The physiological role of titin in striated muscle

Myosin-binding protein C forms C-links and stabilizes OFF states of myosin

Contraction force in muscle is produced by the interaction of myosin motors in the thick filaments and actin in the thin filaments and is fine-tuned by other proteins such as myosin-binding protein C (MyBP-C). One form of control is through the regulation of myosin heads between an ON and OFF state in passive sarcomeres, which leads to their ability or inability to interact with the thin filaments during contraction, respectively. MyBP-C is a flexible and long protein that is tightly bound to the thick filament at its C-terminal end but may be loosely bound at its middle- and N-terminal end (MyBP-CC1C7). Under considerable debate is whether the MyBP-CC1C7 domains directly regulate myosin head ON/OFF states, and/or link thin filaments ("C-links"). Here, we used a combination of mechanics and small-angle X-ray diffraction to study the immediate and selective removal of the MyBP-CC1C7 domains of fast MyBP-C in permeabilized skeletal muscle. After cleavage, the thin filaments were significantly shorter, a result consistent with direct interactions of MyBP-C with thin filaments thus confirming C-links. Ca2+ sensitivity was reduced at shorter sarcomere lengths, and crossbridge kinetics were increased across sarcomere lengths at submaximal activation levels, demonstrating a role in crossbridge kinetics. Structural signatures of the thick filaments suggest that cleavage also shifted myosin heads towards the ON state - a marker that typically indicates increased Ca2+ sensitivity but that may account for increased crossbridge kinetics at submaximal Ca2+ and/or a change in the force transmission pathway. Taken together, we conclude that MyBP-CC1C7 domains play an important role in contractile performance which helps explain why mutations in these domains often lead to debilitating diseases.

Preservation of shortening velocity and power output in single muscle fibres from patients with idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIMs) are autoimmune disorders of skeletal muscle causing weakness and disability. Utilizing single fibre contractility studies, we have previously shown that contractility is affected in muscle fibres from individuals with IIMs. For the current study, we hypothesized that a compensatory increase in shortening velocity occurs in muscle fibres from individuals with IIMs in an effort to maintain power output. We performed in vitro single fibre contractility studies to assess force-velocity relationships and maximum shortening velocity (V_max) of muscle fibres from individuals with IIMs (25 type I and 58 type IIA) and healthy controls (66 type I and 27 type IIA) and calculated maximum power output (W_max) for each fibre. We found significantly higher V_max (mean ± SEM) of fibres from individuals with IIMs, for both type I (1.40 ± 0.31 fibre lengths/s, n = vs. 0.63 ± 0.13 fibre lengths/s; p = 0.0019) and type IIA fibres (2.00 ± 0.17 fibre lengths/s vs 0.77 ± 0.10 fibre lengths/s; p < 0.0001). Furthermore, W_max (mean ± SEM) was maintained compared to fibres from healthy controls, again for both type I and type IIA fibres (4.10 ± 1.00 kN/m²·fibre lengths/s vs. 2.00 ± 0.16 kN/m²·fibre lengths/s; p = ns and 9.00 ± 0.64 kN/m²·fibre lengths/s vs. 6.00 ± 0.67 kN/m²·fibre lengths/s; p = ns respectively). In addition, type I muscle fibres from individuals with IIMs was able to develop maximum power output at lower relative force. The findings of this study suggest that compensatory responses to maintain power output, including increased maximum shortening velocity and improved efficiency, may occur in muscle of individuals with IIMs. The mechanism underlying this response is unclear, and different hypotheses are discussed.

Graded titin cleavage progressively reduces tension and uncovers the source of A-band stability in contracting muscle

The giant muscle protein titin is a major contributor to passive force; however, its role in active force generation is unresolved. Here, we use a novel titin-cleavage (TC) mouse model that allows specific and rapid cutting of elastic titin to quantify how titin-based forces define myocyte ultrastructure and mechanics. We show that under mechanical strain, as TC doubles from heterozygous to homozygous TC muscles, Z-disks become increasingly out of register while passive and active forces are reduced. Interactions of elastic titin with sarcomeric actin filaments are revealed. Strikingly, when titin-cleaved muscles contract, myosin-containing A-bands become split and adjacent myosin filaments move in opposite directions while also shedding myosins. This establishes intact titin filaments as critical force-transmission networks, buffering the forces observed by myosin filaments during contraction. To perform this function, elastic titin must change stiffness or extensible length, unveiling its fundamental role as an activation-dependent spring in contracting muscle.

Structure and Function of Filamin C in the Muscle Z-Disc

Filamin C (FLNC) is one of three filamin proteins (Filamin A (FLNA), Filamin B (FLNB), and FLNC) that cross-link actin filaments and interact with numerous binding partners. FLNC consists of a N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats with two intervening calpain-sensitive hinges separating R15 and R16 (hinge 1) and R23 and R24 (hinge-2). The FLNC subunit is dimerized through R24 and calpain cleaves off the dimerization domain to regulate mobility of the FLNC subunit. FLNC is localized in the Z-disc due to the unique insertion of 82 amino acid residues in repeat 20 and necessary for normal Z-disc formation that connect sarcomeres. Since phosphorylation of FLNC by PKC diminishes the calpain sensitivity, assembly, and disassembly of the Z-disc may be regulated by phosphorylation of FLNC. Mutations of FLNC result in cardiomyopathy and muscle weakness. Although this review will focus on the current understanding of FLNC structure and functions in muscle, we will also discuss other filamins because they share high sequence similarity and are better characterized. We will also discuss a possible role of FLNC as a mechanosensor during muscle contraction.

Effects of a titin mutation on negative work during stretch-shortening cycles in skeletal muscles

Negative work occurs in muscles during braking movements such as downhill walking or landing after a jump. When performing negative work during stretch-shortening cycles, viscoelastic structures within muscles store energy during stretch, return a fraction of this energy during shortening, and dissipate the remaining energy as heat. Because tendons and extracellular matrix are relatively elastic rather than viscoelastic, energy is mainly dissipated by cross bridges and titin. Recent studies demonstrate that titin stiffness increases in active skeletal muscles, suggesting that titin contributions to negative work may have been underestimated in previous studies. The muscular dystrophy with myositis (mdm) mutation in mice results in a deletion in titin that leads to reduced titin stiffness in active muscle, providing an opportunity to investigate the contribution of titin to negative work in stretch-shortening cycles. Using the work loop technique, extensor digitorum longus and soleus muscles from mdm and wild type mice were stimulated during the stretch phase of stretch-shortening cycles to investigate negative work. The results demonstrate that, compared to wild type muscles, negative work is reduced in muscles from mdm mice. We suggest that changes in the viscoelastic properties of mdm titin reduce energy storage by muscles during stretch and energy dissipation during shortening. Maximum isometric stress is also reduced in muscles from mdm mice, possibly due to impaired transmission of cross bridge force, impaired cross bridge function, or both. Functionally, the reduction in negative work could lead to increased muscle damage during eccentric contractions that occur during braking movements.

Titin-Based Cardiac Myocyte Stiffening Contributes to Early Adaptive Ventricular Remodeling After Myocardial Infarction

Rationale:

Myocardial infarction (MI) increases the wall stress in the viable myocardium and initiates early adaptive remodeling in the left ventricle to maintain cardiac output. Later remodeling processes include fibrotic reorganization that eventually leads to cardiac failure. Understanding the mechanisms that support cardiac function in the early phase post MI and identifying the processes that initiate transition to maladaptive remodeling are of major clinical interest.

Objective:

To characterize MI-induced changes in titin-based cardiac myocyte stiffness and to elucidate the role of titin in ventricular remodeling of remote myocardium in the early phase after MI.

Methods and Results:

Titin properties were analyzed in Langendorff-perfused mouse hearts after 20-minute ischemia/60-minute reperfusion (I/R), and mouse hearts that underwent ligature of the left anterior descending coronary artery for 3 or 10 days. Cardiac myocyte passive tension was significantly increased 1 hour after ischemia/reperfusion and 3 and 10 days after left anterior descending coronary artery ligature. The increased passive tension was caused by hypophosphorylation of the titin N2-B unique sequence and hyperphosphorylation of the PEVK (titin domain rich in proline, glutamate, valine, and lysine) region of titin. Blocking of interleukine-6 before left anterior descending coronary artery ligature restored titin-based myocyte tension after MI, suggesting that MI-induced titin stiffening is mediated by elevated levels of the cytokine interleukine-6. We further demonstrate that the early remodeling processes 3 days after MI involve accelerated titin turnover by the ubiquitin–proteasome system.

Conclusions:

We conclude that titin-based cardiac myocyte stiffening acutely after MI is partly mediated by interleukine-6 and is an important mechanism of remote myocardium to adapt to the increased mechanical demands after myocardial injury.

Cellular mechanism of eccentric-induced muscle injury and its relationship with sarcomere heterogeneity

Activity-induced muscle injury and dysfunction have been identified as key components of musculoskeletal injuries. These injuries often occur following eccentric contractions, when the muscle is under tension and stretched by a force that is greater than the force generated by the muscle. Many daily activities require muscles to perform eccentric contractions, including walking (or running) downhill or down stairs, lowering heavy objects, and landing from a jump. Injuries often occur when these activities are performed at high intensity or for prolonged periods of time. General features of eccentric-induced muscle injury are well documented and include disruption of intracellular muscle structure, prolonged muscle weakness and dysfunction, a delayed-onset muscle soreness, and inflammation. Several weeks are required for the affected tissue to fully regenerate and recover from eccentric-induced muscle injury. Possible mechanisms responsible for eccentric-induced muscle injury are activation impairment and structural disruption of the sarcomere. These two factors seem to be the main sources of eccentric-induced muscle injury. Rather than being separate mechanisms they may be complimentary and interact with each other. Therefore, in this review we will focus on the two main cellular mechanism of muscle cell injury following accustomed eccentric contraction.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Four parameters increase the sensitivity and specificity of the exon array analysis and disclose 25 novel aberrantly spliced exons in myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is an RNA gain-of-function disorder in which abnormally expanded CTG repeats of DMPK sequestrate a splicing trans-factor MBNL1 and upregulate another splicing trans-factor CUGBP1. To identify a diverse array of aberrantly spliced genes, we performed the exon array analysis of DM1 muscles. We analyzed 72 exons by RT-PCR and found that 27 were aberrantly spliced, whereas 45 were not. Among these, 25 were novel and especially splicing aberrations of LDB3 exon 4 and TTN exon 45 were unique to DM1. Retrospective analysis revealed that four parameters efficiently detect aberrantly spliced exons: (i) the signal intensity is high; (ii) the ratio of probe sets with reliable signal intensities (that is, detection above background P-value=0.000) is high within a gene; (iii) the splice index (SI) is high; and (iv) SI is deviated from SIs of the other exons that can be estimated by calculating the deviation value (DV). Application of the four parameters gave rise to a sensitivity of 77.8% and a specificity of 95.6% in our data set. We propose that calculation of DV, which is unique to our analysis, is of particular importance in analyzing the exon array data.

Non-uniform distribution of strain during stretch of relaxed skeletal muscle fibers from rat soleus muscle

Tension and regional average sarcomere length (L(s)) behavior were examined during repeated stretches of single, permeabilized, relaxed muscle fibers isolated from the soleus muscles of rats. We tested the hypothesis that during stretches of single permeabilized fibers, the global fiber strain is distributed non-uniformly along the length of a relaxed fiber in a repeatable pattern. Each fiber was subjected to eight constant-velocity stretch and release cycles with a strain of 32% and strain rate of 54% s(-1). Stretch-release cycles were separated by a 4.5 min interval. Throughout each stretch-release cycle, sarcomere lengths were measured using a laser diffraction technique in which 20 contiguous sectors along the entire length of a fiber segment were scanned within 2 ms. The results revealed that: (1) the imposed length change was not distributed uniformly along the fiber, (2) the first stretch-release cycle differed from subsequent cycles in passive tension and in the distribution of global fiber strain, and (3) a characteristic "signature" for the L(s) response emerged after cycle 3. The findings support the conclusions that longitudinal heterogeneity exists in the passive stiffness of individual muscle fibers and that preconditioning of fibers with stretch-release cycles produces a stable pattern of sarcomere strains.

Effects of Eccentric Exercise on Passive Mechanical Properties of Human Gastrocnemius in vivo

INTRODUCTION

In this study, we used a newly developed method for measuring passive length-tension relations of a single human muscle in vivo to quantify changes in the mechanical properties of the human gastrocnemius after eccentric exercise.

METHODS

Twelve subjects performed eccentric exercise on the right leg for 1 h by walking backward downhill on a treadmill. Passive ankle torque was measured as the ankle was rotated within its available range, with the knee in eight different angles. Subjects were studied before exercise, 1 h after exercise, and 24 h later, with further measurements at 48 h and at 1 wk in a subset of six subjects. Subjects also rated the level of perceived muscle soreness on a 10-point scale during walking on flat ground. We examined passive tension in the gastrocnemius at a standard length before and at various times after exercise.

RESULTS

Muscle tension increased significantly at this length 1 h after exercise (34.7 +/- 7.3%; mean +/-SEM), peaked at 24 h (88.4 +/- 12.6%), declined at 48 h (45.5 +/- 4.4%), and returned to the control level at 1 wk. Stiffness of the gastrocnemius in the sitting and standing postures (i.e., at short and long lengths) was derived from passive length-tension relations. Stiffness increased after exercise, and the relative changes in muscle stiffness were similar in both positions. There was no apparent correlation between stiffness and subjective reports of muscle soreness during walking.

CONCLUSION

This study provides the first specific measurements of the increase in stiffness of the human gastrocnemius in vivo after a single bout of eccentric exercise. The increase peaks at 24 h and is nearly fully resolved within 1 wk.

Titin PEVK segment: charge-driven elasticity of the open and flexible polyampholyte

The giant protein titin spans half of the sarcomere length and anchors the myosin thick filament to the Z-line of skeletal and cardiac muscles. The passive elasticity of muscle at a physiological range of stretch arises primarily from the extension of the PEVK segment, which is a polyampholyte with dense and alternating-charged clusters. Force spectroscopy studies of a 51 kDa fragment of the human fetal titin PEVK domain (TP1) revealed that when charge interactions were reduced by raising the ionic strength from 35 to 560 mM, its mean persistence length increased from 0.30 +/- 0.04 nm to 0.60 +/- 0.07 nm. In contrast, when the secondary structure of TP1 was altered drastically by the presence of 40 and 80% (v/v) of trifluoroethanol, its force-extension behavior showed no significant shift in the mean persistence length of approximately approximately 0.18 +/- 0.03 nm at the ionic strength of 15 mM. Additionally, the mean persistence length also increased from 0.29 to 0.41 nm with increasing calcium concentration from pCa 5-8 to pCa 3-4. We propose that PEVK is not a simple entropic spring as is commonly assumed, but a highly evolved, gel-like enthalpic spring with its elasticity dominated by the sequence-specific charge interactions. A single polyampholyte chain may be fine-tuned to generate a broad range of molecular elasticity by varying charge pairing schemes and chain configurations.

Calpain 3: a key regulator of the sarcomere?

Calpain 3 is a 94-kDa calcium-dependent cysteine protease mainly expressed in skeletal muscle. In this tissue, it localizes at several regions of the sarcomere through binding to the giant protein, titin. Loss-of-function mutations in the calpain 3 gene have been associated with limb-girdle muscular dystrophy type 2A (LGMD2A), a common form of muscular dystrophy found world wide. Recently, significant progress has been made in understanding the mode of regulation and the possible function of calpain 3 in muscle. It is now well accepted that it has an unusual zymogenic activation and that cytoskeletal proteins are one class of its substrates. Through the absence of cleavage of these substrates, calpain 3 deficiency leads to abnormal sarcomeres, impairment of muscle contractile capacity, and death of the muscle fibers. These data indicate a role for calpain 3 as a chef d'orchestre in sarcomere remodeling and suggest a new category of LGMD2 pathological mechanisms.

The length dependence of muscle active force: considerations for parallel elastic properties

The purpose of this study was to choose between two popular models of skeletal muscle: one with the parallel elastic component in parallel with both the contractile element and the series elastic component (model A), and the other in which it is in parallel with only the contractile element (model B). Passive and total forces were obtained at a variety of muscle lengths for the medial gastrocnemius muscle in anesthetized rats. Passive force was measured before the contraction (passive A) or was estimated for the fascicle length at which peak total force occurred (passive B). Fascicle length was measured with sonomicrometry. Active force was calculated by subtracting passive (A or B) force from peak total force at each fascicle or muscle length. Optimal length, that fascicle length at which active force is maximized, was 13.1 +/- 1.2 mm when passive A was subtracted and 14.0 +/- 1.1 mm with passive B (P < 0.01). Furthermore, the relationship between double-pulse contraction force and length was broader when calculated with passive B than with passive A. When the muscle was held at a long length, passive force decreased due to stress relaxation. This was accompanied by no change in fascicle length at the peak of the contraction and only a small corresponding decrease in peak total force. There is no explanation for the apparent increase in active force that would be obtained when subtracting passive A from the peak total force. Therefore, to calculate active force, it is appropriate to subtract passive force measured at the fascicle length corresponding to the length at which peak total force occurs, rather than passive force measured at the length at which the contraction begins.

Human motor compensations for thixotropy-dependent changes in resting wrist joint position after large joint movements

AIM

Resting tension of relaxed skeletal muscle fibres held at a given length varies with the immediate previous history of length changes and contractions. The primary aim of this study was to explore the motor control consequences of this history-dependency in healthy subjects.

METHODS

Angular position and passive torque were recorded from the intact wrist joint. Integrated surface electromyography (IEMG) was recorded from wrist extensor and flexor muscles.

RESULTS

In relaxed subjects, wrist joint position was displaced towards dorsiflexion after a single high-amplitude dorsiflexion movement combined with a strong flexor/extensor co-contraction (dorsiflexion conditioning), whereas after volarflexion conditioning there was a shift towards volarflexion. These after-effects could be abruptly cancelled by short periods ( approximately 5 s) of rapid flapping hand movements or forceful isometric co-contractions, findings indicative of muscle thixotropy. The IEMG-evaluated motor after-effects were as follows. A slowly subsiding wrist flexor contraction was needed to restore and maintain the original resting wrist position after dorsiflexion conditioning whereas a slowly subsiding extensor contraction was needed for the same goal after volarflexion conditioning. Furthermore, ongoing wrist extensor IEMG activity required to actively hold the wrist in a moderate dorsiflexed position or to resist a constant volar torque at resting position was temporarily reduced after dorsiflexion conditioning and enhanced (not significantly) after volarflexion conditioning.

CONCLUSION

The results provide evidence that during voluntary maintenance of a desired wrist joint position the motor commands to the position-holding muscles are unconsciously adjusted to compensate for thixotropy-dependent variations in the resting tension of the muscles.

Molecular basis of passive stress relaxation in human soleus fibers: assessment of the role of immunoglobulin-like domain unfolding

Titin (also known as 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 constructed of the PEVK domain and tandem-immunoglobulin segments comprising serially linked immunoglobulin (Ig)-like domains. It is unresolved whether under physiological conditions Ig domains remain folded and act as "spacers" that set the sarcomere length at which the PEVK extends or whether they contribute to titin's extensibility by unfolding. Here we focused on whether Ig unfolding plays a prominent role in stress relaxation (decay of force at constant length after stretch) using mechanical and immunolabeling studies on relaxed human soleus muscle fibers and Monte Carlo simulations. Simulation experiments using Ig-domain unfolding parameters obtained in earlier single-molecule atomic force microscopy experiments recover the phenomenology of stress relaxation and predict large-scale unfolding in titin during an extended period (> approximately 20 min) of relaxation. By contrast, immunolabeling experiments failed to demonstrate large-scale unfolding. Thus, under physiological conditions in relaxed human soleus fibers, Ig domains are more stable than predicted by atomic force microscopy experiments. Ig-domain unfolding did not become more pronounced after gelsolin treatment, suggesting that the thin filament is unlikely to significantly contribute to the mechanical stability of the domains. We conclude that in human soleus fibers, Ig unfolding cannot solely explain stress relaxation.

Binding of copper(II) ions to the polyproline II helices of PEVK modules of the giant elastic protein titin as revealed by ESI-MS, CD, and NMR

Titin, a family of giant elastic proteins, constitutes an elastic sarcomere matrix in striated muscle. In the I-band region of the sarcomere, the titin PEVK segment acts as a molecular spring to generate elasticity as well as sites of adhesion with parallel thin filaments. Previously, we reported that PEVK consists of tandem repeats of 28 residue modules and that the "polyproline II-coil" motif is the fundamental conformational motif of the PEVK module. In order to characterize the factors that may affect and alter the PPII-coil conformational motifs, we have initiated a systematic study of the interaction with divalent cations (Cu2+, Ca2+, Zn2+, and Ni2+) and a conformational profile of PEVK peptides (a representative 28-mer peptide PR: PEPPKEVVPEKKAPVAPPKKPEVPPVKV and its subfragments PR1: kvPEPPKEVVPE, PR2: VPEKKAPVAPPK, PR3: KPEVPPVKV). UV-Vis absorption difference spectra and CD spectra showed that Cu2+ bound to PR1 with high affinity (20 microM), while its binding to PR2 and PR3 as well as the binding of other cations to all four peptides were of lower affinity (>100 microM). Conformational studies by CD revealed that Cu2+ binding to PR1 resulted in a polyproline II to turn transition up to a 1:2 PR1/Cu2+ ratio and a coil to turn transition at higher Cu2+ concentration. ESI-MS provided the stoichiometry of PEVK peptide-Cu2+ complexes at both low and high ion strength, confirming the specific high affinity binding of Cu2+ to PR1 and PR. Furthermore, NMR and ESI-MS/MS fragmentation analysis elucidated the binding sites of the PEVK peptide-Cu2+ complexes at (-2)KVPE2, 8VPE10, 13APV15, and 22EVP24. A potential application of Cu2+ binding in peptide sequencing by mass spectrometry was also revealed. We conclude that Cu2+ binds and bends PEVK peptides to a beta-turn-like structure at specific sites. The specific targeting of Cu2+ towards PPII is likely to be of significant value in elucidating the roles of PPII in titin elasticity as well as in interactions of proline-rich proteins.

Damped elastic recoil of the titin spring in myofibrils of human myocardium

The giant protein titin functions as a molecular spring in muscle and is responsible for most of the passive tension of myocardium. Because the titin spring is extended during diastolic stretch, it will recoil elastically during systole and potentially may influence the overall shortening behavior of cardiac muscle. Here, titin elastic recoil was quantified in single human heart myofibrils by using a high-speed charge-coupled device-line camera and a nanonewtonrange force sensor. Application of a slack-test protocol revealed that the passive shortening velocity (Vp) of nonactivated cardiomyofibrils depends on: (i) initial sarcomere length, (ii) release-step amplitude, and (iii) temperature. Selective digestion of titin, with low doses of trypsin, decelerated myofibrillar passive recoil and eventually stopped it. Selective extraction of actin filaments with a Ca2+-independent gelsolin fragment greatly reduced the dependency of Vp on release-step size and temperature. These results are explained by the presence of viscous forces opposing myofibrillar passive recoil that are caused mainly by weak actin-titin interactions. Thus, Vp is determined by two distinct factors: titin elastic recoil and internal viscous drag forces. The recoil could be modeled as that of a damped entropic spring consisting of independent worm-like chains. The functional importance of myofibrillar elastic recoil was addressed by comparing instantaneous Vp to unloaded shortening velocity, which was measured in demembranated, fully Ca2+-activated, human cardiac fibers. Titin-driven passive recoil was much faster than active unloaded shortening velocity in early phases of isotonic contraction. Damped myofibrillar elastic recoil could help accelerate active contraction speed of human myocardium during early systolic shortening.

Malleable conformation of the elastic PEVK segment of titin: non-co-operative interconversion of polyproline II helix, beta-turn and unordered structures

To understand the structural basis of molecular elasticity and protein interaction of the elastic PEVK (Pro-Glu-Val-Lys) segment of the giant muscle protein titin, we carried out a detailed analysis of a representative PEVK module and a 16-module PEVK protein under various environmental conditions. Three conformational states, polyproline II (PPII) helix, beta-turn and unordered coil were identified by CD and NMR. These motifs interconvert without long-range co-operativity. As a general trend, the relative content of PPII increases with lower temperature and higher polarity, beta-turn increases with lower temperature and lower polarity, and unordered coil increases with higher temperature and higher polarity. NMR studies demonstrate that trans -proline residues are the predominant form at room temperature (22 degrees C), with little trans -to- cis isomerization below 35 degrees C. Ionic strength affects salt bridges between charged side chains, but not the backbone conformation. We conclude that titin PEVK conformation is malleable and responds to subtle environmental changes without co-operativity. This gradual conformational transition may represent a regulatory mechanism for fine-tuning protein interactions and elasticity.

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.

Titin-based contribution to shortening velocity of rabbit skeletal myofibrils

The shortening velocity of skeletal muscle fibres is determined principally by actomyosin cross-bridges. However, these contractile elements are in parallel with elastic elements, whose main structural basis is thought to be the titin filaments. If titin is stretched, it may contribute to sarcomere shortening simply because it can recoil 'passively'. The titin-based contribution to shortening velocity (V(p)) was quantified in single rabbit psoas myofibrils. Non-activated specimens were rapidly released from different initial sarcomere lengths (SLs) by various step amplitudes sufficient to buckle the myofibrils; V(p) was calculated from the release amplitude and the time to slack reuptake. V(p) increased progressively (upper limit of detection, approximately 60 microm s(-1) sarcomere(-1)) between 2.0 and 3.0 microm SL, albeit more steeply than passive tension. At very low passive tension levels already (< 1-2 mN mm(-2)), V(p) could greatly exceed the unloaded shortening velocity measured in fully Ca(2+)-activated skinned rabbit psoas fibres. Degradation of titin in relaxed myofibrils by low doses of trypsin (5 min) drastically decreased V(p). In intact myofibrils, average V(p) was faster, the smaller the release step applied. Also, V(p) was much higher at 30 degrees C than at 15 degrees C (Q(10): 2.0, 3.04 or 6.15, for release steps of 150, 250 or 450 nm sarcomere(-1), respectively). Viscous forces opposing the shortening are likely to be involved in determining these effects. The results support the idea that the contractile system imposes a braking force onto the passive recoil of elastic structures. However, elastic recoil may aid active shortening during phases of high elastic energy utilization, i.e. immediately after the onset of contraction under low or zero load or during prolonged shortening from greater physiological SLs.

Role of titin in vertebrate striated muscle

Titin is a giant muscle protein with a molecular weight in the megaDalton range and a contour length of more than 1 microm. Its size and location within the sarcomere structure determine its important role in the mechanism of muscle elasticity. According to the current consensus, elasticity stems directly from more than one type of spring-like behaviour of the I-band portion of the molecule. Starting from slack length, extension of the sarcomere first causes straightening of the molecule. Further extension then induces local unfolding of a unique sequence, the PEVK region, which is named due to the preponderance of these amino-acid residues. High speeds of extension and/or high forces are likely to lead to unfolding of the beta-sandwich domains from which the molecule is mainly constructed. A release of tension leads to refolding and recoiling of the polypeptide. Here, we review the literature and present new experimental material related to the role of titin in muscle elasticity. In particular, we analyse the possible influence of the arrangement and environment of titin within the sarcomere structure on its extensible behaviour. We suggest that, due to the limited conformational space, elongation and compression of the molecule within the sarcomere occur in a more ordered way or with higher viscosity and higher forces than are observed in solution studies of the isolated protein.

Smitin, a novel smooth muscle titin-like protein, interacts with myosin filaments in vivo and in vitro

Smooth muscle cells use an actin-myosin II-based contractile apparatus to produce force for a variety of physiological functions, including blood pressure regulation and gut peristalsis. The organization of the smooth muscle contractile apparatus resembles that of striated skeletal and cardiac muscle, but remains much more poorly understood. We have found that avian vascular and visceral smooth muscles contain a novel, megadalton protein, smitin, that is similar to striated muscle titin in molecular morphology, localization in a contractile apparatus, and ability to interact with myosin filaments. Smitin, like titin, is a long fibrous molecule with a globular domain on one end. Specific reactivities of an anti-smitin polyclonal antibody and an anti-titin monoclonal antibody suggest that smitin and titin are distinct proteins rather than differentially spliced isoforms encoded by the same gene. Smitin immunofluorescently colocalizes with myosin in chicken gizzard smooth muscle, and interacts with two configurations of smooth muscle myosin filaments in vitro. In physiological ionic strength conditions, smitin and smooth muscle myosin coassemble into irregular aggregates containing large sidepolar myosin filaments. In low ionic strength conditions, smitin and smooth muscle myosin form highly ordered structures containing linear and polygonal end-to-end and side-by-side arrays of small bipolar myosin filaments. We have used immunogold localization and sucrose density gradient cosedimentation analyses to confirm association of smitin with both the sidepolar and bipolar smooth muscle myosin filaments. These findings suggest that the titin-like protein smitin may play a central role in organizing myosin filaments in the contractile apparatus and perhaps in other structures in smooth muscle cells.

Effect of eccentric muscle contractions on Golgi tendon organ responses to passive and active tension in the cat

To investigate the possibility of a peripheral contribution to the perturbations of force sensation reported to occur after eccentric exercise, responses to passive and active tension were recorded from Golgi tendon organs in the medial gastrocnemius muscle of the anaesthetised cat, before and after a series of eccentric contractions. After the eccentric contractions, nearly all tendon organs commenced firing at a shorter muscle length during slow passive stretch than before, probably because of a rise in whole muscle passive tension. There was a small drop in the sensitivity to incremental tension, but no mean change in tension threshold. Following the eccentric contractions, there was a small, but not significant, increase in tendon organ sensitivity to active tension, which was graded using a method of optimised, distributed stimulation of divided ventral roots. Sensitivity was estimated as the mean response over a range of tensions and as the change in discharge rate in response to incremental tension. The experiments provided the opportunity of comparing tendon organ sensitivities to graded passive and active whole muscle tension. In agreement with previous work in which whole muscle nerve stimulation was employed, little difference was found. It was concluded that the peripheral contribution to perturbations of force perception after eccentric exercise is likely to be small and that the centrally derived sense of effort plays the dominant role. Tendon organs appear to be remarkably reliable in signalling whole muscle tension, whether passive or active, and even after the muscle's force production has been disturbed by fatigue or eccentric exercise.

Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications

In eccentric exercise the contracting muscle is forcibly lengthened; in concentric exercise it shortens. While concentric contractions initiate movements, eccentric contractions slow or stop them. A unique feature of eccentric exercise is that untrained subjects become stiff and sore the day afterwards because of damage to muscle fibres. This review considers two possible initial events as responsible for the subsequent damage, damage to the excitation-contraction coupling system and disruption at the level of the sarcomeres. Other changes seen after eccentric exercise, a fall in active tension, shift in optimum length for active tension, and rise in passive tension, are seen, on balance, to favour sarcomere disruption as the starting point for the damage. As well as damage to muscle fibres there is evidence of disturbance of muscle sense organs and of proprioception. A second period of exercise, a week after the first, produces much less damage. This is the result of an adaptation process. One proposed mechanism for the adaptation is an increase in sarcomere number in muscle fibres. This leads to a secondary shift in the muscle's optimum length for active tension. The ability of muscle to rapidly adapt following the damage from eccentric exercise raises the possibility of clinical applications of mild eccentric exercise, such as for protecting a muscle against more major injuries.

Alternative splicing of an amino-terminal PEVK-like region generates multiple isoforms of Drosophila projectin

Drosophila projectin is an extremely large protein found within the muscle sarcomeric unit, parallel with the actin and myosin filaments. Projectin has been suggested as the elastic component of C-filaments in insect indirect flight muscles, which is consistent with its localization from the Z band to the tip of the A band in these muscles. Here, we describe the completion of the projectin sequence analysis, which defines projectin as a 1 MDa protein, composed of 39 immunoglobulin and 39 fibronectin III domains. This analysis led also to the identification of a domain rich in the amino acids P, E, V and K within the NH(2) terminus of projectin. The length of the projectin PEVK-like region varies from 100 to 624 amino acid residues, following a complex pattern of alternative splicing events. PEVK domains were first identified in vertebrate titin and they have been associated with the elasticity of the protein. The PEVK-like domain of the projectin isoforms in indirect flight muscles may contribute to the elastic function of the C-filaments. The synchronous projectin isoforms contain a PEVK-like region, and the possible non-elastic function(s) of this domain in synchronous muscles are discussed.

Passive mechanical properties of the medial gastrocnemius muscle of the cat

1. This is a report on the history dependence of the passive mechanical properties of the medial gastrocnemius muscle of the anaesthetised cat. 2. The muscle was conditioned with an isometric contraction at the test length, or at 3 mm longer than the test length and then returned to the test length, where the level of resting tension was measured, as well as tension changes during a slow stretch. 3. The level of resting tension depended on the form of conditioning and, at the optimum length for active tension, the history-dependent component was 9 % of the total passive tension. 4. During a slow stretch, tension initially rose steeply up to a yield point, beyond which it rose more gradually. The shape of the tension rise depended on the form of conditioning. The level of tension at the yield point consisted of a stretch-dependent component, the 'short-range tension' plus the resting tension for that length. 5. The short-range tension increased with muscle length to peak close to the optimum for active tension. The slope of the tension rise during a stretch, the short-range stiffness, peaked at 2 mm beyond the optimum. 6. The short-range tension was small immediately after a conditioning contraction but grew in size as the interval was increased up to 60 s, with a time constant of 9.9 +/- 0.6 s. After a series of conditioning movements, it recovered more rapidly, with a time constant of 6.6 +/- 0.5 s. 7. The history-dependent changes in passive tension and the response to stretch are interpreted in terms of the presence, in sarcomeres of resting muscle fibres, of crossbridges between actin and myosin which have very slow formation rates, both at rest and during movements.

Single molecule measurements of titin elasticity

Titin, with a massive single chain of 3--4MDa and multiple modular motifs, spans the half-sarcomere of skeletal and cardiac muscles and serves important, multifaceted functions. In recent years, titin has become a favored subject of single molecule observations by atomic force microscopy (AFM) and laser optical trap (LOT). Here we review these single titin molecule extension studies with an emphasis on understanding their relevance to titin elasticity in muscle function. Some fundamental aspects of the methods for single titin molecule investigations, including the application of dynamic force, the elasticity models for filamentous titin motifs, the technical foundations and calibrations of AFM and LOT, and titin sample preparations are provided. A chronological review of major publications on recent single titin extension observations is presented. This is followed by summary evaluations of titin domain folding/unfolding results and of elastic properties of filamentous titin motifs. Implications of these single titin measurements for muscle physiology/pathology are discussed and forthcoming advances in single titin studies are anticipated.

Changes in passive tension of muscle in humans and animals after eccentric exercise

1. This is a report of experiments on ankle extensor muscles of human subjects and a parallel series on the medial gastrocnemius of the anaesthetised cat, investigating the origin of the rise in passive tension after a period of eccentric exercise. 2. Subjects exercised their triceps surae of one leg eccentrically by walking backwards on an inclined, forward-moving treadmill. Concentric exercise required walking forwards on a backwards-moving treadmill. For all subjects the other leg acted as a control. 3. Immediately after both eccentric and concentric exercise there was a significant drop in peak active torque, but only after eccentric exercise was this accompanied by a shift in optimum angle for torque generation and a rise in passive torque. In the eccentrically exercised group some swelling and soreness developed but not until 24 h post-exercise. 4. In the animal experiments the contracting muscle was stretched by 6 mm at 50 mm s(-1) over a length range symmetrical about the optimum length for tension generation. Measurements of passive tension were made before and after the eccentric contractions, using small stretches to a range of muscle lengths, or with large stretches covering the full physiological range. 5. After 150 eccentric contractions, passive tension was significantly elevated over most of the range of lengths. Measurements of work absorption during stretch-release cycles showed significant increases after the contractions. 6. It is suggested that the rise in passive tension in both human and animal muscles after eccentric contractions is the result of development of injury contractures in damaged muscle fibres.

Structural evidence for a possible role of reversible disulphide bridge formation in the elasticity of the muscle protein titin

BACKGROUND

The giant muscle protein titin contributes to the filament system in skeletal and cardiac muscle cells by connecting the Z disk and the central M line of the sarcomere. One of the physiological functions of titin is to act as a passive spring in the sarcomere, which is achieved by the elastic properties of its central I band region. Titin contains about 300 domains of which more than half are folded as immunoglobulin-like (Ig) domains. Ig domain segments of the I band of titin have been extensively used as templates to investigate the molecular basis of protein elasticity.

RESULTS

The structure of the Ig domain I1 from the I band of titin has been determined to 2.1 A resolution. It reveals a novel, reversible disulphide bridge, which is neither required for correct folding nor changes the chemical stability of I1, but it is predicted to contribute mechanically to the elastic properties of titin in active sarcomeres. From the 92 Ig domains in the longest isoform of titin, at least 40 domains have a potential for disulphide bridge formation.

CONCLUSIONS

We propose a model where the formation of disulphide bridges under oxidative stress conditions could regulate the elasticity of the I band in titin by increasing sarcomeric resistance. In this model, the formation of the disulphide bridge could refrain a possible directed motion of the two beta sheets or other mechanically stable entities of the I1 Ig domain with respect to each other when exposed to mechanical forces.

Mechanical design of proteins studied by single-molecule force spectroscopy and protein engineering

Mechanical unfolding and refolding may regulate the molecular elasticity of modular proteins with mechanical functions. The development of the atomic force microscopy (AFM) has recently enabled the dynamic measurement of these processes at the single-molecule level. Protein engineering techniques allow the construction of homomeric polyproteins for the precise analysis of the mechanical unfolding of single domains. alpha-Helical domains are mechanically compliant, whereas beta-sandwich domains, particularly those that resist unfolding with backbone hydrogen bonds between strands perpendicular to the applied force, are more stable and appear frequently in proteins subject to mechanical forces. The mechanical stability of a domain seems to be determined by its hydrogen bonding pattern and is correlated with its kinetic stability rather than its thermodynamic stability. Force spectroscopy using AFM promises to elucidate the dynamic mechanical properties of a wide variety of proteins at the single molecule level and provide an important complement to other structural and dynamic techniques (e.g., X-ray crystallography, NMR spectroscopy, patch-clamp).

Magnetic resonance elastography of skeletal muscle

While the contractile properties of skeletal muscle have been studied extensively, relatively little is known about the elastic properties of muscle in vivo. Magnetic resonance elastography (MRE) is a phase contrast-based method for observing shear waves propagating in a material to determine its stiffness. In this work, MRE is applied to skeletal muscle under load to quantify the change in stiffness with loading. A mathematical model of muscle is developed that predicts a linear relationship between shear stiffness and muscle load. The MRE technique was applied to bovine muscle specimens (N = 10) and human biceps brachii in vivo (N = 5). Muscle stiffness increased linearly for both passive tension (14.5 +/- 1.77 kPa/kg) and active tension, in which the increase in stiffness was dependent upon muscle size, as predicted by the model. A means of noninvasively assessing the viscoelastic pro-perties of skeletal muscle in vivo may provide a useful method for studying muscle biomechanics in health and disease.

Distinct families of Z-line targeting modules in the COOH-terminal region of nebulin

To learn how nebulin functions in the assembly and maintenance of I-Z-I bands, MYC- and GFP- tagged nebulin fragments were expressed in primary cultured skeletal myotubes. Their sites of incorporation were visualized by double staining with anti-MYC, antibodies to myofibrillar proteins, and FITC- or rhodamine phalloidin. Contrary to expectations based on in vitro binding studies, none of the nebulin fragments expressed in maturing myotubes were incorporated selectively into I-band approximately 1.0-micrometer F-alpha-actin-containing thin filaments. Four of the MYC/COOH-terminal nebulin fragments were incorporated exclusively into periodic approximately 0.1-micrometer Z-bands. Whereas both anti-MYC and Rho-phalloidin stained intra-Z-band F-alpha-actin oligomers, only the latter stained the pointed ends of the polarized approximately 1.0-micrometer thin filaments. Z-band incorporation was independent of the nebulin COOH-terminal Ser or SH3 domains. In vitro cosedimentation studies also demonstrated that nebulin SH3 fragments did not bind to F-alpha-actin or alpha-actinin. The remaining six fragments were not incorporated into Z-bands, but were incorporated (a) diffusely throughout the sarcoplasm and into (b) fibrils/patches of varying lengths and widths nested among normal striated myofibrils. Over time, presumably in response to the mediation of muscle-specific homeostatic controls, many of the ectopic MYC-positive structures were resorbed. None of the tagged nebulin fragments behaved as dominant negatives; they neither blocked the assembly nor induced the disassembly of mature striated myofibrils. Moreover, they were not cytotoxic in myotubes, as they were in the fibroblasts and presumptive myoblasts in the same cultures.

Requirements of Kettin, a giant muscle protein highly conserved in overall structure in evolution, for normal muscle function, viability, and flight activity of Drosophila

Kettin is a giant muscle protein originally identified in insect flight muscle Z-discs. Here, we determined the entire nucleotide sequence of Drosophila melanogaster kettin, deduced the amino acid sequence of its protein product (540 kD) along with that of the Caenorhabditis elegans counterpart, and found that the overall primary structure of Kettin has been highly conserved in evolution. The main body of Drosophila Kettin consists of 35 immunoglobulin C2 domains separated by spacers. The central two thirds of spacers are constant in length and share in common two conserved motifs, putative actin binding sites. Neither fibronectin type III nor kinase domains were found. Kettin is present at the Z-disc in several muscle types. Genetic analysis showed that kettin is essential for the formation and maintenance of normal sarcomere structure of muscles and muscle tendons. Accordingly, embryos lacking kettin activity cannot hatch nor can adult flies heterozygous for the kettin mutation fly.