Expression of titin in skeletal muscle varies with hind-limb unloading

Effects of exercise prehabilitation on muscle atrophy and contractile properties in hindlimb-unloaded rats

INTRODUCTION/AIMS

Effective strategies for rapid recovery after surgery are needed. Therefore, we investigated the effects of exercise prehabilitation (EP) and hindlimb unloading (HU) on muscle loss and contractility.

METHODS

Twenty-two Sprague-Dawley rats (12 wk old) were divided into normal control (NCON, n = 5), hindlimb unloading control (HCON, n = 10), and exercise prehabilitation followed by hindlimb unloading (Ex-preH, n = 7) groups. Ex-PreH performed exercise training for 14 days before hindlimb unloading for 14 days. Body composition was evaluated, along with muscle strength and function. The soleus (SOL) and extensor digitorum longus (EDL) muscle contractile properties were analyzed at the whole-muscle level. The titin concentration and myosin heavy chain (MHC) type composition were analyzed.

RESULTS

There were no effects of Ex-preH on total mass, lean mass, or muscle weight. Physical function was significantly higher in the Ex-preH group than in the HCON group (39.5° vs. 35.7°). The SOL twitch force (19.6 vs. 7.1 mN/m2 ) and specific force (107.3 vs. 61.2 mN/m2 ) were greater in Ex-preH group than in HCON group. EDL shortening velocity was higher in Ex-preH group than in HCON group (13.2 vs. 5.0 FL/s). The SOL full-length titin level was higher in Ex-preH group than in HCON group.

DISCUSSION

Exercise prehabilitation did not prevent muscle mass loss followed by muscle wasting, although it minimized the reduction of physical function. Therefore, exercise prehabilitation should be considered for rapid functional recovery after disuse due to surgery and injuries.

Immobility and the High Risk of Not Recovering Function in Older Adults: A Focused Ethnography

PURPOSE

The objective of the study was to explore the experiences of older adults with immobility posthospitalization.

DESIGN

We conducted a focused ethnography qualitative study.

METHODS

Data collection included in-depth interviews with 10 individuals ages 69-82 years who had been hospitalized for at least 1 week, field notes, and observations of mobility. An inductive approach was used to analyze the data.

FINDINGS

Older adults believe mobility is a central element in their life. Hospitalizations lasting 1 week or longer often result in a loss of the older adult's ability to function as before, a pattern that we identified as The Crushing Assault: Consequences of Immobility. Rehabilitation involves a Rocky Road to Regaining Mobility that is challenging but possible.

CONCLUSIONS

Immobility after hospitalization has unexpected and profound consequences that are life-changing and distressing for older adults at home. Recovering function is prolonged and difficult. An individualized training program that includes flexibility and progressive resistance exercises is recommended over longer periods and with extended medical follow-up.

CLINICAL RELEVANCE

Lingering problems with immobility at home are frequently overlooked. Active programming promoting preservation of function and an optimistic attitude as well as focusing on milestones to reach are key to optimal function.

Botulinum Toxin Intervention in Cerebral Palsy-Induced Spasticity Management: Projected and Contradictory Effects on Skeletal Muscles

Spasticity, following the neurological disorder of cerebral palsy (CP), describes a pathological condition, the central feature of which is involuntary and prolonged muscle contraction. The persistent resistance of spastic muscles to stretching is often followed by structural and mechanical changes in musculature. This leads to functional limitations at the respective joint. Focal injection of botulinum toxin type-A (BTX-A) is effectively used to manage spasticity and improve the quality of life of the patients. By blocking acetylcholine release at the neuromuscular junction and causing temporary muscle paralysis, BTX-A aims to reduce spasticity and hereby improve joint function. However, recent studies have indicated some contradictory effects such as increased muscle stiffness or a narrower range of active force production. The potential of these toxin- and atrophy-related alterations in worsening the condition of spastic muscles that are already subjected to changes should be further investigated and quantified. By focusing on the effects of BTX-A on muscle biomechanics and overall function in children with CP, this review deals with which of these goals have been achieved and to what extent, and what can await us in the future.

Calpain-dependent degradation of cytoskeletal proteins as a key mechanism for a reduction in intrinsic passive stiffness of unloaded rat postural muscle

In mammals, prolonged mechanical unloading results in a significant decrease in passive stiffness of postural muscles. The nature of this phenomenon remains unclear. The aim of the present study was to investigate possible causes for a reduction in rat soleus passive stiffness after 7 and 14 days of unloading (hindlimb suspension, HS). We hypothesized that HS-induced decrease in passive stiffness would be associated with calpain-dependent degradation of cytoskeletal proteins or a decrease in actomyosin interaction. Wistar rats were subjected to HS for 7 and 14 days with or without PD150606 (calpain inhibitor) treatment. Soleus muscles were subjected to biochemical analysis and ex vivo measurements of passive tension with or without blebbistatin treatment (an inhibitor of actomyosin interactions). Passive tension of isolated soleus muscle was significantly reduced after 7- and 14-day HS compared to the control values. PD150606 treatment during 7- and 14-day HS induced an increase in alpha-actinin-2 and -3, desmin contents compared to control, partly prevented a decrease in intact titin (T1) content, and prevented a decrease in soleus passive tension. Incubation of soleus muscle with blebbistatin did not affect HS-induced reductions in specific passive tension in soleus muscle. Our study suggests that calpain-dependent breakdown of cytoskeletal proteins, but not a change in actomyosin interaction, significantly contributes to unloading-induced reductions in intrinsic passive stiffness of rat soleus muscle.

Molecular Mechanisms of Muscle Tone Impairment under Conditions of Real and Simulated Space Flight

Kozlovskaya et al. [1] and Grigoriev et al. [2] showed that enormous loss of muscle stiffness (atonia) develops in humans under true (space flight) and simulated microgravity conditions as early as after the first days of exposure. This phenomenon is attributed to the inactivation of slow motor units and called reflectory atonia. However, a lot of evidence indicating that even isolated muscle or a single fiber possesses substantial stiffness was published at the end of the 20th century. This intrinsic stiffness is determined by the active component, i.e. the ability to form actin-myosin cross-bridges during muscle stretch and contraction, as well as by cytoskeletal and extracellular matrix proteins, capable of resisting muscle stretch. The main facts on intrinsic muscle stiffness under conditions of gravitational unloading are considered in this review. The data obtained in studies of humans under dry immersion and rodent hindlimb suspension is analyzed. The results and hypotheses regarding reduced probability of cross-bridge formation in an atrophying muscle due to increased interfilament spacing are described. The evidence of cytoskeletal protein (titin, nebulin, etc.) degradation during gravitational unloading is also discussed. The possible mechanisms underlying structural changes in skeletal muscle collagen and its role in reducing intrinsic muscle stiffness are presented. The molecular mechanisms of changes in intrinsic stiffness during space flight and simulated microgravity are reviewed.

Changes in Muscle Stress and Sarcomere Adaptation in Mice Following Ischemic Stroke

While abnormal muscle tone has been observed in people with stroke, how these changes in muscle tension affect sarcomere morphology remains unclear. The purpose of this study was to examine time-course changes in passive muscle fiber tension and sarcomeric adaptation to these changes post-ischemic stroke in a mouse model by using a novel in-vivo force microscope. Twenty-one mice were evenly divided into three groups based on the time point of testing: 3 days (D3), 10 days (D10), and 20 days (D20) following right middle cerebral artery ligation. At each testing time, the muscle length, width, and estimated volume of the isolated soleus muscle were recorded, subsequently followed by in-vivo muscle tension and sarcomere length measurement. The mass of the soleus muscle was measured at the end of testing to calculate muscle density. Two-way ANOVA with repeated measures was used to examine the differences in each of the dependent variable among the three time-point groups and between the two legs. The passive muscle stress of the impaired limbs in the D3 group (27.65 ± 8.37 kPa) was significantly lower than the less involved limbs (42.03 ± 18.61 kPa; p = 0.05) and the impaired limbs of the D10 (48.92 ± 14.73; p = 0.03) and D20 (53.28 ± 20.54 kPa; p = 0.01) groups. The soleus muscle density of the impaired limbs in the D3 group (0.69 ± 0.12 g/cm3) was significantly lower than the less involved limbs (0.80 ± 0.09 g/cm3; p = 0.04) and the impaired limbs of the D10 (0.87 ± 0.12 g/cm3; p = 0.02) and D20 (1.00 ± 0.14 g/cm3; p < 0.01) groups. The D3 group had a shorter sarcomere length (2.55 ± 0.26 μm) than the D10 (2.83 ± 0.20 μm; p = 0.03) and D20 group (2.81 ± 0.15 μm; p = 0.04). These results suggest that, while ischemic stroke may cause considerable changes in muscle tension and stress, sarcomere additions under increased mechanical loadings may be absent or disrupted post-stroke, which may contribute to muscle spasticity and/or joint contracture commonly observed in patients following stroke.

Cellular Responses of Human Postural Muscle to Dry Immersion

Support withdrawal has been currently considered as one of the main factors involved in regulation of the human locomotor system. For last decades, several authors, including the authors of the present paper, have revealed afferent mechanisms of support perception and introduced the concept of the support afferentation system. The so-called "dry immersion" model which was developed in Russia allows for suspension of subjects in water providing the simulation of the mechanical support withdrawal. The present review is a summary of data allowing to appreciate the value of the "dry" immersion model for the purposes of studying cellular responses of human postural muscle to gravitational unloading. These studies corroborated our hypothesis that the removal of support afferentation inactivates the slow motor unit pool which leads to selective inactivation, and subsequent atony and atrophy, of muscle fibers expressing the slow isoform of myosin heavy chain (which constitutes the majority of soleus muscle fibers). Fibers that have lost a significant part of cytoskeletal molecules are incapable of effective actomyosin motor mobilization which leads to lower calcium sensitivity and lower range of maximal tension in permeabilized fibers. Support withdrawal also leads to lower efficiency of protective mechanisms (nitric oxide synthase) and decreased activity of AMP-activated protein kinase. Thus, "dry" immersion studies have already contributed considerably to the gravitational physiology of skeletal muscle.

Use it or lose it: multiscale skeletal muscle adaptation to mechanical stimuli

Skeletal muscle undergoes continuous turnover to adapt to changes in its mechanical environment. Overload increases muscle mass, whereas underload decreases muscle mass. These changes are correlated with, and enabled by, structural alterations across the molecular, subcellular, cellular, tissue, and organ scales. Despite extensive research on muscle adaptation at the individual scales, the interaction of the underlying mechanisms across the scales remains poorly understood. Here, we present a thorough review and a broad classification of multiscale muscle adaptation in response to a variety of mechanical stimuli. From this classification, we suggest that a mathematical model for skeletal muscle adaptation should include the four major stimuli, overstretch, understretch, overload, and underload, and the five key players in skeletal muscle adaptation, myosin heavy chain isoform, serial sarcomere number, parallel sarcomere number, pennation angle, and extracellular matrix composition. Including this information in multiscale computational models of muscle will shape our understanding of the interacting mechanisms of skeletal muscle adaptation across the scales. Ultimately, this will allow us to rationalize the design of exercise and rehabilitation programs, and improve the long-term success of interventional treatment in musculoskeletal disease.

Adaptive responses of TRPC1 and TRPC3 during skeletal muscle atrophy and regrowth

INTRODUCTION

We assessed the time-dependent changes of transient receptor potential canonical type 1 (TRPC1) and TRPC3 expression and localization associated with muscle atrophy and regrowth in vivo.

METHODS

Mice were subjected to hindlimb unloading for 7 or 14 days (7U, 14U) followed by 3, 7, or 14 days of reloading (3R, 7R, 14R).

RESULTS

Soleus muscle mass and tetanic force were reduced significantly at 7U and 14U and recovered by 14R. Recovery of muscle fiber cross-sectional area was observed by 28R. TRPC1 mRNA was unaltered during the unloading-reloading period. However, protein expression remained depressed through 14R. Decreased localization of TRPC1 to the sarcolemma was observed. TRPC3 mRNA and protein expression levels were decreased significantly during the early phase of reloading.

CONCLUSIONS

Given the known role of these channels in muscle development, changes observed in TRPC1 and TRPC3 may relate closely to muscle atrophy and remodeling processes.

Effects of long-term microgravitation exposure on cell respiration of the rat musculus soleus fibers

Cell respiration of the m. soleus fibers was studied in Wistar rats treated with succinic acid and exposed to microgravitation for 35 days. The results indicated that respiration rates during utilization of endogenous and exogenous substrates and the maximum respiration rate decreased in animals subjected to microgravitation without succinate treatment. The respiration rate during utilization of exogenous substrate did not increase in comparison with that on endogenous substrates. Succinic acid prevented the decrease in respiration rate on endogenous substrates and the maximum respiration rate. On the other hand, the respiration rate on exogenous substrates was reduced in vivarium control rats receiving succinate in comparison with intact control group. That could indicate changed efficiency of complex I of the respiratory chain due to reciprocal regulation of the tricarbonic acid cycle.

Transversal stiffness of fibers and desmin content in leg muscles of rats under gravitational unloading of various durations

The aim of this research was the analysis of structural changes in various parts of the sarcolemma and contractile apparatus of muscle fibers by measuring their transversal stiffness by atomic force microscopy under gravitational unloading. Soleus, medial gastrocnemius, and tibialis anterior muscles of Wistar rats were the objects of the study. Gravitational unloading was carried out by antiorthostatic suspension of hindlimbs for 1, 3, 7, and 12 days. It was shown that the transversal stiffness of different parts of the contractile apparatus of soleus muscle fibers decreases during gravitational unloading in the relaxed, calcium-activated, and rigor states, the fibers of the medial gastrocnemius show no changes, whereas the transversal stiffness of tibialis anterior muscle increases. Thus the transversal stiffness of the sarcolemma in the relaxed state is reduced in all muscles, which may be due to the direct action of gravity as an external mechanical factor that can influence the tension on a membrane. The change of sarcolemma stiffness in activated fibers, which is due probably to the transfer of tension from the contractile apparatus, correlates with the dynamics of changes in the content of desmin.

Calcium-dependent signaling mechanisms and soleus fiber remodeling under gravitational unloading

The decrease in postural muscle fiber size, diminishing of their contractile properties, slow-to-fast shift in myosin heavy chain expression pattern are known to be the main consequences of gravitational unloading. The Ca(2+) role in these processes has been studied for about 20 years. Ingalls et al. [J Appl Physiol 87(1):382-390, 1999] found the resting Ca(2+) level increase in soleus fibers of hindlimb unloaded mice. Results obtained in our laboratory showed that systemic or local application of nifedipine (L-type Ca(2+) channels' blocker) prevents Ca(2+) accumulation in fibers. Thus, activation of dihydropyridine calcium channels can be supposed to promote resting Ca(2+) loading under disuse. So, calcium-dependent signaling pathways may play an important role in the development of some key events observed under unloading. Since 90th the increased activities of Ca(2+)-dependent proteases (calpains) were considered as the crucial effect of hypogravity-induced muscle atrophy, which was proved later. We observed maintenance of titin and nebulin relative content in soleus muscle under unloading combined with Ca(2+) chelators administration. Nifedipine administration was shown to considerably restrict the slow-to-fast transition of myosin heavy chains (MHC) under unloading (at the RNA level and at the protein level as well). To clarify the role of calcineurin/NFAT signaling system in MHC pattern transition under unloading, we blocked this pathway by cyclosporine A application. Hereby, we demonstrated that calcineurin/NFAT pathway possesses a stabilizing function counteracting the myosin phenotype transformation under gravitational unloading.

Contractile and connective tissue protein content of human skeletal muscle: effects of 35 and 90 days of simulated microgravity and exercise countermeasures

We examined the effects of 35 and 90 days of simulated microgravity with or without resistance-exercise (RE) countermeasures on the content of the general skeletal muscle protein fractions (mixed, sarcoplasmic, and myofibrillar) and specific proteins that are critical for muscle function (myosin, actin, and collagen). Subjects from two studies, using either unilateral lower limb suspension (ULLS) or bed rest (BR), comprised four separate groups: 35 days ULLS ( n =11), 35 days ULLS+RE ( n = 10), 90 days BR ( n = 9), and 90 days BR+RE ( n = 8). RE consisted of four sets of seven maximal concentric and eccentric repetitions of the quadriceps femoris muscles that were performed 2 or 3 times per week. Pre- and post-simulated weightlessness muscle biopsies were analyzed from the vastus lateralis of all groups and the soleus of the 35-day ULLS and 90-day BR groups. The general protein fractions and the specific proteins myosin, actin, and collagen of the vastus lateralis were unchanged ( P > 0.05) in both control and countermeasures groups over 35 and 90 days, despite large changes in quadriceps femoris muscle volume (35 days ULLS: −9%, 35 days ULLS+RE: +8%; and 90 days BR: −18%, 90 days BR+RE: −1%). The soleus demonstrated a decrease in mixed (35 days ULLS: −12%, P = 0.0001; 90 days BR: −12%, P = 0.004) and myofibrillar (35 days ULLS: −12%, P = 0.009; 90 days BR: −8%, P = 0.04) protein, along with large changes in triceps surae muscle volume (35 days ULLS: −11%; 90 days BR: −29%). Despite the loss of quadriceps femoris muscle volume or preservation with RE countermeasures during simulated microgravity, the quadriceps femoris muscles are able to maintain the concentrations of the general protein pools and the main contractile and connective tissue elements. Soleus muscle protein composition appears to be disproportionately altered during long-duration simulated weightlessness.

Effects of streptozotocin-induced diabetes and physical training on gene expression of titin-based stretch-sensing complexes in mouse striated muscle

In striated muscle, a sarcomeric noncontractile protein, titin, is proposed to form the backbone of the stress- and strain-sensing structures. We investigated the effects of diabetes, physical training, and their combination on the gene expression of proteins of putative titin stretch-sensing complexes in skeletal and cardiac muscle. Mice were divided into control (C), training (T), streptozotocin-induced diabetic (D), and diabetic training (DT) groups. Training groups performed for 1, 3, or 5 wk of endurance training on a motor-driven treadmill. Muscle samples from T and DT groups together with respective controls were collected 24 h after the last training session. Gene expression of calf muscles (soleus, gastrocnemius, and plantaris) and cardiac muscle were analyzed using microarray and quantitative PCR. Diabetes induced changes in mRNA expression of the proteins of titin stretch-sensing complexes in Z-disc (MLP, myostatin), I-band (CARP, Ankrd2), and M-line (titin kinase signaling). Training alleviated diabetes-induced changes in most affected mRNA levels in skeletal muscle but only one change in cardiac muscle. In conclusion, we showed diabetes-induced changes in mRNA levels of several fiber-type-biased proteins (MLP, myostatin, Ankrd2) in skeletal muscle. These results are consistent with previous observations of diabetes-induced atrophy leading to slower fiber type composition. The ability of exercise to alleviate diabetes-induced changes may indicate slower transition of fiber type.

Isoform diversity of giant proteins in relation to passive and active contractile properties of rabbit skeletal muscles

The active and passive contractile performance of skeletal muscle fibers largely depends on the myosin heavy chain (MHC) isoform and the stiffness of the titin spring, respectively. Open questions concern the relationship between titin-based stiffness and active contractile parameters, and titin's importance for total passive muscle stiffness. Here, a large set of adult rabbit muscles (n = 37) was studied for titin size diversity, passive mechanical properties, and possible correlations with the fiber/MHC composition. Titin isoform analyses showed sizes between ∼3300 and 3700 kD; 31 muscles contained a single isoform, six muscles coexpressed two isoforms, including the psoas, where individual fibers expressed similar isoform ratios of 30:70 (3.4:3.3 MD). Gel electrophoresis and Western blotting of two other giant muscle proteins, nebulin and obscurin, demonstrated muscle type–dependent size differences of ≤70 kD. Single fiber and single myofibril mechanics performed on a subset of muscles showed inverse relationships between titin size and titin-borne tension. Force measurements on muscle strips suggested that titin-based stiffness is not correlated with total passive stiffness, which is largely determined also by extramyofibrillar structures, particularly collagen. Some muscles have low titin-based stiffness but high total passive stiffness, whereas the opposite is true for other muscles. Plots of titin size versus percentage of fiber type or MHC isoform (I-IIB-IIA-IID) determined by myofibrillar ATPase staining and gel electrophoresis revealed modest correlations with the type I fiber and MHC-I proportions. No relationships were found with the proportions of the different type II fiber/MHC-II subtypes. Titin-based stiffness decreased with the slow fiber/MHC percentage, whereas neither extramyofibrillar nor total passive stiffness depended on the fiber/MHC composition. In conclusion, a low correlation exists between the active and passive mechanical properties of skeletal muscle fibers. Slow muscles usually express long titin(s), predominantly fast muscles can express either short or long titin(s), giving rise to low titin-based stiffness in slow muscles and highly variable stiffness in fast muscles. Titin contributes substantially to total passive stiffness, but this contribution varies greatly among muscles.

Gigantic variety: expression patterns of titin isoforms in striated muscles and consequences for myofibrillar passive stiffness

The giant muscle protein titin has become a focus of research interests in the field of muscle mechanics due to its importance for passive muscle stiffness. Here we summarize research activities leading to current understanding of titin's mechanical role in the sarcomere. We then show how low-porosity polyacrylamide-gel electrophoresis, optimised for resolving megadalton proteins, can identify differences in titin-isoform expression in the hearts of 10 different vertebrate species and in several skeletal muscles of the rabbit. A large variety of titin-expression patterns is apparent, which is analysed in terms of its effect on the passive tension of isolated myofibrils obtained from selected muscle types. We show and discuss evidence indicating that vertebrate striated muscle cells are capable of adjusting their passive stiffness in the following ways: (1) Cardiomyocytes co-express long (N2BA) and short (N2B) titin isoform in the same half-sarcomeres and vary the N2BA:N2B ratio to adjust stiffness. Hearts from different mammalian species vary widely in their N2BA:N2B ratio; right ventricles show higher ratios than left ventricles. There is also a significant gradient of N2BA:N2B ratio in a given heart, from basal to apical; transmural ratio differences are less distinct. (2) Skeletal muscles can express longer or shorter I-band-titin (N2A-isoform) to achieve lower or higher titin-derived stiffness, respectively. (3) Some skeletal muscles co-express longer (N2A(L)) and shorter (N2A(S)) titin isoforms, also at the single-fibre level (e.g., rabbit psoas); variations in overall N2A(L):N2A(S) ratio may add to the fine-tuning of titin-based stiffness in the whole muscle. Whereas it is established that titin, together with extracellular collagen, determines the passive tension at physiological sarcomere lengths in cardiac muscle, it remains to be seen to which degree titin and/or extracellular structures are important for the physiological passive-tension generation of whole skeletal muscle.

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.