bims-moremu Biomed News
on Molecular regulators of muscle mass
Issue of 2021‒02‒21
forty-two papers selected by
Anna Vainshtein
Craft Science Inc.

  1. Cell Death Discov. 2021 Feb 17. 7(1): 35
      During differentiation, skeletal muscle develops mature multinucleated muscle fibers, which could contract to exert force on a substrate. Muscle dysfunction occurs progressively in patients with muscular dystrophy, leading to a loss of the ability to walk and eventually to death. The synthetic glucocorticoid dexamethasone (Dex) has been used therapeutically to treat muscular dystrophy by an inhibition of inflammation, followed by slowing muscle degeneration and stabilizing muscle strength. Here, in mice with muscle injury, we found that Dex significantly promotes muscle regeneration via promoting kinesin-1 motor activity. Nevertheless, how Dex promotes myogenesis through kinesin-1 motors remains unclear. We found that Dex directly increases kinesin-1 motor activity, which is required for the expression of a myogenic marker (muscle myosin heavy chain 1/2), and also for the process of myoblast fusion and the formation of polarized myotubes. Upon differentiation, kinesin-1 mediates the recruitment of integrin β1 onto microtubules allowing delivery of the protein into focal adhesions. Integrin β1-mediated focal adhesion signaling then guides myoblast fusion towards a polarized morphology. By imposing geometric constrains via micropatterns, we have proved that cell adhesion is able to rescue the defects caused by kinesin-1 inhibition during the process of myogenesis. These discoveries reveal a mechanism by which Dex is able to promote myogenesis, and lead us towards approaches that are more efficient in improving skeletal muscle regeneration.
  2. Front Physiol. 2020 ;11 596351
      A characteristic feature of eccentric as compared with concentric exercise is the ability to generate greater mechanical loads for lower cardiopulmonary demands. Current evidence concurs to show that eccentric training translates into considerable gains in muscle mass and strength. Less is known, however, regarding its impact on oxygen transport and on factors to be considered for optimizing its prescription and monitoring. This article reviews the existing evidence for endurance eccentric exercise effects on the components of the oxygen transport system from systemic to mitochondria in both humans and animals. In the studies reviewed, specially designed cycle-ergometers or downhill treadmill running were used to generate eccentric contractions. Observations to date indicate that overall, the aerobic demand associated with the eccentric training load was too low to significantly increase peak maximal oxygen consumption. By extension, it can be inferred that the very high eccentric power output that would have been required to solicit a metabolic demand sufficient to enhance peak aerobic power could not be tolerated or sustained by participants. The impact of endurance eccentric training on peripheral flow distribution remains largely undocumented. Given the high damage susceptibility of eccentric exercise, the extent to which skeletal muscle oxygen utilization adaptations would be seen depends on the balance of adverse and positive signals on mitochondrial integrity. The article examines the protection provided by repeated bouts of acute eccentric exercise and reports on the impact of eccentric cycling and downhill running training programs on markers of mitochondrial function and of mitochondrial biogenesis using mostly from animal studies. The summary of findings does not reveal an impact of training on skeletal muscle mitochondrial respiration nor on selected mitochondrial messenger RNA transcripts. The implications of observations to date are discussed within future perspectives for advancing research on endurance eccentric exercise physiological impacts and using a combined eccentric and concentric exercise approach to optimize functional capacity.
    Keywords:  calcium; eccentric training; free radicals; mitochondria; oxygen consumption
  3. Cell Rep. 2021 Jan 26. pii: S2211-1247(20)31645-4. [Epub ahead of print]34(4): 108656
      Muscle satellite cells (SCs) are a quiescent (non-proliferative) stem cell population in uninjured skeletal muscle. Although SCs have been investigated for nearly 60 years, the molecular drivers that transform quiescent SCs into the rapidly dividing (activated) stem/progenitor cells that mediate muscle repair after injury remain largely unknown. Here we identify a prominent FBJ osteosarcoma oncogene (Fos) mRNA and protein signature in recently activated SCs that is rapidly, heterogeneously, and transiently induced by muscle damage. We further reveal a requirement for FOS to efficiently initiate key stem cell functions, including cell cycle entry, proliferative expansion, and muscle regeneration, via induction of "pro-regenerative" target genes that stimulate cell migration, division, and differentiation. Disruption of one of these Fos/AP-1 targets, NAD(+)-consuming mono-ADP-ribosyl-transferase 1 (Art1), in SCs delays cell cycle entry and impedes progenitor cell expansion and muscle regeneration. This work uncovers an early-activated FOS/ART1/mono-ADP-ribosylation (MARylation) pathway that is essential for stem cell-regenerative responses.
    Keywords:  AP-1; ART1; FOS; MARylation; early activation; muscle satellite cells; muscle stem cells; post-translational regulation
  4. Front Physiol. 2020 ;11 615038
      Mitochondrial dysfunction is common to many organ system disorders, including skeletal muscle. Aging muscle and diseases of muscle are often accompanied by defective mitochondrial ATP production. This manuscript will focus on the pre-clinical evidence supporting the use of regular exercise to improve defective mitochondrial metabolism and function in skeletal muscle, through the stimulation of mitochondrial turnover. Examples from aging muscle, muscle-specific mutations and cancer cachexia will be discussed. We will also examine the effects of exercise on the important mitochondrial regulators PGC-1α, and Parkin, and summarize the effects of exercise to reverse mitochondrial dysfunction (e.g., ROS production, apoptotic susceptibility, cardiolipin synthesis) in muscle pathology. This paper will illustrate the breadth and benefits of exercise to serve as "mitochondrial medicine" with age and disease.
    Keywords:  aging; cancer; exercise as medicine; mitochondrial quality control; skeletal muscle
  5. Bone. 2021 Feb 13. pii: S8756-3282(21)00048-X. [Epub ahead of print] 115886
      Magnesium (Mg2+), as an essential mineral, supports and sustains the health and activity of the organs of the human body. Despite some clinical evidence on the association of Mg2+ deficiency with muscle regeneration dysfunction and sarcopenia in older-aged individuals, there is no consensus on the action mode and molecular mechanism by which Mg2+ influences aged muscle size and function. Here, we identified the appropriate Mg2+ environment that promotes the myogenic differentiation and myotube hypertrophy in both C2C12 myoblast and primary aged muscle stem cell (MuSC). Through animal experiments, we demonstrated that Mg2+ supplementation in aged mice significantly promotes muscle regeneration and conserves muscle mass and strength. Mechanistically, Mg2+ stimulation activated the mammalian target of rapamycin (mTOR) signaling, inducing the myogenic differentiation and protein synthesis, which consequently offers protections against the age-related decline in muscle regenerative potential and muscle mass. These findings collectively provide a promising therapeutic strategy for MuSC dysfunction and sarcopenia through Mg2+ supplementation in the elderly.
    Keywords:  Magnesium; aging; muscle regeneration; muscle stem cell; myogenic differentiation; sarcopenia
  6. J Transl Med. 2021 Feb 16. 19(1): 71
      Skeletal muscle aging is associated with a decline in motor function and loss of muscle mass- a condition known as sarcopenia. The underlying mechanisms that drive this pathology are associated with a failure in energy generation in skeletal muscle, either from age-related decline in mitochondrial function, or from disuse. To an extent, lifelong exercise is efficacious in preserving the energetic properties of skeletal muscle and thus may delay the onset of sarcopenia. This review discusses the cellular and molecular changes in skeletal muscle mitochondria during the aging process and how different exercise modalities work to reverse these changes. A key factor that will be described is the efficiency of mitochondrial coupling-ATP production relative to O2 uptake in myocytes and how that efficiency is a main driver for age-associated decline in skeletal muscle function. With that, we postulate the most effective exercise modality and protocol for reversing the molecular hallmarks of skeletal muscle aging and staving off sarcopenia. Two other concepts pertinent to mitochondrial efficiency in exercise-trained skeletal muscle will be integrated in this review, including- mitophagy, the removal of dysfunctional mitochondrial via autophagy, as well as the implications of muscle fiber type changes with sarcopenia on mitochondrial function.
    Keywords:  Aging; Exercise; Mitochondria; Skeletal muscle
  7. Front Physiol. 2021 ;12 620608
      Exercise not only produces beneficial effects on muscle itself via various molecular pathways, but also mediates the interaction between muscles and other organs in an autocrine/paracrine manner through myokines, which plays a positive role in maintaining overall health. Irisin, an exercise-derived myokine, has been found involved in the regulation of some cardiovascular diseases. However, the relationship between irisin and cardiovascular health is not fully elucidated and there are some divergences on the regulation of irisin by exercise. In this review, we present the current knowledge on the origin and physiology of irisin, describe the regulation of irisin by acute and chronic exercises, and discuss the divergences of the related research results. Importantly, we discuss the role of irisin as a biomarker in the diagnosis of cardiovascular diseases and describe its treatment and molecular mechanism in some cardiovascular diseases. It is expected that irisin will be used as a therapeutic agent to combat cardiovascular diseases or other disorders caused by inactivity in the near future.
    Keywords:  cardiovascular health; exercise; irisin; myokine; skeletal muscle
  8. J Biol Regul Homeost Agents. 2021 Feb 17. 35(1):
      Regular exercise induces intramuscular triglyceride accumulation with improved mitochondrial ability, but the mechanism remains unknown. The glycolytic product of exercise, lactate, has long been rec-ognized to suppress lipolysis and promote lipogenesis in adipocytes through inhibition of the cAMP-PKA pathway by activation of the G protein-coupled receptor (GPR81). However, whether lactate results in a similar process in skeletal muscle is unclear. Here, by using intramuscular injection of lactate to the gastrocnemius, the lipid metabolism effects were investigated in rat skeletal muscle. Firstly, the lactate-injection effect was verified by comparing changes in blood lactate levels from injection and exercise (30 min, 31 m/min, treadmill running). After five weeks of lactate intervention, intramuscular triglyceride levels in the gastrocnemius and the proportion of epididymis adipose mass to body weight increased. Chronic intramuscular injection of lactate elevated lactate receptor, GPR81, and reduced cAMP response element-binding (CREB) and P-CREB abundance in the gastrocnemius. Additionally, there was a significant decline in lipolytic-related proteins (AMPK, P-AMPK, P-HSL, CPT-1B, TGF-β2, SDHA) and a significant increase in fat synthesis proteins (SREBP-1C, PPAR-γ). Surprisingly, mitochondrial biomarkers (PGC-1α, CS) were also increased in the gastrocnemius, suggesting that chronic lactate might promote mitochondria biogenesis. Together, these results demonstrated that lactate may play a crucial role in triglyceride storage and mitochondria biogenesis in the skeletal muscle of rat.
    Keywords:  GPR81; Lactate; cAMP; intramuscular triglyceride; skeletal muscle
  9. J Muscle Res Cell Motil. 2021 Feb 19.
      Exercise training is considered as a potential intervention to counteract muscle degeneration in cancer cachexia. However, evidence to support such intervention is equivocal. Therefore, we investigated the effect of exercise training, i.e. voluntary wheel running, on muscle wasting, functional capacity, fiber type composition and vascularization during experimental cancer cachexia in mice. Balb/c mice were injected with PBS (CON) or C26 colon carcinoma cells to induce cancer cachexia (C26). Mice had free access to a running wheel in their home cage (CONEX and C26EX, n = 8-9) or were sedentary (CONS and C26S, n = 8-9). Mice were sacrificed 18 days upon tumor cell injection. Immunohistochemical analyes were performed on m. gastrocnemius and quadriceps, and ex vivo contractile properties were assessed in m. soleus and extensor digitorum longus (EDL). Compared with CON, C26 mice exhibited body weight loss (~ 20 %), muscle atrophy (~ 25 %), reduced grip strength (~ 25 %), and lower twitch and tetanic force (~ 20 %) production in EDL but not in m. soleus. Furthermore, muscle of C26 mice were characterizd by a slow-to-fast fiber type shift (type IIx fibers: +57 %) and increased capillary density (~ 30 %). In C26 mice, wheel running affect neither body weight loss, nor muscle atrophy or functional capacity, nor inhibited tumor growth. However, wheel running induced a type IIb to type IIa fiber shift in m. quadriceps from both CON and C26, but not in m. gastrocnemius. Wheel running does not exacerbate muscular degeneration in cachexic mice, but, when voluntary, is insufficient to improve the muscle phenotype.
    Keywords:  Cancer cachexia; Capillary density; Fiber type composition; Muscle atrophy; Muscle functionality; Voluntary wheel running
  10. Skelet Muscle. 2021 Feb 18. 11(1): 5
      BACKGROUND: Myogenin is a transcription factor that is expressed during terminal myoblast differentiation in embryonic development and adult muscle regeneration. Investigation of this cell state transition has been hampered by the lack of a sensitive reporter to dynamically track cells during differentiation.RESULTS: Here, we report a knock-in mouse line expressing the tdTOMATO fluorescent protein from the endogenous Myogenin locus. Expression of tdTOMATO in MyogntdTom mice recapitulated endogenous Myogenin expression during embryonic muscle formation and adult regeneration and enabled the isolation of the MYOGENIN+ cell population. We also show that tdTOMATO fluorescence allows tracking of differentiating myoblasts in vitro and by intravital imaging in vivo. Lastly, we monitored by live imaging the cell division dynamics of differentiating myoblasts in vitro and showed that a fraction of the MYOGENIN+ population can undergo one round of cell division, albeit at a much lower frequency than MYOGENIN- myoblasts.
    CONCLUSIONS: We expect that this reporter mouse will be a valuable resource for researchers investigating skeletal muscle biology in developmental and adult contexts.
    Keywords:  Intravital imaging; Knock-in mouse; Myogenin; Skeletal muscle; tdTOMATO
  11. Front Physiol. 2021 ;12 626096
      The nuclear receptor subfamily 1, group D member 1 (Nr1d1), plays a role in the skeletal muscle's oxidative capacity, mitochondrial biogenesis, atrophy genes, and muscle fiber size. In light of the effects of physical exercise, the present study investigates the acute response of Nr1d1 and genes related to atrophy and mitochondrial biogenesis on endurance and resistance exercise protocols. In this investigation, we observed, after one bout of endurance exercise, an upregulation of Nr1d1 in soleus muscle, but not in the gastrocnemius, and some genes related to mitochondrial biogenesis and atrophy were enhanced as well. Also, analysis of muscle transcripts from diverse isogenic BXD mice families revealed that the strains with higher Nr1d1 gene expression displayed upregulation of AMPK signaling and mitochondrial-related genes. In summary, a single session of endurance exercise can enhance the Nr1d1 mRNA levels in an oxidative muscle.
    Keywords:  Nr1d1; atrophy; exercise; mitochondrial biogenesis; skeletal muscle
  12. Adv Med Sci. 2021 Feb 13. pii: S1896-1126(21)00007-9. [Epub ahead of print]66(1): 155-161
      PURPOSE: Meteorin-like protein (METRNL) (also known as IL-41), recently identified as a myokine, is released in response to muscle contraction. It improves the skeletal muscle insulin sensitivity through exerting a beneficial anti-inflammatory effect. However, no independent studies have been published to verify the effects of METRNL on human umbilical vein endothelial cells (HUVECs) and THP-1 human monocytes.MATERIALS AND METHODS: The levels of NFκB and IκB phosphorylation as well as the expression of adhesion molecules were assessed by Western blotting analysis. Cell adhesion assay demonstrated the interactions between HUVEC and THP-1 ​cells. We used enzyme-linked immunosorbent assay (ELISA) to measure the levels of TNFα and MCP-1 in culture medium.
    RESULTS: Treatment with METRNL suppressed the secretion of TNFα and MCP-1 as well as NFκB and IκB phosphorylation and inflammatory markers in lipopolysaccharide (LPS)-treated HUVECs and THP-1 ​cells. Furthermore, treatment with METRNL ameliorated LPS-induced attachment of THP-1 monocytes to HUVECs via inhibition of adhesion molecule expression and apoptosis. Treatment of HUVEC and THP-1 ​cells with METRNL enhanced AMPK phosphorylation and PPARδ expression in a dose-dependent manner. Small interference (si) RNA-mediated suppression of AMPK or PPARδ restored all these changes.
    CONCLUSIONS: It has therefore been shown that METRNL ameliorates inflammatory responses through AMPK and PPARδ-dependent pathways in LPS-treated HUVEC. In sum, the current study may suggest the suppressive potential of METRNL against endothelial inflammation.
    Keywords:  AMPK; HUVEC; Inflammation; METRNL; PPARδ; THP-1
  13. J Appl Physiol (1985). 2021 Feb 18.
      Muscle atrophy decreases physical function and overall health. Increased glucocorticoid production and/or use of prescription glucocorticoids can significantly induce muscle atrophy by activating the glucocorticoid receptor thereby transcribing genes that shift protein balance in favor of net protein degradation. While mechanical overload can blunt glucocorticoid-induced atrophy in young muscle, those affected by glucocorticoids generally have impaired force generation. It is unknown whether contractile force alters the ability of resistance exercise to mitigate glucocorticoid receptor translocation and induce a desirable shift in protein balance when glucocorticoids are elevated. In the present study, mice were subjected to a single bout of unilateral, electrically induced muscle contractions by stimulating the sciatic nerve at 100 Hz or 50 Hz frequencies to elicit high force or moderate force contractions of the tibialis anterior, respectively. Dexamethasone was used to activate the glucocorticoid receptor. Dexamethasone increased glucocorticoid signaling, including nuclear translocation of the receptor, but this was mitigated only by high force contractions. The ability of high force contractions to mitigate glucocorticoid receptor translocation coincided with a contraction-mediated increase in muscle protein synthesis, which did not occur in the dexamethasone treated mice subjected to moderate force contractions. Though moderate force contractions failed to increase protein synthesis following dexamethasone treatment, both high and moderate force contractions blunted the glucocorticoid-mediated increase in LC3 II:I marker of autophagy. Thus, these data show that force generation is important for the ability of resistance exercise to mitigate glucocorticoid receptor translocation and promote a desirable shift in protein balance when glucocorticoids are elevated.
    Keywords:  autophagy; dexamethasone; muscle atrophy; protein synthesis
  14. Endocr Res. 2021 Feb 16. 1-15
      Skeletal muscle functions as a locomotory system and maintains whole-body metabolism. Sex differences in such skeletal muscle morphology and function have been documented; however, their underlying mechanisms remain elusive. Glucocorticoids are adrenocortical hormones maintaining homeostasis, including regulating whole-body energy metabolism in addition to stress response. In skeletal muscle, glucocorticoids can reduce the synthesis of muscle proteins and simultaneously accelerate the breakdown of proteins to regulate skeletal muscle mass and energy metabolism via a transcription factor glucocorticoid receptor (GR). We herein evaluated the related contributions of the GR to sex differences of gene expression profiles in skeletal muscle using GR-floxed (GRf/f) and skeletal muscle-specific GR knockout (GRmKO) mice. There were no differences in GR mRNA and protein expression levels in gastrocnemius muscle between males and females. A DNA microarray analysis using gastrocnemius muscle from GRf/f and GRmKO mice revealed that, although most gene expression levels were identical in both sexes, genes related to cholesterol and apolipoprotein synthesis and fatty acid biosynthesis and the immunological system were predominantly expressed in males and females, respectively, in GRf/f but not in GRmKO mice. Moreover, many genes were up-regulated in response to starvation in GRf/f but not in GRmKO mice, many of which were sex-independent and functioned to maintain homeostasis, while genes that showed sex dominance related to a variety of functions. Although the genes expressed in skeletal muscle may be predominantly sex-independent, sex-dominant genes may relate to sex differences in energy metabolism and the immune system and could be controlled by the GR.
    Keywords:  Sex difference; energy metabolism; gene expression; glucocorticoid receptor; skeletal muscle
  15. Cell Mol Life Sci. 2021 Feb 15.
      Human pluripotent stem cells (hPSCs) have attracted considerable interest in understanding the cellular fate determination processes and modeling a number of intractable diseases. In vitro generation of skeletal muscle tissues using hPSCs provides an essential model to identify the molecular functions and gene regulatory networks controlling the differentiation of skeletal muscle progenitor cells. Such a genetic roadmap is not only beneficial to understanding human myogenesis but also to decipher the molecular pathology of many skeletal muscle diseases. The combination of established human in vitro myogenesis protocols and newly developed molecular profiling techniques offers extensive insight into the molecular signatures for the development of normal and disease human skeletal muscle tissues. In this review, we provide a comprehensive overview of the current progress of in vitro skeletal muscle generation from hPSCs and relevant examples of the transcriptional landscape and disease-related transcriptional aberrations involving signaling pathways during the development of skeletal muscle cells.
    Keywords:  Myogenesis; Pluripotent stem cell; Skeletal muscle
  16. J Clin Invest. 2021 Feb 15. pii: 138634. [Epub ahead of print]131(4):
      Patients with neuromuscular disorders suffer from a lack of treatment options for skeletal muscle weakness and disease comorbidities. Here, we introduce as a potential therapeutic agent a heterodimeric ligand-trapping fusion protein, ActRIIB:ALK4-Fc, which comprises extracellular domains of activin-like kinase 4 (ALK4) and activin receptor type IIB (ActRIIB), a naturally occurring pair of type I and II receptors belonging to the TGF-β superfamily. By surface plasmon resonance (SPR), ActRIIB:ALK4-Fc exhibited a ligand binding profile distinctly different from that of its homodimeric variant ActRIIB-Fc, sequestering ActRIIB ligands known to inhibit muscle growth but not trapping the vascular regulatory ligand bone morphogenetic protein 9 (BMP9). ActRIIB:ALK4-Fc and ActRIIB-Fc administered to mice exerted differential effects - concordant with SPR results - on vessel outgrowth in a retinal explant assay. ActRIIB:ALK4-Fc induced a systemic increase in muscle mass and function in wild-type mice and in murine models of Duchenne muscular dystrophy (DMD), amyotrophic lateral sclerosis (ALS), and disuse atrophy. Importantly, ActRIIB:ALK4-Fc improved neuromuscular junction abnormalities in murine models of DMD and presymptomatic ALS and alleviated acute muscle fibrosis in a DMD model. Furthermore, in combination therapy ActRIIB:ALK4-Fc increased the efficacy of antisense oligonucleotide M12-PMO on dystrophin expression and skeletal muscle endurance in an aged DMD model. ActRIIB:ALK4-Fc shows promise as a therapeutic agent, alone or in combination with dystrophin rescue therapy, to alleviate muscle weakness and comorbidities of neuromuscular disorders.
    Keywords:  Drug therapy; Muscle Biology; Neuromuscular disease; Neuroscience; Skeletal muscle
  17. J Cell Physiol. 2021 Feb 15.
      Understanding the role of mechanical loading and exercise in skeletal muscle (SkM) is paramount for delineating the molecular mechanisms that govern changes in muscle mass. However, it is unknown whether loading of bioengineered SkM in vitro adequately recapitulates the molecular responses observed after resistance exercise (RE) in vivo. To address this, the transcriptional and epigenetic (DNA methylation) responses were compared after mechanical loading in bioengineered SkM in vitro and after RE in vivo. Specifically, genes known to be upregulated/hypomethylated after RE in humans were analyzed. Ninety-three percent of these genes demonstrated similar changes in gene expression post-loading in the bioengineered muscle when compared to acute RE in humans. Furthermore, similar differences in gene expression were observed between loaded bioengineered SkM and after programmed RT in rat SkM tissue. Hypomethylation occurred for only one of the genes analysed (GRIK2) post-loading in bioengineered SkM. To further validate these findings, DNA methylation and mRNA expression of known hypomethylated and upregulated genes post-acute RE in humans were also analyzed at 0.5, 3, and 24 h post-loading in bioengineered muscle. The largest changes in gene expression occurred at 3 h, whereby 82% and 91% of genes responded similarly when compared to human and rodent SkM respectively. DNA methylation of only a small proportion of genes analyzed (TRAF1, MSN, and CTTN) significantly increased post-loading in bioengineered SkM alone. Overall, mechanical loading of bioengineered SkM in vitro recapitulates the gene expression profile of human and rodent SkM after RE in vivo. Although some genes demonstrated differential DNA methylation post-loading in bioengineered SkM, such changes across the majority of genes analyzed did not closely mimic the epigenetic response to acute-RE in humans.
    Keywords:  DNA methylation; bioengineering; fibrin; gene expression; mechanical loading; skeletal muscle
  18. Front Cell Dev Biol. 2021 ;9 628339
      Skeletal muscle is a highly heterogeneous tissue that plays a crucial role in mammalian metabolism and motion maintenance. Myogenesis is a complex biological process that includes embryonic and postnatal development, which is regulated by specific signaling pathways and transcription factors. Various non-coding RNAs (ncRNAs) account for the majority of total RNA in cells and have an important regulatory role in myogenesis. In this review, we introduced the research progress in miRNAs, circRNAs, and lncRNAs related to embryonic and postnatal muscle development. We mainly focused on ncRNAs that regulate myoblast proliferation, differentiation, and postnatal muscle development through multiple mechanisms. Finally, challenges and future perspectives related to the identification and verification of functional ncRNAs are discussed. The identification and elucidation of ncRNAs related to myogenesis will enrich the myogenic regulatory network, and the effective application of ncRNAs will enhance the function of skeletal muscle.
    Keywords:  circRNAs; lncRNAs; miRNAs; muscle disease; myogenesis
  19. Front Oncol. 2020 ;10 617109
      Cachexia is a syndrome that affects the entire organism and presents a variable plethora of symptoms in patients, always associated with continuous and involuntary degradation of skeletal muscle mass and function loss. In cancer, this syndrome occurs in 50% of all patients, while prevalence increases to 80% as the disease worsens, reducing quality of life, treatment tolerance, therapeutic response, and survival. Both chronic systemic inflammation and immunosuppression, paradoxically, correspond to important features in cachexia patients. Systemic inflammation in cachexia is fueled by the interaction between tumor and peripheral tissues with significant involvement of infiltrating immune cells, both in the peripheral tissues and in the tumor itself. Autophagy, as a process of regulating cellular metabolism and homeostasis, can interfere with the metabolic profile in the tumor microenvironment. Under a scenario of balanced autophagy in the tumor microenvironment, the infiltrating immune cells control cytokine production and secretion. On the other hand, when autophagy is unbalanced or dysfunctional within the tumor microenvironment, there is an impairment in the regulation of immune cell's inflammatory phenotype. The inflammatory phenotype upregulates metabolic consumption and cytokine production, not only in the tumor microenvironment but also in other tissues and organs of the host. We propose that cachexia-related chronic inflammation can be, at least, partly associated with the failure of autophagic processes in tumor cells. Autophagy endangers tumor cell viability by producing immunogenic tumor antigens, thus eliciting the immune response necessary to counteract tumor progression, while preventing the establishment of inflammation, a hallmark of cachexia. Comprehensive understanding of this complex functional dichotomy may enhance cancer treatment response and prevent/mitigate cancer cachexia. This review summarizes the recent available literature regarding the role of autophagy within the tumor microenvironment and the consequences eliciting the development of cancer cachexia.
    Keywords:  DAMPs; autophagy; cachexia; lymphocyte infiltration; metabolism; systemic inflammations; tumor microenvironment
  20. Cytokine X. 2020 Jun;2(2): 100023
      Interleukin 6 (IL-6) is a secreted cytokine that is an important mediator of the immune response in numerous tissues, including skeletal muscle. IL-6 is considered a myokine as it can be secreted by muscle. IL-6 is secreted following exercise, where it exerts both pro-myogenic effects as well as anti-myogenic effects such as promoting atrophy and muscle wasting. The regulation of IL-6 in skeletal muscle is not well understood. The purpose of this study was to determine if IFN-γ and TNF-ɑ stimulate IL-6 in skeletal muscle. We found that both IFN-γ and TNF-α stimulate IL-6 in skeletal muscle, but the stimulation is not cooperative as seen in monocytes. We have previously shown that the IFN-γ stimulated class II major histocompatibility complex transactivator (CIITA) mediates many of the effects of IFN-γ in skeletal muscle and we show here that CIITA directly stimulates IL-6. The regulation of IL-6 by CIITA is clearly complex, as we found that CIITA both stimulates and restrains IL-6 expression. To show that these effects could be observed in a physiological setting, mice were treated with IFN-γ and we found that both CIITA and IL-6 were upregulated in skeletal muscle.
    Keywords:  CIITA; IFN-γ; IL-6; Skeletal muscle; TNF-ɑ
  21. Biosci Biotechnol Biochem. 2021 Feb 18. 85(2): 440-446
      Polymethoxyflavones (PMFs) contained in the peel of citrus fruits have anti-inflammatory, anticancer, and antidepressant effects. However, their effects on skeletal muscle are unknown. We investigated whether PMFs could prevent skeletal muscle damage induced by eccentric exercise in rats. Downhill running for 90 min increased the levels of the inflammatory cytokines, monocyte chemotactic protein-1 (MCP-1), and interleukin-1β (IL-1β) in skeletal muscles, especially in vastus lateralis, and the plasma creatine kinase levels. These increases were attenuated by a single oral administration of orange peel extract (OPE) 30 min before downhill running. A mixture of nobiletin, sinensetin, 3,5,6,7,8,3',4'-heptamethoxyflavone, and tangeretin, which are the major PMFs of OPE, also showed similar effects on muscle damage. These results suggest that OPE has a protective effect against eccentric exercise-induced skeletal muscle damage, and that the effects may be attributed to the 4 major PMFs.
    Keywords:  downhill running; eccentric exercise; inflammation; polymethoxyflavone; skeletal muscle
  22. Exp Physiol. 2021 Feb 18.
      NEW FINDINGS: What is the central question of this study? Following large traumatic loss of muscle tissue (i.e. volumetric muscle loss (VML)), permanent functional and cosmetic deficits present themselves. Regenerative therapies alone have not been able to generate a robust regenerative response. How the addition of rehabilitative therapies affects the regenerative response is underexplored. What is the main finding and its importance? Using exercise along with autologous muscle repair, we demonstrated accelerated muscle force recovery response post-VML. The accentuated force recovery 2 weeks post-VML would allow patients to return to home sooner than allowed with current therapies.ABSTRACT: Skeletal muscle can regenerate from damage but is overwhelmed with extreme tissue loss, known as volumetric muscle loss (VML). Patients suffering from VML do not fully recover force output in the affected limb. Recent studies show that replacement tissue (i.e. autograph) into the VML defect site plus physical activity show promise for optimizing force recovery post-VML. The purpose of this study was to measure the effects of autologous repair and voluntary wheel running on force recovery post-VML. 32 male Sprague-Dawley rats had 20% of their left tibialis anterior (LTA) excised then replaced and sutured into the intact muscle (autologous repair). The right limb acted as a contralateral control. 16 rats were given free access to a running wheel (Wheel) whereas the other 16 remained in a cage with the running wheel locked (Sed). At 2- and 8-weeks post-VML, the LTA underwent force testing, then the muscle was removed, and morphological and gene expression analysis were conducted. At 2-weeks post-injury, normalized LTA force was 58% greater in the Wheel group compared to the Sed group. At 8-weeks post-VML, LTA force was similar between the Wheel and Sed groups but was still lower than the uninjured RTA. Gene expression analysis at 2 weeks post-VML showed the wheel groups had lower mRNA content of IL-1β, IL-6, and TNF-α compared to the Sed group. Overall, voluntary wheel running promoted early force recovery, but was not sufficient to fully restore force. The accentuated early force recovery is possibly due to a more pro-regenerative microenvironment. This article is protected by copyright. All rights reserved.
    Keywords:  VML; exercise; inflammation; regenerative medicine; rehabilitative medicine
  23. Adv Mater. 2021 Feb 19. e2007946
      Skeletal muscle has an inherent capacity for spontaneous regeneration. However, recovery after severe injuries such as volumetric muscle loss (VML) is limited. There is therefore a need to develop interventions to induce functional skeletal muscle restoration. One suggested approach includes tissue-engineered muscle constructs. Tissue-engineering treatments have so far been impeded by the lack of reliable cell sources and the challenges in engineering of suitable tissue scaffolds. To address these challenges, muscle extracellular matrix (MEM) and induced skeletal myogenic progenitor cells (iMPCs) are integrated within thermally drawn fiber based microchannel scaffolds. The microchannel fibers decorated with MEM enhance differentiation and maturation of iMPCs. Furthermore, engraftment of these bioengineered hybrid muscle constructs induce de novo muscle regeneration accompanied with microvessel and neuromuscular junction formation in a VML mouse model, ultimately leading to functional recovery of muscle activity.
    Keywords:  direct reprogramming; skeletal muscle regeneration; thermal fiber drawing; volumetric muscle loss
  24. Mol Cell Proteomics. 2020 Dec 19. pii: S1535-9476(20)35144-6. [Epub ahead of print]20 100030
      Many cell surface and secreted proteins are modified by the covalent addition of glycans that play an important role in the development of multicellular organisms. These glycan modifications enable communication between cells and the extracellular matrix via interactions with specific glycan-binding lectins and the regulation of receptor-mediated signaling. Aberrant protein glycosylation has been associated with the development of several muscular diseases, suggesting essential glycan- and lectin-mediated functions in myogenesis and muscle development, but our molecular understanding of the precise glycans, catalytic enzymes, and lectins involved remains only partially understood. Here, we quantified dynamic remodeling of the membrane-associated proteome during a time-course of myogenesis in cell culture. We observed wide-spread changes in the abundance of several important lectins and enzymes facilitating glycan biosynthesis. Glycomics-based quantification of released N-linked glycans confirmed remodeling of the glycome consistent with the regulation of glycosyltransferases and glycosidases responsible for their formation including a previously unknown digalactose-to-sialic acid switch supporting a functional role of these glycoepitopes in myogenesis. Furthermore, dynamic quantitative glycoproteomic analysis with multiplexed stable isotope labeling and analysis of enriched glycopeptides with multiple fragmentation approaches identified glycoproteins modified by these regulated glycans including several integrins and growth factor receptors. Myogenesis was also associated with the regulation of several lectins, most notably the upregulation of galectin-1 (LGALS1). CRISPR/Cas9-mediated deletion of Lgals1 inhibited differentiation and myotube formation, suggesting an early functional role of galectin-1 in the myogenic program. Importantly, similar changes in N-glycosylation and the upregulation of galectin-1 during postnatal skeletal muscle development were observed in mice. Treatment of new-born mice with recombinant adeno-associated viruses to overexpress galectin-1 in the musculature resulted in enhanced muscle mass. Our data form a valuable resource to further understand the glycobiology of myogenesis and will aid the development of intervention strategies to promote healthy muscle development or regeneration.
    Keywords:  Glycoproteomics; galectin-1; glycomics; glycosylation; muscle development; myogenesis; myotubes; proteomics
  25. Biosci Biotechnol Biochem. 2021 Feb 03. pii: zbaa059. [Epub ahead of print]
      PGC-1α expression increases in skeletal muscles during exercise and regulates the transcription of many target genes. In this study, we conducted a metabolomic analysis on the blood of transgenic mice overexpressing PGC-1α in its skeletal muscle (PGC-1α-Tg mice) using CE-TOFMS. The blood level of homovanillic acid (dopamine metabolite) and the gene expression of dopamine metabolic enzyme in the skeletal muscle of PGC-1α-Tg mice were high. The blood level of 5-methoxyindoleacetic acid was also high in PGC-1α-Tg mice. The blood levels of branched-chain α-keto acids and β-alanine were low in PGC-1α-Tg mice. These metabolites in the skeletal muscle were present in low concentration. The changes in these metabolites may reflect the skeletal muscle condition with increasing PGC-1α, such as exercise.
    Keywords:  PGC-1α; branched-chain α-keto acids; exercise; homovanillic acid; metabolome
  26. Front Cell Dev Biol. 2020 ;8 616706
      Herein we report that the 18-base telomeric oligodeoxynucleotides (ODNs) designed from the Lactobacillus rhamnosus GG genome promote differentiation of skeletal muscle myoblasts which are myogenic precursor cells. We termed these myogenetic ODNs (myoDNs). The activity of one of the myoDNs, iSN04, was independent of Toll-like receptors, but dependent on its conformational state. Molecular simulation and iSN04 mutants revealed stacking of the 13-15th guanines as a core structure for iSN04. The alkaloid berberine bound to the guanine stack and enhanced iSN04 activity, probably by stabilizing and optimizing iSN04 conformation. We further identified nucleolin as an iSN04-binding protein. Results showed that iSN04 antagonizes nucleolin, increases the levels of p53 protein translationally suppressed by nucleolin, and eventually induces myotube formation by modulating the expression of genes involved in myogenic differentiation and cell cycle arrest. This study shows that bacterial-derived myoDNs serve as aptamers and are potential nucleic acid drugs directly targeting myoblasts.
    Keywords:  berberine (CID: 12457); myogenic differentiation; nucleolin (NCL); oligodeoxynucleotide (ODN); skeletal muscle myoblast
  27. Front Nutr. 2020 ;7 622391
      The term sarcopenia refers to the loss of skeletal muscle mass and strength that generally occurs during aging. The interventions that have proved most effective in reducing the severity and preventing the worsening of sarcopenia include physical exercise, especially resistance, and the administration of dietary supplements in association with a targeted diet; nutritional intervention is the main therapeutic approach for elderly people, since they are very often sedentary (also due to possible disabilities). Among the various nutrients, high biological value proteins and leucine are of particular interest for their demonstrated effects on the health of skeletal muscle. The intake of food containing proteins and leucine during meals stimulates muscle protein synthesis. Lower blood levels of leucine were associated with lower values of the skeletal muscle index, grip strength and performance. The international guidelines recommended that a leucine intake of 3 g at three main meals together with 25-30 g of protein is the goal to be achieved to counteract loss of lean mass in elderly. Food composition databases rarely show the amounts of leucine contained in foods and therefore it becomes difficult to build a diet that follows these guidelines. A table was therefore created for the first time in the literature to collect all the foods richest in leucine, thanks to the union of the most important Italian food databases. Moreover, in order to implement a diet that follows the right recommendations, another tables shows nutritional composition of breakfast, lunch and dinner (that each provide 3 grams of leucine and 25 grams of protein) for seven days.
    Keywords:  diet; elderly; leucine; proteins; sarcopenia
  28. Korean J Physiol Pharmacol. 2021 Mar 01. 25(2): 167-175
      Far-infrared rays (FIR) are known to have various effects on atoms and molecular structures within cells owing to their radiation and vibration frequencies. The present study examined the effects of FIR on gene expression related to glucose transport through microarray analysis in rat skeletal muscle cells, as well as on mitochondrial biogenesis, at high and low glucose conditions. FIR were emitted from a bio-active material coated fabric (BMCF). L6 cells were treated with 30% BMCF for 24 h in medium containing 25 or 5.5 mM glucose, and changes in the expression of glucose transporter genes were determined. The expression of GLUT3 (Slc2a3) increased 2.0-fold (p < 0.05) under 5.5 mM glucose and 30% BMCF. In addition, mitochondrial oxygen consumption and membrane potential (ΔΨm) increased 1.5- and 3.4-fold (p < 0.05 and p < 0.001), respectively, but no significant change in expression of Pgc-1a, a regulator of mitochondrial biogenesis, was observed in 24 h. To analyze the relationship between GLUT3 expression and mitochondrial biogenesis under FIR, GLUT3 was down-modulated by siRNA for 72 h. As a result, the ΔΨm of the GLUT3 siRNA-treated cells increased 3.0-fold (p < 0.001), whereas that of the control group increased 4.6-fold (p < 0.001). Moreover, Pgc-1a expression increased upon 30% BMCF treatment for 72 h; an effect that was more pronounced in the presence of GLUT3. These results suggest that FIR may hold therapeutic potential for improving glucose metabolism and mitochondrial function in metabolic diseases associated with insufficient glucose supply, such as type 2 diabetes.
    Keywords:  Glucose; Glucose transporter type 3; Infrared rays; Mitochondrial biogenesis; Radiation
  29. J Cachexia Sarcopenia Muscle. 2021 Feb;12(1): 209-232
      BACKGROUND: Duchenne muscular dystrophy (DMD) causes severe disability of children and death of young men, with an incidence of approximately 1/5000 male births. Symptoms appear in early childhood, with a diagnosis made mostly around 4 years old, a time where the amount of muscle damage is already significant, preventing early therapeutic interventions that could be more efficient at halting disease progression. In the meantime, the precise moment at which disease phenotypes arise-even asymptomatically-is still unknown. Thus, there is a critical need to better define DMD onset as well as its first manifestations, which could help identify early disease biomarkers and novel therapeutic targets.METHODS: We have used both human tissue-derived myoblasts and human induced pluripotent stem cells (hiPSCs) from DMD patients to model skeletal myogenesis and compared their differentiation dynamics with that of healthy control cells by a comprehensive multi-omic analysis at seven time points. Results were strengthened with the analysis of isogenic CRISPR-edited human embryonic stem cells and through comparisons against published transcriptomic and proteomic datasets from human DMD muscles. The study was completed with DMD knockdown/rescue experiments in hiPSC-derived skeletal muscle progenitor cells and adenosine triphosphate measurement in hiPSC-derived myotubes.
    RESULTS: Transcriptome and miRnome comparisons combined with protein analyses demonstrated that hiPSC differentiation (i) leads to embryonic/foetal myotubes that mimic described DMD phenotypes at the differentiation endpoint and (ii) homogeneously and robustly recapitulates key developmental steps-mesoderm, somite, and skeletal muscle. Starting at the somite stage, DMD dysregulations concerned almost 10% of the transcriptome. These include mitochondrial genes whose dysregulations escalate during differentiation. We also describe fibrosis as an intrinsic feature of DMD skeletal muscle cells that begins early during myogenesis. All the omics data are available online for exploration through a graphical interface at
    CONCLUSIONS: Our data argue for an early developmental manifestation of DMD whose onset is triggered before the entry into the skeletal muscle compartment, data leading to a necessary reconsideration of dystrophin roles during muscle development. This hiPSC model of skeletal muscle differentiation offers the possibility to explore these functions as well as find earlier DMD biomarkers and therapeutic targets.
    Keywords:  Duchenne muscular dystrophy; Human pluripotent stem cells; Myogenesis; Omics
  30. J Biochem Mol Toxicol. 2021 Feb 18. e22744
      Saturated fatty acids, whose circulating levels are markedly increased in the body, significantly affect the growth and functions of skeletal muscle. These fatty acids may exert a detrimental effect on the undifferentiated skeletal myoblasts that may adversely affect their differentiation. In the present study, the exposure of myoblasts to excess palmitic acid caused an elevation of tumor necrosis factor-α expression and an increase in reactive oxygen species levels consistent with the enhanced inflammation and oxidative stress. Various concentrations of palmitic acid significantly decreased the mitochondrial membrane potential, induced the programmed cell death by an increase in the caspase-3 expression, and DNA fragmentation in the myoblasts. These findings suggest that the increased concentrations of saturated fatty acid in the myoblasts increase lipotoxicity by increasing inflammation and oxidative stress, decreasing the mitochondrial function, and inducing apoptosis.
    Keywords:  apoptosis; mitochondrial membrane potential; myoblast; reactive oxygen species; saturated fatty acid
  31. Metabolomics. 2021 Feb 16. 17(3): 26
      INTRODUCTION: Overexpression of lipoprotein lipase (LPL) protects against high-fat-diet (HFD)-induced obesity and insulin resistance in transgenic rabbits; however, the molecular mechanisms remain unclear. Skeletal muscle is a major organ responsible for insulin-stimulated glucose uptake and energy expenditure.OBJECTIVES: The main purpose of the current study was to examine the effects of the overexpression of LPL on the skeletal muscle metabolomic profiles to test our hypothesis that the mitochondrial oxidative metabolism would be activated in the skeletal muscle of LPL transgenic rabbits and that the higher mitochondrial oxidative metabolism activity would confer better phenotypic metabolic outcomes.
    METHODS: Under a HFD, insulin resistance index was measured using the intravenous glucose tolerance test, and total energy expenditure (TEE) was measured by doubly-labeled water in control and LPL transgenic rabbits (n = 12, each group). Serum lipids, such as triglycerides and free fatty acid, were also measured. The skeletal muscle metabolite profile was analyzed using capillary electrophoresis time-of flight mass spectrometry in the two groups (n = 9, each group). A metabolite set enrichment analysis (MSEA) with muscle metabolites and a false discovery rate q < 0.2 was performed to identify significantly different metabolic pathways between the 2 groups.
    RESULTS: The triglycerides and free fatty acid levels and insulin resistance index were lower, whereas the TEE was higher in the LPL transgenic rabbits than in the control rabbits. Among 165 metabolites detected, the levels of 37 muscle metabolites were significantly different between the 2 groups after false discovery rate correction (q < 0.2). The MSEA revealed that the TCA cycle and proteinogenic amino acid metabolism pathways were significantly different between the 2 groups (P < 0.05). In the MSEA, all four selected metabolites for the TCA cycle (2-oxoglutaric acid, citric acid, malic acid, fumaric acid), as well as eight selected metabolites for proteinogenic amino acid metabolism (asparagine, proline, methionine, phenylalanine, histidine, arginine, leucine, isoleucine) were consistently increased in the transgenic rabbits compared with control rabbits, suggesting that these two metabolic pathways were activated in the transgenic rabbits. Some of the selected metabolites, such as citric acid and methionine, were significantly associated with serum lipids and insulin resistance (P < 0.05).
    CONCLUSION: The current results suggest that the overexpression of LPL may lead to increased activities of TCA cycle and proteinogenic amino acid metabolism pathways in the skeletal muscle, and these enhancements may play an important role in the biological mechanisms underlying the anti-obesity/anti-diabetes features of LPL overexpression.
    Keywords:  Diabetes; Insulin resistance; Metabolism; Metabolomics; Obesity; Skeletal muscle
  32. Appl Physiol Nutr Metab. 2021 Feb 16.
      Skeletal muscle microvascular dysfunction and mitochondrial rarefaction feature in type-2 diabetes mellitus (T2DM) linked to low tissue glucose disposal rate (GDR). Exercise training and milk protein supplementation independently promote microvascular and metabolic plasticity in muscle associated with improved nutrient delivery, but combined effects are unknown. In a randomised-controlled trial, 24 men (55.6 y, SD5.7) with T2DM ingested whey protein drinks (protein/carbohydrate/fat: 20/10/3 g; WHEY) or placebo (carbohydrate/fat: 30/3 g; CON) before/after 45 mixed-mode intense exercise sessions over 10 weeks, to study effects on insulin-stimulated (hyperinsulinemic clamp) skeletal-muscle microvascular blood flow (mBF) and perfusion (near-infrared spectroscopy), and histological, genetic, and biochemical markers (biopsy) of microvascular and mitochondrial plasticity. WHEY enhanced insulin-stimulated perfusion (WHEY-CON 5.6%; 90%CI -0.1, 11.3), while mBF was not altered (3.5%; -17.5, 24.5); perfusion, but not mBF, associated (regression) with increased GDR. Exercise training increased mitochondrial (range of means: 40-90%) and lipid density (20-30%), enzyme activity (20-70%), capillary:fiber ratio (~25%), and lowered systolic (~4%) and diastolic (4-5%) blood pressure, but without WHEY effects. WHEY dampened PGC1α -2.9% (90%CI -5.7, -0.2) and NOS3 -6.4% (-1.4, -0.2) expression, but other mRNA were unclear. Skeletal muscle microvascular and mitochondrial exercise adaptations were not accentuated by whey protein ingestion in men with T2DM. Clinical Trial Registration Number: ACTRN12614001197628 Novelty Bullets: • Chronic whey ingestion in T2DM with exercise altered expression of several mitochondrial and angiogenic mRNA. • Whey added no additional benefit to muscle microvascular or mitochondrial adaptations to exercise. • Insulin-stimulated perfusion increased with whey but was without impact on glucose disposal.
  33. J Clin Invest. 2021 Feb 15. pii: 139496. [Epub ahead of print]131(4):
      Skeletal muscle is a major determinant of systemic metabolic homeostasis that plays a critical role in glucose metabolism and insulin sensitivity. By contrast, despite being a major user of fatty acids, and evidence that muscular disorders can lead to abnormal lipid deposition (e.g., nonalcoholic fatty liver disease in myopathies), our understanding of skeletal muscle regulation of systemic lipid homeostasis is not well understood. Here we show that skeletal muscle Krüppel-like factor 15 (KLF15) coordinates pathways central to systemic lipid homeostasis under basal conditions and in response to nutrient overload. Mice with skeletal muscle-specific KLF15 deletion demonstrated (a) reduced expression of key targets involved in lipid uptake, mitochondrial transport, and utilization, (b) elevated circulating lipids, (c) insulin resistance/glucose intolerance, and (d) increased lipid deposition in white adipose tissue and liver. Strikingly, a diet rich in short-chain fatty acids bypassed these defects in lipid flux and ameliorated aspects of metabolic dysregulation. Together, these findings establish skeletal muscle control of lipid flux as critical to systemic lipid homeostasis and metabolic health.
    Keywords:  Fatty acid oxidation; Metabolism; Muscle Biology; Obesity; Skeletal muscle
  34. J Muscle Res Cell Motil. 2021 Feb 17.
      A transduced mechanical signal arriving at its destination in muscle alters sarcomeric structure and function. A major question addressed is how muscle mass and tension generation are optimized to match actual performance demands so that little energy is wasted. Three cases for improved energy efficiency are examined: the troponin complex for tuning force production, control of the myosin heads in a resting state, and the Z-disc proteins for sarcomere assembly. On arrival, the regulation of protein complexes is often controlled by post-translational modification (PTM), of which the most common are phosphorylation by kinases, deacetylation by histone deacetylases and ubiquitination by E3 ligases. Another branch of signals acts not through peptide covalent bonding but via ligand interactions (e.g. Ca2+ and phosphoinositide binding). The myosin head and the regulation of its binding to actin by the troponin complex is the best and earliest example of signal destinations that modify myofibrillar contractility. PTMs in the troponin complex regulate both the efficiency of the contractile function to match physiologic demand for work, and muscle mass via protein degradation. The regulation of sarcomere assembly by integration of incoming signaling pathways causing the same PTMs or ligand binding are discussed in response to mechanical loading and unloading by the Z-disc proteins CapZ, α-actinin, telethonin, titin N-termini, and others. Many human mutations that lead to cardiomyopathy and heart disease occur in the proteins discussed above, which often occur at their PTM or ligand binding sites.
    Keywords:  Mechanotransduction; Myofibrillogenesis; Phosphatidylinositol 4,5-bisphosphate; Proteomics; Signaling pathway
  35. Stem Cell Rev Rep. 2021 Jan 27.
      Epigenetic modification is a crucial mechanism affecting the biological function of stem cells. SETD4 is a histone methyltransferase, and its biological role in bone marrow mesenchymal stem cells (BMSCs) is currently unknown. In this study, we employed CRISPR/Cas9 technology edited mouse model and found that SETD4 knockout significantly promoted the proliferation of BMSCs, impaired BMSCs migration and differentiation potentials of lineages of cardiacmyocyte and smooth muscle cell, and even the angiogenesis via paracrine of VEGF. Through Reduced Representation Bisulfite Sequencing (RRBS) method, we verified that the overall genomic methylation of BMSCs in the SETD4 knockout group only was decreased by 0.47 % compared with wild type. However, the changed genomic methylation covers a total of 96,331 differential methylated CpG sites and 8,692 differential methylation regions (DMRs), with part of them settled in promoter regions. Bioinformatic analysis revealed that differential CpG islands and DMRs in promoter impacted 270 GO functions and 34 KEGG signaling pathways, with some closely related to stem cell biology. Mechanismly, SETD4 knockout inhibited sets of monomethylases and dimethylases for histone lysine, along with significant changes in some factors including Nkx2.5, Gata4, Gli2, Grem2, E2f7, Map7, Nr2f2 and Shox2 that associated with stem cell biology. These results are the first to reveal that even though SETD4 changes the genome's overall methylation to a limited extent in BMSCs, it still affects the numerous cellular functions and signaling pathways, implying SETD4-altered genomic methylation serves a crucial molecular role in BMSCs' biological functions.
    Keywords:  Bioinformatics; Bone marrow mesenchymal stem cells; Cell biology; Genomic methylation; Reduced representation bisulfite sequencing; SETD4
  36. Can J Physiol Pharmacol. 2021 Feb 17.
      α-amyrin, a natural pentacyclic triterpene, have anti-hyperglycemic effect in mice and dual PPARδ/γ action in 3T3-L1 adipocytes, and potential in the control of type 2 diabetes (T2D). About 80% of glucose uptake occurs in skeletal muscle cells, playing a significant role in IR and T2D. Peroxisome-proliferator activated receptors (PPARs), in particular PPARδ and PPARγ, are involved in the regulation of lipids and carbohydrates and, along adenosine-monophosphate (AMP)-activated protein kinase (AMPK) and protein kinase B (Akt/PKB), are implicated in translocation of glucose transporter 4 (GLUT4). However, it is still unknown whether α-amyrin can affect these pathways in skeletal muscle cells. The work's objective was to determine the action of α-amyrin in PPARδ, PPARγ, AMPK, and Akt/PKB in C2C12 myoblasts. The expression of PPARδ, PPARγ, FATP, and GLUT4 was quantified using RT-qPCR and Western blot. α-amyrin increased these markers along with p-AMPK but not p-Akt/PKB. Molecular docking showed that α-amyrin acts as an AMPK-allosteric activator, and may be related to GLUT4 translocation, evidenced by confocal microscopy. These data support that α-amyrin could have an insulin-mimetic action in C2C12 myoblasts and should be considered as a bioactive molecule for new multitarget drugs with utility in T2D and other metabolic diseases.
  37. Methods Mol Biol. 2021 ;2224 203-214
      Duchenne muscular dystrophy (DMD) is a devastating X-linked muscle disorder affecting many children. The disease is caused by the lack of dystrophin production and characterized by muscle wasting. The most common causes of death are respiratory failure and heart failure. Antisense oligonucleotide-mediated exon skipping using a phosphorodiamidate morpholino oligomer (PMO) is a promising therapeutic approach for the treatment of DMD. In preclinical studies, dystrophic mouse models are commonly used for the development of therapeutic oligos. We employ a humanized model carrying the full-length human DMD transgene along with the complete knockout of the mouse Dmd gene. In this model, the effects of human-targeting AOs can be tested without cross-reaction between mouse sequences and human sequences (note that mdx, a conventional dystrophic mouse model, carries a nonsense point mutation in exon 23 and express the full-length mouse Dmd mRNA, which is a significant complicating factor). To determine if dystrophin expression is restored, the Western blotting analysis is commonly performed; however, due to the extremely large protein size of dystrophin (427 kDa), detection and accurate quantification of full-length dystrophin can be a challenge. Here, we present methodologies to systemically inject PMOs into humanized DMD model mice and determine levels of dystrophin restoration via Western blotting. Using a tris-acetate gradient SDS gel and semi-dry transfer with three buffers, including the Concentrated Anode Buffer, Anode Buffer, and Cathode Buffer, less than 1% normal levels of dystrophin expression are easily detectable. This method is fast, easy, and sensitive enough for the detection of dystrophin from both cultured muscle cells and muscle biopsy samples.
    Keywords:  Antisense oligonucleotides (AOs); Antisense therapy; Duchenne/Becker muscular dystrophy (DMD/BMD); Eteplirsen (brand name ExonDys 51); Exon skipping; Golodirsen; NS-065/NCNP-01 (also known as viltolarsen); Nusinersen (brand name Spinraza); Phosphorodiamidate morpholino oligomer (morpholino, or PMO); Retroorbital injection
  38. J Sport Health Sci. 2021 Feb 13. pii: S2095-2546(21)00022-3. [Epub ahead of print]
      BACKGROUND: One of the most life-threatening comorbidities in elderly cancer patients is cancer cachexia, which is characterized by the ongoing loss of skeletal muscular strength and mass and is also associated with aging. There is a lack of recommendations for optimal resistance training (RT) for those patients. The purpose of this study was to systematically review and quantify the effects of RT on muscular strength and hypertrophy in elderly cancer patients.METHODS: Five electronic databases were searched (until January 2020) for studies that met the following criteria: (i) cancer patients aged ≥60 years; (ii) structured and supervised RT intervention for ≥6 weeks; and (iii) measured muscular strength and/or hypertrophy.
    RESULTS: Thirteen studies (717 participants, average age 66 years) met the inclusion criteria. RT significantly increased muscular strength (mean effect size (ES) = 0.87, 95% confidence interval (95%CI): 0.43 to 1.32, p < 0.001) and did not significantly induce muscle hypertrophy (mean ES = 0.09, 95%CI: -0.14 to 0.31, p = 0.45). In subgroup analyses for muscle strength, higher weekly frequency was significantly associated with larger ES. Egger's test showed no significant publication bias for the 2 outcomes.
    CONCLUSION: The results suggest that RT improves muscular strength but does not significantly induce muscle hypertrophy in elderly cancer patients.
    Keywords:  Cancer cachexia; Muscle synthesis; Sarcopenia; Skeletal muscle
  39. J Muscle Res Cell Motil. 2021 Feb 17.
      The classical approach for calibrating non-ratiometric fluorescent Ca2+ dyes entails the measurement of the fluorescence maximum (Fmax) and minimum (Fmin), as well as the dissociation constant (Kd) of the Ca2+-Dye reaction (model 1). An alternative equation does not need the Fmin but requires the rate constants kon and koff (model 2). However, both approaches are experimentally time consuming and the rate constants for several dyes are unknown. Here, we propose a set of equations (model 3) that simplify the calibration of fluorescent Ca2+ transients obtained with non-ratiometric dyes. This equation allows the calibration of signals without using the Fmin: [Ca2+] = Kd(F - Frest/Fmax - F) + [Ca2+]IR(Fmax - Frest/Fmax - F), where [Ca2+]IR is the resting [Ca2+]. If the classical calibration approach is followed, the Fmin can be estimated from: Fmin = Frest - ([Ca2+]IR(Fmax - Frest)/Kd). We tested the models' performance using signals obtained from enzymatically dissociated flexor digitorum brevis fibers of C57BL/6 mice loaded with Fluo-4, AM. Model 3 performed the same as model 2, and both gave peak [Ca2+] values 15 ± 0.3% (n = 3) lower than model 1, when we used our experimental Fmin (1.24 ± 0.11 A.U., n = 4). However, when we used the mathematically estimated Fmin (6.78 ± 0.2 A.U) for model 1, the peak [Ca2+] were similar for all three models. This suggests that the dye leakage makes a correct determination of the Fmin unlikely and induces errors in the estimation of [Ca2+]. In conclusion, we propose simpler and time-saving equations that help to reliably calibrate cytosolic Ca2+ transients obtained with non-ratiometric fluorescent dyes. The use of the estimated Fmin avoids the uncertainties associated with its experimental measurement.
    Keywords:  Ca2+; Calibration; Dyes; Fluorescence; Skeletal muscle
  40. Ageing Res Rev. 2021 Feb 16. pii: S1568-1637(21)00013-1. [Epub ahead of print] 101266
      BACKGROUND: Engaging in physical activity (PA) and avoiding sedentary behavior (SB) are important for healthy ageing with benefits including the mitigation of disability and mortality. Whether benefits extend to key determinants of disability and mortality, namely muscle strength and muscle power, is unclear.AIMS: This systematic review aimed to describe the association of objective measures of PA and SB with measures of skeletal muscle strength and muscle power in community-dwelling older adults.
    METHODS: Six databases were searched from their inception to June 21st, 2020 for articles reporting associations between objectively measured PA and SB and upper body or lower body muscle strength or muscle power in community dwelling adults aged 60 years and older. An overview of associations was visualized by effect direction heat maps, standardized effect sizes were estimated with albatross plots and summarized in box plots. Articles reporting adjusted standardized regression coefficients (β) were included in meta-analyses.
    RESULTS: A total of 112 articles were included representing 43,796 individuals (range: 21 to 3,726 per article) with a mean or median age from 61.0-88.0 years (mean 56.4% female). Higher PA measures and lower SB were associated with better upper body muscle strength (hand grip strength), upper body muscle power (arm curl), lower body muscle strength, and lower body muscle power (chair stand test). Median standardized effect sizes were consistently larger for measures of PA and SB with lower compared to upper body muscle strength and muscle power. The meta-analyses of adjusted β coefficients confirmed the associations between total PA (TPA), moderate-to-vigorous PA (MVPA) and light PA (LPA) with hand grip strength (β = 0.041, β = 0.057, and β = 0.070, respectively, all p ≤ 0.001), and TPA and MVPA with chair stand test (β = 0.199 and β = 0.211, respectively, all p ≤ 0.001).
    CONCLUSIONS: Higher PA and lower SB are associated with greater skeletal muscle strength and muscle power, particularly with the chair stand test.
    Keywords:  accelerometry; aged; muscle contraction; muscle strength; physical activity; sedentary behavior
  41. Nat Neurosci. 2021 Feb 15.
      The spinal cord is a fascinating structure that is responsible for coordinating movement in vertebrates. Spinal motor neurons control muscle activity by transmitting signals from the spinal cord to diverse peripheral targets. In this study, we profiled 43,890 single-nucleus transcriptomes from the adult mouse spinal cord using fluorescence-activated nuclei sorting to enrich for motor neuron nuclei. We identified 16 sympathetic motor neuron clusters, which are distinguishable by spatial localization and expression of neuromodulatory signaling genes. We found surprising skeletal motor neuron heterogeneity in the adult spinal cord, including transcriptional differences that correlate with electrophysiologically and spatially distinct motor pools. We also provide evidence for a novel transcriptional subpopulation of skeletal motor neuron (γ*). Collectively, these data provide a single-cell transcriptional atlas ( ) for investigating the organizing molecular logic of adult motor neuron diversity, as well as the cellular and molecular basis of motor neuron function in health and disease.
  42. J Clin Invest. 2021 Feb 16. pii: 135963. [Epub ahead of print]
      Aberrant lipid metabolism promotes the development of skeletal muscle insulin resistance, but the exact identity of lipid-mediated mechanisms relevant to human obesity remains unclear. A comprehensive lipidomic analysis of primary myocytes from lean insulin-sensitive (LN) and obese insulin-resistant (OB) individuals revealed several species of lysophospholipids (lyso-PL) that were differentially-abundant. These changes coincided with greater expression of lysophosphatidylcholine acyltransferase 3 (LPCAT3), an enzyme involved in phospholipid transacylation (Lands cycle). Strikingly, mice with skeletal muscle-specific knockout of LPCAT3 (LPCAT3-MKO) exhibited greater muscle lyso-PC/PC, concomitant with improved skeletal muscle insulin sensitivity. Conversely, skeletal muscle-specific overexpression of LPCAT3 (LPCAT3-MKI) promoted glucose intolerance. The absence of LPCAT3 reduced phospholipid packing of cellular membranes and increased plasma membrane lipid clustering, suggesting that LPCAT3 affects insulin receptor phosphorylation by modulating plasma membrane lipid organization. In conclusion, obesity accelerates the skeletal muscle Lands cycle, whose consequence might induce the disruption of plasma membrane organization that suppresses muscle insulin action.
    Keywords:  Insulin signaling; Metabolism; Muscle Biology; Skeletal muscle