bims-moremu Biomed News
on Molecular regulators of muscle mass
Issue of 2021‒11‒21
forty-one papers selected by
Anna Vainshtein
Craft Science Inc.

  1. Exp Cell Res. 2021 Nov 15. pii: S0014-4827(21)00463-8. [Epub ahead of print] 112907
      Skeletal muscles are composed of multinuclear cells called myofibers and have unique abilities, one of which is plasticity. In response to the mechanical load induced by physical activity, skeletal muscle exerts several local adaptations, including an increase in myofiber size and myonuclear number, known as muscle hypertrophy. Protein synthesis and muscle satellite cells (MuSCs) are mainly responsible for these adaptations. However, the upstream signaling pathways that promote protein synthesis remain controversial. Further, the necessity of MuSCs in muscle hypertrophy is also a highly debated issue. In this review, we summarized the insulin-like growth factor 1 (IGF-1)/Akt-independent activation of mammalian target of rapamycin (mTOR) signaling in muscle hypertrophy and the involvement of mTOR signaling in age-related loss of skeletal muscle function and mass and in sarcopenia. The roles and behaviors of MuSCs, characteristics of new myonuclei in muscle hypertrophy, and their relevance to sarcopenia have also been updated in this review.
    Keywords:  Akt-independent; Hypertrophy; mTOR; muscle satellite cell; myonuclei; sarcopenia
  2. Cell Death Dis. 2021 Nov 16. 12(12): 1089
      TNF-related apoptosis-inducing ligand (TRAIL) is a protein that induces apoptosis in cancer cells but not in normal ones, where its effects remain to be fully understood. Previous studies have shown that in high-fat diet (HFD)-fed mice, TRAIL treatment reduced body weight gain, insulin resistance, and inflammation. TRAIL was also able to increase skeletal muscle free fatty acid oxidation. The aim of the present work was to evaluate TRAIL actions on skeletal muscle. Our in vitro data on C2C12 cells showed that TRAIL treatment significantly increased myogenin and MyHC and other hallmarks of myogenic differentiation, which were reduced by Dr5 (TRAIL receptor) silencing. In addition, TRAIL treatment significantly increased AKT phosphorylation, which was reduced by Dr5 silencing, as well as glucose uptake (alone and in combination with insulin). Our in vivo data showed that TRAIL increased myofiber size in HFD-fed mice as well as in db/db mice. This was associated with increased myogenin and PCG1α expression. In conclusion, TRAIL/DR5 pathway promotes AKT phosphorylation, skeletal muscle differentiation, and glucose uptake. These data shed light onto a pathway that might hold therapeutic potential not only for the metabolic disturbances but also for the muscle mass loss that are associated with diabetes.
  3. Am J Physiol Regul Integr Comp Physiol. 2021 Nov 17.
      Macrophages are one of the top players when considering immune cells involved with tissue homeostasis. Recently, increasing evidence has demonstrated that these macrophages could also present two major subsets during tissue healing; proliferative macrophages (M1-like), which are responsible for increasing myogenic cell proliferation, and restorative macrophages (M2-like), which are accountable for the end of the mature muscle myogenesis. The participation and characterization of these macrophage subsets is critical during myogenesis, not only to understand the inflammatory role of macrophages during muscle recovery but also to create supportive strategies that can improve mass muscle maintenance. Indeed, most of our knowledge about macrophage subsets comes from skeletal muscle damage protocols, and we still do not know how these subsets can contribute to skeletal muscle adaptation. This narrative review aims to collect and discuss studies demonstrating the involvement of different macrophage subsets during the skeletal muscle damage/regeneration process, showcasing an essential role of these macrophage subsets during muscle adaptation induced by acute and chronic exercise programs.
    Keywords:  damage; exercise; hypertrophy; macrophages; skeletal muscle
  4. Hum Mol Genet. 2021 Nov 15. pii: ddab333. [Epub ahead of print]
      Spinal muscular atrophy (SMA) is characterized by the loss of alpha motor neurons in the spinal cord and a progressive muscle weakness and atrophy. SMA is caused by loss-of-function mutations and/or deletions in the survival of motor neuron (SMN) gene. The role of SMN in motor neurons has been extensively studied, but its function and the consequences of its loss in muscle has also emerged as a key aspect of SMA pathology. In this study, we explore the molecular mechanisms involved in muscle defects in SMA. First, we show in C2C12 myoblasts, that arginine methylation by CARM1 controls myogenic differentiation. More specifically, the methylation of HuR on K217 regulates HuR levels and subcellular localization during myogenic differentiation, and the formation of myotubes. Furthermore, we demonstrate that SMN and HuR interact in C2C12 myoblasts. Interestingly, the SMA-causing E134K point mutation within the SMN Tudor domain, and CARM1 depletion, modulate the SMN-HuR interaction. In addition, using the Smn2B/- mouse model, we report that CARM1 levels are markedly increased in SMA muscles and that HuR fails to properly respond to muscle denervation, thereby affecting the regulation of its mRNA targets. Altogether, our results show a novel CARM1-HuR axis in the regulation of muscle differentiation and plasticity as well as in the aberrant regulation of this axis caused by the absence of SMN in SMA muscle. With the recent developments of therapeutics targeting motor neurons, this study further indicates the need for more global therapeutic approaches for SMA.
  5. Exp Cell Res. 2021 Nov 15. pii: S0014-4827(21)00489-4. [Epub ahead of print] 112933
      The balance between proliferation and differentiation of muscle stem cells is tightly controlled, ensuring the maintenance of a cellular pool needed for muscle growth and repair. Muscle stem cells can proliferate, they can generate differentiating cells, or they self-renew to produce new stem cells. Notch signaling plays a crucial role in this process. Recent studies revealed that expression of the Notch effector HES1 oscillates in activated muscle stem cells. The oscillatory expression of HES1 periodically represses transcription from the genes encoding the myogenic transcription factor MYOD and the Notch ligand DLL1, thereby driving MYOD and DLL1 oscillations. This oscillatory network allows muscle progenitor cells and activated muscle stem cells to remain in a proliferative and 'undecided' state, in which they can either differentiate or self-renew. When HES1 is downregulated, MYOD oscillations become unstable and are replaced by sustained expression, which drives the cells into terminal differentiation. During development and regeneration, proliferating stem cells contact each other and the stability of the oscillatory expression depends on regular DLL1 inputs provided by neighboring cells. In such communities of cells that receive and provide Notch signals, the appropriate timing of DLL1 inputs is important, as sustained DLL1 cannot replace oscillatory DLL1. Thus, in cell communities, DLL1 oscillations ensure the appropriate balance between self-renewal and differentiation. In summary, oscillations in myogenic cells are an important example of dynamic gene expression determining cell fate.
    Keywords:  Dll1; Hes1; MyoD; Myogenesis; Myogenic differentiation; Notch signaling
  6. Neurobiol Dis. 2021 Nov 10. pii: S0969-9961(21)00308-9. [Epub ahead of print] 105559
      Skeletal muscle dysfunction may contribute to the progression and severity of amyotrophic lateral sclerosis (ALS). In the present study, we characterized the skeletal muscle pathophysiology in an inducible transgenic mouse model (rNLS8) that develops a TAR-DNA binding protein (TDP-43) proteinopathy and ALS-like neuropathology and disease progression; representative of >90% of all familial and sporadic ALS cases. As we previously observed elevated levels of miR-23a in skeletal muscle of patients with familial and sporadic ALS, we also investigated the effect of miR-23a suppression on skeletal muscle pathophysiology and disease severity in rNLS8 mice. Five weeks after disease onset TDP-43 protein accumulation was observed in tibialis anterior (TA), quadriceps (QUAD) and diaphragm muscle lysates and associated with skeletal muscle atrophy. In the TA muscle TDP-43 was detected in muscle fibres that appeared atrophied and angular in appearance and that also contained β-amyloid aggregates. These fibres were also positive for neural cell adhesion molecule (NCAM), but not embryonic myosin heavy chain (eMHC), indicating TDP-43/ β-amyloid localization in denervated muscle fibres. There was an upregulation of genes associated with myogenesis and NMJ degeneration and a decrease in the MURF1 atrophy-related protein in skeletal muscle. Suppression of miR-23a impaired rotarod performance and grip strength and accelerated body weight loss during early stages of disease progression. This was associated with increased AchRα mRNA expression and decreased protein levels of PGC-1α. The TDP-43 proteinopathy-induced impairment of whole body and skeletal muscle functional performance is associated with muscle wasting and elevated myogenic and NMJ stress markers. Suppressing miR-23a in the rNLS8 mouse model of ALS contributes to an early acceleration of disease progression as measured by decline in motor function.
    Keywords:  Amyotrophic lateral sclerosis; Motor neuron disease; Myogenic regulatory factors; Skeletal muscle; TDP-43; miR-23a; rNLS8 mice; β-Amyloid
  7. Appl Physiol Nutr Metab. 2021 Nov 16.
      Metformin and exercise both improve glycemic control, but in vitro studies have indicated that an interaction between metformin and exercise occurs in skeletal muscle, suggesting a blunting effect of metformin on exercise training adaptations. Two studies (a double-blind, parallel-group, randomized clinical trial conducted in 29 glucose-intolerant individuals and a double-blind, cross-over trial conducted in 15 healthy lean males) were included in this paper. In both studies, the effect of acute exercise +/- metformin treatment on different skeletal muscle variables, previously suggested to be involved in a pharmaco-physiological interaction between metformin and exercise, was assessed. Furthermore, in the parallel-group trial, the effect of 12 weeks of exercise training was assessed. Skeletal muscle biopsies were obtained before and after acute exercise and 12 weeks of exercise training, and mitochondrial respiration, oxidative stress and AMPK activation was determined. Metformin did not significantly affect the effects of acute exercise or exercise training on mitochondrial respiration, oxidative stress or AMPK activation, indicating that the response to acute exercise and exercise training adaptations in skeletal muscle is not affected by metformin treatment. Further studies are needed to investigate whether an interaction between metformin and exercise is present in other tissues, e.g. the gut. Trial registration: (NCT03316690 and NCT02951260). Novelty bullets • Metformin does not affect exercise-induced alterations in mitochondrial respiratory capacity in human skeletal muscle • Metformin does not affect exercise-induced alterations in systemic levels of oxidative stress nor emission of reactive oxygen species from human skeletal muscle • Metformin does not affect exercise-induced AMPK activation in human skeletal muscle.
  8. FASEB J. 2021 Dec;35(12): e22034
      Mutation to the gene encoding dystrophin can cause Duchenne muscular dystrophy (DMD) and increase the sensitivity to stress in vertebrate species, including the mdx mouse model of DMD. Behavioral stressors can exacerbate some dystrophinopathy phenotypes of mdx skeletal muscle and cause hypotension-induced death. However, we have discovered that a subpopulation of mdx mice present with a wildtype-like response to mild (forced downhill treadmill exercise) and moderate (scruff restraint) behavioral stressors. These "stress-resistant" mdx mice are more physically active, capable of super-activating the hypothalamic-pituitary-adrenal and renin-angiotensin-aldosterone pathways following behavioral stress and they express greater levels of mineralocorticoid and glucocorticoid receptors in striated muscle relative to "stress-sensitive" mdx mice. Stress-resistant mdx mice also presented with a less severe striated muscle histopathology and greater exercise and skeletal muscle oxidative capacity at rest. Most interestingly, female mdx mice were more physically active following behavioral stressors compared to male mdx mice; a response abolished after ovariectomy and rescued with estradiol. We demonstrate that the response to behavioral stress greatly impacts disease severity in mdx mice suggesting the management of stress in patients with DMD be considered as a therapeutic approach to ameliorate disease progression.
    Keywords:  blood pressure; dystrophin; estrogen; hypothalamic pituitary adrenal axis; skeletal muscle
  9. Front Physiol. 2021 ;12 758316
      Obesity and diabetes have been shown to interfere with energy metabolism and cause peripheral insulin resistance in skeletal muscle. However, recent studies have focused on the effect metabolic insult has on the loss of muscle size, strength, and physical function. Contractile dysfunction has been linked to impaired intracellular Ca2+ concentration ([Ca2+]i) regulation. In skeletal muscle, [Ca2+]i homeostasis is highly regulated by Ca2+ transport across the sarcolemma/plasma membrane, the golgi apparatus, sarcoplasmic reticulum (SR), and mitochondria. Particularly, the SR and or mitochondria play an important role in the fine-tuning of this metabolic process. Recent studies showed that obesity and insulin resistance are associated with interactions between the SR and mitochondrial networks (the dynamic tubular reticulum formed by mitochondria), suggesting that metabolic disorders alter Ca2+ handling by these organelles. These interactions are facilitated by specific membrane proteins, including ion channels. This review considers the impact of metabolic disorders, such as obesity and type 2 diabetes, on the regulation of [Ca2+]i in skeletal muscle. It also discusses the mechanisms by which this occurs, focusing chiefly on the SR and mitochondria networks. A deeper understanding of the effect of metabolic disorders on calcium handling might be useful for therapeutic strategies.
    Keywords:  calcium; diabetes; mitochondria; obesity; sarcoplasmic reticulum; skeletal muscle
  10. Stem Cell Reports. 2021 Nov 06. pii: S2213-6711(21)00549-X. [Epub ahead of print]
      One major challenge in realizing cell-based therapy for treating muscle-wasting disorders is the difficulty in obtaining therapeutically meaningful amounts of engraftable cells. We have previously described a method to generate skeletal myogenic progenitors with exceptional engraftability from pluripotent stem cells via teratoma formation. Here, we show that these cells are functionally expandable in vitro while retaining their in vivo regenerative potential. Within 37 days in culture, teratoma-derived skeletal myogenic progenitors were expandable to a billion-fold. Similar to their freshly sorted counterparts, the expanded cells expressed PAX7 and were capable of forming multinucleated myotubes in vitro. Importantly, these cells remained highly regenerative in vivo. Upon transplantation, the expanded cells formed new DYSTROPHIN+ fibers that reconstituted up to 40% of tibialis anterior muscle volume and repopulated the muscle stem cell pool. Our study thereby demonstrates the possibility of producing large quantities of engraftable skeletal myogenic cells for transplantation.
    Keywords:  cell therapy; muscle stem cells; muscular dystrophy; myogenic differentiation; pluripotent stem cells
  11. Kobe J Med Sci. 2021 Sep 30. 67(2): E48-E54
      An experimental animal model that causes mild structural disorders of skeletal muscles is essential to understand general exercise-induced muscle damage. Thermal stimulations such as icing and heating are commonly used as treatments for muscle injuries in sports. We established a downhill running (DR) protocol that leads to structural muscle disorders without sarcolemmal disruption and directly compared the structural changes produced by icing and heating after DR. Male ddY mice were divided into the DR, DR plus icing (Ice), and DR plus heating (Heat) groups. All mice ran at 20 m/min, -20% grade on a treadmill for a total of 90 min (three rounds of 30 min). In the Ice and Heat groups, an ice pack and a hot pack were, respectively, applied to the exercised triceps brachii muscles for 20 min just after DR. The proportion of myofibers with structural disorders was higher in the Ice group than in the DR and Heat groups at days 1 and 7 after DR. Moreover, the structural disorder of myofibers was slightly improved in the Heat group at day 1 after DR compared with the DR group. These findings suggest that icing treatment might aggravate the structural changes after DR.
    Keywords:  Cryotherapy; Downhill running; Exercise-induced muscle damage; Hyperthermia therapy; Structural change
  12. Front Bioeng Biotechnol. 2021 ;9 782333
      In native skeletal muscle, densely packed myofibers exist in close contact with surrounding motor neurons and blood vessels, which are embedded in the fibrous connective tissue. In comparison to conventional two-dimensional (2D) cultures, the three-dimensional (3D) engineered skeletal muscle models allow structural and mechanical resemblance with native skeletal muscle tissue by providing geometric confinement and physiological matrix stiffness to the cells. In addition, various external stimuli applied to these models enhance muscle maturation along with cell-cell and cell-extracellular matrix interaction. Therefore, 3D in vitro muscle models can adequately recapitulate the pathophysiologic events occurring in tissue-tissue interfaces inside the native skeletal muscle such as neuromuscular junction. Moreover, 3D muscle models can induce pathological phenotype of human muscle dystrophies such as Duchenne muscular dystrophy by incorporating patient-derived induced pluripotent stem cells and human primary cells. In this review, we discuss the current biofabrication technologies for modeling various skeletal muscle tissue-related diseases (i.e., muscle diseases) including muscular dystrophies and inflammatory muscle diseases. In particular, these approaches would enable the discovery of novel phenotypic markers and the mechanism study of human muscle diseases with genetic mutations.
    Keywords:  disease modelling; extrusion printing; hiPSC; muscular dystrophy; self-repair; volumetric muscle loss
  13. Physiology (Bethesda). 2021 Nov 15.
      Research conducted over the last 50 years has provided insight into the mechanisms by which insulin stimulates glucose transport across the skeletal muscle cell membrane. Transport alone, however, does not result in net glucose uptake as freeglucose equilibrates across the cell membrane and is not metabolized. Glucose uptake requires that glucose is phosphorylated by hexokinases. Phosphorylated glucosecannot leave the cell and is the substrate for metabolism. It is indisputable that glucose phosphorylation is essential for glucose uptake. Major advances have been made in defining the regulation of the insulin-stimulated glucose transporter, GLUT4, in skeletalmuscle. By contrast, the insulin-regulated hexokinase, hexokinase II parallels RobertFrost's Road Not Taken. Here the case is made that an understanding of glucosephosphorylation by hexokinase II is necessary to define the regulation of skeletal muscle glucose uptake in health and insulin resistance. Results of studies from different physiological disciplines that have elegantly described how hexokinase II can beregulated are summarized to provide a framework for potential application to skeletal muscle. Mechanisms by which hexokinase II is regulated in skeletal muscle await rigorous examination.
    Keywords:  GLUT4; glucose; hexokinase; insulin; muscle
  14. Neoplasia. 2021 Nov 16. pii: S1476-5586(21)00098-1. [Epub ahead of print]23(12): 1307-1317
      Cancer cachexia is characterized by systemic inflammation, protein degradation, and loss of skeletal muscle. Despite extensive efforts to develop therapeutics, only few effective treatments are available to protect against cancer cachexia. Here, we found that gintonin (GT), a ginseng-derived lysophosphatidic acid receptor (LPAR) ligand, protected C2C12 myotubes from tumor necrosis factor α (TNFα)/interferon γ (IFNγ)- induced muscle wasting condition. The activity of GT was found to be dependent on LPAR/Gαi2, as the LPAR antagonist Ki16425 and Gαi2 siRNA abolished the anti-atrophic effects of GT on myotubes. GT suppressed TNFα-induced oxidative stress by reducing reactive oxygen species and suppressing inflammation-related genes, such as interleukin 6 (IL-6) and NADPH oxidase 2 (NOX-2). In addition, GT exhibited anti-atrophy effects in primary normal human skeletal myoblasts. Further, GT protected against Lewis lung carcinoma cell line (LLC1)-induced cancer cachexia in a mouse model. Specifically, GT rescued the lower levels of grip strength, hanging, and cross-sectional area caused by LLC1. Collectively, our findings suggest that GT may be a good therapeutic candidate for protecting against cancer cachexia.
    Keywords:  Cancer cachexia; Gintonin; Muscle atrophy; Oxidative stress
  15. Am J Physiol Cell Physiol. 2021 Nov 17.
      The importance of defining sex differences across various biological and physiological mechanisms is more pervasive now than it has been over the last 15-20 years. As the muscle biology field pushes to identify small molecules and interventions to prevent, attenuate or even reverse muscle wasting, we must consider the effect of sex as a biological variable. It should not be assumed that a therapeutic will affect males and females with equal efficacy or equivalent target affinities under conditions where muscle wasting is observed. With that said, it is not surprising to find that we have an unclear or even a poor understanding of the effects of sex or sex hormones on muscle wasting conditions. Although recent investigations are beginning to establish experimental approaches that will allow investigators to assess the impact of sex-specific hormones on muscle wasting, the field still has not established enough published scientific tools that will allow the field to rigorously address critical hypotheses. Thus, the purpose of this review is to assemble a current summary of knowledge in the area of sexual dimorphism driven by estrogens with an effort to provide insights to interested physiologists on necessary considerations when trying to assess models for potential sex differences in cellular and molecular mechanisms of muscle wasting.
    Keywords:  cachexia; disuse; estradiol; sarcopenia; skeletal muscle
  16. Biochimie. 2021 Nov 16. pii: S0300-9084(21)00258-3. [Epub ahead of print]
      Type 2 diabetes is characterized by reduced insulin sensitivity, elevated blood metabolites, and reduced mitochondrial metabolism. Insulin resistant populations often exhibit reduced expression of genes governing mitochondrial metabolism such as peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). Interestingly, PGC-1α regulates the expression of branched-chain amino acid (BCAA) metabolism, and thus, the consistently observed increased circulating levels of BCAA in diabetics may be partially explained by reduced PGC-1α expression. Conversely, PGC-1α upregulation appears to increase BCAA catabolism. PGC-1α activity is regulated by 5'-AMP-activated protein kinase (AMPK), however, only limited experimental data exists on the effect of AMPK activation in the regulation of BCAA catabolism. The present report examined the effects of the commonly used AMPK activator 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) on the metabolism and expression of several related targets (including BCAA catabolic enzymes) of cultured myotubes. C2C12 myotubes were treated with AICAR at 1 mM for up to 24 h. Mitochondrial and glycolytic metabolism were measured via oxygen consumption and extracellular acidification rate, respectively. Metabolic gene and protein expression were assessed via qRT-PCR and western blot, respectively. AICAR treatment significantly increased mitochondrial content and peak mitochondrial capacity. AICAR treatment also increased AMPK activation and mRNA expression of several regulators of mitochondrial biogenesis but reduced glycolytic metabolism and mRNA expression of several glycolytic enzymes. Interestingly, branched-chain alpha-keto acid dehydrogenase a (BCKDHa) protein was significantly increased following AICAR-treatment suggesting increased overall BCAA catabolic capacity in AICAR-treated cells. Together, these experiments demonstrate AICAR/AMPK activation can upregulate BCAA catabolic machinery in a model of skeletal muscle.
    Keywords:  5′-AMP-activated protein kinase (AMPK); Branched-chain alpha-keto acid dehydrogenase (BCKDH); Mitochondrial biogenesis; Skeletal muscle
  17. Sci Rep. 2021 Nov 16. 11(1): 22360
      The dynamic synergy of genes and pathways in muscles in relation to age affects the muscle characteristics. Investigating the temporal changes in gene expression will help illustrate the molecular mechanisms underlying muscle development. Here we report the gene expression changes in skeletal muscles through successive age groups in Bandur, a meat type sheep of India. RNA sequencing data was generated from the longissimus thoracis muscles from four age groups, ranging from lamb to adult. Analysis of 20 highest expressed genes common across the groups revealed muscle protein, phosphorylation, acetylation, metal binding and transport as significant functions. Maximum differentiation was observed after 2.5-3 years on transition from lambs to adult. Transcriptional regulation by the TFAP2 transcription factors, IL-6 signaling and PI3K/AKT signaling pathways were enriched in younger animals. The gene-protein network demarcated key interactive genes involved in muscle development and proliferation that can be used as candidates for future research on improvement of muscle characteristics.
  18. Front Cell Dev Biol. 2021 ;9 764732
      The neuromuscular junction (NMJ) is a specialized cholinergic synaptic interface between a motor neuron and a skeletal muscle fiber that translates presynaptic electrical impulses into motor function. NMJ formation and maintenance require tightly regulated signaling and cellular communication among motor neurons, myogenic cells, and Schwann cells. Neuromuscular diseases (NMDs) can result in loss of NMJ function and motor input leading to paralysis or even death. Although small animal models have been instrumental in advancing our understanding of the NMJ structure and function, the complexities of studying this multi-tissue system in vivo and poor clinical outcomes of candidate therapies developed in small animal models has driven the need for in vitro models of functional human NMJ to complement animal studies. In this review, we discuss prevailing models of NMDs and highlight the current progress and ongoing challenges in developing human iPSC-derived (hiPSC) 3D cell culture models of functional NMJs. We first review in vivo development of motor neurons, skeletal muscle, Schwann cells, and the NMJ alongside current methods for directing the differentiation of relevant cell types from hiPSCs. We further compare the efficacy of modeling NMDs in animals and human cell culture systems in the context of five NMDs: amyotrophic lateral sclerosis, myasthenia gravis, Duchenne muscular dystrophy, myotonic dystrophy, and Pompe disease. Finally, we discuss further work necessary for hiPSC-derived NMJ models to function as effective personalized NMD platforms.
    Keywords:  disease modeling; drug development; human skeletal muscle; induced pluripotent stem cells; muscular dystrophy; neuromuscular junction; organ on a chip; tissue engineering
  19. Brain. 2021 Nov 11. pii: awab411. [Epub ahead of print]
      Genetic therapy has changed the prognosis of hereditary proximal spinal muscular atrophy, although treatment efficacy has been variable. There is a clear need for deeper understanding of underlying causes of muscle weakness and exercise intolerance in patients with this disease to further optimize treatment strategies. Animal models suggest that in addition to motor neuron and associated musculature degeneration, intrinsic abnormalities of muscle itself including mitochondrial dysfunction contribute to the disease etiology. To test this hypothesis in patients, we conducted the first in vivo clinical investigation of muscle bioenergetics. We recruited 15 patients and 15 healthy age and gender-matched control subjects in this cross-sectional clinico-radiological study. MRI and 31phosphorus magnetic resonance spectroscopy, the modality of choice to interrogate muscle energetics and phenotypic fiber type makeup, was performed of the proximal arm musculature in combination with fatiguing arm-cycling exercise and blood lactate testing. We derived bioenergetic parameter estimates including: blood lactate, intramuscular pH and inorganic phosphate accumulation during exercise, and muscle dynamic recovery constants. Linear correlation was used to test for associations between muscle morphological and bioenergetic parameters and clinico-functional measures of muscle weakness. MRI showed significant atrophy of triceps but not biceps muscles in patients. Maximal voluntary contraction force normalized to muscle cross-sectional area for both arm muscles was 1.4-fold lower in patients than in controls, indicating altered intrinsic muscle properties other than atrophy contributed to muscle weakness in this cohort. In vivo 31phosphorus magnetic resonance spectroscopy identified white-to-red remodeling of residual proximal arm musculature in patients on basis of altered intramuscular inorganic phosphate accumulation during arm-cycling in red versus white and intermediate myofibers. Blood lactate rise during arm-cycling was blunted in patients and correlated with muscle weakness and phenotypic muscle makeup. Post-exercise metabolic recovery was slower in residual intramuscular white myofibers in patients demonstrating mitochondrial ATP synthetic dysfunction in this particular fiber type. This study provides first in vivo evidence in patients that degeneration of motor neurons and associated musculature causing atrophy and muscle weakness in 5q spinal muscular atrophy type 3 and 4 is aggravated by disproportionate depletion of myofibers that contract fastest and strongest. Our finding of decreased mitochondrial ATP synthetic function selectively in residual white myofibers provides both a possible clue to understanding the apparent vulnerability of this particular fiber type in 5q spinal muscular atrophy type 3 and 4 as well as a new biomarker and target for therapy.
    Keywords:  Magnetic Resonance; Spinal muscular atrophy; exercise; metabolism; muscle
  20. J Am Coll Cardiol. 2021 Nov 23. pii: S0735-1097(21)06335-X. [Epub ahead of print]78(21): 2023-2037
      BACKGROUND: The combination of statin therapy and physical activity reduces cardiovascular disease risk in patients with hyperlipidemia more than either treatment alone. However, mitochondrial dysfunction associated with statin treatment could attenuate training adaptations.OBJECTIVES: This study determined whether moderate intensity exercise training improved muscle and exercise performance, muscle mitochondrial function, and fiber capillarization in symptomatic and asymptomatic statin users.
    METHODS: Symptomatic (n = 16; age 64 ± 4 years) and asymptomatic statin users (n = 16; age 64 ± 4 years) and nonstatin using control subjects (n = 20; age 63 ± 5 years) completed a 12-week endurance and resistance exercise training program. Maximal exercise performance (peak oxygen consumption), muscle performance and muscle symptoms were determined before and after training. Muscle biopsies were collected to assess citrate synthase activity, adenosine triphosphate (ATP) production capacity, muscle fiber type distribution, fiber size, and capillarization.
    RESULTS: Type I muscle fibers were less prevalent in symptomatic statin users than control subjects at baseline (P = 0.06). Exercise training improved muscle strength (P < 0.001), resistance to fatigue (P = 0.01), and muscle fiber capillarization (P < 0.01), with no differences between groups. Exercise training improved citrate synthase activity in the total group (P < 0.01), with asymptomatic statin users showing less improvement than control subjects (P = 0.02). Peak oxygen consumption, ATP production capacity, fiber size, and muscle symptoms remained unchanged in all groups following training. Quality-of-life scores improved only in symptomatic statin users following exercise training (P < 0.01).
    CONCLUSIONS: A moderate intensity endurance and resistance exercise training program improves muscle performance, capillarization, and mitochondrial content in both asymptomatic and symptomatic statin users without exacerbating muscle complaints. Exercise training may even increase quality of life in symptomatic statin users. (The Effects of Cholesterol-Lowering Medication on Exercise Performance [STATEX]; NL5972/NTR6346).
    Keywords:  combined exercise training; exercise performance; mitochondrial dysfunction; muscle fiber type distribution; statin-associated muscle symptoms
  21. Life Sci. 2021 Nov 10. pii: S0024-3205(21)01116-4. [Epub ahead of print]287 120129
      AIM: Itaconate (ITA), a derivative of the tricarboxylic acid cycle, has been documented to have a direct antimicrobial effect by inhibiting isocitrate lyase and suppressing proinflammatory cytokines in LPS-treated macrophages. However, the effects of dimethyl ITA (DITA), a membrane-permeable derivative of ITA, on insulin signaling and inflammation in skeletal muscle in an obese state remain to be elucidated. Thus, this study was designed to investigate the effects of DITA on the impairment of insulin signaling and inflammation in palmitate-treated C2C12 myocytes.MATERIALS AND METHODS: Western blotting was used to determine the expression of insulin signaling associated genes, inflammatory markers, fibroblast growth factor 21 (FGF21), and PPARδ expression, as well as AMPK phosphorylation in mouse skeletal muscle cells. Secreted proinflammatory cytokine levels were detected by enzyme-linked immunosorbent assay. Insulin signaling was assessed by glucose uptake assay.
    KEY FINDINGS: Treating C2C12 myocytes with DITA attenuated palmitate-induced aggravation of insulin signaling markers, such as insulin receptor substrate-1 (IRS-1) and Akt phosphorylation and inflammatory markers, such as NFκB and IκB phosphorylation. AMPK phosphorylation, as well as PPARδ and myokine FGF21 expression, were enhanced in C2C12 myocytes by DITA treatment. siRNA-mediated suppression of AMPK or FGF21 expression abolished the effects of DITA on insulin resistance and inflammation in palmitate-treated C2C12 myocytes.
    SIGNIFICANCE: In sum, DITA suppresses inflammation through the AMPK/FGF21/PPARδ signaling, thereby alleviating insulin resistance in palmitate-treated C2C12 myocytes. The current study appears to be an essential basis for performing animal experiments to develop insulin resistance therapeutics.
    Keywords:  AMPK; FGF21; Inflammation; Insulin resistance; Itaconate; PPARδ
  22. Mol Ther Nucleic Acids. 2021 Dec 03. 26 1066-1078
      Muscle atrophy is a common complication of many chronic diseases including heart failure, cancer cachexia, aging, etc. Unhealthy habits and usage of hormones such as dexamethasone can also lead to muscle atrophy. However, the underlying mechanisms of muscle atrophy are not completely understood. Non-coding RNAs (ncRNAs), such as microRNAs (miRNAs), long ncRNAs (lncRNAs), and circular RNAs (circRNAs), play vital roles in muscle atrophy. This review mainly discusses the regulation of ncRNAs in muscle atrophy induced by various factors such as heart failure, cancer cachexia, aging, chronic obstructive pulmonary disease (COPD), peripheral nerve injury (PNI), chronic kidney disease (CKD), unhealthy habits, and usage of hormones; highlights the findings of ncRNAs as common regulators in multiple types of muscle atrophy; and summarizes current therapies and underlying mechanisms for muscle atrophy. This review will deepen the understanding of skeletal muscle biology and provide new strategies and insights into gene therapy for muscle atrophy.
    Keywords:  aging; heart failure; lncRNA; microRNA; muscle atrophy; non-coding RNA
  23. JCI Insight. 2021 Nov 16. pii: e150112. [Epub ahead of print]
      BACKGROUND: Skeletal muscle maladaptation accompanies chronic kidney disease (CKD) and negatively impacts physical function. Emphasis in CKD has historically been placed on muscle fiber intrinsic deficits, such as altered protein metabolism and atrophy. However, targeted treatment of fiber intrinsic dysfunction has produced limited improvement, whereas alterations within the fiber extrinsic environment have scarcely been examined.METHODS: We investigated alterations to the skeletal muscle interstitial environment with deep cellular phenotyping of biopsies from patients with CKD compared to age-matched control participants and performed transcriptome profiling to define the molecular underpinnings of CKD-associated muscle impairments. We further examined changes in the observed muscle maladaptation following initiation of dialysis therapy for kidney failure.
    RESULTS: Patients with CKD exhibited a progressive fibrotic muscle phenotype, which was associated with impaired regenerative capacity and lower vascular density. The severity of these deficits was strongly associated with the degree of kidney dysfunction. Consistent with these profound deficits, CKD was associated with broad alterations to the muscle transcriptome, including altered extracellular matrix organization, downregulated angiogenesis, and altered expression of pathways related to stem cell self-renewal. Remarkably, despite the seemingly advanced nature of this fibrotic transformation, dialysis treatment rescued these deficits, restoring a healthier muscle phenotype. Furthermore, after accounting for muscle atrophy, strength and endurance improved after dialysis initiation.
    CONCLUSION: These data identify a dialysis-responsive muscle fibrotic phenotype in CKD and suggest that the early dialysis window presents a unique opportunity of improved muscle regenerative capacity during which targeted interventions may achieve maximal impact.
    Keywords:  Chronic kidney disease; Extracellular matrix; Muscle Biology; Nephrology; Skeletal muscle
  24. J Physiol. 2021 Nov 17.
      KEY POINTS: Larval muscle contraction force increases with stimulation frequency and duration, revealing substantial plasticity between 5 and 40 Hz. Fictive contraction recordings demonstrate endogenous motoneuron burst frequencies consistent with the neuromuscular system operating within the range of greatest plasticity. Genetic and pharmacological manipulation of critical components of pre- and post-synaptic Ca2+ regulation significantly impact the strength and time-course of muscle contractions. A screen for modulators of the excitation-contraction machinery identified a FMRFa peptide, TPAEDFMRFa, and its associated signaling pathway that dramatically increases muscle performance. Drosophila serves as an excellent model for dissecting components of the excitation-contraction coupling machinery.ABSTRACT: The Drosophila neuromuscular system is widely used to characterize synaptic development and function. However, little is known about how specific synaptic alterations effect neuromuscular transduction and muscle contractility that ultimately dictate behavioural output. Here we develop and use a force transducer system to characterize excitation-contraction coupling at Drosophila larval neuromuscular junctions (NMJs), examining how specific neuronal and muscle manipulations disrupt muscle contractility. Muscle contraction force increased with motoneuron stimulation frequency and duration, showing considerable plasticity between 5-40 Hz and saturating above 50 Hz. Endogenous recordings of fictive contractions revealed average motoneuron burst frequencies of 20-30 Hz, consistent with the system operating within this plastic range of contractility. Temperature was also a key factor in muscle contractility, as force was enhanced at lower temperatures and dramatically reduced with increasing temperatures. Pharmacological and genetic manipulations of critical components of Ca2+ regulation in both pre- and post-synaptic compartments impacted the strength and time-course of muscle contractions. A screen for modulators of muscle contractility led to identification and characterization of the molecular and cellular pathway by which the FMRFa peptide, TPAEDFMRFa, increases muscle performance. These findings indicate Drosophila NMJs provide a robust system to correlate synaptic dysfunction, regulation, and modulation, to alterations in excitation-contraction coupling.   This article is protected by copyright. All rights reserved.
    Keywords:  drosophila; neuromuscular junction; synapse
  25. PLoS Genet. 2021 Nov 15. 17(11): e1009926
      Myofiber atrophy occurs with aging and in many diseases but the underlying mechanisms are incompletely understood. Here, we have used >1,100 muscle-targeted RNAi interventions to comprehensively assess the function of 447 transcription factors in the developmental growth of body wall skeletal muscles in Drosophila. This screen identifies new regulators of myofiber atrophy and hypertrophy, including the transcription factor Deaf1. Deaf1 RNAi increases myofiber size whereas Deaf1 overexpression induces atrophy. Consistent with its annotation as a Gsk3 phosphorylation substrate, Deaf1 and Gsk3 induce largely overlapping transcriptional changes that are opposed by Deaf1 RNAi. The top category of Deaf1-regulated genes consists of glycolytic enzymes, which are suppressed by Deaf1 and Gsk3 but are upregulated by Deaf1 RNAi. Similar to Deaf1 and Gsk3 overexpression, RNAi for glycolytic enzymes reduces myofiber growth. Altogether, this study defines the repertoire of transcription factors that regulate developmental myofiber growth and the role of Gsk3/Deaf1/glycolysis in this process.
  26. J Biophotonics. 2021 Nov 19. e202100219
      Non-invasive promotion of myogenic regulatory factors (MRF), through photobiomodulation therapy (PBMT), may be a viable method of facilitating skeletal muscle regeneration post-injury, given the importance of MRF in skeletal muscle regeneration. The aim of this systematic review was to collate current evidence, identifying key themes and changes in expression of MRF in in vivo models. Web of Science, PubMed, Scopus and Cochrane databases were systematically searched and identified 1,459 studies, of which ten met the inclusion criteria. MyoD was most consistently regulated in response to PBMT treatment, and the expression of remaining MRFs was heterogenous. All studies exhibited a high risk of bias, primarily due to lack of blinding in PBMT application and MRF analysis. Our review suggests that the current evidence base for MRF expression from PBMT is highly variable. Future research should focus on developing a robust methodology for determining the effect of laser therapy on MRF expression, as well as long term assessment of skeletal muscle regeneration. This article is protected by copyright. All rights reserved.
    Keywords:  myogenic regulatory factor; photobiomodulation therapy; systematic review
  27. Front Cell Dev Biol. 2021 ;9 752962
      Autophagy is an evolutionary conserved degradative process contributing to cytoplasm quality control, metabolic recycling and cell defense. Aging is a universal phenomenon characterized by the progressive accumulation of impaired molecular and reduced turnover of cellular components. Recent evidence suggests a unique role for autophagy in aging and age-related disease. Indeed, autophagic activity declines with age and enhanced autophagy may prevent the progression of many age-related diseases and prolong life span. All tissues experience changes during aging, while the role of autophagy in different tissues varies. This review summarizes the links between autophagy and aging in the whole organism and discusses the physiological and pathological roles of autophagy in the aging process in tissues such as skeletal muscle, eye, brain, and liver.
    Keywords:  age-related diseases; aging; autophagy; brain; eye; liver; skeletal muscle
  28. Biophys Rev. 2021 Oct;13(5): 679-695
      All cells sense force and build their cytoskeleton to optimize function. How is this achieved? Two major systems are involved. The first is that load deforms specific protein structures in a proportional and orientation-dependent manner. The second is post-translational modification of proteins as a consequence of signaling pathway activation. These two processes work together in a complex way so that local subcellular assembly as well as overall cell function are controlled. This review discusses many cell types but focuses on striated muscle. Detailed information is provided on how load deforms the structure of proteins in the focal adhesions and filaments, using α-actinin, vinculin, talin, focal adhesion kinase, LIM domain-containing proteins, filamin, myosin, titin, and telethonin as examples. Second messenger signals arising from external triggers are distributed throughout the cell causing post-translational or chemical modifications of protein structures, with the actin capping protein CapZ and troponin as examples. There are numerous unanswered questions of how mechanical and chemical signals are integrated by muscle proteins to regulate sarcomere structure and function yet to be studied. Therefore, more research is needed to see how external triggers are integrated with local tension generated within the cell. Nonetheless, maintenance of tension in the sarcomere is the essential and dominant mechanism, leading to the well-known phrase in exercise physiology: "use it or lose it."
    Keywords:  Adhesome; Costamere; Integrin; Mechanobiology; Mechanotransduction; Sarcomere
  29. J Appl Physiol (1985). 2021 Nov 18.
      Heart failure with preserved ejection fraction (HFpEF) accounts for ~50% of all patients with heart failure and frequently affects postmenopausal women. The HFpEF condition is phenotype-specific, with skeletal myopathy that is crucial for disease development and progression. However, most of the current preclinical models of HFpEF have not addressed the postmenopausal phenotype. We sought to advance a rodent model of postmenopausal HFpEF and examine skeletal muscle abnormalities therein. Female, ovariectomized, spontaneously hypertensive rats (SHR) were fed a high fat, high sucrose diet to induce HFpEF. Controls were female sham-operated Wistar-Kyoto rats on a lean diet. In a complementary, longer-term cohort, controls were female sham-operated SHRs on a lean diet to evaluate the effect of strain difference in the model. Our model developed key features of HFpEF that included increased body weight, glucose intolerance, hypertension, cardiac hypertrophy, diastolic dysfunction, exercise intolerance, and elevated plasma cytokines. In limb skeletal muscle, HFpEF decreased specific force by 15-30% (p < 0.05) and maximal mitochondrial respiration by 40-55% (p < 0.05), increased oxidized glutathione by ~2-fold (p < 0.05), and tended to increase mitochondrial H2O2 emission (p = 0.10). Muscle fiber cross-sectional area, markers of mitochondrial content, and indices of capillarity were not different between control and HFpEF in our short-term cohort. Overall, our model of postmenopausal HFpEF recapitulates several key features of the disease. This new model reveals contractile and mitochondrial dysfunction and redox imbalance that are potential contributors to abnormal metabolism, exercise intolerance, and diminished quality of life in patients with postmenopausal HFpEF.
    Keywords:  high fat diet; hypertension; mitochondria; muscle weakness; oxidants
  30. Biochim Biophys Acta Gen Subj. 2021 Nov 15. pii: S0304-4165(21)00218-X. [Epub ahead of print] 130059
    DIAMATER Study Group
      The angiogenesis process is a phenomenon in which numerous molecules participate in the stimulation of the new vessels' formation from pre-existing vessels. Angiogenesis is a crucial step in tissue regeneration and recovery of organ and tissue function. Muscle diseases affect millions of people worldwide overcome the ability of skeletal muscle to self-repair. Pro-angiogenic therapies are key in skeletal muscle regeneration where both myogenesis and angiogenesis occur. These therapies have been based on mesenchymal stem cells (MSCs), exosomes, microRNAs (miRs) and delivery of biological factors. The use of different calls of biomaterials is another approach, including ceramics, composites, and polymers. Natural polymers are use due its bioactivity and biocompatibility in addition to its use as scaffolds and in drug delivery systems. One of these polymers is the natural rubber latex (NRL) which is biocompatible, bioactive, versatile, low-costing, and capable of promoting tissue regeneration and angiogenesis. In this review, the advances in the field of pro-angiogenic therapies are discussed.
    Keywords:  Angiogenesis; Biomaterials; Mesenchymal stem cells; Natural rubber latex; Skeletal muscle
  31. EMBO Rep. 2021 Nov 15. e53054
      Cancer cells depend on mitochondria to sustain their increased metabolic need and mitochondria therefore constitute possible targets for cancer treatment. We recently developed small-molecule inhibitors of mitochondrial transcription (IMTs) that selectively impair mitochondrial gene expression. IMTs have potent antitumor properties in vitro and in vivo, without affecting normal tissues. Because therapy-induced resistance is a major constraint to successful cancer therapy, we investigated mechanisms conferring resistance to IMTs. We employed a CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats)-(CRISP-associated protein 9) whole-genome screen to determine pathways conferring resistance to acute IMT1 treatment. Loss of genes belonging to von Hippel-Lindau (VHL) and mammalian target of rapamycin complex 1 (mTORC1) pathways caused resistance to acute IMT1 treatment and the relevance of these pathways was confirmed by chemical modulation. We also generated cells resistant to chronic IMT treatment to understand responses to persistent mitochondrial gene expression impairment. We report that IMT1-acquired resistance occurs through a compensatory increase of mitochondrial DNA (mtDNA) expression and cellular metabolites. We found that mitochondrial transcription factor A (TFAM) downregulation and inhibition of mitochondrial translation impaired survival of resistant cells. The identified susceptibility and resistance mechanisms to IMTs may be relevant for different types of mitochondria-targeted therapies.
    Keywords:  CRISPR-Cas9 screen; cancer; chemoresistance; inhibitor of mitochondrial transcription; mtDNA
  32. Commun Biol. 2021 Nov 15. 4(1): 1290
      Mice enter an active hypometabolic state, called daily torpor when they experience a lowered caloric intake under cold ambient temperature. During torpor, the oxygen consumption rate in some animals drops to less than 30% of the normal rate without harming the body. This safe but severe reduction in metabolism is attractive for various clinical applications; however, the mechanism and molecules involved are unclear. Therefore, here we systematically analyzed the gene expression landscape on the level of the RNA transcription start sites in mouse skeletal muscles under various metabolic states to identify torpor-specific transcribed regulatory patterns. We analyzed the soleus muscles from 38 mice in torpid and non-torpid conditions and identified 287 torpor-specific promoters out of 12,862 detected promoters. Furthermore, we found that the transcription factor ATF3 is highly expressed during torpor deprivation and its binding motif is enriched in torpor-specific promoters. Atf3 was also highly expressed in the heart and brown adipose tissue during torpor and systemically knocking out Atf3 affected the torpor phenotype. Our results demonstrate that mouse torpor combined with powerful genetic tools is useful for studying active hypometabolism.
  33. Pol Arch Intern Med. 2021 Nov 15.
      The two main manifestations of wasting disorders in chronic disease are cachexia and sarcopenia. Due to sharing common pathological features, including impairments in systemic inflammation responses, neurohormonal activity, and metabolic systems, the two disorders can present with similar symptoms (tissue depletion, dyspnoea, anorexia, asthenia, fatigue, and impaired physical performance). Wasting disorders are associated with reduced quality of life and increased mortality. Cachexia is characterized by systemic tissue depletion with weight loss and sarcopenia by skeletal muscle loss accompanied by diminished muscular strength and physical performance. Wasting syndromes can be identified through clinical criteria but also through multiple imaging and diagnostic techniques. Additionally, blood biomarkers can be used for diagnosing wasting disorders. In the past decade, intensive research has focused on new therapeutic strategies within a multimodal approach, which embraces nutritional support, physical activity, and targeted pharmacological therapy. Despite some promising first therapeutic results for selected novel agents, a guideline-recommended pharmacological therapy is not yet available for cachexia or sarcopenia. More research is needed to better understand and thereby learn how to treat these wasting disorders.
  34. J Endocrinol. 2021 Nov 01. pii: JOE-21-0242.R1. [Epub ahead of print]
      The aim of this study was to investigate the relationship between mitochondrial content and respiratory function and whole-body insulin resistance in high fat diet (HFD) fed rats. Male Wistar rats were given either a chow diet or a HFD for 12 weeks. After four weeks of the dietary intervention, half of the rats in each group began eight weeks of interval training. In vivo glucose and insulin tolerance were assessed. Mitochondrial respiratory function was assessed in permeabilised soleus and white gastrocnemius (WG) muscles. Mitochondrial content was determined by measurement of citrate synthase (CS) activity and protein expression of components of the electron transport system (ETS). We found HFD rats had impaired glucose and insulin tolerance but increased mitochondrial respiratory function and increased protein expression of components of the ETS. This was accompanied by an increase in CS activity in WG. Exercise training improved glucose and insulin tolerance in the HFD rats. Mitochondrial respiratory function was increased with exercise training in the chow fed animals in soleus muscle. This exercise effect was absent in the HFD animals. In conclusion, exercise training improved insulin resistance in HFD rats, but without changes in mitochondrial respiratory function and content. The lack of an association between mitochondrial characteristics and whole-body insulin resistance was reinforced by the absence of strong correlations between these measures. Our results suggest that improvements in mitochondrial respiratory function and content are not responsible for improvements of whole-body insulin resistance in HFD rats.
  35. Front Physiol. 2021 ;12 735932
      Before major athletic events, a taper is often prescribed to facilitate recovery and enhance performance. However, it is unknown which taper model is most effective for peaking maximal strength and positively augmenting skeletal muscle. Thus, the purpose of this study was to compare performance outcomes and skeletal muscle adaptations following a step vs. an exponential taper in strength athletes. Sixteen powerlifters (24.0 ± 4.0 years, 174.4 ± 8.2 cm, 89.8 ± 21.4 kg) participated in a 6-week training program aimed at peaking maximal strength on back squat [initial 1-repetition-maximum (1RM): 174.7 ± 33.4 kg], bench press (118.5 ± 29.9 kg), and deadlift (189.9 ± 41.2 kg). Powerlifters were matched based on relative maximal strength, and randomly assigned to either (a) 1-week overreach and 1-week step taper or (b) 1-week overreach and 3-week exponential taper. Athletes were tested pre- and post-training on measures of body composition, jumping performance, isometric squat, and 1RM. Whole muscle size was assessed at the proximal, middle, and distal vastus lateralis using ultrasonography and microbiopsies at the middle vastus lateralis site. Muscle samples (n = 15) were analyzed for fiber size, fiber type [myosin-heavy chain (MHC)-I, -IIA, -IIX, hybrid-I/IIA] using whole muscle immunohistochemistry and single fiber dot blots, gene expression, and microRNA abundance. There were significant main time effects for 1RM squat (p < 0.001), bench press (p < 0.001), and deadlift, (p = 0.024), powerlifting total (p < 0.001), Wilks Score (p < 0.001), squat jump peak-power scaled to body mass (p = 0.001), body mass (p = 0.005), fat mass (p = 0.002), and fat mass index (p = 0.002). There were significant main time effects for medial whole muscle cross-sectional area (mCSA) (p = 0.006) and averaged sites (p < 0.001). There was also a significant interaction for MHC-IIA fiber cross-sectional area (fCSA) (p = 0.014) with post hoc comparisons revealing increases following the step-taper only (p = 0.002). There were significant main time effects for single-fiber MHC-I% (p = 0.015) and MHC-IIA% (p = 0.033), as well as for MyoD (p = 0.002), MyoG (p = 0.037), and miR-499a (p = 0.033). Overall, increases in whole mCSA, fCSA, MHC-IIA fCSA, and MHC transitions appeared to favor the step taper group. An overreach followed by a step taper appears to produce a myocellular environment that enhances skeletal muscle adaptations, whereas an exponential taper may favor neuromuscular performance.
    Keywords:  fiber typing; gene expression; mRNA; maximal strength; muscle biopsy; myosin heavy chain; powerlifting; resistance training
  36. J Biol Chem. 2021 Nov 15. pii: S0021-9258(21)01219-9. [Epub ahead of print] 101412
      The N-terminal region (NTR) of ryanodine receptor (RyR) channels is critical for the regulation of Ca2+ release during excitation-contraction (EC) coupling in muscle. The NTR hosts numerous mutations linked to skeletal (RyR1) and cardiac (RyR2) myopathies, highlighting its potential as a therapeutic target. Here, we constructed two biosensors by labeling the mouse RyR2 NTR at domains A, B, and C with FRET pairs. Using fluorescence lifetime (FLT) detection of intramolecular FRET signal, we developed high-throughput screening (HTS) assays with these biosensors to identify small-molecule RyR modulators. We then screened a small validation library and identified several hits. Hits with saturable FRET dose-response profiles and previously unreported effects on RyR were further tested using [3H]ryanodine binding to isolated sarcoplasmic reticulum vesicles to determine effects on intact RyR opening in its natural membrane. We identified three novel inhibitors of both RyR1 and RyR2, and two RyR1-selective inhibitors effective at nanomolar Ca2+. Two of these hits activated RyR1 only at micromolar Ca2+, highlighting them as potential enhancers of EC coupling. To determine whether such hits can inhibit RyR leak in muscle, we further focused on one, an FDA-approved natural antibiotic, fusidic acid (FA). In skinned skeletal myofibers and permeabilized cardiomyocytes, FA was demonstrated to inhibit RyR leak with no detrimental effect on skeletal myofiber EC coupling. However, in intact cardiomyocytes FA induced arrhythmogenic Ca2+ transients, a cautionary observation for a compound with an otherwise solid safety record. These results indicate that HTS campaigns using the NTR biosensor can identify compounds with therapeutic potential.
    Keywords:  FRET; N-terminal region; Ryanodine receptor; fluorescence lifetime; high-throughput screening; myopathy
  37. Biomaterials. 2021 Nov 12. pii: S0142-9612(21)00601-3. [Epub ahead of print] 121244
      Functional recovery following peripheral nerve injury is limited by progressive atrophy of denervated muscle and Schwann cells (SCs) that occurs during the long regenerative period prior to end-organ reinnervation. Insulin-like growth factor 1 (IGF-1) is a potent mitogen with well-described trophic and anti-apoptotic effects on neurons, myocytes, and SCs. Achieving sustained, targeted delivery of small protein therapeutics remains a challenge. We hypothesized that a novel nanoparticle (NP) delivery system can provide controlled release of bioactive IGF-1 targeted to denervated muscle and nerve tissue to achieve improved motor recovery through amelioration of denervation-induced muscle atrophy and SC senescence and enhanced axonal regeneration. Biodegradable NPs with encapsulated IGF-1/dextran sulfate polyelectrolyte complexes were formulated using a flash nanoprecipitation method to preserve IGF-1 bioactivity and maximize encapsulation efficiencies. Under optimized conditions, uniform PEG-b-PCL NPs were generated with an encapsulation efficiency of 88.4%, loading level of 14.2%, and a near-zero-order release of bioactive IGF-1 for more than 20 days in vitro. The effects of locally delivered IGF-1 NPs on denervated muscle and SCs were assessed in a rat median nerve transection-without- repair model. The effects of IGF-1 NPs on axonal regeneration, muscle atrophy, reinnervation, and recovery of motor function were assessed in a model in which chronic denervation is induced prior to nerve repair. IGF-1 NP treatment resulted in significantly greater recovery of forepaw grip strength, decreased denervation-induced muscle atrophy, decreased SC senescence, and improved neuromuscular reinnervation.
    Keywords:  Chronic denervation; IGF-1; Nanoparticles; Nerve regeneration; Peripheral nerve injury
  38. Front Physiol. 2021 ;12 738333
      In response to acute exercise, an array of metabolites, nucleic acids, and proteins are enriched in circulation. Collectively termed "exercise factors," these molecules represent a topical area of research given their speculated contribution to both acute exercise metabolism and adaptation to exercise training. In addition to acute changes induced by exercise, the resting profile of circulating exercise factors may be altered by exercise training. Many exercise factors are speculated to be transported in circulation as the cargo of extracellular vesicles (EVs), and in particular, a sub-category termed "small EVs." This review describes an overview of exercise factors, small EVs and the effects of exercise, but is specifically focused on a critical appraisal of methodological approaches and current knowledge in the context of changes in the resting profile small EVs induced by exercise training, and the potential bioactivities of preparations of these "exercise-trained" small EVs. Research to date can only be considered preliminary, with interpretation of many studies hindered by limited evidence for the rigorous identification of small EVs, and the conflation of acute and chronic responses to exercise due to sample timing in proximity to exercise. Further research that places a greater emphasis on the rigorous identification of small EVs, and interrogation of potential bioactivity is required to establish more detailed descriptions of the response of small EVs to exercise training, and consequent effects on exercise adaptation.
    Keywords:  endurance; exercise factors; exerkines; exosomes; myokines; skeletal muscle
  39. Eur J Appl Physiol. 2021 Nov 18.
      PURPOSE: Circulating testosterone levels are a heritable trait with anabolic properties in various tissues, including skeletal muscle. So far, hundreds of single nucleotide polymorphisms (SNPs) associated with testosterone levels have been identified in nonathletic populations. The aim of the present study was to test the association of 822 testosterone-increasing SNPs with muscle-related traits (muscle fiber size, fat-free mass and handgrip strength) and to validate the identified SNPs in independent cohorts of strength and power athletes.METHODS: One hundred and forty-eight physically active individuals (47 females, 101 males) were assessed for cross-sectional area (CSA) of fast-twitch muscle fibers. Significant SNPs were further assessed for fat-free mass and handgrip strength in > 354,000 participants from the UK Biobank cohort. The validation cohorts included Russian elite athletes.
    RESULTS: From an initial panel of 822 SNPs, we identified five testosterone-increasing alleles (DOCK3 rs77031559 G, ESR1 rs190930099 G, GLIS3 rs34706136 TG, GRAMD1B rs850294 T, TRAIP rs62260729 C) nominally associated (P < 0.05) with CSA of fast-twitch muscle fibers, fat-free mass and handgrip strength. Based on these five SNPs, the number of testosterone-increasing alleles was positively associated with testosterone levels in male athletes (P = 0.048) and greater strength performance in weightlifters (P = 0.017). Moreover, the proportion of participants with ≥ 2 testosterone-increasing alleles was higher in power athletes compared to controls (68.9 vs. 55.6%; P = 0.012).
    CONCLUSION: Testosterone-related SNPs are associated with muscle fiber size, fat-free mass and strength, which combined can partially contribute to a greater predisposition to strength/power sports.
    Keywords:  Athletic performance; Elite athletes; Genetics; Hormones; Polymorphism; Skeletal muscle
  40. J Biol Chem. 2021 Nov 13. pii: S0021-9258(21)01217-5. [Epub ahead of print] 101410
      Pluripotent stem cells are known to shift their mitochondrial metabolism upon differentiation, but the mechanisms underlying such metabolic rewiring are not fully understood. We hypothesized that during differentiation of human induced pluripotent stem cells (hiPSCs), mitochondria undergo mitophagy and are then replenished by the biogenesis of new mitochondria adapted to the metabolic needs of the differentiated cell. To evaluate mitophagy during iPSC differentiation, we performed live cell imaging of mitochondria and lysosomes in hiPSCs differentiating into vascular endothelial cells using confocal microscopy. We observed a burst of mitophagy during the initial phases of hiPSC differentiation into the endothelial lineage, followed by subsequent mitochondrial biogenesis as assessed by the mitochondrial biogenesis biosensor MitoTimer. Furthermore, hiPSCs undergoing differentiation showed greater mitochondrial oxidation of fatty acids and an increase in ATP levels as assessed by an ATP biosensor. We also found that during mitophagy, the mitochondrial phosphatase PGAM5 is cleaved in hiPSC-derived endothelial progenitor cells and in turn activates β-catenin-mediated transcription of the transcriptional co-activator PGC-1α, which upregulates mitochondrial biogenesis. These data suggest that mitophagy itself initiates the increase in mitochondrial biogenesis and oxidative metabolism through transcriptional changes during endothelial cell differentiation. In summary, these findings reveal a mitophagy-mediated mechanism for metabolic rewiring and maturation of differentiating cells via the β-catenin signaling pathway. We propose that such mitochondrial-nuclear crosstalk during hiPSC differentiation could be leveraged to enhance the metabolic maturation of differentiated cells.
    Keywords:  cell differentiation; induced pluripotent stem cells; mitochondrial metabolism; mitophagy; β-catenin
  41. Commun Biol. 2021 Nov 18. 4(1): 1306
      Age is associated with progressively impaired, metabolic, cardiac and vascular function, as well as reduced work/exercise capacity, mobility, and hence quality of life. Exercise exhibit positive effects on age-related dysfunctions and diseases. However, for a variety of reasons many aged individuals are unable to engage in regular physical activity, making the development of pharmacological treatments that mimics the beneficial effects of exercise highly desirable. Here we show that the pan-AMPK activator O304, which is well tolerated in humans, prevented and reverted age-associated hyperinsulinemia and insulin resistance, and improved cardiac function and exercise capacity in aged mice. These results provide preclinical evidence that O304 mimics the beneficial effects of exercise. Thus, as an exercise mimetic in clinical development, AMPK activator O304 holds great potential to mitigate metabolic dysfunction, and to improve cardiac function and exercise capacity, and hence quality of life in aged individuals.