bims-moremu Biomed News
on Molecular regulators of muscle mass
Issue of 2021‒04‒25
forty-six papers selected by
Anna Vainshtein
Craft Science Inc.

  1. Cell Rep. 2021 Apr 20. pii: S2211-1247(21)00332-6. [Epub ahead of print]35(3): 109018
      Physical exercise has profound effects on quality of life and susceptibility to chronic disease; however, the regulation of skeletal muscle function at the molecular level after exercise remains unclear. We tested the hypothesis that the benefits of exercise on muscle function are linked partly to microtraumatic events that result in accumulation of circulating heme. Effective metabolism of heme is controlled by Heme Oxygenase-1 (HO-1, Hmox1), and we find that mouse skeletal muscle-specific HO-1 deletion (Tam-Cre-HSA-Hmox1fl/fl) shifts the proportion of muscle fibers from type IIA to type IIB concomitant with a disruption in mitochondrial content and function. In addition to a significant impairment in running performance and response to exercise training, Tam-Cre-HSA-Hmox1fl/fl mice show remarkable muscle atrophy compared to Hmox1fl/fl controls. Collectively, these data define a role for heme and HO-1 as central regulators in the physiologic response of skeletal muscle to exercise.
    Keywords:  DAMP; exercise training; heme; heme oxygenase-1; hemopexin; mitochondrial dysfunction; muscle atrophy; muscle microtrauma; satellite cells
  2. Mol Ther Nucleic Acids. 2021 Jun 04. 24 352-368
      Skeletal muscle development is a complex and highly orchestrated biological process mediated by a series of myogenesis regulatory factors. Numerous studies have demonstrated that circular RNAs (circRNAs) are involved in muscle differentiation, but the exact molecular mechanisms involved remain unclear. Here, we analyzed the expression of circRNAs at the adult and embryo development stages of cattle musculus longissimus. A stringent set of 1,318 circRNAs candidates were identified, and we found that 495 circRNAs were differentially expressed between embryonic and adult tissue libraries. We subsequently focused on one of the most downregulated circRNAs (using the adult stage expression as control), and this was named muscle differentiation-associated circular RNA (circMYBPC1). With RNA binding protein immunoprecipitation (RIP) and RNA pull-down assays, circMYBPC1 was identified to promote myoblast differentiation by directly binding miR-23a to relieve its inhibition on myosin heavy chain (MyHC). In addition, RIP assays demonstrated that circMYBPC1 could directly bind MyHC protein. In vivo observations also suggested that circMYBPC1 may stimulate skeletal muscle regeneration after muscle damage. These results revealed that the novel non-coding circRNA circMYBPC1 promotes differentiation of myoblasts and may promote skeletal muscle regeneration. Our results provided a basis for in-depth analysis of the role of circRNA in myogenesis and muscle diseases.
    Keywords:  RNA-seq; bovine skeletal muscle; circRNA; muscle differentiation
  3. Neurosci Lett. 2021 Apr 15. pii: S0304-3940(21)00277-9. [Epub ahead of print]754 135899
      The volitional movement of skeletal is controlled by the motor neuron at the site of neuromuscular junction (NMJ) where the retrograde signals are also passed back from muscle to the motor neuron. As the normal function of muscle largely depends on mitochondria that determine the fate of a skeletal muscle myofiber, there must exist a fine-controlled functional coupling between NMJ and mitochondria in myofibers. This mini-review discusses recent publications that reveal how spatiotemporal profiles of intracellular free Ca2+ could couple mitochondrial function with the activity of NMJ in skeletal muscle myofibers.
    Keywords:  Ca(2+) signaling; Mitochondria; Neuromuscular junction; Skeletal muscle
  4. Physiol Genomics. 2021 Apr 19.
      The skeletal muscle hypertrophic response to resistance exercise training (RT) is highly variable across individuals. The molecular underpinnings of this heterogeneity are unclear. This study investigated transcriptional networks linked to RT-induced muscle hypertrophy, classified as (i) predictive of hypertrophy, (ii) responsive to RT independent of muscle hypertrophy, or (iii) plastic with hypertrophy. Older adults (n=31, 18F/13M, 70±4y) underwent 14-wk RT (3d/wk, alternating high-low-high intensity). Muscle hypertrophy was assessed by pre- to post-RT change in mid-thigh muscle cross-sectional area (CSA) [computed tomography (CT), primary outcome], and thigh lean mass [dual-energy x-ray absorptiometry (DXA), secondary outcome]. Transcriptome-wide poly-A RNA-seq was performed on vastus lateralis tissue collected pre- (n=31) and post-RT (n=22). Prediction networks (using only baseline RNAseq) were identified by Weighted Gene Correlation Network Analysis (WGCNA). To identify Plasticity networks, WGCNA change indices for paired samples were calculated and correlated to changes in muscle size outcomes. Pathway-Level Information ExtractoR (PLIER) was applied to identify Response networks and link genes to biological annotation. Predictionnetworks (n=6) confirmed transcripts previously connected to resistance/ aerobic training adaptations in the MetaMEx database while revealing novel member genes that should fuel future research to understand the influence of baseline muscle gene expression on hypertrophy. Response networks (n=6) indicated RT-induced increase in aerobic metabolism and reduced expression of genes associated with spliceosome biology and type-I myofibers. A single exploratory Plasticity network was identified. Findings support that inter-individual differences in baseline gene expression may contribute more than RT-induced changes in gene networks to muscle hypertrophic response heterogeneity.
    Keywords:  RNA-seq; muscle growth; network biology; resistance exercise; skeletal muscle
  5. Phytother Res. 2021 Apr 20.
      Cisplatin (DDP) is widely used in cancer treatment, but DDP can cause skeletal muscle atrophy and cachexia. This study explored the effect and mechanism of daidzein (DAI) in reducing DDP-induced skeletal muscle atrophy and cachexia in vivo and in vitro. DAI alleviated the weight, food intake, muscle, adipose tissue, kidney weight and forelimb grip of LLC tumour-bearing mice after DDP treatment, and did not affect the antitumour effect of DDP. DAI can reduce the decrease of the cross-sectional area of skeletal muscle fibre-induced by DDP and prevent the change of fibre type proportion. In skeletal muscle, it can inhibit Glut4/AMPK/FoxO pathway, down-regulate the expression of atrogin1 and MuRF1, and inhibit skeletal muscle protein degradation. In DDP treated C2C12 myotubes, DAI could inhibit Glut4/AMPK/FoxO pathway to reduce myotubes atrophy, while AMPK agonist MK-3903 could reverse the protective effect of DAI. These results suggest that DAI can alleviate DDP-induced skeletal muscle atrophy by downregulating the expression of Atrogin1 and MuRF1 through the regulation of Glut4/AMPK/FoxO pathway.
    Keywords:  Glut4/AMPK/FoxO pathway; cachexia; cisplatin; daidzein; muscle atrophy; protein degradation
  6. Mol Ther Methods Clin Dev. 2021 Jun 11. 21 274-287
      Gene replacement for laminin-α2-deficient congenital muscular dystrophy 1A (MDC1A) is currently not possible using a single adeno-associated virus (AAV) vector due to the large size of the LAMA2 gene. LAMA2 encodes laminin-α2, a subunit of the trimeric laminin-211 extracellular matrix (ECM) protein that is the predominant laminin expressed in skeletal muscle. LAMA2 expression stabilizes skeletal muscle, in part by binding membrane receptors via its five globular (G) domains. We created a small, AAV-deliverable, micro-laminin gene therapy that expresses these G1-5 domains, LAMA2(G1-5), to test their therapeutic efficacy in the dyW mouse model for MDC1A. We also fused the heparin-binding (HB) domain from HB epidermal growth factor-like growth factor (HB-EGF) to LAMA2(G1-5) to test whether this would increase muscle ECM expression. dyW mice treated intravenously with rAAV9.CMV.HB-LAMA2(G1-5) showed increased muscle ECM expression of transgenic protein relative to mice treated with rAAV9.CMV.LAMA2(G1-5) and showed improved weight-normalized forelimb grip strength relative to untreated dyW mice. Additionally, dyW muscle fibers expressing either micro-laminin protein showed some measures of reduced pathology, although levels of muscle cell apoptosis and inflammation were not decreased. Although systemic expression of rAAV9.CMV.HB-LAMA2(G1-5) did not inhibit all disease phenotypes, these studies demonstrate the feasibility of using a micro-laminin gene therapy strategy to deliver gene replacement for MDC1A.
    Keywords:  AAV; extracellular matrix; gene therapy; laminin; muscular dystrophy
  7. Am J Physiol Cell Physiol. 2021 Apr 21.
      Polyamines have been shown to be absolutely required for protein synthesis and cell growth. The serine/threonine kinase, the mechanistic target of rapamycin complex 1 (mTORC1), also plays a fundamental role in the regulation of protein turnover and cell size, including in skeletal muscle, where mTORC1 is sufficient to increase protein synthesis and muscle fiber size, and is necessary for mechanical overload-induced muscle hypertrophy. Recent evidence suggests that mTORC1 may regulate the polyamine metabolic pathway; however, there is currently no evidence in skeletal muscle. This study examined changes in polyamine pathway proteins during muscle hypertrophy induced by mechanical overload (7 d), with and without the mTORC1 inhibitor, rapamycin, and during muscle atrophy induced by food deprivation (48 h) and denervation (7 d) in mice. Mechanical overload induced an increase in mTORC1 signalling, protein synthesis and muscle mass, and these were associated with rapamycin-sensitive increases in adenosylmethione decarboxylase 1 (Amd1), spermidine synthase (SpdSyn) and c-Myc. Food deprivation decreased mTORC1 signalling, protein synthesis and muscle mass, accompanied by a decrease in spermidine/spermine acetyltransferase 1 (Sat1). Denervation, resulted increased mTORC1 signalling and protein synthesis, and decreased muscle mass, which was associated with an increase in SpdSyn, spermine synthase (SpmSyn) and c-Myc. Combined, these data show that polyamine pathway enzymes are differentially regulated in models of altered mechanical and metabolic stress, and that Amd1 and SpdSyn are, in part, regulated in a mTORC1-dependent manner. Furthermore, these data suggest that polyamines may play a role in the adaptive response to stressors in skeletal muscle.
    Keywords:  mTORC1; muscle atrophy; muscle hypertrophy; polyamine; protein synthesis
  8. Proc Natl Acad Sci U S A. 2021 Apr 27. pii: e2003596118. [Epub ahead of print]118(17):
      Fast skeletal myosin-binding protein-C (fMyBP-C) is one of three MyBP-C paralogs and is predominantly expressed in fast skeletal muscle. Mutations in the gene that encodes fMyBP-C, MYBPC2, are associated with distal arthrogryposis, while loss of fMyBP-C protein is associated with diseased muscle. However, the functional and structural roles of fMyBP-C in skeletal muscle remain unclear. To address this gap, we generated a homozygous fMyBP-C knockout mouse (C2-/-) and characterized it both in vivo and in vitro compared to wild-type mice. Ablation of fMyBP-C was benign in terms of muscle weight, fiber type, cross-sectional area, and sarcomere ultrastructure. However, grip strength and plantar flexor muscle strength were significantly decreased in C2-/- mice. Peak isometric tetanic force and isotonic speed of contraction were significantly reduced in isolated extensor digitorum longus (EDL) from C2-/- mice. Small-angle X-ray diffraction of C2-/- EDL muscle showed significantly increased equatorial intensity ratio during contraction, indicating a greater shift of myosin heads toward actin, while MLL4 layer line intensity was decreased at rest, indicating less ordered myosin heads. Interfilament lattice spacing increased significantly in C2-/- EDL muscle. Consistent with these findings, we observed a significant reduction of steady-state isometric force during Ca2+-activation, decreased myofilament calcium sensitivity, and sinusoidal stiffness in skinned EDL muscle fibers from C2-/- mice. Finally, C2-/- muscles displayed disruption of inflammatory and regenerative pathways, along with increased muscle damage upon mechanical overload. Together, our data suggest that fMyBP-C is essential for maximal speed and force of contraction, sarcomere integrity, and calcium sensitivity in fast-twitch muscle.
    Keywords:  MYBPC2; contraction; distal arthrogryposis; fMyBP-C; skeletal muscle
  9. J Sports Sci. 2021 Apr 19. 1-23
      This systematic review and meta-analysis determined resistance training (RT) load effects on various muscle hypertrophy, strength, and neuromuscular performance task [e.g., countermovement jump (CMJ)] outcomes. Relevent studies comparing higher-load [>60% 1-repetition maximum (RM) or <15-RM] and lower-load (≤60% 1-RM or ≥ 15-RM) RT were identified, with 45 studies (from 4713 total) included in the meta-analysis. Higher- and lower-load RT induced similar muscle hypertrophy at the whole-body (lean/fat-free mass; [ES (95% CI) = 0.05 (-0.20 to 0.29), P = 0.70]), whole-muscle [ES = 0.06 (-0.11 to 0.24), P = 0.47], and muscle fibre [ES = 0.29 (-0.09 to 0.66), P = 0.13] levels. Higher-load RT further improved 1-RM [ES = 0.34 (0.15 to 0.52), P = 0.0003] and isometric [ES = 0.41 (0.07 to 0.76), P = 0.02] strength. The superiority of higher-load RT on 1-RM strength was greater in younger [ES = 0.34 (0.12 to 0.55), P = 0.002] versus older [ES = 0.20 (-0.00 to 0.41), P = 0.05] participants. Higher- and lower-load RT therefore induce similar muscle hypertrophy (at multiple physiological levels), while higher-load RT elicits superior 1-RM and isometric strength. The influence of RT loads on neuromuscular task performance is however unclear.
    Keywords:  Strength; load; muscle hypertrophy; resistance training; systematic review
  10. Front Cell Dev Biol. 2021 ;9 646482
      Skeletal muscle anabolic resistance (i.e., the decrease in muscle protein synthesis (MPS) in response to anabolic stimuli such as amino acids and exercise) has been identified as a major cause of age-related sarcopenia, to which blunted nutrition-sensing contributes. In recent years, it has been suggested that a leucine sensor may function as a rate-limiting factor in skeletal MPS via small-molecule GTPase. Leucine-sensing and response may therefore have important therapeutic potential in the steady regulation of protein metabolism in aging skeletal muscle. This paper systematically summarizes the three critical processes involved in the leucine-sensing and response process: (1) How the coincidence detector mammalian target of rapamycin complex 1 localizes on the surface of lysosome and how its crucial upstream regulators Rheb and RagB/RagD interact to modulate the leucine response; (2) how complexes such as Ragulator, GATOR, FLCN, and TSC control the nucleotide loading state of Rheb and RagB/RagD to modulate their functional activity; and (3) how the identified leucine sensor leucyl-tRNA synthetase (LARS) and stress response protein 2 (Sestrin2) participate in the leucine-sensing process and the activation of RagB/RagD. Finally, we discuss the potential mechanistic role of exercise and its interactions with leucine-sensing and anabolic responses.
    Keywords:  age-related sarcopenia; anabolic resistance; leucine response; leucine-sensing; protein synthesis
  11. Ageing Res Rev. 2021 Apr 16. pii: S1568-1637(21)00091-X. [Epub ahead of print] 101344
      In the United Kingdom (UK), it is projected that by 2035 people aged >65 years will make up 23% of the population, with those aged >85 years accounting for 5% of the total population. Ageing is associated with progressive changes in muscle metabolism and a decline in functional capacity, leading to a loss of independence. Muscle metabolic changes associated with ageing have been linked to alterations in muscle architecture and declines in muscle mass and insulin sensitivity. However, the biological features often attributed to muscle ageing are also seen in controlled studies of physical inactivity (e.g. reduced step-count and bed-rest), and it is currently unclear how many of these ageing features are due to ageing per se or sedentarism. This is particularly relevant at a time of home confinements reducing physical activity levels during the Covid-19 pandemic. Current knowledge gaps include the relative contribution that physical inactivity plays in the development of many of the negative features associated with muscle decline in older age. Similarly, data demonstrating positive effects of government recommended physical activity guidelines on muscle health are largely non-existent. It is imperative therefore that research examining interactions between ageing, physical activity and muscle mass and metabolic health is prioritised so that it can inform on the "normal" muscle ageing process and on strategies for improving health span and well-being. This review will focus on important changes in muscle architecture and metabolism that accompany ageing and highlight the likely contribution of physical inactivity to these changes.
    Keywords:  ageing; metabolism; muscle; physical inactivity; sarcopenia
  12. Geroscience. 2021 Apr 23.
      AMP-activated protein kinase (AMPK) is a central regulator of both lifespan and health across multiple model organisms. β-Guanidinopropionic acid (GPA) is an endogenous AMPK activator previously shown to improve metabolic function in young and obese mice. In this study, we tested whether age of administration significantly affects the physiological outcomes of GPA administration in mice. We report that intervention starting at 7-8 months (young) results in activation of AMPK signaling and a phenotype consisting of lower body mass, improved glucose control, enhanced exercise tolerance, and altered mitochondrial electron transport chain flux similar to previous reports. When GPA treatment is started at 18-19 months (old), the effect of GPA on AMPK signaling is blunted compared to younger mice despite similar accumulation of GPA in skeletal muscle. Even so, GPA administration in older animals delayed age-related declines in lean mass, improved measures of gait performance and circadian rhythm, and increased fat metabolism as measured by respiratory exchange ratio. These results are likely partially driven by the relative difference in basal function and metabolic plasticity between young and old mice. Our results suggest that age-related declines in AMPK sensitivity may limit potential strategies targeting AMPK signaling in older subjects and suggest that further research and development is required for AMPK activators to realize their full potential.
    Keywords:  AMPK; Aging; Beta-guanidinopropionic acid; Healthspan
  13. Life Sci. 2021 Apr 20. pii: S0024-3205(21)00467-7. [Epub ahead of print] 119482
      AIM: Fibrosis is the most common complication from chronic diseases, and yet no therapy capable of mitigating its effects is available. Our goal is to unveil specific signaling regulating the fibrogenic process and to identify potential small molecule candidates that block fibrogenic differentiation of fibro/adipogenic progenitors.METHOD: We performed a large-scale drug screen using muscle-resident fibro/adipogenic progenitors from a mouse model expressing EGFP under the Collagen1a1 promotor. We first confirmed that the EGFP was expressed in response to TGFβ1 stimulation in vitro. Then we treated cells with TGFβ1 alone or with drugs from two libraries of known compounds. The drugs ability to block the fibrogenic differentiation was quantified by imaging and flow cytometry. From a two-rounds screening, positive hits were tested in vivo in the mice model for the Duchenne muscular dystrophy (mdx mice). The histopathology of the muscles was assessed with picrosirius red (fibrosis) and laminin staining (myofiber size).
    KEY FINDINGS: From the in vitro drug screening, we identified 21 drugs and tested 3 in vivo on the mdx mice. None of the three drugs significantly improved muscle histopathology.
    SIGNIFICANCE: The in vitro drug screen identified various efficient compounds, none of them strongly inhibited fibrosis in skeletal muscle of mdx mice. To explain these observations, we hypothesize that in Duchenne Muscular Dystrophy, in which fibrosis is a secondary event due to chronic degeneration and inflammation, the drugs tested could have adverse effect on regeneration or inflammation, balancing off any positive effects and leading to the absence of significant results.
    Keywords:  Drug screening; Fibro/adipogenic progenitors; Fibrosis; Repair; Skeletal muscle
  14. Med Sci Sports Exerc. 2021 Apr 15.
      INTRODUCTION: Whether short-term, single-mode exercise training can improve physical fitness prior to a period of reduced physical activity (e.g. post-surgery recovery) is not well characterized in clinical populations nor middle-age adults. We investigated skeletal muscle adaptive responses following endurance exercise training (ENT), high-intensity interval training (HIIT) or resistance exercise training (RET), and a subsequent period of detraining, in sedentary, middle-age men.METHODS: Thirty-five sedentary, males (39±3 yr) were randomized to parallel groups and undertook six weeks of either ENT (n=12), HIIT (n=12) or RET (n=11) followed by 2.5 weeks of detraining. Skeletal muscle fiber characteristics, body composition, muscle thickness, muscle strength, aerobic capacity, resting energy expenditure and glucose homeostasis were assessed at baseline, and after exercise training and detraining.
    RESULTS: Lean mass increased after RET and HIIT (+3.2±1.6% and +1.6±2.1%, P<0.05). Muscle strength (sum of leg press, leg extension and bench press 1RMs) increased after all training interventions (RET: +25±5%; HIIT: +10±5%; ENT: +7±7%, P<0.05). Aerobic capacity increased only after HIIT and ENT (+14±7% and +11±11%, P<0.05). Type I and II muscle fiber size increased for all groups post-training (main effect of time, P<0.05). Following a period of detraining, the gains in lean mass and maximal muscle strength were maintained in RET and HIIT groups, but maximal aerobic capacity declined below post-training levels in HIIT and ENT (P<0.05).
    CONCLUSION: Six weeks of HIIT induced widespread adaptations prior to detraining in middle-age men. Exercise training-induced increases in aerobic capacity declined during 2.5 weeks of detraining but gains in lean mass and muscle strength were maintained.
  15. iScience. 2021 Apr 23. 24(4): 102303
      Androgens have a robust effect on skeletal muscles to increase muscle mass and strength. The molecular mechanism of androgen/androgen receptor (AR) action on muscle strength is still not well known, especially for the regulation of sarcomeric genes. In this study, we generated androgen-induced hypertrophic model mice, myofiber-specific androgen receptor knockout (cARKO) mice supplemented with dihydrotestosterone (DHT). DHT treatment increased grip strength in control mice but not in cARKO mice. Transcriptome analysis by RNA-seq, using skeletal muscles obtained from control and cARKO mice treated with or without DHT, identified a fast-type muscle-specific novel splicing variant of Myosin light-chain kinase 4 (Mylk4) as a target of AR in skeletal muscles. Mylk4 knockout mice exhibited decreased maximum isometric torque of plantar flexion and passive stiffness of myofibers due to reduced phosphorylation of Myomesin 1 protein. This study suggests that androgen-induced skeletal muscle strength is mediated with Mylk4 and Myomesin 1 axis.
    Keywords:  Animal Physiology; Endocrinology; Molecular Biology; Molecular Physiology
  16. Life Sci. 2021 Apr 19. pii: S0024-3205(21)00505-1. [Epub ahead of print]277 119520
      AIMS: Mouse bone marrow mesenchymal stem cells (BMSCs) are pluripotent cells with self-renewal and differentiation abilities. Since the effects of senescent BMSCs on C2C12 cells are not fully clear, the present study aimed to elucidate these effects.MAIN METHODS: Senescence-associated β-galactosidase staining and western blotting were performed to confirm the senescence of BMSCs. Immunofluorescence and western blotting were used to assess myoblast differentiation in each group. The role of the AKT/P70 signaling pathway and forkhead box O3 (FOXO3) nuclear translocation was explored by western blotting. BMSC-derived exosomes were injected into the tibialis anterior of mice, and RT-qPCR was used to assess the role of exosomes in promoting muscle differentiation.
    KEY FINDINGS: Conditioned medium (CM) from early-senescent BMSCs promoted myogenic differentiation in vitro, which was detected as enhanced expression of myosin heavy chain (MHC), myogenin (MYOG), and myogenic differentiation 1 (MyoD). The AKT signaling pathway was found to be regulated by CM, which inhibited FOXO3 nuclear translocation. RT-qPCR analysis results showed that MHC, MyoD, and MYOG mRNA expression increased in the tibialis anterior of mice after exosome injection.
    SIGNIFICANCE: The present study demonstrated that early-senescent BMSCs accelerated C2C12 cell myogenic differentiation, and the transcription factor, FOXO3, was the target of senescent cells. Collectively, our results suggest that the AKT/P70 signaling pathway mediates the effect of BMSCs on neighboring cells.
    Keywords:  Bone mesenchymal stem cells; C2C12 cells; FOXO3; Myogenic differentiation; Senescence
  17. Nat Commun. 2021 Apr 23. 12(1): 2431
      Insulin resistance and lower muscle quality (strength divided by mass) are hallmarks of type 2 diabetes (T2D). Here, we explore whether alterations in muscle stem cells (myoblasts) from individuals with T2D contribute to these phenotypes. We identify VPS39 as an important regulator of myoblast differentiation and muscle glucose uptake, and VPS39 is downregulated in myoblasts and myotubes from individuals with T2D. We discover a pathway connecting VPS39-deficiency in human myoblasts to impaired autophagy, abnormal epigenetic reprogramming, dysregulation of myogenic regulators, and perturbed differentiation. VPS39 knockdown in human myoblasts has profound effects on autophagic flux, insulin signaling, epigenetic enzymes, DNA methylation and expression of myogenic regulators, and gene sets related to the cell cycle, muscle structure and apoptosis. These data mimic what is observed in myoblasts from individuals with T2D. Furthermore, the muscle of Vps39+/- mice display reduced glucose uptake and altered expression of genes regulating autophagy, epigenetic programming, and myogenesis. Overall, VPS39-deficiency contributes to impaired muscle differentiation and reduced glucose uptake. VPS39 thereby offers a therapeutic target for T2D.
  18. Autophagy. 2021 Apr 20. 1-3
      Autophagic lysosome reformation (ALR) recycles autolysosome membranes formed during autophagy, to make lysosomes and is essential for continued autophagy function. Localized membrane remodeling on autolysosomes leads to the extension of reformation tubules, which undergo scission to form new lysosomes. The phosphoinositides phosphatidylinositol-4-phosphate (PtdIns4P) and phosphatidylinositol-4,5-bisphosphate (PtdIns[4,5]P2) induce this remodeling by recruiting protein effectors to membranes. We identified the inositol polyphosphate 5-phosphatase INPP5K, which converts PtdIns(4,5)P2 to PtdIns4P is essential for ALR in skeletal muscle. INPP5K mutations that reduce its 5-phosphatase activity are known to cause muscular dystrophy, via an undefined mechanism. We generated skeletal muscle-specific inpp5k knockout mice which exhibited severe muscle disease, with lysosome depletion and marked autophagy inhibition. This was due to decreased PtdIns4P and increased PtdIns(4,5)P2 on autolysosomes, causing reduced scission of reformation tubules. ALR was restored in cells with loss of INPP5K by expression of wild-type INPP5K, but not muscle-disease causing mutants. Therefore on autolysosomes, both PtdIns(4,5)P2 generation and its removal by INPP5K is required for completion of ALR. Furthermore, skeletal muscle shows a dependence on the membrane recycling ALR pathway to maintain lysosome homeostasis and ensure the protective role of autophagy against disease.
    Keywords:  Autophagic lysosome reformation; INPP5K; PtdIns(4, 5)P2; PtdIns4P; autophagy; inositol polyphosphate 5-phosphatase; lysosome; muscular dystrophy; phosphoinositide; skeletal muscle
  19. Exp Physiol. 2021 Apr 20.
      NEW FINDINGS: What is the central question of this study? The purpose of this study was to determine if the nucleotides in a nucleoprotein diet could ameliorate the unloading-associated decrease in soleus muscle mass and fiber size. What is the main finding and its importance? The results indicate that the nucleotides in the nucleoprotein-enriched diet could ameliorate the unloading-associated decrease in type I fiber size and muscle mass most likely due to the activation of protein synthesis pathways and satellite cell proliferation and differentiation via ERK1/2 phosphorylation. Thus nucleotide supplementation appears to be an effective countermeasure for muscle atrophy.ABSTRACT: Hindlimb unloading decreases both protein synthesis pathway and satellite cell activation results in muscle atrophy. Nucleotides are included in nucleoprotein and provide the benefits of increasing ERK1/2 phosphorylation. ERK 1/2 phosphorylation is also important in the activation of satellite cells, especially for myoblast proliferation and stimulating protein synthesis pathways. Therefore, we hypothesized that nucleotide in the nucleoproteins would ameliorate muscle atrophy via increasing the protein synthesis pathways and satellite cell activation during hindlimb unloading in rat soleus. Twenty-four female Wistar rats were divided into four groups: control rats fed a basal diet without nucleoprotein (CON), control rats fed a nucleoprotein-enriched diet (CON+NP), hindlimb unloaded rats fed a basal diet (HU), or hindlimb unloaded rats fed a NP diet (HU+NP). HU for two-weeks resulted in decreases in p70S6K- and rpS6-phosphorylation, the numbers of MyoD and myogenin, type I muscle fiber size, and muscle mass. Both CON and HU rats fed the NP diet showed an increase in ERK1/2, p70S6K- and rpS6-phosphorylation, and in the number of MyoD and myogenin compared to their basal diet groups. The NP diet also ameliorated the unloading-associated decrease in type I muscle fiber size and muscle mass. The results indicate that the nucleotides in the nucleoprotein-enriched diet could ameliorate the unloading-associated decrease in type I fiber size and muscle mass most likely due to the activation of protein synthesis pathways and satellite cell proliferation and differentiation via ERK1/2 phosphorylation. Thus nucleotide supplementation appears to be an effective countermeasure for muscle atrophy. This article is protected by copyright. All rights reserved.
    Keywords:  nucleoprotein; protein synthesis pathways; satellite cells
  20. Sci Rep. 2021 Apr 21. 11(1): 8574
      Resistance exercise exerts beneficial effects on glycemic control, which could be mediated by exercise-induced humoral factors released in the bloodstream. Here, we used C57Bl/6 healthy mice, submitted to resistance exercise training for 10 weeks. Trained mice presented higher muscle weight and maximum voluntary carrying capacity, combined with reduced body weight gain and fat deposition. Resistance training improved glucose tolerance and reduced glycemia, with no alterations in insulin sensitivity. In addition, trained mice displayed higher insulinemia in fed state, associated with increased glucose-stimulated insulin secretion. Islets from trained mice showed reduced expression of genes related to endoplasmic reticulum (ER) stress, associated with increased expression of Ins2. INS-1E beta-cells incubated with serum from trained mice displayed similar pattern of insulin secretion and gene expression than isolated islets from trained mice. When exposed to CPA (an ER stress inducer), the serum from trained mice partially preserved the secretory function of INS-1E cells, and prevented CPA-induced apoptosis. These data suggest that resistance training, in healthy mice, improves glucose homeostasis by enhancing insulin secretion, which could be driven, at least in part, by humoral factors.
  21. J Physiol. 2021 Apr 21.
      KEY POINTS: Few days of unloading are sufficient to induce a decline of skeletal muscle mass and function; notably, contractile force is lost at a faster rate than mass. The reasons behind this disproportionate loss of muscle force are still poorly understood. We provide strong evidence of two mechanisms only hypothesized until now for the rapid muscle force loss in only 10 days of bed rest. Our results show that an initial neuromuscular junction instability, accompanied by alterations in the innervation status and impairment of single fibres sarcoplasmic reticulum function contribute to the loss of contractile force in front of a preserved  myofibrillar function and central activation capacity. Early onset of neuromuscular junction instability and impairment in calcium dynamics involved in E-C coupling are proposed as eligible determinants to the greater decline in muscle force than in muscle size during unloading.ABSTRACT: Unloading induces rapid skeletal muscle atrophy and functional decline. Importantly, force is lost at a much higher rate than muscle mass. We aimed to investigate the early determinants of the disproportionate loss of force compared to that of muscle mass in response to unloading. Ten young participants underwent 10-day bed rest (BR). At baseline (BR0) and at 10 days (BR10), quadriceps femoris (QF) volume (VOL) and isometric maximum voluntary contraction (MVC) were assessed. At BR0 and BR10 blood samples and biopsies of vastus lateralis (VL) muscle were collected. Neuromuscular junction (NMJ) stability and myofibre innervation status were assessed, together with single fibre mechanical properties and sarcoplasmic reticulum (SR) calcium handling. From BR0 to BR10, QFVOL and MVC decreased by 5.2% (p = .003) and 14.3% (p<.001), respectively. Initial and partial denervation was detected from increased neural cell adhesion molecule (NCAM) positive myofibres at BR10 compared to BR0 (+3.4%, p = .016). NMJ instability was further inferred from increased C-terminal agrin fragment (CAF) concentration in serum (+19.2% at BR10, p = .031). Fast fibre CSA showed a trend to decrease by 15% (p = .055) at BR10, while single fibre maximal tension (force/CSA) was unchanged. However, at BR10 sarcoplasmic reticulum Ca2+ release in response to caffeine decreased by 35.1% (P<.002) and 30.2% (P<.001) in fast and slow fibres, respectively, pointing to an impaired excitation-contraction coupling. These findings support the view that the early onset of NMJ instability and impairment in SR function are eligible mechanisms contributing to the greater decline in muscle force than in muscle size during unloading. This article is protected by copyright. All rights reserved.
    Keywords:  Ca2+ dynamics; NCAM; Neuromuscular junction instability; muscle atrophy; sarcoplasmic reticulum; single fibres atrophy; single fibres contractile impairment; unloading
  22. Trends Endocrinol Metab. 2021 Apr 19. pii: S1043-2760(21)00068-0. [Epub ahead of print]
      Muscle cachexia has a major detrimental impact on cancer patients, being responsible for 30% of all cancer deaths. It is characterized by a debilitating loss in muscle mass and function, which ultimately deteriorates patients' quality of life and dampens therapeutic treatment efficacy. Muscle cachexia stems from widespread alterations in whole-body metabolism as well as immunity and neuroendocrine functions and these global defects often culminate in aberrant signaling within skeletal muscle, causing muscle protein breakdown and attendant muscle atrophy. This review summarizes recent landmark discoveries that significantly enhance our understanding of the molecular etiology of cancer-driven muscle cachexia and further discuss emerging therapeutic approaches seeking to simultaneously target those newly discovered mechanisms to efficiently curb this lethal syndrome.
    Keywords:  inflammatory cytokines; muscle cachexia; muscle protein breakdown; muscle wasting; therapeutics targeting cachexia
  23. J Physiol. 2021 Apr 19.
       A decrease in protein synthesis plays a major role in the loss of muscle mass that occurs in response to immobilization.  In mice, immobilization leads to a rapid (within 6 hr) and progressive decrease in the rate of protein synthesis and this effect is mediated by a decrease in translational efficiency.  Deep proteomic and phosphoproteomic analyses of mouse skeletal muscles revealed that the rapid immobilization-induced decrease in protein synthesis cannot be explained by changes in the abundance or phosphorylation state of proteins that have been implicated in the regulation of translation. ABSTRACT: The disuse of skeletal muscle, such as that which occurs during immobilization, can lead to the rapid loss of muscle mass and a decrease in the rate of protein synthesis plays a major role in this process. Indeed, current dogma contends that the decrease in protein synthesis is mediated by changes in the activity of protein kinases (e.g., mTOR); however, the validity of this model has not been established. Therefore, to address this, we first subjected mice to 6, 24, or 72 hr of unilateral immobilization and then used the SUnSET technique to measure changes in the relative rate of protein synthesis. The result of our initial experiments revealed that immobilization leads to a rapid (within 6 hr) and progressive decrease in the rate of protein synthesis and that this effect is mediated by a decrease in translational efficiency. We then performed a deep mass spectrometry-based analysis to determine whether this effect could be explained by changes in the expression and/or phosphorylation state of proteins that regulate translation. From this analysis, we were able to quantify 4,320 proteins and 15,020 unique phosphorylation sites, and surprisingly, the outcomes revealed that the rapid immobilization-induced decrease in protein synthesis could not be explained by changes in either the abundance, or phosphorylation state, of proteins. The results of our work not only challenge the current dogma in the field, but also provide an expansive resource of information for future studies that are aimed at defining how disuse leads to loss of muscle mass. This article is protected by copyright. All rights reserved.
    Keywords:  atrophy; immobilization; protein synthesis; proteomics; skeletal muscle
  24. Nutr Metab (Lond). 2021 Apr 21. 18(1): 44
      The protein supplement industry is expanding rapidly and estimated to have a multi-billion market worth. Recent research has centred on understanding how the manufacturing processes of protein supplements may impact muscle recovery and remodeling. The hydrolysed forms of protein undergo a further heating extraction process during production which may contribute to amino acids (AA) appearing in circulation at a slightly quicker rate, or greater amplitude, than the intact form. Whilst the relative significance of the rate of aminoacidemia to muscle protein synthesis is debated, it has been suggested that protein hydrolysates, potentially through the more rapid delivery and higher proportion of di-, tri- and smaller oligo-peptides into circulation, are superior to intact non-hydrolysed proteins and free AAs in promoting skeletal muscle protein remodeling and recovery. However, despite these claims, there is currently insufficient evidence to support superior muscle anabolic properties compared with intact non-hydrolysed proteins and/or free AA controls. Further research is warranted with appropriate protein controls, particularly in populations consuming insufficient amounts of protein, to support and/or refute an important muscle anabolic role of protein hydrolysates. The primary purpose of this review is to provide the reader with a current perspective on the potential anabolic effects of protein hydrolysates in individuals wishing to optimise recovery from, and maximise adaptation to, exercise training.
    Keywords:  Concentrates; Hydrolysates; Isolates; Muscle protein anabolism; Protein synthesis; Supplementation
  25. Exp Gerontol. 2021 Apr 16. pii: S0531-5565(21)00133-9. [Epub ahead of print]150 111358
      INTRODUCTION: Sarcopenia is the loss of skeletal muscle mass and function. It is a major health issue in old age due to lack of understanding of the origin and molecular mechanism. Altered dietary pattern, sedentary lifestyle and physical inactivity have shown adverse effect of skeletal muscle function. Sedentary behaviour and low protein intake have been well associated with sarcopenia. Here, we aim to develop Sarcopenia mimicking murine model to observe the physiological and biochemical changes with physical activity intervention. We also intended to find the association of muscle stem cells and stress induced protein Sestrins in the developed sarcopenic model.METHODS: Male C57BL/6 mice were categorized into 4 groups: young-control (Y-Cntrl), aged-matched control (A-Cntrl), Sarcopenic-model (SAR-model) and Sarcopenic intervention group (SAR-INT) with physical exercises. SAR-model group was kept in a retrofitted confined cage for sedentary lifestyle and was fed with protein-restricted diet. Phenotypic assessment for body mass, grip strength and functional endurance was analysed to confirm the sarcopenic state. Mitochondrial enzymatic assessment, muscle stem cell (MuSCs) proliferation potential and protein quantification of Sestrins expression were performed by enzyme histochemistry, flow cytometry and surface plasmon resonance (SPR), respectively. SAR-model group was given 10 weeks physical activity intervention to assess the physiological and biochemical changes.
    RESULTS: Simultaneous implementation of physical inactivity by sedentary confinement and protein restricted diet led the animals to exhibit the features of sarcopenia. SAR-model group showed a decline of 8.6% (p < 0.0001) in the body weight assessment, 32% decline (p < 0.0001) in the grip strength, 28% increase in time elapsed (p < 0.0001) indicating decline in functional performance. Mitochondrial enzymes (ATPase, NADH-TR and SDH/COX) assessment exhibited low expression in SAR-model group. Ki67 positive muscle stem cell declines around 50% in the model group. SPR quantification of Sestrin 2 showed a decline of 14% which significantly improved to 28% upon physical activity intervention (p = 0.0025) in SAR-INT group.
    CONCLUSION: It can be summarized that the mouse model generated in the present study mimics the feature of human Sarcopenia. Physical activity intervention may improve the sarcopenic status via modulation of Sestrin 2 which can serve as potential molecule for therapeutic implication.
    Keywords:  Aging; Muscle stem cells; Physical activity; Sarcopenia; Sestrins
  26. Mater Sci Eng C Mater Biol Appl. 2021 Apr;pii: S0928-4931(21)00142-9. [Epub ahead of print]123 112003
      Muscle tissue possess an innate regenerative potential that involves an extremely complicated and synchronized process on which resident muscle stem cells play a major role: activate after an injury, differentiate and fuse originating new myofibers for muscle repair. Considerable efforts have been made to design new approaches based on material systems to potentiate muscle repair by engineering muscle extracellular matrix and/or including soluble factors/cells in the media, trying to recapitulate the key biophysical and biochemical cues present in the muscle niche. This work proposes a different and simple approach to potentiate muscle regeneration exploiting the interplay between specific cell membrane receptors. The simultaneous stimulation of borate transporter, NaBC1 (encoded by SLC4A11gene), and fibronectin-binding integrins induced higher number and size of focal adhesions, major cell spreading and actin stress fibers, strengthening myoblast attachment and providing an enhanced response in terms of myotube fusion and maturation. The stimulated NaBC1 generated an adhesion-driven state through a mechanism that involves simultaneous NaBC1/α5β1/αvβ3 co-localization. We engineered and characterized borax-loaded alginate hydrogels for an effective activation of NaBC1 in vivo. After inducing an acute injury with cardiotoxin in mice, active-NaBC1 accelerated the muscle regeneration process. Our results put forward a new biomaterial approach for muscle repair.
    Keywords:  Alginate hydrogels; Borax; Integrins; Muscle regeneration; NaBC1 transporter (SLC4A11)
  27. Elife. 2021 Apr 20. pii: e61138. [Epub ahead of print]10
      Aging is accompanied by disrupted information flow, resulting from accumulation of molecular mistakes. These mistakes ultimately give rise to debilitating disorders including skeletal muscle wasting, or sarcopenia. To derive a global metric of growing 'disorderliness' of aging muscle, we employed a statistical physics approach to estimate the state parameter, entropy, as a function of genes associated with hallmarks of aging. Escalating network entropy reached an inflection point at old age, while structural and functional alterations progressed into oldest-old age. To probe the potential for restoration of molecular 'order' and reversal of the sarcopenic phenotype, we systemically overexpressed the longevity protein, Klotho, via AAV. Klotho overexpression modulated genes representing all hallmarks of aging in old and oldest-old mice, but pathway enrichment revealed directions of changes were, for many genes, age-dependent. Functional improvements were also age-dependent. Klotho improved strength in old mice, but failed to induce benefits beyond the entropic tipping point.
    Keywords:  computational biology; mouse; regenerative medicine; stem cells; systems biology
  28. Mol Ther Nucleic Acids. 2021 Jun 04. 24 403-415
      CRISPR gene therapy is one promising approach for treatment of Duchenne muscular dystrophy (DMD), which is caused by a large spectrum of mutations in the dystrophin gene. To broaden CRISPR gene editing strategies for DMD treatment, we report the efficient restoration of dystrophin expression in induced myotubes by SpCas9 and dual guide RNAs (gRNAs). We first sequenced 32 deletion junctions generated by this editing method and revealed that non-homologous blunt-end joining represents the major indel type. Based on this predictive repair outcome, efficient in-frame deletion of a part of DMD exon 51 was achieved in HEK293T cells with plasmids expressing SpCas9 and dual gRNAs. More importantly, we further corrected a frameshift mutation in human DMD (exon45del) fibroblasts with SpCas9-dual gRNA ribonucleoproteins. The edited DMD fibroblasts were transdifferentiated into myotubes by lentiviral-mediated overexpression of a human MYOD transcription factor. Restoration of DMD expression at both the mRNA and protein levels was confirmed in the induced myotubes. With further development, the combination of SpCas9-dual gRNA-corrected DMD patient fibroblasts and transdifferentiation may provide a valuable therapeutic strategy for DMD.
    Keywords:  CRISPR; Cas9; Duchenne muscular dystrophy; gene therapy; muscle regeneration; regenerative medicine; reprogramming; transdifferentiation
  29. Int J Gen Med. 2021 ;14 1263-1273
      Sarcopenia is a multifactorial disease related to aging, chronic inflammation, insufficient nutrition, and physical inactivity. Previous studies have suggested that there is a relationship between sarcopenia and gut microbiota,namely, the gut-muscle axis. The present review highlights that the gut microbiota can affect muscle mass and muscle function from inflammation and immunity,substance and energy metabolism, endocrine and insulin sensitivity, etc., directly or indirectly establishing a connection with sarcopenia, thereby realizing the "gut-muscle axis".
    Keywords:  gut microbiota; inflammation; metabolism; muscle
  30. Front Endocrinol (Lausanne). 2021 ;12 651211
      Lipid overload of the mitochondria is linked to the development of insulin resistance in skeletal muscle which may be a contributing factor to the progression of type 2 diabetes during obesity. The targeted degradation of mitochondria through autophagy, termed mitophagy, contributes to the mitochondrial adaptive response to changes in dietary fat. Our previous work demonstrates long-term (2-4 months) consumption of a high-fat diet increases mitochondrial lipid oxidation capacity but does not alter markers of mitophagy in mice. The purpose of this study was to investigate initial stages of mitochondrial respiratory adaptations to high-fat diet and the activation of mitophagy. C57BL/6J mice consumed either a low-fat diet (LFD, 10% fat) or high-fat diet (HFD, 60% fat) for 3 or 7 days. We measured skeletal muscle mitochondrial respiration and protein markers of mitophagy in a mitochondrial-enriched fraction of skeletal muscle. After 3 days of HFD, mice had lower lipid-supported oxidative phosphorylation alongside greater electron leak compared with the LFD group. After 7 days, there were no differences in mitochondrial respiration between diet groups. HFD mice had greater autophagosome formation potential (Beclin-1) and greater activation of mitochondrial autophagy receptors (Bnip3, p62) in isolated mitochondria, but no difference in downstream autophagosome (LC3II) or lysosome (Lamp1) abundance after both 3 and 7 days compared with the LFD groups. In cultured myotubes, palmitate treatment decreased mitochondrial membrane potential and hydrogen peroxide treatment increased accumulation of upstream mitophagy markers. We conclude that several days of high-fat feeding stimulated upstream activation of skeletal muscle mitophagy, potentially through lipid-induced oxidative stress, without downstream changes in respiration.
    Keywords:  autophagy; high-fat feeding; mitochondria; reactive oxygen species; respiration
  31. Eur J Sport Sci. 2021 Apr 23. 1-27
      Sex steroids, commonly referred to as sex hormones, are integral to the development and maintenance of the human reproductive system. In addition, male (androgens) and female (estrogens and progestogens) sex hormones promote the development of secondary sex characteristics by targeting a range of other tissues, including skeletal muscle. The role of androgens on skeletal muscle mass, function and metabolism has been well described in males, yet female specific studies are scarce in the literature.This narrative review summarizes the available evidence around the mechanistic role of androgens, estrogens and progestogens in female skeletal muscle. An analysis of the literature indicates that sex steroids play important roles in the regulation of female skeletal muscle mass and function. The free fractions of testosterone and progesterone in serum were consistently associated with the regulation of muscle mass, while estrogens may be primarily involved in mediating the muscle contractile function in conjunction with other sex hormones. Muscle strength was however not directly associated with any hormone in isolation when at physiological concentrations. Importantly, recent evidence suggests that intramuscular sex hormone concentrations may be more strongly associated with muscle size and function than circulating forms, providing interesting opportunities for future research.By combining cross-sectional, interventional and mechanical studies, this review aims to provide a broad, multidisciplinary picture of the current knowledge of the effects of sex steroids on skeletal muscle in females, with a focus on the regulation of muscle size and function and an insight into their clinical implications.
    Keywords:  Body Composition; Endocrinology; Musculoskeletal; Physiology
  32. J Rehabil Med Clin Commun. 2020 ;3 1000041
      Introduction: Spinal and bulbar muscular atrophy is a progressive neuromuscular disease that leads to muscle weakness and reduced physical function. Benefits of physical therapy for people with spinal and bulbar muscular atrophy have not been reported in the literature.Case report: A 62-year-old male patient with spinal and bulbar muscular atrophy reported falling, difficulty walking and completing upright tasks, and showed clinical signs of low baseline function on examination. Transportation challenges made it difficult for this patient to attend frequent one-on-one physical therapy sessions.
    Interventions and outcomes: A minimally supervised home-based exercise intervention was chosen with the goal of safely improving his functional capacity. The 5-visit clinical intervention, spread over 10 months, provided 3 exercise modules: seated-to-standing postural alignment and core muscle activation; upright functional and endurance training; and balance training and rhythmic walking. Post-intervention the patient had increased lower extremity muscle strength, improved balance, and reduced self-reported fatigue.
    Conclusion: Home-based exercises were well tolerated with no increase in creatine kinase. Multiple clinical measures of strength and function improved, possibly related to the patients' excellent motivation and compliance with the programme. Promising utilization of a minimally supervised home-based programme is described here.
    Keywords:  Kennedy’s disease; exercise; motor neurone disease; spinal and bulbar muscular atrophy
  33. J Appl Physiol (1985). 2021 Apr 22.
      While it has long been known that contraction robustly stimulates skeletal muscle glucose uptake, the molecular steps regulating this increase remain incompletely defined. The mammalian ortholog of Sir2, sirtuin 1 (SIRT1), is an NAD+-dependent protein deacetylase that is thought to link perturbations in energy flux associated with exercise to subsequent cellular adaptations. Nevertheless, its role in contraction-stimulated glucose uptake has not been described. The objective of this study was to determine the importance of SIRT1 to contraction-stimulated glucose uptake in mouse skeletal muscle. Using a radioactive 2-deoxyglucose uptake (2DOGU) approach, we measured ex vivo glucose uptake in unstimulated (rested) and electrically-stimulated (100 Hz contraction every 15s for 10 min; contracted) extensor digitorum longus (EDL) and soleus from ~15 week old male and female mice with muscle-specific knockout of SIRT1 deacetylase activity (mKO) and their wildtype (WT) littermates. Skeletal muscle force decreased over the contraction protocol, although there were no differences in the rate of fatigue between genotypes. In EDL and soleus, depletion of SIRT1 deacetylase activity did not affect contraction-induced increase in glucose uptake in either sex. Interestingly, the absolute rate of contraction-stimulated 2DOGU was ~1.4-fold higher in female compared to male mice, regardless of muscle type. Taken together, our findings demonstrate that SIRT1 is not required for contraction-stimulated glucose uptake in mouse skeletal muscle. Moreover, to our knowledge, this is the first demonstration of sex-based differences in contraction-stimulated glucose uptake in mouse skeletal muscle.
    Keywords:  2-deoxyglucose; deacetylase; exercise; sex dimorphism
  34. Am J Physiol Endocrinol Metab. 2021 Apr 19.
      We recently reported that compared to males, female mice have increased hepatic mitochondrial respiratory capacity and are protected against high-fat diet-induced steatosis. Here we sought to determine the role of estrogen in hepatic mitochondrial function, steatosis, and bile acid metabolism in female mice, as well as investigate potential benefits of exercise in the absence or presence of estrogen via ovariectomy (OVX). Female C57BL mice (n=6 per group) were randomly assigned to sham surgery (Sham), ovariectomy (OVX), or OVX plus estradiol replacement therapy (OVX+Est). Half of the mice in each treatment group were sedentary (SED) or had access to voluntary wheel running (VWR). All mice were fed a high-fat diet (HFD) and were housed at thermoneutral temperatures. We assessed isolated hepatic mitochondrial respiratory capacity using the Oroboros O2k with both pyruvate and palmitoylcarnitine as substrates. As expected, OVX mice presented with greater hepatic steatosis, weight gain, and fat mass gain compared to Sham and OVX+Est animals. Hepatic mitochondrial coupling (Basal/State 3 respiration) with pyruvate was impaired following OVX, but both VWR and estradiol treatment rescued coupling to levels greater than or equal to Sham animals. Estradiol and exercise also had different effects on liver electron transport chain protein expression depending on OVX status. Markers of bile acid metabolism and excretion were also impaired by ovariectomy but rescued with estradiol add-back. Together our data suggest that estrogen depletion impairs hepatic mitochondrial function and liver health, and that estradiol replacement and modest exercise can aid in rescuing this phenotype.
    Keywords:  fatty liver; menopause; metabolism
  35. Biomed Pharmacother. 2021 Apr 14. pii: S0753-3322(21)00375-9. [Epub ahead of print]139 111590
      Muscle atrophy and weakness are the adverse effects of long-term or high dose usage of glucocorticoids. In the present study, we explored the effects of fucoxanthin (10 μM) on dexamethasone (10 μM)-induced atrophy in C2C12 myotubes and investigated its underlying mechanisms. The diameter of myotubes was observed under a light microscope, and the expression of myosin heavy chain (MyHC), proteolysis-related, autophagy-related, apoptosis-related, and mitochondria-related proteins was analyzed by western blots or immunoprecipitation. Fucoxanthin alleviates dexamethasone-induced muscle atrophy in C2C12 myotubes, indicated by increased myotubes diameter and expression of MyHC, decreased expression of muscle atrophy F-box (Atrogin-1) and muscle ring finger 1 (MuRF1). Through activating SIRT1, fucoxanthin inhibits forkhead box O (FoxO) transcriptional activity to reduce protein degradation, induces autophagy to enhance degraded protein clearance, promotes mitochondrial function and diminishes apoptosis. In conclusion, we identified fucoxanthin ameliorates dexamethasone induced C2C12 myotubes atrophy through SIRT1 activation.
    Keywords:  Atrophy; C2C12; Dexamethasone; Fucoxanthin; SIRT1
  36. J Clin Endocrinol Metab. 2021 Apr 23. pii: dgab261. [Epub ahead of print]
      CONTEXT: Previous investigations on skeletal muscle health in type 1 diabetes (T1D) has generally focused on later stages of disease progression where comorbidities are present and are posited as a primary mechanism of muscle dysfunction.OBJECTIVE: To investigate skeletal muscle function and morphology across the adult lifespan in those with and without T1D.
    DESIGN: Participants underwent maximal contraction (MVC) testing, resting muscle biopsy and venous blood sampling.
    SETTING: Procedures in this study were undertaken at the McMaster University Medical Centre.
    PARTICIPANTS: Sixty-five healthy adult (18-78 years old) men/males and women/females [T1D=34; control=31] matched for age/biological sex/body mass index (BMI)/self-reported physical activity levels were included.
    MAIN OUTCOME MEASURES: Our primary measure in this study was MVC, with supporting histological/immunofluorescent measures.
    RESULTS: After 35 years of age ('older adults'), MVC declined quicker in T1D subjects compared to controls. Loss of strength in T1D was accompanied by morphological changes associated with accelerated aging. Type 1 myofiber grouping was higher in T1D, and the groups were larger and more numerous than in controls. Older T1D females exhibited more myofibers expressing multiple myosin heavy chain isoforms (hybrid fibers) than controls, another feature of accelerated aging. Conversely, T1D males exhibited a shift towards type 2 fibers, with less evidence of myofiber grouping or hybrid fibers.
    CONCLUSIONS: These data suggest impairments to skeletal muscle function and morphology exist in T1D. The decline in strength with T1D is accelerated after 35 years of age and may be responsible for the earlier onset of frailty which characterizes those with diabetes.
    Keywords:  T1D; T1DM; diabetic myopathy; function; muscle morphometrics; skeletal muscle function
  37. Proteomics. 2021 Apr 24. e2000301
      Spinal muscular atrophy (SMA) is an autosomal recessive disorder that represents a significant cause of infant mortality. SMA is characterized by reduced levels of the Survival Motor Neuron protein leading to the loss of alpha motor neurons in the spinal cord and brain stem as well as defects in peripheral tissues such as skeletal muscle and liver. With progress in promising therapies such as antisense oligonucleotide and gene replacement, there remains a need to better understand disease subtypes and develop biomarkers for improved diagnostics and therapeutic monitoring. In this study, we have examined the utility of extracellular vesicles as a source of biomarker discovery in patient-derived fibroblast cells. Proteome examination utilizing data-independent acquisition and ion mobility mass spectrometry identified 684 protein groups present in all biological replicates tested. Label-free quantitative analysis identified 116 statistically significant protein alterations compared to control cells, including several known SMA biomarkers. Protein level differences were also observed in regulators of Wnt signaling and Cajal bodies. Finally, levels of insulin growth factor binding protein-3 was validated as being significantly higher in extracellular vesicles isolated from SMA cells. We conclude that extracellular vesicles represent a promising source for SMA biomarker discovery as well as a relevant constituent for advancing our understanding of SMA pathophysiology. This article is protected by copyright. All rights reserved.
    Keywords:  data independent acquisition; extracellular vesicles; ion mobility; label-free proteomics; spinal muscular atrophy
  38. Front Cell Dev Biol. 2021 ;9 652652
      PAX3 belongs to the paired-homeobox family of transcription factors and plays a key role as an upstream regulator of muscle progenitor cells during embryonic development. Pax3-mutant embryos display impaired somite development, yet the consequences for myotome formation have not been characterized. The early myotome is formed by PAX3-expressing myogenic cells that delaminate from the dermomyotomal lips and migrate between the dermomyotome and sclerotome where they terminally differentiate. Here we show that in Pax3-mutant embryos, myotome formation is impaired, displays a defective basal lamina and the regionalization of the structural protein Desmin is lost. In addition, this phenotype is more severe in embryos combining Pax3-null and Pax3 dominant-negative alleles. We identify the adhesion molecule M-Cadherin as a PAX3 target gene, the expression of which is modulated in the myotome according to Pax3 gain- and loss-of-function alleles analyzed. Taken together, we identify M-Cadherin as a PAX3-target linked to the formation of the myotome.
    Keywords:  CDH15; M-Cadherin; PAX3; myogenesis; myotome
  39. FASEB J. 2021 May;35(5): e21587
      We examined the association between genotype and resistance training-induced changes (12 wk) in dual x-ray energy absorptiometry (DXA)-derived lean soft tissue mass (LSTM) as well as muscle fiber cross-sectional area (fCSA; vastus lateralis; n = 109; age = 22 ± 2 y, BMI = 24.7 ± 3.1 kg/m2 ). Over 315 000 genetic polymorphisms were interrogated from muscle using DNA microarrays. First, a targeted investigation was performed where single nucleotide polymorphisms (SNP) identified from a systematic literature review were related to changes in LSTM and fCSA. Next, genome-wide association (GWA) studies were performed to reveal associations between novel SNP targets with pre- to post-training change scores in mean fCSA and LSTM. Our targeted investigation revealed no genotype-by-time interactions for 12 common polymorphisms regarding the change in mean fCSA or change in LSTM. Our first GWA study indicated no SNP were associated with the change in LSTM. However, the second GWA study indicated two SNP exceeded the significance level with the change in mean fCSA (P = 6.9 × 10-7 for rs4675569, 1.7 × 10-6 for rs10263647). While the former target is not annotated (chr2:205936846 (GRCh38.p12)), the latter target (chr7:41971865 (GRCh38.p12)) is an intron variant of the GLI Family Zinc Finger 3 (GLI3) gene. Follow-up analyses indicated fCSA increases were greater in the T/C and C/C GLI3 genotypes than the T/T GLI3 genotype (P < .05). Data from the Auburn cohort also revealed participants with the T/C and C/C genotypes exhibited increases in satellite cell number with training (P < .05), whereas T/T participants did not. Additionally, those with the T/C and C/C genotypes achieved myonuclear addition in response to training (P < .05), whereas the T/T participants did not. In summary, this is the first GWA study to examine how polymorphisms associate with the change in hypertrophy measures following resistance training. Future studies are needed to determine if the GLI3 variant differentiates hypertrophic responses to resistance training given the potential link between this gene and satellite cell physiology.
    Keywords:  GLI3; hypertrophy; polymorphisms; skeletal muscle
  40. J Vis Exp. 2021 Apr 03.
      Investigations into both the pathophysiology and therapeutic targets in muscular dystrophies have been hampered by the limited proliferative capacity of human myoblasts. Several mouse models have been created but they either do not truly represent the human physiopathology of the disease or are not representative of the broad spectrum of mutations found in humans. The immortalization of human primary myoblasts is an alternative to this limitation; however, it is still dependent on muscle biopsies, which are invasive and not easily available. In contrast, skin biopsies are easier to obtain and less invasive to patients. Fibroblasts derived from skin biopsies can be immortalized and transdifferentiated into myoblasts, providing a source of cells with excellent myogenic potential. Here, we describe a fast and direct reprogramming method of fibroblast into a myogenic lineage. Fibroblasts are transduced with two lentiviruses: hTERT to immortalize the primary culture and a tet-inducible MYOD, which upon the addition of doxycycline, induces the conversion of fibroblasts into myoblasts and then mature myotubes, which express late differentiation markers. This quick transdifferentiation protocol represents a powerful tool to investigate pathological mechanisms and to investigate innovative gene-based or pharmacological biotherapies for neuromuscular disorders.
  41. Sports Med. 2021 Apr 19.
      INTRODUCTION: Understanding the impact of lockdown upon resistance training (RT), and how people adapted their RT behaviours, has implications for strategies to maintain engagement in similar positive health behaviours. Further, doing so will provide a baseline for investigation of the long-term effects of these public health measures upon behaviours and perceptions, and facilitate future follow-up study.OBJECTIVES: To determine how the onset of coronavirus (COVID-19), and associated 'lockdown', affected RT behaviours, in addition to motivation, perceived effectiveness, enjoyment, and intent to continue, in those who regularly performed RT prior to the pandemic.
    METHODS: We conducted an observational, cross-sectional study using online surveys in multiple languages (English, Danish, French, German, Italian, Portuguese, Slovakian, Swedish, and Japanese) distributed across social media platforms and through authors' professional and personal networks. Adults (n = 5389; median age = 31 years [interquartile range (IQR) = 25, 38]), previously engaged in RT prior to lockdown (median prior RT experience = 7 years [IQR = 4, 12]) participated. Outcomes were self-reported RT behaviours including: continuation of RT during lockdown, location of RT, purchase of specific equipment for RT, method of training, full-body or split routine, types of training, repetition ranges, exercise number, set volumes (per exercise and muscle group), weekly frequency of training, perception of effort, whether training was planned/recorded, time of day, and training goals. Secondary outcomes included motivation, perceived effectiveness, enjoyment, and intent to continue RT.
    RESULTS: A majority of individuals (82.8%) maintained participation in RT during-lockdown. Marginal probabilities from generalised linear models and generalised estimating equations for RT behaviours were largely similar from pre- to during-lockdown. There was reduced probability of training in privately owned gyms (~ 59% to ~ 7%) and increased probability of training at home (~ 18% to ~ 89%); greater probability of training using a full-body routine (~ 38% to ~ 51%); reduced probability of resistance machines (~ 66% to ~ 13%) and free weight use (~ 96% to ~ 81%), and increased probability of bodyweight training (~ 62% to ~ 82%); reduced probability of moderate repetition ranges (~ 62-82% to ~ 55-66%) and greater probability of higher repetition ranges (~ 27% to ~ 49%); and moderate reduction in the perception of effort experienced during-training (r = 0.31). Further, individuals were slightly less likely to plan or record training during lockdown and many changed their training goals. Additionally, perceived effectiveness, enjoyment, and likelihood of continuing current training were all lower during-lockdown.
    CONCLUSIONS: Those engaged in RT prior to lockdown these behaviours with only slight adaptations in both location and types of training performed. However, people employed less effort, had lower motivation, and perceived training as less effective and enjoyable, reporting their likelihood of continuing current training was similar or lower than pre-lockdown. These results have implications for strategies to maintain engagement in positive health behaviours such as RT during-restrictive pandemic-related public health measures. PRE-REGISTRATION: .
    PREPRINT: The preprint version of this work is available on SportRχiv: .
  42. J Neuromuscul Dis. 2021 Mar 27.
      BACKGROUND: Skeletal muscle biopsy is one of the gold standards in the diagnostic workup of muscle disorders. By histopathologic analysis, characteristic features like inflammatory cellular infiltrations, fat and collagen replacement of muscle tissue or structural defects of the myofibers can be detected. In the past years, novel quantitative MRI (qMRI) techniques have been developed to quantify tissue parameters, thus providing a non-invasive diagnostic tool in several myopathies.OBJECTIVE: This proof-of-principle study was performed to validate the qMRI-techniques to skeletal muscle biopsy results.
    METHODS: Ten patients who underwent skeletal muscle biopsy for diagnostic purposes were examined by qMRI. Fat fraction, water T2-time and diffusion parameters were measured in the muscle from which the biopsy was taken. The proportion of fat tissue, the severity of degenerative and inflammatory parameters and the amount of type 1- and type 2- muscle fibers were determined in all biopsy samples. The qMRI-data were then correlated to the histopathological findings.
    RESULTS: The amount of fat tissue in skeletal muscle biopsy correlated significantly with the fat fraction derived from the Dixon sequence. The water T2-time, a parameter for tissue edema, correlated with the amount of vacuolar changes of myofibers and endomysial macrophages in the histopathologic analysis. No significant correlations were found for diffusion parameters.
    CONCLUSION: In this proof-of-principle study, qMRI techniques were related to characteristic histopathologic features in neuromuscular disorders. The study provides the basis for further development of qMRI methods in the follow-up of patients with neuromuscular disorders, especially in the context of emerging treatment strategies.
    Keywords:  Neuromuscular diseases; histology; magnetic resonance imaging; muscle tissue; myositis
  43. Lancet Diabetes Endocrinol. 2021 05;pii: S2213-8587(21)00054-1. [Epub ahead of print]9(5): 304-317
      There has been substantial progress in the knowledge of exercise and type 1 diabetes, with the development of guidelines for optimal glucose management. In addition, an increasing number of people living with type 1 diabetes are pushing their physical limits to compete at the highest level of sport. However, the post-exercise recovery routine, particularly with a focus on sporting performance, has received little attention within the scientific literature, with most of the focus being placed on insulin or nutritional adaptations to manage glycaemia before and during the exercise bout. The post-exercise recovery period presents an opportunity for maximising training adaption and recovery, and the clinical management of glycaemia through the rest of the day and overnight. The absence of clear guidance for the post-exercise period means that people with type 1 diabetes should either develop their own recovery strategies on the basis of individual trial and error, or adhere to guidelines that have been developed for people without diabetes. This Review provides an up-to-date consensus on post-exercise recovery and glucose management for individuals living with type 1 diabetes. We aim to: (1) outline the principles and time course of post-exercise recovery, highlighting the implications and challenges for endurance athletes living with type 1 diabetes; (2) provide an overview of potential strategies for post-exercise recovery that could be used by athletes with type 1 diabetes to optimise recovery and adaptation, alongside improved glycaemic monitoring and management; and (3) highlight the potential for technology to ease the burden of managing glycaemia in the post-exercise recovery period.
  44. Biofabrication. 2021 Apr 22.
      Among various bioreactors used in the field of tissue engineering and regenerative medicine, a magnetic bioreactor is more capable of providing steady force to the cells while avoiding direct manipulation of the materials. However, most of them are complex and difficult to fabricate, with drawbacks in terms of consistency and biocompatibility. In this study, a magnetic bioreactor system and a magnetic hydrogel were manufactured by single-stage three-dimensional printing with digital light processing (DLP) technique for differentiation of myoblast cells. The hydrogel was composed of a magnetic part containing iron oxide and glycidyl-methacrylated silk fibroin, and a cellular part printed by adding mouse myoblast cell (C2C12) to gelatin glycidyl methacrylate, that was placed in the magnetic bioreactor system to stimulate the cells in the hydrogel. The composite hydrogel was steadily printed by a one-stage layering technique using a DLP printer. The magnetic bioreactor offered mechanical stretching of the cells in the hydrogel in three-dimensional ways, so that the cellular differentiation could be executed in three dimensions just like the human environment. Cell viability, as well as gene expression using quantitative reverse transcription-polymerase chain reaction, were assessed after magneto-mechanical stimulation of the myoblast cell-embedded hydrogel in the magnetic bioreactor system. Comparison with the control group revealed that the magnetic bioreactor system accelerated differentiation of mouse myoblast cells in the hydrogel and increased myotube diameter and length in vitro. The DLP-printed magnetic bioreactor and the hydrogel were simply manufactured and easy-to-use, providing an efficient environment for applying noninvasive mechanical force via FDA-approved silk fibroin and iron oxide biocomposite hydrogel, to stimulate cells without any evidence of cytotoxicity, demonstrating the potential for application in muscle tissue engineering.
    Keywords:  3D printing; bioreactors; fibroins; hydrogels; iron oxide; myoblasts; silk
  45. Cureus. 2021 Mar 17. 13(3): e13947
      Soft tissue neoplastic metastases, specifically to the skeletal muscle, are uncommon in comparison to metastases within the abdomen, thorax, or intracranial regions. Specifically, pancreatic adenocarcinoma with skeletal muscle metastasis is a rare clinical phenomenon and is hardly reported. There is a high mortality rate after the diagnosis of metastatic pancreatic adenocarcinoma, attributed to inadequate screening and advanced staging upon incidental discovery. Rarely, metastatic lesions manifest in the skeletal muscle and are hardly documented. Some of the factors that deter skeletal muscle tumor implantation include the deficiency of skeletal muscle mediators and genetic makeup of the primary tumor. Surgical resection of pancreatic adenocarcinoma with adjuvant chemotherapy demonstrates the best prognosis; however, surgical management is usually limited to patients without known metastatic disease. Alternative treatment options such as chemotherapy and radiotherapy are used in the palliative care setting. Here, we present the case of a patient with previously diagnosed and treated pancreatic adenocarcinoma in remission, with biopsy-proven metastases isolated within the skeletal muscle.
    Keywords:  carbohydrate antigen 19-9; carcinoembryonic antigen; computed tomography; core needle biopsy; desmoplastic microenvironment; interventional radiology; pancreatic adenocarcinoma; positron emission tomography; skeletal muscle metastasis; whipple procedure
  46. J Hepatol. 2021 Apr 15. pii: S0168-8278(21)00234-8. [Epub ahead of print]
      BACKGROUND AND AIMS: To evaluate the association between muscle mass and fatty infiltration and biopsy-assessed NAFLD in patients with obesity.MATERIALS AND METHODS: At inclusion (n=184) and 12 months after a dietary intervention (n=15) or a bariatric surgery (n=24), we evaluated NAFLD with liver biopsy, skeletal muscle mass index with computed tomography (CT-SMI) and bioelectrical impedance analysis (BIA-SMI). We developed an index to evaluate absolute fat content in muscle (skeletal muscle fat index; SMFI) from CT-based psoas muscle density (SMFIPsoas).
    RESULTS: Muscle mass was higher in patients with NAFLD than in those without (CT-SMI 56.8±9.9 vs 47.4±6.5 cm2/m,2 p<0.0001). There was no association between sarcopenia and NASH. SMFIPsoas was higher in NASH ≥F2 and early NASH F0-1 than in NAFL (78.5±23.6 and 73.1±15.6 vs 61.2±12.6, p<0.001). One point in score for any of the individual cardinal NASH features (i.e. steatosis, inflammation or ballooning) associated with an increase in SMFIPsoas (all p<0.05). The association between SMFIPsoas and NASH was highly significant even after adjustment for multiple confounders (all p<0.025). After intervention (n=39), NASH improvement, defined by NAS <3 or 2-point NAS score reduction, was achieved in more than 75% of patients (n=25 or n=27, respectively) that had pre-established NASH at inclusion (n=32) and associated with a significant decrease in SMFIPsoas (p<0.001). Strikingly, all patients who had ≥11% reduction in SMFIPsoas achieved NASH improvement (14/14, p<0.05).
    CONCLUSIONS: Muscle fat content, but not muscle mass, is strongly and independently associated with NASH. All individuals who achieved a ≥11% decrease in SMFIPsoas after intervention improved their NASH. These data call to explore muscle fatty infiltration as a potential marker for (and perhaps a pathophysiological contributor to) NASH.
    LAY SUMMARY: The fat content in skeletal muscles is highly reflective of the severity of non-alcoholic fatty liver disease (NAFLD) in patients with morbid obesity. In particular, muscle fat content is strongly associated with non-alcoholic steatohepatitis (NASH) and decreases upon NASH improvement. These data call to explore muscle fatty infiltration as a potential marker and perhaps a pathophysiological contributor to NASH.
    Keywords:  CT-Scan; Muscle composition; Muscle fat; Muscle lipid; Muscle mass; Myosteatosis; NAFLD; NASH; Non-alcoholic steatohepatitis; Obesity; Sarcopenia; liver disease