bims-moremu Biomed News
on Molecular regulators of muscle mass
Issue of 2021‒04‒18
forty-four papers selected by
Anna Vainshtein
Craft Science Inc.

  1. J Endocrinol. 2021 May;pii: JOE-20-0233. [Epub ahead of print]249(2): 113-124
      Vitamin D deficiency is associated with symptoms of skeletal muscle myopathy including muscle weakness and fatigue. Recently, vitamin D-related metabolites have been linked to the maintenance of mitochondrial function within skeletal muscle. However, current evidence is limited to in vitro models and the effects of diet-induced vitamin D deficiency upon skeletal muscle mitochondrial function in vivo have received little attention. In order to examine the role of vitamin D in the maintenance of mitochondrial function in vivo, we utilised an established model of diet-induced vitamin D deficiency in C57BL/6J mice. Mice were either fed a control diet (2200 IU/kg i.e. vitamin D replete) or a vitamin D-deplete (0 IU/kg) diet for periods of 1, 2 and 3 months. Gastrocnemius muscle mitochondrial function and ADP sensitivity were assessed via high-resolution respirometry and mitochondrial protein content via immunoblotting. As a result of 3 months of diet-induced vitamin D deficiency, respiration supported via complex I + II (CI + IIP) and the electron transport chain (ETC) were 35 and 37% lower when compared to vitamin D-replete mice (P < 0.05). Despite functional alterations, citrate synthase activity, AMPK phosphorylation, mitofilin, OPA1 and ETC subunit protein content remained unchanged in response to dietary intervention (P > 0.05). In conclusion, we report that 3 months of diet-induced vitamin D deficiency reduced skeletal muscle mitochondrial respiration in C57BL/6J mice. Our data, when combined with previous in vitro observations, suggest that vitamin D-mediated regulation of mitochondrial function may underlie the exacerbated muscle fatigue and performance deficits observed during vitamin D deficiency.
    Keywords:  mitochondria; skeletal muscle; vitamin D
  2. Curr Opin Pharmacol. 2021 Apr 08. pii: S1471-4892(21)00023-0. [Epub ahead of print]58 1-7
      Maintaining muscle mass is clinically important as muscle helps to regulate metabolic systems of the body as well as support activities of daily living that require mobility, strength, and power. Losing muscle mass decreases an individual's independence and quality of life, and at the same time increases the risk of disease burden. Fibro-adipogenic progenitor (FAP) cells are a group of muscle progenitor cells that play an important role in muscle regeneration and maintenance of skeletal muscle fiber size. These important functions of FAPs are mediated by a complex secretome that interacts in a paracrine manner to stimulate muscle satellite cells to divide and differentiate. Dysregulation of FAP differentiation leads to fibrosis, fatty infiltration, muscle atrophy, and impaired muscle regeneration. Functional deficits in skeletal muscle resulting from atrophy, fibrosis, or fatty infiltration will reduce biomechanical stresses on the skeleton, and both FAP-derived adipocytes and FAPs themselves are likely to secrete factors that can induce bone loss. These findings suggest that FAPs represent a cell population to be targeted therapeutically to improve both muscle and bone health in settings of aging, injury, and disease.
    Keywords:  Adipogenesis; Bone loss; Fibrosis; Muscular dystrophies; Satellite cells
  3. Biochem Biophys Res Commun. 2021 Apr 13. pii: S0006-291X(21)00599-4. [Epub ahead of print]557 33-39
      Cell death and differentiation are closely related at the molecular level. Differentiation of skeletal muscle cells attenuates susceptibility to apoptosis. Necroptosis has recently been recognized as a form of regulated cell death but its role in myogenesis has not been studied. This study aimed to compare the sensitivity to TNF-induced necroptosis in skeletal muscle at the undifferentiated (myoblasts) and differentiated (myotubes) stages. Surprisingly, our results showed that TNF-induced necroptosis was blunted during myoblast differentiation. Moreover, our data revealed that the key molecules involved in necroptosis, including receptor-interacting serine/threonine protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL), were significantly down-regulated during myogenic differentiation, resulting in suppression of necroptosis signal transduction in differentiated myotubes. In addition, RIPK1, RIPK3, and MLKL expression levels were significantly lower in the skeletal muscle of adult mice than in newborn mice, suggesting that the susceptibility to necroptosis might be attenuated in differentiated muscle tissue. In conclusion, this study revealed that expression of key molecules involved in necroptosis is down-regulated during muscle differentiation, which results in the differentiation of muscles becoming insensitive to necroptotic cell death.
    Keywords:  C2C12; Muscle; Myoblast differentiation; Myogenesis; Necroptosis; Programmed cell death
  4. Redox Biol. 2021 Apr 05. pii: S2213-2317(21)00114-2. [Epub ahead of print]43 101966
      Skeletal muscle health is important for the prevention of various age-related diseases. The loss of skeletal muscle mass, which is known as sarcopenia, underlies physical disability, poor quality of life and chronic diseases in elderly people. The transcription factor NRF2 plays important roles in the regulation of the cellular defense against oxidative stress, as well as the metabolism and mitochondrial activity. To determine the contribution of skeletal muscle NRF2 to exercise capacity, we conducted skeletal muscle-specific inhibition of KEAP1, which is a negative regulator of NRF2, and examined the cell-autonomous and non-cell-autonomous effects of NRF2 pathway activation in skeletal muscles. We found that NRF2 activation in skeletal muscles increased slow oxidative muscle fiber type and improved exercise endurance capacity in female mice. We also observed that female mice with NRF2 pathway activation in their skeletal muscles exhibited enhanced exercise-induced mobilization and β-oxidation of fatty acids. These results indicate that NRF2 activation in skeletal muscles promotes communication with adipose tissues via humoral and/or neuronal signaling and facilitates the utilization of fatty acids as an energy source, resulting in increased mitochondrial activity and efficient energy production during exercise, which leads to improved exercise endurance.
    Keywords:  Beta-oxidation; Exercise; Fatty acid; KEAP1-NRF2 system; Skeletal muscle
  5. Arch Biochem Biophys. 2021 Apr 10. pii: S0003-9861(21)00123-5. [Epub ahead of print] 108873
      Glucocorticoids are the drugs most commonly used to manage inflammatory diseases. However, they are prone to inducing muscle atrophy by increasing muscle proteolysis and decreasing protein synthesis. Various studies have demonstrated that antioxidants can mitigate glucocorticoid-induced skeletal muscle atrophy. Here, we investigated the effect of a potent antioxidative natural flavonoid, morin, on the muscle atrophy and oxidative stress induced by dexamethasone (Dex) using mouse C2C12 skeletal myotubes. Dex (10 μM) enhanced the production of reactive oxygen species (ROS) in C2C12 myotubes via glucocorticoid receptor. Moreover, Dex administration reduced the diameter and expression levels of the myosin heavy chain protein in C2C12 myotubes, together with the upregulation of muscle atrophy-associated ubiquitin ligases, such as muscle atrophy F-box protein 1/atrogin-1, muscle ring finger protein-1, and casitas B-lineage lymphoma proto-oncogene-b. Dex also significantly decreased phosphorylated Foxo3a and increased total Foxo3a expression. Interestingly, Dex-induced ROS accumulation and Foxo3a expression were inhibited by morin (10 μM) pretreatment. Morin also prevented the Dex-induced reduction of myotube thickness, together with muscle protein degradation and suppression of the upregulation of atrophy-associated ubiquitin ligases. In conclusion, our results suggest that morin effectively prevents glucocorticoid-induced muscle atrophy by reducing oxidative stress.
    Keywords:  Morin; Mouse C2C12 skeletal myotubes; Muscle atrophy; Reactive oxygen species; Ubiquitin ligases
  6. EMBO J. 2021 Apr 13. e106491
      Exercise can alter the skeletal muscle DNA methylome, yet little is known about the role of the DNA methylation machinery in exercise capacity. Here, we show that DNMT3A expression in oxidative red muscle increases greatly following a bout of endurance exercise. Muscle-specific Dnmt3a knockout mice have reduced tolerance to endurance exercise, accompanied by reduction in oxidative capacity and mitochondrial respiration. Moreover, Dnmt3a-deficient muscle overproduces reactive oxygen species (ROS), the major contributors to muscle dysfunction. Mechanistically, we show that DNMT3A suppresses the Aldh1l1 transcription by binding to its promoter region, altering its epigenetic profile. Forced expression of ALDH1L1 elevates NADPH levels, which results in overproduction of ROS by the action of NADPH oxidase complex, ultimately resulting in mitochondrial defects in myotubes. Thus, inhibition of ALDH1L1 pathway can rescue oxidative stress and mitochondrial dysfunction from Dnmt3a deficiency in myotubes. Finally, we show that in vivo knockdown of Aldh1l1 largely rescues exercise intolerance in Dnmt3a-deficient mice. Together, we establish that DNMT3A in skeletal muscle plays a pivotal role in endurance exercise by controlling intracellular oxidative stress.
    Keywords:  DNA methylation; exercise; oxidative stress
  7. Hum Mol Genet. 2021 Apr 16. pii: ddab108. [Epub ahead of print]
      RNA toxicity underlies the pathogenesis of disorders such as myotonic dystrophy (DM1). Muscular dystrophy is a key element of the pathology of DM1. The means by which RNA toxicity causes muscular dystrophy in DM1 is unclear. Here we have used the DM200 mouse model of RNA toxicity due to the expression of a mutant DMPK 3'UTR mRNA to model the effects of RNA toxicity on muscle regeneration. Using a BaCl2 induced damage model, we find that RNA toxicity leads to decreased expression of PAX7, and decreased numbers of satellite cells, the stem cells of adult skeletal muscle (also known as MuSCs). This is associated with a delay in regenerative response, a lack of muscle fiber maturation, and an inability to maintain a normal number of satellite cells. Repeated muscle damage also elicited key aspects of muscular dystrophy including fat droplet deposition and increased fibrosis and the results represent one of the first times to model these classic markers of dystrophic changes in the skeletal muscles of a mouse model of RNA toxicity. Using a ligand conjugated antisense oligonucleotide (LICA) ASO targeting DMPK sequences for the first time in a mouse model of RNA toxicity in DM1, we find that treatment with IONIS 877864, which targets the DMPK 3'UTR mRNA, is efficacious in correcting the defects in regenerative response and the reductions in satellite cell numbers caused by RNA toxicity. These results demonstrate the possibilities for therapeutic interventions to mitigate the muscular dystrophy associated with RNA toxicity in DM1.
  8. Curr Opin Pharmacol. 2021 Apr 11. pii: S1471-4892(21)00025-4. [Epub ahead of print]58 35-43
      Musculoskeletal conditions are the highest contributor to global disability, accounting for 16% of all ages lived with disability. Volumetric muscle loss (VML) is classified as significant damage to skeletal muscle compartments and motor units, leading to significant tissue loss, functional deficits, and long-term disability. In this review, the current tissue engineering approaches in terms of fabrication techniques, materials, cell sources, and growth factors for enhanced angiogenesis and neuromuscular junction (NMJ) in VML repair, are discussed. Review of results recently published in the literature suggested that bioprinted nanocomposite hydrogels (NC gels) seeded with adult muscle progenitor cells that promote secretion of endogenous vascular growth factors have potential applications in promoting skeletal muscle regeneration, revascularization, and NMJ repair (Figure 1). Despite recent advancements, future research is needed on NC gels and the complex processes underlying vascular infiltration and NMJ repair in VML injuries.
  9. Mol Ther Methods Clin Dev. 2021 Jun 11. 21 144-160
      We tested the hypothesis that voluntary wheel running would complement microdystrophin gene therapy to improve muscle function in young mdx mice, a model of Duchenne muscular dystrophy. mdx mice injected with a single dose of AAV9-CK8-microdystrophin or vehicle at age 7 weeks were assigned to three groups: mdxRGT (run, gene therapy), mdxGT (no run, gene therapy), or mdx (no run, no gene therapy). Wild-type (WT) mice were assigned to WTR (run) and WT (no run) groups. WTR and mdxRGT performed voluntary wheel running for 21 weeks; remaining groups were cage active. Robust expression of microdystrophin occurred in heart and limb muscles of treated mice. mdxRGT versus mdxGT mice showed increased microdystrophin in quadriceps but decreased levels in diaphragm. mdx final treadmill fatigue time was depressed compared to all groups, improved in mdxGT, and highest in mdxRGT. Both weekly running distance (km) and final treadmill fatigue time for mdxRGT and WTR were similar. Remarkably, mdxRGT diaphragm power was only rescued to 60% of WT, suggesting a negative impact of running. However, potential changes in fiber type distribution in mdxRGT diaphragms could indicate an adaptation to trade power for endurance. Post-treatment in vivo maximal plantar flexor torque relative to baseline values was greater for mdxGT and mdxRGT versus all other groups. Mitochondrial respiration rates from red quadriceps fibers were significantly improved in mdxGT animals, but the greatest bioenergetic benefit was observed in the mdxRGT group. Additional assessments revealed partial to full functional restoration in mdxGT and mdxRGT muscles relative to WT. These data demonstrate that voluntary wheel running combined with microdystrophin gene therapy in young mdx mice improved whole-body performance, affected muscle function differentially, mitigated energetic deficits, but also revealed some detrimental effects of exercise. With microdystrophin gene therapy currently in clinical trials, these data may help us understand the potential impact of exercise in treated patients.
    Keywords:  duchenne muscular dystrophy; durability; dystrophic grade; endurance; mitochondrial respiration; muscle pathology; muscle physiology; muscle power; myosin heavy chain; voluntary exercise
  10. Cell Death Discov. 2021 Apr 12. 7(1): 74
      Skeletal muscle denervation occurs in diverse conditions and causes severe muscle atrophy. Signaling by mammalian target of rapamycin complex 1 (mTORC1) plays a central role in the maintenance of skeletal muscle mass by regulating net protein balance; yet, its role in denervation-induced atrophy is unclear. In this study, by using skeletal muscle-specific and inducible raptor knockout mice, we demonstrate that signaling through mTORC1 is activated during denervation and plays an essential role in mitigating the atrophy of non-type IIB muscle fibers. Measurements of protein synthesis rates of individual fibers suggest that denervation increases protein synthesis specifically in non-type IIB muscle fibers and that mTORC1 is required for this event. Furthermore, denervation induced a more pronounced increase in the level of phosphorylated ribosomal S6 protein in non-type IIB muscle fibers than in type IIB muscle fibers. Collectively, our results unveil a novel role for mTORC1 in mediating a fiber type-specific regulation of muscle size and protein synthesis during denervation.
  11. JCI Insight. 2021 Apr 13. pii: 145994. [Epub ahead of print]
      Skeletal muscle can regenerate from muscle stem cells and their myogenic precursor cell progeny, myoblasts. However, precise gene editing in human muscle stem cells for autologous cell replacement therapies of untreatable genetic muscle diseases has not yet been reported. Loss-of-function mutations in SGCA, encoding α-sarcoglycan, cause limb-girdle muscular dystrophy 2D/R3, an early onset, severe and rapidly progressive form of muscular dystrophy affecting equally girls and boys. Patients suffer from muscle degeneration and atrophy affecting the limbs, respiratory muscles, and the heart. We isolated human muscle stem cells from two donors with the common SGCA c.157G>A mutation affecting the last coding nucleotide of exon 2. We found that c.157G>A is an exonic splicing mutation that induces skipping of two co-regulated exons. Using adenine base editing, we corrected the mutation in the cells from both donors with >90% efficiency, thereby rescuing the splicing defect and α-sarcoglycan expression. Base edited patient cells regenerated muscle and contributed to the Pax7 positive satellite cell compartment in vivo in mouse xenografts. We hereby provide the first evidence that autologous gene repaired human muscle stem cells can be harnessed for cell replacement therapies of muscular dystrophies.
    Keywords:  Human stem cells; Monogenic diseases; Skeletal muscle; Stem cells; Therapeutics
  12. J Nutr. 2021 Apr 13. pii: nxab055. [Epub ahead of print]
      BACKGROUND: There is much debate regarding the source/quality of dietary proteins in supporting indices of skeletal muscle anabolism.OBJECTIVE: We performed a systematic review and meta-analysis to determine the effect of protein source/quality on acute muscle protein synthesis (MPS) and changes in lean body mass (LBM) and strength, when combined with resistance exercise (RE).
    METHODS: A systematic search of the literature was conducted to identify studies that compared the effects of ≥2 dose-matched, predominantly isolated protein sources of varying "quality." Three separate models were employed as follows: 1) protein feeding alone on MPS, 2) protein feeding combined with a bout of RE on MPS, and 3) protein feeding combined with longer-term resistance exercise training (RET) on LBM and strength. Further subgroup analyses were performed to compare the effects of protein source/quality between young and older adults. A total of 27 studies in young (18-35 y) and older (≥60 y) adults were included.
    RESULTS: Analysis revealed an effect favoring higher-quality protein for postprandial MPS at rest [mean difference (MD): 0.014%/h; 95% CI: 0.006, 0.021; P < 0.001] and following RE (MD: 0.022%/h; 95% CI: 0.014, 0.030; P < 0.00001) in young (model 1: 0.016%/h; 95% CI: -0.004, 0.036; P = 0.12; model 2: 0.030%/h; 95% CI: 0.015, 0.045; P < 0.0001) and older (model 1: 0.012%/h; 95% CI: 0.006, 0.018; P < 0.001; model 2: 0.014%/h; 95% CI: 0.007, 0.021; P < 0.001) adults. However, although higher protein quality was associated with superior strength gains with RET [standardized mean difference (SMD): 0.24 kg; 95% CI: 0.02, 0.45; P = 0.03)], no effect was observed on changes to LBM (SMD: 0.05 kg; 95% CI: -0.16, 0.25; P = 0.65).
    CONCLUSIONS: The current review suggests that protein quality may provide a small but significant impact on indices of muscle protein anabolism in young and older adults. However, further research is warranted to elucidate the importance of protein source/quality on musculoskeletal aging, particularly in situations of low protein intake.
    Keywords:  aging; lean body mass; muscle protein synthesis; protein; resistance exercise training; sarcopenia; strength
  13. Histol Histopathol. 2021 Apr 12. 18337
      Maternal protein restriction affects postnatal skeletal muscle physiology with impacts that last through senility. To investigate the morphological and molecular characteristics of skeletal muscle in aging rats subjected to maternal protein restriction, we used aged male rats (540 days old) born of dams fed a protein restricted diet (6% protein) during pregnancy and lactation. Using morphological, immunohistochemical and molecular analyses, we evaluated the soleus (SOL) and extensor digitorum longus (EDL) muscles, muscle fiber cross-sectional area (CSA) (n=8), muscle fiber frequency (n=5) and the gene expression (n=8) of the oxidative markers (succinate dehydrogenase-Sdha and citrate synthase-CS) and the glycolytic marker (lactate dehydrogenase-Ldha). Global transcriptome analysis (n=3) was also performed to identify differentially regulated genes, followed by gene expression validation (n=8). The oxidative SOL muscle displayed a decrease in muscle fiber CSA (*p<0.05) and in the expression of oxidative metabolism marker Sdha (***p<0.001), upregulation of the anabolic Igf-1 (**p<0.01), structural Chad (**p<0.01), and Fmod (*p<0.05) genes, and downregulation of the Hspb7 (**p<0.01) gene. The glycolytic EDL muscle exhibited decreased IIA (*p<0.05) and increased IIB (*p<0.05) fiber frequency, and no changes in muscle fiber CSA or in the expression of oxidative metabolism genes. In contrast, the gene expression of Chad (**p<0.01) was upregulated and the Myog (**p<0.01) gene was downregulated. Collectively, our morphological, immunohistochemical and molecular analyses showed that maternal protein restriction induced changes in the expression of metabolic, anabolic, myogenic, and structural genes, mainly in the oxidative SOL muscle, in aged offspring rats.
  14. Exp Gerontol. 2021 Apr 10. pii: S0531-5565(21)00105-4. [Epub ahead of print]150 111330
      BACKGROUND: Sarcopenia, a multi-faceted skeletal muscle disorder in the older population, has poor health outcomes. Some previous observational studies investigated the association between circulating inflammatory markers and sarcopenia components to evaluate chronic inflammation as a risk factor for sarcopenia in the elderly population. Nevertheless, the association between circulating C-reactive protein (CRP) and hs-CRP, as the recognized markers of systemic inflammation and components of sarcopenia, is unclear. This meta-analysis aimed to investigate the association of muscle strength, muscle mass, and muscle function with two serum inflammatory markers, circulating C-reactive protein (CRP) and high-sensitive CRP (hs-CRP).METHODS: We assessed all observational studies across different electronic databases including PubMed, Scopus, and Google Scholar using keywords such as "muscle strength", "muscle mass", "muscle function", CRP and hs-CRP from inception until the 30th of July 2019. Only studies that investigated the association between components of sarcopenia and CRP or hs-CRP levels were included. Participants' country, age, sex, BMI, and screening tool for sarcopenia were retrieved. The correlations between muscle strength, muscle mass, and muscle function with CRP, and hs-CRP were expressed as the correlation coefficient (r) with 95% confidence intervals (CIs). Begg's test and Egger's test were conducted to evaluate risk of publication bias in this study.
    RESULTS: Initially, we found fifty-nine studies for the qualitative synthesis. Ultimately, nineteen adult cross-sectional studies comprising 14,650 subjects were included in the meta-analysis. Of them, fourteen studies measured the correlation between CRP or hs-CRP and muscle strength. There were significant inverse correlation between CRP and hs-CRP concentrations with muscle strength (ES (z) = -0.22; 95% CI = -0.34 to -0.09; P < 0.001), (ES (z) = -0.22; 95% CI = -0.34 to -0.09; P < 0.001), respectively. No publication bias was found between muscle strength and CRP (P = 0.53) or hs-CRP (P = 0.62) respectively.
    CONCLUSION: Among diagnostic components of sarcopenia, impairment of muscle strength was independently associated with both inflammatory biomarkers. However, future cohort studies are essential to clarify the causal correlation.
    Keywords:  C-reactive protein; High-sensitive CRP; Meta-analysis; Muscle mass; Muscle strength; Sarcopenia
  15. Cell Rep. 2021 Apr 13. pii: S2211-1247(21)00311-9. [Epub ahead of print]35(2): 108997
      Despite the well-accepted view that chronic inflammation contributes to the pathogenesis of Duchenne muscular dystrophy (DMD), the function and regulation of eosinophils remain an unclear facet of type II innate immunity in dystrophic muscle. We report the observation that group 2 innate lymphoid cells (ILC2s) are present in skeletal muscle and are the principal regulators of muscle eosinophils during muscular dystrophy. Eosinophils were elevated in DMD patients and dystrophic mice along with interleukin (IL)-5, a major eosinophil survival factor that was predominantly expressed by muscle ILC2s. We also find that IL-33 was upregulated in dystrophic muscle and was predominantly produced by fibrogenic/adipogenic progenitors (FAPs). Exogenous IL-33 and IL-2 complex (IL-2c) expanded muscle ILC2s and eosinophils, decreased the cross-sectional area (CSA) of regenerating myofibers, and increased the expression of genes associated with muscle fibrosis. The deletion of ILC2s in dystrophic mice mitigated muscle eosinophilia and impaired the induction of IL-5 and fibrosis-associated genes. Our findings highlight a FAP/ILC2/eosinophil axis that promotes type II innate immunity, which influences the balance between regenerative and fibrotic responses during muscular dystrophy.
    Keywords:  ILC2; ST2; chemokines; eosinophils; fibro/adipogenic progenitors; interleukin-33; interleukin-5; muscle inflammation; muscular dystrophy; type II innate immunity
  16. JCSM Rapid Commun. 2021 Jan-Jun;4(1):4(1): 24-39
      Background: Loss of skeletal muscle volume and resulting in functional limitations are poor prognostic markers in breast cancer patients. Several molecular defects in skeletal muscle including reduced MyoD levels and increased protein turn over due to enhanced proteosomal activity have been suggested as causes of skeletal muscle loss in cancer patients. However, it is unknown whether molecular defects in skeletal muscle are dependent on tumor etiology.Methods: We characterized functional and molecular defects of skeletal muscle in MMTV-Neu (Neu+) mice (n= 6-12), an animal model that represents HER2+ human breast cancer, and compared the results with well-characterized luminal B breast cancer model MMTV-PyMT (PyMT+). Functional studies such as grip strength, rotarod performance, and ex vivo muscle contraction were performed to measure the effects of cancer on skeletal muscle. Expression of muscle-enriched genes and microRNAs as well as circulating cytokines/chemokines were measured. Since NF-κB pathway plays a significant role in skeletal muscle defects, the ability of NF-κB inhibitor dimethylaminoparthenolide (DMAPT) to reverse skeletal muscle defects was examined.
    Results: Neu+ mice showed skeletal muscle defects similar to accelerated aging. Compared to age and sex-matched wild type mice, Neu+ tumor-bearing mice had lower grip strength (202±6.9 vs. 179±6.8 g grip force, p=0.0069) and impaired rotarod performance (108±12.1 vs. 30±3.9 seconds, P<0.0001), which was consistent with reduced muscle contractibility (p<0.0001). Skeletal muscle of Neu+ mice (n=6) contained lower levels of CD82+ (16.2±2.9 vs 9.0±1.6) and CD54+ (3.8±0.5 vs 2.4±0.4) muscle stem and progenitor cells (p<0.05), suggesting impaired capacity of muscle regeneration, which was accompanied by decreased MyoD, p53 and miR-486 expression in muscles (p<0.05). Unlike PyMT+ mice, which showed skeletal muscle mitochondrial defects including reduced mitochondria levels and Pgc1β, Neu+ mice displayed accelerated aging-associated changes including muscle fiber shrinkage and increased extracellular matrix deposition. Circulating "aging factor" and cachexia and fibromyalgia-associated chemokine Ccl11 was elevated in Neu+ mice (1439.56±514 vs. 1950±345 pg/ml, p<0.05). Treatment of Neu+ mice with DMAPT significantly restored grip strength (205±6 g force), rotarod performance (74±8.5 seconds), reversed molecular alterations associated with skeletal muscle aging, reduced circulating Ccl11 (1083.26 ±478 pg/ml), and improved animal survival.
    Conclusions: These results suggest that breast cancer subtype has a specific impact on the type of molecular and structure changes in skeletal muscle, which needs to be taken into consideration while designing therapies to reduce breast cancer-induced skeletal muscle loss and functional limitations.
    Keywords:  NF-κB; breast cancer; cytokines/chemokines; functional limitations; skeletal muscle
  17. Exp Gerontol. 2021 Apr 13. pii: S0531-5565(21)00123-6. [Epub ahead of print] 111348
      Natural adult aging is associated with declines in skeletal muscle performance, including impaired Ca2+ sensitivity and a slowing of rapid force production (rate of force redevelopment; ktr). The purpose of this study was to investigate the relationship between impaired Ca2+ sensitivity and ktr of single muscle fibres from young and older adults. Participants included 8 young (22-35 yrs) and 8 older (60-81 yrs) males who were living independently. A percutaneous muscle biopsy of the vastus lateralis of each participant was performed. Single muscle fibre mechanical tests included maximal Ca2+-activated force (Po), force-pCa curves, and ktr. We showed a decrease in pCa50 in old type II fibres compared to young, indicating impaired Ca2+ sensitivity in older adults. The ktr behaved in a Ca2+-dependent manner such that with increasing [Ca2+], ktr increases, to a plateau. Interestingly, ktr was not different between young and old muscle fibres. Furthermore, we found strong associations between pCa50 and ktr in both old type I and type II fibres, such that those fibres with lower Ca2+ sensitivity had a slowed ktr. This Ca2+ association, combined with impaired Ca2+-handling in older adults suggests a potential Ca2+-dependent mechanism affecting the transition from weakly- to strongly-bound cross-bridge states, leading to a decline in skeletal muscle performance. Future research is needed to explore the role alterations to Ca2+ sensitivity/handling could be playing in age-related whole muscle performance declines.
    Keywords:  Aging; Calcium sensitivity; Cross-bridge; Muscle contractility; Myosin; Rate of force development
  18. J Neuromuscul Dis. 2021 Apr 02.
      BACKGROUND: The mdx-C57/B6 mouse model does not show the clinical signs of Duchenne muscular dystrophy (DMD), although muscles exhibit hallmarks of permanent regeneration and alterations in muscle function. The DMDmdx4Cv strain exhibits very few revertant dystrophin positive myofibers, making that model suitable for studies on gene and cell therapies.OBJECTIVE: The study appraises the histological evolution of the Tibialis Anterior muscle of WT and DMD mdx4Cv mutant from 1 to 24 months.
    METHODS: Histological analysis included a series of immunostainings of muscle sections for assessing tissue features (fibrosis, lipid deposition, necrosis) and cellular characteristics (size of myofibers, number and distribution of myonuclei, number of satellite cells, vessels, macrophages).
    RESULTS: None of the investigated cell types (satellite cells, endothelial cells, macrophages) showed variations in their density within the tissue in both WT and DMD mdx4Cv muscle. However, analyzing their number per myofiber showed that in DMD mdx4Cv, myofiber capillarization was increased from 1 to 6 months as compared with WT muscle, then dropped from 12 months. Macrophage number did not vary in WT muscle and peaked at 6 months in DMD mdx4Cv muscle. The number of satellite cells per myofiber did not vary in WT muscle while it remained high in DMD mdx4Cv muscle, starting to decrease from 12 months and being significantly lower at 24 months of age. Myofiber size was not different in DMD mdx4Cv from WT except at 24 months, when it strongly decreased in DMD mdx4Cv muscle. Necrosis and lipid deposition were rare in DMD mdx4Cv muscle. Fibrosis did not increase with age in DMD mdx4Cv muscle and was higher than in WT at 6 and 12 months of age.
    CONCLUSIONS: As a whole, the results show a strong decrease of the myofiber size at 24 months, and an increased capillarization until 6 months of age in DMD mdx4Cv as compared with the WT. Thus, DMD mdx4Cv mice poorly recapitulates histological DMD features, and its use should take into account the age of the animals according to the purpose of the investigation.
    Keywords:  aging; duchenne muscular dystrophy; histology; mdx
  19. Front Physiol. 2021 ;12 638983
      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.
    Keywords:  AMPK; autophagy; mTOR; muscle cell homeostasis; skeletal muscle diseases; transcriptional regulation
  20. Am J Physiol Endocrinol Metab. 2021 Apr 12.
      Ketogenic diets (KD) are reported to improve body weight, fat mass, and exercise performance in humans. Unfortunately, most rodent studies have used a low-protein KD, which does not recapitulate diets used by humans. Since skeletal muscle plays a critical role in responding to macronutrient perturbations induced by diet and exercise, the purpose of this study was to test if a normal-protein KD (NPKD) impacts shifts in skeletal muscle substrate oxidative capacity in response to exercise training (ExTr). A high fat, carbohydrate-deficient NPKD (16.1% protein, 83.9% fat, 0% carbohydrate) was given to C57BL/6J male mice for 6 weeks, while controls received a low fat diet with similar protein (15.9% protein, 11.9% fat, 72.2% carbohydrate). On week four of the diet, mice began treadmill training 5 days/week, 60 min/day for 3 weeks. NPKD-fed mice increased body weight and fat mass, while ExTr negated a continued rise in adiposity. ExTr increased intramuscular glycogen, while the NPKD increased intramuscular triglycerides. Neither the NPKD nor ExTr alone altered mitochondrial content; however, in combination, the NPKD-ExTr group showed increases in PGC-1α, as well as markers of mitochondrial fission and fusion. Pyruvate oxidative capacity was unchanged by either intervention, while ExTr increased leucine oxidation in NPKD-fed mice. Lipid metabolism pathways had the most notable changes as the NPKD and ExTr interventions both enhanced mitochondrial and peroxisomal lipid oxidation and many adaptations were additive or synergistic. Overall these results suggest a combination of a NPKD and ExTr induces additive and/or synergistic adaptations in skeletal muscle oxidative capacity.
    Keywords:  exercise; ketogenic diet; mitochondria; peroxisomal; substrate oxidation
  21. Am J Physiol Cell Physiol. 2021 Apr 14.
      During embryogenesis, blood vessels and nerves develop with similar branching structure in response to shared signaling pathways guiding network growth. With both systems integral to physiological homeostasis, dual targeting of blood vessels and nerves to promote neurovascular regeneration following injury is an emerging therapeutic approach in biomedical engineering. A limitation to this strategy is that the nature of crosstalk between emergent vessels and nerves during regeneration in the adult is poorly understood. Following peripheral nerve transection, intraneural vascular cells infiltrate the site of injury to provide a migratory pathway for mobilized Schwann cells of regenerating axons. As Schwann cells demyelinate, they secrete vascular endothelial growth factor, which promotes angiogenesis. Recent advances point to concomitant restoration of neurovascular architecture and function through simultaneous targeting of growth factors and guidance cues shared by both systems during regeneration. In the context of traumatic injury associated with volumetric muscle loss, we consider the nature of biomaterials used to engineer 3-dimensional scaffolds, functionalization of scaffolds with molecular signals that guide and promote neurovascular growth and seeding scaffolds with progenitor cells. Physiological success is defined by each tissue component of the bioconstruct (nerve, vessel, muscle) becoming integrated with that of the host. Advances in microfabrication, cell culture techniques, and progenitor cell biology hold great promise for engineering bioconstructs able to restore organ function after volumetric muscle loss.
    Keywords:  biomaterials; injury; microcirculation; peripheral nerves; regeneration
  22. Stem Cell Res. 2021 Apr 08. pii: S1873-5061(21)00179-3. [Epub ahead of print]53 102333
      Limb-girdle muscular dystrophy recessive 1 (LGMDR1) represents one of the most common types of LGMD in the population, where patients develop a progressive muscle degeneration. The disease is caused by mutations in calpain 3 gene, with over 500 mutations reported to date. However, the molecular events that lead to muscle wasting are not clear, nor the reasons for the great clinical variability among patients, and this has so far hindered the development of effective therapies. Here we generate human induced pluripotent stem cells (iPSCs) from skin fibroblasts of 2 healthy controls and 4 LGMDR1 patients with different mutations. The generated lines were able to differentiate into myogenic progenitors and myotubes in vitro and in vivo, upon a transient PAX7 overexpressing protocol. Thus, we have generated myogenic cellular models of LGMDR1 that harbor different CAPN3 mutations within a human genetic background, and which do not derive from muscular biopsies. These models will allow us to investigate disease mechanisms and test therapies. Despite the variability found among iPSC lines that was unrelated to CAPN3 mutations, we found that patient-derived myogenic progenitors and myotubes express lower levels of DMD, which codes a key protein in satellite cell regulation and myotube maturation.
    Keywords:  CAPN3; Dystrophin; Induced pluripotent stem cells; LGMDR1; Skeletal muscle
  23. Cell Mol Life Sci. 2021 Apr 16.
      Neuromuscular system is constituted of multi-fibrillar muscles, tendons, motor neurons and associated muscle stem cells. Stereotyped pattern of muscle innervation and muscle-specific interactions with tendon cells suggest that neuromuscular system develops in a coordinated way. Remarkably, upon regeneration, coordinated assembly of all neuromuscular components is also critical to rebuild functional muscle. Thus, to ensure muscle function, the neuromuscular system components need to interact both during development and regeneration. Over the last decades, interactions between muscles and tendons, muscles and motor neurons and between muscles and muscle stem cells have been extensively analysed and documented. However, only recent evidence indicates that muscle stem cells interact with motor neurons and that these interactions contribute to building functional muscle both during development and regeneration. From this perspective, we discuss here the relationship between muscle stem cells and motor neurons during Drosophila neuromuscular system development and adverse impact of affected muscle stem cell-motor neuron interactions in regenerating vertebrate muscle.
    Keywords:  AMP; Drosophila; Motor neuron; Muscle stem cell; Satellite cell
  24. Exp Neurol. 2021 Apr 08. pii: S0014-4886(21)00123-0. [Epub ahead of print]341 113717
      Schwann cells are essential for peripheral nerve regeneration but, over short distances in acellular nerve grafts, extracellular matrix (ECM) molecules can support growth. The ECM molecules are present also on denervated muscle surfaces where they can support nerve growth. In this study, we addressed the efficacy of the ECM molecules of denervated muscle to support nerve fiber regeneration and muscle reinnervation. In the hindlimb of Sprague-Dawley rats, the proximal stump of the transected posterior tibial nerve, was cross-sutured to the distal nerve stump (NN) of each of three denervated muscles, tibialis anterior, extensor digitorum longus, and soleus, or implanted onto the denervated muscles' surfaces (N-M), proximal or distal to the endplate zone. Recordings of muscle and motor unit (MU) isometric forces and silver/cholinesterase histochemical staining of longitudinal muscle cryosections were used to determine the numbers of reinnervated MUs and the spatial course of regenerating nerve fibers, respectively. MU numbers declined significantly after N-M (>50%) as compared to those after NN. Muscle forces were reduced despite each nerve reinnervating up to three times the normal MU muscle fiber number. Regenerating nerves 'streamed' from the N-M site either proximal or distal to endplate zones toward the denervated intramuscular endoneurial tubes, with reduced numbers reinnervating endplates. We conclude that there is preferential reinnervation through the endoneurial tube and that it is important to drive implanted nerve fibers to enter endoneurial tubes for optimal muscle reinnervation. Schwann cells play the essential role in guiding regenerating nerve fibers to reinnervate denervated muscle fibers.
    Keywords:  Extracellular matrix; Intramuscular endoneurial tubes; Motor unit; Peripheral nerve regeneration; Schwann cells motor endplate
  25. Aging (Albany NY). 2021 Apr 16. 13
      BACKGROUNDS: A major side effect of statin, a widely used drug to treat hyperlipidemia, is skeletal myopathy through cell apoptosis. The aim of this study is to investigate the roles of microRNA in statin-induced injury.METHODS: Apolipoprotein E knockout (ApoE-/-) mice were administered with simvastatin (20 mg/kg/day) for 8 weeks. Exercise capacity was evaluated by hanging grid test, forelimb grip strength, and running tolerance test.
    RESULTS: In cultured skeletal muscle cells, statin increased the levels of miR-1a but decreased the levels of mitogen-activated protein kinase kinase kinase 1 (MAP3K1) in a time or dose dependent manner. Both computational target-scan analysis and luciferase gene reporter assay indicated that MAP3K1 is the target gene of miR-1a. Statin induced cell apoptosis of skeletal muscle cells, but abolished by downregulating of miR-1a or upregulation of MAP3K1. Further, the effects of miR-1a inhibition on statin-induced cell apoptosis were ablated by MAP3K1 siRNA. In ApoE-/- mice, statin induced cell apoptosis of skeletal muscle cells and decreased exercise capacity in mice infected with vector, but not in mice with lentivirus-mediated miR-1a gene silence.
    CONCLUSION: Statin causes skeletal injury through induction of miR-1a excessive expression to decrease MAP3K1 gene expression.
    Keywords:  apoptosis; microRNA-1a; mitogen-activated protein kinase kinase kinase 1; myopathy; statin
  26. Biochem Biophys Res Commun. 2021 Apr 08. pii: S0006-291X(21)00566-0. [Epub ahead of print]556 127-133
      Nuclear receptor subfamily 4 group A member 1 (NR4A1) acts as a myogenic factor in muscle development and regeneration; however, it remains unclear how Nr4a1 regulates myoblast physiology. In this study, report a role for Nr4a1-mediated regulation of cell adhesion in myoblast and muscle tissue. Nr4a1-overexpression myoblast, Nr4a1-konckdown myoblast and mice gastrocnemius muscle following an injection with an adenovirus vector expression Nr4a1 (Nr4a1-AAV) were used to observe the changes in cell adhesion. Nr4a1 was found to enhance cell-cell contact and adhesion molecule expression in myoblasts. In contrast, the deletion of Nr4a1 expression inhibited junction and adhesion between myoblasts. Moreover, Nr4a1 increased myoblast adhesion via directly binding to an upstream site of zinc finger E-box binding homeobox 1 (Zeb1), which is required for myogenesis in myoblasts. In mice, Zeb1 induced increased cadherin and integrin expression in the gastrocnemius muscle following an injection with an adenovirus vector expressing Nr4a1(Nr4a1-AAV). These data indicate that Nr4a1 regulates myoblast adhesion via Zeb1 expression.
    Keywords:  Cell adhesion; Myoblast; Nr4a1; Zeb1
  27. Biochem Pharmacol. 2021 Apr 09. pii: S0006-2952(21)00153-2. [Epub ahead of print] 114557
      Valdecoxib (VAL), a non-steroidal anti-inflammatory drug, has been widely used for treatment of rheumatoid arthritis, osteoarthritis, and menstrual pain. It is a selective cyclooxygenase-2 inhibitor. The suppressive effects of VAL on cardiovascular diseases and neuroinflammation have been documented; however, its impact on insulin signaling in skeletal muscle has not been studied in detail. The aim of this study was to investigate the effects of VAL on insulin resistance in mouse skeletal muscle. Treatment of C2C12 myocytes with VAL reversed palmitate-induced aggravation of insulin signaling and glucose uptake. Further, VAL attenuated palmitate-induced inflammation and endoplasmic reticulum (ER) stress in a concentration-dependent manner. Treatment with VAL concentration-dependently upregulated AMP-activated protein kinase (AMPK) and heat shock protein beta 1 (HSPB1) expression. In line with in vitro experiments, treatment with VAL augmented AMPK phosphorylation and HSPB1 expression, thereby alleviating high-fat diet-induced insulin resistance along with inflammation and ER stress in mouse skeletal muscle. However, small interfering RNA-mediated inhibition of AMPK abolished the effects of VAL on insulin resistance, inflammation, and ER stress. These results suggest that VAL alleviates insulin resistance through AMPK/HSPB1-mediated inhibition of inflammation and ER stress in skeletal muscle under hyperlipidemic conditions. Hence, VAL could be used as an effective pharmacotherapeutic agent for management of insulin resistance and type 2 diabetes.
    Keywords:  AMPK; ER stress; HSPB1; Valdecoxib; inflammation; insulin resistance
  28. J Clin Invest. 2021 Apr 15. pii: 148176. [Epub ahead of print]131(8):
      It has long been known that fatty acids can either adversely or positively affect insulin signaling in skeletal muscle, depending on chain length or saturation, and can therefore be primary drivers of systemic insulin sensitivity. However, the detailed mechanisms linking fatty acids to insulin signaling in skeletal muscle have been elusive. In this issue of the JCI, Ferrara et al. suggest a model whereby membrane lipid remodeling mediates skeletal muscle insulin sensitivity. The authors demonstrate that membrane glycerophospholipid fatty acid remodeling by lysophosphatidylcholine acyltransferase 3 (LPCAT3) in skeletal muscle from subjects with obesity was induced, suppressing insulin signaling and glucose tolerance. Loss or gain of LPCAT3 function in mouse models showed that Lpcat3 was both required and sufficient for high-fat diet-induced muscle insulin resistance. These results suggest that the physiochemical properties of muscle cell membranes may drive insulin sensitivity and, therefore, systemic glucose intolerance.
  29. Exp Gerontol. 2021 Apr 13. pii: S0531-5565(21)00113-3. [Epub ahead of print] 111338
      Mechanical forces are conducted through myofibers and into nuclei to regulate muscle development, hypertrophy, and homeostasis. We hypothesized that nuclei in aged muscle have changes in the nuclear envelope and associated proteins, resulting in altered markers of mechano-signaling.METHODS: YAP/TAZ protein expression and gene expression of downstream targets, Ankrd1 and Cyr61, were evaluated as mechanotransduction indicators. Expression of proteins in the nuclear lamina and the nuclear pore complex (NPC) were assessed, and nuclear morphology was characterized by electron microscopy. Nuclear envelope permeability was assessed by uptake of 70 kDa fluorescent dextran.
    RESULTS: Nuclear changes with aging included a relative decrease of lamin β1 and Nup107, and a relative increase in Nup93, which could underlie the aberrant nuclear morphology, increased nuclear leakiness, and elevated YAP/TAZ signaling.
    CONCLUSION: Aged muscles have hyperactive nuclear-cytoplasmic signaling, indicative of altered nuclear mechanotransduction. These data highlight a possible role for the nucleus in aging-related aberrant mechano-sensing.
    Keywords:  Biology of aging; Mice; Muscles; Sarcopenia
  30. Sci Adv. 2021 Apr;pii: eabf2856. [Epub ahead of print]7(16):
      Exercise training is a powerful strategy to prevent and combat cardiovascular and metabolic diseases, although the integrative nature of the training-induced adaptations is not completely understood. We show that chronic blockade of histamine H1/H2 receptors led to marked impairments of microvascular and mitochondrial adaptations to interval training in humans. Consequently, functional adaptations in exercise capacity, whole-body glycemic control, and vascular function were blunted. Furthermore, the sustained elevation of muscle perfusion after acute interval exercise was severely reduced when H1/H2 receptors were pharmaceutically blocked. Our work suggests that histamine H1/H2 receptors are important transducers of the integrative exercise training response in humans, potentially related to regulation of optimal post-exercise muscle perfusion. These findings add to our understanding of how skeletal muscle and the cardiovascular system adapt to exercise training, knowledge that will help us further unravel and develop the exercise-is-medicine concept.
  31. Am J Hum Genet. 2021 Apr 09. pii: S0002-9297(21)00130-0. [Epub ahead of print]
      JAG2 encodes the Notch ligand Jagged2. The conserved Notch signaling pathway contributes to the development and homeostasis of multiple tissues, including skeletal muscle. We studied an international cohort of 23 individuals with genetically unsolved muscular dystrophy from 13 unrelated families. Whole-exome sequencing identified rare homozygous or compound heterozygous JAG2 variants in all 13 families. The identified bi-allelic variants include 10 missense variants that disrupt highly conserved amino acids, a nonsense variant, two frameshift variants, an in-frame deletion, and a microdeletion encompassing JAG2. Onset of muscle weakness occurred from infancy to young adulthood. Serum creatine kinase (CK) levels were normal or mildly elevated. Muscle histology was primarily dystrophic. MRI of the lower extremities revealed a distinct, slightly asymmetric pattern of muscle involvement with cores of preserved and affected muscles in quadriceps and tibialis anterior, in some cases resembling patterns seen in POGLUT1-associated muscular dystrophy. Transcriptome analysis of muscle tissue from two participants suggested misregulation of genes involved in myogenesis, including PAX7. In complementary studies, Jag2 downregulation in murine myoblasts led to downregulation of multiple components of the Notch pathway, including Megf10. Investigations in Drosophila suggested an interaction between Serrate and Drpr, the fly orthologs of JAG1/JAG2 and MEGF10, respectively. In silico analysis predicted that many Jagged2 missense variants are associated with structural changes and protein misfolding. In summary, we describe a muscular dystrophy associated with pathogenic variants in JAG2 and evidence suggests a disease mechanism related to Notch pathway dysfunction.
    Keywords:  JAG2, Jagged2, Serrate, Notch signaling pathway, muscular dystrophy, muscle MRI, POGLUT1, MEGF10, exome sequencing, satellite cell
  32. Front Physiol. 2021 ;12 648019
      The sliding filament-swinging cross bridge theory of skeletal muscle contraction provides a reasonable description of muscle properties during isometric contractions at or near maximum isometric force. However, it fails to predict muscle force during dynamic length changes, implying that the model is not complete. Mounting evidence suggests that, along with cross bridges, a Ca2+-sensitive viscoelastic element, likely the titin protein, contributes to muscle force and work. The purpose of this study was to develop a multi-level approach deploying stretch-shortening cycles (SSCs) to test the hypothesis that, along with cross bridges, Ca2+-sensitive viscoelastic elements in sarcomeres contribute to force and work. Using whole soleus muscles from wild type and mdm mice, which carry a small deletion in the N2A region of titin, we measured the activation- and phase-dependence of enhanced force and work during SSCs with and without doublet stimuli. In wild type muscles, a doublet stimulus led to an increase in peak force and work per cycle, with the largest effects occurring for stimulation during the lengthening phase of SSCs. In contrast, mdm muscles showed neither doublet potentiation features, nor phase-dependence of activation. To further distinguish the contributions of cross bridge and non-cross bridge elements, we performed SSCs on permeabilized psoas fiber bundles activated to different levels using either [Ca2+] or [Ca2+] plus the myosin inhibitor 2,3-butanedione monoxime (BDM). Across activation levels ranging from 15 to 100% of maximum isometric force, peak force, and work per cycle were enhanced for fibers in [Ca2+] plus BDM compared to [Ca2+] alone at a corresponding activation level, suggesting a contribution from Ca2+-sensitive, non-cross bridge, viscoelastic elements. Taken together, our results suggest that a tunable viscoelastic element such as titin contributes to: (1) persistence of force at low [Ca2+] in doublet potentiation; (2) phase- and length-dependence of doublet potentiation observed in wild type muscles and the absence of these effects in mdm muscles; and (3) increased peak force and work per cycle in SSCs. We conclude that non-cross bridge viscoelastic elements, likely titin, contribute substantially to muscle force and work, as well as the phase-dependence of these quantities, during dynamic length changes.
    Keywords:  2,3-butanedione monoxime; calcium-dependence; doublet potentiation; history-dependence; skeletal muscle; titin; work loop
  33. Adv Biol (Weinh). 2021 Apr;5(4): e2000280
      Biological tissues comprise complex structural environments known to influence cell behavior via multiple interdependent sensing and transduction mechanisms. Yet, and despite the predominantly nonplanar geometry of these environments, the impact of tissue-size (milliscale) curvature on cell behavior is largely overlooked or underestimated. This study explores how concave, hemicylinder-shaped surfaces 3-50 mm in diameter affect the migration, proliferation, orientation, and differentiation of C2C12 myoblasts. Notably, these milliscale cues significantly affect cell responses compared with planar substrates, with myoblasts grown on surfaces 7.5-15 mm in diameter showing prevalent migration and alignment parallel to the curvature axis. Moreover, surfaces within this curvature range promote myoblast differentiation and the formation of denser, more compact tissues comprising highly oriented multinucleated myotubes. Based on the similarity of effects, it is further proposed that myoblast susceptibility to substrate curvature depends on mechanotransduction signaling. This model thus supports the notion that cellular responses to substrate curvature and compliance share the same molecular pathways and that control of cell behavior can be achieved via modulation of either individual parameter or in combination. This correlation is relevant for elucidating how muscle tissue forms and heals, as well as for designing better biomaterials and more appropriate cell-surface interfaces.
    Keywords:  cell alignment; muscle differentiation; myoblasts; self-organization; surface curvature
  34. PeerJ. 2021 ;9 e10500
      Background: Physical exercise is a health promotion factor regulating gene expression and causing changes in phenotype, varying according to exercise type and intensity. Acute strenuous exercise in sedentary individuals appears to induce different transcriptional networks in response to stress caused by exercise. The objective of this research was to investigate the transcriptional profile of strenuous experimental exercise.Methodology: RNA-Seq was performed with Rattus norvegicus soleus muscle, submitted to strenuous physical exercise on a treadmill with an initial velocity of 0.5 km/h and increments of 0.2 km/h at every 3 min until animal exhaustion. Twenty four hours post-physical exercise, RNA-seq protocols were performed with coverage of 30 million reads per sample, 100 pb read length, paired-end, with a list of counts totaling 12816 genes.
    Results: Eighty differentially expressed genes (61 down-regulated and 19 up-regulated) were obtained. Reactome and KEGG database searches revealed the most significant pathways, for down-regulated gene set, were: PI3K-Akt signaling pathway, RAF-MAP kinase, P2Y receptors and Signaling by Erbb2. Results suggest PI3K-AKT pathway inactivation by Hbegf, Fgf1 and Fgr3 receptor regulation, leading to inhibition of cell proliferation and increased apoptosis. Cell signaling transcription networks were found in transcriptome. Results suggest some metabolic pathways which indicate the conditioning situation of strenuous exercise induced genes encoding apoptotic and autophagy factors, indicating cellular stress.
    Conclusion: Down-regulated networks showed cell transduction and signaling pathways, with possible inhibition of cellular proliferation and cell degeneration. These findings reveal transitory and dynamic process in cell signaling transcription networks in skeletal muscle after acute strenuous exercise.
    Keywords:  Acute strenuous exercise; Soleus muscle; Transcriptome
  35. Biochem Biophys Res Commun. 2021 Apr 09. pii: S0006-291X(21)00546-5. [Epub ahead of print]556 93-98
      Sepsis remains a leading cause of mortality in critically ill patients and is characterized by multi-organ dysfunction. Mitochondrial damage has been proposed to be involved in the pathophysiology of sepsis. In addition to metabolic impairments resulting from mitochondrial dysfunction, mitochondrial DNA (mtDNA) causes systemic inflammation as a damage-associated molecular pattern when it is released to the circulation. Metabolic derangements in skeletal muscle are a major complication of sepsis and negatively affects clinical outcomes of septic patients. However, limited knowledge is available about sepsis-induced mitochondrial damage in skeletal muscle. Here, we show that sepsis induced profound abnormalities in cristae structure, rupture of the inner and outer membranes and enlargement of the mitochondria in mouse skeletal muscle in a time-dependent manner, which was associated with increased plasma mtDNA levels. Farnesyltransferase inhibitor, FTI-277, prevented sepsis-induced morphological aberrations of the mitochondria, and blocked the increased plasma mtDNA levels along with improved survival. These results indicate that protein farnesylation plays a role in sepsis-induced damage of the mitochondria in mouse skeletal muscle. Our findings suggest that mitochondrial disintegrity in skeletal muscle may contribute to elevated circulating mtDNA levels in sepsis.
    Keywords:  Farnesyltransferase inhibitor; Mitochondria; Mitochondrial DNA; Sepsis; Skeletal muscle
  36. Neurosurgery. 2021 Apr 16. pii: nyab105. [Epub ahead of print]
      BACKGROUND: Peripheral nerve injuries result in muscle denervation and apoptosis of the involved muscle, which subsequently reduces mitochondrial content and causes muscle atrophy. The local injection of mitochondria has been suggested as a useful tool for restoring the function of injured nerves or the brain.OBJECTIVE: To determine outcomes following the administration of isolated mitochondria into denervated muscle after nerve injury that have not been investigated.
    METHODS: Muscle denervation was conducted in a sciatic nerve crushed by a vessel clamp and the denervated gastrocnemius muscle was subjected to 195 μg hamster green fluorescent protein (GFP)-mitochondria intramuscular infusion for 10 min.
    RESULTS: The mitochondria were homogeneously distributed throughout the denervated muscle after intramuscular infusion. The increases in caspase 3, 8-oxo-dG, Bad, Bax, and ratio of Bax/Bcl-2 levels in the denervated muscle were attenuated by mitochondrial infusion, and the downregulation of Bcl-2 expression was prevented by mitochondrial infusion. In addition, the decrease in the expression of desmin and the acetylcholine receptor was counteracted by mitochondrial infusion; this effect paralleled the amount of distributed mitochondria. The restoration of the morphology of injured muscles and nerves was augmented by the local infusion of mitochondria. Mitochondrial infusion also led to improvements in sciatic functional indexes, compound muscle action potential amplitudes, and conduction latencies as well as the parameters of CatWalk (Noldus) gait analysis.
    CONCLUSION: The local infusion of mitochondria can successfully prevent denervated muscle atrophy and augment nerve regeneration by reducing oxidative stress in denervated muscle.
    Keywords:  Denervated muscle; Mitochondria transplantation; Nerve crush injury
  37. Genome Med. 2021 Apr 12. 13(1): 57
      BACKGROUND: Mutations in the DMD gene encoding dystrophin-a critical structural element in muscle cells-cause Duchenne muscular dystrophy (DMD), which is the most common fatal genetic disease. Clustered regularly interspaced short palindromic repeat (CRISPR)-mediated gene editing is a promising strategy for permanently curing DMD.METHODS: In this study, we developed a novel strategy for reframing DMD mutations via CRISPR-mediated large-scale excision of exons 46-54. We compared this approach with other DMD rescue strategies by using DMD patient-derived primary muscle-derived stem cells (DMD-MDSCs). Furthermore, a patient-derived xenograft (PDX) DMD mouse model was established by transplanting DMD-MDSCs into immunodeficient mice. CRISPR gene editing components were intramuscularly delivered into the mouse model by adeno-associated virus vectors.
    RESULTS: Results demonstrated that the large-scale excision of mutant DMD exons showed high efficiency in restoring dystrophin protein expression. We also confirmed that CRISPR from Prevotella and Francisella 1(Cas12a)-mediated genome editing could correct DMD mutation with the same efficiency as CRISPR-associated protein 9 (Cas9). In addition, more than 10% human DMD muscle fibers expressed dystrophin in the PDX DMD mouse model after treated by the large-scale excision strategies. The restored dystrophin in vivo was functional as demonstrated by the expression of the dystrophin glycoprotein complex member β-dystroglycan.
    CONCLUSIONS: We demonstrated that the clinically relevant CRISPR/Cas9 could restore dystrophin in human muscle cells in vivo in the PDX DMD mouse model. This study demonstrated an approach for the application of gene therapy to other genetic diseases.
    Keywords:  CRISPR/ Cas12a; CRISPR/Cas9; Duchenne muscular dystrophy; Gene editing; Patient-derived xenograft model
  38. Neurobiol Dis. 2021 Apr 13. pii: S0969-9961(21)00115-7. [Epub ahead of print]155 105366
      Spinal Muscular Atrophy (SMA) is a severe neuromuscular disorder caused by loss of the Survival Motor Neuron 1 gene (SMN1). Due to this depletion of the survival motor neuron (SMN) protein, the disease is characterized by the degeneration of spinal cord motoneurons (MNs), progressive muscular atrophy, and weakness. Nevertheless, the ultimate cellular and molecular mechanisms leading to cell loss in SMN-reduced MNs are only partially known. We have investigated the activation of apoptotic and neuronal survival pathways in several models of SMA cells. Even though the antiapoptotic proteins FAIM-L and XIAP were increased in SMA MNs, the apoptosis executioner cleaved-caspase-3 was also elevated in these cells, suggesting the activation of the apoptosis process. Analysis of the survival pathway PI3K/Akt showed that Akt phosphorylation was reduced in SMA MNs and pharmacological inhibition of PI3K diminished SMN and Gemin2 at transcriptional level in control MNs. In contrast, ERK phosphorylation was increased in cultured mouse and human SMA MNs. Our observations suggest that apoptosis is activated in SMA MNs and that Akt phosphorylation reduction may control cell degeneration, thereby regulating the transcription of Smn and other genes related to SMN function.
    Keywords:  Akt intracellular pathway; Apoptosis; FAIM; Motoneurons; Spinal muscular atrophy; Survival motor neuron
  39. Expert Rev Anti Infect Ther. 2021 Apr 14.
      INTRODUCTION: The coronavirus disease 2019 (COVID-19) is a highly contagious respiratory viral disease for both the general population and health care professionals caring for infected patients. Of particular concern is the potential for significant respiratory, cardiovascular, physical, and psychological dysfunctions.AREAS COVERED: In this context, the current review will focus on the following areas: 1) staying physically active during the COVID-19 pandemic; 2) highlighting the importance of understanding COVID-19 mechanisms; 3) preventing infections for healthcare workers by using personal protective equipment; 4) highlighting importance of respiratory care and physical therapy during hospitalization in patients with COVID-19; and 5) facilitating referral to a rehabilitation program in patients recovering from COVID-19.
    EXPERT OPINION: We recommend daily physical exercise, outdoors or at home, as physical exercise increases the synthesis of anti-inflammatory cytokines; Patients with COVID-19 may develop severe acute respiratory syndrome, hypoxemia, diffuse alveolar damage, ACE2 reduction in the cardiovascular system and muscle weakness acquired through a prolonged hospital stay; The role of the physiotherapist in the hospital environment is of fundamental importance-early mobilization is highly recommended in severe cases of COVID-19.
    Keywords:  COVID-19; Physical therapists; exercise; health care; mobilization
  40. Metabolism. 2021 Apr 12. pii: S0026-0495(21)00075-5. [Epub ahead of print] 154775
      Laminins are extracellular matrix proteins that reside in the basement membrane and provide structural support in addition to promoting cellular adhesion and migration. Through interactions with cell surface receptors, laminins stimulate intracellular signaling cascades which direct specific survival and differentiation outcomes. In metabolic tissues such as the pancreas, adipose, muscle, and liver, laminin isoforms are expressed in discrete temporal and spatial patterns suggesting that certain isoforms may support the development and function of particular metabolic cell types. This review focuses on the research to date detailing the expression of laminin isoforms, their potential function, as well as known pathways involved in laminin signaling in metabolic tissues. We will also discuss the current biomedical therapies involving laminins in these tissues in addition to prospective applications, with the goal being to encourage future investigation of laminins in the context of metabolic disease.
    Keywords:  BAT; ECM; Laminins; WAT; adipose tissue; basement membrane; brown adipose tissue; diabetes; extracellular matrix; insulin resistance; liver; metabolic disease; metabolism; obesity; pancreas; pancreatic islets; signaling; skeletal muscle; stem cells; white adipose tissue
  41. Eur J Appl Physiol. 2021 Apr 17.
      Regular physical activity reduces the risk of several site-specific cancers in humans and suppresses tumour growth in animal models. The mechanisms through which exercise reduces tumour growth remain incompletely understood, but an intriguing and accumulating body of evidence suggests that the incubation of cancer cells with post-exercise serum can have powerful effects on key hallmarks of cancer cell behaviour in vitro. This suggests that exercise can impact tumour biology through direct changes in circulating proteins, RNA molecules and metabolites. Here, we provide a comprehensive narrative overview of what is known about the effects of exercise-conditioned sera on in vitro cancer cell behaviour. In doing so, we consider the key limitations of the current body of literature, both from the perspective of exercise physiology and cancer biology, and we discuss the potential in vivo physiological relevance of these findings. We propose key opportunities for future research in an area that has the potential to identify key anti-oncogenic protein targets and optimise physical activity recommendations for cancer prevention, treatment and survivorship.
    Keywords:  Cancer cell apoptosis; Cancer cell growth; Cancer cell proliferation; Cancer prevention; Cancer therapy; Exercise; Exercise-conditioned serum; Physical activity
  42. Elife. 2021 04 13. pii: e68271. [Epub ahead of print]10
      Risk factors for cardiovascular diseases trigger molecular changes that harm the endothelial cells in the heart, but exercise can suppress these effects.
    Keywords:  aging; chromosomes; endothelium; exercise; gene expression; heart; human; medicine; mouse; obesity
  43. Elife. 2021 04 13. pii: e62293. [Epub ahead of print]10
      Aging is associated with complex molecular and cellular processes that are poorly understood. Here we leveraged the Tabula Muris Senis single-cell RNA-seq data set to systematically characterize gene expression changes during aging across diverse cell types in the mouse. We identified aging-dependent genes in 76 tissue-cell types from 23 tissues and characterized both shared and tissue-cell-specific aging behaviors. We found that the aging-related genes shared by multiple tissue-cell types also change their expression congruently in the same direction during aging in most tissue-cell types, suggesting a coordinated global aging behavior at the organismal level. Scoring cells based on these shared aging genes allowed us to contrast the aging status of different tissues and cell types from a transcriptomic perspective. In addition, we identified genes that exhibit age-related expression changes specific to each functional category of tissue-cell types. Altogether, our analyses provide one of the most comprehensive and systematic characterizations of the molecular signatures of aging across diverse tissue-cell types in a mammalian system.
    Keywords:  aging; computation; computational biology; mouse; single cell; systems biology
  44. PLoS Biol. 2021 Apr 12. 19(4): e3001148
      Sarcomeres, the basic contractile units of striated muscle cells, contain arrays of thin (actin) and thick (myosin) filaments that slide past each other during contraction. The Ig-like domain-containing protein myotilin provides structural integrity to Z-discs-the boundaries between adjacent sarcomeres. Myotilin binds to Z-disc components, including F-actin and α-actinin-2, but the molecular mechanism of binding and implications of these interactions on Z-disc integrity are still elusive. To illuminate them, we used a combination of small-angle X-ray scattering, cross-linking mass spectrometry, and biochemical and molecular biophysics approaches. We discovered that myotilin displays conformational ensembles in solution. We generated a structural model of the F-actin:myotilin complex that revealed how myotilin interacts with and stabilizes F-actin via its Ig-like domains and flanking regions. Mutant myotilin designed with impaired F-actin binding showed increased dynamics in cells. Structural analyses and competition assays uncovered that myotilin displaces tropomyosin from F-actin. Our findings suggest a novel role of myotilin as a co-organizer of Z-disc assembly and advance our mechanistic understanding of myotilin's structural role in Z-discs.