Myofiber Type Composition Research Articles

Succinate dehydrogenase-complex II regulates skeletal muscle cellular respiration and contractility but not muscle mass in genetically induced pulmonary emphysema.

Reduced skeletal muscle mass and oxidative capacity coexist in patients with pulmonary emphysema and are independently associated with higher mortality. If reduced cellular respiration contributes to muscle atrophy in that setting remains unknown. Using a mouse with genetically induced pulmonary emphysema that recapitulates muscle dysfunction, we found that reduced activity of succinate dehydrogenase (SDH) is a hallmark of its myopathic changes. We generated an inducible, muscle-specific SDH knockout mouse that demonstrates lower mitochondrial oxygen consumption, myofiber contractility, and exercise endurance. Respirometry analyses show that in vitro complex I respiration is unaffected by loss of SDH subunit C in muscle mitochondria, which is consistent with the pulmonary emphysema animal data. SDH knockout initially causes succinate accumulation associated with a down-regulated transcriptome but modest proteome effects. Muscle mass, myofiber type composition, and overall body mass constituents remain unaltered in the transgenic mice. Thus, while SDH regulates myofiber respiration in experimental pulmonary emphysema, it does not control muscle mass or other body constituents.

Caveolin-3 regulates slow oxidative myofiber formation in female mice

Abstract Objectives Caveolin-3 (Cav-3) plays a pivotal role in maintaining skeletal muscle mass and function. Mutations or deletions of Cav-3 can result in the development of various forms of myopathy, which affect the integrity and repair capacity of muscle fiber membranes. However, the potential effect of Cav-3 on myofiber type composition remains unclear. Methods To investigate the effect of Cav-3 on muscle strength and running capacity, we examined the grip force test and the low/high-speed running test. Oxidative and glycolytic myofiber-related genes, proteins, and skeletal muscle fiber composition were measured to determine the role of the Cav-3 in oxidative myofiber formation. Results We report the impact of Cav-3 on enhancing muscle endurance performance in female mice, and the discovery of a new physiological function to increase the proportion of slow-twitch oxidative muscle fiber by analyzing the gastrocnemius and soleus. Skeletal muscle-specific ablation of Cav-3 in female mice increased oxidative myofiber-related gene expression and type I oxidative myofiber composition, with resultant improvements in endurance performance. In male mice, the absence of Cav-3 in skeletal muscle reduced in the expression of glycolytic fiber-related genes and proteins. Conclusions This study identified Cav-3 as a regulator of slow-twitch oxidative muscle fiber formation acting on female mice, which may provide a potential target for improving muscle oxidative function.

Comparative transcriptome analysis of slow-twitch and fast-twitch muscles in Kazakh horses

This study conducted a thorough analysis of the myofiber type composition in the extensor digitorum longus muscle (EDL) and soleus muscle (SOL) of Kazakh horses, across different genders (male and female). The results showed significant differences in myofiber type composition between EDL and SOL, with a higher proportion of Type I fibers in SOL muscles and a greater prevalence of Type II fibers in EDL muscles. Additionally, the myofiber diameter in Kazakh horses was relatively small, potentially related to the tenderness and edible quality of their muscles. Using high-throughput sequencing technology, we constructed 32 cDNA sequencing libraries and obtained high-quality read data. Gene expression analysis revealed 278 and 372 differentially expressed genes (DEGs) in EDL and SOL muscles, respectively, including genes related to muscle contraction, metabolism, and development. Intersection analysis of DEGs between genders showed that 60 DEGs were significantly different in both male and female horses. GO annotation and KEGG analysis further elucidated the roles of these DEGs in muscle structure, function, and cellular signaling. Protein-protein interaction (PPI) network analysis and identification of hub genes provided new insights into the molecular mechanisms underlying muscle growth and development. Finally, the reliability of the DEGs data was validated through quantitative real-time PCR (qRT-PCR). This study not only enhances our understanding of the biological characteristics of horse muscles but also provides potential molecular targets for improving horse muscle performance and health.

Vitamin A regulates mitochondrial biogenesis and function through p38 MAPK-PGC-1α signaling pathway and alters the muscle fiber composition of sheep

BackgroundVitamin A (VA) and its metabolite, retinoic acid (RA), are of great interest for their wide range of physiological functions. However, the regulatory contribution of VA to mitochondrial and muscle fiber composition in sheep has not been reported.MethodLambs were injected with 0 (control) or 7,500 IU VA palmitate into the biceps femoris muscle on d 2 after birth. At the age of 3 and 32 weeks, longissimus dorsi (LD) muscle samples were obtained to explore the effect of VA on myofiber type composition. In vitro, we investigated the effects of RA on myofiber type composition and intrinsic mechanisms.ResultsThe proportion of type I myofiber was greatly increased in VA-treated sheep in LD muscle at harvest. VA greatly promoted mitochondrial biogenesis and function in LD muscle of sheep. Further exploration revealed that VA elevated PGC-1α mRNA and protein contents, and enhanced the level of p38 MAPK phosphorylation in LD muscle of sheep. In addition, the number of type I myofibers with RA treatment was significantly increased, and type IIx myofibers was significantly decreased in primary myoblasts. Consistent with in vivo experiment, RA significantly improved mitochondrial biogenesis and function in primary myoblasts of sheep. We then used si-PGC-1α to inhibit PGC-1α expression and found that si-PGC-1α significantly abrogated RA-induced the formation of type I myofibers, mitochondrial biogenesis, MitoTracker staining intensity, UQCRC1 and ATP5A1 expression, SDH activity, and enhanced the level of type IIx muscle fibers. These data suggested that RA improved mitochondrial biogenesis and function by promoting PGC-1α expression, and increased type I myofibers. In order to prove that the effect of RA on the level of PGC-1α is caused by p38 MAPK signaling, we inhibited the p38 MAPK signaling using a p38 MAPK inhibitor, which significantly reduced RA-induced PGC-1α and MyHC I levels.ConclusionVA promoted PGC-1α expression through the p38 MAPK signaling pathway, improved mitochondrial biogenesis, and altered the composition of muscle fiber type.Graphical

Nutritional regulation of skeletal muscle energy metabolism, lipid accumulation and meat quality in pigs.

The quality of pork determines consumers' purchase intention, which directly affects the economic value of pork. Minimizing the proportion of inferior pork and producing high quality pork are the ultimate goals of the pig industry. Muscle energy metabolism, serving as a regulative hub in organism energy expenditure and storage as a fat deposit, is compatible with myofiber type composition, affecting meat color, intramuscular fat content, tenderness, pH values and drip loss. Increasing data illustrate that dietary nutrients and bioactive ingredients affect muscle energy metabolism, white adipose browning and fat distribution, and myofiber type composition in humans, and rodents. Recently, some studies have shown that modulating muscle energy metabolism and lipid accumulation through nutritional approaches could effectively improve meat quality. This article reviews the progress and development in this field, and specifically discusses the impacts of dietary supply of amino acids, lipids, and gut microbiota as well as maternal nutrition on skeletal muscle energy metabolism, lipid accumulation and meat quality of pigs, so as to provide comprehensive overview with respect to effective avenues for improving meat quality.

Emergence and Progression of Behavioral Motor Deficits and Skeletal Muscle Atrophy across the Adult Lifespan of the Rat.

The facultative loss of muscle mass and function during aging (sarcopenia) poses a serious threat to our independence and health. When activities of daily living are impaired (clinical phase), it appears that the processes leading to sarcopenia have been ongoing in humans for decades (preclinical phase). Here, we examined the natural history of sarcopenia in male outbred rats to compare the occurrence of motor behavioral deficits with the degree of muscle wasting and to explore the muscle-associated processes of the preclinical and clinical phases, respectively. Selected metrics were validated in female rats. We used the soleus muscle because of its long duty cycles and its importance in postural control. Results show that gait and coordination remain intact through middle age (40-60% of median lifespan) when muscle mass is largely preserved relative to body weight. However, the muscle shows numerous signs of remodeling with a shift in myofiber-type composition toward type I. As fiber-type prevalence shifted, fiber-type clustering also increased. The number of hybrid fibers, myofibers with central nuclei, and fibers expressing embryonic myosin increased from being barely detectable to a significant number (5-10%) at late middle age. In parallel, TGFβ1, Smad3, FBXO32, and MuRF1 mRNAs increased. In early (25-month-old) and advanced (30-month-old) aging, gait and coordination deteriorate with the progressive loss of muscle mass. In late middle age and early aging due to type II atrophy (>50%) followed by type I atrophy (>50%), the number of myofibers did not correlate with this process. In advanced age, atrophy is accompanied by a decrease in SCs and βCatenin mRNA, whereas several previously upregulated transcripts were downregulated. The re-expression of embryonic myosin in myofibers and the upregulation of mRNAs encoding the γ-subunit of the nicotinic acetylcholine receptor, the neuronal cell adhesion molecule, and myogenin that begins in late middle age suggest that one mechanism driving sarcopenia is the disruption of neuromuscular connectivity. We conclude that sarcopenia in rats, as in humans, has a long preclinical phase in which muscle undergoes extensive remodeling to maintain muscle mass and function. At later time points, these adaptive mechanisms fail, and sarcopenia becomes clinically manifest.

Quantitative analysis of myofiber type composition in skeletal muscles

Quantitative analysis of myofiber type composition in skeletal muscles

Muscle fiber composition affects the postmortem redox characteristics of yak beef

Yak infraspinatus (IS), longissimus thoracis (LT), and semitendinosus (ST) muscles were used to explore relationships between myofiber types and postmortem redox characteristics. IS mainly consisted of myofiber type Ⅰ (49.2%), LT has a vast majority of type IIa (72.7%), while ST possessed a similar percentage of type IIa (44.7%) and IIb (51.5%). Compared with LT and ST, IS exhibited higher initial H2O2, myoglobin (T-Mb), and metmyoglobin (MetMb) contents that provoked severe protein oxidation despite higher endogenous antioxidative capacity (e.g., glutathione antioxidative system). Three muscles showed specificities in myofiber type composition and redox characteristics, which were strongly correlated. Specifically, type Ⅰ myofiber positively correlated with H2O2, T-Mb, and multiple antioxidase activity/content while negatively correlated with lipid peroxidation, MetMb, and lactic dehydrogenase activity; also, they tended to employ more SFAs in more intermuscular fat assembly. Overall, muscle-specific myofiber type/redox attributes required differentiated processing to prevent meat oxidation and produce standardized products.

MYTHO: A novel regulator of autophagy and skeletal muscle health

RationaleThe loss of skeletal muscle function can have dramatic health consequence. Skeletal muscle dysfunction are found in a wide range of clinical conditions, including cancer cachexia, starvation, sepsis, and aging. Autophagy is an essential catabolic process that remove damaged cytosolic components. Accumulating evidence indicates that insufficient or excessive autophagy can contribute to the development of skeletal muscle atrophy. We recently identified a novel FoxO‐dependent gene, which we named MYTHO, with the potential to regulate autophagy in skeletal muscles. However, to date, the roles that MYTHO plays in skeletal muscles remain unknown. In this study, we evaluated the functional importance of MYTHO in skeletal muscle structure and function.MethodsTo study the functional importance of MYTHO in skeletal muscle mass and function, we used a combination of genetic strategies in mice to delete or overexpress MYTHO.ResultsMYTHO expression is induced in various muscle wasting conditions, including starvation, denervation, cancer cachexia, and sepsis. Using different genetics approaches, we confirmed that MYTHO is an important regulator of autophagy in skeletal muscle. Importantly, we also found that MYTHO knockdown in skeletal muscles caused various myopathic features, including muscle weakness, accumulation of tubular aggregates, a shift in myofiber type composition and nuclei mispositioning in myofibers. Using animals with muscle specific Atg7 deletion (a model of autophagy inactivation), we demonstrated that the pathological features associated with MYTHO knockdown are independent of its role in regulating autophagy.ConclusionWe conclude that MYTHO is a central player in regulating skeletal muscle mass and integrity.

Neuromuscular Pathology of Extrinsic Tongue Muscles in the Pink1‐/‐ Rat Model of Parkinson Disease

IntroductionParkinson disease (PD) affects the central and peripheral nervous systems resulting in widespread pathology. Recently, more research attention has been directed toward peripheral nerves and muscles as potential sources of pathology relating to oromotor control and swallowing. Swallowing impairment (dysphagia) presents during the prodromal phase of the disease, worsens over time, and can result in serious health and quality of life impairment. Post‐mortem human analyses show pathology of nerves and muscles of the pharynx and larynx. However less is known about tongue muscles. To study clinically relevant oromotor and swallow deficits across disease progression, we use the Pink1‐/‐ rat model of early‐onset autosomal dominant PD. We have shown oromotor and swallow dysfunction in the prodromal and early stages of disease (4‐8 months) in Pink1‐/‐ rats. At 6 months there were no differences between Pink1‐/‐ and WT genioglossus (GG) muscle in many measures of muscle biology, nor was there evidence of denervation. However, we speculate this may occur later in disease progression. We hypothesize that Pink1‐/‐ GG muscles will have smaller myofiber sizes with a shift in myofiber heavy chain isoform profiles toward slower profiles compared to WT GG muscles. Additionally, we hypothesize that that Pink1‐/‐ GG NMJs will be fewer in total number with less overlap of pre‐ and post‐synaptic structures and greater motor endplate area and dispersion ratios compared to WT GG NMJs. Data and analyses are ongoing; results with expanded methodology will be presented.MethodsWe evaluated the GG muscles of 10‐month‐old rats, Pink1‐/‐ (n=2) WT (n=2)(Sage Labs, Boyertown, PA, USA). Fixed tissues were flash frozen and sectioned at 20 microns. We labeled pre‐synaptic structures with primary antibodies to NF‐M & ZNP‐1 (red fluorophore secondaries) and post‐synaptic acetylcholine receptors at the motor endplate with alpha‐bungarotoxin‐AF488 (green fluorophore). Images were taken at 20x using a fluorescent microscope (Olympus, Bx53, USA) and analyzed with ImageJ (imagej.nih.gov). Then we counted the total number of motor endplates (green) in each tissue section. We also counted how many motor endplates co‐localized with nerve terminals (red) to quantify the number of structurally intact NMJs. Here we report effects sizes due to an incomplete data set.ResultsPreliminary results demonstrated modest effect size with trends towards WT rats having more total NMJs (Cohen’s D=0.359) which was compatible with our hypothesis, but a lower proportion of structurally intact NMJs (Cohen’s D=0.454) contrary to our hypothesis.DiscussionPreliminary results suggest differences in total NMJs and NMJ structure between WT and Pink1‐/‐ rats. We will report data sets of myofiber cross‐sectional area, myofiber type composition, and the NMJ parameters of total number, overlap of pre‐ and post‐synaptic structures, motor endplate area, and dispersion ratio. These results will provide a substantial presentation of the neuromuscular phenotype present at mid‐disease timepoints in Pink1‐/‐ rats aged 10 months. Investigating neuromuscular biology of extrinsic tongue muscles and swallow‐related structures is vital for understanding of the onset, sequelae, and underlying pathology of dysphagia in PD. These results are important to an ongoing effort to develop novel therapies targeted at the devastating swallowing deficits in PD.

Improving Molecular and Histopathology in Diaphragm Muscle of the Double Transgenic ACTA1-MCM/FLExDUX4 Mouse Model of FSHD with Systemic Antisense Therapy.

Facioscapulohumeral muscular dystrophy (FSHD) is a rare muscle dystrophy causing muscle weakness initially in the face, shoulders, and upper arms, and extends to lower body muscles as the disease progresses. Respiratory restriction in FSHD is increasingly reported to be more common and severe than previously thought, with the involvement of diaphragm weakness in pulmonary insufficiency being under debate. As aberrant expression of the double homeobox 4 (DUX4) gene is the prime cause of FSHD, we and others have developed numerous strategies and reported promising results on downregulating DUX4 expression in both cellular and animal models of FSHD. However, the effect of DUX4 and anti-DUX4 approaches on diaphragm muscle has not been elucidated. In this study, we show that toxic DUX4 expression causes pathology that affects the diaphragm of ACTA1-MCM/FLExDUX4 mouse model of FSHD at both molecular and histological levels. Of importance, a systemic antisense treatment that suppresses DUX4 and target genes expression by 50% significantly improves muscle regeneration and muscle fibrosis, and prevents modification in myofiber type composition, supporting its development as a treatment for FSHD.

MiR-183 and miR-96 orchestrate both glucose and fat utilization in skeletal muscle.

Our knowledge of the coordination of fuel usage in skeletal muscle is incomplete. Whether and how microRNAs are involved in the substrate selection for oxidation is largely unknown. Here we show that mice lacking miR-183 and miR-96 have enhanced muscle oxidative phenotype and altered glucose/lipid homeostasis. Moreover, loss of miR-183 and miR-96 results in a shift in substrate utilization toward fat relative to carbohydrates in mice. Mechanistically, loss of miR-183 and miR-96 suppresses glucose utilization in skeletal muscle by increasing PDHA1 phosphorylation via targeting FoxO1 and PDK4. On the other hand, loss of miR-183 and miR-96 promotes fat usage in skeletal muscle by enhancing intramuscular lipolysis via targeting FoxO1 and ATGL. Thus, our study establishes miR-183 and miR-96 as master coordinators of fuel selection and metabolic homeostasis owing to their capability of modulating both glucose utilization and fat catabolism. Lastly, weshow that loss of miR-183 and miR-96 can alleviate obesity andimprove glucose metabolism in high-fat diet-induced mice, suggesting that miR-183 and miR-96 may serve as therapeutic targets for metabolic diseases.

LncMyoD Promotes Skeletal Myogenesis and Regulates Skeletal Muscle Fiber-Type Composition by Sponging miR-370-3p.

The development of skeletal muscle is a highly ordered and complex biological process. Increasing evidence has shown that noncoding RNAs, especially long-noncoding RNAs (lncRNAs) and microRNAs, play a vital role in the development of myogenic processes. In this study, we observed that lncMyoD regulates myogenesis and changes myofiber-type composition. miR-370-3p, which is directly targeted by lncMyoD, promoted myoblast proliferation and inhibited myoblast differentiation in the C2C12 cell line, which serves as a valuable model for studying muscle development. In addition, the inhibition of miR-370-3p promoted fast-twitch fiber transition. Further analysis indicated that acyl-Coenzyme A dehydrogenase, short/branched chain (ACADSB) is a target gene of miR-370-3p, which is also involved in myoblast differentiation and fiber-type transition. Furthermore, our data suggested that miR-370-3p was sponged by lncMyoD. In contrast with miR-370-3p, lncMyoD promoted fast-twitch fiber transition. Taken together, our results suggest that miR-370-3p regulates myoblast differentiation and muscle fiber transition and is sponged by lncMyoD.

Histopathological and aquaporin7 mRNA expression analyzes in the skeletal and cardiac muscles of obese db/db mice.

The aim of this study is to examine 1) muscle fiber type composition, 2) myofiber diameter, and 3) aquaporin (AQP) 7 and AQP 9 mRNA expressions by quantitative PCR in muscles of obese db/db mice. The myofiber type composition of skeletal muscle was not statistically significantly different between db/db mice and control mice; while the average myofiber diameter ratio showed a decrease in db/db mice. The expression of AQP7 but not AQP9 mRNA in the skeletal and cardiac muscles was significantly upregulated in db/db mice. Thus this study revealed quantitatively that type 2 myofiber atrophy was shown in the skeletal muscles of db/db mice. AQP7 mRNA expression was upregulated in the skeletal and cardiac muscles of db/db mice.

Effect of maternal nutrient restriction on expression of glucose transporters (SLC2A4 and SLC2A1) and insulin signaling in skeletal muscle of SGA and Non-SGA sheep fetuses

Maternal nutrient restriction (NR) causes small for gestational age (SGA) offspring, which are at higher risk for accelerated postnatal growth and developing insulin resistance in adulthood. Skeletal muscle is essential for whole-body glucose metabolism, as 80% of insulin-mediated glucose uptake occurs in this tissue. Maternal NR can alter fetal skeletal muscle mass, expression of glucose transporters, insulin signaling, and myofiber type composition. It also leads to accumulation of intramuscular triglycerides (IMTG), which correlates to insulin resistance. Using a 50% NR treatment from gestational day (GD) 35 to GD 135 in sheep, we routinely observe a spectral phenotype of fetal weights within the NR group. Thus, we classified those fetuses into NR(Non-SGA; n = 11) and NR(SGA; n = 11). The control group (n = 12) received 100% of nutrient requirements throughout pregnancy. At GD 135, fetal plasma and gastrocnemius and soleus muscles were collected. In fetal plasma, total insulin was lower in NR(SGA) fetuses compared NR(Non-SGA) and control fetuses (P < 0.01), whereas total IGF-1 was lower in NR(SGA) fetuses compared with control fetuses (P < 0.05). Within gastrocnemius, protein expression of insulin receptor (INSRB; P < 0.05) and the glucose transporters, solute carrier family 2 member 1 and solute carrier family 2 member 4, was higher (P < 0.05) in NR(SGA) fetuses compared with NR(Non-SGA) fetuses; IGF-1 receptor protein was increased (P < 0.01) in NR(SGA) fetuses compared with control fetuses, and a lower (P < 0.01) proportion of type I myofibers (insulin sensitive and oxidative) was observed in SGA fetuses. For gastrocnemius muscle, the expression of lipoprotein lipase (LPL) messenger RNA (mRNA) was upregulated (P < 0.05) in both NR(SGA) and NR(Non-SGA) fetuses compared with control fetuses, whereas carnitine palmitoyltransferase 1B (CPT1B) mRNA was higher (P < 0.05) in NR(Non-SGA) fetuses compared with control fetuses, but there were no differences (P > 0.05) for protein levels of LPL or CPT1B. Within soleus, there were no differences (P > 0.05) for any characteristic except for the proportion of type I myofibers, which was lower (P < 0.05) in NR(SGA) fetuses compared with control fetuses. Accumulation of IMTG did not differ (P > 0.05) in gastrocnemius or soleus muscles. Collectively, the results indicate molecular differences between SGA and Non-SGA fetuses for most characteristics, suggesting that maternal NR induces a spectral phenotype for the metabolic programming of those fetuses.

Motoneuron deafferentation and gliosis occur in association with neuromuscular regressive changes during ageing in mice.

BackgroundThe cellular mechanisms underlying the age‐associated loss of muscle mass and function (sarcopenia) are poorly understood, hampering the development of effective treatment strategies. Here, we performed a detailed characterization of age‐related pathophysiological changes in the mouse neuromuscular system.MethodsYoung, adult, middle‐aged, and old (1, 4, 14, and 24–30 months old, respectively) C57BL/6J mice were used. Motor behavioural and electrophysiological tests and histological and immunocytochemical procedures were carried out to simultaneously analyse structural, molecular, and functional age‐related changes in distinct cellular components of the neuromuscular system.ResultsAgeing was not accompanied by a significant loss of spinal motoneurons (MNs), although a proportion (~15%) of them in old mice exhibited an abnormally dark appearance. Dark MNs were also observed in adult (~9%) and young (~4%) animals, suggesting that during ageing, some MNs undergo early deleterious changes, which may not lead to MN death. Old MNs were depleted of cholinergic and glutamatergic inputs (~40% and ~45%, respectively, P < 0.01), suggestive of age‐associated alterations in MN excitability. Prominent microgliosis and astrogliosis [~93% (P < 0.001) and ~100% (P < 0.0001) increase vs. adults, respectively] were found in old spinal cords, with increased density of pro‐inflammatory M1 microglia and A1 astroglia (25‐fold and 4‐fold increase, respectively, P < 0.0001). Ageing resulted in significant reductions in the nerve conduction velocity and the compound muscle action potential amplitude (~30%, P < 0.05, vs. adults) in old distal plantar muscles. Compared with adult muscles, old muscles exhibited significantly higher numbers of both denervated and polyinnervated neuromuscular junctions, changes in fibre type composition, higher proportion of fibres showing central nuclei and lipofuscin aggregates, depletion of satellite cells, and augmented expression of different molecules related to development, plasticity, and maintenance of neuromuscular junctions, including calcitonin gene‐related peptide, growth associated protein 43, agrin, fibroblast growth factor binding protein 1, and transforming growth factor‐β1. Overall, these alterations occurred at varying degrees in all the muscles analysed, with no correlation between the age‐related changes observed and myofiber type composition or muscle topography.ConclusionsOur data provide a global view of age‐associated neuromuscular changes in a mouse model of ageing and help to advance understanding of contributing pathways leading to development of sarcopenia.

Reactive Jumps Preserve Skeletal Muscle Structure, Phenotype, and Myofiber Oxidative Capacity in Bed Rest.

Identification of countermeasures able to prevent disuse-induced muscle wasting is crucial to increase performance of crew members during space flight as well as ameliorate patient’s clinical outcome after long immobilization periods. We report on the outcome of short but high-impact reactive jumps (JUMP) as countermeasure during 60 days of 6° head-down tilt (HDT) bed rest on myofiber size, type composition, capillarization, and oxidative capacity in tissue biopsies (pre/post/recovery) from the knee extensor vastus lateralis (VL) and deep calf soleus (SOL) muscle of 22 healthy male participants (Reactive jumps in a sledge, RSL-study 2015–2016, DLR:envihab, Cologne). Bed rest induced a slow-to-fast myofiber shift (type I –>II) with an increased prevalence of hybrid fibers in SOL after bed rest without jumps (control, CTRL, p = 0.016). In SOL, JUMP countermeasure in bed rest prevented both fast and slow myofiber cross-sectional area (CSA) decrements (p = 0.005) in CTRL group. In VL, bed rest only induced capillary rarefaction, as reflected by the decrease in local capillary-to-fiber ratio (LCFR) for both type II (pre vs. post/R + 10, p = 0.028/0.028) and type I myofibers (pre vs. R + 10, p = 0.012), which was not seen in the JUMP group. VO2maxFiber (pL × mm–1 × min–1) calculated from succinate dehydrogenase (SDH)-stained cryosections (OD660 nm) showed no significant differences between groups. High-impact jump training in bed rest did not prevent disuse-induced myofiber atrophy in VL, mitigated phenotype transition (type I – >II) in SOL, and attenuated capillary rarefaction in the prime knee extensor VL however with little impact on oxidative capacity changes.

Maternal Nutrient Restriction and Skeletal Muscle Development: Consequences for Postnatal Health.

Severe undernutrition and famine continue to be a worldwide concern, as cases have been increasing in the past 5years, particularly in developing countries. The occurrence of nutrient restriction (NR) during pregnancy affects fetal growth, leading to small for gestational age (SGA) or intrauterine growth restricted (IUGR) offspring. During adulthood, SGA and IUGR offspring are at a higher risk for the development of metabolic syndrome. Skeletal muscle is particularly sensitive to prenatal NR. This tissue plays an essential role in oxidation and glucose metabolism because roughly 80% of insulin-mediated glucose uptake occurs in muscle, and it represents around 40% of body weight. Alterations in myofiber number, hypertrophy and myofiber type composition, decreased protein synthesis, lower mitochondrial content and activity of oxidative enzymes, and increased accumulation of intramuscular triglycerides are among the described programming effects of maternal NR on skeletal muscle. Together, these features would add to a phenotype that is prone to insulin resistance, type 2 diabetes, obesity, and metabolic syndrome. Insights from diverse animal models (i.e. ovine, swine, and rodent) have provided valuable information regarding the molecular mechanisms behind those altered developmental pathways. Understanding those molecular signatures supports the development of efficient treatments to counteract the effects of maternal NR on skeletal muscle, and its negative implications for postnatal health.

Methyltransferase-like 21e inhibits 26S proteasome activity to facilitate hypertrophy of type IIb myofibers.

Skeletal muscles contain heterogeneous myofibers that are different in size and contractile speed, with type IIb myofiber being the largest and fastest. Here, we identify methyltransferase-like 21e (Mettl21e), a member of newly classified nonhistone methyltransferases, as a gene enriched in type IIb myofibers. The expression of Mettl21e was strikingly up-regulated in hypertrophic muscles and during myogenic differentiation in vitro and in vivo. Knockdown (KD) of Mettl21e led to atrophy of cultured myotubes, and targeted mutation of Mettl21e in mice reduced the size of IIb myofibers without affecting the composition of myofiber types. Mass spectrometry and methyltransferase assay revealed that Mettl21e methylated valosin-containing protein (Vcp/p97), a key component of the ubiquitin-proteasome system. KD or knockout of Mettl21e resulted in elevated 26S proteasome activity, and inhibition of proteasome activity prevented atrophy of Mettl21e KD myotubes. These results demonstrate that Mettl21e functions to maintain myofiber size through inhibiting proteasome-mediated protein degradation.-Wang, C., Zhang, B., Ratliff, A. C., Arrington, J., Chen, J., Xiong, Y., Yue, F., Nie, Y., Hu, K., Jin, W., Tao, W. A., Hrycyna, C. A., Sun, X., Kuang, S. Methyltransferase-like 21e inhibits 26S proteasome activity to facilitate hypertrophy of type IIb myofibers.

Associations of the expression levels of genes involved in CFL2b and MyHC isoform type changes in longissimus dorsi muscle of HeBao and Large White pigs (Sus scrofa) during postnatal growth

This study aimed to compare the patterns of postnatal transformation of myosin heavy chain (MyHC) isoform types in the longissimus dorsi (LD) muscle between HeBao (HB) and Large White (LW) pigs, and assess the association of porcine cofilin2b (CFL2b) mRNA abundance with changes of myofiber type composition. The four MyHC isoforms (MyHC-1, -2a, -2b, and -2x) of the LD muscle were assessed for mRNA levels in 28 HB and 28 LW pigs by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The associations of CFL2b mRNA expression and myofiber type (MyHC-1, -2a, -2b, and -2x) changes were analyzed by RT-qPCR. Although the mRNA expression patterns of MyHCs were different between the two breeds, they had similar expression levels. During postnatal growth, relative CFL2b abundance was gradually increased, with dramatic changes observed after 90 d between the two breeds (P &lt; 0.01). Further analysis revealed significant positive correlations of CFL2b gene expression with MyHC-1/slow (HB: r = 0.871), MyHC-2b [LW: r = 0.881 (P &lt; 0.01)], and MyHC-2x (HB: r = 0.795, LW: r = 0.814), and a significant negative correlation with MyHC-1/slow [r = −0.938 (P &lt; 0.01)] in LW. No significant associations of CFL2b expression with MyHC-2a (HB: r = −0. 195, r = −0.697) and MyHC-2b (HB: r = 0.493) were found. Our findings suggested that HB pigs had different muscle development mechanisms in the LD muscle compared with LW, and the CFL2b expression difference could affect the levels of myofiber types which could account for meat quality differences. HB pigs possessed less glycolytic, with more oxidative metabolism and better meat quality traits compared with LW pigs at different growth stages.