Processes In Skeletal Muscle Research Articles

Microproteins in skeletal muscle: hidden keys in muscle physiology.

Recent advances in the transcriptomics, translatomics, and proteomics have led us to the exciting new world of functional endogenous microproteins. These microproteins have a small size and are derived from small open reading frames (smORFs) of RNAs previously annotated as non‐coding (e.g. lncRNAs and circRNAs) as well as from untranslated regions and canonical mRNAs. The presence of these microproteins reveals a much larger translatable portion of the genome, shifting previously defined dogmas and paradigms. These findings affect our view of organisms as a whole, including skeletal muscle tissue. Emerging evidence demonstrates that several smORF‐derived microproteins play crucial roles during muscle development (myogenesis), maintenance, and regeneration, as well as lipid and glucose metabolism and skeletal muscle bioenergetics. These microproteins are also involved in processes including physical activity capacity, cellular stress, and muscular‐related diseases (i.e. myopathy, cachexia, atrophy, and muscle wasting). Given the role of these small proteins as important key regulators of several skeletal muscle processes, there are rich prospects for the discovery of new microproteins and possible therapies using synthetic microproteins.

Lactobacillus plantarum HY7715 Ameliorates Sarcopenia by Improving Skeletal Muscle Mass and Function in Aged Balb/c Mice.

Sarcopenia is a loss of muscle mass and function in elderly people and can lead to physical frailty and fall-related injuries. Sarcopenia is an inevitable event of the aging process that substantially impacts a person’s quality of life. Recent studies to improve muscle function through the intake of various functional food materials are attracting attention. However, it is not yet known whether probiotics can improve muscle mass and muscle strength and affect physical performance. Lactobacillus plantarum HY7715 (HY7715) is a lactic acid bacteria isolated from kimchi. The present research shows that L. plantarum HY7715 increases physical performance and skeletal muscle mass in 80-week-old aged Balb/c male mice. HY7715 not only induces myoblast differentiation and mitochondrial biogenesis but also inhibits the sarcopenic process in skeletal muscle. In addition, HY7715 recovers the microbiome composition and beta-diversity shift. Therefore, HY7715 has promise as a functional probiotic supplement to improve the degeneration of muscle function that is associated with aging.

The Role of cAMP-PKA Pathway in Lactate-Induced Intramuscular Triglyceride Accumulation and Mitochondria Content Increase in Mice.

The glycolytic product of exercise, lactate, has long been recognized to promote lipid accumulation by activation of G-protein-coupled receptor 81 (GPR81) and inhibition of the cyclic adenosine monophosphate-protein kinase A (cAMP –PKA) pathway in adipose tissue. Whether lactate causes a similar process in skeletal muscle is unclear. Lactate might also improve mitochondria content in skeletal muscle; however, the mechanism is not clarified either. In this study, using intramuscular injection of lactate to the gastrocnemius and intraperitoneal injection of forskolin (activator of cAMP-PKA pathway), we identified the role of the cAMP-PKA pathway in lactate-induced intramuscular triglyceride accumulation and mitochondrial content increase. The intramuscular triglyceride level in the gastrocnemius increased after 5weeks of lactate injection (p<0.05), and this effect was blocked by forskolin injection (p<0.05). Corresponding expression level changes of GPR81, P-PKA/PKA, P-CREB/cAMP-response element binding protein (CREB), and proteins related to lipid metabolism suggest that lactate could induce intramuscular triglyceride accumulation partly through the inhibition of the cAMP-PKA pathway. Meanwhile, the intramuscular expression of citrate synthase (CS) and the activity of CS increased after 5weeks of lactate injection (p<0.05), but the change of CS expression was not blocked by forskolin injection, suggesting other mechanisms might exist. Consequently, exploration for other potential mechanisms that might contribute to the lactate-induced mitochondria content increase was conducted. We found an increase in the contents of lactate-related metabolites in skeletal muscle mitochondria after acute lactate injection (the p-value of each analysis is less than 0.05). LHDA was also validated to exist in mitochondria in this study. These results provide a possibility for metabolism-related mechanisms of lactate-induced mitochondria content increase. Future study is needed to validate this hypothesis. In conclusion, lactate-induced intramuscular triglyceride accumulation is achieved by inhibition of lipolysis, and this process is regulated by the cAMP-PKA pathway. Promoted lipogenesis also contributes to lactate-induced triglyceride accumulation, and this process might also be regulated by the cAMP-PKA pathway. Lactate injection might increase mitochondria content and cAMP-PKA pathway might have a limited contribution, while other metabolism-related mechanisms might play a prominent role.

The muscle clock: how does muscle physiology change in 24h in response to exercise?

The muscle clock: how does muscle physiology change in 24h in response to exercise?

The Role of Autophagy in Skeletal Muscle Diseases.

Skeletal muscle is the most abundant type of tissue in human body, being involved in diverse activities and maintaining a finely tuned metabolic balance. Autophagy, characterized by the autophagosome–lysosome system with the involvement of evolutionarily conserved autophagy-related genes, is an important catabolic process and plays an essential role in energy generation and consumption, as well as substance turnover processes in skeletal muscles. Autophagy in skeletal muscles is finely tuned under the tight regulation of diverse signaling pathways, and the autophagy pathway has cross-talk with other pathways to form feedback loops under physiological conditions and metabolic stress. Altered autophagy activity characterized by either increased formation of autophagosomes or inhibition of lysosome-autophagosome fusion can lead to pathological cascades, and mutations in autophagy genes and deregulation of autophagy pathways have been identified as one of the major causes for a variety of skeleton muscle disorders. The advancement of multi-omics techniques enables further understanding of the molecular and biochemical mechanisms underlying the role of autophagy in skeletal muscle disorders, which may yield novel therapeutic targets for these disorders.

Lactate augments intramuscular triglyceride accumulation and mitochondrial biogenesis in rats.

Regular exercise induces intramuscular triglyceride accumulation with improved mitochondrial ability, but the mechanism remains unknown. The glycolytic product of exercise, lactate, has long been rec-ognized to suppress lipolysis and promote lipogenesis in adipocytes through inhibition of the cAMP-PKA pathway by activation of the G protein-coupled receptor (GPR81). However, whether lactate results in a similar process in skeletal muscle is unclear. Here, by using intramuscular injection of lactate to the gastrocnemius, the lipid metabolism effects were investigated in rat skeletal muscle. Firstly, the lactate-injection effect was verified by comparing changes in blood lactate levels from injection and exercise (30 min, 31 m/min, treadmill running). After five weeks of lactate intervention, intramuscular triglyceride levels in the gastrocnemius and the proportion of epididymis adipose mass to body weight increased. Chronic intramuscular injection of lactate elevated lactate receptor, GPR81, and reduced cAMP response element-binding (CREB) and P-CREB abundance in the gastrocnemius. Additionally, there was a significant decline in lipolytic-related proteins (AMPK, P-AMPK, P-HSL, CPT-1B, TGF-β2, SDHA) and a significant increase in fat synthesis proteins (SREBP-1C, PPAR-γ). Surprisingly, mitochondrial biomarkers (PGC-1α, CS) were also increased in the gastrocnemius, suggesting that chronic lactate might promote mitochondria biogenesis. Together, these results demonstrated that lactate may play a crucial role in triglyceride storage and mitochondria biogenesis in the skeletal muscle of rat.

Bioimpedance analysis of body composition in elderly and old men with polymorbid pathology

Aim to compare the parameters of bioimpedance analysis of body composition in elderly and old men with polymorbid pathology.&#x0D; Material and methods. The study involved 123 men aged from 65 to 84 years (mean age 73.7 5.9 years) with polymorbid pathology. The Group 1 included 65 elderly men; the Group 2 consisted of 58 old men.&#x0D; Results. The study revealed the decrease in strength, mass and muscle function parameters related to aging. The bioimpedance phase angle was lower in the group of old men than in the group of elderly men. The correlations of the phase angle with anthropometric and functional parameters, as well as with body composition were revealed.&#x0D; Conclusion. The decrease in muscle mass parameters and low phase angle may be associated with degenerative processes in skeletal muscles due to aging and polymorbidity. The bioimpedance analysis of body composition can be used in geriatric practice for the diagnosis of sarcopenia syndrome in elderly and old men.

STUDY OF BONE MARROW ASPIRATE INJECTION EFFECT ON DENERVATED MUSCLE ACCORDING TO ELECTROMYOGRAPHY STUDIES

The aim. To study in experiment the effect of bone marrow aspirate injection on the course of denervation-reinnervation processes in skeletal muscle using neurophysiological research method. Materials and methods. Experimental study was performed on 36 mature rabbits weighing 3-4 kg. Experimental model of the peripheral nerve acute injury and neurorrhaphy is taken as a basis. Animals were divided into 4 groups of 9 animals per group (3 animals for each experimental period). At 8, 12, and 16 weeks after surgery, all rabbits underwent needle EMG of the target muscles (m. gastrocnemius, and m. tibialis anterior) to study denervation-reinnervation changes. Results. Results of target muscles electromyographic examination are shown. As norm, we took motor unit potentials parameters, which were registered during the study of contralateral (not operated) pelvic limbs. Largest number of registered MUPs at different stages of the denervation-reinnervation process was observed, and was significantly higher (α = 0.07) in the groups in which bone marrow aspirate was injected in relation to group without bone marrow aspirate injection. Changes in the parameters of the registered MUPs in all groups corresponded to the general characteristics of the denervation-reinnervation process. Conclusions. Injection of bone marrow aspirate into the target muscles during surgery and in the early stages of reinnervation (in experimental study it is 7 weeks after surgery) – reliably (α = 0.07) promotes improvement of reinnervation processes in muscles, which is manifested by registration of more motor unit potentials.

The paradoxical lean phenotype of hypothyroid mice is marked by increased adaptive thermogenesis in the skeletal muscle

Obesity is a major health problem worldwide, given its growing incidence and its association with a variety of comorbidities. Weight gain results from an increase in energy intake without a concomitant increase in energy expenditure. To combat the obesity epidemic, many studies have focused on the pathways underlying satiety and hunger signaling, while other studies have concentrated on the mechanisms involved in energy expenditure, most notably adaptive thermogenesis. Hypothyroidism in humans is typically associated with a decreased basal metabolic rate, lower energy expenditure, and weight gain. However, hypothyroid mouse models have been reported to have a leaner phenotype than euthyroid controls. To elucidate the mechanism underlying this phenomenon, we used a drug-free mouse model of hypothyroidism: mice lacking the sodium/iodide symporter (NIS), the plasma membrane protein that mediates active iodide uptake in the thyroid. In addition to being leaner than euthyroid mice, owing in part to reduced food intake, these hypothyroid mice show signs of compensatory up-regulation of the skeletal-muscle adaptive thermogenic marker sarcolipin, with an associated increase in fatty acid oxidation (FAO). Neither catecholamines nor thyroid-stimulating hormone (TSH) are responsible for sarcolipin expression or FAO stimulation; rather, thyroid hormones are likely to negatively regulate both processes in skeletal muscle. Our findings indicate that hypothyroidism in mice results in a variety of metabolic changes, which collectively lead to a leaner phenotype. A deeper understanding of these changes may make it possible to develop new strategies against obesity.

Photobiomodulation modulates the expression of inflammatory cytokines during the compensatory hypertrophy process in skeletal muscle.

Compensatory hypertrophy (CH) occurs due to excessive mechanical load on a muscle, promoting an increase in the size of muscle fibers. In clinical practice, situations such as partial nerve injuries, denervation, and muscle imbalance caused by trauma to muscles and nerves or diseases that promote the loss of nerve conduction can induce CH in muscle fibers. Photobiomodulation (PBM) has demonstrated beneficial effects on muscle tissue during CH. The aim of the present study was to evaluate the effect of PBM on the inflammatory cytokines interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) as well as type 2 metalloproteinases (MMP-2) during the process of CH due to excessive load on the plantaris muscle in rats. Forty-five Wistar rats weighing 250g were divided into three groups: control group (n = 10), hypertrophy (H) group (n = 40), and H + PBM group (n = 40). CH was induced through the ablation of synergist muscles of the plantaris muscle. The tendons of the gastrocnemius and soleus muscles were isolated and sectioned to enable the partial removal of each of muscle. The preserved plantaris muscle below the removed muscles was submitted to excessive functional load. PBM was performed with low-level laser (AsGaAl, λ = 780nm; 40mW; energy density: 10J/cm2; 10s on each point, 8 points; 3.2J). Animals from each group were euthanized after 7 and 14days. The plantaris muscles were carefully removed and sent for analysis of the gene and protein expression of IL-6 and TNF-α using qPCR and ELISA, respectively. MMP-2 activity was analyzed using zymography. The results were submitted to statistical analysis (ANOVA + Tukey's test, p < 0.05). The protein expression analysis revealed an increase in IL-6 levels in the H + PBM group compared to the H group and a reduction in the H group compared to the control group. A reduction in TNF-α was found in the H and H + PBM groups compared to the control group at 7days. The gene expression analysis revealed an increase in IL-6 in the H + PBM group compared to the H group at 14days as well as an increase in TNF-α in the H + PBM group compared to the H group at 7days. Increases in MMP-2 were found in the H and H + PBM groups compared to the control group at both 7 and 14days. Based on findings in the present study, it is concluded that PBM was able to modulate pro-inflammatory cytokines that are essential for the compensatory hypertrophy process. However, it has not shown a modulation effect directly in MMP-2 activity during the same period evaluated.

Bioinformatics Analysis Of MicroRNA In Skeletal Muscle Of Mice Related To Aerobic Exercise

MicroRNAs (miRNAs) are increasingly being studied as regulators for biological processes in skeletal muscle. However, little information relating to biological process regulated by aerobic exercise-affected miRNA is available. Bioinformatics analysis provides a perspective on the direction of future research. PURPOSE: To analyze the functions of miRNAs which were affected by 8-week aerobic exercise in skeletal muscle of mice. METHODS: Twenty C57BL/6J mice were randomly divided into exercise (E) and control (C) groups. The E group were trained on a treadmill with a program of slope 0°, 12m/min, 60min/day, and 5 days/week. The C group lived normally without training. After 8-week of training, skeletal muscles were harvested for RNA extraction. Subsequently, miScript miRNA PCR Arrays was conducted to identify the differentially expressed miRNAs between two groups. Target gene prediction was performed by using TargetScan and microT-CDS tools. Gene Ontology analysis (GO) on the target gene was performed with Cytoscape Bingo. MirPath v.3 from the DIANA tools was used to execute the Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses. RESULTS: 44 miRNAs were upregulated (Fold Change≥2.0, P<0.05) and 14 miRNAs were downregulated (Fold Change<-2.0, P<0.05) after the aerobic exercise. Prediction of target genes showed that there were a total of 9122 target genes (Ptpn9, Gria3, Zfp219, etc.) for the up-regulated miRNAs, while 3636 target genes (Nrf1, Lmbr1l, Zbp1, etc.) for the down-regulated miRNAs. GO analysis revealed that the differentially expressed miRNAs participated in a variety of molecular functions and biological processes including nucleic acid binding, protein binding, regulation of gene expression, macromolecule metabolic process and others. KEGG analysis showed that the up-regulated miRNAs were significantly enriched in MAPK, Wnt and mTOR signaling pathway. The down-regulated miRNAs were mainly enriched in MAPK, Wnt signaling pathway. CONCLUSION: MiRNAs induced by aerobic exercise potentially regulate the biological processes of skeletal muscle mainly through MAPK and Wnt signaling pathways. Supported by Key R&D Projects of the Ministry of Science and Technology (grant no 2018YFC2000600), and Fundamental Research Funds for the Central Universities (grant no. 2016SYS002).

Early adjustments in mitochondrial structure and function in skeletal muscle to high altitude: design and rationale of the first study from the Kilimanjaro Biobank.

The physiological acclimatisation and adaptation processes in skeletal muscle at high altitude are of high medical and social relevance not only to understand limitations in physical performance at high altitude but also to understand the consequences of hypoxemia and tissue hypoxia in critically ill patients. Of particular importance in these processes are the alterations in content and function of mitochondria and myoglobin. The majority of studies on oxygen delivery to the tissues and utilisation by the cellular metabolism at high altitude were performed after prolonged stay at high altitude and in altitude-adapted highlanders. However, these studies do not provide insight in the sequence of events during the physiological acclimatisation and adaptation processes. Therefore, it is important to identify the early alterations in structure and function of the major determinants of the oxygen transport via myoglobin and oxygen utilisation by the mitochondria in skeletal muscle at high altitude. To achieve this goal, it is of interest to collect, analyse and compare quadriceps muscle biopsies and venous blood samples of climbers, guides and porters before and after climbing Mount Kilimanjaro and in participants of the Kilimanjaro Marathon before and after the run. The samples will be carefully documented and stored in the Kilimanjaro Biobank and will be made available to other research groups.

Gene therapy for overexpressing Neuregulin 1 type I in skeletal muscles promotes functional improvement in the SOD1G93A ALS mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motoneurons (MNs), with no effective treatment currently available. The molecular mechanisms that are involved in MN death are complex and not fully understood, with partial contributions of surrounding glial cells and skeletal muscle to the disease. Neuregulin 1 (NRG1) is a trophic factor highly expressed in MNs and neuromuscular junctions. Recent studies have suggested a crucial role of the isoform I (NRG1-I) in the collateral reinnervation process in skeletal muscle, and NRG1-III in the preservation of MNs in the spinal cord, opening a window for developing novel therapies for neuromuscular diseases like ALS. In this study, we overexpressed NRG1-I widely in the skeletal muscles of the SOD1G93A transgenic mouse. The results show that NRG1 gene therapy activated the survival pathways in muscle and spinal cord, increasing the number of surviving MNs and neuromuscular junctions and reducing the astroglial reactivity in the spinal cord of the treated SOD1G93A mice. Furthermore, NRG1-I overexpression preserved motor function and delayed the onset of clinical disease. In summary, our data indicates that NRG1 plays an important role on MN survival and muscle innervation in ALS, and that viral-mediated overexpression of NRG1 isoforms may be considered as a promising approach for ALS treatment.

Expression and functional analysis of the hydrogen peroxide biosensors HyPer and HyPer2 in C2C12 myoblasts/myotubes and single skeletal muscle fibres

Hydrogen peroxide (H2O2) is generated in cells and plays an important role as a signalling molecule. It has been reported that H2O2 is involved in physiological and pathological processes in skeletal muscle. However, H2O2 detection in cells with traditional techniques produces frequent artefacts. Currently, the HyPer biosensor detects intracellular H2O2 specifically in real time using fluorescence microscopy. The aim of this study was to develop and optimize approaches used to express the HyPer biosensor in different models of skeletal muscle cells, such as the C2C12 myoblast/myotube cell line and mature skeletal muscle fibres isolated from C57BL/6J mice, and to measure intracellular H2O2 in real time in these cells. The results show that the expression of the HyPer biosensor in skeletal muscle cells is possible. In addition, we demonstrate that HyPer is functional and that this biosensor detects changes and fluctuations in intracellular H2O2 in a reversible manner. The HyPer2 biosensor, which is a more advanced version of HyPer, presents improved properties in terms of sensitivity in detecting lower concentrations of H2O2 in skeletal muscle fibres. In conclusion, the expression of the HyPer biosensor in the different experimental models combined with fluorescence microscopy techniques is a powerful methodology to monitor and register intracellular H2O2 specifically in skeletal muscle. The innovation of the methodological approaches presented in this study may present new avenues for studying the role of H2O2 in skeletal muscle pathophysiology. Furthermore, the methodology may potentially be adapted to yield other specific biosensors for different reactive oxygen and nitrogen species or metabolites involved in cellular functions.

Refining genome-wide associated loci for serum uric acid in individuals with African ancestry.

Serum uric acid is the end-product of purine metabolism and at high levels is a risk factor for several human diseases including gout and cardiovascular disease. Heritability estimates range from 0.32 to 0.63. Genome-wide association studies (GWAS) provide an unbiased approach to identify loci influencing serum uric acid. Here, we performed the first GWAS for serum uric acid in continental Africans, with replication in African Americans. Africans (n = 4126) and African Americans (n = 5007) were genotyped on high-density GWAS arrays. Efficient mixed model association, a variance component approach, was used to perform association testing for a total of ~ 18 million autosomal genotyped and imputed variants. CAVIARBF was used to fine map significant regions. We identified two genome-wide significant loci: 4p16.1 (SLC2A9) and 11q13.1 (SLC22A12). At SLC2A9, the most strongly associated SNP was rs7683856 (P = 1.60 × 10-44). Conditional analysis revealed a second signal indexed by rs6838021 (P = 5.75 × 10-17). Gene expression and regulatory motif data prioritized a single-candidate causal variant for each signal. At SLC22A12, the most strongly associated SNP was rs147647315 (P = 6.65 × 10-25). Conditional analysis and functional annotation prioritized the missense variant rs147647315 (R (Arg) > H (His)) as the sole causal variant. Functional annotation of these three signals implicated processes in skeletal muscle, subcutaneous adipose tissue and the kidneys, respectively. This first GWAS of serum uric acid in continental Africans identified three associations at two loci, SLC2A9 and SLC22A12. The combination of weak linkage disequilibrium in Africans and functional annotation led to the identification of candidate causal SNPs for all three signals. Each candidate causal variant implicated a different cell type. Collectively, the three associations accounted for 4.3% of the variance of serum uric acid.

Comparative profiling of skeletal muscle models reveals heterogeneity of transcriptome and metabolism

Rat L6, mouse C2C12, and primary human skeletal muscle cells (HSMCs) are commonly used to study biological processes in skeletal muscle, and experimental data on these models are abundant. However, consistently matched experimental data are scarce, and comparisons between the different cell types and adult tissue are problematic. We hypothesized that metabolic differences between these cellular models may be reflected at the mRNA level. Publicly available data sets were used to profile mRNA levels in myotubes and skeletal muscle tissues. L6, C2C12, and HSMC myotubes were assessed for proliferation, glucose uptake, glycogen synthesis, mitochondrial activity, and substrate oxidation, as well as the response to in vitro contraction. Transcriptomic profiling revealed that mRNA of genes coding for actin and myosin was enriched in C2C12, whereas L6 myotubes had the highest levels of genes encoding glucose transporters and the five complexes of the mitochondrial electron transport chain. Consistently, insulin-stimulated glucose uptake and oxidative capacity were greatest in L6 myotubes. Insulin-induced glycogen synthesis was highest in HSMCs, but C2C12 myotubes had higher baseline glucose oxidation. All models responded to electrical pulse stimulation-induced glucose uptake and gene expression but in a slightly different manner. Our analysis reveals a great degree of heterogeneity in the transcriptomic and metabolic profiles of L6, C2C12, or primary human myotubes. Based on these distinct signatures, we provide recommendations for the appropriate use of these models depending on scientific hypotheses and biological relevance.

RNA-seq profiling, and impaired autophagic process in skeletal muscle of MELAS

Mitochondrial myopathy, Encephalopathy, Lactic Acidosis, and Stroke-like episodes (MELAS) is a common subtype of mitochondrial disease with high disability and mortality rate. The molecular mechanisms of MELAS are largely unknown and whether autophagy is activated in this disease remains controversial. In this work, we reported whole transcriptome profiling of skeletal muscle of MELAS patients and age-matched controls. Analyses revealed that MELAS patients had 224 differentially expressed genes (174 down-regulated, 50 up-regulated) compared to age-matched controls. Most of these genes relevant to MELAS are involved in signal transduction, metabolic process and immune system process. However, the RNA-seq data indicated that autophagy was not altered in MELAS. Functional assays showed that increased reactive oxygen species (ROS), decreased ATP production and decreased lysosome content in fibroblasts derived from MELAS patients, suggesting that mitochondrial dysfunction and degradation deficiency in MELAS. Furthermore, Western-blot analyses using skeletal muscle and fibroblasts derived from MELAS patients showed that autophagy was impaired in MEALS since two important autophagic genes: Beclin-1 and LC3-II, were significantly down-regulated. In conclusion, our study identified molecules and pathways involved in the mechanisms of MELAS, and the impairment of autophagy in this disease, which may serve as the potential therapeutic target for MELAS.

Presence of metalloproteinases 2 and 9 and 8-OHdG in the fibrotic process in skeletal muscle of Mdx mice

Inflammation and oxidative stress occurs in muscle of Duchenne muscular dystrophy (DMD). The relationship between a panel of biomarkers and the DMD outcome is necessary to indicate of disease progression and response to rehabilitation programs. The aim was to analyze the connective tissue of muscle of Mdx mice and immunoexpression of MMP-2, MMP-9, and 8-OHdG, which signalizes oxidative stress related to DNA damage. Biceps brachii of male C57BL/10 and C57BL/10-Dmdmdx mice was submitted to Hematoxylin-Eosin, Sirius red and immunohistochemistry (MMP-2, MMP-9 and 8-OHdG) analysis. Mdx showed focal lesions with intense inflammation and fibrosis related to immunoexpression of MMP-2 and MMP-9, proving the hypothesis that these MMPs are linked to muscular tissue degeneration, which can be regenerated by their inhibition, improving the treatment of DMD carriers. Histopathological findings related to centralized nuclei increase were related to higher 8-OHdG immunomarked nuclei in Mdx, which signalizes oxidative stress associated with DNA damage provoked by DMD. Such result shows that the evaluation of 8-OHdG during the evolution of the disease could be a method to evaluate DMD disease progression.

Mechanosensitive pathways controlling translation regulatory processes in skeletal muscle and implications for adaptation.

The ability of myofibers to sense and respond appropriately to mechanical signals is one of the primary determinants of the skeletal muscle phenotype. In response to a change in mechanical load, muscle cells alter their protein metabolism, primarily through the regulation of protein synthesis rate. Protein synthesis rates are determined by both translation efficiency and translational capacity within the muscle. Translational capacity is strongly determined by the ribosome content of the muscle; thus the regulation of ribosomal biogenesis by mechanical inputs has been an area of recent interest. Despite the clear association between mechanical signals and changes in protein metabolism, the molecular pathways that link these events are still not fully elucidated. This review focuses on recent studies looking at how mechanosignaling impacts translational events. The role of impaired mechanotransduction in aging is discussed, as is the connection between age-dependent signaling defects and compromised ribosomal biogenesis during mechanical overload. Finally, emerging evidence suggests that the nucleus can act as a mechanosensitive element and that this mode of mechanotransduction may have an important role in skeletal muscle physiology and adaptation.

Isoproterenol induces an increase in muscle fiber size by the proliferation of Pax7-positive cells and in a mTOR-independent mechanism.

β‐Adrenergic signaling regulates many physiological processes in skeletal muscles. A wealth of evidence has shown that β‐agonists can increase skeletal muscle mass in vertebrates. Nevertheless, to date, the specific role of β‐adrenergic receptors in different cell phenotypes (myoblasts, fibroblasts, and myotubes) and during the different steps of embryonic skeletal muscle differentiation has not been studied. Therefore, here we address this question through the analysis of embryonic chick primary cultures of skeletal muscle cells during the formation of multinucleated myotubes. We used isoproterenol (ISO), a β‐adrenergic receptor agonist, to activate the β‐adrenergic signaling and quantified several aspects of muscle differentiation. ISO induced an increase in myoblast proliferation, in the percentage of Pax7‐positive myoblasts and in the size of skeletal muscle fibers, suggesting that ISO activates a hyperplasic and hypertrophic muscle response. Interestingly, treatment with ISO did not alter the number of fibroblast cells, suggesting that ISO effects are specific to muscle cells in the case of chick myogenic cell culture. We also show that rapamycin, an inhibitor of the mammalian target of rapamycin signaling pathway, did not prevent the effects of ISO on chick muscle fiber size. The collection of these results provides new insights into the role of β‐adrenergic signaling during skeletal muscle proliferation and differentiation and specifically in the regulation of skeletal muscle hyperplasia and hypertrophy.