Functional heterogeneity of mammalian single muscle fibres: do myosin isoforms tell the whole story?

The Influence of Pedaling Frequency on Blood Lactate Accumulation in Cycling Sprints

Anaerobic performance diagnostics in athletes relies on accurate measurements of blood lactate concentration and the calculation of blood lactate accumulation resulting from glycolytic processes. In this study, we investigated the impact of pedaling frequency on blood lactate accumulation during 10-second maximal isokinetic cycling sprints. Thirteen trained males completed five 10-second maximal isokinetic cycling sprints on a bicycle ergometer at different pedaling frequencies (90 rpm, 110 rpm, 130 rpm, 150 rpm, 170 rpm) with continuous power and frequency measurement. Capillary blood samples were taken pre-exercise and up to 30 minutes post-exercise to determine the maximum blood lactate concentration.Blood lactate accumulation was calculated as the difference between maximal post-exercise and pre-start blood lactate concentration. Repeated measurement ANOVA with Bonferroni-adjusted post hoc t-tests revealed significant progressive increases in maximal blood lactate concentration and accumulation with higher pedaling frequencies (p<0.001; η2+>+0.782).The findings demonstrate a significant influence of pedaling frequency on lactate accumulation, emphasizing its relevance in anaerobic diagnostics. Optimal assessment of maximal lactate formation rate is suggested to require a pedaling frequency of at least 130 rpm or higher, while determining metabolic thresholds using the maximal lactate formation rate may benefit from a slightly lower pedaling frequency.

Physiological and molecular predictors of cycling sprint performance

The study aimed to identify novel muscle phenotypic factors that could determine sprint performance using linear regression models including the lean mass of the lower extremities (LLM), myosin heavy chain composition (MHC), and proteins and enzymes implicated in glycolytic and aerobic energy generation (citrate synthase, OXPHOS proteins), oxygen transport and diffusion (myoglobin), ROS sensing (Nrf2/Keap1), antioxidant enzymes, and proteins implicated in calcium handling. For this purpose, body composition (dual-energy X-ray absorptiometry) and sprint performance (isokinetic 30-s Wingate test: peak and mean power output, Wpeak and Wmean ) were measured in young physically active adults (51 males and 10 females), from which a resting muscle biopsy was obtained from the musculus vastus lateralis. Although females had a higher percentage of MHC I, SERCA2, pSer16 /Thr17 -phospholamban, and Calsequestrin 2 protein expressions (all p < 0.05), and 18.4% lower phosphofructokinase 1 protein expression than males (p < 0.05), both sexes had similar sprint performance when it was normalized to body weight or LLM. Multiple regression analysis showed that Wpeak could be predicted from LLM, SDHB, Keap1, and MHC II % (R 2  = 0.62, p < 0.001), each variable contributing to explain 46.4%, 6.3%, 4.4%, and 4.3% of the variance in Wpeak , respectively. LLM and MHC II % explained 67.5% and 2.1% of the variance in Wmean , respectively (R 2  = 0.70, p < 0.001). The present investigation shows that SDHB and Keap1, in addition to MHC II %, are relevant determinants of peak power output during sprinting.

Fiber type and metabolic characteristics of skeletal muscle in 16 breeds of domestic dogs

The domestic dog (Canis lupus familiaris) species comprises hundreds of breeds, each differing in physical characteristics, behavior, strength, and running capability. Very little is known about the skeletal muscle composition and metabolism between the different breeds, which may explain disease susceptibility. Muscle samples from the triceps brachii (TB) and vastus lateralis (VL) were collected post mortem from 35 adult dogs, encompassing 16 breeds of varying ages and sex. Samples were analyzed for fiber type composition, fiber size, oxidative, and glycolytic metabolic capacity (citrate synthase [CS], 3-hydroxyacetyl-coA dehydrogenase [3HAD], creatine kinase [CK], and lactate dehydrogenase [LDH] enzyme activities). There was no significant difference between the TB and VL in any of the measurements. However, there were large intra species variation, with some variables confirming the physical attributes of a specific breed. Collectively, type IIA was the predominant fiber type followed by type I and type IIX. The cross-sectional areas (CSA) of the fibers were all smaller when compared to humans and similar to other wild animals. There was no difference in the CSA between the fiber types and muscle groups. Metabolically, the muscle of the dog displayed high oxidative capacity with high activities for CS and 3HAD. Lower CK and higher LDH activities than humans indicate a lower and higher flux through the high energy phosphate and glycolytic pathways, respectively. The high variability found across the different breeds may be attributed to genetics, function or lifestyle which have largely been driven through human intervention. This data may provide a foundation for future research into the role of these parameters in disease susceptibility, such as insulin resistance and diabetes, across breeds.

Non-invasive estimation of muscle fibre size from high-density electromyography

Because of the biophysical relation between muscle fibre diameter and the propagation velocity of action potentials along the muscle fibres, motor unit conduction velocity could be a non-invasive index of muscle fibre size in humans. However, the relation between motor unit conduction velocity and fibre size has been only assessed indirectly in animal models and in human patients with invasive intramuscular EMG recordings, or it has been mathematically derived from computer simulations. By combining advanced non-invasive techniques to record motor unit activity in vivo, i.e. high-density surface EMG, with the gold standard technique for muscle tissue sampling, i.e. muscle biopsy, here we investigated the relation between the conduction velocity of populations of motor units identified from the biceps brachii muscle, and muscle fibre diameter. We demonstrate the possibility of predicting muscle fibre diameter (R2  = 0.66) and cross-sectional area (R2  = 0.65) from conduction velocity estimates with low systematic bias (∼2% and ∼4% respectively) and a relatively low margin of individual error (∼8% and ∼16%, respectively). The proposed neuromuscular interface opens new perspectives in the use of high-density EMG as a non-invasive tool to estimate muscle fibre size without the need of surgical biopsy sampling. The non-invasive nature of high-density surface EMG for the assessment of muscle fibre size may be useful in studies monitoring child development, ageing, space and exercise physiology, although the applicability and validity of the proposed methodology need to be more directly assessed in these specific populations by future studies. KEY POINTS: Because of the biophysical relation between muscle fibre size and the propagation velocity of action potentials along the sarcolemma, motor unit conduction velocity could represent a potential non-invasive candidate for estimating muscle fibre size in vivo. This relation has been previously assessed in animal models and humans with invasive techniques, or it has been mathematically derived from simulations. By combining high-density surface EMG with muscle biopsy, here we explored the relation between the conduction velocity of populations of motor units and muscle fibre size in healthy individuals. Our results confirmed that motor unit conduction velocity can be considered as a novel biomarker of fibre size, which can be adopted to predict muscle fibre diameter and cross-sectional area with low systematic bias and margin of individual error. The proposed neuromuscular interface opens new perspectives in the use of high-density EMG as a non-invasive tool to estimate muscle fibre size without the need of surgical biopsy sampling.

A noninvasive test for estimating myosin heavy chain of the vastus lateralis in females with mechanomyography

This study examined relationships between percent myosin heavy chain (%MHC) expression and mechanomyographic amplitude (MMG_RMS). Fifteen females (age ± SD=21.3 ± 5.3 yrs) completed isometric trapezoidal contractions at 30% and 70% maximal voluntary contraction (MVC). MMG was recorded from the vastus lateralis (VL). Participants gave a muscle biopsy of the VL post-testing. MMG_RMS-torque relationships during the linearly varying segments were log-transformed and linear regressions were applied to calculate b terms (slopes). For the steady torque segment, MMG_RMS was averaged. Correlations were performed for type I%MHC with the MMG variables. Multiple regression was utilized to examine prediction equations for type I%MHC. Type I%MHC was significantly correlated with the b terms during the increasing segment of the 70% MVC (p = 0.003; r = -0.718), and MMG_RMS during steady torque at 30% (p = 0.008; r = -0.652) and 70% MVC (p = 0.040; r = -0.535). Type I%MHC reduced the linearity of the MMG_RMS-torque relationship during the high-intensity linearly increasing segment, and MMG_RMS at a low- and high-intensity steady torque. A combination of MMG variables estimated type I%MHC expression with 81.2% accuracy. MMG recorded during a low- and high-intensity isometric trapezoidal contraction may offer a simple, noninvasive test for estimating type I%MHC expression of the VL in sedentary females.

Suboccipital Muscles, Forward Head Posture, and Cervicogenic Dizziness

Dizziness or vertigo can be caused by dysfunction of the vestibular or non-vestibular systems. The diagnosis, treatment, and mechanism of dizziness or vertigo caused by vestibular dysfunction have been described in detail. However, dizziness by the non-vestibular system, especially cervicogenic dizziness, is not well known. This paper explained the cervicogenic dizziness caused by abnormal sensory input with references to several studies. Among head and neck muscles, suboccipital muscles act as stabilizers and controllers of the head. Structural and functional changes of the suboccipital muscles can induce dizziness. Especially, myodural bridges and activation of trigger point stimulated by abnormal head posture may be associated with cervicogenic dizziness.

Protein O-GlcNAcylation in Metabolic Modulation of Skeletal Muscle: A Bright but Long Way to Go

O-GlcNAcylation is an atypical, dynamic and reversible O-glycosylation that is critical and abundant in metazoan. O-GlcNAcylation coordinates and receives various signaling inputs such as nutrients and stresses, thus spatiotemporally regulating the activity, stability, localization and interaction of target proteins to participate in cellular physiological functions. Our review discusses in depth the involvement of O-GlcNAcylation in the precise regulation of skeletal muscle metabolism, such as glucose homeostasis, insulin sensitivity, tricarboxylic acid cycle and mitochondrial biogenesis. The complex interaction and precise modulation of O-GlcNAcylation in these nutritional pathways of skeletal muscle also provide emerging mechanical information on how nutrients affect health, exercise and disease. Meanwhile, we explored the potential role of O-GlcNAcylation in skeletal muscle pathology and focused on its benefits in maintaining proteostasis under atrophy. In general, these understandings of O-GlcNAcylation are conducive to providing new insights into skeletal muscle (patho) physiology.

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

The excitation–contraction coupling (ECC) in skeletal muscle refers to the Ca²⁺-mediated link between the membrane excitation and the mechanical contraction. The initiation and propagation of an action potential through the membranous system of the sarcolemma and the tubular network lead to the activation of the Ca²⁺-release units (CRU): tightly coupled dihydropyridine and ryanodine (RyR) receptors. The RyR gating allows a rapid, massive, and highly regulated release of Ca²⁺ from the sarcoplasmic reticulum (SR). The release from triadic places generates a sarcomeric gradient of Ca²⁺ concentrations ([Ca²⁺]) depending on the distance of a subcellular region from the CRU. Upon release, the diffusing Ca²⁺ has multiple fates: binds to troponin C thus activating the contractile machinery, binds to classical sarcoplasmic Ca²⁺ buffers such as parvalbumin, adenosine triphosphate and, experimentally, fluorescent dyes, enters the mitochondria and the SR, or is recycled through the Na⁺/Ca²⁺ exchanger and store-operated Ca²⁺ entry (SOCE) mechanisms. To commemorate the 7^th decade after being coined, we comprehensively and critically reviewed “old”, historical landmarks and well-established concepts, and blended them with recent advances to have a complete, quantitative-focused landscape of the ECC. We discuss the: 1) elucidation of the CRU structures at near-atomic resolution and its implications for functional coupling; 2) reliable quantification of peak sarcoplasmic [Ca²⁺] using fast, low affinity Ca²⁺ dyes and the relative contributions of the Ca²⁺-binding mechanisms to the whole concert of Ca²⁺ fluxes inside the fibre; 3) articulation of this novel quantitative information with the unveiled structural details of the molecular machinery involved in mitochondrial Ca²⁺ handing to understand how and how much Ca²⁺ enters the mitochondria; 4) presence of the SOCE machinery and its different modes of activation, which awaits understanding of its magnitude and relevance in situ; 5) pharmacology of the ECC, and 6) emerging topics such as the use and potential applications of super-resolution and induced pluripotent stem cells (iPSC) in ECC. Blending the old with the new works better!

A novel description of the Vastus lateralis morphology of the Temminck's Ground Pangolin (Manis temminckii)

The pangolin (Manidae family) is the world's most trafficked animal, yet very little is known about its physiology and metabolism primarily due to its inconspicuous and solitary nature. Skeletal muscle samples from the Vastus lateralis were collected postmortem from a single female Temminck's ground pangolin (Manis temminckii). Samples were analyzed for fiber type composition, fiber size and myosin heavy chain isoform content. The oxidative and glycolytic metabolic capacity was determined through citrate synthase, 3-hydroxyacetyl co A dehydrogenase, creatine kinase, lactate dehydrogenase, phosphofructokinase and glycogen phosphorylase enzyme activities. Lastly, antioxidant capacity was determined through superoxide dismutase and catalase enzyme activities, and the total antioxidant capacity. The pangolin metabolic profile was then compared to other endurance and non-endurance mammals, in which data was standardized relative to human endurance athletes in order to provide context. Slow twitch type I fibers, rich in mitochondria were the predominant fiber type within the pangolin indicating a reliance on oxidative derived energy from fats and carbohydrates. This suggests that the pangolin has a high endurance capability when compared to other wild animals and human endurance athletes. This is the first study to investigate the skeletal muscle physiology and metabolism of any pangolin species, in an attempt to further understand this endangered animal and aid with conservation efforts.

Skeletal muscle cellular contractile dysfunction after anterior cruciate ligament reconstruction contributes to quadriceps weakness at 6-month follow-up

Muscle dysfunction following anterior cruciate ligament reconstruction (ACLR) may evolve from alterations in muscle contractility at the myofilament protein level. Using a prospective, within-subject case-control design, we evaluated cellular-level contractility, cross-sectional area (CSA), and myosin heavy chain (MHC) isoform expression on single muscle fibers 3 weeks post ACLR, and evaluated their relationship to whole muscle strength and patient-oriented outcomes 6 months post operation. Biopsies of the vastus lateralis were performed 3 weeks post ACLR in 11 subjects (5 females, mean age ± SD = 24.7 ± 6.5 years, height = 172.7 ± 8.2 cm, mass = 75.7 ± 12.5 kg) following first-time ACL rupture and whole muscle strength and self-reported pain, function, and quality of life assessed 6 months post ACLR. At 3 weeks post ACLR, force production was reduced (p < 0.01) in MHC I (-36%) and IIA (-48%) fibers compared with the non-injured leg. When force production was expressed relative to CSA to account for fiber atrophy, reductions remained in MHC IIA fibers (-40%; p < 0.001), but MHC I fibers showed only a trend toward being lower (-13%; p = 0.09). Finally, skeletal muscle fiber functional deficits at 3 weeks post ACLR were associated with whole muscle weakness and less favorable patient-reported outcomes at 6-month follow-up. Thus, ACLR promotes early cellular contractile dysfunction that may contribute to decreased whole muscle strength and patient function, and increased patient-reported symptoms, at 6-month follow-up.

Rbfox1 is required for myofibril development and maintaining fiber type-specific isoform expression in Drosophila muscles

Protein isoform transitions confer muscle fibers with distinct properties and are regulated by differential transcription and alternative splicing. RNA-binding Fox protein 1 (Rbfox1) can affect both transcript levels and splicing, and is known to contribute to normal muscle development and physiology in vertebrates, although the detailed mechanisms remain obscure. In this study, we report that Rbfox1 contributes to the generation of adult muscle diversity in Drosophila Rbfox1 is differentially expressed among muscle fiber types, and RNAi knockdown causes a hypercontraction phenotype that leads to behavioral and eclosion defects. Misregulation of fiber type-specific gene and splice isoform expression, notably loss of an indirect flight muscle-specific isoform of Troponin-I that is critical for regulating myosin activity, leads to structural defects. We further show that Rbfox1 directly binds the 3'-UTR of target transcripts, regulates the expression level of myogenic transcription factors myocyte enhancer factor 2 and Salm, and both modulates expression of and genetically interacts with the CELF family RNA-binding protein Bruno1 (Bru1). Rbfox1 and Bru1 co-regulate fiber type-specific alternative splicing of structural genes, indicating that regulatory interactions between FOX and CELF family RNA-binding proteins are conserved in fly muscle. Rbfox1 thus affects muscle development by regulating fiber type-specific splicing and expression dynamics of identity genes and structural proteins.

Hind foot drumming: Myosin heavy chain muscle fiber distribution in the hind limb muscles of three African mole-rat species (Bathyergidae)

Hind foot drumming as a form of seismic signaling plays a pivotal role in the communication of various mammalian species including Bathyergidae (African mole-rats). The aim of the present study was to histologically determine if the action of hind foot drumming would influence the number of type II fibers present in the hind limb muscles of two drumming (Georychus capensis and Bathyergus suillus) and one non-drumming (Cryptomys hottentotus natalensis) bathyergid species. Twenty-one frozen muscles of each species were selected for the purpose of mid-belly cryostat sections. These sections were immunohistochemically labeled for myosin heavy chain slow muscle fibers (MHCs). In addition, oxidative capacity was determined by means of histochemical staining. A high percentage of fast type II muscle fibers was found in all the functional muscle groups, although there were no statistical differences between the drumming and non-drumming species. Bathyergus suillus had significantly fewer type II fibers in mm. semitendinosus, gluteofemoralis, tibialis cranialis, plantaris, and the medial head of m. gastrocnemius compared to the other two species. In all three species, the majority of the muscle fibers in all functional muscle groups demonstrated low oxidative capacity which correlated with the expression of type II muscle fibers. It therefore seems likely that the number of type II muscle fibers in the hind limb muscles of the Bathyergidae species studied here is more influenced by either body size or digging strategy rather than being an adaptation for hind foot drumming.

Reorganization of chromatin architecture during prenatal development of porcine skeletal muscle

Myofibres (primary and secondary myofibre) are the basic structure of muscle and the determinant of muscle mass. To explore the skeletal muscle developmental processes from primary myofibres to secondary myofibres in pigs, we conducted an integrative three-dimensional structure of genome and transcriptomic characterization of longissimus dorsi muscle of pig from primary myofibre formation stage [embryonic Day 35 (E35)] to secondary myofibre formation stage (E80). In the hierarchical genomic structure, we found that 11.43% of genome switched compartment A/B status, 14.53% of topologically associating domains are changed intradomain interactions (D-scores) and 2,730 genes with differential promoter–enhancer interactions and (or) enhancer activity from E35 to E80. The alterations of genome architecture were found to correlate with expression of genes that play significant roles in neuromuscular junction, embryonic morphogenesis, skeletal muscle development or metabolism, typically, NEFL, MuSK, SLN, Mef2D and GCK. Significantly, Sox6 and MATN2 play important roles in the process of primary to secondary myofibres formation and increase the regulatory potential score and genes expression in it. In brief, we reveal the genomic reorganization from E35 to E80 and construct genome-wide high-resolution interaction maps that provide a resource for studying long-range control of gene expression from E35 to E80.

Comparative analysis of the transcriptomes of EDL, psoas, and soleus muscles from mice

BACKGROUND

Individual skeletal muscles have evolved to perform specific tasks based on their molecular composition. In general, muscle fibers are characterized as either fast-twitch or slow-twitch based on their myosin heavy chain isoform profiles. This approach made sense in the early days of muscle studies when SDS-PAGE was the primary tool for mapping fiber type. However, Next Generation Sequencing tools permit analysis of the entire muscle transcriptome in a single sample, which allows for more precise characterization of differences among fiber types, including distinguishing between different isoforms of specific proteins. We demonstrate the power of this approach by comparing the differential gene expression patterns of extensor digitorum longus (EDL), psoas, and soleus from mice using high throughput RNA sequencing.

RESULTS

EDL and psoas are typically classified as fast-twitch muscles based on their myosin expression pattern, while soleus is considered a slow-twitch muscle. The majority of the transcriptomic variability aligns with the fast-twitch and slow-twitch characterization. However, psoas and EDL exhibit unique expression patterns associated with the genes coding for extracellular matrix, myofibril, transcription, translation, striated muscle adaptation, mitochondrion distribution, and metabolism. Furthermore, significant expression differences between psoas and EDL were observed in genes coding for myosin light chain, troponin, tropomyosin isoforms, and several genes encoding the constituents of the Z-disk.

CONCLUSIONS

The observations highlight the intricate molecular nature of skeletal muscles and demonstrate the importance of utilizing transcriptomic information as a tool for skeletal muscle characterization.

Unique functional properties of slow skeletal muscle tropomyosin

Tropomyosin (Tpm) is an α-helical coiled-coil actin-binding protein playing an essential role in the regulation of muscle contraction. The α- (Tpm 1.1) and γ- (Tpm 3.12) Tpm isoforms are expressed in fast and slow human skeletal muscles, respectively, while β-Tpm (Tpm 2.2) is expressed in both muscle types. This results in the formation of Tpm αα- and γγ-homodimers as well as αβ- and γβ-heterodimers. The properties of αα-homodimer are well studied, whereas very little is known about the functional properties of γγ-homodimer and γβ-heterodimer. We investigated interaction characteristics of Tpm γγ-homodimer and γβ-heterodimer with actin filaments and Ca²⁺-regulation of actin-myosin interaction on myosin from fast and slow skeletal muscles. The results showed that complexes formed by γγ-Tpm and γβ-Tpm with F-actin are more stable than those with αα-Tpm and αβ-Tpm. The maximum sliding speed of regulated thin filaments with either γγ-Tpm or γβ-Tpm moving over skeletal myosin was significantly less than that of the filaments with αα-Tpm or αβ-Tpm. The results indicate that isoforms of Tpm along with isoforms of myosin determine of functional properties of skeletal muscles and support an idea on the combined expression of myosin and Tpm isoforms.

Defective sarcomere organization and reduced larval locomotion and fish survival in slow muscle heavy chain 1 (smyhc1) mutants

Zebrafish skeletal muscles are broadly divided into slow-twitch and fast-twitch muscle fibers. The slow fibers, which express a slow fiber-specific myosin heavy chain 1 (Smyhc1), are the first group of muscle fibers formed during myogenesis. To uncover Smyhc1 function in muscle growth, we generated three mutant alleles with reading frame shift mutations in the zebrafish smyhc1 gene using CRISPR. The mutants showed shortened sarcomeres with no thick filaments and M-lines in slow fibers of the mutant embryos. However, the formation of slow muscle precursors and expression of other slow muscle genes were not affected and fast muscles appeared normal. The smyhc1 mutant embryos and larvae showed reduced locomotion and food intake. The mutant larvae exhibited increased lethality of incomplete penetrance. Approximately 2/5 of the homozygous mutants were viable and grew into reproductive adults. These adult mutants displayed a typical pattern of slow and fast muscle fiber distribution, and regained normal slow muscle formation. Together, our studies indicate that Smyhc1 is essential for myogenesis in embryonic slow muscles, and loss of Smyhc1 results in defective sarcomere assembly, reduces larval motility and fish survival, but has no visible impact on muscle growth in juvenile and adult zebrafish that escape the larval lethality.

Wild antelope skeletal muscle antioxidant enzyme activities do not correlate with muscle fibre type or oxidative metabolism

Wild antelope are some of the fastest land animals in the world, presenting with high oxidative and glycolytic skeletal muscle metabolism. However, no study has investigated their muscle antioxidant capacity, and may assist in understanding their physical ability and certain pathophysiological manifestations, such as capture myopathy. Therefore, the primary aim of this study was to determine the antioxidant activities superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR), as well as five key regulatory enzymes that serve as markers of glycolysis (phosphofructokinase (PFK) and lactate dehydrogenase (LDH)), the tricarboxylic acid cycle (citrate synthase (CS)), β-oxidation (3-hydroxyacetyl CoA dehydrogenase (3HAD)) and the phosphagen pathway (creatine kinase (CK)), in the Vastus lateralis muscle of six southern African wild antelope species (mountain reedbuck, springbok, blesbok, fallow deer, black wildebeest and kudu). Four different muscle groups from laboratory rats served as reference values for the enzyme activities. SOD, CS and LDH activities were the highest in the wild antelope, whereas CK appeared highest in rat fast twitch muscles. Between the wild antelope species, differences exist for SOD, CAT, PFK, CK and LDH, but not for CS, 3HAD and GR. CAT and GR correlated positively only with type I fibres. No correlations could be found between muscle fibre type and the oxidative enzymes, CS and 3HAD, from the wild animals, concurring with previous studies on porcine and rats. However, wild antelope and rat muscle CK and SOD strongly correlated, hinting towards an antioxidant role for CK.

Cheap labor: myosin fiber type expression and enzyme activity in the forelimb musculature of sloths (Pilosa: Xenarthra)

Sloths are canopy-dwelling inhabitants of American neotropical rainforests that exhibit suspensory behaviors. These abilities require both strength and muscular endurance to hang for extended periods of time; however, the skeletal muscle mass of sloths is reduced, thus requiring modifications to muscle architecture and leverage for large joint torque. We hypothesize that intrinsic muscle properties are also modified for fatigue resistance and predict a heterogeneous expression of slow/fast myosin heavy chain (MHC) fibers that utilize oxidative metabolic pathways for economic force production. MHC fiber type distribution and energy metabolism in the forelimb muscles of three-toed ( Bradypus variegatus, n = 5) and two-toed ( Choloepus hoffmanni, n = 4) sloths were evaluated using SDS-PAGE, immunohistochemistry, and enzyme activity assays. The results partially support our hypothesis by a primary expression of the slow MHC-1 isoform as well as moderate expression of fast MHC-2A fibers, whereas few hybrid MHC-1/2A fibers were found in both species. MHC-1 fibers were larger in cross-sectional area (CSA) than MHC-2A fibers and comprised the greatest percentage of CSA in each muscle sampled. Enzyme assays showed elevated activity for the anaerobic enzymes creatine kinase and lactate dehydrogenase compared with low activity for aerobic markers citrate synthase and 3-hydroxyacetyl CoA dehydrogenase. These findings suggest that sloth forelimb muscles may rely heavily on rapid ATP resynthesis pathways, and lactate accumulation may be beneficial. The intrinsic properties observed match well with suspensory requirements, and these modifications may have further evolved in unison with low metabolism and slow movement patterns as means to systemically conserve energy. NEW & NOTEWORTHY Myosin heavy chain (MHC) fiber type and fiber metabolic properties were evaluated to understand the ability of sloths to remain suspended for extended periods without muscle fatigue. Broad distributions of large, slow MHC-1 fibers as well as small, fast MHC-2A fibers are expressed in sloth forelimbs, but muscle metabolism is generally not correlated with myosin fiber type or body size. Sloth muscles rely on rapid, anaerobic pathways to resist fatigue and sustain force production.

Distribution of muscle fibre conduction velocity for representative samples of motor units in the full recruitment range of the tibialis anterior muscle

AIM

Motor units are recruited in an orderly manner according to the size of motor neurones. Moreover, because larger motor neurones innervate fibres with larger diameters than smaller motor neurones, motor units should be recruited orderly according to their conduction velocity (MUCV). Because of technical limitations, these relations have been previously tested either indirectly or in small motor unit samples that revealed weak associations between motor unit recruitment threshold (RT) and MUCV. Here, we analyse the relation between MUCV and RT for large samples of motor units.

METHODS

Ten healthy volunteers completed a series of isometric ankle dorsiflexions at forces up to 70% of the maximum. Multi-channel surface electromyographic signals recorded from the tibialis anterior muscle were decomposed into single motor unit action potentials, from which the corresponding motor unit RT, MUCV and action potential amplitude were estimated. Established relations between muscle fibre diameter and CV were used to estimate the fibre size.

RESULTS

Within individual subjects, the distributions of MUCV and fibre diameters were unimodal and did not show distinct populations. MUCV was strongly correlated with RT (mean (SD) R²  = 0.7 (0.09), P < 0.001; 406 motor units), which supported the hypothesis that fibre diameter is associated with RT.

CONCLUSION

The results provide further evidence for the relations between motor neurone and muscle fibre properties for large samples of motor units. The proposed methodology for motor unit analysis has also the potential to open new perspectives in the study of chronic and acute neuromuscular adaptations to ageing, training and pathology.

Exogenous and Endogenous Factors Influencing Color of Fresh Meat from Ungulates

Biochemistry of post-mortem muscle tissue is complex, and several factors affect the fresh meat color and color stability, both of which influence consumer acceptance. Therefore, improving meat color and meat color stability is of significant value to the meat industry and consumers. While extensive literature is available on the color and color stability of domestic ungulates, literature on wild ungulates is notably lacking. With an increasing global demand for meats from wild ungulates, it is critical to identify the knowledge gaps regarding their color and color stability. The objective of this paper is to overview the exogenous and endogenous factors influencing the color and color stability of fresh meats from domestic and wild ungulates. The literature highlighted that the pre- and post-harvest factors influencing meat color and meat color stability are interrelated and not mutually exclusive. Current research indicates that the effects of several of these factors are specific to species, breed, and muscle source. Novel ways to manipulate these factors using a biosystems approach should be explored to improve color attributes of fresh ungulate meats.

Imbalances in the Development of Muscle and Tendon as Risk Factor for Tendinopathies in Youth Athletes: A Review of Current Evidence and Concepts of Prevention

Tendons feature the crucial role to transmit the forces exerted by the muscles to the skeleton. Thus, an increase of the force generating capacity of a muscle needs to go in line with a corresponding modulation of the mechanical properties of the associated tendon to avoid potential harm to the integrity of the tendinous tissue. However, as summarized in the present narrative review, muscle and tendon differ with regard to both the time course of adaptation to mechanical loading as well as the responsiveness to certain types of mechanical stimulation. Plyometric loading, for example, seems to be a more potent stimulus for muscle compared to tendon adaptation. In growing athletes, the increased levels of circulating sex hormones might additionally augment an imbalanced development of muscle strength and tendon mechanical properties, which could potentially relate to the increasing incidence of tendon overload injuries that has been indicated for adolescence. In fact, increased tendon stress and strain due to a non-uniform musculotendinous development has been observed recently in adolescent volleyball athletes, a high-risk group for tendinopathy. These findings highlight the importance to deepen the current understanding of the interaction of loading and maturation and demonstrate the need for the development of preventive strategies. Therefore, this review concludes with an evidence-based concept for a specific loading program for increasing tendon stiffness, which could be implemented in the training regimen of young athletes at risk for tendinopathy. This program incorporates five sets of four contractions with an intensity of 85–90% of the isometric voluntary maximum and a movement/contraction duration that provides 3 s of high magnitude tendon strain.

Modulators of actin-myosin dissociation: basis for muscle type functional differences during fatigue

The muscle types present with variable fatigue tolerance, in part due to the myosin isoform expressed. However, the critical steps that define “fatigability” in vivo of fast vs. slow myosin isoforms, at the molecular level, are not yet fully understood. We examined the modulation of the ATP-induced myosin subfragment 1 (S1) dissociation from pyrene-actin by inorganic phosphate (P_i), pH, and temperature using a specially modified stopped-flow system that allowed fast kinetics measurements at physiological temperature. We contrasted the properties of rabbit psoas (fast) and bovine masseter (slow) myosins (obtained from samples collected from New Zealand rabbits and from a licensed abattoir, respectively, according to institutional and national ethics permits). To identify ATP cycling biochemical intermediates, we assessed ATP binding to a preequilibrated mixture of actomyosin and variable [ADP], pH (pH 7 vs. pH 6.2), and P_i (zero, 15, or 30 added mM P_i) in a range of temperatures (5 to 45°C). Temperature and pH variations had little, if any, effect on the ADP dissociation constant (K_ADP) for fast S1, but for slow S1, K_ADP was weakened with increasing temperature or low pH. In the absence of ADP, the dissociation constant for phosphate (K_Pi) was weakened with increasing temperature for fast S1. In the presence of ADP, myosin type differences were revealed at the apparent phosphate affinity, depending on pH and temperature. Overall, the newly revealed kinetic differences between myosin types could help explain the in vivo observed muscle type functional differences at rest and during fatigue.

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.

Comparative analysis of DNA methylome and transcriptome of skeletal muscle in lean-, obese-, and mini-type pigs

DNA methylation plays a pivotal role in biological processes by affecting gene expression. However, how DNA methylation mediates phenotype difference of skeletal muscle between lean-, obese-, and mini-type pigs remains unclear. We systematically carried out comparative analysis of skeletal muscle by integrating analysis of genome-wide DNA methylation, mRNA, lncRNA and miRNA profiles in three different pig breeds (obese-type Tongcheng, lean-type Landrace, and mini-type Wuzhishan pigs). We found that the differentially methylated genes (DMGs) were significantly associated with lipid metabolism, oxidative stress and muscle development. Among the identified DMGs, 253 genes were related to body-size and obesity. A set of lncRNAs and mRNAs including UCP3, FHL1, ANK1, HDAC4, and HDAC5 exhibited inversely changed DNA methylation and expression level; these genes were associated with oxidation reduction, fatty acid metabolism and cell proliferation. Gene regulatory networks involved in phenotypic variation of skeletal muscle were related to lipid metabolism, cellular movement, skeletal muscle development, and the p38 MAPK signaling pathway. DNA methylation potentially influences the propensity for obesity and body size by affecting gene expression in skeletal muscle. Our findings provide an abundant information of epigenome and transcriptome that will be useful for animal breeding and biomedical research.

Fast-to-Slow Transition of Skeletal Muscle Contractile Function and Corresponding Changes in Myosin Heavy and Light Chain Formation in the R6/2 Mouse Model of Huntington's Disease

Huntington´s disease (HD) is a hereditary neurodegenerative disease resulting from an expanded polyglutamine sequence (poly-Q) in the protein huntingtin (HTT). Various studies report atrophy and metabolic pathology of skeletal muscle in HD and suggest as part of the process a fast-to-slow fiber type transition that may be caused by the pathological changes in central motor control or/and by mutant HTT in the muscle tissue itself. To investigate muscle pathology in HD, we used R6/2 mice, a common animal model for a rapidly progressing variant of the disease expressing exon 1 of the mutant human gene. We investigated alterations in the extensor digitorum longus (EDL), a typical fast-twitch muscle, and the soleus (SOL), a slow-twitch muscle. We focussed on mechanographic measurements of excised muscles using single and repetitive electrical stimulation and on the expression of the various myosin isoforms (heavy and light chains) using dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of whole muscle and single fiber preparations. In EDL of R6/2, the functional tests showed a left shift of the force-frequency relation and decrease in specific force. Moreover, the estimated relative contribution of the fastest myosin isoform MyHC IIb decreased, whereas the contribution of the slower MyHC IIx isoform increased. An additional change occurred in the alkali MyLC forms showing a decrease in 3f and an increase in 1f level. In SOL, a shift from fast MyHC IIa to the slow isoform I was detectable in male R6/2 mice only, and there was no evidence of isoform interconversion in the MyLC pattern. These alterations point to a partial remodeling of the contractile apparatus of R6/2 mice towards a slower contractile phenotype, predominantly in fast glycolytic fibers.

Myosin content of single muscle fibers following short-term disuse and active recovery in young and old healthy men

Short-term disuse and subsequent recovery affect whole muscle and single myofiber contractile function in young and old. While the loss and recovery of single myofiber specific force (SF) following disuse and rehabilitation has been shown to correlate with alterations in myosin concentrations in young, it is unknown whether similar relationships exist in old. Therefore, the purpose of the present study was to examine the effect of 14days lower limb disuse followed by 28days of active recovery on single muscle fiber myosin content in old (68yrs) and young (24yrs) recreationally physically active healthy men. Following disuse, myosin content decreased (p<0.05) in MHC 1 (young -28%, old -19%) and 2a fibers (young -23%, old -32%). In old, myosin content decreased more (p<0.05) in MHC 2a vs 1 fibers. Following recovery, myosin content increased (p<0.05) and returned to pre-disuse levels for both young and old in both fiber types, with MHC 2a fibers demonstrating an overshooting in young (+31%, p<0.05) but not old. Strong correlations were observed between myosin content and single fiber SF in both young and old, with greater slope steepness in MHC 2a vs 1 fibers indicating an enhanced intrinsic contractile capacity of MHC 2a fibers. In conclusion, adaptive changes in myofiber myosin content appear to occur rapidly following brief periods of disuse (2wks) and after subsequent active recovery (4wks) in young and old, which contribute to alterations in contractile function at the single muscle fiber level. Changes in myosin content appear to occur independently of age, while influenced by fiber type (MHC isoform) in young but not old.

Functional Segregation within the Muscles of Aquatic Propulsion in the Asiatic Water Monitor (Varanus salvator)

Water monitor lizards (Varanus salvator) swim using sinusoidal oscillations generated at the base of their long (50% of total body length) tail. In an effort to determine which level of the structural/organizational hierarchy of muscle is associated with functional segregation between the muscles of the tail base, an array of muscle features-myosin heavy chain profiles, enzymatic fiber types, twitch and tetanic force production, rates of fatigue, muscle compliance, and electrical activity patterns-were quantitated. The two examined axial muscles, longissimus, and iliocaudalis, were generally similar at the molecular, biochemical, and physiological levels, but differed at the biomechanics level and in their activation pattern. The appendicular muscle examined, caudofemoralis, differed from the axial muscles particularly at the molecular and physiological levels, and it exhibited a unique compliance profile and pattern of electrical activation. There were some apparent contradictions between the different structural/organizational levels examined. These contradictions, coupled with a unique myosin heavy chain profile, lead to the hypothesis that there are previously un-described molecular/biochemical specializations within varanid skeletal muscles.

Contractile properties of motor units and expression of myosin heavy chain isoforms in rat fast-type muscle after volitional weight-lifting training

Dynamic resistance training increases the force and speed of muscle contraction, but little is known about modifications to the contractile properties of the main physiological types of motor units (MUs) that contribute to these muscle adaptations. Although the contractile profile of MU muscle fibers is tightly coupled to myosin heavy chain (MyHC) protein expression, it is not well understood if MyHC transition is a prerequisite for modifications to the contractile characteristics of MUs. In this study, we examined MU contractile properties, the mRNA expression of MyHC, parvalbumin, and sarcoendoplasmic reticulum Ca²⁺ pump isoforms, as well as the MyHC protein content after 5 wk of volitional progressive weight-lifting training in the medial gastrocnemius muscle in rats. The training had no effect on MyHC profiling or Ca²⁺-handling protein gene expression. Maximum force increased in slow (by 49%) and fast (by 21%) MUs. Within fast MUs, the maximum force increased in most fatigue-resistant and intermediate but not most fatigable MUs. Twitch contraction time was shortened in slow and fast fatigue-resistant MUs. Twitch half-relaxation was shortened in fast most fatigue-resistant and intermediate MUs. The force-frequency curve shifted rightward in fast fatigue-resistant MUs. Fast fatigable MUs fatigued less within the initial 15 s while fast fatigue-resistant units increased the ability to potentiate the force within the first minute of the standard fatigue test. In conclusion, at the early stage of resistance training, modifications to the contractile characteristics of MUs appear in the absence of MyHC transition and the upregulation of Ca²⁺-handling genes.

Influence of the contractile properties of muscle on motor unit firing rates during a moderate-intensity contraction in vivo

It is suggested that firing rate characteristics of motor units (MUs) are influenced by the physical properties of the muscle. However, no study has correlated MU firing rates at recruitment, targeted force, or derecruitment with the contractile properties of the muscle in vivo. Twelve participants (age = 20.67 ± 2.35 yr) performed a 40% isometric maximal voluntary contraction of the leg extensors that included linearly increasing, steady force, and decreasing segments. Muscle biopsies were collected with myosin heavy chain (MHC) content quantified, and surface electromyography (EMG) was recorded from the vastus lateralis. The EMG signal was decomposed into the firing events of single MUs. Slopes and y-intercepts were calculated for 1) firing rates at recruitment vs. recruitment threshold, 2) mean firing rates at steady force vs. recruitment threshold, and 3) firing rates at derecruitment vs. derecruitment threshold relationships for each subject. Correlations among type I %MHC isoform content and the slopes and y-intercepts from the three relationships were examined. Type I %MHC isoform content was correlated with MU firing rates at recruitment (y-intercepts: r = -0.577; slopes: r = 0.741) and targeted force (slopes: r = 0.853) vs. recruitment threshold and MU firing rates at derecruitment (y-intercept: r = -0.597; slopes: r = 0.701) vs. derecruitment threshold relationships. However, the majority of the individual MU firing rates vs. recruitment and derecruitment relationships were not significant (P > 0.05) and, thus, revealed no systematic pattern. In contrast, MU firing rates during the steady force demonstrated a systematic pattern with higher firing rates for the lower- than higher-threshold MUs and were correlated with the physical properties of MUs in vivo.

Skeletal muscle dysfunction in the db/db mouse model of type 2 diabetes

INTRODUCTION

In this study we examined the mechanisms of motor dysfunction in type 2 diabetes.

METHODS

Contractile force was measured in isolated nerve-muscle preparations of db/db mice using various protocols for electrical stimulation. Sarcoplasmic reticulum Ca(2+) adenosine triphosphatase protein (SERCA) was quantified by comparing Ca(2+) -dependent and non-specific phosphorylation.

RESULTS

Compared with controls, the muscle-nerve preparations of db/db mice displayed muscle atrophy, reduced axonal excitability, and force deficit when stimulated via the nerve. Muscle relaxation after contraction was slowed, and SERCA content was reduced. In contrast, the sensitivity of the neuromuscular junction to tubocurarine and muscle fiber excitability were not affected.

CONCLUSIONS

The force deficit in db/db muscles was caused by atrophy and failure of neuromuscular signal transmission related to motor nerve axonal dysfunction. The slowed relaxation rate generally observed in diabetic muscles can, to a large extent, be explained by decreased SERCA pump content. Muscle Nerve 54: 460-468, 2016.

Age-related functional changes and susceptibility to eccentric contraction-induced damage in skeletal muscle cell

Depending upon external loading conditions, skeletal muscles can either shorten, lengthen, or remain at a fixed length as they produce force. Fixed-end or isometric contractions stabilize joints and allow muscles to act as active struts during locomotion. Active muscles dissipate energy when they are lengthened by an external force that exceeds their current force producing capacity. These unaccustomed eccentric activities often lead to muscle weakness, soreness, and inflammation. During aging, the ability to produce force under these conditions is reduced and appears to be due to not only reductions in muscle mass but also to alterations in the basic mechanisms of contraction. These alterations include impairments in the excitation–contraction process, and the action of the cross-bridges. Also, it is well known that age-related skeletal muscle atrophy is characterized by a preferential atrophy of fast fibers, and increased susceptibility to fast muscle fiber when aged muscles are exposed to eccentric contraction followed by the impaired recovery process has been reported. Taken together, the selective loss of fast muscle fiber in aged muscle could be affected by eccentric-induced muscle damage, which has significant implication to identify the etiology of the age-related functional changes. Therefore, in this review the alteration of age-related muscle function and its impact to/of eccentric induced muscle damage and recovery will be addressed in detail.

Early phase interference between low-intensity running and power training in moderately trained females

PURPOSE

The aim of the study was to investigate the effects of low-intensity running performed immediately after lower-body power-training sessions on power development.

METHODS

Twenty young females participated in 6 weeks, 3/week, of either lower body power training (PT) or lower body power training followed by 30 min of low-intensity running (PET) eliciting 60-70 % of maximal heart rate. The following were measured before and after the training period: counter-movement jump, isometric leg press force and rate of force development (RFD), half squat 1-RM, vastus lateralis fiber type composition and cross sectional area, resting intramuscular fiber conduction velocity (MFCV), and heart rate during the modified Bruce treadmill test.

RESULTS

Counter-movement jump height and peak power increased after PT (10.7 ± 6.2 and 12.9 ± 18.7 %, p < 0.05) but not after PET (3.4 ± 7.6 and 5.11 ± 10.94 %, p > 0.05). Maximum isometric force, RFD, and half squat 1-RM increased similarly in both groups. Muscle fiber type composition was not altered in either group. Muscle fiber cross sectional area increased only after PT (17.5 ± 17.4, 14.5 ± 10.4, 20.36 ± 11.3 %, in type I, IIA, and IIX fibers, respectively, p < 0.05). Likewise, mean MFCV increased with PT only (before: 4.53 ± 0.38 m s(-1), after: 5.09 ± 0.39 m s(-1), p = 0.027). Submaximal heart rate during the Bruce treadmill test remained unchanged after PT but decreased after PET.

CONCLUSION

These results suggest that low-intensity running performed after lower-body power training impairs the exercise-induced adaptation in stretch-shortening cycle jumping performance (vertical jump height, peak power), during the first 6 weeks of training, which may be partially linked to inhibited muscle fiber hypertrophy and muscle fiber conduction velocity.

Evidence for ACTN3 as a Speed Gene in Isolated Human Muscle Fibers

Purpose

To examine the effect of α-actinin-3 deficiency due to homozygosity for the ACTN3 577X-allele on contractile and morphological properties of fast muscle fibers in non-athletic young men.

Methods

A biopsy was taken from the vastus lateralis of 4 RR and 4 XX individuals to test for differences in morphologic and contractile properties of single muscle fibers. The cross-sectional area of the fiber and muscle fiber composition was determined using standard immunohistochemistry analyses. Skinned single muscle fibers were subjected to active tests to determine peak normalized force (P₀), maximal unloading velocity (V₀) and peak power. A passive stretch test was performed to calculate Young’s Modulus and hysteresis to assess fiber visco-elasticity.

Results

No differences were found in muscle fiber composition. The cross-sectional area of type II_a and II_x fibers was larger in RR compared to XX individuals (P<0.001). P₀ was similar in both groups over all fiber types. A higher V₀ was observed in type II_a fibers of RR genotypes (P<0.001) but not in type I fibers. The visco-elasticity as determined by Young’s Modulus and hysteresis was unaffected by fiber type or genotype.

Conclusion

The greater V₀ and the larger fast fiber CSA in RR compared to XX genotypes likely contribute to enhanced whole muscle performance during high velocity contractions.

The effect of serum origin on tissue engineered skeletal muscle function

Skeletal muscle phenotype is regulated by a complex interaction between genetic, hormonal, and electrical inputs. However, because of the interrelatedness of these factors in vivo it is difficult to determine the importance of one over the other. Over the last 5 years, we have engineered skeletal muscles in the European Union (EU) and the United States (US) using the same clone of C2C12 cells. Strikingly, the dynamics of contraction of the muscles was dramatically different. Therefore, in this study we sought to determine whether the hormonal milieu (source of fetal bovine serum (FBS)) could alter engineered muscle phenotype. In muscles engineered in serum of US origin time-to-peak tension (2.2-fold), half relaxation (2.6-fold), and fatigue resistance (improved 25%) all showed indications of a shift towards a slower phenotype. Even though there was a dramatic shift in the rate of contraction, myosin heavy chain expression was the same. The contraction speed was instead related to a shift in calcium release/sensitivity proteins (DHPR = 3.1-fold lower, slow CSQ = 3.4-fold higher, and slow TnT = 2.4-fold higher) and calcium uptake proteins (slow SERCA = 1.7-fold higher and parvalbumin = 41-fold lower). These shifts in calcium dynamics were accompanied by a partial shift in metabolic enzymes, but could not be explained by purported regulators of muscle phenotype. These data suggest that hormonal differences in serum of USDA and EU origin cause a shift in calcium handling resulting in a dramatic change in engineered muscle function.

Transition of myosin heavy chain isoforms in human laryngeal abductors following denervation

The objective of this study was to investigate the myofiber subtype transition of human posterior cricoarytenoid (PCA) muscle after the injury to recurrent laryngeal nerve (RLN). PCA muscle specimens were obtained from 38 bilateral vocal fold paralysis patients underwent arytenoidectomy. According to the duration of RLN injury, all the cases were divided into five denervation groups: 6-12 months, 1-2, 2-3, 3-6, and >6 years. The normal PCA muscles from total laryngectomy patients were chosen as controls. Immunofluorescence was adopted to detect the expression level of myosin heavy chain (MHC)-I and MHC-II in PCA muscle. Quantitative real-time PCR was also used to assess the transcriptional level of MHC subtypes (MHC-I, MHC-IIa, MHC-IIb, MHC-IIx, embryonic-MHC, and peri-natal-MHC). Immunofluorescence showed that MHC-I-positive myofibers in denervation groups were much lower than control group, respectively, while MHC-II-positive myofibers were significantly higher than control group (P < 0.05). With the extension of denervation, the number of MHC-I-positive myofibers gradually decreased, while MHC-II gradually increased and peaked in 1- to 2-year group. Transcriptional level of MHC-I, MHC-IIa, and MHC-IIb in denervation groups significantly down-regulated compared with the control (P < 0.05), respectively. However, MHC-IIx, embryonic-MHC, and peri-natal-MHC significantly up-regulated in all denervation groups, and the highest level was in 1- to 2-year denervation group. Data from the present study demonstrated that the maximum transition of MHC subtypes in human PCA muscles occurred in 1-2 years after denervation, suggesting that laryngeal reinnervation before the occurrence of irreversible transition of MHC subtypes could maintain the structural integrity of laryngeal PCA muscles.

Skeletal muscle signature of a champion sprint runner

We had the unique opportunity to study the skeletal muscle characteristics, at the single fiber level, of a world champion sprint runner who is the current indoor world record holder in the 60-m hurdles (7.30 s) and former world record holder in 110-m hurdles (12.91 s). Muscle biopsies were obtained from the vastus lateralis at rest and 4 h after a high-intensity exercise challenge (4 × 7 repetitions of resistance exercise). Single muscle fiber analyses were conducted for fiber type distribution (myosin heavy chain, MHC), fiber size, contractile function (strength, speed, and power) and mRNA expression (before and after the exercise bout). The world-class sprinter's leg muscle had a high abundance (24%) of the pure MHC IIx muscle fibers with a total fast-twitch fiber population of 71%. Power output of the MHC IIx fibers (35.1 ± 1.4 W/l) was 2-fold higher than MHC IIa fibers (17.1 ± 0.5 W/l) and 14-fold greater than MHC I fibers (2.5 ± 0.1 W/l). Additionally, the MHC IIx fibers were highly responsive to intense exercise at the transcriptional level for genes involved with muscle growth and remodeling (Fn14 and myostatin). To our knowledge, the abundance of pure MHC IIx muscle fibers is the highest observed in an elite sprinter. Further, the power output of the MHC IIa and MHC IIx muscle fibers was greater than any human values reported to date. These data provide a myocellular basis for the high level of sprinting success achieved by this individual.

Resistance to high-fat diet-induced obesity in male heterozygous Pprc1 knockout mice

Peroxisome proliferator-activated receptor gamma, co-activator-related 1 (Pprc1) is the third member of the Pgc1 family. Other than the well-characterized Pgc1a and Pgc1b that act as regulators of mitochondrial biogenesis and oxidative metabolism, the function of Pprc1 in vivo is rarely reported, due to embryonic lethality of whole-body Pprc1 knockout mice. To investigate the biological and physiological function of Pprc1 in metabolic processes, male Pprc1(+/-) mice fed with a high fat diet (HFD) showed resistance to diet-induced obesity with a decrease of adipose tissue in Pprc1(+/-) mice, which was a result of elevated energy expenditure. In skeletal muscle of Pprc1(+/-) mice, Pprc1 level showed haplo-insufficiency with down-regulation of Pgc1b and Pgc1a, whereas in adipose tissue, Pprc1 expression remained normal, with significant compensatory increase of other Pgc1 family members to induce an up-regulation of respiratory chain genes. Taken together, as the first report on the metabolic roles of Pprc1 in vivo, these results indicated an elevated basal metabolic rate and lipid metabolic alteration of male Pprc1(+/-) mice on HFD, suggesting the significant role of Pprc1 in controlling mitochondrial gene expression and energy metabolic processes, synergistically with Pgc1a and Pgc1b.

Dormancy and quiescence of skeletal muscle stem cells

The skeletal muscle of vertebrates has a huge regenerative capacity. When destroyed after different types of injury, this organ can regenerate very quickly (less than 20 days following myotoxin injection in the mouse) ad integrum and repeatedly. The cell responsible for this regeneration is the so-called satellite cell, the muscle stem cell that lies on top of the muscle fibre, a giant, multinucleated cell that contains the contractile material. When injected in the muscle, satellite cells can efficiently differentiate into contractile muscle fibres. The satellite cell shows great therapeutic potential; and its regenerative capacity has triggered particular interest in the field of muscular degeneration. In this review we will focus on one particular property of the satellite cell: its quiescence and dormancy. Indeed adult satellite cells are quiescent; they lie between the basal lamina and the basement membrane of the muscle fibre, ready to proliferate, and fuse in order to regenerate myofibers upon injury. It has recently been shown that a subpopulation of satellite cells is able to enter dormancy in human and mice cadavers. Dormancy is defined by a low metabolic state, low mobility, and a long lag before division when plated in vitro, compared to quiescent cells. This definition is also based on current knowledge about long-term hematopoietic stem cells, a subpopulation of stem cells that are described as dormant based on the same criteria (rare division and low metabolism when compared to progeny which are dividing more often). In the first part of this review, we will provide a description of satellite cells which addresses their quiescent state. We will then focus on the uneven distribution of satellite cells in the muscle and describe evidence that suggests that their dormancy differs from one muscle to the next and that one should be cautious when making generalisations regarding this cellular state. In a second part, we will discuss the transition between active dividing cells in developing animals to quiescence. This mechanism could be used or amplified in the switch from quiescence to dormancy. In a third part, we will review the signals and dynamics that actively maintain the satellite cell quiescent. The in-depth understanding of these mechanisms is key to describing how dormancy relies on quiescent state of the cells. In a fourth part, we will deal with dormancy per se: how dormant satellite cells can be obtained, their characteristics, their metabolic profile, and their molecular signature as compared to quiescent cells. Here, we will highlight one of the most important recent findings: that quiescence is a prerequisite for the entry of the satellite cell into dormancy. Since dormancy is a newly discovered phenomenon, we will review the mechanisms responsible for quiescence and activation, as these two cellular states are better known and key to understanding satellite cell dormancy. This will allow us to describe dormancy and its prerequisites.

Actin sliding velocity on pure myosin isoforms from hindlimb unloaded mice

AIM

Notwithstanding the widely accepted idea that following disuse skeletal muscles become faster, an increase in shortening velocity was previously observed mostly in fibres containing type 1 myosin, whereas a decrease was generally found in fibres containing type 2B myosin. In this study, unloaded shortening velocity of pure type 1 and 2B fibres from hindlimb unloaded mice was determined and a decrease in type 2B fibres was found.

METHODS

To clarify whether the decrease in shortening velocity could depend on alterations of myosin motor function, an in vitro motility assay approach was applied to study pure type 1 and pure type 2B myosin from hindlimb unloaded mice. The latter approach, assessing actin sliding velocity on isolated myosin in the absence of other myofibrillar proteins, enabled to directly investigate myosin motor function.

RESULTS

Actin sliding velocity was significantly lower on type 2B myosin following unloading (2.70 ± 0.32 μm s(-1)) than in control conditions (4.11 ± 0.35 μm s(-1)), whereas actin sliding velocity of type 1 myosin was not different following unloading (0.89 ± 0.04 μm s(-1)) compared with control conditions (0.84 ± 0.17 μm s(-1)). Myosin light chain (MLC) isoform composition of type 2B myosin from hindlimb unloaded and control mice was not different. No oxidation of either type 1 or 2B myosin was observed. Higher phosphorylation of regulatory MLC in type 2B myosin after unloading was found.

CONCLUSION

Results suggest that the observed lower shortening velocity of type 2B fibres following unloading could be related to slowing of acto-myosin kinetics in the presence of MLC phosphorylation.

McArdle disease does not affect skeletal muscle fibre type profiles in humans

Patients suffering from glycogen storage disease V (McArdle disease) were shown to have higher surface electrical activity in their skeletal muscles when exercising at the same intensity as their healthy counterparts, indicating more muscle fibre recruitment. To explain this phenomenon, this study investigated whether muscle fibre type is shifted towards a predominance in type I fibres as a consequence of the disease. Muscle biopsies from the Biceps brachii (BB) (n = 9) or Vastus lateralis (VL) (n = 8) were collected over a 13-year period from male and female patients diagnosed with McArdle disease, analysed for myosin heavy chain (MHC) isoform content using SDS-PAGE, and compared to healthy controls (BB: n = 3; VL: n = 10). All three isoforms were expressed and no difference in isoform expression in VL was found between the McArdle patients and healthy controls (MHC I: 33±19% vs. 43±7%; MHC IIa: 52±9% vs. 40±7%; MHC IIx: 15±18% vs. 17±9%). Similarly, the BB isoform content was also not different between the two groups (MHC I: 33±14% vs. 30±11%; MHC IIa: 46±17% vs. 39±5%; MHC IIx: 21±13% vs. 31±14%). In conclusion, fibre type distribution does not seem to explain the higher surface EMG in McArdle patients. Future studies need to investigate muscle fibre size and contractility of McArdle patients.

Insights into the skeletal muscle characteristics of three southern African antelope species

Skeletal muscle fibre type, cross-sectional area (CSA), maximum enzyme capacities and fibre oxidative capacities were investigated in three southern African antelope species. Muscle samples from blesbok (Damaliscus pygargus phillipsi), mountain reedbuck (Redunca fulvorufula) and greater kudu (Tragelaphus strepsiceros) were collected post mortem from the Vastus lateralis and analysed for myosin heavy chain (MHC) isoform content, citrate synthase (CS), 3-hydroxyacyl Co A dehydrogenase (3-HAD), phosphofructokinase (PFK), lactate dehydrogenase (LDH) and creatine kinase (CK) activities. Histochemistry and immunohistochemistry were performed to determine relative fibre oxidative capacity, fibre type and cross-sectional area (CSA). Type IIX fibres were the most abundant fibre type in all three species, ranging from 43 to 57%. Kudu had less type IIX fibres than mountain reedbuck and blesbok (P<0.05), values confirmed by their respective MHC isoform content. Blesbok had the smallest fibres, followed by mountain reedbuck and finally kudu (P<0.001). Overall, all three species had high oxidative and glycolytic capacities, but species differences were found. Kudu had the lowest CS activity, followed by blesbok and mountain reedbuck, but the highest PFK, LDH and CK activities. This study confirmed large variation in oxidative capacities within a single fibre type, as well as overlap between the fibre types with no distinct differences between the three species. The fibre type profile of each species is discussed and confirms some of their physical attributes and capabilities.

Effects of chronic administration of arachidonic acid on lipid profiles and morphology in the skeletal muscles of aged rats

Arachidonic acid (20:4n-6, ARA) is a major component of the cell membrane, whereas ARA-derived eicosanoids are formed when cells are damaged. Aging is associated with an accretion of oxidative stress in skeletal muscles. In this study, we examined the effects of chronic administration (13 weeks) of ARA (240 mg/kg/day) on fatty acid composition, antioxidative status, and morphology of slow (soleus muscles) and fast (extensor digitorum longus muscles; EDL)-twitch muscles in aged rats (21 months old). The level of reactive oxygen species was higher in the EDL of ARA-administered rats than in that of control rats. ARA administration decreased the muscle cell volumes and increased the number of slow myosin heavy chain (MHC)-positive cells in the EDL. The relative content of MHC2X was increased whereas the relative content of MHC2B was decreased in the EDL of ARA-administered rats. These results suggest that ARA deposition in the fast-twitch muscle of aged rats reduced cell volume with an increase in oxidative stress.