Myonuclei acquired by overload exercise precede hypertrophy and are not lost on detraining

Mechanical loading stimulates hypertrophy in tissue-engineered skeletal muscle: Molecular and phenotypic responses

Mechanical loading of skeletal muscle results in molecular and phenotypic adaptations typified by enhanced muscle size. Studies on humans are limited by the need for repeated sampling, and studies on animals have methodological and ethical limitations. In this investigation, three‐dimensional skeletal muscle was tissue‐engineered utilizing the murine cell line C2C12, which bears resemblance to native tissue and benefits from the advantages of conventional in vitro experiments. The work aimed to determine if mechanical loading induced an anabolic hypertrophic response, akin to that described in vivo after mechanical loading in the form of resistance exercise. Specifically, we temporally investigated candidate gene expression and Akt‐mechanistic target of rapamycin 1 signalling along with myotube growth and tissue function. Mechanical loading (construct length increase of 15%) significantly increased insulin‐like growth factor‐1 and MMP‐2 messenger RNA expression 21 hr after overload, and the levels of the atrophic gene MAFbx were significantly downregulated 45 hr after mechanical overload. In addition, p70S6 kinase and 4EBP‐1 phosphorylation were upregulated immediately after mechanical overload. Maximal contractile force was augmented 45 hr after load with a 265% increase in force, alongside significant hypertrophy of the myotubes within the engineered muscle. Overall, mechanical loading of tissue‐engineered skeletal muscle induced hypertrophy and improved force production.

Myonuclear accretion is a determinant of exercise-induced remodeling in skeletal muscle

Skeletal muscle adapts to external stimuli such as increased work. Muscle progenitors (MPs) control muscle repair due to severe damage, but the role of MP fusion and associated myonuclear accretion during exercise are unclear. While we previously demonstrated that MP fusion is required for growth using a supra-physiological model (Goh and Millay, 2017), questions remained about the need for myonuclear accrual during muscle adaptation in a physiological setting. Here, we developed an 8 week high-intensity interval training (HIIT) protocol and assessed the importance of MP fusion. In 8 month-old mice, HIIT led to progressive myonuclear accretion throughout the protocol, and functional muscle hypertrophy. Abrogation of MP fusion at the onset of HIIT resulted in exercise intolerance and fibrosis. In contrast, ablation of MP fusion 4 weeks into HIIT, preserved exercise tolerance but attenuated hypertrophy. We conclude that myonuclear accretion is required for different facets of exercise-induced adaptive responses, impacting both muscle repair and hypertrophic growth.

Effects of training, detraining, and retraining on strength, hypertrophy, and myonuclear number in human skeletal muscle

Previously trained mouse muscles acquire strength and volume faster than naïve muscles; it has been suggested that this is related to increased myonuclear density. The present study aimed to determine whether a previously strength-trained leg (mem-leg) would respond better to a period of strength training than a previously untrained leg (con-leg). Nine men and 10 women performed unilateral strength training (T1) for 10 wk, followed by 20 wk of detraining (DT) and a 5-wk bilateral retraining period (T2). Muscle biopsies were taken before and after each training period and analyzed for myonuclear number, fiber volume, and cross-sectional area (CSA). Ultrasound and one repetition of maximum leg extension were performed to determine muscle thickness (MT) and strength. CSA (~17%), MT (~10%), and strength (~20%) increased during T1 in the mem-leg. However, the myonuclear number and fiber volume did not change. MT and CSA returned to baseline values during DT, but strength remained elevated (~60%), supporting previous findings of a long-lasting motor learning effect. MT and strength increased similarly in the mem-leg and con-leg during T2, whereas CSA, fiber volume, and myonuclear number remained unaffected. In conclusion, training response during T2 did not differ between the mem-leg and con-leg. However, this does not discount the existence of human muscle memory, since no increase in the number of myonuclei was detected during T1 and no clear detraining effect was observed for cell size during DT; thus, the present data did not allow for a rigorous test of the muscle memory hypothesis. NEW & NOTEWORTHY If a long-lasting intramuscular memory exists in humans, this will affect strength-training advice for both athletes and the public. Based on animal experiments, we hypothesized that such a memory exists and that it is related to the myonuclear number. However, a period of unilateral strength training, followed by detraining, did not increase the myonuclear number. The training response, during a subsequent bilateral retraining period, was not enhanced in the previously trained leg.

Comparative Transcriptome and Methylome Analysis in Human Skeletal Muscle Anabolism, Hypertrophy and Epigenetic Memory

Transcriptome wide changes in human skeletal muscle after acute (anabolic) and chronic resistance exercise (RE) induced hypertrophy have been extensively determined in the literature. We have also recently undertaken DNA methylome analysis (850,000 + CpG sites) in human skeletal muscle after acute and chronic RE, detraining and retraining, where we identified an association between DNA methylation and epigenetic memory of exercise induced skeletal muscle hypertrophy. However, it is currently unknown as to whether all the genes identified in the transcriptome studies to date are also epigenetically regulated at the DNA level after acute, chronic or repeated RE exposure. We therefore aimed to undertake large scale bioinformatical analysis by pooling the publicly available transcriptome data after acute (110 samples) and chronic RE (181 samples) and comparing these large data sets with our genome-wide DNA methylation analysis in human skeletal muscle after acute and chronic RE, detraining and retraining. Indeed, after acute RE we identified 866 up- and 936 down-regulated genes at the expression level, with 270 (out of the 866 up-regulated) identified as being hypomethylated, and 216 (out of 936 downregulated) as hypermethylated. After chronic RE we identified 2,018 up- and 430 down-regulated genes with 592 (out of 2,018 upregulated) identified as being hypomethylated and 98 (out of 430 genes downregulated) as hypermethylated. After KEGG pathway analysis, genes associated with ‘cancer’ pathways were significantly enriched in both bioinformatic analysis of the pooled transcriptome and methylome datasets after both acute and chronic RE. This resulted in 23 (out of 69) and 28 (out of 49) upregulated and hypomethylated and 12 (out of 37) and 2 (out of 4) downregulated and hypermethylated ‘cancer’ genes following acute and chronic RE respectively. Within skeletal muscle tissue, these ‘cancer’ genes predominant functions were associated with matrix/actin structure and remodelling, mechano-transduction (e.g. PTK2/Focal Adhesion Kinase and Phospholipase D- following chronic RE), TGF-beta signalling and protein synthesis (e.g. GSK3B after acute RE). Interestingly, 51 genes were also identified to be up/downregulated in both the acute and chronic RE pooled transcriptome analysis as well as significantly hypo/hypermethylated after acute RE, chronic RE, detraining and retraining. Five genes; FLNB, MYH9, SRGAP1, SRGN, ZMIZ1 demonstrated increased gene expression in the acute and chronic RE transcriptome and also demonstrated hypomethylation in these conditions. Importantly, these 5 genes demonstrated retained hypomethylation even during detraining (following training induced hypertrophy) when exercise was ceased and lean mass returned to baseline (pre-training) levels, identifying them as genes associated with epigenetic memory in skeletal muscle. Importantly, for the first time across the transcriptome and epigenome combined, this study identifies novel differentially methylated genes associated with human skeletal muscle anabolism, hypertrophy and epigenetic memory.

Elevated myonuclear density during skeletal muscle hypertrophy in response to training is reversed during detraining

Myonuclei gained during exercise-induced skeletal muscle hypertrophy may be long-lasting and could facilitate future muscle adaptability after deconditioning, a concept colloquially termed "muscle memory." The evidence for this is limited, mostly due to the lack of a murine exercise-training paradigm that is nonsurgical and reversible. To address this limitation, we developed a novel progressive weighted-wheel-running (PoWeR) model of murine exercise training to test whether myonuclei gained during exercise persist after detraining. We hypothesized that myonuclei acquired during training-induced hypertrophy would remain following loss of muscle mass with detraining. Singly housed female C57BL/6J mice performed 8 wk of PoWeR, while another group performed 8 wk of PoWeR followed by 12 wk of detraining. Age-matched sedentary cage-dwelling mice served as untrained controls. Eight weeks of PoWeR yielded significant plantaris muscle fiber hypertrophy, a shift to a more oxidative phenotype, and greater myonuclear density than untrained mice. After 12 wk of detraining, the plantaris muscle returned to an untrained phenotype with fewer myonuclei. A finding of fewer myonuclei simultaneously with plantaris deconditioning argues against a muscle memory mechanism mediated by elevated myonuclear density in primarily fast-twitch muscle. PoWeR is a novel, practical, and easy-to-deploy approach for eliciting robust hypertrophy in mice, and our findings can inform future research on the mechanisms underlying skeletal muscle adaptive potential and muscle memory.

Cachexia does not induce loss of myonuclei or muscle fibres during xenografted prostate cancer in mice

AIM

Cachexia is a severe wasting disorder involving loss of body- and muscle mass reducing survival and quality of life in cancer patients. We aim at determining if cachexia is a mere perturbation of the protein balance or if the condition also involves a degenerative loss of myonuclei within the fibre syncytia or loss of whole muscle fibres.

METHODS

We induced cachexia by xenografting PC3 prostate cancer cells in nu/nu mice. Six weeks later, we counted myonuclei by in vivo microscopic imaging of single live fibres in the extensor digitorum longus muscle (EDL), and the EDL, soleus and tibialis anterior muscles were also harvested for ex vivo histology.

RESULTS

The mice lost on average 15% of the whole-body wt. The muscle wet weight of the glycolytic, fast EDL was reduced by 14%, the tibialis anterior by 17%, and the slow, oxidative soleus by 6%. The fibre cross-sectional area in the EDL was reduced by 21% with no loss of myonuclei or any significant reduction in the number of muscle fibres. TUNEL-positive nuclei or fibres with embryonic myosin were rare both in cachectic and control muscles, and haematoxylin-eosin staining revealed no clear signs of muscle pathology.

CONCLUSION

The data suggest that the cachexia induced by xenografted prostate tumours induces a pronounced atrophy not accompanied by a loss of myonuclei or a loss of muscle fibres. Thus, stem cell related treatment might be redundant, and the quest for treatment options should rather focus on intervening with intracellular pathways regulating muscle fibre size.

Skeletal Muscles Do Not Undergo Apoptosis During Either Atrophy or Programmed Cell Death-Revisiting the Myonuclear Domain Hypothesis

Skeletal muscles are the largest cells in the body and are one of the few syncytial ones. There is a longstanding belief that a given nucleus controls a defined volume of cytoplasm, so when a muscle grows (hypertrophy) or shrinks (atrophy), the number of myonuclei change accordingly. This phenomenon is known as the “myonuclear domain hypothesis.” There is a general agreement that hypertrophy is accompanied by the addition of new nuclei from stem cells to help the muscles meet the enhanced synthetic demands of a larger cell. However, there is a considerable controversy regarding the fate of pre-existing nuclei during atrophy. Many researchers have reported that atrophy is accompanied by the dramatic loss of myonuclei via apoptosis. However, since there are many different non-muscle cell populations that reside within the tissue, these experiments cannot easily distinguish true myonuclei from those of neighboring mononuclear cells. Recently, two independent models, one from rodents and the other from insects, have demonstrated that nuclei are not lost from skeletal muscle fibers when they undergo either atrophy or programmed cell death. These and other data argue against the current interpretation of the myonuclear domain hypothesis and suggest that once a nucleus has been acquired by a muscle fiber it persists.

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

Lactate Promotes Myoblast Differentiation and Myotube Hypertrophy via a Pathway Involving MyoD In Vitro and Enhances Muscle Regeneration In Vivo

Lactate is a metabolic substrate mainly produced in muscles, especially during exercise. Recently, it was reported that lactate affects myoblast differentiation; however, the obtained results are inconsistent and the in vivo effect of lactate remains unclear. Our study thus aimed to evaluate the effects of lactate on myogenic differentiation and its underlying mechanism. The differentiation of C2C12 murine myogenic cells was accelerated in the presence of lactate and, consequently, myotube hypertrophy was achieved. Gene expression analysis of myogenic regulatory factors showed significantly increased myogenic determination protein (MyoD) gene expression in lactate-treated cells compared with that in untreated ones. Moreover, lactate enhanced gene and protein expression of myosin heavy chain (MHC). In particular, lactate increased gene expression of specific MHC isotypes, MHCIIb and IId/x, in a dose-dependent manner. Using a reporter assay, we showed that lactate increased promoter activity of the MHCIIb gene and that a MyoD binding site in the promoter region was necessary for the lactate-induced increase in activity. Finally, peritoneal injection of lactate in mice resulted in enhanced regeneration and fiber hypertrophy in glycerol-induced regenerating muscles. In conclusion, physiologically high lactate concentrations modulated muscle differentiation by regulating MyoD-associated networks, thereby enhancing MHC expression and myotube hypertrophy in vitro and, potentially, in vivo.

Effects of Exergaming in Cancer Related Fatigue in the Quality of Life and Electromyography of the Middle Deltoid of People with Cancer in Treatment: A Controlled Trial

Objective:

In the present study, we aimed to evaluate the effects an exergaming protocol for cancer patients who undergo or have already undergone cancer treatment. We sought to evaluate changes in cancer-related fatigue, function, and ability to perform daily activities, in addition to changes in the electromyographic pattern of the middle deltoid muscle.

Methods:

We conducted a controlled trial. Nineteen volunteers in the cancer group (aged 61 ± 9 years; body mass index28 ± 5) and 19 in the control group (aged 58 ± 8 years); body mass index 28 ± 4) participated in the study. They were evaluated by means of a sociodemographic and clinical questionnaire, the Functional Assessment of Chronic Therapy-Fatigue (FACIT-F) questionnaire, and surface electromyography in the deltoid muscle at three moments: before the beginning of the exergaming protocol, after 10 training sessions, and after 20 sessions. The protocol consisted of practicing exergaming using Xbox 360^® (Microsoft, Redmond, USA) with Kinect^®. The game “Your Shape Fitness Evolved” (Ubisoft, Rennes, France) was used.

Results:

Total FACIT-F scores, fatigue subscale scores, and median frequency values observed in the cancer group were lower than those in the control group. These values improved in relation to the initial evaluation in the cancer group after the practice of the exergaming protocol.

Conclusion:

The exergaming protocol used in this study was effective for reducing reported symptoms of fatigue, increasing perceived quality of life, and improving the pattern of deltoid muscle contraction in cancer patients.

Exercise-induced muscle damage: What is it, what causes it and what are the nutritional solutions?

Exercise-induced muscle damage (EIMD) is characterized by symptoms that present both immediately and for up to 14 days after the initial exercise bout. The main consequence of EIMD for the athlete is the loss of skeletal muscle function and soreness. As such, numerous nutrients and functional foods have been examined for their potential to ameliorate the effects of EIMD and accelerate recovery, which is the purpose of many nutritional strategies for the athlete. However, the trade-off between recovery and adaptation is rarely considered. For example, many nutritional interventions described in this review target oxidative stress and inflammation, both thought to contribute to EIMD but are also crucial for the recovery and adaptation process. This calls into question whether long term administration of supplements and functional foods used to target EIMD is indeed best practice. This rapidly growing area of sports nutrition will benefit from careful consideration of the potential hormetic effect of long term use of nutritional aids that ameliorate muscle damage. This review provides a concise overview of what EIMD is, its causes and consequences and critically evaluates potential nutritional strategies to ameliorate EIMD. We present a pragmatic practical summary that can be adopted by practitioners and direct future research, with the purpose of pushing the field to better consider the fine balance between recovery and adaptation and the potential that nutritional interventions have in modulating this balance.

A cellular mechanism of muscle memory facilitates mitochondrial remodelling following resistance training

KEY POINTS

Referring to the muscle memory theory, previously trained muscles acquire strength and volume much faster than naive muscles. Using extreme experimental models such as synergist ablation or steroid administration, previous studies have demonstrated that the number of nuclei increases when a muscle becomes enlarged, which serves as a cellular muscle memory mechanism for the muscle. In the present study, we found that, when rats were subjected to physiologically relevant resistance training, the number of myonuclei increased and was retained during a long-term detraining period. The acquired myonuclei were related to a greater degree of muscle hypertrophic and mitochondrial biogenesis processes following subsequent hypertrophic conditions. Our data suggest a cellular mechanism supporting the notion that exposing young muscles to resistance training would help to restore age-related muscle loss coupled with mitochondrial dysfunction in later life.

ABSTRACT

Muscle hypertrophy induced by resistance training is accompanied by an increase in the number of myonuclei. The acquired myonuclei are viewed as a cellular component of muscle memory by which muscle enlargement is promoted during a re-training period. In the present study, we investigated the effect of exercise preconditioning on mitochondrial remodelling induced by resistance training. Sprague-Dawley rats were divided into four groups: untrained control, training, pre-training or re-training. The training groups were subjected to weight loaded-ladder climbing exercise training. Myonuclear numbers were significantly greater (up to 20%) in all trained muscles compared to untrained controls. Muscle mass was significantly higher in the re-training group compared to the training group (∼2-fold increase). Mitochondrial content, mitochondrial biogenesis gene expression levels and mitochondrial DNA copy numbers were significantly higher in re-trained muscles compared to the others. Oxidative myofibres (type I) were significantly increased only in the re-trained muscles. Furthermore, in vitro studies using insulin-like growth factor-1-treated L6 rat myotubes demonstrated that myotubes with a higher myonuclear number confer greater expression levels of both mitochondrial and nuclear genes encoding for constitutive and regulatory mitochondrial proteins, which also showed a greater mitochondrial respiratory function. These data suggest that myonuclei acquired from previous training facilitate mitochondrial biogenesis in response to subsequent retraining by (at least in part) enhancing cross-talk between mitochondria and myonuclei in the pre-conditioned myofibres.

Amount of daily exercise is an essential stimulation to alter the epigenome of skeletal muscle in rats

Long-term running training causes epigenetic changes in the skeletal muscles. Here we tested the effects of the total amount or duration of running training on the distribution of histones in the rat plantaris muscle. Post-weaned young rats were assigned to 3 different training groups: Run-1, 30 min/day running exercise for 8 wk using an animal treadmill at 24 m/min; Run-2, 15 min/day for 8 wk; and Run-3, 60 min/day for 4 wk. Citrate synthase activity was not significantly changed by running training, although the slight increase was observed in Run-3. Genes that were previously defined as showing the typical responses to running training were targeted to measure the distribution of histones using chromatin immunoprecipitation. The distribution of acetylated histone 3 was elevated in Run-2 and Run-3, but not in Run-1. Incorporation of H3.3 into the nucleosome was stimulated in Run-1, whereas H3.3 distribution was unchanged in Run-2 or downregulated in Run-3. Significant downregulation of H3.3 expression was also detected in Run-3. We further checked the responses of the target genes during acute running. Target genes were transcriptionally activated and histone acetylation was stimulated at the loci in response to acute running. These results suggested that the exchange of the histone component to H3.3 was stimulated by running training, inhibiting the accumulation of acetylated histones in Run-1. Additionally, it was further suggested that the enhanced daily amount of running caused changes in the H3.3 expression, affecting the rate of the histone exchange in Run-3. NEW & NOTEWORTHY Chromatin remodeling in the skeletal muscle is a potent mechanism preventing disuse atrophy in later life that can be acquired via long-term exercise training. Here we demonstrated in rats that daily exercise amount is a key factor in the development of epigenetic changes in the skeletal muscle. To acquire a health benefit, our research suggests the importance of considering the time endurance for daily exercise bouts.

Specific labelling of myonuclei by an antibody against pericentriolar material 1 on skeletal muscle tissue sections

AIM

Skeletal muscle is a heterogeneous tissue containing several different cell types, and only about 40%-50% of the cell nuclei within the tissue belong to myofibres. Existing technology, attempting to distinguish myonuclei from other nuclei at the light microscopy level, has led to controversies in our understanding of the basic cell biology of muscle plasticity. This study aims at demonstrating that an antibody against the protein pericentriolar material 1 (PCM1) can be used to reliably identify myonuclei on histological cross sections from humans, mice and rats.

METHODS

Cryosections were labelled with a polyclonal antibody against PCM1. The specificity of the labelling for myonuclei was verified using 3D reconstructions of confocal z-stacks triple-labelled for DNA, dystrophin and PCM1, and by co-localization with nuclear mCherry driven by the muscle-specific Alpha-Actin-1 promoter after viral transduction.

RESULTS

The PCM1 antibody specifically labelled all myonuclei, and myonuclei only, in cryosections of muscles from rats, mice and men. Nuclei in other cell types including satellite cells were not labelled. Both normal muscles and hypertrophic muscles after synergist ablation were investigated.

CONCLUSION

Pericentriolar material 1 can be used as a specific histological marker for myonuclei in skeletal muscle tissue without relying on counterstaining of other structures or cumbersome and subjective analysis of nuclear positioning.

Myonuclear Domain Flexibility Challenges Rigid Assumptions on Satellite Cell Contribution to Skeletal Muscle Fiber Hypertrophy

Satellite cell-mediated myonuclear accretion is thought to be required for skeletal muscle fiber hypertrophy, and even drive hypertrophy by preceding growth. Recent studies in humans and rodents provide evidence that challenge this axiom. Specifically, Type 2 muscle fibers reliably demonstrate a substantial capacity to hypertrophy in the absence of myonuclear accretion, challenging the notion of a tightly regulated myonuclear domain (i.e., area that each myonucleus transcriptionally governs). In fact, a “myonuclear domain ceiling”, or upper limit of transcriptional output per nucleus to support hypertrophy, has yet to be identified. Satellite cells respond to muscle damage, and also play an important role in extracellular matrix remodeling during loading-induced hypertrophy. We postulate that robust satellite cell activation and proliferation in response to mechanical loading is largely for these purposes. Future work will aim to elucidate the mechanisms by which Type 2 fibers can hypertrophy without additional myonuclei, the extent to which Type 1 fibers can grow without myonuclear accretion, and whether a true myonuclear domain ceiling exists.

Effects of detraining and retraining on muscle energy-sensing network and meteorin-like levels in obese mice

Background

Increased intramuscular peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) with exercise directly or indirectly affects other tissues, but the effector pathway of PGC-1α has not been clearly elucidated. The purpose of this study was to investigate the effect of exercise and/or dietary change on the protein levels of the soleus muscle energy-sensing network and meteorin-like (Metrnl), and additionally to analyze the detraining and retraining effects in high-fat diet (HFD)-induced obese mice.

Methods

One hundred male C57BL/6 mice were divided into normal-diet + sedentary (CO, n = 20) and HFD + sedentary (HF, n = 80) groups, and obesity was induced in the HF group through consumption of a 45% HFD for 6 weeks. The HF group was subdivided into HF only (n = 20), HF + training (HFT, n = 20), dietary change + sedentary (HFND, n = 20), and HFND + training (HFNDT, n = 20) groups, and the mice in the training groups underwent a treadmill training for 8 weeks, 5 times per week, 40 min per day. The HFT and HFNDT groups underwent 8-week training, 8-week detraining, and 4-week retraining.

Results

An 8-week training was effective in increasing the protein levels of soleus muscle AMP-activated protein kinase (AMPK), PGC-1α, and plasma Metrnl in the obese mice (P < 0.05). Moreover, exercise in obesity reduced body weight (P < 0.05), and exercise with dietary conversion was effective in reducing body weight (P < 0.05) and fat mass (P < 0.05) after 8-week training. 8-week detraining restored the increased protein level to the pre-exercise state, but, the previous exercise effect in body weight and fat mass (P < 0.05) of the HFNDT group remained until the end of 4-week detraining. 4-week retraining was effective in increasing the protein levels of soleus muscle AMPK, PGC-1α, blood Metrnl (P < 0.05), and reducing in body weight (P < 0.05) and fat mass (P < 0.05), when retraining with dietary change.

Conclusions

The results of this study suggest that regular exercise is indispensable to reduce body weight and fat mass through upregulation of the muscle energy-sensing network and Metrnl protein levels, and retraining with dietary change is necessary to obtain the retraining effects more quickly.

Electronic supplementary material

The online version of this article (10.1186/s12944-018-0751-3) contains supplementary material, which is available to authorized users.

Human Skeletal Muscle Possesses an Epigenetic Memory of Hypertrophy

It is unknown if adult human skeletal muscle has an epigenetic memory of earlier encounters with growth. We report, for the first time in humans, genome-wide DNA methylation (850,000 CpGs) and gene expression analysis after muscle hypertrophy (loading), return of muscle mass to baseline (unloading), followed by later hypertrophy (reloading). We discovered increased frequency of hypomethylation across the genome after reloading (18,816 CpGs) versus earlier loading (9,153 CpG sites). We also identified AXIN1, GRIK2, CAMK4, TRAF1 as hypomethylated genes with enhanced expression after loading that maintained their hypomethylated status even during unloading where muscle mass returned to control levels, indicating a memory of these genes methylation signatures following earlier hypertrophy. Further, UBR5, RPL35a, HEG1, PLA2G16, SETD3 displayed hypomethylation and enhanced gene expression following loading, and demonstrated the largest increases in hypomethylation, gene expression and muscle mass after later reloading, indicating an epigenetic memory in these genes. Finally, genes; GRIK2, TRAF1, BICC1, STAG1 were epigenetically sensitive to acute exercise demonstrating hypomethylation after a single bout of resistance exercise that was maintained 22 weeks later with the largest increase in gene expression and muscle mass after reloading. Overall, we identify an important epigenetic role for a number of largely unstudied genes in muscle hypertrophy/memory.

Comparative Analysis of Muscle Hypertrophy Models Reveals Divergent Gene Transcription Profiles and Points to Translational Regulation of Muscle Growth through Increased mTOR Signaling

Skeletal muscle mass is a result of the balance between protein breakdown and protein synthesis. It has been shown that multiple conditions of muscle atrophy are characterized by the common regulation of a specific set of genes, termed atrogenes. It is not known whether various models of muscle hypertrophy are similarly regulated by a common transcriptional program. Here, we characterized gene expression changes in three different conditions of muscle growth, examining each condition during acute and chronic phases. Specifically, we compared the transcriptome of Extensor Digitorum Longus (EDL) muscles collected (1) during the rapid phase of postnatal growth at 2 and 4 weeks of age, (2) 24 h or 3 weeks after constitutive activation of AKT, and (3) 24 h or 3 weeks after overload hypertrophy caused by tenotomy of the Tibialis Anterior muscle. We observed an important overlap between significantly regulated genes when comparing each single condition at the two different timepoints. Furthermore, examining the transcriptional changes occurring 24 h after a hypertrophic stimulus, we identify an important role for genes linked to a stress response, despite the absence of muscle damage in the AKT model. However, when we compared all different growth conditions, we did not find a common transcriptional fingerprint. On the other hand, all conditions showed a marked increase in mTORC1 signaling and increased ribosome biogenesis, suggesting that muscle growth is characterized more by translational, than transcriptional regulation.

The effects of dietary β-guanidinopropionic acid on growth and muscle fiber development in juvenile red porgy, Pagrus pagrus

β-guanidinopropionic acid (β-GPA) has been used in mammalian models to reduce intracellular phosphocreatine (PCr) concentration, which in turn lowers the energetic state of cells. This leads to changes in signaling pathways that attempt to re-establish energetic homeostasis. Changes in those pathways elicit effects similar to those of exercise such as changes in body and muscle growth, metabolism, endurance and health. Generally, exercise effects are beneficial to fish health and aquaculture, but inducing exercise in fishes can be impractical. Therefore, this study evaluated the potential use of supplemental β-GPA to induce exercise-like effects in a rapidly growing juvenile teleost, the red porgy (Pagrus pagrus). We demonstrate for the first time that β-GPA can be transported into teleost muscle fibers and is phosphorylated, and that this perturbs the intracellular energetic state of the cells, although to a lesser degree than typically seen in mammals. β-GPA did not affect whole animal growth, nor did it influence skeletal muscle fiber size or myonuclear recruitment. There was, however, an increase in mitochondrial volume within myofibers in treated fish. GC/MS metabolomic analysis revealed shifts in amino acid composition of the musculature, putatively reflecting increases in connective tissue and decreases in protein synthesis that are associated with β-GPA treatment. These results suggest that β-GPA modestly affects fish muscle in a manner similar to that observed in mammals, and that β-GPA may have application to aquaculture by providing a more practical means of generating some of the beneficial effects of exercise in fishes.

MyD88 promotes myoblast fusion in a cell-autonomous manner

Myoblast fusion is an indispensable step for skeletal muscle development, postnatal growth, and regeneration. Myeloid differentiation primary response gene 88 (MyD88) is an adaptor protein that mediates Toll-like receptors and interleukin-1 receptor signaling. Here we report a cell-autonomous role of MyD88 in the regulation of myoblast fusion. MyD88 protein levels are increased during in vitro myogenesis and in conditions that promote skeletal muscle growth in vivo. Deletion of MyD88 impairs fusion of myoblasts without affecting their survival, proliferation, or differentiation. MyD88 regulates non-canonical NF-κB and canonical Wnt signaling during myogenesis and promotes skeletal muscle growth and overload-induced myofiber hypertrophy in mice. Ablation of MyD88 reduces myofiber size during muscle regeneration, whereas its overexpression promotes fusion of exogenous myoblasts to injured myofibers. Our study shows that MyD88 modulates myoblast fusion and suggests that augmenting its levels may be a therapeutic approach to improve skeletal muscle formation in degenerative muscle disorders.

Inducible satellite cell depletion attenuates skeletal muscle regrowth following a scald-burn injury

KEY POINTS

Severe burns result in significant skeletal muscle cachexia that impedes recovery. Activity of satellite cells, skeletal muscle stem cells, is altered following a burn injury and likely hinders regrowth of muscle. Severe burn injury induces satellite cell proliferation and fusion into myofibres with greater activity in muscles proximal to the injury site. Conditional depletion of satellite cells attenuates recovery of myofibre area and volume following a scald burn injury in mice. Skeletal muscle regrowth following a burn injury requires satellite cell activity, underscoring the therapeutic potential of satellite cells in the prevention of prolonged frailty in burn survivors.

ABSTRACT

Severe burns result in profound skeletal muscle atrophy; persistent muscle atrophy and weakness are major complications that hamper recovery from burn injury. Many factors contribute to the erosion of muscle mass following burn trauma, and we have previously shown concurrent activation and apoptosis of muscle satellite cells following a burn injury in paediatric patients. To determine the necessity of satellite cells during muscle recovery following a burn injury, we utilized a genetically modified mouse model (Pax7CreER -DTA) that allows for the conditional depletion of satellite cells in skeletal muscle. Additionally, mice were provided 5-ethynyl-2'-deoxyuridine to determine satellite cell proliferation, activation and fusion. Juvenile satellite cell-wild-type (SC-WT) and satellite cell-depleted (SC-Dep) mice (8 weeks of age) were randomized to sham or burn injury consisting of a dorsal scald burn injury covering 30% of total body surface area. Both hindlimb and dorsal muscles were studied at 7, 14 and 21 days post-burn. SC-Dep mice had >93% depletion of satellite cells compared to SC-WT (P < 0.05). Burn injury induced robust atrophy in muscles located both proximal and distal to the injury site (∼30% decrease in fibre cross-sectional area, P < 0.05). Additionally, burn injury induced skeletal muscle regeneration, satellite cell proliferation and fusion. Depletion of satellite cells impaired post-burn recovery of both muscle fibre cross-sectional area and volume (P < 0.05). These findings support an integral role for satellite cells in the aetiology of lean tissue recovery following a severe burn injury.

Skeletal Muscle Remodeling: Interconnections Between Stem Cells and Protein Turnover

Nutrition and exercise are important components of a healthy lifestyle to improve rates of hypertrophic and nonhypertrophic skeletal muscle remodeling. We provide evidence to support the hypothesis that muscle stem cells and protein turnover are collaborative, not separate, mechanisms supporting muscle remodeling by facilitating protein, nuclear, and cellular turnover in response to the ingestion of protein dense foods and exercise.

Satellite cell depletion prevents fiber hypertrophy in skeletal muscle

The largest mammalian cells are the muscle fibers, and they have multiple nuclei to support their large cytoplasmic volumes. During hypertrophic growth, new myonuclei are recruited from satellite stem cells into the fiber syncytia, but it was recently suggested that such recruitment is not obligatory: overload hypertrophy after synergist ablation of the plantaris muscle appeared normal in transgenic mice in which most of the satellite cells were abolished. When we essentially repeated these experiments analyzing the muscles by immunohistochemistry and in vivo and ex vivo imaging, we found that overload hypertrophy was prevented in the satellite cell-deficient mice, in both the plantaris and the extensor digitorum longus muscles. We attribute the previous findings to a reliance on muscle mass as a proxy for fiber hypertrophy, and to the inclusion of a significant number of regenerating fibers in the analysis. We discuss that there is currently no model in which functional, sustainable hypertrophy has been unequivocally demonstrated in the absence of satellite cells; an exception is re-growth, which can occur using previously recruited myonuclei without addition of new myonuclei.

Requirement of myomaker-mediated stem cell fusion for skeletal muscle hypertrophy

Fusion of skeletal muscle stem/progenitor cells is required for proper development and regeneration, however the significance of this process during adult muscle hypertrophy has not been explored. In response to muscle overload after synergist ablation in mice, we show that myomaker, a muscle specific membrane protein essential for myoblast fusion, is activated mainly in muscle progenitors and not myofibers. We rendered muscle progenitors fusion-incompetent through genetic deletion of myomaker in muscle stem cells and observed a complete reduction of overload-induced hypertrophy. This blunted hypertrophic response was associated with a reduction in Akt and p70s6k signaling and protein synthesis, suggesting a link between myonuclear accretion and activation of pro-hypertrophic pathways. Furthermore, fusion-incompetent muscle exhibited increased fibrosis after muscle overload, indicating a protective role for normal stem cell activity in reducing myofiber strain associated with hypertrophy. These findings reveal an essential contribution of myomaker-mediated stem cell fusion during physiological adult muscle hypertrophy.

Peroxisome Proliferator Activated Receptor Beta (PPARβ) activity increases the immune response and shortens the early phases of skeletal muscle regeneration

Peroxisome Proliferator-Activated Receptor Beta (PPARβ) is a transcription factor playing an important role in both muscle myogenesis and remodeling, and in inflammation. However, its role in the coordination of the transient muscle inflammation and reparation process following muscle injury has not yet been fully determined. We postulated that activation of the PPARβ pathway alters the early phase of the muscle regeneration process, i.e. when immune cells infiltrate in injured muscle. Tibialis anteriors of C57BL6/J mice treated or not with the PPARβ agonist GW0742 were injected with cardiotoxin (or with physiological serum for the contralateral muscle). Muscle regeneration was monitored on days 4, 7, and 14 post-injury. We found that treatment of mice with GW0742 increased, at day 4 post-damage, the recruitment of immune cells (M1 and M2 macrophages) and upregulated the expression of the anti-inflammatory cytokine IL-10 and TGF-β mRNA. Those effects were accompanied by a significant increase at day 4 of myogenic regulatory factors (Pax7, MyoD, Myf5, Myogenin) mRNA in GW0742-treated mice. However, we showed an earlier return (7 days vs 14 days) of Myf5 and Myogenin to basal levels in GW0742- compared to DMSO-treated mice. Differential effects of GW0742 observed during the regeneration were associated with variations of PPARβ pathway activity. Collectively, our findings indicate that PPARβ pathway activity shortens the early phases of skeletal muscle regeneration by increasing the immune response.

Myogenic Progenitor Cells Control Extracellular Matrix Production by Fibroblasts during Skeletal Muscle Hypertrophy

Satellite cells, the predominant stem cell population in adult skeletal muscle, are activated in response to hypertrophic stimuli and give rise to myogenic progenitor cells (MPCs) within the extracellular matrix (ECM) that surrounds myofibers. This ECM is composed largely of collagens secreted by interstitial fibrogenic cells, which influence satellite cell activity and muscle repair during hypertrophy and aging. Here we show that MPCs interact with interstitial fibrogenic cells to ensure proper ECM deposition and optimal muscle remodeling in response to hypertrophic stimuli. MPC-dependent ECM remodeling during the first week of a growth stimulus is sufficient to ensure long-term myofiber hypertrophy. MPCs secrete exosomes containing miR-206, which represses Rrbp1, a master regulator of collagen biosynthesis, in fibrogenic cells to prevent excessive ECM deposition. These findings provide insights into how skeletal stem and progenitor cells interact with other cell types to actively regulate their extracellular environments for tissue maintenance and adaptation.

Training at non-damaging intensities facilitates recovery from muscle atrophy

INTRODUCTION

Resistance training promotes recovery from muscle atrophy, but optimum training programs have not been established. We aimed to determine the optimum training intensity for muscle atrophy.

METHODS

Mice recovering from atrophied muscles after 2 weeks of tail suspension underwent repeated isometric training with varying joint torques 50 times per day.

RESULTS

Muscle recovery assessed by maximal isometric contraction and myofiber cross-sectional areas (CSAs) were facilitated at 40% and 60% maximum contraction strength (MC), but at not at 10% and 90% MC. At 60% and 90% MC, damaged and contained smaller diameter fibers were observed. Activation of myogenic satellite cells and a marked increase in myonuclei were observed at 40%, 60%, and 90% MC.

CONCLUSIONS

The increases in myofiber CSAs were likely caused by increased myonuclei formed through fusion of resistance-induced myofibers with myogenic satellite cells. These data indicate that resistance training without muscle damage facilitates efficient recovery from atrophy. Muscle Nerve 55: 243-253, 2017.

Muscle memory and a new cellular model for muscle atrophy and hypertrophy

Memory is a process in which information is encoded, stored, and retrieved. For vertebrates, the modern view has been that it occurs only in the brain. This review describes a cellular memory in skeletal muscle in which hypertrophy is 'remembered' such that a fibre that has previously been large, but subsequently lost its mass, can regain mass faster than naive fibres. A new cell biological model based on the literature, with the most reliable methods for identifying myonuclei, can explain this phenomenon. According to this model, previously untrained fibres recruit myonuclei from activated satellite cells before hypertrophic growth. Even if subsequently subjected to grave atrophy, the higher number of myonuclei is retained, and the myonuclei seem to be protected against the elevated apoptotic activity observed in atrophying muscle tissue. Fibres that have acquired a higher number of myonuclei grow faster when subjected to overload exercise, thus the nuclei represent a functionally important 'memory' of previous strength. This memory might be very long lasting in humans, as myonuclei are stable for at least 15 years and might even be permanent. However, myonuclei are harder to recruit in the elderly, and if the long-lasting muscle memory also exists in humans, one should consider early strength training as a public health advice. In addition, myonuclei are recruited during steroid use and encode a muscle memory, at least in rodents. Thus, extending the exclusion time for doping offenders should be considered.

Sex differences in the response to resistance exercise training in older people

Resistance exercise training is known to be effective in increasing muscle mass in older people. Acute measurement of protein metabolism data has indicated that the magnitude of response may differ between sexes. We compared adaptive responses in muscle mass and function to 18 weeks resistance exercise training in a cohort of older (>65 years) men and women. Resistance exercise training improved knee extensor maximal torque, 4 m walk time, time to complete five chair rises, muscle anatomical cross‐sectional area (ACSA) and muscle quality with no effect on muscle fat/water ratio or plasma glucose, insulin, triacylglycerol, IL‐6, and TNF‐α. Differences between sexes were observed for knee extensor maximal torque and muscle quality with greater increases observed in men versus women (P < 0.05). Maximal torque increased by 15.8 ± 10.6% in women and 41.7 ± 25.5% in men, whereas muscle quality increased by 8.8 ± 17.5% in women and by 33.7 ± 25.6% in men. In conclusion, this study has demonstrated a difference in the magnitude of adaptation, of some of the outcome measures employed, in response to 18 weeks of resistance exercise training between men and women. The mechanisms underlying this observation remain to be established.

Coupling of myocardial stress resistance and signalling to voluntary activity and inactivity

AIMS

We examined coupling of myocardial ischaemic tolerance to physical activity and inactivity, and whether this involves modulation of survival (AKT, AMPK, ERK1/2, HSP27, EGFR) and injury (GSK3β) proteins implicated in ischaemic preconditioning and calorie restriction.

METHODS

Proteomic modifications were assessed in ventricular myocardium, and tolerance to 25-min ischaemia in ex vivo perfused hearts from C57Bl/6 mice subjected to 14-day voluntary activity in running-naïve animals (Active); 7 days of subsequent inactivity (Inactive); brief (day 3) restoration of running (Re-Active); or time-matched inactivity.

RESULTS

Active mice increased running speed and distance by 75-150% over 14 days (to ~40 m min(-1) and 10 km day(-1) ), with Active hearts resistant to post-ischaemic dysfunction (40-50% improvements in ventricular pressure development, diastolic pressure and dP/dt). Cardioprotection was accompanied by ~twofold elevations in AKT, AMPK, HSP27 and GSK3β phosphorylation and EGFR expression. Ischaemic tolerance was reversed in Inactive hearts, paralleling reduced EGFR expression and GSK3β and ERK1/2 phosphorylation (AKT, AMPK, HSP27 phosphorylation unaltered). Running characteristics, ischaemic tolerance, EGFR expression and GSK3β phosphorylation returned to Active levels within 1-3 days of restored activity (without changes in AKT, AMPK or HSP27 phosphorylation). Transcriptional responses included activity-dependent Anp induction vs. Hmox1 and Sirt3 suppression, and inactivity-dependent Adora2b induction.

CONCLUSIONS

Data confirm the sensitive coupling of ischaemic tolerance to activity: voluntary running induces cardioprotection that dissipates within 1 week of inactivity yet recovers rapidly upon subsequent activity. While exercise in naïve animals induces a molecular profile characteristic of preconditioning/calorie restriction, only GSK3β and EGFR modulation consistently parallel activity- and inactivity-dependent ischaemic tolerance.

Reduced Appendicular Lean Body Mass, Muscle Strength, and Size of Type II Muscle Fibers in Patients with Spondyloarthritis versus Healthy Controls: A Cross-Sectional Study

Introduction. The purpose of this study was to investigate body composition, muscle function, and muscle morphology in patients with spondyloarthritis (SpA). Methods. Ten male SpA patients (mean ± SD age 39 ± 4.1 years) were compared with ten healthy controls matched for sex, age, body mass index, and self-reported level of physical exercise. Body composition was measured by dual energy X-ray absorptiometry. Musculus quadriceps femoris (QF) strength was assessed by maximal isometric contractions prior to test of muscular endurance. Magnetic resonance imaging of QF was used to measure muscle size and calculate specific muscle strength. Percutaneous needle biopsy samples were taken from m. vastus lateralis. Results. SpA patients presented with significantly lower appendicular lean body mass (LBM) (p = 0.02), but there was no difference in bone mineral density, fat mass, or total LBM. Absolute QF strength was significantly lower in SpA patients (p = 0.03) with a parallel trend for specific strength (p = 0.08). Biopsy samples from the SpA patients revealed significantly smaller cross-sectional area (CSA) of type II muscle fibers (p = 0.04), but no difference in CSA type I fibers. Conclusions. Results indicate that the presence of SpA disease is associated with reduced appendicular LBM, muscle strength, and type II fiber CSA.

Satellite Cells Contribution to Exercise Mediated Muscle Hypertrophy and Repair

Satellite cells (SCs) are the most abundant skeletal muscle stem cells. They are widely recognized for their contributions to maintenance of muscle mass, regeneration and hypertrophy during the human life span. These cells are good candidates for cell therapy due to their self-renewal capabilities and presence in an undifferentiated form. Presently, a significant gap exists between our knowledge of SCs behavior and their application as a means for human skeletal muscle tissue repair and regeneration. Both physiological and pathological stimuli potentially affect SCs activation, proliferation, and terminal differentiation the former category being the focus of this article. Activation of SCs occurs following exercise, post-training micro-injuries, and electrical stimulation. Exercise, as a potent and natural stimulus, is at the center of numerous studies on SC activation and relevant fields. According to research, different exercise modalities end with various effects. This review article attempts to picture the state of the art of the SCs life span and their engagement in muscle regeneration and hypertrophy in exercise.

The Impact of Endurance Training on Human Skeletal Muscle Memory, Global Isoform Expression and Novel Transcripts

Regularly performed endurance training has many beneficial effects on health and skeletal muscle function, and can be used to prevent and treat common diseases e.g. cardiovascular disease, type II diabetes and obesity. The molecular adaptation mechanisms regulating these effects are incompletely understood. To date, global transcriptome changes in skeletal muscles have been studied at the gene level only. Therefore, global isoform expression changes following exercise training in humans are unknown. Also, the effects of repeated interventions on transcriptional memory or training response have not been studied before. In this study, 23 individuals trained one leg for three months. Nine months later, 12 of the same subjects trained both legs in a second training period. Skeletal muscle biopsies were obtained from both legs before and after both training periods. RNA sequencing analysis of all 119 skeletal muscle biopsies showed that training altered the expression of 3,404 gene isoforms, mainly associated with oxidative ATP production. Fifty-four genes had isoforms that changed in opposite directions. Training altered expression of 34 novel transcripts, all with protein-coding potential. After nine months of detraining, no training-induced transcriptome differences were detected between the previously trained and untrained legs. Although there were several differences in the physiological and transcriptional responses to repeated training, no coherent evidence of an endurance training induced transcriptional skeletal muscle memory was found. This human lifestyle intervention induced differential expression of thousands of isoforms and several transcripts from unannotated regions of the genome. It is likely that the observed isoform expression changes reflect adaptational mechanisms and processes that provide the functional and health benefits of regular physical activity.

Skeletal muscle is the most abundant tissue of the healthy human body. It is also highly adaptable to different environmental stimuli, e.g. regular exercise. Exercise training improves overall health and muscle function, and can be used to prevent and treat several common diseases e.g. cardiovascular disease and type II diabetes. Therefore, it is of great importance to understand the molecular mechanisms behind adaptation processes in human skeletal muscle. In this study, we show that different expression variants from the same gene can be regulated in different directions with training, implicating alternative protein functions from one single gene. Such findings are emblematic of the complex mechanisms regulating the effects of training. We also find that training changes the activity of functionally unknown parts of the genome, with the potential for new proteins involved in the health-enhancing effects of exercise. Additionally, our results challenge the belief of a skeletal muscle memory, where previous training can affect the response to a subsequent training period. Overall, we provide understanding of the skeletal muscle biology and novel insights into the mechanisms behind the massive benefits of regular exercise on the human skeletal muscle transcriptome, inspiring further studies for deeper investigation.

Muscle fiber type diversification during exercise and regeneration

The plasticity of skeletal muscle can be traced down to extensive metabolic, structural and molecular remodeling at the single fiber level. Skeletal muscle is comprised of different fiber types that are the basis of muscle plasticity in response to various functional demands. Resistance and endurance exercises are two external stimuli that differ in their duration and intensity of contraction and elicit markedly different responses in muscles adaptation. Further, eccentric contractions that are associated with exercise-induced injuries, elicit varied muscle adaptation and regenerative responses. Most adaptive changes are fiber type-specific and are highly influenced by diverse structural, metabolic and functional characteristics of individual fiber types. Regulation of signaling pathways by reactive oxygen species (ROS) and oxidative stress also plays an important role in muscle fiber adaptation during exercise. This review focuses on cellular and molecular responses that regulate the adaptation of skeletal muscle to exercise and exercise-related injuries.

The myonuclear domain is not maintained in skeletal muscle during either atrophy or programmed cell death

Skeletal muscle mass can increase during hypertrophy or decline dramatically in response to normal or pathological signals that trigger atrophy. Many reports have documented that the number of nuclei within these cells is also plastic. It has been proposed that a yet-to-be-defined regulatory mechanism functions to maintain a relatively stable relationship between the cytoplasmic volume and nuclear number within the cell, a phenomenon known as the "myonuclear domain" hypothesis. While it is accepted that hypertrophy is typically associated with the addition of new nuclei to the muscle fiber from stem cells such as satellite cells, the loss of myonuclei during atrophy has been controversial. The intersegmental muscles from the tobacco hawkmoth Manduca sexta are composed of giant syncytial cells that undergo sequential developmental programs of atrophy and programmed cell death at the end of metamorphosis. Since the intersegmental muscles lack satellite cells or regenerative capacity, the tissue is not "contaminated" by these nonmuscle nuclei. Consequently, we monitored muscle mass, cross-sectional area, nuclear number, and cellular DNA content during atrophy and the early phases of cell death. Despite a ∼75-80% decline in muscle mass and cross-sectional area during the period under investigation, there were no reductions in nuclear number or DNA content, and the myonuclear domain was reduced by ∼85%. These data suggest that the myonuclear domain is not an intrinsic property of skeletal muscle and that nuclei persist through atrophy and programmed cell death.

Eccentric resistance training increases and retains maximal strength, muscle endurance, and hypertrophy in trained men

The aim of the present study was to evaluate the effects of different resistance training protocols on muscle strength, endurance, and hypertrophy after training and detraining. Thirty-four resistance-trained males were randomized in concentric-only (CONC), eccentric-only (ECC), traditional concentric-eccentric (TRAD) bench press resistance training or control group. The training volume was equalized among the intervention groups. Bench press of 1-repetition maximum (1RM)/body mass, maximum number of repetitions (MNR), and chest circumference were evaluated at the baseline, after 6 weeks of training, and after 6 weeks of detraining. All intervention groups reported significant 1RM/body mass increases after training (CONC baseline: 1.04 ± 0.06, post-training: 1.12 ± 0.08, p < 0.05; ECC baseline: 1.08 ± 0.04, post-training: 1.15 ± 0.05, p < 0.05; TRAD baseline: 1.06 ± 0.08, post-training: 1.11 ± 0.10, p < 0.05). After detraining, only ECC retained 1RM/body mass above the baseline (1.17 ± 0.07, p < 0.05), while CONC and TRAD returned to baseline values. Only ECC improved and retained MNR (baseline: 22 ± 3; post-training: 25 ± 3, and post-detraining: 25 ± 4, p < 0.05 compared with baseline) and chest circumference (baseline: 98.3 ± 2.4 cm, post-training: 101.7 ± 2.2 cm and post-detraining: 100.7 ± 2.3 cm. p < 0.05 compared with baseline), while no significant changes occurred in both CONC and TRAD. The incorporation of eccentric training can be recommended for counteracting the negative effects of detraining or forced physical inactivity.

Increased hypertrophic response with increased mechanical load in skeletal muscles receiving identical activity patterns

It is often assumed that mechanical factors are important for effects of exercise on muscle, but during voluntary training and most experimental conditions the effects could solely be attributed to differences in electrical activity, and direct evidence for a mechanosensory pathway has been scarce. We here show that, in rat muscles stimulated in vivo under deep anesthesia with identical electrical activity patterns, isometric contractions induced twofold more hypertrophy than contractions with 50-60% of the isometric force. The number of myonuclei and the RNA levels of myogenin and myogenic regulatory factor 4 were increased with high load, suggesting that activation of satellite cells is mechano dependent. On the other hand, training induced a major shift in fiber type distribution from type 2b to 2x that was load independent, indicating that the electrical signaling rather than mechanosignaling controls fiber type. RAC-α serine/threonine-protein kinase (Akt) and ribosomal protein S6 kinase β-1 (S6K1) were not significantly differentially activated by load, suggesting that the differences in mechanical factors were not important for activating the Akt/mammalian target of rapamycin/S6K1 pathway. The transmembrane molecule syndecan-4 implied in overload hypertrophy in cardiac muscle was not load dependent, suggesting that mechanosignaling in skeletal muscle is different.

Does skeletal muscle have an 'epi'-memory? The role of epigenetics in nutritional programming, metabolic disease, aging and exercise

Skeletal muscle mass, quality and adaptability are fundamental in promoting muscle performance, maintaining metabolic function and supporting longevity and healthspan. Skeletal muscle is programmable and can ‘remember’ early‐life metabolic stimuli affecting its function in adult life. In this review, the authors pose the question as to whether skeletal muscle has an ‘epi’‐memory? Following an initial encounter with an environmental stimulus, we discuss the underlying molecular and epigenetic mechanisms enabling skeletal muscle to adapt, should it re‐encounter the stimulus in later life. We also define skeletal muscle memory and outline the scientific literature contributing to this field. Furthermore, we review the evidence for early‐life nutrient stress and low birth weight in animals and human cohort studies, respectively, and discuss the underlying molecular mechanisms culminating in skeletal muscle dysfunction, metabolic disease and loss of skeletal muscle mass across the lifespan. We also summarize and discuss studies that isolate muscle stem cells from different environmental niches in vivo (physically active, diabetic, cachectic, aged) and how they reportedly remember this environment once isolated in vitro. Finally, we will outline the molecular and epigenetic mechanisms underlying skeletal muscle memory and review the epigenetic regulation of exercise‐induced skeletal muscle adaptation, highlighting exercise interventions as suitable models to investigate skeletal muscle memory in humans. We believe that understanding the ‘epi’‐memory of skeletal muscle will enable the next generation of targeted therapies to promote muscle growth and reduce muscle loss to enable healthy aging.

Prior swimming exercise favors muscle recovery in adult female rats after joint immobilization

[Purpose] To evaluate the efficacy of pre-exercise on immobilization and subsequent recovery of white gastrocnemius (WG) and soleus (SOL) muscles of female rats. [Subjects and Methods] Thirty, 8-month-old, female Wistar rats were randomly and evenly allocated to six groups: sedentary (S); immobilized sedentary (IS); immobilized/rehabilitated sedentary (IRS); trained (T); immobilized trained (IT); and immobilized/rehabilitated trained (IRT). For four months, T, IT and IRT group animals performed swimming exercise (three sessions per week, 60 minutes per session), while S, IS and IRS groups animals remained housed in cages. After this period, the left hindlimb of the animals from the IS, IRS, IT and IRT groups was immobilized for five days, with the ankle at 90°. After removal of the orthosis, animals from the IRS and IRT groups followed a rehabilitation program based on swimming (five sessions per week, 60 minutes per session) for two weeks. [Results] Immobilization significantly reduced the cross-sectional area of the white gastrocnemius muscle; no changes were observed in the soleus muscles of the trained animals. Transforming growth factor-β1 protein levels were similar among the trained groups. [Conclusion] Prior swimming prevents hypotrophy of the soleus muscle after immobilization, and protein levels reflected the adaptive capacity of the skeletal muscle.

On Having No Head: Cognition throughout Biological Systems

The central nervous system (CNS) underlies memory, perception, decision-making, and behavior in numerous organisms. However, neural networks have no monopoly on the signaling functions that implement these remarkable algorithms. It is often forgotten that neurons optimized cellular signaling modes that existed long before the CNS appeared during evolution, and were used by somatic cellular networks to orchestrate physiology, embryonic development, and behavior. Many of the key dynamics that enable information processing can, in fact, be implemented by different biological hardware. This is widely exploited by organisms throughout the tree of life. Here, we review data on memory, learning, and other aspects of cognition in a range of models, including single celled organisms, plants, and tissues in animal bodies. We discuss current knowledge of the molecular mechanisms at work in these systems, and suggest several hypotheses for future investigation. The study of cognitive processes implemented in aneural contexts is a fascinating, highly interdisciplinary topic that has many implications for evolution, cell biology, regenerative medicine, computer science, and synthetic bioengineering.

Electrical stimulation influences chronic intermittent hypoxia-hypercapnia induction of muscle fibre transformation by regulating the microRNA/Sox6 pathway

Chronic obstructive pulmonary disease can cause muscle fibre transformation due to chronic intermittent hypoxia-hypercapnia (CIHH). Studies have shown that high expression of Sox6 in muscle could suppress type-I fibres through downregulating the PPARβ (peroxisome proliferator-activated receptor β)/ERRγ (oestrogen-related receptor γ)/microRNA pathway. However, whether this pathway is involved in CIHH-induced muscle fibre transformation is unknown. Electrical stimulation (ES) is an effective approach to ameliorate muscle dysfunction. Here, we explored the effects of ES on CIHH-induced muscle fibre transformation and the microRNA/Sox6 pathway. After CIHH exposure, both the soleus (SOL) and gastrocnemius (GC) muscles showed decreased type-I fibres. The PPARβ/ERRγ/mir-499&208b (PEM, for GC) and PPARβ/mir-499&208b (PM, for SOL) signalling cascades were suppressed, followed by elevated Sox6 expression. Low frequency electrical stimulation (LFES) activated the PEM/PM pathway and enhanced type-I fibre numbers through suppressing Sox6 in SOL and GC. High frequency electrical stimulation (HFES) promoted type-I fibre expression through activating the PEM pathway in GC. Although PPARβ expression and type-I fibres were suppressed in SOL after HFES, no significant change was found in mir-499&208b/Sox6 expression. These results suggest that the microRNA/Sox6 pathway is disturbed after CIHH. Both low and high frequency electrical stimulations induce muscle fibre transformation partly through regulating the microRNA/Sox6 pathway.