bims-moremu 2022-09-18 papers

bims-moremu

Biomed News

on Molecular regulators of muscle mass

Issue of 2022‒09‒18
thirty-two papers selected by
Anna Vainshtein
Craft Science Inc.

Transplantation to study satellite cell heterogeneity in skeletal muscle.
Effects of PGC-1α overexpression on the myogenic response during skeletal muscle regeneration.
Sensing local energetics to acutely regulate mitophagy in skeletal muscle.
Drugs, clocks and exercise in aging: hype and hope, facts and fiction.
Estrogen-related receptor alpha is an AMPK-regulated factor that promotes ischemic muscle revascularization and recovery in diet-induced obese mice.
Sirtuin 6 inhibition protects against glucocorticoid-induced skeletal muscle atrophy by regulating IGF/PI3K/AKT signaling.
miR-539-5p regulates Srebf1 transcription in the skeletal muscle of diabetic mice by targeting DNA methyltransferase 3b.
Differential microRNA profiles of intramuscular and secreted extracellular vesicles in human tissue-engineered muscle.
Skeletal muscle oxidative stress and inflammation in aging: Focus on antioxidant and anti-inflammatory therapy.
Myeloid cell-specific mutation of Spi1 selectively reduces M2-biased macrophage numbers in skeletal muscle, reduces age-related muscle fibrosis and prevents sarcopenia.
Pathophysiological levels of GDF11 activate Smad2/Smad3 signaling and induce muscle atrophy in human iPSC-derived myocytes.
Motor unit dysregulation following 15 days of unilateral lower limb immobilisation.
Non-myogenic mesenchymal cells contribute to muscle degeneration in facioscapulohumeral muscular dystrophy patients.
Tiny changes in cytoplasmic [Ca2+] cause large changes in mitochondrial Ca2+: what are the triggers and functional implications?
Skeletal muscle contraction kinetics and AMPK responses are modulated by the adenine nucleotide degrading enzyme AMPD1.
Computational Model of the Effect of Mitochondrial Dysfunction on Excitation-Contraction Coupling in Skeletal Muscle.
Higher bone remodeling biomarkers are related to a higher muscle function in older adults: Effects of acute exercise.
Carm1 and the Epigenetic Control of Stem Cell Function.
Rates of protein synthesis are maintained in brain but reduced in skeletal muscle during dietary sulfur amino acid restriction.
Altered skeletal muscle SERCA calcium transport efficiency following a thermogenic stimulus.
Lactate-induced lactylation in skeletal muscle is associated with insulin resistance in humans.
Pharmacological TRPC6 inhibition improves survival and muscle function in mice with duchenne muscular dystrophy.
Overexpression of UCP3 decreases mitochondrial efficiency in mouse skeletal muscle in vivo.
Global chromatin accessibility profiling analysis reveals a chronic activation state in aged muscle stem cells.
Phosphatidylserine orchestrates Myomerger membrane insertions to drive myoblast fusion.
Sclerostin aggravates insulin signaling in skeletal muscle and hepatic steatosis via upregulation of ER stress by mTOR-mediated inhibition of autophagy under hyperlipidemic conditions.
Metabolomics reveals mouse plasma metabolite responses to acute exercise and effects of disrupting AMPK-glycogen interactions.
Development of skeletal muscle atrophy and intermuscular adipose tissue in early breast cancer patients treated with chemotherapy.
Individual-specific levels of CTG•CAG somatic instability are shared across multiple tissues in myotonic dystrophy type 1.
Lysosomal glycogen accumulation in Pompe disease results in disturbed cytoplasmic glycogen metabolism.
Irisin Attenuates Muscle Impairment during Bed Rest through Muscle-Adipose Tissue Crosstalk.
The role of exercise-induced myokines in promoting angiogenesis.

Front Cell Dev Biol. 2022 ;10 902225

Transplantation to study satellite cell heterogeneity in skeletal muscle.

Bahareh Hekmatnejad, Michael A Rudnicki.

  Skeletal muscle has a remarkable capacity to regenerate throughout life, which is mediated by its resident muscle stem cells, also called satellite cells. Satellite cells, located periphery to the muscle fibers and underneath the basal lamina, are an indispensable cellular source for muscle regeneration. Satellite cell transplantation into regenerating muscle contributes robustly to muscle repair, thereby indicating that satellite cells indeed function as adult muscle stem cells. Moreover, satellite cells are a heterogenous population in adult tissue, with subpopulations that can be distinguished based on gene expression, cell-cycle progression, ability to self-renew, and bi-potential ability. Transplantation assays provide a powerful tool to better understand satellite cell function in vivo enabling the separation of functionally distinct satellite cell subpopulations. In this review, we focus on transplantation strategies to explore satellite cells' functional heterogeneity, approaches targeting the recipient tissue to improve transplantation efficiency, and common strategies to monitor the behaviour of the transplanted cells. Lastly, we discuss some recent approaches to overcome challenges to enhance the transplantation potential of muscle stem cells.

Keywords:  differentiation; engraftment; heterogeneity; muscle stem cell; satellite cells; self-renewal; transplantation

DOI:  https://doi.org/10.3389/fcell.2022.902225
Sports Med Health Sci. 2022 Sep;4(3): 198-208

Effects of PGC-1α overexpression on the myogenic response during skeletal muscle regeneration.

Tyrone A Washington, Wesley S Haynie, Eleanor R Schrems, Richard A Perry, Lemuel A Brown, Breanna M Williams, Megan E Rosa-Caldwell, David E Lee, Jacob L Brown.

  The ability of skeletal muscle to regenerate from injury is crucial for locomotion, metabolic health, and quality of life. Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1A) is a transcriptional coactivator required for mitochondrial biogenesis. Increased mitochondrial biogenesis is associated with improved muscle cell differentiation, however PGC1A's role in skeletal muscle regeneration following damage requires further investigation. The purpose of this study was to investigate the role of skeletal muscle-specific PGC1A overexpression during regeneration following damage. 22 C57BL/6J (WT) and 26 PGC1A muscle transgenic (A1) mice were injected with either phosphate-buffered saline (PBS, uninjured control) or Bupivacaine (MAR, injured) into their tibialis anterior (TA) muscle to induce skeletal muscle damage. TA muscles were extracted 3- or 28-days post-injury and analyzed for markers of regenerative myogenesis and protein turnover. Pgc1a mRNA was ∼10-20 fold greater in A1 mice. Markers of protein synthesis, AKT and 4EBP1, displayed decreases in A1 mice compared to WT at both timepoints indicating a decreased protein synthetic response. Myod mRNA was ∼75% lower compared to WT 3 days post-injection. WT mice exhibited decreased cross-sectional area of the TA muscle at 28 days post-injection with bupivacaine compared to all other groups. PGC1A overexpression modifies the myogenic response during regeneration.

Keywords:  Muscle regeneration; PGC-1α; Protein turnover; Satellite cells; Skeletal muscle; p38 MAPK

DOI:  https://doi.org/10.1016/j.smhs.2022.06.005
Front Cell Dev Biol. 2022 ;10 987317

Sensing local energetics to acutely regulate mitophagy in skeletal muscle.

Anna S Nichenko, Kalyn S Specht, Siobhan M Craige, Joshua C Drake.

  The energetic requirements of skeletal muscle to sustain movement, as during exercise, is met largely by mitochondria, which form an intricate, interconnected reticulum. Maintenance of a healthy mitochondrial reticulum is essential for skeletal muscle function, suggesting quality control pathways are spatially governed. Mitophagy, the process by which damaged and/or dysfunctional regions of the mitochondrial reticulum are removed and degraded, has emerged as an integral part of the molecular response to exercise. Upregulation of mitophagy in response to acute exercise is directly connected to energetic sensing mechanisms through AMPK. In this review, we discuss the connection of mitophagy to muscle energetics and how AMPK may spatially control mitophagy through multiple potential means.

Keywords:  AMPK; energetic stress; mitochondria; mitophagy; reactive oxygen species

DOI:  https://doi.org/10.3389/fcell.2022.987317
J Physiol. 2022 Sep 16.

Drugs, clocks and exercise in aging: hype and hope, facts and fiction.

Regula Furrer, Christoph Handschin.

  Aging is a biological process that is linked to a functional decline, ultimately resulting in death. Large interindividual differences exist in terms of life- and healthspan, representing life expectancy and the number of years spent in the absence of major diseases, respectively. The genetic and molecular mechanisms that are involved in the regulation of the aging process, and those that render age the main risk factor for many diseases are still poorly understood. Nevertheless, a growing number of compounds have been put forward to affect this process. However, for scientists and laypeople alike, it is difficult to separate facts from fiction, and hype from hope. In this review, we discuss the currently pursued pharmacological anti-aging approaches. These are compared to non-pharmacological interventions, some of which confer powerful effects on health and well-being, in particular an active lifestyle and exercise. Moreover, functional parameters and biological clocks as well as other molecular marks are compared in terms of predictive power of morbidity and mortality. Then, conceptual aspects and roadblocks in the development of anti-aging drugs are outlined. Finally, an overview on current and future strategies to mitigate age-related pathologies and the extension of life- and healthspan is provided. Abstract figure legend Modulation of the human aging process. Aging leads to a functional decline with increased frailty and morbidity, ultimately resulting in death. This process is affected by the genetic setup, epigenetic predisposition, and random events, leading to large interindividual differences in regard to health, well-being and life expectancy. Exercise, diet and other lifestyle-based interventions as well as societal measures can help to maximize the chance for healthy aging. Proposed drugs, cellular, epigenetic, and other non-pharmacological approaches such as caloric restriction await validation for safe and efficacious application in human geroprotection. This article is protected by copyright. All rights reserved.

Keywords:  aging; exercise; healthspan; lifespan; longevity; rapamycin; rejuvenation; reprogramming; senolytics

DOI:  https://doi.org/10.1113/JP282887
FASEB Bioadv. 2022 Sep;4(9): 602-618

Estrogen-related receptor alpha is an AMPK-regulated factor that promotes ischemic muscle revascularization and recovery in diet-induced obese mice.

Danesh H Sopariwala, Andrea S Rios, Mi Kyung Park, Min Sup Song, Ashok Kumar, Vihang A Narkar.

  Obesity and type II diabetes are leading causes of peripheral arterial disease (PAD), which is characterized by vascular insufficiency and ischemic damage in the limb skeletal muscle. Glycemic control is not sufficient to prevent progression of PAD, and molecular targets that can promote muscle neo-angiogenesis in obesity and diabetes remain poorly defined. Here, we have investigated whether nuclear receptor estrogen-related receptor alpha (ERRα) can promote ischemic revascularization in the skeletal muscles of diet-induced obese (DIO) mice. Using muscle-specific ERRα transgenic mice, we found that ERRα overexpression promotes revascularization, marked by increased capillary staining and muscle perfusion in DIO mice after hindlimb ischemic injury. Furthermore, ERRα facilitates repair and restoration of skeletal muscle myofiber size after limb ischemia in DIO mice. The ameliorative effects of ERRα overexpression did not involve the prevention of weight gain, hyperglycemia or glucose/insulin intolerance, suggesting a direct role for ERRα in promoting angiogenesis. Interestingly, levels of endogenous ERRα protein are suppressed in the skeletal muscles of DIO mice compared to lean controls, coinciding with the suppression of angiogenic gene expression, and reduced AMPK signaling in the DIO skeletal muscles. Upon further investigating the link between AMPK and ERRα, we found that AMPK activation increases the expression and recruitment of ERRα protein to specific angiogenic gene promoters in muscle cells. Further, the induction of angiogenic factors by AMPK activators in muscle cells is blocked by repressing ERRα. In summary, our results identify an AMPK/ERRα-dependent angiogenic gene program in the skeletal muscle, which is repressed by DIO, and demonstrate that forced ERRα activation can promote ischemic revascularization and muscle recovery in obesity.

Keywords:  AMPK; angiogenesis; ischemia; nuclear receptors; obesity; skeletal muscle

DOI:  https://doi.org/10.1096/fba.2022-00015
Nat Commun. 2022 Sep 15. 13(1): 5415

Sirtuin 6 inhibition protects against glucocorticoid-induced skeletal muscle atrophy by regulating IGF/PI3K/AKT signaling.

Sneha Mishra, Claudia Cosentino, Ankit Kumar Tamta, Danish Khan, Shalini Srinivasan, Venkatraman Ravi, Elena Abbotto, Bangalore Prabhashankar Arathi, Shweta Kumar, Aditi Jain, Anand S Ramaian, Shruti M Kizkekra, Raksha Rajagopal, Swathi Rao, Swati Krishna, Ninitha Asirvatham-Jeyaraj, Elizabeth R Haggerty, Dafne M Silberman, Irwin J Kurland, Ravindra P Veeranna, Tamilselvan Jayavelu, Santina Bruzzone, Raul Mostoslavsky, Nagalingam R Sundaresan.

Chronic activation of stress hormones such as glucocorticoids leads to skeletal muscle wasting in mammals. However, the molecular events that mediate glucocorticoid-induced muscle wasting are not well understood. Here, we show that SIRT6, a chromatin-associated deacetylase indirectly regulates glucocorticoid-induced muscle wasting by modulating IGF/PI3K/AKT signaling. Our results show that SIRT6 levels are increased during glucocorticoid-induced reduction of myotube size and during skeletal muscle atrophy in mice. Notably, overexpression of SIRT6 spontaneously decreases the size of primary myotubes in a cell-autonomous manner. On the other hand, SIRT6 depletion increases the diameter of myotubes and protects them against glucocorticoid-induced reduction in myotube size, which is associated with enhanced protein synthesis and repression of atrogenes. In line with this, we find that muscle-specific SIRT6 deficient mice are resistant to glucocorticoid-induced muscle wasting. Mechanistically, we find that SIRT6 deficiency hyperactivates IGF/PI3K/AKT signaling through c-Jun transcription factor-mediated increase in IGF2 expression. The increased activation, in turn, leads to nuclear exclusion and transcriptional repression of the FoxO transcription factor, a key activator of muscle atrophy. Further, we find that pharmacological inhibition of SIRT6 protects against glucocorticoid-induced muscle wasting in mice by regulating IGF/PI3K/AKT signaling implicating the role of SIRT6 in glucocorticoid-induced muscle atrophy.

DOI: https://doi.org/10.1038/s41467-022-32905-w
Mol Ther Nucleic Acids. 2022 Sep 13. 29 718-732

miR-539-5p regulates Srebf1 transcription in the skeletal muscle of diabetic mice by targeting DNA methyltransferase 3b.

Devesh Kesharwani, Amit Kumar, Ashima Rizvi, Malabika Datta.

  Aberrant DNA methylation is associated with diabetes, but the precise regulatory events that control the levels and activity of DNA methyltransferases (DNMTs) is not well understood. Here we show that miR-539-5p targets Dnmt3b and regulates its cellular levels. miR-539-5p and Dnmt3b show inverse patterns of expression in skeletal muscle of diabetic mice. By binding to the 3' UTR of Dnmt3b, miR-539-5p downregulates its levels in C2C12 cells and in human primary skeletal muscle cells. miR-539-5p-Dnmt3b interaction regulates Srebf1 transcription by altering methylation at CpG islands within Srebf1 in C2C12 cells. Dnmt3b inhibition alone was sufficient to upregulate Srebf1 transcription. In vivo antagonism of miR-539-5p in normal mice induced hyperglycemia and hyperinsulinemia and impaired oral glucose tolerance. These mice had elevated Dnmt3b and decreased Srebf1 levels in skeletal muscle. db/db mice injected with miR-539-5p mimics showed improved circulatory glucose and cholesterol levels. Oral glucose tolerance improved together with normalization of Dnmt3b and Srebf1 levels in skeletal muscle. Our results support a critical role of miR-539-5p and Dnmt3b in aberrant skeletal muscle metabolism during diabetes by regulating Srebf1 transcription; modulating the miR-539-5p-Dnmt3b axis might have therapeutic potential for addressing altered skeletal muscle physiology during insulin resistance and type 2 diabetes.

Keywords:  DNA methylation; DNMT3b; Srebf1; diabetes; miR-539-5p; skeletal muscle

DOI:  https://doi.org/10.1016/j.omtn.2022.08.013
Front Physiol. 2022 ;13 937899

Differential microRNA profiles of intramuscular and secreted extracellular vesicles in human tissue-engineered muscle.

Christopher G Vann, Xin Zhang, Alastair Khodabukus, Melissa C Orenduff, Yu-Hsiu Chen, David L Corcoran, George A Truskey, Nenad Bursac, Virginia B Kraus.

  Exercise affects the expression of microRNAs (miR/s) and muscle-derived extracellular vesicles (EVs). To evaluate sarcoplasmic and secreted miR expression in human skeletal muscle in response to exercise-mimetic contractile activity, we utilized a three-dimensional tissue-engineered model of human skeletal muscle ("myobundles"). Myobundles were subjected to three culture conditions: no electrical stimulation (CTL), chronic low frequency stimulation (CLFS), or intermittent high frequency stimulation (IHFS) for 7 days. RNA was isolated from myobundles and from extracellular vesicles (EVs) secreted by myobundles into culture media; miR abundance was analyzed by miRNA-sequencing. We used edgeR and a within-sample design to evaluate differential miR expression and Pearson correlation to evaluate correlations between myobundle and EV populations within treatments with statistical significance set at p < 0.05. Numerous miRs were differentially expressed between myobundles and EVs; 116 miRs were differentially expressed within CTL, 3 within CLFS, and 2 within IHFS. Additionally, 25 miRs were significantly correlated (18 in CTL, 5 in CLFS, 2 in IHFS) between myobundles and EVs. Electrical stimulation resulted in differential expression of 8 miRs in myobundles and only 1 miR in EVs. Several KEGG pathways, known to play a role in regulation of skeletal muscle, were enriched, with differentially overrepresented miRs between myobundle and EV populations identified using miEAA. Together, these results demonstrate that in vitro exercise-mimetic contractile activity of human engineered muscle affects both their expression of miRs and number of secreted EVs. These results also identify novel miRs of interest for future studies of the role of exercise in organ-organ interactions in vivo.

Keywords:  engineered tissue; extracellular vescicles; miRNA sequencing; microRNA; skeletal muscle

DOI:  https://doi.org/10.3389/fphys.2022.937899
Front Cell Dev Biol. 2022 ;10 964130

Skeletal muscle oxidative stress and inflammation in aging: Focus on antioxidant and anti-inflammatory therapy.

Mingming Chen, Yiyi Wang, Shoulong Deng, Zhengxing Lian, Kun Yu.

  With aging, the progressive loss of skeletal muscle will have negative effect on multiple physiological parameters, such as exercise, respiration, thermoregulation, and metabolic homeostasis. Accumulating evidence reveals that oxidative stress and inflammation are the main pathological characteristics of skeletal muscle during aging. Here, we focus on aging-related sarcopenia, summarize the relationship between aging and sarcopenia, and elaborate on aging-mediated oxidative stress and oxidative damage in skeletal muscle and its critical role in the occurrence and development of sarcopenia. In addition, we discuss the production of excessive reactive oxygen species in aging skeletal muscle, which reduces the ability of skeletal muscle satellite cells to participate in muscle regeneration, and analyze the potential molecular mechanism of ROS-mediated mitochondrial dysfunction in aging skeletal muscle. Furthermore, we have also paid extensive attention to the possibility and potential regulatory pathways of skeletal muscle aging and oxidative stress mediate inflammation. Finally, in response to the abnormal activity of oxidative stress and inflammation during aging, we summarize several potential antioxidant and anti-inflammatory strategies for the treatment of sarcopenia, which may provide beneficial help for improving sarcopenia during aging.

Keywords:  aging; inflammation; oxidative stress; skeletal muscle; treatment strategy

DOI:  https://doi.org/10.3389/fcell.2022.964130
Aging Cell. 2022 Sep 13. e13690

Myeloid cell-specific mutation of Spi1 selectively reduces M2-biased macrophage numbers in skeletal muscle, reduces age-related muscle fibrosis and prevents sarcopenia.

Ying Wang, Steven S Welc, Michelle Wehling-Henricks, Ying Kong, Connor Thomas, Enca Montecino-Rodriguez, Kenneth Dorshkind, James G Tidball.

  Intramuscular macrophages play key regulatory roles in determining the response of skeletal muscle to injury and disease. Recent investigations showed that the numbers and phenotype of intramuscular macrophages change during aging, suggesting that those changes could influence the aging process. We tested that hypothesis by generating a mouse model that harbors a myeloid cell-specific mutation of Spi1, which is a transcription factor that is essential for myeloid cell development. The mutation reduced the numbers of macrophages biased to the CD163+/CD206+ M2 phenotype in muscles of aging mice without affecting the numbers of CD68-expressing macrophages and reduced the expression of transcripts associated with the M2-biased phenotype. The mutation did not affect the colony-forming ability or the frequency of specific subpopulations of bone marrow hematopoietic cells and did not affect myeloid/lymphoid cell ratios in peripheral blood leukocyte populations. Cellularity of most myeloid lineage cells was not influenced by the mutation. The Spi1 mutation in bone marrow-derived macrophages in vitro also did not affect expression of transcripts that indicate the M2-biased phenotype. Thus, myeloid cell-targeted mutation of Spi1 influences macrophage phenotype in muscle but did not affect earlier stages of differentiation of cells in the macrophage lineage. The mutation reduced age-related muscle fibrosis, which is consistent with the reduction of M2-biased macrophages, and reduced expression of the pro-fibrotic enzyme arginase. Most importantly, the mutation prevented sarcopenia. Together, our observations indicate that intramuscular, M2-biased macrophages play significant roles in promoting detrimental, age-related changes in muscle.

Keywords:  Sarcopenia; aging; macrophage; skeletal muscle

DOI:  https://doi.org/10.1111/acel.13690
Am J Physiol Cell Physiol. 2022 Sep 12.

Pathophysiological levels of GDF11 activate Smad2/Smad3 signaling and induce muscle atrophy in human iPSC-derived myocytes.

Mikako Honda, Takumi Makino, Xiaolin Zhao, Mariko Matsuto, Hidetoshi Sakurai, Yu Takahashi, Makoto Shimizu, Ryuichiro Sato, Yoshio Yamauchi.

  Skeletal muscle mass is negatively regulated by several TGF-β superfamily members. Myostatin (MSTN) is the most prominent negative regulator of muscle mass. Recent studies show that in addition to MSTN, GDF11, which shares high sequence identity with MSTN, induces muscle atrophy in vitro and in vivo at supraphysiological levels, whereas controversy regarding its roles exists. Furthermore, higher circulating GDF11 levels associate with frailty in humans. On the other hand, little is known about the effect of pathophysiological levels of GDF11 on muscle atrophy. Here we seek to determine whether pathophysiological levels of GDF11 are sufficient to activate Smad2/Smad3 signaling and induce muscle atrophy using human iPSC-derived myocytes (hiPSC-myocytes). We first show that incubating hiPSC-myocytes with pathophysiologic concentrations of GDF11 significantly reduces myocyte diameters. We next demonstrate that pathophysiological levels of GDF11 are sufficient to activate Smad2/3 signaling. Finally, we show that pathophysiological levels of GDF11 are capable of inducing the expression of Atrogin-1, an atrophy-promoting E3 ubiquitin ligase and that FOXO1 blockage reverses the GDF11-induced Atrogin-1 expression and atrophic phenotype. Collectively, our results suggest that GDF11 induces skeletal muscle atrophy at the pathophysiological level through the GDF11-FOXO1 axis.

Keywords:  GDF11; human iPSCs; muscle atrophy; myostatin; skeletal muscle

DOI:  https://doi.org/10.1152/ajpcell.00341.2022
J Physiol. 2022 Sep 11.

Motor unit dysregulation following 15 days of unilateral lower limb immobilisation.

Thomas B Inns, Joseph J Bass, Edward J O Hardy, Daniel J Wilkinson, Daniel W Stashuk, Philip J Atherton, Bethan E Phillips, Mathew Piasecki.

  KEY POINTS: Muscle mass and function decline rapidly in situations of disuse such as bed rest and limb immobilisation. The reduction in muscle function commonly exceeds that of muscle mass, which may be associated with the dysregulation of neural input to muscle. We have used intramuscular electromyography to sample individual motor unit and near fibre potentials from the vastus lateralis following 15 days of unilateral limb immobilisation. Following disuse, the disproportionate loss of muscle strength when compared to size coincided with suppressed motor unit firing rate. These motor unit adaptations were observed at multiple contraction levels and in the immobilised limb only. Our findings demonstrate neural dysregulation as a key component of functional loss following muscle disuse in humans.ABSTRACT: Disuse atrophy, caused by situations of unloading such as limb immobilisation, causes a rapid yet diverging reduction in skeletal muscle function when compared to muscle mass. While mechanistic insight into the loss of mass is well studied, deterioration of muscle function with a focus towards the neural input to muscle remains underexplored. This study aimed to determine the role of motor unit adaptation in disuse-induced neuromuscular deficits. Ten young, healthy male volunteers underwent 15 days of unilateral lower limb immobilisation with intramuscular electromyography (iEMG) bilaterally recorded from the vastus lateralis (VL) during knee extensor contractions normalised to maximal voluntary contraction (MVC), pre and post disuse. Muscle cross-sectional area was determined by ultrasound. Individual MUs were sampled and analysed for changes in motor unit (MU) discharge and MU potential (MUP) characteristics. VL CSA was reduced by approximately 15% which was exceeded by a two-fold decrease of 31% in muscle strength in the immobilised limb, with no change in either parameter in the non-immobilised limb. Parameters of MUP size were reduced by 11 to 24% with immobilisation, while neuromuscular junction (NMJ) transmission instability remained unchanged, and MU firing rate decreased by 8 to 11% at several contraction levels. All adaptations were observed in the immobilised limb only. These findings highlight impaired neural input following immobilisation reflected by suppressed MU firing rate which may underpin the disproportionate reductions of strength relative to muscle size. Abstract figure legend Ten healthy young males underwent 15 days of unilateral lower limb immobilisation with an irremovable leg brace. Muscle size, strength and neuromuscular characteristics were measured bilaterally. Muscle strength reduced to a greater extent than muscle size in the immobilised leg while remaining unaltered in the non-immobilised leg. Motor unit firing rate, measured bilaterally using intramuscular electromyography, was also reduced in the immobilised leg only. This occurred at contraction intensities both relative to follow-up muscle strength and muscle strength normalised to pre-immobilisation. These findings suggest that neural dysregulation contributes to the loss of muscle strength observed in situations of disuse atrophy in humans. This article is protected by copyright. All rights reserved.

Keywords:  NMJ; electromyography; motor unit; muscle disuse

DOI:  https://doi.org/10.1113/JP283425
Cell Death Dis. 2022 Sep 16. 13(9): 793

Non-myogenic mesenchymal cells contribute to muscle degeneration in facioscapulohumeral muscular dystrophy patients.

Lorena Di Pietro, Flavia Giacalone, Elvira Ragozzino, Valentina Saccone, Federica Tiberio, Marco De Bardi, Mario Picozza, Giovanna Borsellino, Wanda Lattanzi, Enrico Guadagni, Sara Bortolani, Giorgio Tasca, Enzo Ricci, Ornella Parolini.

Muscle-resident non-myogenic mesenchymal cells play key roles that drive successful tissue regeneration within the skeletal muscle stem cell niche. These cells have recently emerged as remarkable therapeutic targets for neuromuscular disorders, although to date they have been poorly investigated in facioscapulohumeral muscular dystrophy (FSHD). In this study, we characterised the non-myogenic mesenchymal stromal cell population in FSHD patients' muscles with signs of disease activity, identified by muscle magnetic resonance imaging (MRI), and compared them with those obtained from apparently normal muscles of FSHD patients and from muscles of healthy, age-matched controls. Our results showed that patient-derived cells displayed a distinctive expression pattern of mesenchymal markers, along with an impaired capacity to differentiate towards mature adipocytes in vitro, compared with control cells. We also demonstrated a significant expansion of non-myogenic mesenchymal cells (identified as CD201- or PDGFRA-expressing cells) in FSHD muscles with signs of disease activity, which correlated with the extent of intramuscular fibrosis. In addition, the accumulation of non-myogenic mesenchymal cells was higher in FSHD muscles that deteriorate more rapidly. Our results prompt a direct association between an accumulation, as well as an altered differentiation, of non-myogenic mesenchymal cells with muscle degeneration in FSHD patients. Elucidating the mechanisms and cellular interactions that are altered in the affected muscles of FSHD patients could be instrumental to clarify disease pathogenesis and identifying reliable novel therapeutic targets.

DOI: https://doi.org/10.1038/s41419-022-05233-6
Am J Physiol Cell Physiol. 2022 Sep 12.

Tiny changes in cytoplasmic [Ca2+] cause large changes in mitochondrial Ca2+: what are the triggers and functional implications?

Crystal Seng, Luke Pearce, Aldo Meizoso-Huesca, Daniel P Singh, Robyn M Murphy, Cedric Lamboley, Bradley S Launikonis.

  Ca2+ is an integral component of the functional and developmental regulation of the mitochondria. In skeletal muscle, Ca2+ is reported to modulate the rate of ATP resynthesis, regulate the expression of PGC1α following exercise and drive the generation of reactive oxygen species (ROS). Due to the latter, mitochondrial Ca2+ overload is recognized as a pathophysiological event but the former events represent important physiological functions in need of tight regulation. Recently, we described the relationship between [Ca2+]mito and resting [Ca2+]cyto and other mitochondrial Ca2+-handling properties of skeletal muscle. An important next step is to understand the triggers for Ca2+ redistribution between intracellular compartments, which determine the mitochondrial Ca2+ load. These triggers in both physiological and pathophysiological scenarios can be traced to the coupled activity of the ryanodine receptor 1 (RyR1) and store-operated Ca2+ entry (SOCE) in the resting muscle. In this piece we will discuss some issues regarding Ca2+ measurements relevant to mitochondrial Ca2+-handling, the steady state relationship between cytoplasmic and mitochondrial Ca2+ and the potential implications for Ca2+ handling by muscle mitochondria and cellular function.

Keywords:  calcium; cytoplasm; mitochondria; muscle; ryanodine receptor

DOI:  https://doi.org/10.1152/ajpcell.00092.2022
J Appl Physiol (1985). 2022 Sep 15.

Skeletal muscle contraction kinetics and AMPK responses are modulated by the adenine nucleotide degrading enzyme AMPD1.

Paul S Hafen, Andrew S Law, Catalina Matias, Spencer G Miller, Jeffrey J Brault.

  AMP deaminase 1 (AMPD1; AMP → IMP + NH3) deficiency in skeletal muscle results in an inordinate accumulation of AMP during strenuous exercise, with some but not all studies reporting premature fatigue and reduced work capacity. To further explore these inconsistencies, we investigated the extent to which AMPD1 deficiency impacts skeletal muscle contractile function of different muscles and the [AMP]/AMPK responses to different intensities of fatiguing contractions. To reduce AMPD1 protein, we electroporated either an inhibitory AMPD1-specific miRNA encoding plasmid, or a control plasmid, into contralateral EDL and SOL muscles of C57BL/6J mice (n = 48 male, 24 female). After ten days, isolated muscles were assessed for isometric twitch, tetanic, and repeated fatiguing contraction characteristics using one of four (None, LOW, MOD, HIGH) duty cycles. AMPD1 knockdown (~35%) had no effect on twitch force or twitch contraction/relaxation kinetics. However, during maximal tetanic contractions, AMPD1 knockdown impaired both time-to-peak tension (TPT) and half-relaxation time (1/2 RT) in EDL, but not SOL muscle. In addition, AMPD1 knockdown in EDL exaggerated the AMP response to contractions at LOW (+100%) and MOD (+54%) duty cycles, but not HIGH. This accumulation of AMP was accompanied by increased AMPK phosphorylation (Thr-172; LOW +25%, MOD +34%) and downstream substrate phosphorylation (LOW +15%, MOD +17%). These responses to AMPD1 knockdown were not different between males and females. Our findings demonstrate that AMPD1 plays a role in maintaining skeletal muscle contractile function and regulating the energetic responses associated with repeated contractions in a muscle- but not sex-specific manner.

Keywords:  AMP deaminase; AMP-activated Protein kinase; Fatigue; Muscle contraction; Sex differences

DOI:  https://doi.org/10.1152/japplphysiol.00035.2022
Bull Math Biol. 2022 Sep 17. 84(11): 123

Computational Model of the Effect of Mitochondrial Dysfunction on Excitation-Contraction Coupling in Skeletal Muscle.

Sageanne Senneff, Madeleine M Lowery.

  It has become well established that mitochondria not only regulate myoplasmic calcium in skeletal muscle, but also use that calcium to stimulate oxidative phosphorylation (OXPHOS). While experimental approaches have allowed for imaging of mitochondrial calcium and membrane potentials in isolated fibers, capturing the role of mitochondria and the impact of mitochondrial impairments on excitation-contraction coupling (ECC) remains difficult to explore in intact muscle. Computational models have been widely used to examine the structure and function of skeletal muscle contraction; however, models of ECC to date lack communication between the myoplasm and mitochondria for regulating calcium and ATP during sustained contractions. To address this, a mathematical model of mitochondrial calcium handling and OXPHOS was integrated into a physiological model of ECC incorporating action potential propagation, calcium handling between the sarcoplasmic reticulum (SR) and the myoplasm, and crossbridge cycling. The model was used to examine the protective role of mitochondria during repeated stimulation and the impact of mitochondrial dysfunction on ECC resulting from progressive OXPHOS inhibition. Pathological myoplasmic calcium accumulation occurred through distinct mechanisms in the model in the case of either electron transport chain, F1F0 ATP synthase, or adenine nucleotide transporter impairments. To investigate the effect of each impairment on force, a model of calcium-stimulated apoptosis was utilized to capture dysfunction-induced reductions in muscle mass, driving whole muscle force loss. The model presented in this study can be used to examine the role of mitochondria in the regulation of calcium, ATP, and force generation during voluntary contraction.

Keywords:  Apoptosis; Calcium dynamics; Force; Mathematical modeling; Muscle weakness

DOI:  https://doi.org/10.1007/s11538-022-01079-3
Bone. 2022 Sep 12. pii: S8756-3282(22)00222-8. [Epub ahead of print] 116545

Higher bone remodeling biomarkers are related to a higher muscle function in older adults: Effects of acute exercise.

Cassandra Smith, Danielle Hiam, Alexander Tacey, Xuzhu Lin, Mary N Woessner, Navabeh Zarekookandeh, Andrew Garnham, Paul Chubb, Joshua R Lewis, Marc Sim, Markus Herrmann, Gustavo Duque, Itamar Levinger.

  Bone and muscle are closely linked mechanically and biochemically. Bone hormones secreted during bone remodeling might be linked to muscle mass and strength maintenance. Exercise elicits high mechanical strain and is essential for bone health. However, the relationship between commonly used bone turnover markers (BTMs) and muscle function in community dwelling older adults remains unclear. It is also unknown how acute exercise with differing mechanical strain may affect BTMs, and whether baseline muscle function alters BTM responses differently. We tested the hypothesis that BTMs are associated with muscle function, and that acute exercise could change the circulating levels of BTMs. Thirty-five older adults (25 females/10 males, 72.8 ± 6.0 years) participated. Baseline assessments included body composition (DXA), handgrip strength and a physical performance test (PPT) (gait speed, timed-up-and-go [TUG], stair ascent/descent). Leg muscle quality (LMQ) and stair climb power (SCP) were calculated. Participants performed (randomized) 30 min aerobic (AE) (cycling 70%HRPeak) and resistance (RE) (leg press 70%RM, jumping) exercise. C-terminal telopeptide of type I collagen (CTX), procollagen of type I propeptide (P1NP), total osteocalcin (t)OC and ucOC were assessed at baseline and post-exercise. Data were analyzed using linear mixed models and simple regressions, adjusted for sex. At baseline, higher muscle strength (LMQ, handgrip) was related to higher P1NP, higher SCP was related to higher P1NP and β-CTX, and better physical performance (lower PPT) related to higher P1NP and β-CTX (p < 0.05). Exercise, regardless of mode, decreased β-CTX and tOC (all p < 0.05), while P1NP and ucOC remained unaltered. Higher baseline handgrip strength, SCP and LMQ was associated with lower post-exercise β-CTX responses, and poorer baseline mobility (increased TUG time) was associated with higher post-exercise β-CTX. Independently of exercise mode, acute exercise decreased β-CTX and tOC. Our data suggest that in older adults at baseline, increased BTM levels were linked to better muscle function. Altogether, our data strengthens the evidence for bone-muscle interaction, however, mechanisms behind this specific component of bone-muscle crostalk remain unclear.

Keywords:  Aging; Biochemical markers of bone turnover; Bone-muscle interactions; Exercise; Skeletal muscle

DOI:  https://doi.org/10.1016/j.bone.2022.116545
Stem Cells Transl Med. 2022 Sep 14. pii: szac068. [Epub ahead of print]

Carm1 and the Epigenetic Control of Stem Cell Function.

John Saber, Michael A Rudnicki.

  Coactivator-associated arginine methyltransferase 1 (CARM1) is a methyltransferase whose function has been highly studied in the context of nuclear receptor signaling. However, CARM1 is known to epigenetically regulate expression of several myogenic genes involved in differentiation such as Myog and MEF2C. CARM1 also acts to regulate myogenesis through its influence on various cellular processes from embryonic to adult myogenesis. First, CARM1 has a crucial role in establishing polarity-regulated gene expression during an asymmetric satellite cell division by methylating PAX7, leading to the expression of Myf5. Second, satellite cells express the CARM1-FL and CARM1-ΔE15 isoforms. The former has been shown to promote pre-mRNA splicing through its interaction with CA150 and U1C, leading to their methylation and increased activity, while the latter displays a reduction in both metrics, thus, modulating alternative pre-mRNA splice forms in muscle cells. Third, CARM1 is a regulator of autophagy through its positive reinforcement of AMPK activity and gene expression. Autophagy already has known implications in ageing and disease, and CARM1 could follow suite. Thus, CARM1 is a central regulator of several important processes impacting muscle stem cell function and myogenesis.

Keywords:  Carm1; asymmetric division; autophagy; muscle stem cells; pre-mRNA splicing

DOI:  https://doi.org/10.1093/stcltm/szac068
Front Aging. 2022 ;3 975129

Rates of protein synthesis are maintained in brain but reduced in skeletal muscle during dietary sulfur amino acid restriction.

Wenceslao Martinez, Qian Zhang, Melissa A Linden, Nate Schacher, Sanna Darvish, Emily T Mirek, Jordan L Levy, William O Jonsson, Tracy G Anthony, Karyn L Hamilton.

  Dietary interventions such as sulfur amino acid restriction (SAAR) target multiple drivers of aging, and show promise for preventing or delaying the onset of chronic diseases. SAAR promotes metabolic health and longevity in laboratory animals. The effects of SAAR on proteostasis remain relatively unexplored. We previously reported that SAAR promotes mitochondrial proteostatic maintenance, despite suppression of global protein synthesis, in two peripheral tissues, the liver and skeletal muscle. However, the brain, a tissue vulnerable to age-related neurodegenerative diseases due to the loss of proteostasis, has not been thoroughly studied. Therefore, we sought to reveal proteostatic responses in the brains of mice fed SAAR for 35 days. Here, we demonstrate that male C57Bl/6J mice fed two levels of SAAR maintained rates of protein synthesis in all sub-cellular fractions of the pre-frontal cortex. In comparison, rates of skeletal muscle protein synthesis in SAAR fed mice were slower than control-fed mice. To gain mechanistic insight, we examined several key nutrient/energy sensitive signaling proteins: AMP-activated protein kinase (AMPK), eukaryotic initiation factor 2 (eIF2), and ribosomal protein S6 (rpS6). SAAR had minimal to modest effects on the total abundance and phosphorylation of these proteins in both tissues. Our results indicate that the pre-frontal cortex in brain is resistant to perturbations in protein synthesis in mice fed SAAR, unlike skeletal muscle, which had a reduction in global protein synthesis. The results from this study demonstrate that proteostatic control in brain is of higher priority than skeletal muscle during dietary SAAR.

Keywords:  amino acids; dietary intervention; energy sensing; healthspan; methionine restriction; proteostasis

DOI:  https://doi.org/10.3389/fragi.2022.975129
Am J Physiol Regul Integr Comp Physiol. 2022 Sep 12.

Altered skeletal muscle SERCA calcium transport efficiency following a thermogenic stimulus.

Lydia A Heemstra, Lauren G Koch, Steven L Britton, Colleen M Novak.

  Exposure to predator threat induces a rapid and robust increase in skeletal muscle thermogenesis in rats. The central nervous system relays threat information to skeletal muscle through activation of the sympathetic nervous system, but muscle mechanisms mediating this thermogenesis remain unidentified. Given the relevance of sarcolipin-mediated futile calcium cycling through the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump to mammalian muscle non-shivering thermogenesis, we hypothesized that this plays a role in contextually induced muscle thermogenesis as well. This was assessed by measuring enzymatic activity of SERCA and sarcoplasmic reticulum Ca2+ transport, where the apparent coupling ratio-Ca2+ uptake rate divided by ATPase activity rate at a standard Ca2+ concentration-was predicted to decrease in association with muscle thermogenesis. Sprague Dawley rats exposed to predator (ferret) odor (PO) showed a rapid decrease in the apparent coupling ratio in the soleus muscle, indicating SERCA uncoupling compared to control-odor-exposed rats. A rat model of high aerobic fitness and elevated muscle thermogenesis also demonstrated soleus muscle SERCA uncoupling relative to their obesity-prone, low-fitness counterparts. Both the high- and low-aerobic fitness rats showed soleus SERCA uncoupling with exposure to PO. Lastly, no increase in sarcolipin expression in soleus muscle was detected with PO exposure. This dataset implicates muscle uncoupling of SERCA Ca2+ transport and ATP hydrolysis, likely through altered SERCA or sarcolipin function outside of translational regulation, as one contributor to the muscle thermogenesis provoked by exposure to predator threat. These data support the involvement of SERCA uncoupling in both muscle thermogenic induction and enhanced aerobic capacity.

Keywords:  aerobic fitness; futile calcium cycling; high- and low-capacity runners; sarcolipin; uncoupling

DOI:  https://doi.org/10.1152/ajpregu.00173.2022
Front Physiol. 2022 ;13 951390

Lactate-induced lactylation in skeletal muscle is associated with insulin resistance in humans.

Dominic Maschari, Gunjan Saxena, Timothy D Law, Erin Walsh, Mason C Campbell, Leslie A Consitt.

  Elevated circulating lactate has been associated with obesity and insulin resistance. The aim of the current study was to determine if lactate-induced lysine lactylation (kla), a post-translational modification, was present in human skeletal muscle and related to insulin resistance. Fifteen lean (Body Mass Index: 22.1 ± 0.5 kg/m2) and fourteen obese (40.6 ± 1.4 kg/m2) adults underwent a muscle biopsy and 2-h oral glucose tolerance test. Skeletal muscle lactylation was increased in obese compared to lean females (19%, p < 0.05) and associated with insulin resistance (r = 0.37, p < 0.05) in the whole group. Skeletal muscle lactylation levels were significantly associated with markers of anaerobic metabolism (plasma lactate and skeletal muscle lactate dehydrogenase [LDH], p < 0.05) and negatively associated with markers of oxidative metabolism (skeletal muscle cytochrome c oxidase subunit 4 and Complex I [pyruvate] OXPHOS capacity, p < 0.05). Treatment of primary human skeletal muscle cells (HSkMC) with sodium lactate for 24 h increased protein lactylation and IRS-1 serine 636 phosphorylation in a similar dose-dependent manner (p < 0.05). Inhibition of glycolysis (with 2-deoxy-d-glucose) or LDH-A (with sodium oxamate or LDH-A siRNA) for 24 h reduced HSkMC lactylation which paralleled reductions in culture media lactate accumulation. This study identified the existence of a lactate-derived post-translational modification in human skeletal muscle and suggests skeletal muscle lactylation could provide additional insight into the regulation of skeletal muscle metabolism, including insulin resistance.

Keywords:  insulin sensitivity; lactate; lactylation; obesity; post-translation modification; skeletal muscle

DOI:  https://doi.org/10.3389/fphys.2022.951390
JCI Insight. 2022 Sep 13. pii: e158906. [Epub ahead of print]

Pharmacological TRPC6 inhibition improves survival and muscle function in mice with duchenne muscular dystrophy.

Brian L Lin, Joseph Y Shin, William Pd Jeffreys, Nadan Wang, Clarisse A Lukban, Megan C Moorer, Esteban Velarde, Olivia A Hanselman, Seoyoung Kwon, Suraj Kannan, Ryan C Riddle, Christopher W Ward, Steven S Pullen, Antonio Filareto, David A Kass.

  Gene mutations causing loss of dystrophin result in the severe muscle disease known as Duchenne muscular dystrophy (DMD). Despite efforts at genetic repair, DMD therapy remains largely palliative. Loss of dystrophin destabilizes the sarcolemmal membrane impacting mechanosensitive cation channels to increase calcium entry, promoting cell damage, and eventually muscle dysfunction. One putative channel is transient receptor potential canonical 6 (TRPC6) that we showed contributes to abnormal force and calcium stress-responses in mouse cardiomyocytes lacking dystrophin and haplodeficient in utrophin mdx/utrn+/- (HET). Here, we show in both HET and the far more severe homozygous mdx/utrn-/- (DKO) mouse that TRPC6 gene deletion or its selective pharmacologic inhibition (BI 749327) prolongs survival 2-3-fold, improving skeletal and cardiac muscle and bone defects. Gene pathways reduced by BI 749327 treatment most prominently regulate fat metabolism and TGFβ1 signaling. These results support the testing of TRPC6 inhibitors in human trials for other diseases as a novel DMD therapy.

Keywords:  Calcium channels; Cardiology; Cardiovascular disease; Muscle; Muscle Biology

DOI:  https://doi.org/10.1172/jci.insight.158906
FEBS Lett. 2022 Sep 16.

Overexpression of UCP3 decreases mitochondrial efficiency in mouse skeletal muscle in vivo.

Roberto Codella, Tiago C Alves, Douglas E Befroy, Cheol Soo Choi, Livio Luzi, Douglas L Rothman, Richard G Kibbey, Gerald I Shulman.

  Uncoupling protein-3 (UCP3) is a mitochondrial transmembrane protein highly expressed in muscle that has been implicated in regulating the efficiency of mitochondrial oxidative phosphorylation. Increasing UCP3 expression in skeletal muscle enhances proton leak across the inner mitochondrial membrane and increases oxygen consumption in isolated mitochondria, but its precise function in vivo has yet to be fully elucidated. To examine whether muscle-specific overexpression of UCP3 modulates muscle mitochondrial oxidation in vivo, rates of ATP synthesis were assessed by 31 P magnetic resonance spectroscopy (MRS) and rates of mitochondrial oxidative metabolism were measured by assessing the rate of [2-13 C]acetate incorporation into muscle [4-13 C]- , [3-13 C]-glutamate and [4-13 C]-glutamine by high resolution 13 C/1 H MRS. Using this approach we found that overexpression of UCP3 in skeletal muscle was accompanied by increased muscle mitochondrial inefficiency in vivo as reflected by a 42% reduction in the ratio of ATP synthesis to mitochondrial oxidation.

Keywords:  magnetic resonance spectroscopy; mitochondrial efficiency; mitochondrial uncoupling; muscle energy metabolism

DOI:  https://doi.org/10.1002/1873-3468.14494
iScience. 2022 Sep 16. 25(9): 104954

Global chromatin accessibility profiling analysis reveals a chronic activation state in aged muscle stem cells.

Anqi Dong, Jing Liu, Kangning Lin, Wenshu Zeng, Wai-Kin So, Shenyuan Hu, Tom H Cheung.

  Regulation of chromatin accessibility is critical for cell fate decisions. Chromatin structure responds to extrinsic environments rapidly. The traditional adult stem cell isolation approach requires tissue dissociation, which triggers stem cell activation and leads to alterations in chromatin structure. To preserve the in vivo chromatin states, we utilized the PFA-perfusion-based isolation approach and characterized the DNA regulatory landscapes during muscle stem cell quiescence exit and aging. We showed that aged SCs display a chronically activated chromatin signature. Detailed analysis of the chromatin accessibility profiles identified key enhancer elements for SC quiescence. Constant activation of the enhancer elements promotes stemness and prevents SCs from differentiation, whereas genetic deletion causes cell-cycle arrest and leads to defects in activation. Our comprehensive characterization of the chromatin accessibility and transcriptomic landscapes in SC quiescence and aging broadens our understanding of these processes and identifies key distal regulatory elements for SC function.

Keywords:  Cell biology; Omics; Stem cells research; Transcriptomics

DOI:  https://doi.org/10.1016/j.isci.2022.104954
Proc Natl Acad Sci U S A. 2022 Sep 20. 119(38): e2202490119

Phosphatidylserine orchestrates Myomerger membrane insertions to drive myoblast fusion.

Dilani G Gamage, Kamran Melikov, Paola Munoz-Tello, Tanner J Wherley, Leah C Focke, Evgenia Leikina, Elliana Huffman, Jiajie Diao, Douglas J Kojetin, Vikram Prasad, Leonid V Chernomordik, Douglas P Millay.

  Muscle cell fusion is a multistep process where the final step of the reaction drives progression beyond early hemifusion events to complete fusion. This step requires activity of the muscle-specific fusogen Myomerger, a single-pass transmembrane protein containing 84 amino acids with an ectodomain that includes two α-helices. Previous studies have demonstrated that Myomerger acts by destabilizing membranes through generation of elastic stresses in the outer leaflet of the plasma membrane. An obvious question is how such destabilizing activity might be regulated to avoid membrane and cellular damage, and how the two juxtaposed helices cooperate in fusion. Using cellular fusion assays and in vitro liposome assays, we report that the two helices possess unique characteristics, both of which are needed for full activity of the protein. We demonstrate that externalized phosphatidylserine (PS), a lipid previously implicated in myoblast fusion, has a determinant role in the regulation of Myomerger activity. The membrane-proximal, amphipathic Helix-1 is normally disordered and its α-helical structure is induced by PS, making membrane interactions more efficacious. The distal, more hydrophobic Helix-2 is intrinsically ordered, possesses an ability to insert into membranes, and augments the membrane-stressing effects of Helix-1. These data reveal that Myomerger fusogenic activity is an exquisitely orchestrated event involving its two ectodomain helices, which are controlled by membrane lipid composition, providing an explanation as to how its membrane-stressing activity is spatially and temporally regulated during the final step of myoblast fusion.

Keywords:  Myomerger/Myomixer; membrane fusion; myoblast fusion; phosphatidylserine

DOI:  https://doi.org/10.1073/pnas.2202490119
J Cell Physiol. 2022 Sep 10.

Sclerostin aggravates insulin signaling in skeletal muscle and hepatic steatosis via upregulation of ER stress by mTOR-mediated inhibition of autophagy under hyperlipidemic conditions.

Heeseung Oh, Seung Yeon Park, Wonjun Cho, A M Abd El-Aty, Ahmet Hacimuftuoglu, Chang Hyuk Kwon, Ji Hoon Jeong, Tae Woo Jung.

  Recently, sclerostin (SCL), a circulating glycoprotein, was proposed to be a novel myokine involved in developing metabolic disorders. The association between SCL levels and insulin resistance in skeletal muscle, liver, and adipose tissue was studied in individuals with aggravated glucose tolerance. Thus, we hypothesized that elevated circulating SCL might affect skeletal muscle insulin signaling and hepatic lipid metabolism, and aimed to investigate the effects of SCL on skeletal muscle insulin resistance and hepatic steatosis in obesity using in vitro and in vivo experimental models under hyperlipidemic conditions. In the current study, we found elevated SCL messenger RNA expression levels in myocytes in obese patients. In addition to a higher blood level, SCL was expressed at an elevated level in the skeletal muscle of mice fed a high-fat diet (HFD). Higher SCL release levels and expression were also noticed in palmitate-treated C2C12 myocytes. SCL suppression by in vivo transfection improves skeletal muscle insulin resistance and hepatic steatosis in HFD-fed mice. The treatment of C2C12 myocytes with recombinant SCL aggravated insulin signaling. Furthermore, treatment with SCL augmented lipogenic lipid deposition in human primary hepatocytes. Treatment with SCL upregulated mammalian target of rapamycin (mTOR) phosphorylation and suppressed autophagy markers, thereby causing endoplasmic reticulum (ER) stress. 4-Phenylbutyric acid, a pharmacological ER stress inhibitor, abolished the effects of SCL on insulin signaling in C2C12 myocytes and lipid accumulation in primary hepatocytes. In conclusion, SCL promotes skeletal muscle insulin resistance and hepatic steatosis by upregulating ER stress via the mTOR/autophagy-mediated pathway. The present study suggests that antagonizing SCL might be a novel therapeutic strategy for simultaneously managing insulin resistance and hepatic steatosis in obesity.

Keywords:  AMPK; ER stress; NAFLD; SIRT6; autophagy; sclerostin

DOI:  https://doi.org/10.1002/jcp.30873
Front Mol Biosci. 2022 ;9 957549

Metabolomics reveals mouse plasma metabolite responses to acute exercise and effects of disrupting AMPK-glycogen interactions.

Mehdi R Belhaj, Nathan G Lawler, John A Hawley, David I Broadhurst, Nolan J Hoffman, Stacey N Reinke.

  Introduction: The AMP-activated protein kinase (AMPK) is a master regulator of energy homeostasis that becomes activated by exercise and binds glycogen, an important energy store required to meet exercise-induced energy demands. Disruption of AMPK-glycogen interactions in mice reduces exercise capacity and impairs whole-body metabolism. However, the mechanisms underlying these phenotypic effects at rest and following exercise are unknown. Furthermore, the plasma metabolite responses to an acute exercise challenge in mice remain largely uncharacterized. Methods: Plasma samples were collected from wild type (WT) and AMPK double knock-in (DKI) mice with disrupted AMPK-glycogen binding at rest and following 30-min submaximal treadmill running. An untargeted metabolomics approach was utilized to determine the breadth of plasma metabolite changes occurring in response to acute exercise and the effects of disrupting AMPK-glycogen binding. Results: Relative to WT mice, DKI mice had reduced maximal running speed (p < 0.0001) concomitant with increased body mass (p < 0.01) and adiposity (p < 0.001). A total of 83 plasma metabolites were identified/annotated, with 17 metabolites significantly different (p < 0.05; FDR<0.1) in exercised (↑6; ↓11) versus rested mice, including amino acids, acylcarnitines and steroid hormones. Pantothenic acid was reduced in DKI mice versus WT. Distinct plasma metabolite profiles were observed between the rest and exercise conditions and between WT and DKI mice at rest, while metabolite profiles of both genotypes converged following exercise. These differences in metabolite profiles were primarily explained by exercise-associated increases in acylcarnitines and steroid hormones as well as decreases in amino acids and derivatives following exercise. DKI plasma showed greater decreases in amino acids following exercise versus WT. Conclusion: This is the first study to map mouse plasma metabolomic changes following a bout of acute exercise in WT mice and the effects of disrupting AMPK-glycogen interactions in DKI mice. Untargeted metabolomics revealed alterations in metabolite profiles between rested and exercised mice in both genotypes, and between genotypes at rest. This study has uncovered known and previously unreported plasma metabolite responses to acute exercise in WT mice, as well as greater decreases in amino acids following exercise in DKI plasma. Reduced pantothenic acid levels may contribute to differences in fuel utilization in DKI mice.

Keywords:  AMP-activated protein kinase; acylcarnitines; amino acids; exercise metabolism; glycogen; metabolomics; pantothenic acid; plasma metabolite

DOI:  https://doi.org/10.3389/fmolb.2022.957549
Am J Physiol Cell Physiol. 2022 Sep 12.

Development of skeletal muscle atrophy and intermuscular adipose tissue in early breast cancer patients treated with chemotherapy.

Joris Mallard, Elyse Hucteau, Laura Bender, Anouk Charlot, Léa Debrut, Carole Pflumio, Philippe Trensz, Roland Schott, Fabrice Favret, Xavier Pivot, Thomas J Hureau, Allan F Pagano.

  BACKGROUND: Chemotherapy is a common therapy to treat breast cancer patients but also leads to skeletal muscle deconditioning. Skeletal muscle deconditioning is multifactorial and intermuscular adipose tissue (IMAT) accumulation is closely linked to muscle dysfunction. To date, there is no clinical study available investigating IMAT development through a longitudinal protocol and the underlying mechanisms remain unknown. Our study was dedicated to investigating IMAT content in early breast cancer patients treated with chemotherapy and exploring the subsequent cellular mechanisms involved in its development.METHODS: We included thirteen women undergoing chemotherapy. Muscle biopsies and ultrasonography assessment were performed before and after chemotherapy completion. Histological and western blotting analyses were conducted.
RESULTS: We found a substantial increase in protein levels of three mature adipocyte markers (perilipin, +901%; adiponectin, +135%; FABP4, +321%; p<0.05). These results were supported by an increase in Oil red O-positive staining (+358%; p<0.05). A substantial increase in PDGFRα protein levels was observed (+476%; p<0.05) highlighting an increase in Fibro-adipogenic progenitors (FAPs) content. The cross-sectional area of the vastus lateralis muscle fibers substantially decreased (-21%; p<0.01), and muscle architecture was altered, as shown by a decrease in fascicle length (-15%; p<0.05) and a decreasing trend in muscle thickness -8%; p=0.08).
CONCLUSIONS: We demonstrated both IMAT development and muscle atrophy in breast cancer patients treated with chemotherapy. FAPs, critical stem cells inducing both IMAT development and skeletal muscle atrophy, also increased, suggesting that FAPs likely play a critical role in the skeletal muscle deconditioning observed in breast cancer patients treated with chemotherapy.

Keywords:  Fatty Infiltrations; Fibro-Adipogenic Progenitors (FAPs); IMAT; Muscle Architecture; Skeletal Muscle Deconditioning

DOI:  https://doi.org/10.1152/ajpcell.00373.2022
Hum Mol Genet. 2022 Sep 13. pii: ddac231. [Epub ahead of print]

Individual-specific levels of CTG•CAG somatic instability are shared across multiple tissues in myotonic dystrophy type 1.

Marigold Myotonic Dystrophy Biomarkers Discovery Initiative-MMDBDI

Myotonic dystrophy type 1 is a complex disease caused by a genetically unstable CTG repeat expansion in the 3'-untranslated region of the DMPK gene. Age-dependent, tissue-specific somatic instability has confounded genotype-phenotype associations, but growing evidence suggests that it also contributes directly toward disease progression. Using a well-characterized clinical cohort of DM1 patients from Costa Rica, we quantified somatic instability in blood, buccal cells, skin and skeletal muscle. Whilst skeletal muscle showed the largest expansions, modal allele lengths in skin were also very large and frequently exceeded 2000 CTG repeats. Similarly, the degree of somatic expansion in blood, muscle and skin were associated with each other. Notably, we found that the degree of somatic expansion in skin was highly predictive of that in skeletal muscle. More importantly, we established that individuals whose repeat expanded more rapidly than expected in one tissue (after correction for progenitor allele length and age), also expanded more rapidly than expected in other tissues. We also provide evidence suggesting that individuals in whom the repeat expanded more rapidly than expected in skeletal muscle, have an earlier age-at-onset than expected (after correction for the progenitor allele length). Pyrosequencing analyses of the genomic DNA flanking the CTG repeat revealed that the degree of methylation in muscle was well predicted by the muscle modal allele length and age, but that neither methylation of the flanking DNA, nor levels of DMPK sense and anti-sense transcripts, could obviously explain individual- or tissue-specific patterns of somatic instability.

DOI: https://doi.org/10.1093/hmg/ddac231
J Inherit Metab Dis. 2022 Sep 16.

Lysosomal glycogen accumulation in Pompe disease results in disturbed cytoplasmic glycogen metabolism.

Rodrigo Canibano-Fraile, Laurike Harlaar, Carlos A Dos Santos, Marianne Hoogeveen-Westerveld, Jeroen A A Demmers, Tim Snijders, Philip Lijnzaad, Robert M Verdijk, Nadine A M E van der Beek, Pieter A van Doorn, Ans T van der Ploeg, Esther Brusse, W W M Pim Pijnappel, Gerben J Schaaf.

  Pompe disease is an inherited metabolic myopathy caused by deficiency of acid α-glucosidase (GAA), resulting in lysosomal glycogen accumulation. Residual GAA enzyme activity affects disease onset and severity, although other factors, including dysregulation of cytoplasmic glycogen metabolism, are suspected to modulate the disease course. In this study, performed in mice and patient biopsies, we found elevated protein levels of enzymes involved in glucose uptake and cytoplasmic glycogen synthesis in skeletal muscle from mice with Pompe disease, including glycogenin (GYG1), glycogen synthase (GS), glucose transporter 4 (GLUT4), glycogen branching enzyme (GBE1), and UDP-glucose pyrophosphorylase (UGP2). Expression levels were elevated before the loss of muscle mass and function. For first time, quantitative mass spectrometry in skeletal muscle biopsies from five adult patients with Pompe disease showed increased expression of glycogen branching enzyme protein relative to healthy controls at the group level. Paired analysis of individual patients who responded well to treatment with enzyme replacement therapy (ERT) showed reduction of glycogen synthase, glycogenin, and glycogen branching enzyme 1 in all patients after start of ERT compared to baseline. These results indicate that metabolic changes precede muscle wasting in Pompe disease, and imply a positive feedforward loop in Pompe disease, in which lysosomal glycogen accumulation promotes cytoplasmic glycogen synthesis and glucose uptake, resulting in aggravation of the disease phenotype.

Keywords:  Pompe disease; Skeletal muscle; glycogen metabolism; lysosomal storage disorder; metabolic myopathy

DOI:  https://doi.org/10.1002/jimd.12560
Biology (Basel). 2022 Jun 30. pii: 999. [Epub ahead of print]11(7):

Irisin Attenuates Muscle Impairment during Bed Rest through Muscle-Adipose Tissue Crosstalk.

Andrea D'Amuri, Juana Maria Sanz, Stefano Lazzer, Rado Pišot, Bostjan Šimunič, Gianni Biolo, Giovanni Zuliani, Mladen Gasparini, Marco Narici, Bruno Grassi, Carlo Reggiani, Edoardo Dalla Nora, Angelina Passaro.

  The detrimental effect of physical inactivity on muscle characteristics are well known. Irisin, an exercise-induced myokine cleaved from membrane protein fibronectin type III domain-containing protein-5 (FNDC5), mediates at least partially the metabolic benefits of exercise. This study aimed to assess the interplay between prolonged inactivity, circulating irisin, muscle performance, muscle fibers characteristics, as well as the FNDC5 gene expression (FNDC5ge) in muscle and adipose tissue among healthy subjects. Twenty-three healthy volunteers were tested before and after 14 days of Bed Rest, (BR). Post-BR circulating levels of irisin significantly increased, whereas body composition, muscle performance, and muscle fiber characteristics deteriorated. Among the subjects achieving the highest post-BR increase of irisin, the lowest reduction in maximal voluntary contraction and specific force of Fiber Slow/1, the highest increase of FNDC5ge in adipose tissue, and no variation of FNDC5ge in skeletal muscle were recorded. Subjects who had the highest FNDC5ge in adipose tissue but not in muscle tissue showed the highest circulating irisin levels and could better withstand the harmful effect of BR.

Keywords:  FNDC5 gene expression; inactivity; muscle fiber; myokines; sarcopenia

DOI:  https://doi.org/10.3390/biology11070999
Front Physiol. 2022 ;13 981577

The role of exercise-induced myokines in promoting angiogenesis.

Chao Qi, Xianjing Song, He Wang, Youyou Yan, Bin Liu.

  Ischemic diseases are a major cause of mortality or disability in the clinic. Surgical or medical treatment often has poor effect on patients with tissue and organ ischemia caused by diffuse stenoses. Promoting angiogenesis is undoubtedly an effective method to improve perfusion in ischemic tissues and organs. Although many animal or clinical studies tried to use stem cell transplantation, gene therapy, or cytokines to promote angiogenesis, these methods could not be widely applied in the clinic due to their inconsistent experimental results. However, exercise rehabilitation has been written into many authoritative guidelines in the treatment of ischemic diseases. The function of exercise in promoting angiogenesis relies on the regulation of blood glucose and lipids, as well as cytokines that secreted by skeletal muscle, which are termed as myokines, during exercise. Myokines, such as interleukin-6 (IL-6), chemokine ligand (CXCL) family proteins, irisin, follistatin-like protein 1 (FSTL1), and insulin-like growth factor-1 (IGF-1), have been found to be closely related to the expression and function of angiogenesis-related factors and angiogenesis in both animal and clinical experiments, suggesting that myokines may become a new molecular target to promote angiogenesis and treat ischemic diseases. The aim of this review is to show current research progress regarding the mechanism how exercise and exercise-induced myokines promote angiogenesis. In addition, the limitation and prospect of researches on the roles of exercise-induced myokines in angiogenesis are also discussed. We hope this review could provide theoretical basis for the future mechanism studies and the development of new strategies for treating ischemic diseases.

Keywords:  angiogenesis; cytokines; exercise; ischemic diseases; myokine

DOI:  https://doi.org/10.3389/fphys.2022.981577