Muscle memory in humans: evidence for myonuclear permanence and long-term transcriptional regulation after strength training.

The objective of this work was to investigate myonuclear permanence and transcriptional regulation as mechanisms for cellular muscle memory after strength training in humans. Twelve untrained men and women performed 10 weeks of unilateral elbow-flexor strength training followed by 16 weeks of de-training. Thereafter, 10 weeks' re-training was conducted with both arms: the previously trained arm and the contralateral untrained control arm. Muscle biopsies were taken from the trained arm before and after both training periods and from the control arm before and after re-training. Muscle biopsies were analysed for fibre cross-sectional area (fCSA), myonuclei and global transcriptomics (RNA sequencing). During the first training period, myonuclei increased in type 1 (13 ± 17%) and type 2 (33 ± 23%) fibres together with a 30 ± 43% non-significant increase in mixed fibre fCSA (P = 0.069). Following de-training, fCSA decreased in both fibre types, whereas myonuclei were maintained, resulting in 33% higher myonuclear number in previously trained vs. control muscle in type 2 fibres. Furthermore, in the previously trained muscle, three differentially expressed genes (DEGs; EGR1, MYL5 and COL1A1) were observed. Following re-training, the previously trained muscle showed larger type 2 fCSA compared to the control (P = 0.035). However, delta change in type 2 fCSA was not different between muscles. Gene expression was more dramatically changed in the control arm (1338 DEGs) than in the previously trained arm (822 DEGs). The sustained higher number of myonuclei in the previously trained muscle confirms myonuclear accretion and permanence in humans. Nevertheless, because of the unclear effect on the subsequent hypertrophy with re-training, the physiological benefit remains to be determined. KEY POINTS: Muscle memory is a cellular mechanism that describes the capacity of skeletal muscle fibres to respond differently to training stimuli if the stimuli have been previously encountered. This study overcomes past methodological limitations related to the choice of muscles and analytical procedures. We show that myonuclear number is increased after strength training and maintained during de-training. Increased myonuclear number and differentially expressed genes related to muscle performance and development in the previously trained muscle did not translate into a clearly superior responses during re-training. Because of the unclear effect on the subsequent hypertrophy and muscle strength gain with re-training, the physiological benefit remains to be determined.

Association analysis of indel variants and gene expression identifies MDM4 as a novel locus for skeletal muscle hypertrophy and power athlete status.

Insertions and deletions (indels) are the second most common type of variation in the human genome. However, limited data on their associations with exercise-related phenotypes have been documented. The aim of the present study was to examine the association between 18,370 indel variants and power athlete status, followed by additional studies in 357,246 individuals. In the discovery phase, the D allele of the MDM4 gene rs35493922 I/D polymorphism was over-represented in power athletes compared with control subjects (P = 7.8 × 10-9) and endurance athletes (P = 0.0012). These findings were replicated in independent cohorts, showing a higher D allele frequency in power athletes compared with control subjects (P = 0.016) and endurance athletes (P = 0.031). Furthermore, the D allele was positively associated (P = 0.0013) with greater fat-free mass in the UK Biobank. MDM4 encodes a protein that inhibits the activity of p53, which induces muscle fibre atrophy. Accordingly, we found that MDM4 expression was significantly higher in the vastus lateralis of power athletes compared with endurance athletes (P = 0.0009) and was positively correlated with the percentage of fast-twitch muscle fibres (P = 0.0062) and the relative area occupied by fast-twitch muscle fibres (P = 0.0086). The association between MDM4 gene expression and an increased proportion of fast-twitch muscle fibres was confirmed in two additional cohorts. Finally, we found that the MDM4 DD genotype was associated with increased MDM4 gene expression in vastus lateralis and greater cross-sectional area of fast-twitch muscle fibres. In conclusion, MDM4 is suggested to be a potential regulator of muscle fibre specification and size, with its indel variant being associated with power athlete status. HIGHLIGHTS: What is the central question of this study? Which indel variants are functional and associated with sport- and exercise-related traits? What is the main finding and its importance? Out of 18,370 tested indels, the MDM4 gene rs35493922 I/D polymorphism was found to be the functional variant (affecting gene expression) and the most significant, with the deletion allele showing associations with power athlete status, fat-free mass and cross-sectional area of fast-twitch muscle fibres. Furthermore, the expression of MDM4 was positively correlated with the percentage of fast-twitch muscle fibres and the relative area occupied by fast-twitch muscle fibres.

Levosimendan's Effects on Length-Dependent Activation in Murine Fast-Twitch Skeletal Muscle.

Levosimendan's calcium sensitizing effects in heart muscle cells are well established; yet, its potential impact on skeletal muscle cells has not been evidently determined. Despite controversial results, levosimendan is still expected to interact with skeletal muscle through off-target sites (further than troponin C). Adding to this debate, we investigated levosimendan's acute impact on fast-twitch skeletal muscle biomechanics in a length-dependent activation study by submersing single muscle fibres in a levosimendan-supplemented solution. We employed our MyoRobot technology to investigate the calcium sensitivity of skinned single muscle fibres alongside their stress-strain response in the presence or absence of levosimendan (100 µM). While control data are in agreement with the theory of length-dependent activation, levosimendan appears to shift the onset of the 'descending limb' of active force generation to longer sarcomere lengths without notably improving myofibrillar calcium sensitivity. Passive stretches in the presence of levosimendan yielded over twice the amount of enlarged restoration stress and Young's modulus in comparison to control single fibres. Both effects have not been described before and may point towards potential off-target sites of levosimendan.

Transcriptome analysis of long non-coding RNA associated with embryonic muscle development in chickens.

1. Skeletal muscle is an important component of chicken carcass. In chickens, the number of muscle fibres is fixed during the embryonic period, and muscle development during the embryonic period determines the muscle development potential after hatching.2. Beijing-You (BY) and Cornish (CN) chickens show completely different growth rates and body types, and two breeds were used in this study to explore the role of lncRNAs in muscle development during different chicken embryonic periods. A systematic analysis of lncRNAs and mRNAs were conducted in the pectoral muscle tissues of BY and CN chickens at embryonic days 11 (ED11), 13 (ED13), 15 (ED15), 17 (ED17), and 1-day-old (D1) using RNA-seq. A total of 4,104 differentially expressed transcripts (DETs) were identified among the five stages, including 2,359 lncRNAs and 1,745 mRNAs.3. The number of DETs between the two breeds at ED17 (1,658 lncRNAs and 1,016 mRNAs) was much higher than the total number of DET at all the other stages (692 lncRNAs and 729 mRNAs), indicating that the two breeds show the largest difference in gene regulation at ED17.4. Correlation analysis was performed for all differentially expressed lncRNAs and mRNAs during the five periods. Forty-three, cis interaction pairs of lncRNA-mRNA related to chicken muscle development were predicted. The expression of four pairs was verified, and the results showed MSTRG.12395.2-FGFBP2 and MSTRG.18590.6-FMOD were significantly up-regulated in CN at ED11 compared to BY and might be important candidate genes for embryonic muscle development.

Pikeperch muscle tissues: a comparative study of structure, enzymes, genes, and proteins in wild and farmed fish.

Pikeperch (Sander lucioperca) is a freshwater species and an internationally highly demanded fish in aquaculture. Despite intensive research efforts on this species, fundamental knowledge of skeletal muscle biology and structural characteristics is missing. Therefore, we conducted a comprehensive analysis of skeletal muscle parameters in adult pikeperch from two different origins, wild-caught specimens from a lake and those reared in a recirculating aquaculture system. The analyses comprised the biochemical characteristics (nucleic acid, protein content), enzyme activities (creatine kinase, lactate dehydrogenase, NADP-dependent isocitrate dehydrogenase), muscle-specific gene and protein expression (related to myofibre formation, regeneration and permanent growth, muscle structure), and muscle fibre structure. The findings reveal distinct differences between the skeletal muscle of wild and farmed pikeperch. Specifically, nucleic acid content, enzyme activity, and protein expression varied significantly. The higher enzyme activity observed in wild pikeperch suggests greater metabolically activity in their muscles. Conversely, farmed pikeperch indicated a potential for pronounced muscle growth. As the data on pikeperch skeletal muscle characteristics is sparse, the purpose of our study is to gain fundamental insights into the characteristics of adult pikeperch muscle. The presented data serve as a foundation for further research on percids' muscle biology and have the potential to contribute to advancements and adaptations in aquaculture practices.

Effects of dietary Allium mongolicum Regel powder supplementation on the growth performance, meat quality, antioxidant capacity and muscle fibre characteristics of fattening Angus calves under heat stress conditions.

The aim of this study was to investigate the effects of dietary Allium mongolicum Regel powder (AMRP) supplementation on the growth performance, meat quality, antioxidant capacity and muscle fibre characteristics of fattening Angus calves. Growth performance data and longissimus thoracis (LT) samples were collected from four groups of fattening Angus, which were fed either a basal diet (CON) or a basal diet supplemented with an AMRP dose of 10 (LAMR), 15 (MAMR), or 20 g/animal/day AMRP (HAMR) for 120 days before slaughter. AMRP addition to the feed improved growth performance and meat quality and altered muscle fibre type. Some responses to AMRP supplementation were dose dependent, whereas others were not. Together, the results of this study demonstrated that dietary supplementation with 10 g/animal/day AMRP was the optimal dose in terms of fattening calf growth performance, while 20 g/animal/day AMRP supplementation was the optimal dose in terms of meat quality.

Transcriptomics reveals transient and dynamic muscle fibrosis and atrophy differences following spinal cord injury in rats.

BACKGROUND: The rate and magnitude of skeletal muscle wasting after severe spinal cord injury (SCI) exceeds most other disuse conditions. Assessing the time course of molecular changes can provide insight into the progression of muscle wasting post-SCI. The goals of this study were (1) to identify potential targets that may prevent the pathologic features of SCI in soleus muscles and (2) to establish therapeutic windows for treating these pathologic changes. METHODS: Four-month-old Sprague-Dawley male rats received T9 laminectomy (SHAM surgery) or severe contusion SCI. Hindlimb locomotor function was assessed weekly, with soleus muscles obtained 1 week, 2 weeks, 1 month and 3 months post-surgery (n = 6-7 per group per timepoint). RNA was extracted from muscles for bulk RNA-sequencing analysis (n = 3-5 per group per timepoint). Differentially expressed genes (DEGs) were evaluated between age-matched SHAM and SCI animals. Myofiber size, muscle fibre type and fibrosis were assessed on contralateral muscles. RESULTS: SCI produced immediate and persistent hindlimb paralysis, with Basso-Beattie-Bresnahan locomotor scores remaining below 7 throughout the study, contributing to a progressive 25-50% lower soleus mass and myofiber atrophy versus SHAM (P < 0.05 at all timepoints). Transcriptional comparisons of SCI versus SHAM resulted in 184 DEGs (1 week), 436 DEGs (2 weeks), 133 DEGs (1 month) and 1200 DEGs (3 months). Upregulated atrophy-related genes included those associated with cell senescence, nuclear factor kappa B, ubiquitin proteasome and unfolded protein response pathways, along with upregulated genes that negatively influence muscle growth through the transforming growth factor beta pathway and inhibition of insulin-like growth factor-I/Akt/mechanistic target of rapamycin and p38/mitogen-activated protein kinase signalling. Genes associated with extracellular matrix (ECM), including collagens, collagen crosslinkers, proteoglycans and those regulating ECM integrity, were enriched within upregulated DEGs at 1 week but subsequently downregulated at 2 weeks and 3 months and were accompanied by >50% higher ECM areas and hydroxyproline levels in SCI muscles (P < 0.05). Myofiber remodelling genes were enriched in upregulated DEGs at 2 weeks and 1 month and were downregulated at 3 months. Genes that regulate neuromuscular junction remodelling were evident in muscles post-SCI, along with slow-to-fast fibre-type shifts: 1 week and 2 weeks SCI muscles were composed of 90% myosin heavy chain (MHC) type I fibres, which decreased to only 16% at 3 months and were accompanied by 50% fibres containing MHC IIX (P < 0.05). Metabolism genes were enriched in upregulated DEGs at 1 month and were further enriched at 3 months. CONCLUSIONS: Our results substantiate many known pathologic features of SCI-induced wasting in rat skeletal muscle and identify a progressive and dynamic transcriptional landscape within the post-SCI soleus. Future studies are warranted to consider these therapeutic treatment windows when countering SCI muscle pathology.

Diaphragm weakness in late-onset Pompe disease: A complex interplay between lower motor neuron and muscle fibre degeneration.

BACKGROUND: Late-onset Pompe disease (LOPD) patients may still need ventilation support at some point of their disease course, despite regular recombinant human alglucosidase alfa treatment. This suggest that other pathophysiological mechanisms than muscle fibre lesion can contribute to the respiratory failure process. We investigate through neurophysiology whether spinal phrenic motor neuron dysfunction could contribute to diaphragm weakness in LOPD patients. MATERIAL AND METHODS: A group of symptomatic LOPD patients were prospectively studied in our centre from January 2022 to April 2023. We collected both demographic and clinical data, as well as neurophysiological parameters. Phrenic nerve conduction studies and needle EMG sampling of the diaphragm were perfomed. RESULTS: Eight treated LOPD patients (3 males, 37.5%) were investigated. Three patients (37.5%) with no respiratory involvement had normal phrenic nerve motor responses [median phrenic compound muscle action potential (CMAP) amplitude of 0.49 mV; 1st-3rd interquartile range (IQR), 0.48-0.65]. Those with respiratory failure (under nocturnal non-invasive ventilation) had abnormal phrenic nerve motor responses (median phrenic CMAP amplitude of 0 mV; 1st-3rd IQR, 0-0.15), and were then investigated with EMG. Diaphragm needle EMG revealed both myopathic and neurogenic changes in 3 (60%) and myopathic potentials in 1 patient. In the last one, no motor unit potentials could be recruited. CONCLUSIONS: Our study provide new insights regarding respiratory mechanisms in LOPD, suggesting a contribution of spinal phrenic motor neuron dysfunction for diaphragm weakness. If confirmed in further studies, our results recommend the need of new drugs crossing the blood-brain barrier.

Muscle fibre typology affects whole-body metabolic rate during isolated muscle contractions and human locomotion.

The wide variation in muscle fibre type distribution across individuals, along with the very different energy consumption rates in slow versus fast muscle fibres, suggests that muscle fibre typology contributes to inter-individual differences in metabolic rate during exercise. However, this has been hard to demonstrate due to the gap between a single muscle fibre and full-body exercises. We investigated the isolated effect of triceps surae muscle contraction velocity on whole-body metabolic rate during cyclic contractions in individuals a priori selected for their predominantly slow (n = 11) or fast (n = 10) muscle fibre typology by means of proton magnetic resonance spectroscopy (1H-MRS). Subsequently, we examined their whole-body metabolic rate during walking and running at 2 m/s, exercises with comparable metabolic rates but distinct triceps surae muscle force and velocity demands (walking: low force, high velocity; running: high force, low velocity). Increasing triceps surae contraction velocity during cyclic contractions elevated net whole-body metabolic rate for both typology groups. However, the slow group consumed substantially less net metabolic energy at the slowest contraction velocity, but the metabolic difference between groups diminished at faster velocities. Consistent with the more economic force production during slow contractions, the slow group exhibited lower metabolic rates than the fast group while running, whereas metabolic rates were similar during walking. These findings provide important insights into the influence of muscle fibre typology on whole-body metabolic rate and emphasize the importance of considering muscle mechanical demands to understand muscle fibre typology related differences in whole-body metabolic rates. KEY POINTS: Muscle fibre typology is often suggested to affect whole-body metabolic rate, yet convincing in vivo evidence is lacking. Using isolated plantar flexor muscle contractions in individuals a priori selected for their predominantly slow or fast muscle fibre typology, we demonstrated that having predominantly slow muscle fibres provides a metabolic advantage during slow muscle contractions, but this benefit disappeared at faster contractions. We extended these results to full-body exercises, where we demonstrated that higher proportions of slow fibres associated with better economy during running but not when walking. These findings provide important insights into the influence of muscle fibre typology on whole-body metabolic rate and emphasize the importance of considering muscle mechanical demands to understand muscle fibre typology related differences in whole-body metabolic rate.

The Effect of Birth Weight on Fattening Performance, Meat Quality, and Muscle Fibre Characteristics in Lambs of the Karayaka Native Breed.

This investigation aimed to assess the influence of birth weight on post-weaning fattening performance, meat quality, muscle fibre characteristics, and carcass traits in Karayaka lambs. The study categorized the lambs into three distinct groups based on birth weight: low birth weight (LBW), medium birth weight (MBW), and high birth weight (HBW). Throughout the fattening phase, the lambs were given ad libitum access to food and water, culminating in the slaughter at the end of the study. Following slaughter, warm and cold carcasses were weighted, and specific muscles (longissimus thoracis et lumborum [LTL], semitendinosus [ST], and semimembranosus [SM]) were isolated for the evaluation of muscle weights, muscle fibre types (Type I, Type IIA, and Type IIB), and muscle fibre numbers. Carcass characteristics were also determined, including eye muscle (LTL) fat, loin thickness, and meat quality characteristics, such as pH, colour, texture, cooking loss, and water-holding capacity. The statistical analysis revealed highly significant differences among the experimental groups concerning muscle weights and warm and cold carcass weights (p < 0.01), with the lambs in the HBW group exhibiting a notably higher carcass yield (in females: 45.65 ± 1.34% and in males: 46.18 ± 0.77%) and LTL, ST, and SM (except for female lambs) muscle weights than the lambs in LBW group (p < 0.01). However, apart from the texture of LTL and ST muscles, no significant differences in meat quality parameters were observed among the treatment groups (p > 0.05). Notably, the birth weight of lambs did not impart a discernible effect on the total number and metabolic activity of muscle fibres in LTL, ST, and SM muscles. Nonetheless, a noteworthy distinction in the fibre area of Type I fibres in the LTL muscle of male lambs (LBW: 30.4 ± 8.9, MBW: 29.1 ± 7.3 and HBW; 77.3 ± 15.4) and in the ST muscle of female lambs (LBW: 44.1 ± 8.1, MBW: 38.8 ± 7.7 and HBW: 36.9 ± 7.1) were evident among the birth weight groups (p < 0.05). The study also found that the mean fat thickness values of eye muscles in Karayaka lambs, as obtained by ultrasonic tests, were below the typical range for sheep. In synthesis, the outcomes of this study underscore the considerable impact of birth weight on slaughtered and carcass weights, emphasizing the positive association between higher birth weights and enhanced carcass yield. Remarkably, despite these pronounced effects on carcass traits, the birth weight did not demonstrate a statistically significant influence on meat quality or overall muscle fibre characteristics, except for the area of Type I fibres in the LTL muscle. This nuanced understanding contributes valuable insights into the intricate relationship between birth weight and various physiological and carcass parameters in Karayaka lambs undergoing post-weaning fattening.

Daily blood flow restriction does not preserve muscle mass and strength during 2 weeks of bed rest.

We measured the impact of blood flow restriction on muscle protein synthesis rates, muscle mass and strength during 2 weeks of strict bed rest. Twelve healthy, male adults (age: 24 ± 3 years, body mass index: 23.7 ± 3.1 kg/m2 ) were subjected to 14 days of strict bed rest with unilateral blood flow restriction performed three times daily in three 5 min cycles (200 mmHg). Participants consumed deuterium oxide and we collected blood and saliva samples throughout 2 weeks of bed rest. Before and immediately after bed rest, lean body mass (dual-energy X-ray absorptiometry scan) and thigh muscle volume (magnetic resonance imaging scan) were assessed in both the blood flow restricted (BFR) and control (CON) leg. Muscle biopsies were collected and unilateral muscle strength (one-repetition maximum; 1RM) was assessed for both legs before and after the bed rest period. Bed rest resulted in 1.8 ± 1.0 kg lean body mass loss (P < 0.001). Thigh muscle volume declined from 7.1 ± 1.1 to 6.7 ± 1.0 L in CON and from 7.0 ± 1.1 to 6.7 ± 1.0 L in BFR (P < 0.001), with no differences between treatments (P = 0.497). In addition, 1RM leg extension strength decreased from 60.2 ± 10.6 to 54.8 ± 10.9 kg in CON and from 59.2 ± 12.1 to 52.9 ± 12.0 kg in BFR (P = 0.014), with no differences between treatments (P = 0.594). Muscle protein synthesis rates during bed rest did not differ between the BFR and CON leg (1.11 ± 0.12 vs. 1.08 ± 0.13%/day, respectively; P = 0.302). Two weeks of bed rest substantially reduces skeletal muscle mass and strength. Blood flow restriction during bed rest does not modulate daily muscle protein synthesis rates and does not preserve muscle mass or strength. KEY POINTS: Bed rest, often necessary for recovery from illness or injury, leads to the loss of muscle mass and strength. It has been postulated that blood flow restriction may attenuate the loss of muscle mass and strength during bed rest. We investigated the effect of blood flow restriction on muscle protein synthesis rates, muscle mass and strength during 2 weeks of strict bed rest. Blood flow restriction applied during bed rest does not modulate daily muscle protein synthesis rates and does not preserve muscle mass or strength. Blood flow restriction is not effective in preventing muscle atrophy during a prolonged period of bed rest.

Mechanism of post-tetanic depression of slow muscle fibres.

A brief tetanic stimulation has a very different effect on the subsequent isometric twitch force of fast and slow skeletal muscles. Fast muscle responds with an enhanced twitch force which doubles that of the pre-tetanic value, whereas slow muscle depresses the post-tetanic twitch by about 20%. Twitch potentiation of fast muscle has long been known to be due to myosin light chain 2 phosphorylation. It is proposed that post-tetanic twitch depression in slow muscle is due to the dephosphorylation of the slow isoform of the thick filament protein, myosin-binding protein-C, by Ca2+/calmodulin-activated phosphatase calcineurin, whilst its phosphorylation underlies the force enhancement due to ß-adrenergic stimulation in slow and fast muscle.

Bacillus subtilis and Enterococcus faecium co-fermented feed alters antioxidant capacity, muscle fibre characteristics and lipid profiles of finishing pigs.

This study aimed to assess how Bacillus subtilis and Enterococcus faecium co-fermented feed (FF) affects the antioxidant capacity, muscle fibre types and muscle lipid profiles of finishing pigs. In this study, a total of 144 Duroc × Berkshire × Jiaxing Black finishing pigs were randomly assigned into three groups with four replicates (twelve pigs per replication). The three treatments were a basal diet (0 % FF), basal diet + 5 % FF and basal diet + 10 % FF, respectively. The experiment lasted 38 d after 4 d of acclimation. The study revealed that 10 % FF significantly increased the activity of superoxide dismutase (SOD) and catalase (CAT) compared with 0 % FF group, with mRNA levels of up-regulated antioxidant-related genes (GPX1, SOD1, SOD2 and CAT) in 10 % FF group. 10 % FF also significantly up-regulated the percentage of slow-twitch fibre and the mRNA expression of MyHC I, MyHC IIa and MyHC IIx, and slow MyHC protein expression while reducing MyHC IIb mRNA expression. Lipidomics analysis showed that 5 % FF and 10 % FF altered lipid profiles in longissimus thoracis. 10 % FF particularly led to an increase in the percentage of TAG. The Pearson correlation analysis indicated that certain molecular markers such as phosphatidic acid (PA) (49:4), Hex2Cer (d50:6), cardiolipin (CL) (72:8) and phosphatidylcholine (PC) (33:0e) could be used to indicate the characteristics of muscle fibres and were closely related to meat quality. Together, our findings suggest that 10 % FF improved antioxidant capacity, enhanced slow-twitch fibre percentage and altered muscle lipid profiles in finishing pigs.

MYH13, a superfast myosin expressed in extraocular, laryngeal and syringeal muscles.

MYH13 is a unique type of sarcomeric myosin heavy chain (MYH) first detected in mammalian extraocular (EO) muscles and later also in vocal muscles, including laryngeal muscles of some mammals and syringeal muscles of songbirds. All these muscles are specialized in generating very fast contractions while producing relatively low force, a design appropriate for muscles acting against a much lower load than most skeletal muscles inserting into the skeleton. The definition of the physiological properties of muscle fibres containing MYH13 has been complicated by the mixed fibre type composition of EO muscles and the coexistence of different MYH types within the same fibre. A major advance in this area came from studies on isolated recombinant myosin motors and the demonstration that the affinity of actin-bound human MYH13 for ADP is much weaker than those of fast-type MYH1 (type 2X) and MYH2 (type 2A). This property is consistent with a very fast detachment of myosin from actin, a major determinant of shortening velocity. The MYH13 gene arose early during vertebrate evolution but was characterized only in mammals and birds and appears to have been lost in some teleost fish. The MYH13 gene is located at the 3' end of the mammalian fast/developmental gene cluster and in a similar position to the orthologous cluster in syntenic regions of the songbird genome. MYH13 gene regulation is controlled by a super-enhancer in the mammalian locus and deletion of the neighbouring fast MYH1 and MYH4 genes leads to abnormal MYH13 expression in mouse leg muscles.

Fusion of myofibre branches is a physiological feature of healthy human skeletal muscle regeneration.

BACKGROUND: The occurrence of hyperplasia, through myofibre splitting, remains a widely debated phenomenon. Structural alterations and fibre typing of skeletal muscle fibres, as seen during regeneration and in certain muscle diseases, can be challenging to interpret. Neuromuscular electrical stimulation can induce myofibre necrosis followed by changes in spatial and temporal cellular processes. Thirty days following electrical stimulation, remnants of regeneration can be seen in the myofibre and its basement membrane as the presence of small myofibres and encroachment of sarcolemma and basement membrane (suggestive of myofibre branching/splitting). The purpose of this study was to investigate myofibre branching and fibre type in a systematic manner in human skeletal muscle undergoing adult regenerative myogenesis. METHODS: Electrical stimulation was used to induce myofibre necrosis to the vastus lateralis muscle of one leg in 5 young healthy males. Muscle tissue samples were collected from the stimulated leg 30 days later and from the control leg for comparison. Biopsies were sectioned and stained for dystrophin and laminin to label the sarcolemma and basement membrane, respectively, as well as ATPase, and antibodies against types I and II myosin, and embryonic and neonatal myosin. Myofibre branches were followed through 22 serial Sects. (264 µm). Single fibres and tissue blocks were examined by confocal and electron microscopy, respectively. RESULTS: Regular branching of small myofibre segments was observed (median length 144 µm), most of which were observed to fuse further along the parent fibre. Central nuclei were frequently observed at the point of branching/fusion. The branch commonly presented with a more immature profile (nestin + , neonatal myosin + , disorganised myofilaments) than the parent myofibre, together suggesting fusion of the branch, rather than splitting. Of the 210 regenerating muscle fibres evaluated, 99.5% were type II fibres, indicating preferential damage to type II fibres with our protocol. Furthermore, these fibres demonstrated 7 different stages of "fibre-type" profiles. CONCLUSIONS: By studying the regenerating tissue 30 days later with a range of microscopy techniques, we find that so-called myofibre branching or splitting is more likely to be fusion of myotubes and is therefore explained by incomplete regeneration after a necrosis-inducing event.

Restricted maternal nutrition and supplementation of propylene glycol, monensin sodium and rumen-protected choline chloride during late pregnancy does not affect muscle fibre characteristics of offspring.

BACKGROUND: Grazing in arid and semi-arid regions faces pregnant ewes with feed restrictions and hence affects the offspring muscle fibre characteristics. Using feed additives that enhance nutrient availability during foetal muscle development is expected to alter offspring skeletal muscle characteristics. OBJECTIVES: This study evaluated the effect of maternal restricted nutrition and supplementation of propylene glycol, monensin sodium and rumen-protected choline chloride on lamb's muscle fibre characteristics. METHODS: Forty-eight Ghezel ewes were randomly allocated to one of six diets (N = 8) during the last 6 weeks of gestation: ad libitum feed intake (AL); restricted feeding (RF); restricted feeding containing propylene glycol (PG); restricted feeding containing propylene glycol and monensin sodium (MS); restricted feeding containing propylene glycol and rumen-protected choline chloride (RPC); restricted feeding containing propylene glycol, monensin sodium and rumen-protected choline chloride (PMC). The muscle samples were obtained from the semitendinosus muscle of 2-week-old male lambs (n = 5/treatment) via biopsy and were stained and classified as fibre types I, IIA and IIB. RESULTS: Pre-parturient maternal feed restriction and administration of propylene glycol, monensin sodium and rumen-protected choline chloride had no significant effect on fibre-type composition, fibre density of muscle, muscle cross-sectional area and volume density of fibres (p > 0.05). CONCLUSIONS: Either maternal dietary restriction or supplementation of nutrient flux-involved additives during late pregnancy did not alter muscle fibre development and had no short-term effects on muscle properties of the resulting offspring as myogenesis occurs in early and mid-gestation, not late gestation. Therefore, maternal nutrition may not be a problematic issue in sheep production in arid and semi-arid areas.

Quantitative analysis of MR T2 relaxation times in neck muscles.

T2 relaxation times (T2 times) are different between resting and exercised muscles and between muscles of healthy subjects and subjects with muscle pathology. However, studies specifically focusing on neck muscles are lacking. Furthermore, normative neck muscle T2 times are not well defined and methodology used to analyse T2 times in neck muscles is not robust. We analysed T2 times in key neck muscles and explored factors affecting variability between muscles. 20 healthy subjects were recruited. Two circular regions of interest (ROIs) were drawn in two mutually exclusive regions within neck muscles on T2 weighted images and values averaged. ROI measurements were performed by a co-investigator, supervised by a neuro-radiologist. For the first ten subjects, measurements were done from C1-T1. For the remaining subjects, ROIs were drawn at two pre-determined levels. Two MRIs were repeated at 31 degrees acquisition to evaluate the effect of muscle fibre orientation. ROI values were translated into T2 times. Results showed semispinalis capitis had the longest T2 times (range 46.88-51.42 ms), followed by splenius capitis (range 47.37-48.33 ms), trapezius (range 45.27-47.46 ms), levator scapulae (range 43.17-45.63 ms) and sternocleidomastoid (range 38.45-42.91 ms). T2 times did not vary along length of muscles and were unaffected by muscle fibre orientation (P > 0.05). T2 times of splenius capitis correlated significantly with age at C2/C3 and C5/C6 levels and trapezius at C7/T1 level. Gender did not influence relaxation times (P > 0.05). In conclusion, results of normative neck muscle T2 time values and factors influencing the T2 times could serve as a reference for future MR analysis of neck muscles. The methodology used may also be useful for related studies of neck muscles.

Effects of Atmospheric Ammonia on Skeletal Muscle Growth in Broilers.

Ammonia, one of the most polluted gases in poultry houses, has always been an urgent problem to solve. Exposure to ammonia can threaten the respiratory tract, induce inflammation, and decrease growth performance. To date, there are few studies investigating the effects of ammonia on skeletal muscle growth. In this experiment, a total of 144 broilers were randomly divided into two groups, and 0 ppm and 35 ppm atmospheric ammonia were administered in the chambers. The trial lasted for 21 days. The breast muscle, thigh muscle, dressed weight, and serum biochemical indexes were measured. The skeletal muscle fibre morphology was observed using light microscopy, and the expressions of genes associated with skeletal muscle development and myosin heavy chain genes were assessed. After 7 days of ammonia exposure, the broilers' weight in the ammonia group decreased. On the 21st day of the experiment, in the ammonia group, the breast muscle weight, thigh muscle weight, and dressed weight decreased, the blood urea nitrogen content increased, skeletal muscle fibre diameter shortened, the expression of myostatin increased, and the expression of myosin heavy chain-FWM and myosin heavy chain-FRM decreased significantly. This article suggests that 35 ppm atmospheric ammonia seriously affects the skeletal muscle gain rate of broilers, and the myostatin pathway could be a potential regulation of the growth rate of muscle fibre under ammonia exposure.

MicroRNA-27a inhibits porcine type I muscle fibre gene expression by directly targeting peroxisome proliferator-activated receptor-γ coactivator-1α.

MicroRNAs are one of the key determinants of muscle fibre development and phenotype in mammals. The preliminary experiment implied that microRNA-27a (miR-27a) might involve in regulation of muscle fibre type composition of pigs. Thereby, the present study aimed to confirm the regulatory effect of miR-27a on porcine type I muscle fibre-encoding gene (myosin heavy chain gene 7, MYH7) expression and its related mechanism. We firstly observed opposite expression patterns between miR-27a and MYH7 as well as between miR-27a and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) during differentiation of porcine skeletal muscle satellite cells. Through the subsequent transfection analysis in porcine myotubes, we found that miR-27a suppressed the expression of MYH7 and PGC-1α. Besides, miR-27a induced inhibition of PGC-1α downstream targets, namely myocyte enhancer factor-2C (MEF2C) along with mitochondrial biogenesis and oxidative metabolism-related factors such as nuclear respiratory factor 1 (NRF-1), mitochondrial transcription factor A (mtTFA), cytochrome c (Cytc) and cytochrome oxidase IV (COX Ⅳ) and succinodehydrogenase (SDH). Dual-luciferase reporter analysis revealed that miR-27a could bind to the predicted target site in the 3'-untranslated regions of PGC-1α mRNA, confirming a direct targeting of PGC-1α by miR-27a. Moreover, PGC-1α silencing abolished the promotive effects of miR-27a inhibitor on MYH7, PGC-1α and its downstream targets (MEF2C, NRF-1, mtTFA, COX Ⅳ, Cytc and SDH) in porcine myotubes. Collectively, miR-27a inhibits porcine MYH7 expression by negatively regulating PGC-1α and PGC-1α-controlled MEF2C expression as well as mitochondrial biogenesis and oxidative metabolism. Our findings may provide a molecular target for genetic or nutritional control of muscle fibre phenotype of pigs, probably having an important implication for regulating pork quality.