Lateral transmission of force is impaired in skeletal muscles of dystrophic mice and very old rats

Treadmill running and mechanical overloading improved the strength of the plantaris muscle in the dystrophin-desmin double knockout (DKO) mouse

Limited knowledge exists regarding the chronic effect of muscular exercise on muscle function in a murine model of severe Duchenne muscular dystrophy (DMD). Here we determined the effects of 1 month of voluntary wheel running (WR), 1 month of enforced treadmill running (TR) and 1 month of mechanical overloading resulting from the removal of the synergic muscles (OVL) in mice lacking both dystrophin and desmin (DKO). Additionally, we examined the effect of activin receptor administration (AR). DKO mice, displaying severe muscle weakness, atrophy and greater susceptibility to contraction-induced functional loss, were exercised or treated with AR at 1 month of age and in situ force production of lower leg muscle was measured at the age of 2 months. We found that TR and OVL increased absolute maximal force and the rate of force development of the plantaris muscle in DKO mice. In contrast, those of the tibialis anterior (TA) muscle remained unaffected by TR and WR. Furthermore, the effects of TR and OVL on plantaris muscle function in DKO mice closely resembled those in mdx mice, a less severe murine DMD model. AR also improved absolute maximal force and the rate of force development of the TA muscle in DKO mice. In conclusion, exercise training improved plantaris muscle weakness in severely affected dystrophic mice. Consequently, these preclinical results may contribute to fostering further investigations aimed at assessing the potential benefits of exercise for DMD patients, particularly resistance training involving a low number of intense muscle contractions. KEY POINTS: Very little is known about the effects of exercise training in a murine model of severe Duchenne muscular dystrophy (DMD). One reason is that it is feared that chronic muscular exercise, particularly that involving intense muscle contractions, could exacerbate the disease. In DKO mice lacking both dystrophin and desmin, characterized by severe lower leg muscle weakness, atrophy and fragility in comparison to the less severe DMD mdx model, we found that enforced treadmill running improved absolute maximal force of the plantaris muscle, while that of tibialis anterior muscle remained unaffected by both enforced treadmill and voluntary wheel running. Furthermore, mechanical overloading, a non-physiological model of chronic resistance exercise, reversed plantaris muscle weakness. Consequently, our findings may have the potential to alleviate concerns and pave the way for exploring the prescription of endurance and resistance training as a viable therapeutic approach for the treatment of dystrophic patients. Additionally, such interventions may serve in mitigating the pathophysiological mechanisms induced by physical inactivity.

Bioreactor development for skeletal muscle hypertrophy and atrophy by manipulating uniaxial cyclic strain: proof of concept

Skeletal muscles overcome terrestrial, gravitational loading by producing tensile forces that produce movement through joint rotation. Conversely, the microgravity of spaceflight reduces tensile loads in working skeletal muscles, causing an adaptive muscle atrophy. Unfortunately, the design of stable, physiological bioreactors to model skeletal muscle tensile loading during spaceflight experiments remains challenging. Here, we tested a bioreactor that uses initiation and cessation of cyclic, tensile strain to induce hypertrophy and atrophy, respectively, in murine lineage (C2C12) skeletal muscle myotubes. Uniaxial cyclic stretch of myotubes was conducted using a StrexCell® (STB-1400) stepper motor system (0.75 Hz, 12% strain, 60 min day^-1). Myotube groups were assigned as follows: (a) quiescent over 2- or (b) 5-day (no stretch), (c) experienced 2-days (2dHY) or (d) 5-days (5dHY) of cyclic stretch, or (e) 2-days of cyclic stretch followed by a 3-day cessation of stretch (3dAT). Using ß-sarcoglycan as a sarcolemmal marker, mean myotube diameter increased significantly following 2dAT (51%) and 5dAT (94%) vs. matched controls. The hypertrophic, anabolic markers talin and Akt phosphorylation (Thr308) were elevated with 2dHY but not in 3dAT myotubes. Inflammatory, catabolic markers IL-1ß, IL6, and NF-kappaB p65 subunit were significantly higher in the 3dAT group vs. all other groups. The ratio of phosphorylated FoxO3a/total FoxO3a was significantly lower in 3dAT than in the 2dHY group, consistent with elevated catabolic signaling during unloading. In summary, we demonstrated proof-of-concept for a spaceflight research bioreactor, using uniaxial cyclic stretch to produce myotube hypertrophy with increased tensile loading, and myotube atrophy with subsequent cessation of stretch.

Glycative stress inhibits hypertrophy and impairs cell membrane integrity in overloaded mouse skeletal muscle

BACKGROUND

Glycative stress, characterized by the formation and accumulation of advanced glycation end products (AGEs) associated with protein glycation reactions, has been implicated in inducing a decline of muscle function. Although the inverse correlation between glycative stress and muscle mass and strength has been demonstrated, the underlying molecular mechanisms are not fully understood. This study aimed to elucidate how glycative stress affects the skeletal muscle, particularly the adaptive muscle response to hypertrophic stimuli and its molecular mechanism.

METHODS

Male C57BL/6NCr mice were randomly divided into the following two groups: the bovine serum albumin (BSA)-treated and AGE-treated groups. Mice in the AGE-treated group were intraperitoneally administered AGEs (0.5 mg/g) once daily, whereas those in the BSA-treated group received an equal amount of BSA (0.5 mg/g) as the vehicle control. After 7 days of continuous administration, the right leg plantaris muscle of mice in each group underwent functional overload treatment by synergist ablation for 7 days to induce muscle hypertrophy. In in vitro studies, cultured C2C12 myocytes were treated with AGEs (1 mg/mL) to examine cell adhesion and cell membrane permeability.

RESULTS

Continuous AGE administration increased the levels of fluorescent AGEs, Nε-(carboxymethyl) lysine, and methylglyoxal-derived hydroimidazolone-1 in both plasma and skeletal muscle. Plantaris muscle weight, muscle fibre cross-sectional area, protein synthesis rate, and the number of myonuclei increased with functional overload in both groups; however, the increase was significantly reduced by AGE treatment. Some muscles of AGE-treated mice were destroyed by functional overload. Proteomic analysis was performed to explore the mechanisms of muscle hypertrophy suppression and myofibre destruction by AGEs. When principal component analysis was performed on 4659 data obtained by proteomic analysis, AGE treatment was observed to affect protein expression only in functionally overloaded muscles. Enrichment analysis of the 436 proteins extracted using the K-means method further identified a group of proteins involved in cell adhesion. Consistent with this finding, dystrophin-glycoprotein complex proteins and cell adhesion-related proteins were confirmed to increase with functional overload; however, this was attenuated by AGE treatment. Additionally, the treatment of C2C12 muscle cells with AGEs inhibited their ability to adhere and increased cell membrane permeability.

CONCLUSIONS

This study indicates that glycative stress may be a novel pathogenic factor in skeletal muscle dysfunctions by causing loss of membrane integrity and preventing muscle mass gain.

The role of the extracellular matrix in the reduction of lateral force transmission in muscle bundles: A finite element analysis

BACKGROUND AND OBJECTIVE

Aging is associated with a reduction in muscle performance, but muscle weakness is characterized by a much greater loss of force loss compared to mass loss. The aim of this work is to assess the contribution of the extracellular matrix (ECM) to the lateral transmission of force in humans and the loss of transmitted force due to age-related modifications.

METHODS

Finite element models of muscle bundles are developed for young and elderly human subjects, by considering a few fibers connected through an ECM layer. Bundles of young and elderly subjects are assumed to differ in terms of ECM thickness, as observed experimentally. A three-element-based Hill model is adopted to describe the active behavior of muscle fibers, while the ECM is modeled assuming an isotropic hyperelastic neo-Hookean constitutive formulation. Numerical analyses are carried out by mimicking, at the scale of a bundle, two experimental protocols from the literature.

RESULTS

When comparing numerical results obtained for bundles of young and elderly subjects, a greater reduction in the total transmitted force is observed in the latter. The loss of transmitted force is 22 % for the elderly subjects, while it is limited to 7.5 % for the young subjects. The result for the elderly subjects is in line with literature studies on animal models, showing a reduction in the range of 20-34 %. This can be explained by an alteration in the mechanism of lateral force transmission due to the lower shear stiffness of the ECM in elderly subjects, related to its higher thickness.

CONCLUSIONS

Computational modeling allows to evaluate at the bundle level how the age-related increase of the ECM amount between fibers affects the lateral transmission of force. The results suggest that the observed increase in ECM thickness in aging alone can explain the reduction of the total transmitted force, due to the impaired lateral transmission of force of each fiber.

Muscle Morphology Does Not Solely Determine Knee Flexion Weakness After Anterior Cruciate Ligament Reconstruction with a Semitendinosus Tendon Graft: A Combined Experimental and Computational Modeling Study

The distal semitendinosus tendon is commonly harvested for anterior cruciate ligament reconstruction, inducing substantial morbidity at the knee. The aim of this study was to probe how morphological changes of the semitendinosus muscle after harvest of its distal tendon for anterior cruciate ligament reconstruction affects knee flexion strength and whether the knee flexor synergists can compensate for the knee flexion weakness. Ten participants 8-18 months after anterior cruciate ligament reconstruction with an ipsilateral distal semitendinosus tendon autograft performed isometric knee flexion strength testing (15°, 45°, 60°, and 90°; 0° = knee extension) positioned prone on an isokinetic dynamometer. Morphological parameters extracted from magnetic resonance images were used to inform a musculoskeletal model. Knee flexion moments estimated by the model were then compared with those measured experimentally at each knee angle position. A statistically significant between-leg difference in experimentally-measured maximal isometric strength was found at 60° and 90°, but not 15° or 45°, of knee flexion. The musculoskeletal model matched the between-leg differences observed in experimental knee flexion moments at 15° and 45° but did not well estimate between-leg differences with a more flexed knee, particularly at 90°. Further, the knee flexor synergists could not physiologically compensate for weakness in deep knee flexion. These results suggest additional factors other than knee flexor muscle morphology play a role in knee flexion weakness following anterior cruciate ligament reconstruction with a distal semitendinosus tendon graft and thus more work at neural and microscopic levels is required for informing treatment and rehabilitation in this demographic.

Multimodal and conventional resistance training interventions improve muscle function in older adults: Findings from the Training IMCT study

Age-associated remodeling processes affect the intramuscular connective tissue (IMCT) network, which may significantly impair muscle function. Thus, we aimed to test whether including exercises shown to efficiently target the IMCT to a conventional resistance exercise intervention (CONV) would result in greater functional gains as compared to CONV alone. Fifty-three men and women (66.2 ± 3.3 years) were assigned to either CONV (n = 15), multimodal training (MULTI; n = 17) or a control (CTRL; n = 21) group. All subjects were tested at baseline, and those assigned to CONV or MULTI underwent a 16-week training intervention. The CONV group followed a progressive resistance training program, in which the number of weekly training sessions gradually increased from 1 to 3. In the MULTI group, one of these sessions was replaced with plyometric training, followed by self-myofascial release. Testing included maximal strength and power, imaging-based muscle volume, architecture, and functional performance. The intervention effects were analyzed using two- or three-way repeated measures ANOVA models (α = 0.05). Briefly, the maximal knee extension isometric contraction, one-repetition maximum, and isokinetic peak torque increased in all groups (p < 0.05), albeit to a lesser extent in CTRL. On the other hand, quadriceps femoris muscle volume (p = 0.019) and vastus lateralis pennation angle (p < 0.001) increased only in the MULTI group. Handgrip strength did not change in response to the intervention (p = 0.312), whereas Sit-to-Stand performance improved in all groups after the first 8-wks, but only in MULTI and CONV after 16-wks (all p < 0.001). In conclusion, we found that a resistance training intervention in which one weekly training session is replaced by plyometric training is feasible and as effective as a program consisting solely of conventional strength training sessions for inducing gains in muscle strength and function in older adults. Muscle size and architecture improved only in the MULTI group. German Clinical Trials: DRKS00015750.

One Week of Single-Leg Immobilization Lowers Muscle Connective Protein Synthesis Rates in Healthy, Young Adults

PURPOSE

Short periods of limb immobilization lower myofibrillar protein synthesis rates. Within skeletal muscle, the extracellular matrix of connective proteins is recognized as an important factor determining the capacity to transmit contractile force. Little is known regarding the impact of immobilization and subsequent recovery on muscle connective protein synthesis rates. This study examined the impact of 1 wk of leg immobilization and 2 wk of subsequent ambulant recovery on daily muscle connective protein synthesis rates.

METHODS

Thirty healthy, young (24 ± 5 yr) men were subjected to 7 d of one-legged knee immobilization followed by 14 d of ambulant recovery. Deuterium oxide ingestion was applied over the entire period, and muscle biopsy samples were collected before immobilization, after immobilization, and after recovery to measure muscle connective protein synthesis rates and mRNA expression of key extracellular matrix proteins (collagen I, collagen III), glycoproteins (fibronectin, tenascin-C), and proteoglycans (fibromodulin, and decorin). A two-way repeated-measures (time-leg) ANOVA was used to compare changes in muscle connective protein synthesis rates during immobilization and recovery.

RESULTS

During immobilization, muscle connective protein synthesis rates were lower in the immobilized (1.07 ± 0.30%·d -1 ) compared with the nonimmobilized (1.48 ± 0.44%·d -1 ; P < 0.01) leg. When compared with the immobilization period, connective protein synthesis rates in the immobilized leg increased during subsequent recovery (1.48 ± 0.64%·d -1 ; P < 0.01). After recovery, skeletal muscle collagen I, collagen III, fibronectin, fibromodulin, and decorin mRNA expression increased when compared with the postimmobilization time point (all P < 0.001).

CONCLUSIONS

One week of leg immobilization lowers muscle connective protein synthesis rates. Muscle connective protein synthesis rates increase during subsequent ambulant recovery, which is accompanied by increased mRNA expression of key extracellular matrix proteins.

The unconditioned fear response in vertebrates deficient in dystrophin

Dystrophin loss due to mutations in the Duchenne muscular dystrophy (DMD) gene is associated with a wide spectrum of neurocognitive comorbidities, including an aberrant unconditioned fear response to stressful/threat stimuli. Dystrophin-deficient animal models of DMD demonstrate enhanced stress reactivity that manifests as sustained periods of immobility. When the threat is repetitive or severe in nature, dystrophinopathy phenotypes can be exacerbated and even cause sudden death. Thus, it is apparent that enhanced sensitivity to stressful/threat stimuli in dystrophin-deficient vertebrates is a legitimate cause of concern for patients with DMD that could impact neurocognition and pathophysiology. This review discusses our current understanding of the mechanisms and consequences of the hypersensitive fear response in preclinical models of DMD and the potential challenges facing clinical translatability.

The interaction of in vivo muscle operating lengths and passive stiffness in rat hindlimbs

The operating length of a muscle is a key determinant of its ability to produce force in vivo. Muscles that operate near the peak of their force-length relationship will generate higher forces whereas muscle operating at relatively short length may be safe from sudden lengthening perturbations and subsequent damage. At longer lengths, passive mechanical properties have the potential to contribute to force or constrain operating length with stiffer muscle-tendon units theoretically being restricted to shorter lengths. Connective tissues typically increase in density during aging, thus increasing passive muscle stiffness and potentially limiting the operating lengths of muscle during locomotion. Here, we compare in vivo and in situ muscle strain from the medial gastrocnemius in young (7 months old) and aged (30-32 months old) rats presumed to have varying passive tissue stiffness to test the hypothesis that stiffer muscles operate at shorter lengths relative to their force-length relationship. We measured in vivo muscle operating length during voluntary locomotion on inclines and flat trackways and characterized the muscle force-length relationship of the medial gastrocnemius using fluoromicrometry. Although no age-related results were evident, rats of both age groups demonstrated a clear relationship between passive stiffness and in vivo operating length, such that shorter operating lengths were significantly correlated with greater passive stiffness. Our results suggest that increased passive stiffness may restrict muscles to operating lengths shorter than optimal lengths, potentially limiting force capacity during locomotion.

Ex-vivo validation of spatial gain sonography for the quantification of echo intensity in fascicle-aligned ultrasound images in ten anatomical muscles in Bos taurus

This study aimed to validate the concept of spatial gain sonography for quantifying texture-related echo intensity in B-mode ultrasound of skeletal muscle. Fifty-one bovine muscles were scanned postmortem using B-mode ultrasonography at varying fascicle probe angles (FPA). The relationship between mean gray values (MGV) and FPA was fitted with a sinusoidal and a linear function, the slope of which was defined as tilt echo gain (TEG). Macroscopic muscle cross sections were optically analyzed for intramuscular connective tissue (IMCT) content which was plotted against MGV at 0° FPA (MGV_00). MGV peaked at FPA 0°. Sine fits were superior to linear fits (adjusted r2-values 0.647 vs. 0.613), especially for larger FPAs. In mixed models, the pennation angle was related to TEG (P < 0.001) and MGV_00 (P = 0.035). Age was relevant for MGV_00 (P < 0.001), but not TEG (P > 0.10). The correlation between the IMCT percentage and MGV_00 was significant but weak (P = 0.026; adjusted r2 = 0.103). The relationship between fascicle probe angle and echo intensity in B-mode ultrasound can be modeled more accurately with a sinusoidal but more practically for clinical use with a linear fit. The peak mean gray value MGV_00 can be used to compare echo intensity across muscles without the bias of pennation angle.

Re‑examining the mechanism of eccentric exercise‑induced skeletal muscle damage from the role of the third filament, titin (Review)

Intense and unaccustomed eccentric exercise has been extensively studied for its ability to induce muscle damage. However, the underlying mechanism of this phenomenon still requires further clarification. This knowledge gap arises from the need for explanation of the eccentric contraction through the sliding filament theory. The two-filament sarcomere model, which is consisted of thin and thick filaments, forms the basis of the sliding filament theory. The mechanisms of concentric and isometric contractions at the cellular and molecular levels are effectively described by this model. However, when relying solely on the cross-bridge swing, the sliding filament theory fails to account for specific observations, such as the stability of the descending limb of the force-length relationship curve. Recent evidence indicated that titin and the extracellular matrix (ECM) may play a protective role by interacting with the thick and thin filaments. During an eccentric contraction, titin serves as a third filament in the sarcomere, which helps regulate changes in passive force. The two-filament sarcomere model has limitations in explaining eccentric contraction, thus this compensates for those shortcomings. The present review explored the potential of replacing the two-filament sarcomere model with a three-filament sarcomere model, incorporating thin filaments, thick filaments and titin. This revised model offers a more comprehensive explanation of eccentric contraction phenomena. Furthermore, the sliding filament theory was investigated in the context of the three-filament sarcomere model. The double-layer protection mechanism, which involves increased titin stiffness and the ECM during eccentric contraction was explored. This mechanism may enhance lateral force transmission between muscle fibers and the ECM, resulting in sarcolemma and ECM shear deformation. These findings provided insight into the mechanism of eccentric exercise-induced skeletal muscle damage. Considering the three-filament sarcomere model and the double-layer protection mechanism, the present review offered a more logical and comprehensive understanding of the mechanism behind eccentric exercise-induced muscle damage.

On a three-dimensional model for the description of the passive characteristics of skeletal muscle tissue

In this work, a three-dimensional model was developed to describe the passive mechanical behaviour of anisotropic skeletal muscle tissue. To validate the model, orientation-dependent axial ([Formula: see text], [Formula: see text], [Formula: see text]) and semi-confined compression experiments (mode I, II, III) were performed on soleus muscle tissue from rabbits. In the latter experiments, specimen deformation is prescribed in the loading direction and prevented in an additional spatial direction, fibre compression at [Formula: see text] (mode I), fibre elongation at [Formula: see text] (mode II) and a neutral state of the fibres at [Formula: see text] where their length is kept constant (mode III). Overall, the model can adequately describe the mechanical behaviour with a relatively small number of model parameters. The stiffest tissue response during orientation-dependent axial compression ([Formula: see text] kPa) occurs when the fibres are oriented perpendicular to the loading direction ([Formula: see text]) and are thus stretched during loading. Semi-confined compression experiments yielded the stiffest tissue ([Formula: see text] kPa) in mode II when the muscle fibres are stretched. The extensive data set collected in this study allows to study the different error measures depending on the deformation state or the combination of deformation states.

Effects of Aging on Collagen in the Skeletal Muscle of Mice

Aging affects several tissues in the body, including skeletal muscle. Multiple types of collagens are localized in the skeletal muscle and contribute to the maintenance of normal muscle structure and function. Since the effects of aging on muscle fibers vary by muscle fiber type, it is expected that the effects of aging on intramuscular collagen might be influenced by muscle fiber type. In this study, we examined the effect of aging on collagen levels in the soleus (slow-twitch muscle) and gastrocnemius (fast-twitch muscle) muscles of 3-, 10-, 24-, and 28-month-old male C57BL/6J mice using molecular and morphological analysis. It was found that aging increased collagen I, III, and VI gene expression and immunoreactivity in both slow- and fast-twitch muscles and collagen IV expression in slow-twitch muscles. However, collagen IV gene expression and immunoreactivity in fast-twitch muscle were unaffected by aging. In contrast, the expression of the collagen synthesis marker heat shock protein 47 in both slow- and fast-twitch muscles decreased with aging, while the expression of collagen degradation markers increased with aging. Overall, these results suggest that collagen gene expression and immunoreactivity are influenced by muscle fiber type and collagen type and that the balance between collagen synthesis and degradation tends to tilt toward degradation with aging.

The extracellular matrix of dystrophic mouse diaphragm accounts for the majority of its passive stiffness and is resistant to collagenase digestion

The healthy skeletal muscle extracellular matrix (ECM) has several functions including providing structural integrity to myofibers, enabling lateral force transmission, and contributing to overall passive mechanical properties. In diseases such as Duchenne Muscular dystrophy, there is accumulation of ECM materials, primarily collagen, which results in fibrosis. Previous studies have shown that fibrotic muscle is often stiffer than healthy muscle, in part due to the increased number and altered architecture of collagen fibers within the ECM. This would imply that the fibrotic matrix is stiffer than the healthy matrix. However, while previous studies have attempted to quantify the extracellular contribution to passive stiffness in muscle, the outcomes are dependent on the type of method used. Thus, the goals of this study were to compare the stiffness of healthy and fibrotic muscle ECM and to demonstrate the efficacy of two methods for quantifying extracellular-based stiffness in muscle, namely decellularization and collagenase digestion. These methods have been demonstrated to remove the muscle fibers or ablate collagen fiber integrity, respectively, while maintaining the contents of the extracellular matrix. Using these methods in conjunction with mechanical testing on wildtype and D2.mdx mice, we found that a majority of passive stiffness in the diaphragm is dependent on the ECM, and the D2.mdx diaphragm ECM is resistant to digestion by bacterial collagenase. We propose that this resistance is due to the increased collagen cross-links and collagen packing density in the ECM of the D2.mdx diaphragm. Taken altogether, while we did not find increased stiffness of the fibrotic ECM, we did observe that the D2.mdx diaphragm conveyed resistance against collagenase digestion. These findings demonstrate how different methods for measuring ECM-based stiffness each have their own limitations and can produce different results.

Use of a novel technique to assess impact of age-related denervation on mouse soleus muscle function

Denervation contributes to loss of force-generating capacity in aged skeletal muscles, but problems with quantification of denervated fibers mean the precise impact of denervation on muscle function remains unclear. This study therefore looked to develop a reliable assay for identifying denervated muscle fibers, and used this to explore the impact of denervation on age-related force-generation in mouse skeletal muscle. Thirteen young (6-month-old) and 10 old (24-months-old) C57Bl/6 J female mice were utilized. Anaesthetized mice were infused with the fluorescent deoxyglucose analog 2[N-(7-nitrobenz-2-oxa-1,2-diaxol-4-yl)amino]-2-deoxyglucose (2-NBDG) and the tibial nerve was repeatedly stimulated to label active skeletal muscle fibers by activity-dependent uptake of 2-NBDG. Data on muscle force generation were acquired as part of the stimulation routine. Labeled muscles were removed, snap frozen, sectioned, and slide mounted. Sections were imaged to show accumulation of 2-NBDG in activated fibers and lack of 2-NBDG accumulation in quiescent (denervated) fibers, then processed using immunohistochemistry to allow collection of data on fiber number and morphology. Soleus muscles from older mice had nine times as many denervated fibers as those from young mice (average n = 36 vs 4, old vs young). Older muscles developed significantly more passive force and less specific force, but denervation only partly accounted for age-related deficits in specific force. Further investigations are required to definitively identify contributors to the decrease in force generation that remain unaccounted for.

In vivoestimation of anisotropic mechanical properties of the gastrocnemius during functional loading with MR elastography

Objective.In vivoimaging assessments of skeletal muscle structure and function allow for longitudinal quantification of tissue health. Magnetic resonance elastography (MRE) non-invasively quantifies tissue mechanical properties, allowing for evaluation of skeletal muscle biomechanics in response to loading, creating a better understanding of muscle functional health.Approach. In this study, we analyze the anisotropic mechanical response of calf muscles using MRE with a transversely isotropic, nonlinear inversion algorithm (TI-NLI) to investigate the role of muscle fiber stiffening under load. We estimate anisotropic material parameters including fiber shear stiffness (μ1), substrate shear stiffness (μ2), shear anisotropy (ϕ), and tensile anisotropy (ζ) of the gastrocnemius muscle in response to both passive and active tension.Main results. In passive tension, we found a significant increase inμ1,ϕ,andζwith increasing muscle length. While in active tension, we observed increasingμ2and decreasingϕandζduring active dorsiflexion and plantarflexion-indicating less anisotropy-with greater effects when the muscles act as agonist.Significance. The study demonstrates the ability of this anisotropic MRE method to capture the multifaceted mechanical response of skeletal muscle to tissue loading from muscle lengthening and contraction.

Eccentric contraction-induced strength loss in dystrophin-deficient muscle: Preparations, protocols, and mechanisms

The absence of dystrophin hypersensitizes skeletal muscle of lower and higher vertebrates to eccentric contraction (ECC)-induced strength loss. Loss of strength can be accompanied by transient and reversible alterations to sarcolemmal excitability and disruption, triad dysfunction, and aberrations in calcium kinetics and reactive oxygen species production. The degree of ECC-induced strength loss, however, appears dependent on several extrinsic and intrinsic factors such as vertebrate model, skeletal muscle preparation (in vivo, in situ, or ex vivo), skeletal muscle hierarchy (single fiber versus whole muscle and permeabilized versus intact), strength production, fiber branching, age, and genetic background, among others. Consistent findings across research groups show that dystrophin-deficient fast(er)-twitch muscle is hypersensitive to ECCs relative to wildtype muscle, but because preparations are highly variable and sensitivity to ECCs are used repeatedly to determine efficacy of many preclinical treatments, it is critical to evaluate the impact of skeletal muscle preparations on sensitivity to ECC-induced strength loss in dystrophin-deficient skeletal muscle. Here, we review and discuss variations in skeletal muscle preparations to evaluate the factors responsible for variations and discrepancies between research groups. We further highlight that dystrophin-deficiency, or loss of the dystrophin-glycoprotein complex in skeletal muscle, is not a prerequisite for accelerated strength loss-induced by ECCs.

VEGFA rs2010963 GG genotype is associated with superior adaptations to resistance versus endurance training in the same group of healthy, young men

PURPOSE

We used a within-subject, cross-over study to determine the relationship between the intra-individual adaptations to four weeks' resistance (RT) versus four weeks' endurance (END) training, and we investigated whether three single nucleotide polymorphisms (SNPs) were associated with these adaptations.

METHODS

Thirty untrained, healthy, young men completed a cycling test to exhaustion to determine peak oxygen uptake (V̇O_2peak), and a knee extension (KE) maximum voluntary isometric contraction (MVIC) of the right leg before and after four weeks' supervised RT (four sets of 10 repetitions at 80% single repetition maximum unilateral KE exercise, three times weekly) and four weeks' supervised END (30 min combined continuous/interval cycling, three times weekly), separated by a three-week washout phase. Participants were genotyped for the ACTN3 rs1815739, NOS3 rs2070744 and VEGFA rs2010963 SNPs.

RESULTS

The intra-individual adaptations regarding percentage changes in MVIC force and V̇O_2peak following RT and END, respectively, were unrelated (r² = 0.003; P = 0.79). However, a VEGFA genotype × training modality interaction (P = 0.007) demonstrated that VEGFA GG homozygotes increased their MVIC force after RT (+ 20.9 ± 13.2%) more than they increased their V̇O_2peak after END (+ 8.4 ± 9.1%, P = 0.005), and more than VEGFA C-allele carriers increased their MVIC force after RT (+ 12.2 ± 8.1%, P = 0.04). There were no genotype × training modality interactions for the ACTN3 or NOS3 SNPs.

CONCLUSION

High/low responders to RT were not consequently high/low responders to END or vice versa. However, preferential adaptation of VEGFA rs2010963 GG homozygotes to RT over END, and their greater adaptation to RT compared to VEGFA C-allele carriers, indicate a novel genetic predisposition for superior RT adaptation.

Skeletal Muscle Stem Cells in Aging: Asymmetric/Symmetric Division Switching

In aged muscle, satellite cells’ symmetric and asymmetric divisions are impaired, and intrinsic and extrinsic complex mechanisms govern these processes. This review presents many updated aspects regarding muscle stem cells’ fate in normal and aging conditions. The balance between self-renewal and commitment divisions contributes to muscle regeneration, muscle homeostasis, aging, and disease. Stimulating muscle regeneration in aging could be a therapeutic target, but there is still a need to understand the many mechanisms that influence each other in satellite cells and their niche. We highlight here the general outlines regarding satellite cell divisions, the primary markers present in muscle stem cells, the aging aspects concerning signaling pathways involved in symmetric/asymmetric divisions, the regenerative capacity of satellite cells and their niche alteration in senescent muscle, genetics and epigenetics mechanisms implied in satellite cells aging and exercise effect on muscle regeneration in the elderly.

The Effects of Aging on the Intramuscular Connective Tissue

The intramuscular connective tissue plays a critical role in maintaining the structural integrity of the muscle and in providing mechanical support. The current study investigates age-related changes that may contribute to passive stiffness and functional impairment of skeletal muscles. Variations in the extracellular matrix in human quadriceps femoris muscles in 10 young men, 12 elderly males and 16 elderly females, and in the hindlimb muscles of 6 week old, 8 month old and 2 year old C57BL/6J male mice, were evaluated. Picrosirius red, Alcian blue and Weigert Van Gieson stainings were performed to evaluate collagen, glycosamynoglycans and elastic fibers. Immunohistochemistry analyses were carried out to assess collagen I, collagen III and hyaluronan. The percentage area of collagen was significantly higher with aging (p < 0.01 in humans, p < 0.001 in mice), mainly due to an increase in collagen I, with no differences in collagen III (p > 0.05). The percentage area of elastic fibers in the perimysium was significantly lower (p < 0.01) in elderly men, together with a significant decrease in hyaluronan content both in humans and in mice. No significant differences were detected according to gender. The accumulation of collagen I and the lower levels of hyaluronan and elastic fibers with aging could cause a stiffening of the muscles and a reduction of their adaptability.

Stretching muscle cells induces transcriptional and splicing transitions and changes in SR proteins

Alternative splicing is an RNA processing mechanism involved in skeletal muscle development and pathology. Muscular diseases exhibit splicing alterations and changes in mechanobiology leading us to investigate the interconnection between mechanical forces and RNA processing. We performed deep RNA-sequencing after stretching muscle cells. First, we uncovered transcriptional changes in genes encoding proteins involved in muscle function and transcription. Second, we observed that numerous mechanosensitive genes were part of the MAPK pathway which was activated in response to stretching. Third, we revealed that stretching skeletal muscle cells increased the proportion of alternatively spliced cassette exons and their inclusion. Fourth, we demonstrated that the serine and arginine-rich proteins exhibited stronger transcriptional changes than other RNA-binding proteins and that SRSF4 phosphorylation is mechanosensitive. Identifying SRSF4 as a mechanosensitive RNA-binding protein that might contribute to crosstalk between mechanotransduction, transcription, and splicing could potentially reveal novel insights into muscular diseases, particularly those with unknown etiologies.

Characterisation of Progressive Skeletal Muscle Fibrosis in the Mdx Mouse Model of Duchenne Muscular Dystrophy: An In Vivo and In Vitro Study

Duchenne muscular dystrophy (DMD) is a rare genetic disease leading to progressive muscle wasting, respiratory failure, and cardiomyopathy. Although muscle fibrosis represents a DMD hallmark, the organisation of the extracellular matrix and the molecular changes in its turnover are still not fully understood. To define the architectural changes over time in muscle fibrosis, we used an mdx mouse model of DMD and analysed collagen and glycosaminoglycans/proteoglycans content in skeletal muscle sections at different time points during disease progression and in comparison with age-matched controls. Collagen significantly increased particularly in the diaphragm, quadriceps, and gastrocnemius in adult mdx, with fibrosis significantly correlating with muscle degeneration. We also analysed collagen turnover pathways underlying fibrosis development in cultured primary quadriceps-derived fibroblasts. Collagen secretion and matrix metalloproteinases (MMPs) remained unaffected in both young and adult mdx compared to wt fibroblasts, whereas collagen cross-linking and tissue inhibitors of MMP (TIMP) expression significantly increased. We conclude that, in the DMD model we used, fibrosis mostly affects diaphragm and quadriceps with a higher collagen cross-linking and inhibition of MMPs that contribute differently to progressive collagen accumulation during fibrotic remodelling. This study offers a comprehensive histological and molecular characterisation of DMD-associated muscle fibrosis; it may thus provide new targets for tailored therapeutic interventions.

Intramuscular connective tissue content and mechanical properties: Influence of aging and physical activity in mice

Aging is accompanied by morphological and mechanical changes to the intramuscular connective tissue (IMCT) of skeletal muscles, but whether physical exercise can influence these changes is debated. We investigated the effects of aging and exercise with high or low resistance on composition and mechanical properties of the IMCT, including direct measurements on isolated IMCT which has rarely been reported. Middle-aged (11 months, n = 24) and old (22 months, n = 18) C57BL/6 mice completed either high (HR) or low (LR) resistance voluntary wheel running or were sedentary (SED) for 10 weeks. Passive mechanical properties of the intact soleus and plantaris muscles and the isolated IMCT of the plantaris muscle were measured in vitro. IMCT thickness was measured on picrosirius red stained cross sections of the gastrocnemius and soleus muscle and for the gastrocnemius hydroxyproline content was quantified biochemically and advanced glycation end-products (AGEs) estimated by fluorometry. Mechanical stiffness, IMCT content and total AGEs were all elevated with aging in agreement with previous findings but were largely unaffected by training. Conclusion: IMCT accumulated with aging with a proportional increase in mechanical stiffness, but even the relatively high exercise volume achieved with voluntary wheel-running with or without resistance did not significantly influence these changes.

Ubiquitin Ligases in Longevity and Aging Skeletal Muscle

The development and prevalence of diseases associated with aging presents a global health burden on society. One hallmark of aging is the loss of proteostasis which is caused in part by alterations to the ubiquitin-proteasome system (UPS) and lysosome-autophagy system leading to impaired function and maintenance of mass in tissues such as skeletal muscle. In the instance of skeletal muscle, the impairment of function occurs early in the aging process and is dependent on proteostatic mechanisms. The UPS plays a pivotal role in degradation of misfolded and aggregated proteins. For the purpose of this review, we will discuss the role of the UPS system in the context of age-related loss of muscle mass and function. We highlight the significant role that E3 ubiquitin ligases play in the turnover of key components (e.g., mitochondria and neuromuscular junction) essential to skeletal muscle function and the influence of aging. In addition, we will briefly discuss the contribution of the UPS system to lifespan. By understanding the UPS system as part of the proteostasis network in age-related diseases and disorders such as sarcopenia, new discoveries can be made and new interventions can be developed which will preserve muscle function and maintain quality of life with advancing age.

Multiscale modeling of passive material influences on deformation and force output of skeletal muscles

Passive materials in human skeletal muscle tissues play an important role in force output of skeletal muscles. This paper introduces a multiscale modeling framework to investigate how age-associated variations on microscale passive muscle components, including microstructural geometry (e.g., connective tissue thickness) and material properties (e.g., anisotropy), influence the force output and deformations of the continuum skeletal muscle. We first define a representative volume element (RVE) for the microstructure of muscle and determine the homogenized macroscale mechanical properties of the RVE from the separate mechanical properties of the individual components of the RVE, including muscle fibers and connective tissue with its associated collagen fibers. The homogenized properties of the RVE are then used to define the elements of the continuum muscle model to evaluate the force output and deformations of the whole muscle. Conversely, the regional deformations of the continuum model are fed back to the RVE model to determine the responses of the individual microscale components. Simulations of muscle isometric contractions at a range of muscle lengths are performed to investigate the effects of muscle architectural changes (e.g., pennation angles) due to aging on force output and muscle deformation. The correlations between the pennation angle, the shear deformation in the microscale connective tissue (an indicator for the lateral force transmission), the angle difference between the fiber direction and principal strain direction and the resulting shear deformation at the continuum scale, as well as the force output of the skeletal muscle are also discussed.

Beneficial impacts of neuromuscular electrical stimulation on muscle structure and function in the zebrafish model of Duchenne muscular dystrophy

Neuromuscular electrical stimulation (NMES) allows activation of muscle fibers in the absence of voluntary force generation. NMES could have the potential to promote muscle homeostasis in the context of muscle disease, but the impacts of NMES on diseased muscle are not well understood. We used the zebrafish Duchenne muscular dystrophy (dmd) mutant and a longitudinal design to elucidate the consequences of NMES on muscle health. We designed four neuromuscular stimulation paradigms loosely based on weightlifting regimens. Each paradigm differentially affected neuromuscular structure, function, and survival. Only endurance neuromuscular stimulation (eNMES) improved all outcome measures. We found that eNMES improves muscle and neuromuscular junction morphology, swimming, and survival. Heme oxygenase and integrin alpha7 are required for eNMES-mediated improvement. Our data indicate that neuromuscular stimulation can be beneficial, suggesting that the right type of activity may benefit patients with muscle disease.

[Feeling stiff…but what does it mean objectively? : Can you measure muscle tension?]

Almost everyone is familiar with "tense muscles", but what is muscle tension physiologically behind? Are tense muscles more active; do they have problems relaxing? Are they harder or stiffer than asymptomatic muscles? In this work, current evidence regarding the activity and stiffness of tense neck muscles is presented. Further, measurement methods and their limitations are explained. These limitations reveal the shortcomings of the current knowledge and the need for further research. Finally, a recently funded research project on the measurement of tense muscles is presented.

Diaphragm muscle fibrosis involves changes in collagen organization with mechanical implications in Duchenne muscular dystrophy

In Duchenne muscular dystrophy (DMD), diaphragm muscle dysfunction results in respiratory insufficiency, a leading cause of death in patients. Increased muscle stiffness occurs with buildup of fibrotic tissue, characterized by excessive accumulation of extracellular matrix (ECM) components such as collagen, and prevents the diaphragm from achieving the excursion lengths required for respiration. However, changes in mechanical properties are not explained by collagen amount alone and we must consider the complex structure and mechanics of fibrotic tissue. The goals of our study were to 1) determine if and how collagen organization changes with the progression of DMD in diaphragm muscle tissue and 2) predict how collagen organization influences the mechanical properties of the ECM. We first visualized collagen structure with scanning electron microscopy (SEM) images and then developed an analysis framework to quantify collagen organization and generate image-based finite-element models. Image analysis revealed increased collagen fiber straightness and alignment in mdx over wild type (WT) at 3 mo (straightness: mdx = 0.976 ± 0.0108, WT = 0.887 ± 0.0309, alignment: mdx = 0.876 ± 0.0333, WT = 0.759 ± 0.0416) and 6 mo (straightness: mdx = 0.942 ± 0.0182, WT = 0.881 ± 0.0163, alignment: mdx = 0.840 ± 0.0315, WT = 0.759 ± 0.0368). Collagen fibers retained a transverse orientation relative to muscle fibers (70°-90°) in all groups. Mechanical models predicted an increase in the transverse relative to longitudinal (muscle fiber direction) stiffness, with stiffness ratio (transverse/longitudinal) increased in mdx over WT at 3 mo (mdx = 5.45 ± 2.04, WT = 1.97 ± 0.670) and 6 mo (mdx = 4.05 ± 0.985, WT = 1.96 ± 0.506). This study revealed changes in diaphragm ECM structure and mechanics during disease progression in the mdx muscular dystrophy mouse phenotype, highlighting the need to consider the role of collagen organization on diaphragm muscle function.NEW & NOTEWORTHY Scanning electron microscopy images of decellularized diaphragm muscle from WT and mdx, Duchenne muscular dystrophy model, mice revealed that collagen fibers in the epimysium are oriented transverse to muscle fibers, with age- and disease-dependent changes in collagen arrangement. Finite-element models generated from these images predicted that changes in collagen arrangement during disease progression influence the mechanical properties of the extracellular matrix. Thus, changes in collagen fiber-level structure are implicated on tissue-level properties during fibrosis.

Spin Lattice (T1) and Magnetization Transfer Saturation (MTsat) Imaging to Monitor Age-Related Differences in Skeletal Muscle Tissue

Background: The aim was to compare spin-lattice relaxation (T1) mapping from sequences with no fat suppression and three fat suppression methods and Magnetization Transfer Saturation (MT_sat) mapping, to identify regional and age-related differences in calf muscle. These differences may be of clinical significance in age-related loss of muscle force. Methods: Ten young and seven senior subjects were imaged on a 3T MRI scanner using a 3D Fast Low Angle Shot sequence without and with different fat suppression and with MT saturation pulse. Bland–Altman plots were used to assess T1 maps using the fat unsuppressed sequence as the reference image. Age and regional differences in T1 and in MT_sat were assessed using two-way factorial analyses of variance (ANOVAs) with Bonferroni-adjusted independent sample t-tests for post hoc analyses. Results: A significant age-related increase in T1 and decrease in MT_sat was seen in the calf muscles. The largest size effect was observed in the T1 sequence with fat saturation. Conclusions: T1 increase with age may reflect increase in inflammatory processes while the decrease in MTsat may indicate that magnetization transfer may also be associated with muscle fiber macromolecules. T1 and MT_sat maps of calf muscle have the potential to detect regional and age-related compositional differences in calf muscle.

Reversal of deficits in aged skeletal muscle during disuse and recovery in response to treatment with a secrotome product derived from partially differentiated human pluripotent stem cells

Aged individuals are at risk to experience slow and incomplete muscle recovery following periods of disuse atrophy. While several therapies have been employed to mitigate muscle mass loss during disuse and improve recovery, few have proven effective at both. Therefore, the purpose of this study was to examine the effectiveness of a uniquely developed secretome product (STEM) on aged skeletal muscle mass and function during disuse and recovery. Aged (22 months) male C57BL/6 were divided into PBS or STEM treatment (n = 30). Mice within each treatment were assigned to either ambulatory control (CON; 14 days of normal cage ambulation), 14 days of hindlimb unloading (HU), or 14 days of hindlimb unloading followed by 7 days of recovery (recovery). Mice were given an intramuscular delivery into the hindlimb muscle of either PBS or STEM every other day for the duration of their respective treatment group. We found that STEM-treated mice compared to PBS had greater soleus muscle mass, fiber cross-sectional area (CSA), and grip strength during CON and recovery experimental conditions and less muscle atrophy and weakness during HU. Muscle CD68 +, CD11b + and CD163 + macrophages were more abundant in STEM-treated CON mice compared to PBS, while only CD68 + and CD11b + macrophages were more abundant during HU and recovery conditions with STEM treatment. Moreover, STEM-treated mice had lower collagen IV and higher Pax7 + cell content compared to PBS across all experimental conditions. As a follow-up to examine the cell autonomous role of STEM on muscle, C2C12 myotubes were given STEM or horse serum media to examine myotube fusion/size and effects on muscle transcriptional networks. STEM-treated C2C12 myotubes were larger and had a higher fusion index and were related to elevated expression of transcripts associated with extracellular matrix remodeling. Our results demonstrate that STEM is a unique cocktail that possesses potent immunomodulatory and cytoskeletal remodeling properties that may have translational potential to improve skeletal muscle across a variety of conditions that adversely effect aging muscle.

Measurement of Lateral Transmission of Force in the Extensor Digitorum Longus Muscle of Young and Old Mice

The main function of skeletal muscles is to generate force. The force developed by myofiber contraction is transmitted to the tendon. There are two pathways of force transmission from myofibers to tendons: longitudinal transmission that depends on tension elicited via the myotendinous junction and lateral transmission that depends on shear elicited via the interface between the myofiber surface and surrounding connective tissue. Experiments using animal muscle and mathematical models indicated that lateral transmission is the dominant pathway in muscle force transmission. Studies using rat muscle showed that the efficiency of lateral force transmission declines with age. Here, the lateral transmission of force was measured using the extensor digitorum longus muscle from young and old mice. Dependence on longitudinal transmission increased in the old muscle, and there was a trend for lower efficiency of lateral force transmission in the old muscle compared to the young muscle. There was a noticeable increase in the connective tissue volume in the old muscle; however, there was no significant change in the expression of dystrophin, a critical molecule for the link between the myofiber cytoskeleton and extracellular matrix. This study demonstrates the measurement of lateral force transmission in mouse muscles and that alteration in force transmission property may underlie age-related muscle weakness.

Age-dependent mechanical and microstructural properties of the rabbit soleus muscle

During growth there are serious changes in the skeletal muscles to compensate for the changed requirements in terms of body weight and size. In this study, the age-dependent (between 21 and 100 days) mechanical and microstructural properties of rabbit soleus muscle tissue were investigated. For this purpose, morphological properties (animal mass, soleus muscle mass, tibial length) were measured at 5 different times during aging. On the other hand, fibre orientation-dependent axial and semi-confined compression experiments were realised. In addition, the essential components (muscle fibres, extracellular matrix, remaining components), dominating the microstructure of muscle tissue, were analysed. While the mechanical results show hardly any age-dependent differences, the morphological and microstructural results show clear age-dependent differences. All morphological parameters increase significantly (animal mass by 839.2%, muscle mass 1050.6%, tibial length 233.6%). In contrast, microstructural parameters change differently. The percentage of fibres (divided into slow-twitch (ST) and fast-twitch (FT) fibres) increases significantly (137.6%), while the proportions of the extracellular matrix and the remaining components (48.2% and 46.1%) decrease. At the same time, the cross-sectional area of the fibres increases significantly (697.9%). The totality of this age-dependent information provides a deeper understanding of age-related changes in muscle structure and function and may contribute to successful development and validation of growth models in the future. STATEMENT OF SIGNIFICANCE: This article reports the first comprehensive data set on age-dependent morphological (animal mass, soleus muscle mass, tibial length), mechanical (axial and semi-confined compression), and microstructural (muscle fibres, extracellular matrix, remaining components) properties of the rabbit soleus muscle. On the one hand, the results of this study contribute to the understanding of muscle mechanics and thus to understanding of load transfer mechanisms inside the muscle tissue during growth. On the other hand, these results are relevant to the fields of constitutive formulation of age-dependent muscle tissue.

The impact of collagen protein ingestion on musculoskeletal connective tissue remodeling: a narrative review

Collagen is the central structural component of extracellular connective tissue, which provides elastic qualities to tissues. For skeletal muscle, extracellular connective tissue transmits contractile force to the tendons and bones. Connective tissue proteins are in a constant state of remodeling and have been shown to express a high level of plasticity. Dietary-protein ingestion increases muscle protein synthesis rates. High-quality, rapidly digestible proteins are generally considered the preferred protein source to maximally stimulate myofibrillar (contractile) protein synthesis rates. In contrast, recent evidence demonstrates that protein ingestion does not increase muscle connective tissue protein synthesis. The absence of an increase in muscle connective tissue protein synthesis after protein ingestion may be explained by insufficient provision of glycine and/or proline. Dietary collagen contains large amounts of glycine and proline and, therefore, has been proposed to provide the precursors required to facilitate connective tissue protein synthesis. This literature review provides a comprehensive evaluation of the current knowledge on the proposed benefits of dietary collagen consumption to stimulate connective tissue remodeling to improve health and functional performance.

Biomechanical Properties of the Sarcolemma and Costameres of Skeletal Muscle Lacking Desmin

Intermediate filaments (IFs), composed primarily by desmin and keratins, link the myofibrils to each other, to intracellular organelles, and to the sarcolemma. There they may play an important role in transfer of contractile force from the Z-disks and M-lines of neighboring myofibrils to costameres at the membrane, across the membrane to the extracellular matrix, and ultimately to the tendon (“lateral force transmission”). We measured the elasticity of the sarcolemma and the connections it makes at costameres with the underlying contractile apparatus of individual fast twitch muscle fibers of desmin-null mice. By positioning a suction pipet to the surface of the sarcolemma and applying increasing pressure, we determined the pressure at which the sarcolemma separated from nearby sarcomeres, P_separation, and the pressure at which the isolated sarcolemma burst, P_bursting. We also examined the time required for the intact sarcolemma-costamere-sarcomere complex to reach equilibrium at lower pressures. All measurements showed the desmin-null fibers to have slower equilibrium times and lower P_separation and P_bursting than controls, suggesting that the sarcolemma and its costameric links to nearby contractile structures were weaker in the absence of desmin. Comparisons to earlier values determined for muscles lacking dystrophin or synemin suggest that the desmin-null phenotype is more stable than the former and less stable than the latter. Our results are consistent with the moderate myopathy seen in desmin-null muscles and support the idea that desmin contributes significantly to sarcolemmal stability and lateral force transmission.

Morphological Changes in the Myotendinous Junction of mdx Mice

The myotendinous junction (MTJ) is the interface between muscle and tendon, and it is the main area of force transmission of the locomotor apparatus. Dystrophic processes promote pathological injury which affects the skeletal muscle and can influence the morphology of the MTJ. This study aimed to investigate the adaptations in MTJ morphology of mdx mice in the tibialis anterior muscle. Male mice (n = 24) were divided into Control—C57bl/10 and mdx—C57bl/10mdx (Duchenne muscular dystrophy experimental model). In the mdx group, centralized nuclei with a large area and greater deposition of type III collagen (fibrosis) were observed. Also, shorter sarcomeres and sarcoplasmatic projections of MTJ were observed. We concluded that the adaptations in mdx mice demonstrated extensive impairment in the MTJ region with reduced ultrastructures.

Biochemical and structural basis of the passive mechanical properties of whole skeletal muscle

Passive mechanical properties of whole skeletal muscle are not as well understood as active mechanical properties. Both the structural basis for passive mechanical properties and the properties themselves are challenging to determine because it is not clear which structures within skeletal muscle actually bear passive loads and there are not established standards by which to make mechanical measurements. Evidence suggests that titin bears the majority of the passive load within the single muscle cell. However, at larger scales, such as fascicles and muscles, there is emerging evidence that the extracellular matrix bears the major part of the load. Complicating the ability to quantify and compare across size scales, muscles and species, definitions of muscle passive properties such as stress, strain, modulus and stiffness can be made relative to many reference parameters. These uncertainties make a full understanding of whole muscle passive mechanical properties and modelling these properties very difficult. Future studies defining the specific load bearing structures and their composition and organization are required to fully understand passive mechanics of the whole muscle and develop therapies to treat disorders in which passive muscle properties are altered such as muscular dystrophy, traumatic laceration, and contracture due to upper motor neuron lesion as seen in spinal cord injury, stroke and cerebral palsy.

The Neuromuscular Junction: Roles in Aging and Neuromuscular Disease

The neuromuscular junction (NMJ) is a specialized synapse that bridges the motor neuron and the skeletal muscle fiber and is crucial for conversion of electrical impulses originating in the motor neuron to action potentials in the muscle fiber. The consideration of contributing factors to skeletal muscle injury, muscular dystrophy and sarcopenia cannot be restricted only to processes intrinsic to the muscle, as data show that these conditions incur denervation-like findings, such as fragmented NMJ morphology and corresponding functional changes in neuromuscular transmission. Primary defects in the NMJ also influence functional loss in motor neuron disease, congenital myasthenic syndromes and myasthenia gravis, resulting in skeletal muscle weakness and heightened fatigue. Such findings underscore the role that the NMJ plays in neuromuscular performance. Regardless of cause or effect, functional denervation is now an accepted consequence of sarcopenia and muscle disease. In this short review, we provide an overview of the pathologic etiology, symptoms, and therapeutic strategies related to the NMJ. In particular, we examine the role of the NMJ as a disease modifier and a potential therapeutic target in neuromuscular injury and disease.

Lower Extremity Muscle Involvement in the Intermediate and Bethlem Myopathy Forms of COL6-Related Dystrophy and Duchenne Muscular Dystrophy: A Cross-Sectional Study

Collagen VI-related dystrophies (COL6-RDs) and Duchenne muscular dystrophy (DMD) cause progressive muscle weakness and disability. COL6-RDs are caused by mutations in the COL6 genes (COL6A1, COL6A2 and COL6A3) encoding the extracellular matrix protein collagen VI, and DMD is caused by mutations in the DMD gene encoding the cytoplasmic protein dystrophin. Both COL6-RDs and DMD are characterized by infiltration of the muscles by fatty and fibrotic tissue. This study examined the effect of disease pathology on skeletal muscles in lower extremity muscles of COL6-RDs using timed functional tests, strength measures and qualitative/ quantitative magnetic resonance imaging/spectroscopy measures (MRI/MRS) in comparison to unaffected (control) individuals. Patients with COL6-RD were also compared to age and gender matched patients with DMD.Patients with COL6-RD presented with a typical pattern of fatty infiltration of the muscle giving rise to an apparent halo effect around the muscle, while patients with DMD had evidence of fatty infiltration throughout the muscle areas imaged. Quantitatively, fat fraction, and transverse relaxation time (T2) were elevated in both COL6-RD and DMD patients compared to unaffected (control) individuals. Patients with COL6-RD had widespread muscle atrophy, likely contributing to weakness. In contrast, patients with DMD revealed force deficits even in muscle groups with increased contractile areas.

Vector-mediated expression of muscle specific kinase restores specific force to muscles in the mdx mouse model of Duchenne muscular dystrophy

NEW FINDINGS

What is the central question of this study? The (dystrophin-deficient) muscles of mdx mice generate less contractile force per cross-sectional area (specific force) than those of healthy wild-type mice: what is the influence of muscle specific kinase (MuSK) upon the properties of the tibialis anterior (TA) muscle in mdx mice? What is the main finding and its importance? Injection of adeno-associated viral vector encoding MuSK into the TA muscle of young mdx mice increased the specific force of the muscle, suggesting the MuSK signalling system has the potential to restore healthy growth to dystrophin-deficient muscles.

ABSTRACT

In the mdx mouse model of Duchenne muscular dystrophy, muscle fibres are fragile and prone to injury and degeneration. Compared to wild-type mice, muscles of mdx mice also develop less specific force (contractile force/cross-sectional area). We recently reported that injecting adeno-associated viral vector encoding muscle specific kinase (AAV-MuSK) into muscles of mdx mice increased utrophin expression and made the muscles more resistant to acute stretch-induced injury. Here we injected AAV-MuSK unilaterally into the tibialis anterior muscle of mdx mice at a younger age (4 weeks), and recorded contraction force from the muscles in situ at 12 weeks of age. Compared to contralateral empty-vector control muscles, muscles injected with AAV-MuSK produced 28% greater specific force (P = 0.0005). They did not undergo the compensatory hypertrophy that normally occurs in muscles of mdx mice. Injection of AAV encoding rapsyn (a downstream effector of MuSK signalling) caused no such improvement in muscle strength. Muscles injected with AAV-MuSK displayed a 10% reduction in the number of fibres with centralized nuclei (P = 0.0015). Our results in mdx mice suggest that elevating the expression of MuSK can reduce the incidence of muscle fibre regeneration and improve the strength of dystrophin-deficient muscles.

One-to-one innervation of vocal muscles allows precise control of birdsong

The motor control resolution of any animal behavior is limited to the minimal force step available when activating muscles, which is set by the number and size distribution of motor units (MUs) and muscle-specific force. Birdsong is an excellent model system for understanding acquisition and maintenance of complex fine motor skills, but we know surprisingly little about how the motor pool controlling the syrinx is organized and how MU recruitment drives changes in vocal output. Here we developed an experimental paradigm to measure MU size distribution using spatiotemporal imaging of intracellular calcium concentration in cross-sections of living intact syrinx muscles. We combined these measurements with muscle stress and an in vitro syrinx preparation to determine the control resolution of fundamental frequency (f_o), a key vocal parameter, in zebra finches. We show that syringeal muscles have extremely small MUs, with 40%-50% innervating ≤3 and 13%-17% innervating a single muscle fiber. Combined with the lowest specific stress (5 mN/mm²) known to skeletal vertebrate muscle, small force steps by the major f_o controlling muscle provide control of 50-mHz to 7.3-Hz steps per MU. We show that the song system has the highest motor control resolution possible in the vertebrate nervous system and suggest this evolved due to strong selection on fine gradation of vocal output. Furthermore, we propose that high-resolution motor control was a key feature contributing to the radiation of songbirds that allowed diversification of song and speciation by vocal space expansion.

Effect of a 12-week endurance training program on force transfer and membrane integrity proteins in lean, obese, and type 2 diabetic subjects

The mechanisms accounting for the loss of muscle function with obesity and type 2 diabetes are likely the result of a combination of neural and muscular factors. One muscular factor that is important, yet has received little attention, is the protein machinery involved in longitudinal and lateral force transmission. The purpose of this study was to compare the levels of force transfer and membrane integrity proteins before and after a 12‐week endurance training program in lean, obese, and obese type 2 diabetic adults. Nineteen sedentary subjects (male = 8 and female = 11) were divided into three groups: Lean (n = 7; 50.3 ± 4.1 y; 69.1 ± 7.2 kg); Obese (n = 6; 49.8 ± 4.1 y; 92.9 ± 19.5 kg); and Obese with type 2 diabetes (n = 6; 51.5 ± 7.9 years; 88.9 ± 15.1 kg). Participants trained 150 min/week between 55% and 75% of VO_2max for 12 weeks. Skeletal muscle biopsies were taken before and after the training intervention. Baseline dystrophin and muscle LIM protein levels were higher (~50% p < .01) in lean compared to obese and type 2 diabetic adults, while the protein levels of the remaining force transfer and membrane integrity proteins were similar between groups. After training, obese individuals decreased (−53%; p < .01) the levels of the muscle ankyrin repeat protein and lean individuals decreased dystrophin levels (−45%; p = .01), while the levels of the remaining force transfer and membrane integrity proteins were not affected by training. These results suggest that there are modest changes to force transfer and membrane integrity proteins in middle‐aged individuals as a result of 12 weeks of lifestyle and training interventions.

The Human Muscle Size and Strength Relationship: Effects of Architecture, Muscle Force, and Measurement Location

PURPOSE

This study aimed to determine the best muscle size index of muscle strength by establishing if incorporating muscle architecture measurements improved the human muscle size-strength relationship. The influence of calculating muscle force, and the location of anatomical cross-sectional area (ACSA) measurements on this relationship were also examined.

METHODS

Fifty-two recreationally active males completed unilateral isometric knee extension strength assessments and MRI scans of the dominant thigh and knee to determine quadriceps femoris (QF) size variables (ACSA along the length of the femur, maximum ACSA [ACSAMAX] and volume [VOL]) and patellar tendon moment arm. Ultrasound images (2 sites per constituent muscle) were analyzed to quantify muscle architecture (fascicle length, pennation angle), and when combined with VOL (from MRI), facilitated calculation of QF effective PCSA (EFFPCSA) as potentially the best muscle size determinant of strength. Muscle force was calculated by dividing maximum voluntary torque (MVT) by the moment arm and addition of antagonist torque (derived from hamstring EMG).

RESULTS

The associations of EFFPCSA (r=0.685), ACSAMAX (r=0.697), or VOL (r=0.773) with strength did not differ, although qualitatively VOL explained 59.8% of the variance in strength, ~11-13% greater than EFFPCSA or ACSAMAX. All muscle size variables had weaker associations with muscle force than MVT. The association of strength-ACSA at 65% of femur length (r=0.719) was greater than for ACSA measured between 10-55% and 75-90% (r=-0.042-0.633) of femur length.

CONCLUSIONS

In conclusion, using contemporary methods to assess muscle architecture and calculate EFFPCSA did not enhance the muscle strength-size association. For understanding/monitoring muscle size, the major determinant of strength, these findings support the assessment of muscle volume, that is independent of architecture measurements, and was most highly correlated to strength.

Muscle Proteomic and Transcriptomic Profiling of Healthy Aging and Metabolic Syndrome in Men

(1) Background: Aging is associated with a progressive decline in muscle mass and function. Aging is also a primary risk factor for metabolic syndrome, which further alters muscle metabolism. However, the molecular mechanisms involved remain to be clarified. Herein we performed omic profiling to decipher in muscle which dominating processes are associated with healthy aging and metabolic syndrome in old men. (2) Methods: This study included 15 healthy young, 15 healthy old, and 9 old men with metabolic syndrome. Old men were selected from a well-characterized cohort, and each vastus lateralis biopsy was used to combine global transcriptomic and proteomic analyses. (3) Results: Over-representation analysis of differentially expressed genes (ORA) and functional class scoring of pathways (FCS) indicated that healthy aging was mainly associated with upregulations of apoptosis and immune function and downregulations of glycolysis and protein catabolism. ORA and FCS indicated that with metabolic syndrome the dominating biological processes were upregulation of proteolysis and downregulation of oxidative phosphorylation. Proteomic profiling matched 586 muscle proteins between individuals. The proteome of healthy aging revealed modifications consistent with a fast-to-slow transition and downregulation of glycolysis. These transitions were reduced with metabolic syndrome, which was more associated with alterations in NADH/NAD+ shuttle and β-oxidation. Proteomic profiling further showed that all old muscles overexpressed protein chaperones to preserve proteostasis and myofiber integrity. There was also evidence of aging-related increases in reactive oxygen species but better detoxifications of cytotoxic aldehydes and membrane protection in healthy than in metabolic syndrome muscles. (4) Conclusions: Most candidate proteins and mRNAs identified herein constitute putative muscle biomarkers of healthy aging and metabolic syndrome in old men.

Influence of Woody Breast Myopathy on Sarcomere Length and Tensile Strength in Commercial Broiler Pectoralis major Muscle

Woody breast syndrome is characterized by degenerative changes at the muscle fiber level and accumulation of connective tissue between the fibers. This study explored effects of the syndrome on muscle properties by focusing on a comparison of the sarcomere lengths between normal and woody breast muscles, including cranial and middle parts, surface and deeper layers, electrically stimulated and nonstimulated muscles, and their combinations. Tensile strengths longitudinally and transversally to the muscle fiber direction in the cranial and middle parts of the muscles were determined. The overall sarcomere lengths of woody breasts were longer than normal muscles (1.93 μ m vs. 1.88 μm; P &lt; 0.05). The surface layer had overall longer sarcomere lengths than the deeper layer (cranial surface vs. cranial deeper layer: 1.97 vs. 1.89 μ m; middle surface vs. middle deeper layer: 1.93 vs.1.84 μ m; P &lt; 0.05). Compared with normal breast muscles, woody breast muscles had longer sarcomeres in the surface layer; however, sarcomere length did not differ in the deeper layer. Electrically stimulated chicken breasts generally had longer sarcomere lengths (2.00 vs. 1.82 μm; P &lt; 0.001). There was no significant difference in tensile strength between normal and focal woody breast (mild local lesion, usually in the cranial end) samples when fiber direction or sample location was studied (P &gt; 0.05), but there was a clear difference between normal and focal versus diffuse (severe woody breast lesion throughout the muscle) samples (P &lt; 0.001). Tensile strength was much greater in diffuse woody breast muscles when extended longitudinally or transversely to the fiber direction. In conclusion, although this study did not show sarcomere lengths in living muscle, it suggests an imbalance in sarcomere lengths in different parts of the breast muscle, which may induce a reduction in the functionality and strength of the muscle.

Extracellular matrix: an important regulator of cell functions and skeletal muscle development

Extracellular matrix (ECM) is a kind of connective tissue in the cell microenvironment, which is of great significance to tissue development. ECM in muscle fiber niche consists of three layers: the epimysium, the perimysium, and the endomysium (basal lamina). These three layers of connective tissue structure can not only maintain the morphology of skeletal muscle, but also play an important role in the physiological functions of muscle cells, such as the transmission of mechanical force, the regeneration of muscle fiber, and the formation of neuromuscular junction. In this paper, detailed discussions are made for the structure and key components of ECM in skeletal muscle tissue, the role of ECM in skeletal muscle development, and the application of ECM in biomedical engineering. This review will provide the reader with a comprehensive overview of ECM, as well as a comprehensive understanding of the structure, physiological function, and application of ECM in skeletal muscle tissue.

The Myotendinous Junction-A Vulnerable Companion in Sports. A Narrative Review

The incidence of strain injuries continues to be high in many popular sports, especially hamstring strain injuries in football, despite a documented important effect of eccentric exercise to prevent strains. Studies investigating the anatomical properties of these injuries in humans are sparse. The majority of strains are seen at the interface between muscle fibers and tendon: the myotendinous junction (MTJ). It has a unique morphology with a highly folded muscle membrane filled with invaginations of collagen fibrils from the tendon, establishing an increased area of force transmission between muscle and tendon. There is a very high rate of remodeling of the muscle cells approaching the MTJ, but little is known about how the tissue adapts to exercise and which structural changes heavy eccentric exercise may introduce. This review summarizes the current knowledge about the anatomy, composition and adaptability of the MTJ, and discusses reasons why strain injuries can be prevented by eccentric exercise.

Translational research on Myology and Mobility Medicine: 2021 semi-virtual PDM3 from Thermae of Euganean Hills, May 26 - 29, 2021

On 19-21 November 2020, the meeting of the 30 years of the Padova Muscle Days was virtually held while the SARS-CoV-2 epidemic was hitting the world after a seemingly quiet summer. During the 2020-2021 winter, the epidemic is still active, despite the start of vaccinations. The organizers hope to hold the 2021 Padua Days on Myology and Mobility Medicine in a semi-virtual form (2021 S-V PDM3) from May 26 to May 29 at the Thermae of Euganean Hills, Padova, Italy. Here the program and the Collection of Abstracts are presented. Despite numerous world problems, the number of submitted/selected presentations (lectures and oral presentations) has increased, prompting the organizers to extend the program to four dense days.

Exercise Plus Presleep Protein Ingestion Increases Overnight Muscle Connective Tissue Protein Synthesis Rates in Healthy Older Men

Protein ingestion and exercise stimulate myofibrillar protein synthesis rates. When combined, exercise further increases the postprandial rise in myofibrillar protein synthesis rates. It remains unclear whether protein ingestion with or without exercise also stimulates muscle connective tissue protein synthesis rates. The authors assessed the impact of presleep protein ingestion on overnight muscle connective tissue protein synthesis rates at rest and during recovery from resistance-type exercise in older men. Thirty-six healthy, older men were randomly assigned to ingest 40 g intrinsically L-[1-13C]-phenylalanine and L-[1-13C]-leucine-labeled casein protein (PRO, n = 12) or a nonprotein placebo (PLA, n = 12) before going to sleep. A third group performed a single bout of resistance-type exercise in the evening before ingesting 40 g intrinsically-labeled casein protein prior to sleep (EX+PRO, n = 12). Continuous intravenous infusions of L-[ring-2H5]-phenylalanine and L-[1-13C]-leucine were applied with blood and muscle tissue samples collected throughout overnight sleep. Presleep protein ingestion did not increase muscle connective tissue protein synthesis rates (0.049 ± 0.013 vs. 0.060 ± 0.024%/hr in PLA and PRO, respectively; p = .73). Exercise plus protein ingestion resulted in greater overnight muscle connective tissue protein synthesis rates (0.095 ± 0.022%/hr) when compared with PLA and PRO (p < .01). Exercise increased the incorporation of dietary protein-derived amino acids into muscle connective tissue protein (0.036 ± 0.013 vs. 0.054 ± 0.009 mole percent excess in PRO vs. EX+PRO, respectively; p < .01). In conclusion, resistance-type exercise plus presleep protein ingestion increases overnight muscle connective tissue protein synthesis rates in older men. Exercise enhances the utilization of dietary protein-derived amino acids as precursors for de novo muscle connective tissue protein synthesis during overnight sleep.