Diaphragm Fatigue in SMNΔ7 Mice and Its Molecular Determinants: An Underestimated Issue.

Spinal muscular atrophy (SMA) is a genetic disorder characterized by the loss of spinal motor neurons leading to muscle weakness and respiratory failure. Mitochondrial dysfunctions are found in the skeletal muscle of patients with SMA. For obvious ethical reasons, the diaphragm muscle is poorly studied, notwithstanding the very important role that respiratory involvement plays in SMA mortality. The main goal of this study was to investigate diaphragm functionality and the underlying molecular adaptations in SMNΔ7 mice, a mouse model that exhibits symptoms similar to that of patients with intermediate type II SMA. Functional, biochemical, and molecular analyses on isolated diaphragm were performed. The obtained results suggest the presence of an intrinsic energetic imbalance associated with mitochondrial dysfunction and a significant accumulation of reactive oxygen species (ROS). In turn, ROS accumulation can affect muscle fatigue, cause diaphragm wasting, and, in the long run, respiratory failure in SMNΔ7 mice. Exposure to the antioxidant molecule ergothioneine leads to the functional recovery of the diaphragm, confirming the presence of mitochondrial impairment and redox imbalance. These findings suggest the possibility of carrying out a dietary supplementation in SMNΔ7 mice to preserve their diaphragm function and increase their lifespan.

Structural and functional impairments of skeletal muscle in patients with postacute sequelae of SARS-CoV-2 infection.

Following acute coronavirus disease 2019 (COVID-19), a substantial proportion of patients showed symptoms and sequelae for several months, namely the postacute sequelae of COVID-19 (PASC) syndrome. Major phenomena are exercise intolerance, muscle weakness, and fatigue. We aimed to investigate the physiopathology of exercise intolerance in patients with PASC syndrome by structural and functional analyses of skeletal muscle. At least 3 mo after infection, nonhospitalized patients with PASC (n = 11, age: 54 ± 11 yr; PASC) and patients without long-term symptoms (n = 12, age: 49 ± 9 yr; CTRL) visited the laboratory on four nonconsecutive days. Spirometry, lung diffusion capacity, and quality of life were assessed at rest. A cardiopulmonary incremental exercise test was performed. Oxygen consumption (V̇o2) kinetics were determined by moderate-intensity exercises. Muscle oxidative capacity (k) was assessed by near-infrared spectroscopy. Histochemical analysis, O2 flux (JO2) by high-resolution respirometry, and quantification of key molecular markers of mitochondrial biogenesis and dynamics were performed in vastus lateralis biopsies. Pulmonary and cardiac functions were within normal range in all patients. V̇o2peak was lower in PASC than CTRL (24.7 ± 5.0 vs. 32.9 ± 7.4 mL·min-1·kg-1, respectively, P < 0.05). V̇o2 kinetics was slower in PASC than CTRL (41 ± 12 vs. 30 ± 9 s-1, P < 0.05). k was lower in PASC than CTRL (1.54 ± 0.49 vs. 2.07 ± 0.51 min-1, P < 0.05). Citrate synthase, peroxisome proliferator-activated receptor-γ coactivator (PGC)1α, and JO2 for mitochondrial complex II were significantly lower in PASC vs. CTRL (all P values <0.05). In our cohort of patients with PASC, we showed limited exercise tolerance mainly due to "peripheral" determinants. Substantial reductions were observed for biomarkers of mitochondrial function, content, and biogenesis. PASC syndrome, therefore, appears to negatively impact skeletal muscle function, although the disease is a heterogeneous condition.NEW & NOTEWORTHY Several months after mild acute SARS-CoV-2 infection, a substantial proportion of patients present persisting, and often debilitating, symptoms and sequelae. These patients show reduced quality of life due to exercise intolerance, muscle weakness, and fatigue. The present study supports the hypothesis that "peripheral" impairments at skeletal muscle level, namely, reduced mitochondrial function and markers of mitochondrial biogenesis, are major determinants of exercise intolerance and fatigue, "central" phenomena at respiratory, and cardiac level being less relevant.

Age-related decline in murine heart and skeletal muscle performance is attenuated by reduced Ahnak1 expression.

BACKGROUND: Aging is associated with a progressive reduction in cellular function leading to poor health and loss of physical performance. Mitochondrial dysfunction is one of the hallmarks of aging; hence, interventions targeting mitochondrial dysfunction have the potential to provide preventive and therapeutic benefits to elderly individuals. Meta-analyses of age-related gene expression profiles showed that the expression of Ahnak1, a protein regulating several signal-transduction pathways including metabolic homeostasis, is increased with age, which is associated with low VO2MAX and poor muscle fitness. However, the role of Ahnak1 in the aging process remained unknown. Here, we investigated the age-related role of Ahnak1 in murine exercise capacity, mitochondrial function, and contractile function of cardiac and skeletal muscles. METHODS: We employed 15- to 16-month-old female and male Ahnak1-knockout (Ahnak1-KO) and wild-type (WT) mice and performed morphometric, biochemical, and bioenergetics assays to evaluate the effects of Ahnak1 on exercise capacity and mitochondrial morphology and function in cardiomyocytes and tibialis anterior (TA) muscle. A human left ventricular (LV) cardiomyocyte cell line (AC16) was used to investigate the direct role of Ahnak1 in cardiomyocytes. RESULTS: We found that the level of Ahnak1 protein is significantly up-regulated with age in the murine LV (1.9-fold) and TA (1.8-fold) tissues. The suppression of Ahnak1 was associated with improved exercise tolerance, as all aged adult Ahnak1-KO mice (100%) successfully completed the running programme, whereas approximately 31% male and 8% female WT mice could maintain the required running speed and distance. Transmission electron microscopic studies showed that LV and TA tissue specimens of aged adult Ahnak1-KO of both sexes have significantly more enlarged/elongated mitochondria and less small mitochondria compared with WT littermates (P < 0.01 and P < 0.001, respectively) at basal level. Further, we observed a shift in mitochondrial fission/fusion balance towards fusion in cardiomyocytes and TA muscle from aged adult Ahnak1-KO mice. The maximal and reserve respiratory capacities were significantly higher in cardiomyocytes from aged adult Ahnak1-KO mice compared with the WT counterparts (P < 0.05 and P < 0.01, respectively). Cardiomyocyte contractility and fatigue resistance of TA muscles were significantly increased in Ahnak1-KO mice of both sexes, compared with the WT groups. In vitro studies using AC16 cells have confirmed that the alteration of mitochondrial function is indeed a direct effect of Ahnak1. Finally, we presented Ahnak1 as a novel cardiac mitochondrial membrane-associated protein. CONCLUSIONS: Our data suggest that Ahnak1 is involved in age-related cardiac and skeletal muscle dysfunction and could therefore serve as a promising therapeutical target.

Rebalancing expression of HMGB1 redox isoforms to counteract muscular dystrophy.

Muscular dystrophies (MDs) are a group of genetic diseases characterized by progressive muscle wasting associated to oxidative stress and persistent inflammation. It is essential to deepen our knowledge on the mechanism connecting these two processes because current treatments for MDs have limited efficacy and/or are associated with side effects. Here, we identified the alarmin high-mobility group box 1 (HMGB1) as a functional link between oxidative stress and inflammation in MDs. The oxidation of HMGB1 cysteines switches its extracellular activities from the orchestration of tissue regeneration to the exacerbation of inflammation. Extracellular HMGB1 is present at high amount and undergoes oxidation in patients with MDs and in mouse models of Duchenne muscular dystrophy (DMD) and limb-girdle muscular dystrophy 3 (LGMDR3) compared to controls. Genetic ablation of HMGB1 in muscles of DMD mice leads to an amelioration of the dystrophic phenotype as evidenced by the reduced inflammation and muscle degeneration, indicating that HMGB1 oxidation is a detrimental process in MDs. Pharmacological treatment with an engineered nonoxidizable variant of HMGB1, called 3S, improves functional performance, muscle regeneration, and satellite cell engraftment in dystrophic mice while reducing inflammation and fibrosis. Overall, our data demonstrate that the balance between HMGB1 redox isoforms dictates whether skeletal muscle is in an inflamed or regenerating state, and that the nonoxidizable form of HMGB1 is a possible therapeutic approach to counteract the progression of the dystrophic phenotype. Rebalancing the HMGB1 redox isoforms may also be a therapeutic strategy for other disorders characterized by chronic oxidative stress and inflammation.

Exercise Preconditioning Blunts Early Atrogenes Expression and Atrophy in Gastrocnemius Muscle of Hindlimb Unloaded Mice.

A large set of FoxOs-dependent genes play a primary role in controlling muscle mass during hindlimb unloading. Mitochondrial dysfunction can modulate such a process. We hypothesized that endurance exercise before disuse can protect against disuse-induced muscle atrophy by enhancing peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) expression and preventing mitochondrial dysfunction and energy-sensing AMP-activated protein kinase (AMPK) activation. We studied cross sectional area (CSA) of muscle fibers of gastrocnemius muscle by histochemistry following 1, 3, 7, and 14 days of hindlimb unloading (HU). We used Western blotting and qRT-PCR to study mitochondrial dynamics and FoxOs-dependent atrogenes' expression at 1 and 3 days after HU. Preconditioned animals were submitted to moderate treadmill exercise for 7 days before disuse. Exercise preconditioning protected the gastrocnemius from disuse atrophy until 7 days of HU. It blunted alterations in mitochondrial dynamics up to 3 days after HU and the expression of most atrogenes at 1 day after disuse. In preconditioned mice, the activation of atrogenes resumed 3 days after HU when mitochondrial dynamics, assessed by profusion and pro-fission markers (mitofusin 1, MFN1, mitofusin 2, MFN2, optic atrophy 1, OPA1, dynamin related protein 1, DRP1 and fission 1, FIS1), PGC1α levels, and AMPK activation were at a basal level. Therefore, the normalization of mitochondrial dynamics and function was not sufficient to prevent atrogenes activation just a few days after HU. The time course of sirtuin 1 (SIRT1) expression and content paralleled the time course of atrogenes' expression. In conclusion, seven days of endurance exercise counteracted alterations of mitochondrial dynamics and the activation of atrogenes early into disuse. Despite the normalization of mitochondrial dynamics, the effect on atrogenes' suppression died away within 3 days of HU. Interestingly, muscle protection lasted until 7 days of HU. A longer or more intense exercise preconditioning may prolong atrogenes suppression and muscle protection.

Ultra-fast force-clamp spectroscopy data on the interaction between skeletal muscle myosin and actin.

Ultrafast force-clamp spectroscopy is a single molecule technique based on laser tweezers with sub-millisecond and sub-nanometer resolution. The technique has been successfully applied to investigate the rapid conformational changes that occur when a myosin II motor from skeletal muscle interacts with an actin filament. Here, we share data on the kinetics of such interaction and experimental records collected under different forces [1]. The data can be valuable for researchers interested in the mechanosensitive properties of myosin II, both from an experimental and modeling point of view. The data is related to the research article "ultrafast force-clamp spectroscopy of single molecules reveals load dependence of myosin working stroke" [2].

Recovery from 6-month spaceflight at the International Space Station: muscle-related stress into a proinflammatory setting.

The Sarcolab pilot study of 2 crewmembers, investigated before and after a 6-mo International Space Station mission, has demonstrated the substantial muscle wasting and weakness, along with disruption of muscle's oxidative metabolism. The present work aimed at evaluating the pro/anti-inflammatory status in the same 2 crewmembers (A, B). Blood circulating (c-)microRNAs (miRs), c-proteasome, c-mitochondrial DNA, and cytokines were assessed by real-time quantitative PCR or ELISA tests. Time series analysis was performed ( i.e., before flight and after landing) at 1 and 15 d of recovery (R+1 and R+15, respectively). C-biomarkers were compared with an age-matched control population and with 2-dimensional proteomic analysis of the 2 crewmembers' muscle biopsies. Striking differences were observed between the 2 crewmembers at R+1, in terms of inflamma-miRs (c-miRs-21-5p, -126-3p, and -146a-5p), muscle specific (myo)-miR-206, c-proteasome, and IL-6/leptin, thus making the 2 astronauts dissimilar to each other. Final recovery levels of c-proteasome, c-inflamma-miRs, and c-myo-miR-206 were not reverted to the baseline values in crewmember A. In both crewmembers, myo-miR-206 changed significantly after recovery. Muscle biopsy of astronaut A showed an impressive 80% increase of α-1-antitrypsin, a target of miR-126-3p. These results point to a strong stress response induced by spaceflight involving muscle tissue and the proinflammatory setting, where inflamma-miRs and myo-miR-206 mediate the systemic recovery phase after landing.-Capri, M., Morsiani, C., Santoro, A., Moriggi, M., Conte, M., Martucci, M., Bellavista, E., Fabbri, C., Giampieri, E., Albracht, K., Flück, M., Ruoss, S., Brocca, L., Canepari, M., Longa, E., Di Giulio, I., Bottinelli, R., Cerretelli, P., Salvioli, S., Gelfi, C., Franceschi, C., Narici, M., Rittweger, J. Recovery from 6-month spaceflight at the International Space Station: muscle-related stress into a proinflammatory setting.

Reduction of Movement in Neurological Diseases: Effects on Neural Stem Cells Characteristics.

Both astronauts and patients affected by chronic movement-limiting pathologies face impairment in muscle and/or brain performance. Increased patient survival expectations and the expected longer stays in space by astronauts may result in prolonged motor deprivation and consequent pathological effects. Severe movement limitation can influence not only the motor and metabolic systems but also the nervous system, altering neurogenesis and the interaction between motoneurons and muscle cells. Little information is yet available about the effect of prolonged muscle disuse on neural stem cells characteristics. Our in vitro study aims to fill this gap by focusing on the biological and molecular properties of neural stem cells (NSCs). Our analysis shows that NSCs derived from the SVZ of HU mice had shown a reduced proliferation capability and an altered cell cycle. Furthermore, NSCs obtained from HU animals present an incomplete differentiation/maturation. The overall results support the existence of a link between reduction of exercise and muscle disuse and metabolism in the brain and thus represent valuable new information that could clarify how circumstances such as the absence of load and the lack of movement that occurs in people with some neurological diseases, may affect the properties of NSCs and contribute to the negative manifestations of these conditions.

High mobility group box 1 orchestrates tissue regeneration via CXCR4.

Inflammation and tissue regeneration follow tissue damage, but little is known about how these processes are coordinated. High Mobility Group Box 1 (HMGB1) is a nuclear protein that, when released on injury, triggers inflammation. We previously showed that HMGB1 with reduced cysteines is a chemoattractant, whereas a disulfide bond makes it a proinflammatory cytokine. Here we report that fully reduced HMGB1 orchestrates muscle and liver regeneration via CXCR4, whereas disulfide HMGB1 and its receptors TLR4/MD-2 and RAGE (receptor for advanced glycation end products) are not involved. Injection of HMGB1 accelerates tissue repair by acting on resident muscle stem cells, hepatocytes, and infiltrating cells. The nonoxidizable HMGB1 mutant 3S, in which serines replace cysteines, promotes muscle and liver regeneration more efficiently than the wild-type protein and without exacerbating inflammation by selectively interacting with CXCR4. Overall, our results show that the reduced form of HMGB1 coordinates tissue regeneration and suggest that 3S may be used to safely accelerate healing after injury in diverse clinical contexts.

Structure and function of human muscle fibres and muscle proteome in physically active older men.

KEY POINTS: Loss of muscle mass and strength in the growing population of elderly people is a major health concern for modern societies. This condition, termed sarcopenia, is a major cause of falls and of the subsequent increase in morbidity and mortality. Despite numerous studies on the impact of ageing on individual muscle fibres, the contribution of single muscle fibre adaptations to ageing-induced atrophy and functional impairment is still unsettled. The level of physical function and disuse is often associated with ageing. We studied relatively healthy older adults in order to understand the effects of ageing per se without the confounding impact of impaired physical function. We found that in healthy ageing, structural and functional alterations of muscle fibres occur. Protein post-translational modifications, oxidation and phosphorylation contribute to such alterations more than loss of myosin and other muscle protein content. ABSTRACT: Contradictory results have been reported on the impact of ageing on structure and functions of skeletal muscle fibres, likely to be due to a complex interplay between ageing and other phenomena such as disuse and diseases. Here we recruited healthy, physically and socially active young (YO) and elderly (EL) men in order to study ageing per se without the confounding effects of impaired physical function. In vivo analyses of quadriceps and in vitro analyses of vastus lateralis muscle biopsies were performed. In EL subjects, our results show that (i) quadriceps volume, maximum voluntary contraction isometric torque and patellar tendon force were significantly lower; (ii) muscle fibres went through significant atrophy and impairment of specific force (isometric force/cross-sectional area) and unloaded shortening velocity; (iii) myosin/actin ratio and myosin content in individual muscle fibres were not altered; (iv) the muscle proteome went through quantitative adaptations, namely an up-regulation of the content of several groups of proteins among which were myofibrillar proteins and antioxidant defence systems; (v) the muscle proteome went through qualitative adaptations, namely phosphorylation of several proteins, including myosin light chain-2 slow and troponin T and carbonylation of myosin heavy chains. The present results indicate that impairment of individual muscle fibre structure and function is a major feature of ageing per se and that qualitative adaptations of muscle proteome are likely to be more involved than quantitative adaptations in determining such a phenomenon.

Autophagy impairment in muscle induces neuromuscular junction degeneration and precocious aging.

The cellular basis of age-related tissue deterioration remains largely obscure. The ability to activate compensatory mechanisms in response to environmental stress is an important factor for survival and maintenance of cellular functions. Autophagy is activated both under short and prolonged stress and is required to clear the cell of dysfunctional organelles and altered proteins. We report that specific autophagy inhibition in muscle has a major impact on neuromuscular synaptic function and, consequently, on muscle strength, ultimately affecting the lifespan of animals. Inhibition of autophagy also exacerbates aging phenotypes in muscle, such as mitochondrial dysfunction, oxidative stress, and profound weakness. Mitochondrial dysfunction and oxidative stress directly affect acto-myosin interaction and force generation but show a limited effect on stability of neuromuscular synapses. These results demonstrate that age-related deterioration of synaptic structure and function is exacerbated by defective autophagy.

Molecular mechanism regulating myosin and cardiac functions by ELC.

The essential myosin light chain (ELC) is involved in modulation of force generation of myosin motors and cardiac contraction, while its mechanism of action remains elusive. We hypothesized that ELC could modulate myosin stiffness which subsequently determines its force production and cardiac contraction. Therefore, we generated heterologous transgenic mouse (TgM) strains with cardiomyocyte-specific expression of ELC with human ventricular ELC (hVLC-1; TgM(hVLC-1)) or E56G-mutated hVLC-1 (hVLC-1(E56G); TgM(E56G)). hVLC-1 or hVLC-1(E56G) expression in TgM was around 39% and 41%, respectively of total VLC-1. Laser trap and in vitro motility assays showed that stiffness and actin sliding velocity of myosin with hVLC-1 prepared from TgM(hVLC-1) (1.67 pN/nm and 2.3 µm/s, respectively) were significantly higher than myosin with hVLC-1(E56G) prepared from TgM(E56G) (1.25 pN/nm and 1.7 µm/s, respectively) or myosin with mouse VLC-1 (mVLC-1) prepared from C57/BL6 (1.41 pN/nm and 1.5 µm/s, respectively). Maximal left ventricular pressure development of isolated perfused hearts in vitro prepared from TgM(hVLC-1) (80.0 mmHg) were significantly higher than hearts from TgM(E56G) (66.2 mmHg) or C57/BL6 (59.3±3.9 mmHg). These findings show that ELCs decreased myosin stiffness, in vitro motility, and thereby cardiac functions in the order hVLC-1>hVLC-1(E56G)≈mVLC-1. They also suggest a molecular pathomechanism of hypertrophic cardiomyopathy caused by hVLC-1 mutations.

Ultrafast force-clamp spectroscopy of single molecules reveals load dependence of myosin working stroke.

We describe a dual-trap force-clamp configuration that applies constant loads between a binding protein and an intermittently interacting biological polymer. The method has a measurement delay of only ∼10 µs, allows detection of interactions as brief as ∼100 µs and probes sub-nanometer conformational changes with a time resolution of tens of microseconds. We tested our method on molecular motors and DNA-binding proteins. We could apply constant loads to a single motor domain of myosin before its working stroke was initiated (0.2-1 ms), thus directly measuring its load dependence. We found that, depending on the applied load, myosin weakly interacted (<1 ms) with actin without production of movement, fully developed its working stroke or prematurely detached (<5 ms), thus reducing the working stroke size with load. Our technique extends single-molecule force-clamp spectroscopy and opens new avenues for investigating the effects of forces on biological processes.

Actomyosin kinetics of pure fast and slow rat myosin isoforms studied by in vitro motility assay approach.

An in vitro motility assay approach was used to investigate the mechanisms of the functional differences between myosin isoforms, by studying the effect of MgATP and MgADP on actin sliding velocity (V(f)) of pure slow and fast rat skeletal myosin at different temperatures. The value of V(f) depended on [MgATP] according to Michaelis-Menten kinetics, with an apparent constant (K(m)) of 54.2, 64.4 and 200 µm for the fast isoform and 18.6, 36.5 and 45.5 µM for the slow isoform at 20, 25 and 35°C, respectively. The presence of 2 mM MgADP decreased V(f) and yielded an inhibition constant (K(i)) of 377, 463 and 533 µM for the fast isoform at 20, 25 and 35°C, respectively, and 120 and 355 µM for the slow isoform at 25 and 35°C, respectively. The analysis of K(m) and K(i) suggested that slow and fast isoforms differ in the kinetics limiting V(f). Moreover, the higher sensitivity of the fast myosin isoform to a drop in [MgATP] is consistent with the higher fatigability of fast fibres than slow fibres. From the Michaelis-Menten relation in the absence of MgADP, we calculated the rate of actomyosin dissociation by MgATP (k(+ATP)) and the rate of MgADP release (k(-ADP)). We found values of k(+ATP) of 4.8 × 10(6), 6.5 × 10(6) and 6.6 × 10(6) M(-1) s(-1) for the fast isoform and 3.3 × 10(6), 2.9 × 10(6) and 6.7 × 10(6) M(-1) s(-1) for the slow isoform and values of k(-ADP) of 263, 420 and 1320 s(-1) for the fast isoform and 62, 107 and 306 s(-1) for the slow isoform at 20, 25 and 35°C, respectively. The results suggest that k(-ADP) could be the major determinant of functional differences between the fast and slow myosin isoforms at physiological temperatures.

Actin sliding velocity on pure myosin isoforms from dystrophic mouse muscles.

Duchenne muscular dystrophy (DMD) is a genetic disease characterized by skeletal muscle wasting and atrophy. Recent evidence suggests that the impaired skeletal muscle performance in DMD is not solely dependent on a loss of contractile muscle mass. In this study the myosin motor function of mdx and control (wildtype, WT) mice was compared using pure myosin isoforms in an "in vitro motility assay" (IVMA). Actin sliding velocity (Vf) on myosin 2B extracted from single muscle fibers of gastrocnemius muscles was significantly lower in mdx mice (3.48 +/- 0.13 microm/s, n = 18) than in WT mice (4.02 +/- 0.19 microm/s, n = 10). No difference in Vf was found between myosin 1 extracted from soleus muscles of mdx (0.84 +/- 0.04 microm/s, n = 13) and of WT (0.89 +/- 0.04 microm/s, n = 10). The results suggest that the dystrophic process alters myosin molecular function, and this contributes to the functional impairment in dystrophic muscles. Muscle Nerve 40: 249-256, 2009.

Myosin as a potential redox-sensor: an in vitro study.

A balanced redox status is necessary to optimize force production in contractile apparatus, where free radicals generated by skeletal muscle are involved in some basic physiological processes like excitation-contraction coupling. Protein glutathionylation has a key role in redox regulation of proteins and signal transduction. Here we show that myosin is sensitive to in vitro glutathionylation and MALDI-TOF analysis identified three potential sites of glutathione binding, two of them locating on the myosin head. Glutathionylation of myosin has an important impact on the protein structure, as documented by the lower fluorescence quantum yield of glutathionylated myosin and its increased susceptibility to the proteolytic cleavage. Myosin function is also sensitive to glutathionylation, which modulates its ATPase activity depending on GSSG redox balance. Thus, like the phosphorylation/dephosphorylation cycle, glutathionylation may represent a mechanism by which glutathione modulates sarcomere functions depending on the tissue redox state, and myosin may constitute a muscle redox-sensor.

Fiber types in canine muscles: myosin isoform expression and functional characterization.

This study was aimed to achieve a definitive and unambiguous identification of fiber types in canine skeletal muscles and of myosin isoforms that are expressed therein. Correspondence of canine myosin isoforms with orthologs in other species as assessed by base sequence comparison was the basis for primer preparation and for expression analysis with RT-PCR. Expression was confirmed at protein level with histochemistry, immunohistochemistry, and SDS-PAGE combined together and showed that limb and trunk muscles of the dog express myosin heavy chain (MHC) type 1, 2A, and 2X isoforms and the so-called "type 2dog" fibers express the MHC-2X isoform. MHC-2A was found to be the most abundant isoform in the trunk and limb muscle. MHC-2X was expressed in most but not all muscles and more frequently in hybrid 2A-2X fibers than in pure 2X fibers. MHC-2B was restricted to specialized extraocular and laryngeal muscles, although 2B mRNA, but not 2B protein, was occasionally detected in the semimembranosus muscle. Isometric tension (P(o)) and maximum shortening velocity (V(o)) were measured in single fibers classified on the basis of their MHC isoform composition. Purified myosin isoforms were extracted from single muscle fibers and characterized by the speed (V(f)) of actin filament sliding on myosin in an in vitro motility assay. A close proportionality between V(o) and V(f) indicated that the diversity in V(o) was due to the different myosin isoform composition. V(o) increased progressively in the order 1/slow < 2A < 2X < 2B, thus confirming the identification of the myosin isoforms and providing their first functional characterization of canine muscle fibers.

Skeletal muscle hypertrophy and structure and function of skeletal muscle fibres in male body builders.

Needle biopsy samples were taken from vastus lateralis muscle (VL) of five male body builders (BB, age 27.4+/-0.93 years; mean+/-s.e.m.), who had being performing hypertrophic heavy resistance exercise (HHRE) for at least 2 years, and from five male active, but untrained control subjects (CTRL, age 29.9+/-2.01 years). The following determinations were performed: anatomical cross-sectional area and volume of the quadriceps and VL muscles in vivo by magnetic resonance imaging (MRI); myosin heavy chain isoform (MHC) distribution of the whole biopsy samples by SDS-PAGE; cross-sectional area (CSA), force (Po), specific force (Po/CSA) and maximum shortening velocity (Vo) of a large population (n=524) of single skinned muscle fibres classified on the basis of MHC isoform composition by SDS-PAGE; actin sliding velocity (Vf) on pure myosin isoforms by in vitro motility assays. In BB a preferential hypertrophy of fast and especially type 2X fibres was observed. The very large hypertrophy of VL in vivo could not be fully accounted for by single muscle fibre hypertrophy. CSA of VL in vivo was, in fact, 54% larger in BB than in CTRL, whereas mean fibre area was only 14% larger in BB than in CTRL. MHC isoform distribution was shifted towards 2X fibres in BB. Po/CSA was significantly lower in type 1 fibres from BB than in type 1 fibres from CTRL whereas both type 2A and type 2X fibres were significantly stronger in BB than in CTRL. Vo of type 1 fibres and Vf of myosin 1 were significantly lower in BB than in CTRL, whereas no difference was observed among fast fibres and myosin 2A. The findings indicate that skeletal muscle of BB was markedly adapted to HHRE through extreme hypertrophy, a shift towards the stronger and more powerful fibre types and an increase in specific force of muscle fibres. Such adaptations could not be fully accounted for by well known mechanisms of muscle plasticity, i.e. by the hypertrophy of single muscle fibre (quantitative mechanism) and by a regulation of contractile properties of muscle fibres based on MHC isoform content (qualitative mechanism). Two BB subjects took anabolic steroids and three BB subjects did not. The former BB differed from the latter BB mostly for the size of their muscles and muscle fibres.

Fast fibres in a large animal: fibre types, contractile properties and myosin expression in pig skeletal muscles.

Little is known about the influence of Myosin Heavy Chain (MHC) isoforms on the contractile properties of single muscle fibres in large animals. We have studied MHC isoform composition and contractile properties of single muscle fibres from the pig. Masseter, diaphragm, longissimus, semitendinosus, rectractor bulbi and rectus lateralis were sampled in female pigs (aged 6 months, mass 160 kg). RT-PCR, histochemistry, immunohistochemistry and gel electrophoresis were combined to identify and separate four MHC isoforms: MHC-slow and three fast MHC (2A, 2X, 2B). Maximum shortening velocity (V(o)) and isometric tension (P(o)) were measured in single muscle fibres with known MHC isoform composition. Six groups of fibres (pure: slow, 2A, 2X and 2B, and hybrid: 2A-2X and 2X-2B) with large differences in V(o) and P(o) were identified. Slow fibres had mean V(o)=0.17+/-0.01 length s(-1) and P(o)=25.1+/-3.3 mN mm(-2). For fast fibres 2A, 2X and 2B, mean V(o) values were 1.86+/-0.18, 2.55+/-0.19 and 4.06+/-0.33 length s(-1) and mean P(o) values 74.93+/-8.36, 66.85+/-7.58 and 32.96+/-7.47 mN mm(-2), respectively. An in vitro motility assay confirmed that V(o) strictly reflected the functional properties of the myosin isoforms. We conclude that pig muscles express high proportions of fast MHC isoforms, including MHC-2B, and that V(o) values are higher than expected on the basis of the scaling relationship between contractile parameters and body size.

The effect of ageing and immobilization on structure and function of human skeletal muscle fibres.

Biopsy samples were taken from vastus lateralis muscle of seven young (YO, age 30.2 +/- 2.2 years), and seven elderly (EL, age 72.7 +/- 2.3 years) subjects and two elderly subjects whose right leg had been immobilized for 3.5 months (EL-IMM, ages 70 and 75). The following main parameters were studied: (1) myosin heavy chain (MHC) isoform distribution of the samples, determined by SDS-PAGE; (2) cross-sectional area (CSA), specific force (Po/CSA) and maximum shortening velocity (Vo) of a large population (n = 593) of single skinned muscle fibres, classified on the basis of MHC isoform composition determined by SDS-PAGE; (3) actin sliding velocity (Vf) on pure myosin isoforms determined by in vitro motility assays; (4) myosin concentration in single fibres determined by quantitative SDS-PAGE. MHC isoform distribution was shifted towards fast isoforms in EL and to a larger extent in EL-IMM. In EL and, more consistently, in EL-IMM we observed a higher percentage of hybrid fibres than in YO, and noted the presence of MHC-neonatal and of unusual hybrid fibres containing more than two MHC isoforms. Po/CSA significantly decreased in type 1 and 2A fibres in the order YO EL EL-IMM. Vo of type 1 and 2A fibres was significantly lower in EL and higher in EL-IMM than in YO, i.e. immobilization more than counteracted the age-dependent decrease in Vo. The latter phenomenon was not observed for Vf. Vf on myosin 1 was lower in both EL and EL-IMM than in YO. Vf on myosin 2X was lower in EL than in YO, and a similar trend was observed for myosin 2A. Myosin concentration decreased in type 1 and 2A fibres in the order YO EL EL-IMM and was linearly related to the Po/CSA values of corresponding fibre types from the same subjects. The experiments suggest that (1) myosin concentration is a major determinant of the lower Po/CSA of single fibres in ageing and especially following immobilization and (2) ageing is associated with lower Vo of single fibres due to changes in the properties of myosin itself, whereas immobilization is associated with higher Vo in the absence of a change in myosin function.