bims-moremu Biomed News
on Molecular regulators of muscle mass
Issue of 2021‒11‒14
forty-nine papers selected by
Anna Vainshtein
Craft Science Inc.

  1. Cell Rep. 2021 Nov 09. pii: S2211-1247(21)01450-9. [Epub ahead of print]37(6): 109971
      Skeletal muscle atrophy is a debilitating condition that occurs with aging and disease, but the underlying mechanisms are incompletely understood. Previous work determined that common transcriptional changes occur in muscle during atrophy induced by different stimuli. However, whether this holds true at the proteome level remains largely unexplored. Here, we find that, contrary to this earlier model, distinct atrophic stimuli (corticosteroids, cancer cachexia, and aging) induce largely different mRNA and protein changes during muscle atrophy in mice. Moreover, there is widespread transcriptome-proteome disconnect. Consequently, atrophy markers (atrogenes) identified in earlier microarray-based studies do not emerge from proteomics as generally induced by atrophy. Rather, we identify proteins that are distinctly modulated by different types of atrophy (herein defined as "atroproteins") such as the myokine CCN1/Cyr61, which regulates myofiber type switching during sarcopenia. Altogether, these integrated analyses indicate that different catabolic stimuli induce muscle atrophy via largely distinct mechanisms.
    Keywords:  CCN1/Cyr61; aging; atroprotein; cancer-induced cachexia; dexamethasone; muscle atrophy; myofiber atrophy; myokine; proteomics; sarcopenia
  2. Mech Ageing Dev. 2021 Nov 03. pii: S0047-6374(21)00167-6. [Epub ahead of print]200 111595
      Cellular senescence is a state of cell cycle arrest induced by several forms of metabolic stress. Senescent cells accumulate with advancing age and have a distinctive phenotype, characterized by profound chromatin alterations and a robust senescence-associated secretory phenotype (SASP) that exerts negative effects on tissue health, both locally and systemically. In preclinical models, pharmacological agents that eliminate senescent cells (senotherapeutics) restore health and youthful properties in multiple tissues. To date, however, very little is understood about the vulnerability of terminally-differentiated skeletal muscle fibers and the resident mononuclear cells that populate the interstitial microenvironment of skeletal muscle to senescence, and their contribution to the onset and progression of skeletal muscle loss and dysfunction with aging. Scientific advances in these areas have the potential to highlight new therapeutic approaches to optimize late-life muscle health. To this end, this review highlights the current evidence and the key questions that need to be addressed to advance the field's understanding of cellular senescence as a mediator of skeletal muscle aging and the potential for emerging senescent cell-targeting therapies to counter age-related deficits in muscle mass, strength, and function.
    Keywords:  Fibroadipogenic progenitor cells; Muscle fiber; Sarcopenia; Satellite cells; Senescence-associated secretory phenotype; Senolytics
  3. Int J Mol Sci. 2021 Oct 26. pii: 11568. [Epub ahead of print]22(21):
      miRNAs and lncRNAs do not encode proteins, but they play an important role in the regulation of gene expression. They differ in length, biogenesis, and mode of action. In this work, we focus on the selected miRNAs and lncRNAs involved in the regulation of myogenesis and muscle regeneration. We present selected miRNAs and lncRNAs that have been shown to control myogenic differentiation and show that manipulation of their levels could be used to improve myogenic differentiation of various types of stem and progenitor cells. Finally, we discuss how physical activity affects miRNA and lncRNA expression and how it affects muscle well-being.
    Keywords:  human; lncRNA; mammals; miRNA; mouse; myogenesis; regeneration; skeletal muscle; training
  4. Cell Signal. 2021 Nov 06. pii: S0898-6568(21)00275-8. [Epub ahead of print] 110186
      Rare gain of function mutations in the gene encoding Dyrk1b, a key regulator of skeletal muscle differentiation, have been associated with sarcopenic obesity (SO) and metabolic syndrome (MetS) in humans. So far, the global gene networks regulated by Dyrk1b during myofiber differentiation have remained elusive. Here, we have performed untargeted proteomics to determine Dyrk1b-dependent gene-network in differentiated C2C12 myofibers. This analysis led to identification of translational inhibitor, 4e-bp1 as a post-transcriptional target of Dyrk1b in C2C12 cells. Accordingly, CRISPR/Cas9 mediated knockout of Dyrk1b in zebrafish identified 4e-bp1 as a downstream target of Dyrk1b in-vivo. The Dyrk1b knockout zebrafish embryos exhibited markedly reduced myosin heavy chain 1 expression in poorly developed myotomes and were embryonic lethal. Using knockdown and overexpression approaches in C2C12 cells, we found that 4e-bp1 enhances autophagy and mediates the effects of Dyrk1b on skeletal muscle differentiation. Dyrk1bR102C, the human sarcopenic obesity-associated mutation impaired muscle differentiation via excessive activation of 4e-bp1/autophagy axis in C2C12 cells. Strikingly, the defective muscle differentiation in Dyrk1bR102C cells was rescued by reduction of autophagic flux. The identification of Dyrk1b-4e-bp1-autophagy axis provides significant insight into pathways that are relevant to human skeletal muscle development and disorders.
    Keywords:  4e-bp1; Myogenesis; Sarcopenic obesity; dyrk1b
  5. Skelet Muscle. 2021 Nov 12. 11(1): 25
      As a key regulator of cellular calcium homeostasis, the Sarcoendoplasmic Reticulum Calcium ATPase (SERCA) pump acts to transport calcium ions from the cytosol back to the sarcoplasmic reticulum (SR) following muscle contraction. SERCA function is closely associated with muscle health and function, and SERCA activity is susceptible to muscle pathogenesis. For example, it has been well reported that pathological conditions associated with aging, neurodegeneration, and muscular dystrophy (MD) significantly depress SERCA function with the potential to impair intracellular calcium homeostasis and further contribute to muscle atrophy and weakness. As a result, targeting SERCA activity has attracted attention as a therapeutical method for the treatment of muscle pathologies. The interventions include activation of SERCA activity and genetic overexpression of SERCA. This review will focus on SERCA function and regulation mechanisms and describe how those mechanisms are affected under muscle pathological conditions including elevated oxidative stress induced by aging, muscle disease, or neuromuscular disorders. We also discuss the current progress and therapeutic approaches to targeting SERCA in vivo.
    Keywords:  Aging; Calcium homeostasis; Neuromuscular disorder; Oxidative stress; SERCA; Skeletal muscle
  6. J Gen Physiol. 2022 Sep 05. pii: e2021ecc21. [Epub ahead of print]154(9):
      Proper skeletal muscle development, maintenance, and function is necessary for movement. Decline in muscle function with age and disease is directly associated with a diminished quality of life. Radiation therapy is commonly used to treat certain forms of childhood cancer based on the cytotoxic effects of radiation on cancerous tissue. However, the adverse effects elicited by radiation are not always constrained to the diseased tissue and can accelerate muscle wasting and decline, which is particularly detrimental to juvenile cancer survivors. Exercise is effective at limiting muscle decline and improving muscle function in various diseases. Thus, we hypothesized 1 mo of voluntary endurance exercise following juvenile radiation treatment will reduce muscle damage and restore functional deficits that occur following radiation. Here, we show that following juvenile radiation, 1 mo of voluntary wheel running significantly improved muscle function in mice by promoting adaptations in intracellular calcium handling, improving mitochondrial turnover and reducing oxidative stress resulting from radiation-induced mitochondrial damage. These findings help guide caregivers in their approach to childhood cancer survivor recovery and have implications for other diseases where similar mechanisms of calcium handling and mitochondrial function are disrupted.
  7. J Cachexia Sarcopenia Muscle. 2021 Nov 09.
      BACKGROUND: Skeletomuscular diseases result in significant muscle loss and decreased performance, paralleled by a loss in mitochondrial and oxidative capacity. Insulin and insulin-like growth factor-1 (IGF-1) are two potent anabolic hormones that activate a host of signalling intermediates including the serine/threonine kinase AKT to influence skeletal muscle physiology. Defective AKT signalling is associated with muscle pathology, including cachexia, sarcopenia, and disuse; however, the mechanistic underpinnings remain unresolved.METHODS: To elucidate the role of AKT signalling in muscle mass and physiology, we generated both congenital and inducible mouse models of skeletal muscle-specific AKT deficiency. To understand the downstream mechanisms mediating AKT's effects on muscle biology, we generated mice lacking AKT1/2 and FOXO1 (M-AKTFOXO1TKO and M-indAKTFOXO1TKO) to inhibit downstream FOXO1 signalling, AKT1/2 and TSC1 (M-AKTTSCTKO and M-indAKTTSCTKO) to activate mTORC1, and AKT1/2, FOXO1, and TSC1 (M-QKO and M-indQKO) to simultaneously activate mTORC1 and inhibit FOXO1 in AKT-deficient skeletal muscle. Muscle proteostasis and physiology were assessed using multiple assays including metabolic labelling, mitochondrial function, fibre typing, ex vivo physiology, and exercise performance.
    RESULTS: Here, we show that genetic ablation of skeletal muscle AKT signalling resulted in decreased muscle mass and a loss of oxidative metabolism and muscle performance. Specifically, deletion of muscle AKT activity during development or in adult mice resulted in a significant reduction in muscle growth by 30-40% (P  < 0.0001; n = 12-20) and 15% (P < 0.01 and P < 0.0001; n = 20-30), respectively. Interestingly, this reduction in muscle mass was primarily due to an ~40% reduction in protein synthesis in both M-AKTDKO and M-indAKTDKO muscles (P < 0.05 and P < 0.01; n = 12-20) without significant changes in proteolysis or autophagy. Moreover, a significant reduction in oxidative capacity was observed in both M-AKTDKO (P < 0.05, P < 0.01 and P < 0.001; n = 5-12) and M-indAKTDKO (P < 0.05 and P < 0.01; n = 4). Mechanistically, activation and inhibition of mTORC1/FOXO1, respectively, but neither alone, were sufficient to restore protein synthesis, muscle oxidative capacity, and muscle function in the absence of AKT in vivo. In a mouse model of disuse-induced muscle loss, simultaneous activation of mTORC1 and inhibition of FOXO1 preserved muscle mass following immobilization (~5-10% reduction in casted M-indFOXO1TSCDKO muscles vs. ~30-40% casted M-indControl muscles, P < 0.05 and P < 0.0001; n = 8-16).
    CONCLUSIONS: Collectively, this study provides novel insights into the AKT-dependent mechanisms that underlie muscle protein homeostasis, function, and metabolism in both normal physiology and disuse-induced muscle wasting.
    Keywords:  AKT signalling; Disuse-induced muscle wasting; Fibre specification; Insulin action
  8. Nutrition. 2021 Jul 15. pii: S0899-9007(21)00274-4. [Epub ahead of print]93 111412
      OBJECTIVES: Skeletal muscle accounts for 80% of whole body insulin-stimulated glucose uptake, and it plays a key role in preventing and curing obesity and insulin resistance (IR). Vitamin K2 (VK2) plays a beneficial role in improving mitochondrial function through SIRT1 signaling in high-fat diet (HFD)-induced mice and palmitate acid (PA)-treated C2C12 cells. A previous study also found VK2 increases oxidative muscle fibers and decreases glycolytic muscle fibers in obesity-induced mice, however, the underlying molecular mechanism of effect of VK2 on increasing oxidative fibers have not been well established.METHODS: C57BL/6 male mice were induced IR using HFD fed. Animals received HFD for eight weeks, and different doses of VK2 were supplemented by oral gavage for the last eight weeks were randomly and equally divided into seven groups. C2C12 cells were exposed to different doses of PA for 16 h to mimic insulin resistance in vivo. Skeletal muscle types and mitochondrial function evaluated. C2C12 cells were transfected with SIRT1 siRNA.
    RESULTS: The present study first revealed that VK2 intervention also alleviated plasma non-esterified fatty acid levels that contribute to obesity-induced IR, VK2 administration also could effectively increase the proportion of slow-twitch fibers by improving mitochondrial function via SIRT1 signaling pathway in both HFD-fed mice and PA-exposed cells. However, the benefits of VK2 were abrogated in C2C12 transfected with SIRT1 siRNA in PA-treated C2C12 cells. Thus, SIRT1 is partially required for VK2 improvement the proportion of slow-twitch fiber in PA-treated C2C12 cells.
    CONCLUSION: Naturally occurring VK2 increases slow-twitch fibers by improving mitochondrial function and decreasing non-esterified fatty acid levels via partially SIRT1/SIRT3 signaling pathway. These data have potential importance for the therapy for a number of muscular and neuromuscular diseases in humans.
    Keywords:  Mitochondrial function; Muscle fibers; SIRT1 signaling; Vitamin K(2)
  9. J Gen Physiol. 2022 Sep 05. pii: e2021ecc23. [Epub ahead of print]154(9):
      Pannexins are plasma membrane heptameric channels mediating ATP release from the cytosol to the extracellular space. Skeletal muscle activity is associated with Pannexin 1 (Panx1) channels activation, ATP release out to the extracellular space and subsequent activation of purinergic signaling pathways. In agreement, recent evidence has shown molecular and functional interactions between Panx1 and the excitation-contraction (EC) coupling machinery of skeletal muscle. In this framework, we tested whether pharmacological effectors of Panx1 affect EC coupling in differentiated muscle fibers. Using confocal detection of cytosolic Ca2+ in voltage-clamped mouse muscle fibers, we found that the Panx1 blocker probenecid (1 mM) affects intracellular Ca2+ handling and EC coupling: acute application of probenecid generates a rise in resting Ca2+ that also occurs in nominally Ca2+-free extracellular medium. This effect is associated with a reduction of Ca2+ release through the sarcoplasmic reticulum (SR) Ca2+ channel RYR1. The effect of probenecid persists with time, with muscle fibers incubated for 30 min in the presence of the drug exhibiting a 40% reduction in peak SR Ca2+ release. Under the same conditions, the other Panx1 blocker carbenoxolone (50 µM) produced a 70% reduction in peak SR Ca2+ release. Application of probenecid on electrically stimulated whole mouse muscle induced a slight rise in resting tension and a >50% reduction of tetanic force after 30 min of incubation. Our results provide further support for the strong links between Panx1 function and EC coupling. Because probenecid is used both in the clinic for several types of therapeutic benefits and as a hiding agent for doping in sport, our results question whether potential adverse muscular effects may have, so far, been overlooked.
  10. Cold Spring Harb Perspect Biol. 2021 Nov 08. pii: a040824. [Epub ahead of print]
      Despite the evolutionary loss of tissue regenerative potential, robust skeletal muscle repair processes are largely retained even in higher vertebrates. In mammals, the skeletal muscle regeneration program is driven by resident stem cells termed satellite cells, guided by the coordinated activity of multiple intrinsic and extrinsic factors and other cell types. A thorough understanding of muscle repair mechanisms is crucial not only for combating skeletal myopathies, but for its prospective aid in devising therapeutic strategies to endow regenerative potential on otherwise regeneration-deficient organs. In this review, we discuss skeletal muscle regeneration from an evolutionary perspective, summarize the current knowledge of cellular and molecular mechanisms, and highlight novel paradigms of muscle repair revealed by explorations of the recent decade.
  11. FASEB J. 2021 Dec;35(12): e22031
      Loss of skeletal muscle mass and force is of critical importance in numerous pathologies, like age-related sarcopenia or cancer. It has been shown that the Akt-mTORC1 pathway is critical for stimulating adult muscle mass and function, however, it is unknown if mTORC1 is the only mediator downstream of Akt and which intracellular processes are required for functional muscle growth. Here, we show that loss of Raptor reduces muscle hypertrophy after Akt activation and completely prevents increases in muscle force. Interestingly, the residual hypertrophy after Raptor deletion can be completely prevented by administration of the mTORC1 inhibitor rapamycin. Using a quantitative proteomics approach we find that loss of Raptor affects the increases in mitochondrial proteins, while rapamycin mainly affects ribosomal proteins. Taken together, these results suggest that mTORC1 is the key mediator of Akt-dependent muscle growth and its regulation of the mitochondrial proteome is critical for increasing muscle force.
    Keywords:  Raptor; hypertrophy; mTOR; mitochondria; rapamycin; skeletal muscle
  12. Elife. 2021 Nov 10. pii: e66107. [Epub ahead of print]10
      Abnormalities in skeletal muscle repair can lead to poor function and complications such as scarring or heterotopic ossification (HO). Here, we use fibrodysplasia ossificans progressiva (FOP), a disease of progressive HO caused by ACVR1R206H (Activin receptor type-1 receptor) mutation, to elucidate how ACVR1 affects skeletal muscle repair. Rare and unique primary FOP human muscle stem cells (Hu-MuSCs) isolated from cadaveric skeletal muscle demonstrated increased ECM marker expression, showed skeletal muscle-specific impaired engraftment and regeneration ability. Human induced pluripotent stem cell (iPSC)-derived muscle stem/progenitor cells (iMPCs) single cell transcriptome analyses from FOP also revealed unusually increased ECM and osteogenic marker expression compared to control iMPCs. These results show that iMPCs can recapitulate many aspects of Hu-MuSCs for detailed in vitro study, that ACVR1 is a key regulator of Hu-MuSC function and skeletal muscle repair; and that ACVR1 activation in iMPCs or Hu-MuSCs may contribute to HO by changing the local tissue environment.
    Keywords:  cell biology; human; regenerative medicine; stem cells
  13. Curr Res Physiol. 2021 ;4 202-208
      Calorie restriction (CR) involves a reductions of calorie intake without altering the nutritional balance, and has many beneficial effects, such as improving oxidative metabolism and extending lifespan. However, CR decreases in skeletal muscle mass and fat mass in correlation with the reduction in food intake. Lactate is known to have potential as a signaling molecule rather than a metabolite during exercise. In this study, we examined the effects of the combination of caloric restriction and lactate administration on skeletal muscle adaptation in order to elucidate a novel role of lactate. We first demonstrated that daily lactate administration (equivalent to 1 g/kg of body weight) for 2 weeks suppressed CR-induced muscle atrophy by activating mammalian/mechanistic target of rapamycin (mTOR) signaling, a muscle protein synthesis pathway, and inhibited autophagy-induced muscle degradation. Next, we found that lactate administration under calorie restriction enhanced mitochondrial enzyme activity (citrate synthase and succinate dehydrogenase) and the expression of oxidative phosphorylation (OXPHOS) protein expression. Our results suggest that lactate administration under caloric restriction not only suppresses muscle atrophy but also improves mitochondrial function.
    Keywords:  Autophagy; Calorie restriction; Keywards; Lactate; Skeletal muscle; mTOR signaling
  14. J Gen Physiol. 2022 Sep 05. pii: e2021ecc14. [Epub ahead of print]154(9):
      The zebrafish has emerged as a very relevant animal model to decipher the pathophysiology of human muscle disorders. However, the vast majority of studies on zebrafish skeletal muscle have investigated genetic, histological, and molecular aspects, but functional approaches at the cellular level, especially in the field of excitation-contraction (EC) coupling, are scarcer and generally limited to cultured myotubes or fibers from embryonic zebrafish. Considering that zebrafish undergoes profound metamorphosis during transition from larval to adult stage and that number of muscle pathologies come up at ages far beyond embryonic stages, there is an actual need to investigate EC coupling in fully differentiated zebrafish skeletal muscle. In the present study, we were able to implement current and voltage clamp combined with intracellular Ca2+ measurements using the intracellularly loaded Ca2+ dye indo-1 in enzymatically isolated fast skeletal muscle fibers from 1-yr old zebrafish. Recording of action potentials (AP) in current-clamp conditions revealed very fast kinetics of the repolarization phase of AP. Measurements of intramembrane charge movements in voltage-clamp conditions showed that charge movement density was half that measured in mammalian fibers, but they displayed much faster kinetics. Ca2+ transients elicited by depolarization displayed a voltage-dependent phase of activation and voltage- and time-dependent phase of inactivation. Recording of Ca2+ signals elicited by trains of AP at different rates in current-clamp conditions indicated that Ca2+ signals fused at very high stimulation frequencies with no sign of Ca2+ signal decay for the entire 0.5 s duration of the stimulation, giving evidence that fibers were still able to generate AP and the sarcoplasmic reticulum to release Ca2+ with stimulation rates as high as 200 Hz. These data indicate that adult zebrafish fast skeletal muscle fibers exhibit strikingly fast kinetics of EC coupling from AP firing to charge movements and sarcoplasmic reticulum Ca2+ release.
  15. FASEB J. 2021 Dec;35(12): e21999
      The Creb-Regulated Transcriptional Coactivator (Crtc) family of transcriptional coregulators drive Creb1-mediated transcription effects on metabolism in many tissues, but the in vivo effects of Crtc2/Creb1 transcription on skeletal muscle metabolism are not known. Skeletal muscle-specific overexpression of Crtc2 (Crtc2 mice) induced greater mitochondrial activity, metabolic flux capacity for both carbohydrates and fats, improved glucose tolerance and insulin sensitivity, and increased oxidative capacity, supported by upregulation of key metabolic genes. Crtc2 overexpression led to greater weight loss during alternate day fasting (ADF), selective loss of fat rather than lean mass, maintenance of higher energy expenditure during the fast and reduced binge-eating during the feeding period. ADF downregulated most of the mitochondrial electron transport genes, and other regulators of mitochondrial function, that were substantially reversed by Crtc2-driven transcription. Glucocorticoids acted with AMPK to drive atrophy and mitophagy, which was reversed by Crtc2/Creb1 signaling. Crtc2/Creb1-mediated signaling coordinates metabolic adaptations in skeletal muscle that explain how Crtc2/Creb1 regulates metabolism and weight loss.
    Keywords:  Creb1; Crtc2; intermittent fasting; skeletal muscle
  16. Clin Sci (Lond). 2021 Nov 09. pii: CS20210894. [Epub ahead of print]
      Individuals born preterm show reduced exercise capacity and increased risk for pulmonary and cardiovascular diseases, but the impact of preterm birth on skeletal muscle, an inherently critical part of cardiorespiratory fitness, remains unknown. We evaluated the impacts of preterm birth-related conditions on the development, growth, and function of skeletal muscle using a recognized preclinical rodent model in which newborn rats are exposed to 80% oxygen from day 3 to 10 of life. We analyzed different hindlimb muscles of male and female rats at 10 days (neonatal), 4 weeks (juvenile) and 16 weeks (young adults). Neonatal high oxygen exposure increased the generation of reactive oxygen species and the signs of inflammation in skeletal muscles, which was associated with muscle fiber atrophy, fiber type shifting (reduced proportion of type I slow fibers and increased proportion of type IIb fast-fatigable fibers), and impairment in muscle function. These effects were maintained until adulthood. Fast-twitch muscles were more vulnerable to the effects of hyperoxia than slow-twitch muscles. Male rats, which expressed lower antioxidant defenses, were more susceptible than females to oxygen-induced myopathy. Overall, preterm birth-related conditions have long-lasting effects on the composition, morphology, and function of skeletal muscles; and these effects are sex-specific. Oxygen-induced changes in skeletal muscles could contribute to the reduced exercise capacity and to increased risk of diseases of preterm born individuals.
    Keywords:  Muscle atrophy; Preterm birth; Skeletal muscle; oxidative stress
  17. Front Physiol. 2021 ;12 770455
      Aging is associated with a decline in skeletal muscle mass and function-termed sarcopenia-as mediated, in part, by muscle anabolic resistance. This metabolic phenomenon describes the impaired response of muscle protein synthesis (MPS) to the provision of dietary amino acids and practice of resistance-based exercise. Recent observations highlight the gut-muscle axis as a physiological target for combatting anabolic resistance and reducing risk of sarcopenia. Experimental studies, primarily conducted in animal models of aging, suggest a mechanistic link between the gut microbiota and muscle atrophy, mediated via the modulation of systemic amino acid availability and low-grade inflammation that are both physiological factors known to underpin anabolic resistance. Moreover, in vivo and in vitro studies demonstrate the action of specific gut bacteria (Lactobacillus and Bifidobacterium) to increase systemic amino acid availability and elicit an anti-inflammatory response in the intestinal lumen. Prospective lifestyle approaches that target the gut-muscle axis have recently been examined in the context of mitigating sarcopenia risk. These approaches include increasing dietary fiber intake that promotes the growth and development of gut bacteria, thus enhancing the production of short-chain fatty acids (SCFA) (acetate, propionate, and butyrate). Prebiotic/probiotic/symbiotic supplementation also generates SCFA and may mitigate low-grade inflammation in older adults via modulation of the gut microbiota. Preliminary evidence also highlights the role of exercise in increasing the production of SCFA. Accordingly, lifestyle approaches that combine diets rich in fiber and probiotic supplementation with exercise training may serve to produce SCFA and increase microbial diversity, and thus may target the gut-muscle axis in mitigating anabolic resistance in older adults. Future mechanistic studies are warranted to establish the direct physiological action of distinct gut microbiota phenotypes on amino acid utilization and the postprandial stimulation of muscle protein synthesis in older adults.
    Keywords:  anabolic resistance; dietary fiber; exercise; gut microbiota; sarcopenia; skeletal muscle
  18. J Gen Physiol. 2022 Sep 05. pii: e2021ecc41. [Epub ahead of print]154(9):
      An important question in neuromuscular biology is how skeletal muscle cells decipher the stimulation pattern coming from motoneurons to define their phenotype-activating transcriptional changes in a process named excitation-transcription coupling. We have shown in adult muscle fibers that 20 Hz electrical stimulation (ES) activates a signaling cascade that starts with Cav1.1 activation, ATP release trough pannexin-1 channel, activation of purinergic receptors, and IP3-dependent Ca2+ signals inducing transcriptional changes related to muscle plasticity from fast to slow phenotype. Extracellular addition of 30 µM ATP mimics transcriptional changes induced by ES at 20 Hz. ATP release occurs in two peaks, the first around 15 s after ES and a second around 300 s after ES. In the present work, we used apyrase to hydrolyze ATP 60 s after ES, maintaining the first peak and eliminating the second peak. In this condition, transcriptional changes were abolished, indicating that the second peak is the one crucial to activate transcription. Additionally, we observed a small depolarization of fibers after ES. The addition of 30 to 100 µM external ATP also induced depolarization of muscle fibers. This depolarization was unable to activate contraction but was able to induce transcriptional changes induced by 20 Hz ES. These changes were completely inhibited by the IP3R blocker xestospongin B, suggesting that IP3-dependent events are triggered at these membrane depolarization values. Moreover, transcriptional changes induced by addition of 30 µM extracellular ATP was blocked by incubation of fibers with 25 µM Nifedipine. These results suggest that the second ATP peak observed after 20 Hz ES is responsible for transcriptional activation by inducing small depolarizations of fiber membranes that are also sensed by Cav1.1. Finally, we show evidence that downstream of purinergic receptors, PKC is activated, likely causing phosphorylation of ClC-1 chloride channels, possibly responsible for depolarization after 20 Hz.
  19. Acta Histochem Cytochem. 2021 Oct 29. 54(5): 167-175
      The basement membrane (BM)-related factors, including collagen IV, are important for the maintenance and recovery of skeletal muscles. Aging impairs the expression of BM-related factors during recovery after disuse atrophy. Muscle activity facilitates collagen synthesis that constitutes the BM. However, the effect of endurance exercise on the BM of aged muscles is unclear. Thus, to understand the effect of endurance exercise on the BM of the skeletal muscle in aged rats, we prescribed treadmill running in aged rats and compared the differences in the expression of BM-related factors between the aged rats with and without exercise habits. Aged rats were subjected to endurance exercise via treadmill running. Exercise increased the mRNA expression levels of the BM-related factors, the area and intensity of collagen IV-immunoreactivity and the width of lamina densa in the soleus muscle of aged rats. These finding suggests that endurance exercise promotes BM construction in aged rats.
    Keywords:  aging; basement membrane; endurance exercise; soleus muscle
  20. Int J Mol Sci. 2021 Oct 30. pii: 11789. [Epub ahead of print]22(21):
      Curcumin administration attenuates muscle disuse atrophy, but its effectiveness against aging-induced, selective loss of mass or force (presarcopenia or asthenia/dynopenia), or combined loss (sarcopenia), remains controversial. A new systemic curcumin treatment was developed and tested in 18-month-old C57BL6J and C57BL10ScSn male mice. The effects on survival, liver toxicity, loss of muscle mass and force, and satellite cell responsivity and commitment were evaluated after 6-month treatment. Although only 24-month-old C57BL10ScSn mice displayed age-related muscle impairment, curcumin significantly increased survival of both strains (+20-35%), without signs of liver toxicity. Treatment prevented sarcopenia in soleus and presarcopenia in EDL of C57BL10ScSn mice, whereas it did not affect healthy-aged muscles of C57BL6J. Curcumin-treated old C57BL10ScSn soleus preserved type-1 myofiber size and increased type-2A one, whereas EDL maintained adult values of total myofiber number and fiber-type composition. Mechanistically, curcumin only partially prevented the age-related changes in protein level and subcellular distribution of major costamere components and regulators. Conversely, it affected satellite cells, by maintaining adult levels of myofiber maturation in old regenerating soleus and increasing percentage of isolated, MyoD-positive satellite cells from old hindlimb muscles. Therefore, curcumin treatment successfully prevents presarcopenia and sarcopenia development by improving satellite cell commitment and recruitment.
    Keywords:  Grp94; aging; dystrophin; gp96; melusin; nNOS; sarcopenia; satellite cells; senescence
  21. Int J Mol Sci. 2021 Oct 21. pii: 11377. [Epub ahead of print]22(21):
      Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.
    Keywords:  amphiphysin; autophagy; centronuclear myopathy; dynamin; membrane trafficking; myotubular myopathy; myotubularin; ryanodine receptor; satellite cell; triads
  22. Curr Res Physiol. 2020 Dec;3 34-43
      Lactate is not merely a metabolic intermediate that serves as an oxidizable and glyconeogenic substrate, but it is also a potential signaling molecule. The objectives of this study were to investigate whether lactate administration enhances post-exercise glycogen repletion in association with cellular signaling activation in different types of skeletal muscle. Eight-week-old male ICR mice performed treadmill running (20 m/min for 60 min) following overnight fasting (16 h). Immediately after the exercise, animals received an intraperitoneal injection of phosphate-buffered saline or sodium lactate (equivalent to 1 g/kg body weight), followed by oral ingestion of water or glucose (2 g/kg body weight). At 60 min of recovery, glucose ingestion enhanced glycogen content in the soleus, plantaris, and gastrocnemius muscles. In addition, lactate injection additively increased glycogen content in the plantaris and gastrocnemius muscles, but not in the soleus muscle. Nevertheless, lactate administration did not significantly alter protein levels related to glucose uptake and oxidation in the plantaris muscle, but enhanced phosphorylation of TBC1D1, a distal protein regulating GLUT4 translocation, was observed in the soleus muscle. Muscle FBP2 protein content was significantly higher in the plantaris and gastrocnemius muscles than in the soleus muscle, whereas MCT1 protein content was significantly higher in the soleus muscle than in the plantaris and gastrocnemius muscles. The current findings suggest that an elevated blood lactate concentration and post-exercise glucose ingestion additively enhance glycogen recovery in glycolytic phenotype muscles. This appears to be associated with glyconeogenic protein content, but not with enhanced glucose uptake, attenuated glucose oxidation, or lactate transport protein.
    Keywords:  Exercise; Glycogen; Glyconeogenesis; Lactate; Recovery; Skeletal muscle
  23. Biochim Biophys Acta Mol Cell Res. 2021 Nov 08. pii: S0167-4889(21)00224-X. [Epub ahead of print] 119170
      Skeletal muscles represent a complex and highly organised tissue responsible for all voluntary body movements. Developed through an intricate and tightly controlled process known as myogenesis, muscles form early in development and are maintained throughout life. Due to the constant stresses that muscles are subjected to, skeletal muscles maintain a complex course of regeneration to both replace and repair damaged myofibers and to form new functional myofibers. This process, made possible by a pool of resident muscle stem cells, termed satellite cells, and controlled by an array of transcription factors, is additionally reliant on a diverse range of cell adhesion molecules and the numerous signaling cascades that they initiate. This article will review the literature surrounding adhesion molecules and their roles in skeletal muscle myogenesis and repair.
    Keywords:  Cell adhesion molecules; Myogenesis; Satellite cells
  24. Commun Biol. 2021 Nov 12. 4(1): 1280
      Skeletal muscle repair is driven by the coordinated self-renewal and fusion of myogenic stem and progenitor cells. Single-cell gene expression analyses of myogenesis have been hampered by the poor sampling of rare and transient cell states that are critical for muscle repair, and do not inform the spatial context that is important for myogenic differentiation. Here, we demonstrate how large-scale integration of single-cell and spatial transcriptomic data can overcome these limitations. We created a single-cell transcriptomic dataset of mouse skeletal muscle by integration, consensus annotation, and analysis of 23 newly collected scRNAseq datasets and 88 publicly available single-cell (scRNAseq) and single-nucleus (snRNAseq) RNA-sequencing datasets. The resulting dataset includes more than 365,000 cells and spans a wide range of ages, injury, and repair conditions. Together, these data enabled identification of the predominant cell types in skeletal muscle, and resolved cell subtypes, including endothelial subtypes distinguished by vessel-type of origin, fibro-adipogenic progenitors defined by functional roles, and many distinct immune populations. The representation of different experimental conditions and the depth of transcriptome coverage enabled robust profiling of sparsely expressed genes. We built a densely sampled transcriptomic model of myogenesis, from stem cell quiescence to myofiber maturation, and identified rare, transitional states of progenitor commitment and fusion that are poorly represented in individual datasets. We performed spatial RNA sequencing of mouse muscle at three time points after injury and used the integrated dataset as a reference to achieve a high-resolution, local deconvolution of cell subtypes. We also used the integrated dataset to explore ligand-receptor co-expression patterns and identify dynamic cell-cell interactions in muscle injury response. We provide a public web tool to enable interactive exploration and visualization of the data. Our work supports the utility of large-scale integration of single-cell transcriptomic data as a tool for biological discovery.
  25. J Gen Physiol. 2022 Sep 05. pii: e2021ecc35. [Epub ahead of print]154(9):
      Synergistic ablation (SA) is widely used to induce muscle hypertrophy in rodent studies. However, it has been demonstrated that SA-induced compensatory hypertrophy induces increases in maximum isometric force that are smaller in magnitude than the increase in muscle cross-sectional area, suggesting a reduction in the specific force production due to intrinsic contractile dysfunction in the hypertrophied fibers. Here, by using the mechanical skinned fibers, we investigated the mechanisms behind the reduction in specific force in the compensatory hypertrophied muscles. Rats had unilateral surgical ablation of the gastrocnemius and soleus muscles to induce the compensatory hypertrophy in the plantaris muscles. Two wk after surgery, the mean fiber diameter was increased by 19% in the SA group compared with the contralateral control (CNT) group. In contrast, compared with the CNT group, both the depolarization-induced force (-51%) and the Ca2+-activated maximum specific force (-32%) were markedly reduced in skinned fibers from the SA group. These deleterious functional alterations were accompanied by decreases in the amount of DHPRα1, RYR, junctophilin 1, and SH3 and cysteine-rich domain 3 (STAC3) in SA muscles. Thus, these data clearly show that SA induces not only an increase in skeletal muscle fiber hypertrophy but also leads to a reduction in the intrinsic contractile dysfunction due to the excitation-contraction uncoupling and impaired force-generating capacity.
  26. J Gen Physiol. 2022 Sep 05. pii: e2021ecc38. [Epub ahead of print]154(9):
      Eccentric contractions, in which the muscle is stretched during contraction, cause substantially greater damage than isometric (ISO) contractions, in which the length of the muscle does not change during contraction. Here, we tested the hypothesis that ISO training improves fatigue resistance in skeletal muscle from dystrophin-deficient mdx52 mice (15-22 wk old). ISO training (100 Hz stimulation frequency, 0.25-s contractions every 0.5 s, 6 sets of 60 contractions) was performed on the left plantar flexor muscles in vivo with supramaximal electrical stimulation every other day for 4 wk. Compared with the normal control muscle, resistance to fatigue was reduced in the nontrained muscle from mdx52 mice, which was accompanied by a reduction in citrate synthase activity and the LC3BII/I ratio and an increase in the phosphorylation levels of Akt Ser473 and the expression levels of p62. ISO training restored these alterations and markedly increased in vivo fatigue resistance and PGC-1α expression in mdx52 muscles. Moreover, an increased number of Evans Blue dye-positive fibers was significantly reduced by ISO training in mdx52 muscles. In contrast, ISO training did not restore a reduction in the amount of SH3 and cysteine-rich domain 3 in mdx muscles. Thus, our data suggest that mitochondrial function is impaired in dystrophin-deficient muscles, which is likely to be induced by the defective autophagy due to persistent activation of Akt. ISO training inhibits the aberrant activation of Akt presumably by up-regulating the PGC-1α expression, which results in improved mitochondrial function and thus fatigue resistance in dystrophin-deficient muscles.
  27. Am J Physiol Endocrinol Metab. 2021 Nov 08.
      Sprint interval training (SIT) is a time efficient alternative to endurance exercise, conferring beneficial skeletal muscle metabolic adaptations. Current literature has investigated the nutritional regulation of acute and chronic exercise-induced metabolic adaptations in muscle following endurance exercise, principally comparing the impact of training in fasted and carbohydrate-fed (CHO) conditions. Alternative strategies such as exercising in low CHO, protein-fed conditions remain poorly characterised, specifically pertaining to adaptations associated with SIT. Thus, this study aimed to compare the metabolic and performance adaptations to acute and short term SIT in the fasted state with pre-exercise hydrolysed (WPH) or concentrate (WPC) whey protein supplementation. In healthy males, pre-exercise protein ingestion did not alter exercise-induced increases in PGC-1α, PDK4, SIRT1, and PPAR-δ mRNA expression following acute SIT. However, supplementation of WPC and WPH beneficially altered acute exercise-induced SIRT4 and CD36 mRNA expression, respectively. Pre-exercise protein ingestion attenuated acute exercise-induced increases in muscle pan-acetylation, and PARP1 protein content compared with fasted SIT. Acute serum metabolomic differences confirmed greater pre-exercise amino acid delivery in protein-fed compared with fasted conditions. Following 3 weeks of SIT, training-induced increases in mitochondrial enzymatic activity and exercise performance were similar across nutritional groups. Interestingly, resting muscle acetylation status was favourably regulated in WPH conditions following training. Such findings suggest pre-exercise WPC and WPH ingestion positively influences metabolic adaptations to SIT compared to fasted training, resulting in either similar or enhanced performance adaptations. Future studies investigating nutritional modulation of metabolic adaptations to exercise are warranted to build upon these novel findings.
    Keywords:  exercise performance; mitochondrial biogenesis; skeletal muscle metabolism; whey protein
  28. Nat Rev Nephrol. 2021 Nov 08.
      Loss of muscle proteins is a deleterious consequence of chronic kidney disease (CKD) that causes a decrease in muscle strength and function, and can lead to a reduction in quality of life and increased risk of morbidity and mortality. The effectiveness of current treatment strategies in preventing or reversing muscle protein losses is limited. The limitations largely stem from the systemic nature of diseases such as CKD, which stimulate skeletal muscle protein degradation pathways while simultaneously activating mechanisms that impair muscle protein synthesis and repair. Stimuli that initiate muscle protein loss include metabolic acidosis, insulin and IGF1 resistance, changes in hormones, cytokines, inflammatory processes and decreased appetite. A growing body of evidence suggests that signalling molecules secreted from muscle can enter the circulation and subsequently interact with recipient organs, including the kidneys, while conversely, pathological events in the kidney can adversely influence protein metabolism in skeletal muscle, demonstrating the existence of crosstalk between kidney and muscle. Together, these signals, whether direct or indirect, induce changes in the levels of regulatory and effector proteins via alterations in mRNAs, microRNAs and chromatin epigenetic responses. Advances in our understanding of the signals and processes that mediate muscle loss in CKD and other muscle wasting conditions will support the future development of therapeutic strategies to reduce muscle loss.
  29. Diabetes. 2021 Nov 09. pii: db210601. [Epub ahead of print]
      One exercise session can elevate insulin-stimulated glucose uptake (ISGU) in skeletal muscle, but the mechanisms remain elusive. Circumstantial evidence suggests a role for Akt substrate of 160 kDa (AS160 or TBC1D4). We used genetic approaches to rigorously test this idea. The initial experiment evaluated AS160's role for the postexercise increase in ISGU using muscles from male wildtype (WT) and AS160-knockout (AS160-KO) rats. The next experiment used AS160-KO rats with an adeno-associated virus (AAV) approach to determine if rescuing muscle AS160 deficiency could restore exercise's ability to improve ISGU. The third experiment tested if eliminating the muscle GLUT4 deficit in AS160-KO rats via AAV-delivered GLUT4 would enable postexercise enhancement of ISGU. The final experiment employed AS160-KO rats and AAV-delivery of AS160 mutated to prevent phosphorylation of Ser588, Thr642, and Ser704 to evaluate their role in postexercise ISGU. We discovered: 1) AS160 expression was essential for postexercise increase in ISGU; 2) rescuing muscle AS160 expression of AS160-KO rats restored postexercise enhancement of ISGU; 3) restoring GLUT4 expression in AS160-KO muscle did not rescue the postexercise increase in ISGU; and 4) although AS160 phosphorylation on 3 key sites was not required for postexercise elevation in ISGU, it was essential for the full-exercise effect.
  30. J Appl Physiol (1985). 2021 Nov 11.
      Cancer cachexia (CC) results in impaired muscle function and quality of life and is the primary cause of death for ~20-30% of cancer patients. We demonstrated mitochondrial degeneration as a precursor to CC in male mice, however, if such alterations occur in females is currently unknown. The purpose of this study was to elucidate muscle alterations in CC development in female tumor-bearing mice. 60 female C57BL/6J mice were injected with PBS or Lewis Lung Carcinoma at 8-week age, and tumors developed for 1, 2, 3, or 4 weeks to assess the time course of cachectic development. In vivo muscle contractile function, protein fractional synthetic rate (FSR), protein turnover, and mitochondrial health were assessed. 3- and 4-week tumor-bearing mice displayed a dichotomy in tumor growth and were reassigned to High Tumor (HT) and Low Tumor (LT) groups. HT mice exhibited lower soleus, TA, and fat weights compared to PBS. HT mice showed lower peak isometric torque and slower one-half relaxation time compared to PBS. HT mice had lower FSR compared to PBS while E3 ubiquitin ligases were greater in HT compared to other groups. Bnip3 (mitophagy) and pMitoTimer red puncta (mitochondrial degeneration) were greater in HT while Pgc1α1 and Tfam (mitochondrial biogenesis) were lower in HT compared to PBS. We demonstrate alterations in female tumor-bearing mice where HT exhibited greater protein degradation, impaired muscle contractility, and mitochondrial degeneration compared to other groups. Our data provide novel evidence for a distinct cachectic development in tumor-bearing female mice compared to previous male studies.
    Keywords:  Lewis Lung Carcinoma; Mitochondrial function; mitochondrial quality control; muscle atrophy; skeletal muscle contractility
  31. J Biophotonics. 2021 Nov 09. e202100246
      Exercise, especially anaerobic one, can gradually increase muscle mass over time as a result of adaptive responses of muscle cells to ensure metabolic homeostasis in the tissue. Low-level light therapy (LLLT) or photobiomodulation (PBM) exhibits beneficial effects on promoting muscular functions, regeneration, and recovery from exhausting exercise, although the underlying cellular mechanisms remain poorly understood. We found that hypoxia, a condition following anaerobic exercise, significantly impeded myotube differentiation from myoblasts. However, this adverse effect was blunted greatly by pre-exposure of myoblast cells to a 980 nm laser at 0.1 J/cm2 , resulting in almost nearly normal myotube differentiation. LLL pre-treatment enhanced myotube formation by 80%, with a tubular diameter of 4.28±0.11 μm on average, representative of a 53.4% increase over sham light treatment. The normalized myoblast differentiation concurred with 68% more mitochondrial mass and myogenin expression over controls. Moreover, LLL-pretreatment appeared to enhance glucose uptake, prevent energy metabolic switch from oxidative phosphorylation to glycolysis, and diminish lactate production under hypoxic conditions. The observation provides valuable guidance with respect to the timing of LLLT and its potential effects on muscle strengths in concert with anaerobic exercise. This article is protected by copyright. All rights reserved.
  32. Nanomedicine (Lond). 2021 Nov 08.
      Skeletal muscle is integral to the functioning of the human body. Several pathological conditions, such as trauma (primary lesion) or genetic diseases such as Duchenne muscular dystrophy (DMD), can affect and impair its functions or exceed its regeneration capacity. Tissue engineering (TE) based on natural, synthetic and hybrid biomaterials provides a robust platform for developing scaffolds that promote skeletal muscle regeneration, strength recovery, vascularization and innervation. Recent 3D-cell printing technology and the use of nanocarriers for the release of drugs, peptides and antisense oligonucleotides support unique therapeutic alternatives. Here, the authors present recent advances in scaffold biomaterials and nano-based therapeutic strategies for skeletal muscle regeneration and perspectives for future endeavors.
    Keywords:  muscle wasting; nanocarriers; tissue engineering; volumetric muscle loss
  33. Biochem J. 2021 Nov 12. 478(21): 3809-3826
      While the etiology of type 2 diabetes is multifaceted, the induction of insulin resistance in skeletal muscle is a key phenomenon, and impairments in insulin signaling in this tissue directly contribute to hyperglycemia. Despite the lack of clarity regarding the specific mechanisms whereby insulin signaling is impaired, the key role of a high lipid environment within skeletal muscle has been recognized for decades. Many of the proposed mechanisms leading to the attenuation of insulin signaling - namely the accumulation of reactive lipids and the pathological production of reactive oxygen species (ROS), appear to rely on this high lipid environment. Mitochondrial biology is a central component to these processes, as these organelles are almost exclusively responsible for the oxidation and metabolism of lipids within skeletal muscle and are a primary source of ROS production. Classic studies have suggested that reductions in skeletal muscle mitochondrial content and/or function contribute to lipid-induced insulin resistance; however, in recent years the role of mitochondria in the pathophysiology of insulin resistance has been gradually re-evaluated to consider the biological effects of alterations in mitochondrial content. In this respect, while reductions in mitochondrial content are not required for the induction of insulin resistance, mechanisms that increase mitochondrial content are thought to enhance mitochondrial substrate sensitivity and submaximal adenosine diphosphate (ADP) kinetics. Thus, this review will describe the central role of a high lipid environment in the pathophysiology of insulin resistance, and present both classic and contemporary views of how mitochondrial biology contributes to insulin resistance in skeletal muscle.
    Keywords:  diabetes; insulin; metabolic syndromes; metabolism; mitochondria; muscle
  34. Cannabis Cannabinoid Res. 2021 Nov 11.
      Background: Cannabidiol (CBD) is becoming increasingly popular for the treatment of clinical conditions including as an aid for muscle recovery. Previous work demonstrated that CBD exhibited mild effects on skeletal muscle, with a tendency to increase anabolic signaling and decrease inflammatory signaling. Methods: To gain mechanistic insight and extend these findings, we conducted a set of experiments using C2C12 myotubes. Results: Increasing the dose of CBD (1-5 μM) provided with insulin-like growth factor 1 (IGF-1) showed no effect on anabolic signaling through mTORC1 (S6K1 [Thr389], p=0.27; rpS6 [Ser240/244], p=0.81; or 4E-BP1 [Thr37/46], p=0.87). Similarly, inflammatory signaling through nuclear factor kappa B (NF-κB) (p105, p=0.88; p50, p=0.93; or phosphorylated p65 [Ser536], p=0.84) in response to tumor necrosis factor α (TNFα) was unaffected by CBD (2.5 μM), whereas dioscin, a natural product that blocks NF-κB signaling, reduced p105 and phosphorylated p65 (Ser536) compared with the TNFα and the TNFα + CBD condition (p<0.01 and p<0.05, respectively). Finally, cannabinoid receptor type 1 (CB1) receptor levels were measured in C2C12 cells, murine skeletal muscle, cortex, and hippocampus. Although CB1 was not detectable in muscle cells or muscle tissue, high levels were observed in brain tissue. Conclusion: In conclusion, CBD does not directly modulate anabolic or inflammatory signaling in myotubes in vitro, which can likely be explained by the lack of functional receptors.
    Keywords:  C2C12; CBD; NF-κB; inflammation; mTORC1; muscle
  35. Int J Mol Sci. 2021 Oct 29. pii: 11764. [Epub ahead of print]22(21):
      It is well established that microgravity exposure causes significant muscle weakness and atrophy via muscle unloading. On Earth, muscle unloading leads to a disproportionate loss in muscle force and size with the loss in muscle force occurring at a faster rate. Although the exact mechanisms are unknown, a role for Ca2+ dysregulation has been suggested. The sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) pump actively brings cytosolic Ca2+ into the SR, eliciting muscle relaxation and maintaining low intracellular Ca2+ ([Ca2+]i). SERCA dysfunction contributes to elevations in [Ca2+]i, leading to cellular damage, and may contribute to the muscle weakness and atrophy observed with spaceflight. Here, we investigated SERCA function, SERCA regulatory protein content, and reactive oxygen/nitrogen species (RONS) protein adduction in murine skeletal muscle after 35-37 days of spaceflight. In male and female soleus muscles, spaceflight led to drastic impairments in Ca2+ uptake despite significant increases in SERCA1a protein content. We attribute this impairment to an increase in RONS production and elevated total protein tyrosine (T) nitration and cysteine (S) nitrosylation. Contrarily, in the tibialis anterior (TA), we observed an enhancement in Ca2+ uptake, which we attribute to a shift towards a faster muscle fiber type (i.e., increased myosin heavy chain IIb and SERCA1a) without elevated total protein T-nitration and S-nitrosylation. Thus, spaceflight affects SERCA function differently between the soleus and TA.
    Keywords:  calcium ATPase; calcium handling; muscle fiber type; neuronatin; phospholamban; sarcolipin; sarcoplasmic reticulum; spaceflight; unloading
  36. Hum Mol Genet. 2021 Nov 11. pii: ddab326. [Epub ahead of print]
      The common null polymorphism (R577X) in the ACTN3 gene is present in over 1.5 billion people worldwide and results in the absence of the protein α-actinin-3 from the Z-discs of fast-twitch skeletal muscle fibres. We have previously reported that this polymorphism is a modifier of dystrophin deficient Duchenne Muscular Dystrophy. To investigate the mechanism underlying this we use a double knockout (dk)Actn3KO/mdx (dKO) mouse model which lacks both dystrophin and sarcomere α-actinin-3. We used dKO mice and mdx dystrophic mice at 12 months (aged) to investigate the correlation between morphological changes to the fast-twitch dKO EDL and the reduction in force deficit produced by an in vitro eccentric contraction protocol. In the aged dKO mouse we found a marked reduction in fibre branching complexity that correlated with protection from eccentric contraction induced force deficit. Complex branches in the aged dKO EDL fibres (28%) were substantially reduced compared to aged mdx EDL fibres (68%) and this correlates with a graded force loss over three eccentric contractions for dKO muscles (~35% after first contraction, ~ 66% overall) compared to an abrupt drop in mdx upon the first eccentric contraction (~73% after first contraction, ~ 89% after three contractions). In dKO protection from eccentric contraction damage was linked with a doubling of SERCA1 pump density the EDL. We propose that the increased oxidative metabolism of fast-twitch glycolytic fibres characteristic of the null polymorphism (R577X) and increase in SR Ca2+ pump proteins reduces muscle fibre branching and decreases susceptibility to eccentric injury in the dystrophinopathies.
  37. J Gen Physiol. 2022 Sep 05. pii: e2021ecc33. [Epub ahead of print]154(9):
      Septins are considered as the fourth component of the cytoskeleton, with septin-7 isoform playing a critical role in myogenic cell division and fusion. Skeletal muscle regeneration is a highly orchestrated process that requires many steps, including proper cell division to achieve functional recovery. Here, the role of septin-7 was investigated in this complex process. To this end, muscle injury was induced in wild type BL6/C57 and septin-7-conditional (mer-Cre-mer) knock-down mice by in vivo BaCl2 injection to the left m. tibialis anterior muscle (TA) of the mice (the right m. tibialis anterior muscle was nontreated control). Mice were sacrificed 4 and 14 d later to reflect the early (monitored by PAX7 level) and late (monitored by myogenin level) phases of muscle regeneration. Western blotting was used to follow the changes of septin-7, PAX7, and myogenin expression at the protein level, while changes of mRNA were detected by qPCR. Morphological differences were visualized by HE staining. Levels of septin-7 protein increased 4 and 14 d after injury in BL6/C57 mice and mRNA expression of SEPT7 showed significant elevation both 4 and 14 d after injection in Cre+ mice only, considered to be a compensatory increase of mRNA expression of SEPT7 in order to ensure the appropriate regeneration process. Furthermore, up-regulation of septin-7 protein was more pronounced on day 14 in both Cre- and Cre+ mice, which may indicate its importance in the later phase of regeneration. Level of PAX7 and myogenin were also increased 4 and 14 d after injury in BL6/C57, Cre-, and Cre+ mice, respectively. Taken together, our data suggest the importance of septin-7 in skeletal muscle regeneration.
  38. Trends Mol Med. 2021 Nov 05. pii: S1471-4914(21)00272-0. [Epub ahead of print]
      Aside from myofibers, numerous mononucleated cells reside in the skeletal muscle. These include the mesenchymal cells called fibro-adipogenic progenitors (FAPs), that support muscle development and regeneration in adult muscles. Recent evidence shows that defects in FAP function contributes to chronic muscle diseases and targeting FAPs offers avenues for treating these diseases.
    Keywords:  adipogenesis; fibrosis; injury; muscular dystrophy; osteogenesis; stromal cells
  39. J Gen Physiol. 2022 Sep 05. pii: e2021ecc16. [Epub ahead of print]154(9):
      Duchenne muscular dystrophy (DMD) is a fatal X-linked genetic disease characterized by progressive loss of skeletal muscle. The mechanisms underlying the DMD pathology likely involve the complex interaction between reactive oxygen species (ROS) impaired Ca2+ handling and chronic inflammation, characterized by the presence of immune cells such as neutrophils. Hypochlorous acid (HOCl) is a highly reactive form of ROS produced endogenously via the actions of myeloperoxidase, an enzyme secreted by neutrophils. Myeloperoxidase activity is significantly elevated in dystrophic muscle. This study aimed to determine the effect of HOCl exposure on excitation-contraction coupling and its potential contribution to the dystrophic pathology. Isolated extensor digitorum longus (EDL) muscles and single fibers from C57 (wild type) and mdx (dystrophic) mice were used to investigate the effects of HOCl on whole muscle function, intracellular Ca2+ handling, and myofilament force production. HOCl exposure significantly decreased maximum specific force in isolated EDL muscles by 26% and 49%, respectively, in C57 and mdx mice (P < 0.0001). In single interosseous fibers, HOCl exposure significantly increased resting intracellular Ca2+ concentration by ∼17-19% (P < 0.05) and decreased the amplitude of electrically induced Ca2+ transients by ∼45% and 50%, respectively, in C57 and mdx fibers (C57, P < 0.05; mdx, P < 0.01). These effects of HOCl on resting Ca2+ could be blocked via application of tetracaine (ryanodine receptor blocker) or Gd3+ (stretch-activated channel blocker; C57, P < 0.01; mdx, P < 0.01 for both). The effect of HOCl on Ca2+ transient amplitude was significantly reduced by Gd3+ (C57, P < 0.05; mdx, P < 0.01). In chemically skinned EDL fibers, HOCl exposure decreased maximum Ca2+-activated force by ∼40% in both C57 and mdx fibers (P < 0.001). These results indicate that HOCl potently affects excitation-contraction coupling via impaired Ca2+ handling and myofilament force production. Hence, HOCl potentially links the chronic inflammation, oxidative stress, and impaired Ca2+ handling that underlies the dystrophic pathology.
  40. Biochem J. 2021 Nov 12. 478(21): 3827-3846
      The interaction between insulin and exercise is an example of balancing and modifying the effects of two opposing metabolic regulatory forces under varying conditions. While insulin is secreted after food intake and is the primary hormone increasing glucose storage as glycogen and fatty acid storage as triglycerides, exercise is a condition where fuel stores need to be mobilized and oxidized. Thus, during physical activity the fuel storage effects of insulin need to be suppressed. This is done primarily by inhibiting insulin secretion during exercise as well as activating local and systemic fuel mobilizing processes. In contrast, following exercise there is a need for refilling the fuel depots mobilized during exercise, particularly the glycogen stores in muscle. This process is facilitated by an increase in insulin sensitivity of the muscles previously engaged in physical activity which directs glucose to glycogen resynthesis. In physically trained individuals, insulin sensitivity is also higher than in untrained individuals due to adaptations in the vasculature, skeletal muscle and adipose tissue. In this paper, we review the interactions between insulin and exercise during and after exercise, as well as the effects of regular exercise training on insulin action.
    Keywords:  exercise; glucose uptake; insulin; insulin sensitivity; muscle metabolism; physical activity
  41. Curr Res Physiol. 2021 ;4 47-59
      μ-Crystallin, encoded by the CRYM gene, binds the thyroid hormones, T3 and T4. Because T3 and T4 are potent regulators of metabolism and gene expression, and CRYM levels in human skeletal muscle can vary widely, we investigated the effects of overexpression of Crym. We generated transgenic mice, Crym tg, that expressed Crym protein specifically in skeletal muscle at levels 2.6-147.5 fold higher than in controls. Muscular functions, Ca2+ transients, contractile force, fatigue, running on treadmills or wheels, were not significantly altered, although T3 levels in tibialis anterior (TA) muscle were elevated ~190-fold and serum T4 was decreased 1.2-fold. Serum T3 and thyroid stimulating hormone (TSH) levels were unaffected. Crym transgenic mice studied in metabolic chambers showed a significant decrease in the respiratory exchange ratio (RER) corresponding to a 13.7% increase in fat utilization as an energy source compared to controls. Female but not male Crym tg mice gained weight more rapidly than controls when fed high fat or high simple carbohydrate diets. Although labeling for myosin heavy chains showed no fiber type differences in TA or soleus muscles, application of machine learning algorithms revealed small but significant morphological differences between Crym tg and control soleus fibers. RNA-seq and gene ontology enrichment analysis showed a significant shift towards genes associated with slower muscle function and its metabolic correlate, β-oxidation. Protein expression showed a similar shift, though with little overlap. Our study shows that μ-crystallin plays an important role in determining substrate utilization in mammalian muscle and that high levels of μ-crystallin are associated with a shift toward greater fat metabolism.
    Keywords:  Glycolysis; Proteomics; RER; RNA-seq; Thyroid hormone; β-oxidation
  42. Biophys Rev. 2021 Oct;13(5): 653-677
      Muscle specific signaling has been shown to originate from myofilaments and their associated cellular structures, including the sarcomeres, costameres or the cardiac intercalated disc. Two signaling hubs that play important biomechanical roles for cardiac and/or skeletal muscle physiology are the N2B and N2A regions in the giant protein titin. Prominent proteins associated with these regions in titin are chaperones Hsp90 and αB-crystallin, members of the four-and-a-half LIM (FHL) and muscle ankyrin repeat protein (Ankrd) families, as well as thin filament-associated proteins, such as myopalladin. This review highlights biological roles and properties of the titin N2B and N2A regions in health and disease. Special emphasis is placed on functions of Ankrd and FHL proteins as mechanosensors that modulate muscle-specific signaling and muscle growth. This region of the sarcomere also emerged as a hotspot for the modulation of passive muscle mechanics through altered titin phosphorylation and splicing, as well as tethering mechanisms that link titin to the thin filament system.
    Keywords:  Mechanosensor; Muscle; Muscle mechanics; Sarcomere; Signaling; Titin
  43. J Physiol. 2021 Nov 11.
      Sex differences in exercise physiology, such as substrate metabolism and skeletal muscle fatigability, stem from inherent biological factors, including endogenous hormones and genetics. Studies investigating exercise physiology frequently include only males or do not take sex differences into consideration. Although there is still an under-representation of female participants in exercise research, existing studies have identified sex differences in physiological and molecular responses to exercise training. The observed sex differences in exercise physiology are underpinned by the sex chromosome complement, sex hormones, and on a molecular level, the epigenome and transcriptome. Future research in the field should aim to include both sexes, control for menstrual cycle factors, conduct large-scale and ethnically diverse studies, conduct meta-analyses to consolidate findings from various studies, leverage unique cohorts (such as post-menopausal, transgender, and those with sex chromosome abnormalities), as well as integrate tissue and cell-specific -omics data. This knowledge is essential for developing deeper insight into sex-specific physiological responses to exercise training, thus directing future exercise physiology studies and practical application. This article is protected by copyright. All rights reserved.
  44. Am J Physiol Regul Integr Comp Physiol. 2021 Nov 10.
      Eccentric contractions (ECC) facilitate cytosolic calcium ion (Ca2+) release from the sarcoplasmic reticulum (SR) and Ca2+ influx from extracellular space. Ca2+ is a vital signaling messenger that regulates multiple cellular processes via its spatial and temporal concentration ([Ca2+]i) dynamics. We hypothesized that: 1) a specific pattern of spatial/temporal intramyocyte Ca2+ dynamics portends muscle damage following ECC, and 2) these dynamics would be regulated by the ryanodine receptor (RyR). [Ca2+]i in the tibialis anterior muscles of anesthetized adult Wistar rats was measured by ratiometric (i.e. ratio, R, 340/380 nm excitation) in vivo bioimaging with Fura-2 pre-ECC and at 5 and 24 hours post-ECC (5 x 40 contractions). Rats received RyR inhibitor dantrolene (DAN; 10 mg/kg i.p.) immediately post-ECC (+DAN). Muscle damage was evaluated by histological analysis on hematoxylin-eosin stained muscle sections. Compared to control (CONT, no ECC), [Ca2+]i distribution was heterogeneous with increased % total area of high [Ca2+]i sites (operationally defined as R > 1.39 i.e., > 1 SD of mean control) 5 hours post-ECC (CONT, 14.0 ± 8.0; ECC5h: 52.0 ± 7.4%, p < 0.01). DAN substantially reduced the high [Ca2+]i area 5 hours post-ECC (ECC5h+DAN: 6.4 ± 3.1%, p < 0.01) and myocyte damage (ECC24h, 63.2 ± 1.0%; ECC24h+DAN, 29.1 ± 2.2%, p < 0.01). Temporal and spatially-amplified [Ca2+]i fluctuations occurred regardless of DAN (ECC vs ECC+DAN, p > 0.05). These results suggest that the RyR-mediated local high [Ca2+]i itself is related to the magnitude of muscle damage while the [Ca2+]i fluctuation is an RyR-independent phenomenon.
    Keywords:  calcium dynamics; in vivo bioimaging; muscle damage; ryanodine receptor
  45. Int J Mol Sci. 2021 Oct 27. pii: 11587. [Epub ahead of print]22(21):
      Muscle fatigue (MF) declines the capacity of muscles to complete a task over time at a constant load. MF is usually short-lasting, reversible, and is experienced as a feeling of tiredness or lack of energy. The leading causes of short-lasting fatigue are related to overtraining, undertraining/deconditioning, or physical injury. Conversely, MF can be persistent and more serious when associated with pathological states or following chronic exposure to certain medication or toxic composites. In conjunction with chronic fatigue, the muscle feels floppy, and the force generated by muscles is always low, causing the individual to feel frail constantly. The leading cause underpinning the development of chronic fatigue is related to muscle wasting mediated by aging, immobilization, insulin resistance (through high-fat dietary intake or pharmacologically mediated Peroxisome Proliferator-Activated Receptor (PPAR) agonism), diseases associated with systemic inflammation (arthritis, sepsis, infections, trauma, cardiovascular and respiratory disorders (heart failure, chronic obstructive pulmonary disease (COPD))), chronic kidney failure, muscle dystrophies, muscle myopathies, multiple sclerosis, and, more recently, coronavirus disease 2019 (COVID-19). The primary outcome of displaying chronic muscle fatigue is a poor quality of life. This type of fatigue represents a significant daily challenge for those affected and for the national health authorities through the financial burden attached to patient support. Although the origin of chronic fatigue is multifactorial, the MF in illness conditions is intrinsically linked to the occurrence of muscle loss. The sequence of events leading to chronic fatigue can be schematically denoted as: trigger (genetic or pathological) -> molecular outcome within the muscle cell -> muscle wasting -> loss of muscle function -> occurrence of chronic muscle fatigue. The present review will only highlight and discuss current knowledge on the molecular mechanisms that contribute to the upregulation of muscle wasting, thereby helping us understand how we could prevent or treat this debilitating condition.
    Keywords:  atrophy; fatigue; muscle function; skeletal muscle
  46. J Gen Physiol. 2022 Sep 05. pii: e2021ecc18. [Epub ahead of print]154(9):
      This work describes a simple way to identify fiber types in living muscles by fluorescence lifetime imaging microscopy (FLIM). We quantified the mean values of lifetimes derived from a two-exponential fit (τ1 and τ2) in freshly dissected mouse FDB and soleus muscles. While τ1 values did not change between muscles, the distribution of τ2 shifted to higher values in FDB. To understand the origin of this difference, we obtained maps of autofluorescence lifetimes in cryosections of both muscles and paired them with immunofluorescence images of myosin heavy chain isoforms (MHC), which allow identification of fiber types. In soleus, τ2 was 3.1 ns for type I (SEM = 0.009, n = 49), 3.4 ns for type IIA (SEM = 0.01, n = 30), and 3.3 ns for type IIX (SEM = 0.01, n = 21). In FDB muscle, τ2 was 3.17 ns for type I (SEM = 0.04, n = 18), 3.5 ns for type IIA (SEM = 0.03, n = 27), and 3.62 ns for type IIX (SEM = 0.03, n = 22). From the distribution of measures, it follows that an FDB fiber with τ2 >3.3 ns is expected to be of type II, and of type I otherwise. This simple classification method has first- and second-class errors estimated at 0.06 and 0.27, respectively. Studies in progress aim at further elucidating the reasons for the different lifetimes, not just among fiber types but between fibers of the same type in the two muscles. Preliminary results point at differences in both the oxidation-reduction and protein-bound versus free states of flavins as causes for the observed divergence of fluorescence lifetimes. Lifetime maps of autofluorescence therefore constitute a tool to identify fiber type that, being practical, fast, and noninvasive, can be applied in living tissue without compromising other experimental interventions.
  47. Semin Cell Dev Biol. 2021 Oct 29. pii: S1084-9521(21)00272-X. [Epub ahead of print]
      A significant amount of attention has been brought to the endocrine-like function of skeletal muscle on various tissues, particularly with bone. Several lines of investigation indicate that the physiology of both bone and muscle systems may be regulated by a given stimulus, such as exercise, aging, and inactivity. Moreover, emerging evidence indicates that bone is heavily influenced by soluble factors derived from skeletal muscle (i.e., muscle-to-bone communication). The purpose of this review is to discuss the regulation of bone remodeling (formation and/or resorption) through skeletal muscle-derived cytokines (hereafter myokines) including the anti-inflammatory cytokine METRNL and pro-inflammatory cytokines (e.g., TNF-α, IL-6, FGF-2 and others). Our goal is to highlight possible therapeutic opportunities to improve muscle and bone health in aging.
    Keywords:  Bone; Crosstalk; Exercise; Myokine; Skeletal muscle
  48. J Gen Physiol. 2022 Sep 05. pii: e2021ecc11. [Epub ahead of print]154(9):
      Cardiac RYR2-mediated sarcoplasmic Ca2+ (SR) release is essential for matching increased energy demand during fight-or-flight response with mitochondrial metabolic output by delivering Ca2+ into the mitochondrial matrix to activate Ca2+-dependent Krebs cycle dehydrogenases. RYR2 complex gain-of-function mutations associated with catecholaminergic polymorphic ventricular tachycardia (CPVT) have been linked to mitochondrial structural damage and enhanced production of reactive oxygen species (ROS). Despite being critical for arrhythmogenesis in CPVT, the exact causes of these phenomena remain undetermined. Taking advantage of a new rat model of CPVT induced by heterozygous RYR2 gain-of-function mutation S2222L, we tested how RYR2 overactivity alters mitochondrial Ca2+ and ROS handling, and how these changes cause mitochondrial structural defects. Injection of epinephrine (1 mg/kg) and caffeine (120 mg/kg) induced bigamy and bidirectional VT in vivo in 100% of CPVT rats. Simultaneous whole-cell patch clamp and confocal Ca2+-imaging demonstrated that under β-adrenergic stimulation with isoproterenol (50 nM), CPVT ventricular myocytes (VMs) exhibited severe Ca2+ mishandling and high propensity for generation of spontaneous Ca2+ waves (SCWs) that cause arrhythmogenic afterdepolarizations. Diminished Ca2+ transient amplitude in CPVT VMs resulted in a significant reduction in mitochondrial matrix-[Ca2+], and thereby a mito-ROS surge, visualized using matrix-targeted biosensors mtRCaMP1h and MLS-HyPer, respectively. Importantly, using novel Ca2+-biosensors targeted to intermembrane space (IMS-GECO), we uncovered that [Ca2+] in this compartment reaches 1 µM, sufficient for activation of Ca2+-dependent protease μ-calpain. Adenoviral overexpression of IMS-targeted calpastatin, an endogenous calpain inhibitor, reduced mito-ROS, restored cytosolic Ca2+ transient amplitude and SR Ca2+ content, and reduced RYR2-mediated SCWs in CPVT VMs. These changes were paralleled by restored expression levels of OPA1, a mitochondrial structural protein responsible for tight cristae organization. Our data suggest that enhanced mito-ROS due to matrix-[Ca2+] reduction in CPVT VMs and unexpectedly high IMS-[Ca2+] promotes IMS-calpain-mediated degradation of OPA1, resulting in mitochondrial structural damage that contributes to proarrhythmic remodeling.
  49. PLoS Genet. 2021 Nov 09. 17(11): e1009907
      Muscle cells have different phenotypes adapted to different usage, and can be grossly divided into fast/glycolytic and slow/oxidative types. While most muscles contain a mixture of such fiber types, we aimed at providing a genome-wide analysis of the epigenetic landscape by ChIP-Seq in two muscle extremes, the fast/glycolytic extensor digitorum longus (EDL) and slow/oxidative soleus muscles. Muscle is a heterogeneous tissue where up to 60% of the nuclei can be of a different origin. Since cellular homogeneity is critical in epigenome-wide association studies we developed a new method for purifying skeletal muscle nuclei from whole tissue, based on the nuclear envelope protein Pericentriolar material 1 (PCM1) being a specific marker for myonuclei. Using antibody labelling and a magnetic-assisted sorting approach, we were able to sort out myonuclei with 95% purity in muscles from mice, rats and humans. The sorting eliminated influence from the other cell types in the tissue and improved the myo-specific signal. A genome-wide comparison of the epigenetic landscape in EDL and soleus reflected the differences in the functional properties of the two muscles, and revealed distinct regulatory programs involving distal enhancers, including a glycolytic super-enhancer in the EDL. The two muscles were also regulated by different sets of transcription factors; e.g. in soleus, binding sites for MEF2C, NFATC2 and PPARA were enriched, while in EDL MYOD1 and SIX1 binding sites were found to be overrepresented. In addition, more novel transcription factors for muscle regulation such as members of the MAF family, ZFX and ZBTB14 were identified.