Disuse-induced Muscle Atrophy Research Articles

5′-UMP inhibited muscle atrophy due to detraining: a randomized, double-blinded, placebo-controlled, parallel-group comparative study

PurposeA short period of disuse of 1–2 weeks due to factors such as illness or injury can lead to muscle atrophy, affecting both athletic performance and health. Recent research has shown that uridine 5′-monophosphate (5′-UMP) can counteract disuse-induced muscle atrophy by increasing PGC-1α expression and inhibiting atrogin-1 expression. However, the effect of 5′-UMP on disuse muscle atrophy in humans remains unknown. Therefore, the aimed of this study was to explore the effects of 5′-UMP supplementation during detraining on short-term disuse muscle atrophy in healthy men.MethodsFollowing a 6-week resistance training program on upper arm, healthy men were randomized to either a UMP group (n = 11) or a placebo group (n = 10), taking their respective supplements during the 2-week detraining period. Muscle thickness, an indicator of muscle hypertrophy and atrophy, was measured at 3 positions (MT50, MT60, and MT70) at baseline, 1 week, and 2 weeks after detraining.ResultsBoth groups showed a significant decrease in muscle thickness at MT70. The relative decrease was greater in the placebo group (2.4 ± 2.8%) than in the UMP group (0.0 ± 2.0%), significantly (p = 0.034) at 1 week. However, no significant difference was observed at MT50 and MT60.ConclusionAfter the hypertrophy, 5′-UMP may prevent muscle atrophy due to the detraining within the first week.

Deciphering the Therapeutic Role of Lactate in Combating Disuse-Induced Muscle Atrophy: An NMR-Based Metabolomic Study in Mice.

Disuse muscle atrophy (DMA) is a significant healthcare challenge characterized by progressive loss of muscle mass and function resulting from prolonged inactivity. The development of effective strategies for muscle recovery is essential. In this study, we established a DMA mouse model through hindlimb suspension to evaluate the therapeutic potential of lactate in alleviating the detrimental effects on the gastrocnemius muscle. Using NMR-based metabolomic analysis, we investigated the metabolic changes in DMA-injured gastrocnemius muscles compared to controls and evaluated the beneficial effects of lactate treatment. Our results show that lactate significantly reduced muscle mass loss and improved muscle function by downregulating Murf1 expression, decreasing protein ubiquitination and hydrolysis, and increasing myosin heavy chain levels. Crucially, lactate corrected perturbations in four key metabolic pathways in the DMA gastrocnemius: the biosynthesis of phenylalanine, tyrosine, and tryptophan; phenylalanine metabolism; histidine metabolism; and arginine and proline metabolism. In addition to phenylalanine-related pathways, lactate also plays a role in regulating branched-chain amino acid metabolism and energy metabolism. Notably, lactate treatment normalized the levels of eight essential metabolites in DMA mice, underscoring its potential as a therapeutic agent against the consequences of prolonged inactivity and muscle wasting. This study not only advances our understanding of the therapeutic benefits of lactate but also provides a foundation for novel treatment approaches aimed at metabolic restoration and muscle recovery in conditions of muscle wasting.

Transcriptome and Metabolome Profiling Provide New Insights into Disuse Muscle Atrophy in Chicken: The Potential Role of Fast-Twitch Muscle Fibers.

Disuse muscle atrophy is a disease caused by restricted activity, affecting human health and animal protein quality. While extensive research on its mechanism has been studied in mammals, comparatively little is known about this process in chickens, which are a significant source of protein for human consumption worldwide. Understanding the mechanisms underlying skeletal muscle atrophy in chickens is crucial for improving poultry health and productivity, as well as for developing strategies to mitigate muscle loss. In this study, two groups of chickens were subjected to limb immobilization for two and four weeks, respectively, in order to induce disuse muscle atrophy and uniformly sampled gastrocnemius muscle at the fourth week. A combined analysis of the transcriptome and metabolome was conducted to investigate the mechanisms of disuse-induced muscle atrophy. Through H&E staining and immunofluorescence, we found that, compared to slow-twitch muscle fibers, the fast-twitch muscle fibers showed a greater reduction in cross-sectional area in the immobilized leg, and were also the main driver of changes in cross-sectional area observed in the non-immobilized leg. Integrated analysis revealed that differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) were mainly enriched in pathways related to energy metabolism, such as fatty acid metabolism, oxidative phosphorylation (OXPHOS), and glycolysis. These results provide important insights for further research on disuse muscle atrophy.

The effect of protein or amino acid provision on immobilization-induced muscle atrophy in healthy adults: A systematic review and meta-analysis.

Bed rest and limb immobilization are models of muscle disuse associated with skeletal muscle atrophy and reduced strength. The purpose of this systematic review was to examine the impact of protein or amino acid provision before and/or during a period of muscle disuse on muscle atrophy (primary outcome), strength and muscle protein synthesis (secondary outcomes) following a disuse period. We performed a systematic review of Embase, MEDLINE, Web of Science, PubMed and Clinical Trials in December 2022. Eligible studies were randomized controlled trials that combined a dietary protein or amino acid intervention versus control during an experimental model of disuse (bed rest or unilateral limb immobilization) in healthy individuals aged ≥18 years. Nine articles from eight independent trials were identified and rated for risk of bias by two authors. A meta-analysis of muscle mass data revealed no effect (standardized mean difference: 0.2; 95% confidence interval: -0.18 to 0.57, P=0.31) of protein/amino acid intervention in preventing disuse-induced muscle atrophy. Although the meta-analysis was not conducted on strength or muscle protein synthesis data, there was insufficient evidence in the reviewed articles to support the use of protein/amino acid provision in mitigating the disuse-induced decline in either outcome measurement. Additional high-quality studies, including the reporting of randomization procedures and blinding procedures and the provision of statistical analysis plans, might be required to determine whether protein or amino acid provision serves as an effective strategy to attenuate muscle atrophy during periods of disuse.

Eldecalcitol prevents muscle loss and osteoporosis in disuse muscle atrophy via NF-κB signaling in mice

We investigated the effect of eldecalcitol on disuse muscle atrophy. C57BL/6J male mice aged 6 weeks were randomly assigned to control, tail suspension (TS), and TS-eldecalcitol–treated groups and were injected intraperitoneally twice a week with either vehicle (control and TS) or eldecalcitol at 3.5 or 5 ng for 3 weeks. Grip strength and muscle weights of the gastrocnemius (GAS), tibialis anterior (TA), and soleus (SOL) were determined. Oxidative stress was evaluated by malondialdehyde, superoxide dismutase, glutathione peroxidase, and catalase. Bone microarchitecture was analyzed using microcomputed tomography. The effect of eldecalcitol on C2C12 myoblasts was analyzed by measuring myofibrillar protein MHC and the atrophy markers Atrogin-1 and MuRF-1 using immunofluorescence. The influence of eldecalcitol on NF-κB signaling pathway and vitamin D receptor (VDR) was assessed through immunofluorescence, (co)-immunoprecipitation, and VDR knockdown studies. Eldecalcitol increased grip strength (P < 0.01) and restored muscle loss in GAS, TA, and SOL (P < 0.05 to P < 0.001) induced by TS. An improvement was noted in bone mineral density and bone architecture in the eldecalcitol group. The impaired oxidative defense system was restored by eldecalcitol (P < 0.05 to P < 0.01 vs. TS). Eldecalcitol (10 nM) significantly inhibited the expression of MuRF-1 (P < 0.001) and Atrogin-1 (P < 0.01), increased the diameter of myotubes (P < 0.05), inhibited the expression of P65 and P52 components of NF-κB and P65 nuclear location, thereby inhibiting NF-κB signaling. Eldecalcitol promoted VDR binding to P65 and P52. VDR signaling is required for eldecalcitol-mediated anti-atrophy effects. In conclusion, eldecalcitol exerted its beneficial effects on disuse-induced muscle atrophy via NF-κB inhibition.

A non-invasive mouse model that recapitulates disuse-induced muscle atrophy in immobilized patients

Disuse muscle atrophy occurs consequent to prolonged limb immobility or bed rest, which represents an unmet medical need. As existing animal models of limb immobilization often cause skin erosion, edema, and other untoward effects, we here report an alternative method via thermoplastic immobilization of hindlimbs in mice. While significant decreases in the weight and fiber size were noted after 7 days of immobilization, no apparent skin erosion or edema was found. To shed light onto the molecular mechanism underlying this muscle wasting, we performed the next-generation sequencing analysis of gastrocnemius muscles from immobilized versus non-mobilized legs. Among a total of 55,487 genes analyzed, 787 genes were differentially expressed (> fourfold; 454 and 333 genes up- and down-regulated, respectively), which included genes associated with muscle tissue development, muscle system process, protein digestion and absorption, and inflammation-related signaling. From a clinical perspective, this model may help understand the molecular/cellular mechanism that drives muscle disuse and identify therapeutic strategies for this debilitating disease.

Integrated metabolomics and proteomics analysis to understand muscle atrophy resistance in hibernating Spermophilus dauricus

Hibernating Spermophilus dauricus experiences minor muscle atrophy, which is an attractive anti-disuse muscle atrophy model. Integrated metabolomics and proteomics analysis was performed on the hibernating S. dauricus during the pre-hibernation (PRE) stage, torpor (TOR) stage, interbout arousal (IBA) stage, and post-hibernation (POST) stage. Time course stage transition-based (TOR vs. PRE, IBA vs. TOR, POST vs. IBA) differential expression analysis was performed based on the R limma package. A total of 14 co-differential metabolites were detected. Among these, l-cystathionine, l-proline, ketoleucine, serine, and 1-Hydroxy-3,6,7-Trimethoxy-2, 8-Diprenylxanthone demonstrated the highest levels in the TOR stage; Beta-Nicotinamide adenine dinucleotide, Dihydrozeatin, Pannaric acid, and Propionylcarnitine demonstrated the highest levels in the IBA stage; Adrenosterone, PS (18:0/14,15-EpETE), S-Carboxymethylcysteine, TxB2, and 3-Phenoxybenzylalcohol demonstrated the highest levels in the POST stage. Kyoto Encyclopedia of Genes and Genomes pathways annotation analysis indicated that biosynthesis of amino acids, ATP-binding cassette transporters, and cysteine and methionine metabolism were co-differential metabolism pathways during the different stages of hibernation. The stage-specific metabolism processes and integrated enzyme-centered metabolism networks in the different stages were also deciphered. Overall, our findings suggest that (1) the periodic change of proline, ketoleucine, and serine contributes to the hindlimb lean tissue preservation; and (2) key metabolites related to the biosynthesis of amino acids, ATP-binding cassette transporters, and cysteine and methionine metabolism may be associated with muscle atrophy resistance. In conclusion, our co-differential metabolites, co-differential metabolism pathways, stage-specific metabolism pathways, and integrated enzyme-centered metabolism networks are informative for biologists to generate hypotheses for functional analyses to perturb disuse-induced muscle atrophy.

Biological sex divergence in transcriptomic profiles during the onset of hindlimb unloading-induced atrophy.

Disuse-induced muscle atrophy is a common clinical problem observed mainly in older adults, intensive care units patients, or astronauts. Previous studies presented biological sex divergence in progression of disuse-induced atrophy along with differential changes in molecular mechanisms possibly underlying muscle atrophy. The aim of this study was to perform transcriptomic profiling of male and female mice during the onset and progression of unloading disuse-induced atrophy. Male and female mice underwent hindlimb unloading (HU) for 24, 48, 72, and 168 h (n = 8/group). Muscles were weighed for each cohort and gastrocnemius was used for RNA-sequencing analysis. Females exhibited muscle loss as early as 24 h of HU, whereas males after 168 h of HU. In males, pathways related to proteasome degradation were upregulated throughout 168 h of HU, whereas in females these pathways were upregulated up to 72 h of HU. Lcn2, a gene contributing to regulation of myogenesis, was upregulated by 6.46- to 19.86-fold across all time points in females only. A reverse expression of Fosb, a gene related to muscle degeneration, was observed between males (4.27-fold up) and females (4.57-fold down) at 24-h HU. Mitochondrial pathways related to tricarboxylic acid (TCA) cycle were highly downregulated at 168 h of HU in males, whereas in females this downregulation was less pronounced. Collagen-related pathways were consistently downregulated throughout 168 h of HU only in females, suggesting a potential biological sex-specific protective mechanism against disuse-induced fibrosis. In conclusion, females may have protection against HU-induced skeletal muscle mitochondrial degeneration and fibrosis through transcriptional mechanisms, although they may be more vulnerable to HU-induced muscle wasting compared with males.NEW & NOTEWORTHY Herein, we have assessed the transcriptomic response across biological sexes during the onset and progression of unloading disuse-induced atrophy in mice. We have demonstrated an inverse expression of Fosb between males and females, as well as differentially timed patterns of expressing atrophy-related pathways between sexes that are concomitant to the accelerated atrophy in females. We also identified in females signs of mechanisms to combat disuse-induced mitochondrial degeneration and fibrosis.

Acute effect of electrical stimulation on muscle protein synthesis and break-down in the soleus muscle of hindlimb unloaded rats.

Electrical stimulation (ES) is effective for disuse-induced muscle atrophy. However, the acute effect of ES on muscle protein synthesis (MPS) and muscle protein breakdown (MPB) remains unclear. We investigated the effect of a single-session ES treatment on mTORC1 signaling, MPS, and MPB in the soleus muscle of 2-week hindlimb unloaded rats. Sprague Dawley rats (n = 12 male) were randomly divided into control (CON) and hindlimb unloaded (HU) groups. After 2 weeks, the right soleus muscle was percutaneously stimulated and underwent supramaximal isometric contractions. The left soleus muscle served as an internal control. We collected soleus muscle samples 6 h after ES. Two weeks of HU decreased p70S6K and S6rp activation, downstream factors for mTORC1 signaling, and SUnSET method-assessed MPS, but increased the LC3-II/I ratio, an indicator of autophagy. ES on disused muscle successfully activated mTORC1 signaling but did not affect MPS. Contrary, ES decreased ubiquitinated proteins expression and LC3B-II/I ratio. HU might affect mTORC1 activation and MPS differently in response to acute ES possibly due to excessive ROS production caused by ES. Our findings suggest that ES applied to disused skeletal muscles may suppress MPB, but its effect on MPS appears to be attenuated.

Obstructive Sleep Apnea and Venous Thromboembolism: Unraveling the Emerging Association.

Oxidative stress has emerged as a significant contributor to skeletal muscle atrophy, influencing cellular processes that underlie muscle wasting. This review article delves into the intricate interplay between oxidative stress and muscle atrophy, shedding light on its mechanisms and implications. We begin by outlining the fundamental concepts of oxidative stress, delineating reactive oxygen species (ROS) and reactive nitrogen species (RNS), their sources, and the ensuing oxidative damage to cellular components. Subsequently, we delve into skeletal muscle atrophy, elucidating its diverse forms, molecular pathways, key signaling cascades, and the role of inflammation in exacerbating muscle wasting. Bridging these concepts, we explore the connections between oxidative stress and muscle atrophy, unveiling how oxidative stress impacts muscle protein synthesis and breakdown, perturbs cellular signaling pathways, and contributes to mitochondrial dysfunction. The review underscores the complexity of quantifying and interpreting oxidative stress markers, highlighting the challenges posed by the dynamic nature of oxidative stress and the presence of basal ROS levels. Addressing the specificity of oxidative stress markers, we emphasize the importance of selecting markers pertinent to muscle tissue and considering systemic influences. Standardization of experimental protocols emerges as a critical need to ensure consistency and reproducibility across studies. Looking ahead, we discuss the implications of oxidative stress in diverse scenarios, encompassing age-related muscle loss (sarcopenia), muscle wasting in chronic diseases like cancer cachexia, and disuse-induced muscle atrophy. Additionally, we delve into potential therapeutic strategies, including antioxidant supplementation, exercise, pharmacological interventions, nutritional approaches, and lifestyle modifications, as avenues to mitigate oxidative stress-driven muscle atrophy. The review concludes by outlining promising future directions in this field, calling for deeper exploration of specific oxidative stress markers, understanding the temporal dynamics of oxidative stress, validation through translational studies in humans, and the development of targeted therapeutic interventions. By advancing our understanding of the intricate relationship between oxidative stress and skeletal muscle atrophy, this review contributes to paving the way for innovative strategies to address muscle wasting and improve muscle health.

Mechanosensory Structures in the Mechanotransduction System of Muscle Fibers

The ability of skeletal muscles to sense mechanical stimuli and respond to them by changing intracellular electrochemical and biochemical processes (mechanotransduction) is of crucial importance for the regulation of physiological processes in muscle fibers. This review describes the main sarcolemmal, sarcomeric, and cytoskeletal mechanosensitive structures and analyzes mechano-dependent signaling pathways and mechanisms involved in the regulation of gene expression as well as muscle protein synthesis and degradation. The final part of the review formulates specific questions in the field of muscle mechanotransduction that need to be addressed in future studies. Understanding of skeletal muscle mechanotransduction is necessary for the development of effective measures aimed at the treatment of muscular dystrophies, sarcopenia, and prevention of disuse-induced muscle atrophy.

ER Stress is Activated and Involved in Disuse-Induced Muscle Atrophy.

Muscle atrophy resulting wholly or partially from disuse represents a serious medical complication that decreases quality of life and increases morbidity and mortality. The accumulation of misfolded/unfolded proteins disrupts endoplasmic reticulum (ER) homeostasis and thus causes ER stress. Growing evidence indicates that ER stress plays an essential role in skeletal muscle remodeling under various physiological or pathophysiological conditions. However, whether ER stress is involved in disuse-induced muscle atrophy remains unclear. To induce muscle atrophy, 8-week-old C57BL/6JNifdc male mice were subjected to 3, 7, or 14 days of hindlimb unloading (HU), and rhesus macaques (Macaca mulatta) were subjected to 10∘ head-down tilted bed rest (HDBR) for 6 weeks. Tauroursodeoxycholic acid (TUDCA) (500 mg/kg/d) was orally administered to mice during HU to inhibit ER stress. Quantitative PCR, Western blotting, and immunohistochemistry were conducted to evaluate gene, protein, and structural changes, respectively. ER stress marker genes were rapidly induced by HU in a similar trend to that observed with atrophy-related genes such as Atrogin-1, muscle RING finger 1 (MuRF1), and muscle ubiquitin ligase of SCF complex in atrophy-1 (MUSA1). Inhibition of ER stress with TUDCA, a pan-ER stress inhibitor, attenuated HU-induced muscle atrophy and the upregulation of ubiquitin ligases via the AKT/forkhead box O3a pathway. In addition, the oxidative-to-glycolytic myofiber type transition caused by HU was also inhibited by TUDCA treatment. ER stress activation was also confirmed in HDBR-induced rhesus soleus muscle atrophy. The strong positive correlation between ER stress activation and both HU- and HDBR-induced muscle atrophy indicates that ER stress activation is ubiquitously involved in disuse-induced muscle atrophy, regardless of species. Thus, inhibiting ER stress may be an effective therapeutic strategy to prevent muscle atrophy during disuse.

The effect of combined β-lactoglobulin supplementation and resistance exercise training prior to limb immobilisation on muscle protein synthesis rates in healthy young adults: study protocol for a randomised controlled trial

BackgroundThe decline in skeletal muscle mass experienced following a short-term period (days to weeks) of muscle disuse is mediated by impaired rates of muscle protein synthesis (MPS). Previous RCTs of exercise or nutrition prehabilitation interventions designed to mitigate disuse-induced muscle atrophy have reported limited efficacy. Hence, the aim of this study is to investigate the impact of a complex prehabilitation intervention that combines β-lactoglobulin (a novel milk protein with a high leucine content) supplementation with resistance exercise training on disuse-induced changes in free-living integrated rates of MPS in healthy, young adults.Methods/designTo address this aim, we will recruit 24 healthy young (18–45 years) males and females to conduct a parallel, double-blind, 2-arm, randomised placebo-controlled trial. The intervention group will combine a 7-day structured resistance exercise training programme with thrice daily dietary supplementation with 23 g of β-lactoglobulin. The placebo group will combine the same training programme with an energy-matched carbohydrate (dextrose) control. The study protocol will last 16 days for each participant. Day 1 will be a familiarisation session and days 2–4 will be the baseline period. Days 5–11 represent the ‘prehabilitation period’ whereby participants will combine resistance training with their assigned dietary supplementation regimen. Days 12–16 represent the muscle disuse-induced ‘immobilisation period’ whereby participants will have a single leg immobilised in a brace and continue their assigned dietary supplementation regimen only (i.e. no resistance training). The primary endpoint of this study is the measurement of free-living integrated rates of MPS using deuterium oxide tracer methodology. Measurements of MPS will be calculated at baseline, over the 7-day prehabilitation period and over the 5-day immobilisation period separately. Secondary endpoints include measurements of muscle mass and strength that will be collected on days 4 (baseline), 11 (end of prehabilitation) and 16 (end of immobilisation).DiscussionThis novel study will establish the impact of a bimodal prehabilitation strategy that combines ß-lactoglobulin supplementation and resistance exercise training in modulating MPS following a short-term period of muscle disuse. If successful, this complex intervention may be translated to clinical practice with application to patients scheduled to undergo, for example, hip or knee replacement surgery.Trial registrationNCT05496452. Registered on August 10, 2022.Protocol version: 16-12-2022/1

Saponin extract from Achyranthes bidentata Blume alleviates disuse-induced muscle atrophy through PI3K/Akt signaling pathway

Ethnopharmacological relevanceThe roots of Achyranthes bidentata Blume are one of the regularly used herbal drugs in Chinese medicine, and has been applied for strengthening the muscle and bone for a long time. However, its effect on muscle remains unclear. Aim of the studyThis paper aims to explore the anti-muscle atrophy effect of A. bidentata, and to clarify the possible signaling pathways involved. Materials and methodsThe saponin extract of the roots of A. bidentata (ABSE) was prepared and analyzed, and its activity on myoblast differentiation was assayed with C2C12 cell culture. ABSE was then orally administered at dosage of 35, 70 and 140 mg/kg/day to disuse-induced muscle atrophy mice. The studies on mice body weight and muscle quality were conducted, and Western blot was used for exploring the possible signaling pathways involved in the muscle protective action aided with transcriptome analysis. ResultsThe total saponin content of ABSE was 59.1%. ABSE promoted the C2C12 cells differentiation to myotube in C2C12 differentiation assay. Further study with disuse-induced muscle atrophy mice model demonstrated that ABSE significantly increased muscle fiber diameter as well as the proportion of slow muscle fibers. Possible mechanism study aided with transcriptome analysis revealed that ABSE alleviated muscle atrophy at least through activation of PI3K/Akt pathway in vivo & vitro. ConclusionsThe saponin extract of the root of A. bidentata (ABSE) has a protective effect on muscle atrophy, and showed a considerable potential in prevention and treatment of muscle atrophy.

Kampo formula hochu-ekki-to (Bu-Zhong-Yi-Qi-Tang, TJ-41) ameliorates muscle atrophy by modulating atrogenes and AMPK in vivo and in vitro

BackgroundMuscle disuse results in loss of skeletal muscle mass and function. Hochu-ekki-to (TJ-41; Bu-Zhong-Yi-Qi-Tang in Chinese) is an herbal medicinal formulation used to treat patients with frailty, fatigue and appetite loss. It has been suggested that two atrogenes, atrogin-1 and muscle Ring finger 1 (MuRF1), are ubiquitin ligases involved in disuse-induced muscle atrophy and that 5’ adenosine monophosphate-activated protein kinase (AMPK) is involved in skeletal muscle metabolism. Effects of TJ-41 on disuse-induced muscle atrophy are unclear.MethodsWe subjected differentiated C2C12 myotubes to serum starvation, then examined the effects of TJ-41 on atrogenes expression, AMPK activity and the morphology of the myotubes. Male C57BL/6J mice were subjected to tail-suspension to induce hindlimb atrophy. We administered TJ-41 by gavage to the control group and the tail-suspended group, then examined the effects of TJ-41 on atrogene expression, AMPK activity, and the muscle weight.ResultsSerum starvation induced the expression of atrogin-1 and MuRF1 in C2C12 myotubes, and TJ-41 significantly downregulated the expression of atrogin-1. Tail-suspension of the mice induced the expression of atrogin-1 and MuRF1 in skeletal muscle as well as its muscle atrophy, whereas TJ-41 treatment significantly downregulated the expression of atrogin-1 and ameliorated the loss of the muscle weight. In addition, TJ-41 also activated AMPK and inactivated Akt and mTOR in skeletal muscle in vivo.ConclusionTJ-41 inhibited atrogenes in an Akt-independent manner as well as activating AMPK in skeletal muscles in vivo, further implying the therapeutic potential of TJ-41 against disuse-induced muscle atrophy and other atrogenes-dependent atrophic conditions.

Loss of 4E-BPs prevents the hindlimb immobilization-induced decrease in protein synthesis in skeletal muscle.

The present study was designed to test the hypothesis that upregulating protein synthesis attenuates the loss of muscle mass in a model of disuse atrophy. The studies compared the effect of unilateral hindlimb immobilization in wild-type (WT) mice and double-knockout (DKO) mice lacking the translational regulators 4E-BP1 and 4E-BP2. Immobilization-induced downregulation of protein synthesis occurred in both groups of mice, but protein synthesis was higher in gastrocnemius muscle from the immobilized hindlimb of fasted DKO compared with WT mice. Surprisingly, although protein synthesis was partially elevated in DKO compared with WT mice, atrophy occurred to the same extent in both groups of animals. This may be partially due to impaired leucine-induced stimulation of protein synthesis in DKO compared with WT mice due to downregulated eukaryotic initiation factor eIF4E expression in muscle of DKO compared with WT mice. Expression of the E3 ubiquitin ligases MAFbx and MuRF-1 mRNAs and total protein ubiquitylation was upregulated in the immobilized compared with the nonimmobilized hindlimb of both WT and DKO mice, with little difference in the magnitude of the upregulation between genotypes. Analysis of newly synthesized proteins revealed downregulation of several glycolytic enzymes in the gastrocnemius of DKO mice compared with WT mice, as well as in the immobilized compared with the nonimmobilized hindlimb. Overall, the results suggest that the elevated rate of protein synthesis during hindlimb immobilization in fasted DKO mice is insufficient to prevent disuse-induced muscle atrophy, probably due to induction of compensatory mechanisms including downregulation of eIF4E expression.NEW & NOTEWORTHY Basal rates of protein synthesis are elevated in skeletal muscle in the immobilized leg of mice lacking the translational repressors, 4E-BP1 and 4E-BP2 (knockout mice), compared with wild-type mice. However, disuse-induced muscle atrophy occurs to the same extent in both wild-type and knockout mice suggesting that compensatory mechanisms are induced that overcome the upregulation of muscle protein synthesis. Proteomic analysis revealed that mRNAs encoding several glycolytic enzymes are differentially translated in wild-type and knockout mice.

Chicken or Egg? Mitochondrial Phospholipids and Oxidative Stress in Disuse-Induced Skeletal Muscle Atrophy.

Significance: Accumulation of reactive oxygen species (ROS) is known to promote cellular damage in multiple cell types. In skeletal muscle, ROS has been implicated in disuse-induced muscle atrophy. However, the molecular origin and mechanism of how disuse promotes ROS and muscle dysfunction remains unclear. Recent Advances: Recently, we implicated membrane lipids of mitochondria to be a potential source of ROS to promote muscle atrophy. Critical Issues: In this review, we discuss evidence that changes in mitochondrial lipids represent a physiologically relevant process by which disuse promotes mitochondrial electron leak and oxidative stress. Future Directions: We further discuss lipid hydroperoxides as a potential downstream mediator of ROS to induce muscle atrophy. Antioxid. Redox Signal. 38, 338-351.

Mitigating sarcoplasmic reticulum stress limits disuse-induced muscle loss in hindlimb unloaded mice

Muscle disuse in the hindlimb unloaded (HU) mice causes significant atrophy and weakness. However, the cellular and molecular mechanisms driving disuse-muscle atrophy remain elusive. We investigated the potential contribution of proteins dysregulation by sarcoplasmic reticulum (SR), a condition called SR stress, to muscle loss during HU. Male, c57BL/6j mice were assigned to ground-based controls or HU groups treated with vehicle or 4-phenylbutyrate (4-PBA), a potent inhibitor of SR stress, once a day for three weeks. We report that the 4-PBA reduced the SR stress and partly reversed the muscle atrophy and weakness in the HU mice. Transcriptome analysis revealed that several genes were switched on (n = 3688) or differentially expressed (n = 1184) due to HU. GO, and KEGG term analysis revealed alterations in pathways associated with the assembly of cilia and microtubules, extracellular matrix proteins regulation, calcium homeostasis, and immune modulation during HU. The muscle restoration with 4-PBA partly reversed these changes along with differential and unique expression of several genes. The analysis of genes among the two comparisons (HU-v vs. control and HU-t vs. HU-v.) shows 841 genes were overlapped between the two comparisons and they may be regulated by 4-PBA. Altogether, our findings suggest that the pharmacological suppression of SR stress may be an effective strategy to prevent disuse-induced muscle weakness and atrophy.

4E‐BP1/2 Deletion Enhances mRNA Cap‐binding Complex Assembly and Protein Synthesis in Immobilized Skeletal Muscle But is Not Sufficient to Prevent Muscle Atrophy

Skeletal muscle atrophy in long‐term care patients is positively correlated with worse health outcomes and patient mortality. Immobilization‐induced muscle atrophy is due to an imbalance between protein synthesis and protein degradation. The imbalance favors degradation due to anabolic resistance within the muscle. Targeting regulators of protein synthesis to increase mRNA translation under immobilized conditions may improve the protein synthesis/degradation imbalance and ameliorate immobilization‐induced skeletal muscle atrophy. 4E‐BP1 and 4E‐BP2 (eukaryotic initiation factor 4E (eIF4E) binding proteins 1 and 2) are important regulators of mRNA translation initiation that bind to eIF4E and prevents its binding to the scaffold protein eIF4G, an essential and rate‐limiting step of cap‐dependent mRNA translation. Previous studies have implicated eIF4E and its binding proteins in disuse‐induced repression of muscle protein synthesis. Consequently, we hypothesize that eliminating 4E‐BP1/2 may enhance mRNA translation and protect against disuse‐induced muscle atrophy.The hindlimb muscles of 8–9‐week‐old C57Bl/6 mixed‐sex wild‐type (WT) (20.25±2.39 g; n=19) and global 4E‐BP1/4E‐BP2 double knockout (DKO) (20.55±2.65 g; n=19) mice were compared after 3 days of unilateral hindlimb immobilization. Each mouse had one hindlimb immobilized in the plantarflexed position while the contralateral nonimmobilized hindlimb served as a control. Food and water were provided to the mice ad libitum. The hindlimb muscles were removed after a 4‐6 h fast, and the weights were compared between both genotypes for both the immobilized and free limbs. The gastrocnemius muscles were homogenized, eIF4E was immunoprecipitated, and eIF4E, eIF4G, and 4E‐BP1 in the immunoprecipitated sample was assessed by western blot analysis. In the immobilized gastrocnemius of the DKO mice the amount of eIF4G associated with eIF4E was higher than in control mice (p=0.001). In addition, mRNA translation, as assessed in a separate study by the incorporation of puromycin (0.04 μmol puromycin/g of body mass) into protein in the gastrocnemius, was also higher in muscle of the immobilized leg of DKO (n=24) compared to control mice (n=26) (17.1% increase, p=0.012). Surprisingly, eIF4E binding protein deficiency did not prevent muscle atrophy, as the mass of the gastrocnemius, plantaris, and soleus muscles was reduced to a similar extent in the immobilized compared to the control leg of both DKO and wildtype mice. To assess a possible role for upregulation of protein degradation in disuse‐induced muscle atrophy, expression of the E3 ubiquitin ligase, MAFbx‐1 (muscle atrophy f‐box 1) was also assessed. MAFbx‐1 expression was elevated in the gastrocnemius of the immobilized compared to the contralateral non‐immobilized limb, though no significant difference was found between genotypes. More research into other components of the mRNA cap binding complex, e.g., eIF4A, and additional biomarkers of protein degradative pathways may reveal more details on the mechanism of immobilization‐induced muscle atrophy.

Costly immunometabolic remodelling in disused muscle buildup through physical exercise.

The mechanisms underlying the immunometabolic disturbances during skeletal muscle atrophy caused by a plethora of circumstances ranging from hospitalization to spaceflight missions remain unknown. Here, we outline the possible pathways that might be dysregulated in such conditions and assess the potential of physical exercise to mitigate and promote the recovery of muscle morphology, metabolism and function after intervals of disuse. Studies applying exercise to attenuate disuse-induced muscle atrophy have shown a pivotal role of circulating myokines in the activation of anabolic signalling pathways. These muscle-derived factors induce accretion of contractile proteins in the myofibers, and at the same time decrease protein breakdown and loss. Regular exercise plays a crucial role in re-establishing adequate immunometabolism and increasing the migration and presence in the muscle of macrophages with an anti-inflammatory phenotype (M2) and T regulatory cells (Tregs) after disease-induced muscle loss. Additionally, the switch in metabolic pathways (glycolysis to oxidative phosphorylation [OXPHOS]) is important for achieving rapid metabolic homeostasis during muscle regeneration. In this review, we discuss the molecular aspects of the immunometabolic response elicited by exercise during skeletal muscle regeneration. There is not, nevertheless, consensus on a single optimal intensity of exercise required to improve muscle strength, mass and functional capacity owing to the wide range of exercise protocols studied so far. Despite the absence of agreement on the specific strategy, physical exercise appears as a powerful complementary strategy to attenuate the harmful effects of muscle disuse in different scenarios.