inelastic region of I-band titin over the physiological range. Each extension probably comprises part of three to six titin molecules depending on the complement of titin in the thick filament, as previously suggested. A polymer formed from several strands of Ig domains would make for a relatively rigid structure which would resist folding or stretching when subjected to the small passive forces which pertain over the physiological range of sarcomere lengths. The relationship of the N2-line with the end-filament has also been studied. The N2-line position varies with sarcomere length in an elastic manner. Only at short sarcomere lengths does the end of the end-filament coincide with the position of the N2-line. Taking into account recent work on the elasticity of titin in the I-band we conclude that the N2-line corresponds to part of the elastic PEVK region of titin and not the region of titin sequence designated N2.

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.

Direct visualization of extensibility in isolated titin molecules

"Molecular combing" induced by a receding meniscus has been shown to extend individual titin molecules. Electron microscopy reveals that both ends of the molecule tend to attach to a mica substrate, probably due to their local positive charges. This leaves the remainder of the molecule free to be straightened and extended by forces of up to approximately 800 pN. A small region in the I-band part of the molecule, which probably corresponds to sequence high in P, E, V and K residues, is the most compliant and appears to extend by an unfolding of the polypeptide chain. Other parts of the molecule are also capable of extension. These mechanical extensions in titin are probably reversible.

Nonuniform elasticity of titin in cardiac myocytes: a study using immunoelectron microscopy and cellular mechanics

Titin (also known as connectin) is a muscle-specific giant protein found inside the sarcomere, spanning from the Z-line to the M-line. The I-band segment of titin is considered to function as a molecular spring that develops tension when sarcomeres are stretched (passive tension). Recent studies on skeletal muscle indicate that it is not the entire I-band segment of titin that behaves as a spring; some sections are inelastic and do not take part in the development of passive tension. To better understand the mechanism of passive tension development in the heart, where passive tension plays an essential role in the pumping function, we investigated titin's elastic segment in cardiac myocytes using structural and mechanical techniques. Single cardiac myocytes were stretched by various amounts and then immunolabeled and processed for electron microscopy in the stretched state. Monoclonal antibodies that recognize different titin epitopes were used, and the locations of the titin epitopes in the sarcomere were studied as a function of sarcomere length. We found that only a small region of the I-band segment of titin is elastic; its contour length is estimated at approximately 75 nm, which is only approximately 40% of the total I-band segment of titin. Passive tension measurements indicated that the fundamental determinant of how much passive tension the heart develops is the strain of titin's elastic segment. Furthermore, we found evidence that in sarcomeres that are slack (length, approximately 1.85 microns) the elastic titin segment is highly folded on top of itself. Based on the data, we propose a two-stage mechanism of passive tension development in the heart, in which, between sarcomere lengths of approximately 1.85 microns and approximately 2.0 microns, titin's elastic segment straightens and, at lengths longer than approximately 2.0 microns, the molecular domains that make up titin's elastic segment unravel. Sarcomere shortening to lengths below slack (approximately 1.85 microns) also results in straightening of the elastic titin segment, giving rise to a force that opposes shortening and that tends to bring sarcomeres back to their slack length.

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

Titin is a striated muscle-specific giant protein (M(r) approximately 3,000,000) that consists predominantly of two classes of approximately 100 amino acid motifs, class I and class II, that repeat along the molecule. Titin is found inside the sarcomere, in close proximity to both actin and myosin filaments. Several biochemical studies have found that titin interacts with myosin and actin. In the present work we investigated whether this biochemical interaction is functionally significant by studying the effect of titin on actomyosin interaction in an in vitro motility assay where fluorescently labeled actin filaments are sliding on top of a lawn of myosin molecules. We used genetically expressed titin fragments containing either a single class I motif (Ti I), a single class II motif (Ti II), or the two motifs linked together (Ti I-II). Neither Ti I nor Ti II alone affected actin-filament sliding on either myosin, heavy meromyosin, or myosin subfragment-1. In contrast, the linked fragment (Ti I-II) strongly inhibited actin sliding. Ti I-II-induced inhibition was observed with full-length myosin, heavy meromyosin, and myosin subfragment-1. The degree of inhibition was largest with myosin subfragment-1, intermediate with heavy meromyosin, and smallest with myosin. In vitro binding assays and electrophoretic analyses revealed that the inhibition is most likely caused by interaction between the actin filament and the titin I-II fragment. The physiological relevance of the novel finding of motility inhibition by titin fragments is discussed.

Passive tension in cardiac muscle: contribution of collagen, titin, microtubules, and intermediate filaments

The passive tension-sarcomere length relation of rat cardiac muscle was investigated by studying passive (or not activated) single myocytes and trabeculae. The contribution of collagen, titin, microtubules, and intermediate filaments to tension and stiffness was investigated by measuring (1) the effects of KCl/KI extraction on both trabeculae and single myocytes, (2) the effect of trypsin digestion on single myocytes, and (3) the effect of colchicine on single myocytes. It was found that over the working range of sarcomeres in the heart (lengths approximately 1.9-2.2 microns), collagen and titin are the most important contributors to passive tension with titin dominating at the shorter end of the working range and collagen at longer lengths. Microtubules made a modest contribution to passive tension in some cells, but on average their contribution was not significant. Finally, intermediate filaments contributed about 10% to passive tension of trabeculae at sarcomere lengths from approximately 1.9 to 2.1 microns, and their contribution dropped to only a few percent at longer lengths. At physiological sarcomere lengths of the heart, cardiac titin developed much higher tensions (> 20-fold) than did skeletal muscle titin at comparable lengths. This might be related to the finding that cardiac titin has a molecular mass of 2.5 MDa, 0.3-0.5 MDa smaller than titin of mammalian skeletal muscle, which is predicted to result in a much shorter extensible titin segment in the I-band of cardiac muscle. Passive stress plotted versus the strain of the extensible titin segment showed that the stress-strain relationships are similar in cardiac and skeletal muscle. The difference in passive stress between cardiac and skeletal muscle at the sarcomere level predominantly resulted from much higher strains of the I-segment of cardiac titin at a given sarcomere length. By expressing a smaller titin isoform, without changing the properties of the molecule itself, cardiac muscle is able to develop significant levels of passive tension at physiological sarcomere lengths.

Structure and function of titin and nebulin

Recent investigations of titin anchorage and elasticity have been supplemented with in vitro expression studies on isolated domains of titin and nebulin. These have yielded new insights into the molecular basis of the functions of these proteins in muscle. The characterization of a cellular (non-muscle) isoform of titin has extended the functional relevance of this family of proteins beyond the realm of muscle.

Connectin, an elastic protein of striated muscle

Connectin, also called titin, a giant elastic protein of striated muscle (approximately 3000 kDa) mainly consists of fibronectin type III and immunoglobulin C2 globular domains, the beta-sheets of which are parallel to the main axis of the molecule. One connectin molecule runs through the I band and binds onto the myosin filament up to the M line starting from the Z line. It positions the myosin filament at the center of a sarcomere. Connectin is also responsible for resting tension generation. Biodiversity of the connectin family exists in invertebrate muscle.

Immunoglobulin-type domains of titin: same fold, different stability?

Titin is a 3-MDa protein thought to form a fibrous intracellular system in vertebrate striated muscle and to play an important role in sarcomere alignment during muscle contraction. It has also been implicated as a "molecular ruler", regulating the assembly and the precise length of the thick filaments [Whiting, A. J., Wardale, J., & Trinick, J. (1989) J. Mol. Biol. 205, 163-169]. Partial sequencing of titin-encoding cDNAs suggests that the protein is organized in a modular fashion, containing two classes of approximately 100-residue repeats [Labeit, S., Barlow, D. P., Gautel, M., Gibson, T., Holt, J., Hsieh, C. L., Francke, U., Leonard, K., Wardale, J., Whiting, A., & Trinick, J. (1990) Nature 345, 273-276]. These motifs, referred to as type I and type II modules, show sequence homology to the fibronectin III and immunoglobulin C2 superfamilies, respectively. Since the type II modules represent the most widely occurring motifs along the titin molecule, we expressed in Escherichia coli three domains of this type spanning different regions of the sarcomere (A-band and M-line) and studied their structure and stability. Using circular dichroism, nuclear magnetic resonance, and fluorescence spectroscopy, we showed that all the fragments examined are independently folded in solution and possess a beta-sheet conformation. Furthermore, employing NMR analysis, we identified an overall folding pattern present in all modules and related to the Ig fold, as previously suggested by theoretical predictions. The stability of the modules over a range of conditions was investigated by measuring key thermodynamic parameters for both thermal and chemical denaturation and by monitoring amide proton exchange as a function of time.(ABSTRACT TRUNCATED AT 250 WORDS)

Passive tension and stiffness of vertebrate skeletal and insect flight muscles: the contribution of weak cross-bridges and elastic filaments

Tension and dynamic stiffness of passive rabbit psoas, rabbit semitendinosus, and waterbug indirect flight muscles were investigated to study the contribution of weak-binding cross-bridges and elastic filaments (titin and minititin) to the passive mechanical behavior of these muscles. Experimentally, a functional dissection of the relative contribution of actomyosin cross-bridges and titin and minititin was achieved by 1) comparing mechanically skinned muscle fibers before and after selective removal of actin filaments with a noncalcium-requiring gelsolin fragment (FX-45), and 2) studying passive tension and stiffness as a function of sarcomere length, ionic strength, temperature, and the inhibitory effect of a carboxyl-terminal fragment of smooth muscle caldesmon. Our data show that weak bridges exist in both rabbit skeletal muscle and insect flight muscle at physiological ionic strength and room temperature. In rabbit psoas fibers, weak bridge stiffness appears to vary with both thin-thick filament overlap and with the magnitude of passive tension. Plots of passive tension versus passive stiffness are multiphasic and strikingly similar for these three muscles of distinct sarcomere proportions and elastic proteins. The tension-stiffness plot appears to be a powerful tool in discerning changes in the mechanical behavior of the elastic filaments. The stress-strain and stiffness-strain curves of all three muscles can be merged into one, by normalizing strain rate and strain amplitude of the extensible segment of titin and minititin, further supporting the segmental extension model of resting tension development.

Characterization of beta-connectin (titin 2) from striated muscle by dynamic light scattering

Connectin (titin) is a large filamentous protein (single peptide) with a molecular mass of approximately 3 MDa, contour length approximately 900 nm, and diameter approximately 4 nm, and resides in striated muscle. Connectin links the thick filaments to the Z-lines in a sarcomere and produces a passive elastic force when muscle fiber is stretched. The aim of this study is to elucidate some aspects of physical properties of isolated beta-connectin (titin 2), a proteolytic fragment of connectin, by means of dynamic light-scattering (DLS) spectroscopy. The analysis of DLS spectra for beta-connectin gave the translational diffusion coefficient of 3.60 x 10(-8) cm2/s at 10 degrees C (or the hydrodynamic radius of 44.1 nm), molecular mass little smaller than 3.0 MDa (for a literature value of sedimentation coefficient), the root-mean-square end-to-end distance of 163 nm (or the radius of gyration of 66.6 nm), and the Kuhn segment number of 30 and segment length of 30 nm (or the persistence length of 15 nm). These results permitted to estimate the flexural rigidity of 6.0 x 10(-20) dyn x cm2 for filament bending, and the elastic constant of 7 dyn/cm for extension of one persistence length. Based on a simple model, implications of the present results in muscle physiology are discussed.

Interplay between passive tension and strong and weak binding cross-bridges in insect indirect flight muscle. A functional dissection by gelsolin-mediated thin filament removal

The interplay between passive and active mechanical properties of indirect flight muscle of the waterbug (Lethocerus) was investigated. A functional dissection of the relative contribution of cross-bridges, actin filaments, and C filaments to tension and stiffness of passive, activated, and rigor fibers was carried out by comparing mechanical properties at different ionic strengths of sarcomeres with and without thin filaments. Selective thin filament removal was accomplished by treatment with the actin-severving protein gelsolin. Thin filament, removal had no effect on passive tension, indicating that the C filament and the actin filament are mechanically independent and that passive tension is developed by the C filament in response to sarcomere stretch. Passive tension increased steeply with sarcomere length until an elastic limit was reached at only 6-7% sarcomere extension, which corresponds to an extension of 350% of the C filament. The passive tension-length relation of insect flight muscle was analyzed using a segmental extension model of passive tension development (Wang, K, R. McCarter, J. Wright, B. Jennate, and R Ramirez-Mitchell. 1991. Proc. Natl. Acad. Sci. USA. 88:7101-7109). Thin filament removal greatly depressed high frequency passive stiffness (2.2 kHz) and eliminated the ionic strength sensitivity of passive stiffness. It is likely that the passive stiffness component that is removed by gelsolin is derived from weak-binding cross-bridges, while the component that remains is derived from the C filament. Our results indicate that a significant number of weak-binding cross-bridges exist in passive insect muscle at room temperature and at an ionic strength of 195 mM. Analysis of rigor muscle indicated that while rigor tension is entirely actin based, rigor stiffness contains a component that resists gelsolin treatment and is therefore likely to be C filament based. Active tension and active stiffness of unextracted fibers were directly proportional to passive tension before activation. Similarly, passive stiffness due to weak bridges also increased linearly with passive tension, up to a limit. These correlations lead us to propose a stress-activation model for insect flight muscle in which passive tension is a prerequisite for the formation of both weak-binding and strong-binding cross-bridges.

Elastic filaments in situ in cardiac muscle: deep-etch replica analysis in combination with selective removal of actin and myosin filaments

To clarify the full picture of the connectin (titin) filament network in situ, we selectively removed actin and myosin filaments from cardiac muscle fibers by gelsolin and potassium acetate treatment, respectively, and observed the residual elastic filament network by deep-etch replica electron microscopy. In the A bands, elastic filaments of uniform diameter (6-7 nm) projecting from the M line ran parallel, and extended into the I bands. At the junction line in the I bands, which may correspond to the N2 line in skeletal muscle, individual elastic filaments branched into two or more thinner strands, which repeatedly joined and branched to reach the Z line. Considering that cardiac muscle lacks nebulin, it is very likely that these elastic filaments were composed predominantly of connectin molecules; indeed, anti-connectin monoclonal antibody specifically stained these elastic filaments. Further, striations of approximately 4 nm, characteristic of isolated connectin molecules, were also observed in the elastic filaments. Taking recent analyses of the structure of isolated connectin molecules into consideration, we concluded that individual connectin molecules stretched between the M and Z lines and that each elastic filament consisted of laterally-associated connectin molecules. Close comparison of these images with the replica images of intact and S1-decorated sarcomeres led us to conclude that, in intact sarcomeres, the elastic filaments were laterally associated with myosin and actin filaments in the A and I bands, respectively. Interestingly, it was shown that the elastic property of connectin filaments was not restricted by their lateral association with actin filaments in intact sarcomeres. Finally, we have proposed a new structural model of the cardiac muscle sarcomere that includes connectin filaments.