Skeletal Muscle Atrophy: Discovery of Mechanisms and Potential Therapies

Kaempferol Improves Exercise Performance by Regulating Glucose Uptake, Mitochondrial Biogenesis, and Protein Synthesis via PI3K/AKT and MAPK Signaling Pathways

Kaempferol is a natural flavonoid with reported bioactivities found in many fruits, vegetables, and medicinal herbs. However, its effects on exercise performance and muscle metabolism remain inconclusive. The present study investigated kaempferol's effects on improving exercise performance and potential mechanisms in vivo and in vitro. The grip strength, exhaustive running time, and distance of mice were increased in the high-dose kaempferol group (p < 0.01). Also, kaempferol reduced fatigue-related biochemical markers and increased the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) related to antioxidant capacity. Kaempferol also increased the glycogen and adenosine triphosphate (ATP) content in the liver and skeletal muscle, as well as glucose in the blood. In vitro, kaempferol promoted glucose uptake, protein synthesis, and mitochondrial function and decreased oxidative stress in both 2D and 3D C2C12 myotube cultures. Moreover, kaempferol activated the PI3K/AKT and MAPK signaling pathways in the C2C12 cells. It also upregulated the key targets of glucose uptake, mitochondrial function, and protein synthesis. These findings suggest that kaempferol improves exercise performance and alleviates physical fatigue by increasing glucose uptake, mitochondrial biogenesis, and protein synthesis and by decreasing ROS. Kaempferol's molecular mechanism may be related to the regulation of the PI3K/AKT and MAPK signaling pathways.

Characterization of sensory and motor dysfunction and morphological alterations in late stages of type 2 diabetic mice

Diabetic neuropathy is the most common complication of diabetes and lacks effective treatments. Although sensory dysfunction during the early stages of diabetes has been extensively studied in various animal models, the functional and morphological alterations in sensory and motor systems during late stages of diabetes remain largely unexplored. In the current work, we examined the influence of diabetes on sensory and motor function as well as morphological changes in late stages of diabetes. The obese diabetic Leprdb/db mice (db/db) were used for behavioral assessments and subsequent morphological examinations. The db/db mice exhibited severe sensory and motor behavioral defects at the age of 32 weeks, including significantly higher mechanical withdrawal threshold and thermal latency of hindpaws compared with age-matched nondiabetic control animals. The impaired response to noxious stimuli was mainly associated with the remarkable loss of epidermal sensory fibers, particularly CGRP-positive nociceptive fibers. Unexpectedly, the area of CGRP-positive terminals in the spinal dorsal horn was dramatically increased in diabetic mice, which was presumably associated with microglial activation. In addition, the db/db mice showed significantly more foot slips and took longer time during the beam-walking examination compared with controls. Meanwhile, the running duration in the rotarod test was markedly reduced in db/db mice. The observed sensorimotor deficits and motor dysfunction were largely attributed to abnormal sensory feedback and muscle atrophy as well as attenuated neuromuscular transmission in aged diabetic mice. Morphological analysis of neuromuscular junctions (NMJs) demonstrated partial denervation of NMJs and obvious fragmentation of acetylcholine receptors (AChRs). Intrafusal muscle atrophy and abnormal muscle spindle innervation were also detected in db/db mice. Additionally, the number of VGLUT1-positive excitatory boutons on motor neurons was profoundly increased in aged diabetic mice as compared to controls. Nevertheless, inhibitory synaptic inputs onto motor neurons were similar between the two groups. This excitation-inhibition imbalance in synaptic transmission might be implicated in the disturbed locomotion. Collectively, these results suggest that severe sensory and motor deficits are present in late stages of diabetes. This study contributes to our understanding of mechanisms underlying neurological dysfunction during diabetes progression and helps to identify novel therapeutic interventions for patients with diabetic neuropathy.

Ursolic Acid Induces Beneficial Changes in Skeletal Muscle mRNA Expression and Increases Exercise Participation and Performance in Dogs with Age-Related Muscle Atrophy

Muscle atrophy and weakness are prevalent and debilitating conditions in dogs that cannot be reliably prevented or treated by current approaches. In non-canine species, the natural dietary compound ursolic acid inhibits molecular mechanisms of muscle atrophy, leading to improvements in muscle health. To begin to translate ursolic acid to canine health, we developed a novel ursolic acid dietary supplement for dogs and confirmed its safety and tolerability in dogs. We then conducted a randomized, placebo-controlled, proof-of-concept efficacy study in older beagles with age-related muscle atrophy, also known as sarcopenia. Animals received placebo or ursolic acid dietary supplements once a day for 60 days. To assess the study's primary outcome, we biopsied the quadriceps muscle and quantified atrophy-associated mRNA expression. Additionally, to determine whether the molecular effects of ursolic acid might have functional correlates consistent with improvements in muscle health, we assessed secondary outcomes of exercise participation and T-maze performance. Importantly, in canine skeletal muscle, ursolic acid inhibited numerous mRNA expression changes that are known to promote muscle atrophy and weakness. Furthermore, ursolic acid significantly improved exercise participation and T-maze performance. These findings identify ursolic acid as a natural dietary compound that inhibits molecular mechanisms of muscle atrophy and improves functional performance in dogs.

Advances in research on cell models for skeletal muscle atrophy

Skeletal muscle, the largest organ in the human body, plays a crucial role in supporting and defending the body and is essential for movement. It also participates in regulating the processes of protein synthesis and degradation. Inhibition of protein synthesis and activation of degradation metabolism can both lead to the development of skeletal muscle atrophy, a pathological condition characterized by a decrease in muscle mass and fiber size. Many physiological and pathological conditions can cause a decline in muscle mass, but the underlying mechanisms of its pathogenesis remain incompletely understood, and the selection of treatment strategies and efficacy evaluations vary. Moreover, the early symptoms of this condition are often not apparent, making it easily overlooked in clinical practice. Therefore, it is necessary to develop and use cell models to understand the etiology and influencing factors of skeletal muscle atrophy. In this review, we summarize the methods used to construct skeletal muscle cell models, including hormone, inflammation, cachexia, genetic engineering, drug, and physicochemical models. We also analyze, compare, and evaluate the various construction and assessment methods.

Reprogramming of cis-regulatory networks during skeletal muscle atrophy in male mice

A comprehensive atlas of cis-regulatory elements and their dynamic activity is necessary to understand the transcriptional basis of cellular structure maintenance, metabolism, and responses to the environment. Here we show, using matched single-nucleus chromatin accessibility and RNA-sequencing from juvenile male C57BL6 mice, an atlas of accessible chromatin regions in both normal and denervated skeletal muscles. We identified cell-type-specific cis-regulatory networks, highlighting the dynamic regulatory circuits mediating transitions between myonuclear types. Through comparison of normal and perturbed muscle, we delineated the reprogramming of cis-regulatory networks in response to denervation, described the interplay of promoters/enhancers and target genes. We further unveil a hierarchical structure of transcription factors that delineate a regulatory network in atrophic muscle, identifying ELK4 as a key atrophy-related transcription factor that instigates muscle atrophy through TGF-β1 regulation. This study furnishes a rich genomic resource, essential for decoding the regulatory dynamics of skeletal muscle in both physiological and pathological states.

Shared Gene Targets of the ATF4 and p53 Transcriptional Networks

The master tumor suppressor p53 regulates multiple cell fate decisions, such as cell cycle arrest and apoptosis, via transcriptional control of a broad gene network. Dysfunction in the p53 network is common in cancer, often through mutations that inactivate p53 or other members of the pathway. Induction of tumor-specific cell death by restoration of p53 activity without off-target effects has gained significant interest in the field. In this study, we explore the gene regulatory mechanisms underlying a putative anticancer strategy involving stimulation of the p53-independent integrated stress response (ISR). Our data demonstrate the p53 and ISR pathways converge to independently regulate common metabolic and proapoptotic genes. We investigated the architecture of multiple gene regulatory elements bound by p53 and the ISR effector ATF4 controlling this shared regulation. We identified additional key transcription factors that control basal and stress-induced regulation of these shared p53 and ATF4 target genes. Thus, our results provide significant new molecular and genetic insight into gene regulatory networks and transcription factors that are the target of numerous antitumor therapies.

The transcription regulator ATF4 is a mediator of skeletal muscle aging

Aging slowly erodes skeletal muscle strength and mass, eventually leading to profound functional deficits and muscle atrophy. The molecular mechanisms of skeletal muscle aging are not well understood. To better understand mechanisms of muscle aging, we investigated the potential role of ATF4, a transcription regulatory protein that can rapidly promote skeletal muscle atrophy in young animals deprived of adequate nutrition or activity. To test the hypothesis that ATF4 may be involved in skeletal muscle aging, we studied fed and active muscle-specific ATF4 knockout mice (ATF4 mKO mice) at 6 months of age, when wild-type mice have achieved peak muscle mass and function, and at 22 months of age, when wild-type mice have begun to manifest age-related muscle atrophy and weakness. We found that 6-month-old ATF4 mKO mice develop normally and are phenotypically indistinguishable from 6-month-old littermate control mice. However, as ATF4 mKO mice become older, they exhibit significant protection from age-related declines in strength, muscle quality, exercise capacity, and muscle mass. Furthermore, ATF4 mKO muscles are protected from some of the transcriptional changes characteristic of normal muscle aging (repression of certain anabolic mRNAs and induction of certain senescence-associated mRNAs), and ATF4 mKO muscles exhibit altered turnover of several proteins with important roles in skeletal muscle structure and metabolism. Collectively, these data suggest ATF4 as an essential mediator of skeletal muscle aging and provide new insight into a degenerative process that impairs the health and quality of life of many older adults.

Shared gene targets of the ATF4 and p53 transcriptional networks

The master tumor suppressor p53 regulates multiple cell fate decisions, like cell cycle arrest and apoptosis, via transcriptional control of a broad gene network. Dysfunction in the p53 network is common in cancer, often through mutations that inactivate p53 or other members of the pathway. Induction of tumor-specific cell death by restoration of p53 activity without off-target effects has gained significant interest in the field. In this study, we explore the gene regulatory mechanisms underlying a putative anti-cancer strategy involving stimulation of the p53-independent Integrated Stress Response (ISR). Our data demonstrate the p53 and ISR pathways converge to independently regulate common metabolic and pro-apoptotic genes. We investigated the architecture of multiple gene regulatory elements bound by p53 and the ISR effector ATF4 controlling this shared regulation. We identified additional key transcription factors that control basal and stress-induced regulation of these shared p53 and ATF4 target genes. Thus, our results provide significant new molecular and genetic insight into gene regulatory networks and transcription factors that are the target of numerous antitumor therapies.

Genome-Wide Association Analysis Identifies Genomic Regions and Candidate Genes for Growth and Fatness Traits in Diannan Small-Ear (DSE) Pigs

In the livestock industry, the growth and fatness traits are directly related to production efficiency and economic profits. As for Diannan small-ear (DSE) pigs, a unique indigenous breed, the genetic architecture of growth and fatness traits is still elusive. The aim of this study was to search the genetic loci and candidate genes associated with phenotypic traits in DSE pigs using GWAS based on the Geneseek Porcine 50K SNP Chip data. A total of 22,146 single nucleotide polymorphisms (SNPs) were detected in 265 DSE pigs and used for Genome-wide association studies (GWAS) analysis. Seven SNPs were found to be associated with back height, chest circumference, cannon bone circumference, and backfat thickness at the suggestive significance level. Based on gene annotation results, these seven SNPs were, respectively, mapped to the following candidate genes, VIPR2, SLC10A2, NUCKS1, MCT1, CHCHD3, SMOX, and GPR1, which are mainly involved with adipocyte differentiation, lipid metabolism, skeletal muscle development, and average daily weight gain. Our work offers novel insights into the genetic architecture of economically important traits in swine and may play an important role in breeding using molecular markers in the DSE breed.

Propolis Ethanolic Extract Attenuates D-gal-induced C2C12 Cell Injury by Modulating Nrf2/HO-1 and p38/p53 Signaling Pathways

Previous study has shown that propolis ethanolic extract (PEE) has a protective effect on aging skeletal muscle atrophy. However, the exact molecular mechanism remains unclear. This study aimed to investigate the effect of PEE on D-galactose (D-gal)-induced damage in mouse C2C12 cells. The results revealed that PEE increased the viability of senescent C2C12 cells, decreased the number of senescence-associated β-galactosidase (SA-β-Gal)-positive cells and promoted the differentiation of C2C12 cells. PEE resisted oxidative stress caused by D-gal by activating the Nrf2/HO-1 signaling pathway and maintained the differentiation ability of C2C12 cells. PEE inhibited apoptosis by suppressing p38 phosphorylation and reducing p53 expression. In summary, our findings reveal the molecular mechanism by which PEE protects D-gal-induced C2C12 cells, providing a theoretical basis for the development of PEE for the alleviation of muscle atrophy.

Effects on Goat Meat Extracts on α-Glucosidase Inhibitory Activity, Expression of Bcl-2-Associated X (BAX), p53, and p21 in Cell Line and Expression of Atrogin-1, Muscle Atrophy F-Box (MAFbx), Muscle RING-Finger Protein-1 (MuRF-1), and Myosin Heavy Chain-7 (MYH-7) in C2C12 Myoblsts

This study examined the α-glucosidase inhibitory, and apoptosis- and anti-muscular-related factors of goat meat extracts from forelegs, hind legs, loin, and ribs. The goat meat extracts were evaluated for their α-glucosidase inhibitory activity. The gene and protein expression levels of Bcl-2-associated X (bax), p53, and p21 were examined by reverse transcription polymerase chain reaction (RT-PCR) and immunoblotting in AGS and HT-29 cells. The expression levels of Atrogin-1 and MHC1b were examined by RT-PCR in C2C12 myoblasts, and the expression levels of Atrogin-1, muscle atrophy F-box (MAFbx), muscle RING-finger protein-1 (MuRF-1), and myosin heavy chain-7 were investigated by immunoblotting. α-Glucosidase inhibitory activity was higher in ethanol extract than in hydrous and hot water extracts. BAX and p53 expression levels were higher (p<0.05) in AGS cells treated with goat meat extract than those of cells treated with no goat meat extract. In HT-29 cells, the protein expression levels of BAX, p53, and p21 were higher (p<0.05) in the cells treated with goat meat extract than those of cells not treated with goat meat extract. In dexamethasone-treated C2C12 cells, goat meat extract treatment lower (p<0.05) the expression of Atrogin-1 and lower (p<0.05) the expression of MAFbx and MuRF-1. The results of the present study indicate that goat meat extracts have α-glucosidase inhibitory activity in vitro. In addition, apoptosis was induced in AGS cells and HT-29 cells treated with goat meat extract, and anti-muscular atrophy activity was also observed in C2C12 cells treated with goat meat extract.

Transcriptome-wide association study identifies novel genes associated with bone mineral density and lean body mass in children

OBJECTIVE

To identify novel candidate genes whose expression is associated with bone mineral density (BMD) and body lean mass (LM) in children.

METHODS

A tissue-specific transcriptome-wide association study (TWAS) was conducted utilizing a large-scale genome-wide association study (GWAS) dataset associated with BMD and LM and involving 10,414 participants. The measurement of BMD and LM phenotypes was made based on total-body dual-energy X-ray absorptiometry (TB-DXA) scans. TWAS was conducted by using FUSION software. Reference panels for muscle skeleton (MS), peripheral blood (NBL) and whole blood (YBL) were used for TWAS analysis. Functional enrichment and protein-protein interaction (PPI) analyses of the genes identified by TWAS were performed by using the online tool Metascape ( http://metascape.org ).

RESULTS

For BMD, we identified 174 genes with P < 0.05, such as IKZF1 (P = 1.46 × 10^-9) and CHKB (P = 8.31 × 10^-7). For LM, we identified 208 genes with P < 0.05, such as COPS5 (P = 3.03 × 10^-12) and MRPS33 (P = 5.45 × 10^-10). Gene ontology (GO) enrichment analysis of the BMD-associated genes revealed 200 GO terms, such as protein catabolic process (Log P = -5.09) and steroid hormone-mediated signaling pathway (Log P = -3.13). GO enrichment analysis of the LM-associated genes detected 287 GO terms, such as the apoptotic signaling pathway (Log P = -8.08) and lipid storage (Log P = -3.55).

CONCLUSION

This study identified several candidate genes for BMD and LM in children, providing novel clues to the genetic mechanisms underlying the development of childhood BMD and LM.

HMGB1 Promotes In Vitro and In Vivo Skeletal Muscle Atrophy through an IL-18-Dependent Mechanism

Skeletal muscle atrophy occurs due to muscle wasting or reductions in protein associated with aging, injury, and inflammatory processes. High-mobility group box-1 (HMGB1) protein is passively released from necrotic cells and actively secreted by inflammatory cells, and is implicated in the pathogenesis of various inflammatory and immune diseases. HMGB1 is upregulated in muscle inflammation, and circulating levels of the proinflammatory cytokine interleukin-18 (IL-18) are upregulated in patients with sarcopenia, a muscle-wasting disease. We examined whether an association exists between HMGB1 and IL-18 signaling in skeletal muscle atrophy. HMGB1-induced increases of IL-18 levels enhanced the expression of muscle atrophy markers and inhibited myogenic marker expression in C2C12 and G7 myoblast cell lines. HMGB1-induced increases of IL-18 production in C2C12 cells involved the RAGE/p85/Akt/mTOR/c-Jun signaling pathway. HMGB1 short hairpin RNA (shRNA) treatment rescued the expression of muscle-specific differentiation markers in murine C2C12 myotubes and in mice with glycerol-induced muscle atrophy. HMGB1 and IL-18 signaling was suppressed in the mice after HMGB1 shRNA treatment. These findings suggest that the HMGB1/IL-18 axis is worth targeting for the treatment of skeletal muscle atrophy.

PGC-1β modulates catabolism and fiber atrophy in the fasting-response of specific skeletal muscle beds

OBJECTIVE

Skeletal muscle is a pivotal organ for the coordination of systemic metabolism, constituting one of the largest storage site for glucose, lipids and amino acids. Tight temporal orchestration of protein breakdown in times of fasting has to be balanced with preservation of muscle mass and function. However, the molecular mechanisms that control the fasting response in muscle are poorly understood.

METHODS

We now have identified a role for the peroxisome proliferator-activated receptor γ coactivator 1β (PGC-1β) in the regulation of catabolic pathways in this context in muscle-specific loss-of-function mouse models.

RESULTS

Muscle-specific knockouts for PGC-1β experience mitigated muscle atrophy in fasting, linked to reduced expression of myostatin, atrogenes, activation of AMP-dependent protein kinase (AMPK) and other energy deprivation signaling pathways. At least in part, the muscle fasting response is modulated by a negative effect of PGC-1β on the nuclear factor of activated T-cells 1 (NFATC1).

CONCLUSIONS

Collectively, these data highlight the complex regulation of muscle metabolism and reveal a new role for muscle PGC-1β in the control of proteostasis in fasting.

Solanaceae glycoalkaloids: α-solanine and α-chaconine modify the cardioinhibitory activity of verapamil

CONTEXT

Solanaceae glycoalkaloids (SGAs) possess cardiomodulatory activity.

OBJECTIVE

This study investigated the potential interaction between verapamil and glycoalkaloids.

MATERIAL AND METHODS

The cardioactivity of verapamil and glycoalkaloids (α-solanine and α-chaconine) was tested in adult beetle (Tenebrio molitor) myocardium in vitro using microdensitometric methods. The myocardium was treated with pure substances and mixtures of verapamil and glycoalkaloids for 9 min with saline as a control. Two experimental variants were used: simultaneous application of verapamil and glycoalkaloids or preincubation of the myocardium with one of the compounds followed by perfusion with a verapamil solution. We used 9 × 10^-6-5 × 10^-5 M and 10^-9-10^-5 M concentration for verapamil and glycoalkaloids, respectively.

RESULTS

Verapamil, α-solanine and α-chaconine showed cardioinhibitory activity with IC₅₀ values equal to 1.69 × 10^-5, 1.88 × 10^-7 and 7.48 × 10^-7 M, respectively. When the glycoalkaloids were applied simultaneously with verapamil, an antagonistic effect was observed with a decrease in the maximal inhibitory effect and prolongation of t₅₀ and the recovery time characteristic of verapamil. We also confirmed the expression of two transcript forms of the gene that encodes the α1 subunit of L-type calcium channels in the myocardium and brain with equal transcription levels of both forms in the myocardium and significant domination of the shorter form in the brain of the insect species tested.

DISCUSSION AND CONCLUSIONS

The results show that attention to the composition of the daily diet during therapy with various drugs is particularly important. In subsequent studies, the nature of interaction between verapamil and SGAs on the molecular level should be checked, and whether this interaction decreases the efficiency of cardiovascular therapy with verapamil in humans.

Effect of Intake of Leucine-Rich Protein Supplement in Parallel with Resistance Exercise on the Body Composition and Function of Healthy Adults

Although sarcopenia has been dealt with in several studies, the standardized guidelines for preventing sarcopenia resulting from increased life expectancy are still insufficient. Therefore, this study evaluated the effects of daily resistance exercise and the intake of leucine-rich protein supplements daily for 12 weeks on the body composition and physical function of healthy adults aged >50 years living in Korea. The study analyzed 50 healthy people without medical conditions, who were randomly assigned to two groups (taking either protein powder or placebo powder) twice a day for 12 weeks. All participants performed resistance exercises regularly that could be repeated 8−12 times using a TheraBand for 12 weeks. A total of 41 participants completed the study. When measured via bioimpedance analysis (BIA), body fat mass (kg) and body fat (%) significantly decreased, and lean body mass (LBM) (kg) and skeletal muscle mass (SMM) (kg) significantly increased, in both groups. However, when measured via dual-energy X-ray absorptiometry (DXA), LBM was significantly increased only in the protein powder group. The LBM and SMM change measured via BIA was significantly greater in the protein powder group than in the placebo powder group (LBM: 0.95 ± 0.91 kg in the protein powder group vs. 0.38 ± 1.06 kg in the placebo powder group, p = 0.043; SMM: 0.69 ± 0.58 kg in the protein powder group vs. 0.29 ± 0.65 kg in the placebo powder group, p = 0.039, respectively). In the senior fitness test (SFT), significant functional improvement was found within the two groups, but no significant difference was found between the groups in the degree of improvement. In conclusion, in older people aged >50, to prevent sarcopenia, is more effective to combine resistance exercise and leucine-rich protein supplementation than to simply perform resistance exercise.

The effect of resistance and endurance training with ursolic acid on atrophy-related biomarkers in muscle tissue of diabetic male rats induced by streptozotocin and a high-fat diet

In this study, the effect of resistance and endurance training with/without ursolic acid supplementation was evaluated to identify atrophy-related biomarkers in elderly rats induced by diabetes and a high-fat diet (HFD) based on in silico analysis algorithms and pharmaceutical methods. The visualizer software found differential gene expression levels in skeletal muscle atrophy via computed hub gene network parameters. Also, the impact of ursolic acid, as a potent inducer of the Trp53 protein in ameliorating decreased muscle mass, was analyzed in diabetic rats. Fifty-six-old male Wistar rats were randomly assigned into seven groups, including healthy control (Control), diabetic control (DM), Ursolic acid supplementation (UA), resistance training (RT), endurance training (ET), resistance training+ Ursolic acid supplementation (RT + U), and endurance training in combination with Ursolic acid supplementation (ET + U). Exercise intervention included 8 weeks of resistance or endurance training programs. Biomedical informatic outputs determined the P53 signaling pathway as a remarkable causative factor in the pathomechanism of atrophy. In addition, the results demonstrated that exercise and supplementation of UA impeded the interactions among p53/ATF4/p21. Moreover, ET and ursolic acid had a synergetic effect on the signaling pathway of p53/ATF4/p21 and probably could inhibit the aging process and modulate the p53/ATF4/p21 molecular pathway. The interaction between UA and endurance exercise significantly modified the activity of the p53/ATF4/p21 signaling pathway. Based on in silico studies, the p53/ATF4/p21 pathway plays an essential role in aging, and the inhibition of this pathway would be beneficial in decelerating the aging process. PRACTICAL APPLICATIONS: Ursolic acid (UA) is a natural pentacyclic triterpenoid carboxylic acid found in apples (a major compound of apple wax) and other fruits; it is known to improve skeletal muscle function and reduce the muscular atrophy pathways. We indicated that p53/ATF4/p21 signaling is an essential factor in aging, and the suppression of this pathway could be beneficial in the deceleration of the aging process. Therefore, this work would shed light on understanding the effect of exercise and nutrition interventions on preventing atrophy markers of skeletal muscle in diabetic rats. Further studies are needed to seek the precise mechanism of the synergism between UA and exercise in ameliorating atrophy markers.

CORP: Gene delivery into murine skeletal muscle using in vivo electroporation

The strategy of gene delivery into skeletal muscles has provided exciting avenues in identifying new potential therapeutics towards muscular disorders and addressing basic research questions in muscle physiology through overexpression and knockdown studies. In vivo electroporation methodology offers a simple, rapidly effective technique for the delivery of plasmid DNA into post-mitotic skeletal muscle fibers and the ability to easily explore the molecular mechanisms of skeletal muscle plasticity. The purpose of this review is to describe how to robustly electroporate plasmid DNA into different hindlimb muscles of rodent models. Further, key parameters (e.g., voltage, hyaluronidase, plasmid concentration) which contribute to the successful introduction of plasmid DNA into skeletal muscle fibers will be discussed. In addition, details on processing tissue for immunohistochemistry and fiber cross-sectional area (CSA) analysis will be outlined. The overall goal of this review is to provide the basic and necessary information needed for successful implementation of in vivo electroporation of plasmid DNA and thus open new avenues of discovery research in skeletal muscle physiology.

Natural products: Potential therapeutic agents to prevent skeletal muscle atrophy

The skeletal muscle (SkM) is the largest organ, which plays a vital role in controlling musculature, locomotion, body heat regulation, physical strength, and metabolism of the body. A sedentary lifestyle, aging, cachexia, denervation, immobilization, etc. Can lead to an imbalance between protein synthesis and degradation, which is further responsible for SkM atrophy (SmA). To date, the understanding of the mechanism of SkM mass loss is limited which also restricted the number of drugs to treat SmA. Thus, there is an urgent need to develop novel approaches to regulate muscle homeostasis. Presently, some natural products attained immense attraction to regulate SkM homeostasis. The natural products, i.e., polyphenols (resveratrol, curcumin), terpenoids (ursolic acid, tanshinone IIA, celastrol), flavonoids, alkaloids (tomatidine, magnoflorine), vitamin D, etc. exhibit strong potential against SmA. Some of these natural products have been reported to have equivalent potential to standard treatments to prevent body lean mass loss. Indeed, owing to the large complexity, diversity, and slow absorption rate of bioactive compounds made their usage quite challenging. Moreover, the use of natural products is controversial due to their partially known or elusive mechanism of action. Therefore, the present review summarizes various experimental and clinical evidence of some important bioactive compounds that shall help in the development of novel strategies to counteract SmA elicited by various causes.

Disuse-induced skeletal muscle atrophy in disease and non-disease states in humans: mechanisms, prevention, and recovery strategies

Decreased skeletal muscle contractile activity (disuse) or unloading leads to muscle mass loss, also known as muscle atrophy. The balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB) is the primary determinant of skeletal muscle mass. A reduced mechanical load on skeletal muscle is one of the main external factors leading to muscle atrophy. However, endocrine and inflammatory factors can act synergistically in catabolic states, amplifying the atrophy process and accelerating its progression. Additionally, older individuals display aging-induced anabolic resistance, which can predispose this population to more pronounced effects when exposed to periods of reduced physical activity or mechanical unloading. Different cellular mechanisms contribute to the regulation of muscle protein balance during skeletal muscle atrophy. This review summarizes the effects of muscle disuse on muscle protein balance and the molecular mechanisms involved in muscle atrophy in the absence or presence of disease. Finally, a discussion of the current literature describing efficient strategies to prevent or improve the recovery from muscle atrophy is also presented.

Effect of C60 Fullerene on Recovery of Muscle Soleus in Rats after Atrophy Induced by Achillotenotomy

Biomechanical and biochemical changes in the muscle soleus of rats during imitation of hind limbs unuse were studied in the model of the Achilles tendon rupture (Achillotenotomy). Oral administration of water-soluble C₆₀ fullerene at a dose of 1 mg/kg was used as a therapeutic agent throughout the experiment. Changes in the force of contraction and the integrated power of the muscle, the time to reach the maximum force response, the mechanics of fatigue processes development, in particular, the transition from dentate to smooth tetanus, as well as the levels of pro- and antioxidant balance in the blood of rats on days 15, 30 and 45 after injury were described. The obtained results indicate a promising prospect for C₆₀ fullerene use as a powerful antioxidant for reducing and correcting pathological conditions of the muscular system arising from skeletal muscle atrophy.

The gender-specific adverse association of polycyclic aromatic hydrocarbons on skeletal muscle mass and strength in the general adults and the possible mechanisms in experimental rats

Whether polycyclic aromatic hydrocarbons (PAHs) exposure is associated with muscle mass and muscle strength has been scantly investigated. The cross-sectional associations of urinary PAH metabolites with appendicular skeletal muscle mass and hand grip strength in adults were first investigated in the National Health and Nutrition Examination Survey (NHANES). Laboratory study was further carried out to examine the effect of PAHs on skeletal muscle mass and strength. 2742 and 2462 US adults were finally analyzed for muscle mass and muscle strength, respectively. In male participants, urinary PAH metabolites were found to show an inverse relationship with muscle mass and grip strength. In female participants, no significant relationship was found between urinary PAH metabolites with muscle mass or grip strength. In male Sprague-Dawley (SD) rats, administration of B [a]P induced muscle atrophy when compared with the control. However, muscle mass and strength were not significantly altered in female rats. The variations in muscle morphology parameters were accompanied by significant decrease in plasma testosterone levels in the B [a]P-treated male rats. Testosterone co-treatment significantly mitigated B [a]P mediated damages in skeletal muscle in male rats. The results of the present study indicate that there may be a gender-specific causal relationship between the PAHs and muscle atrophy.

Aging Aggravates Cachexia in Tumor-Bearing Mice

Simple Summary

Cachexia is a deadly disease that accompanies many different types of cancers. Animal studies on cachexia have so far mostly been conducted using young mice, while cancer in humans is a disease of high age. Mouse models used to date may therefore not be suitable to study cachexia with relevance to patients. By comparing young and old mice of three different strains and two different tumor types, we here show that the age of mice has a substantial effect on cachexia progression (specifically body weight, tissue weight, fiber size, molecular markers) that is dependent on the mouse strain studied. This is independent of glucose tolerance. The cachexia markers IL6 and GDF15 differ between ages in both mice and patients. Future studies on cachexia should consider the age and strain of mice.

Abstract

Background: Cancer is primarily a disease of high age in humans, yet most mouse studies on cancer cachexia are conducted using young adolescent mice. Given that metabolism and muscle function change with age, we hypothesized that aging may affect cachexia progression in mouse models. Methods: We compare tumor and cachexia development in young and old mice of three different strains (C57BL/6J, C57BL/6N, BALB/c) and with two different tumor cell lines (Lewis Lung Cancer, Colon26). Tumor size, body and organ weights, fiber cross-sectional area, circulating cachexia biomarkers, and molecular markers of muscle atrophy and adipose tissue wasting are shown. We correlate inflammatory markers and body weight dependent on age in patients with cancer. Results: We note fundamental differences between mouse strains. Aging aggravates weight loss in LLC-injected C57BL/6J mice, drives it in C57BL/6N mice, and does not influence weight loss in C26-injected BALB/c mice. Glucose tolerance is unchanged in cachectic young and old mice. The stress marker GDF15 is elevated in cachectic BALB/c mice independent of age and increased in old C57BL/6N and J mice. Inflammatory markers correlate significantly with weight loss only in young mice and patients. Conclusions: Aging affects cachexia development and progression in mice in a strain-dependent manner and influences the inflammatory profile in both mice and patients. Age is an important factor to consider for future cachexia studies.

Integration of proteomic and genetic approaches to assess developmental muscle atrophy

Muscle atrophy, or a decline in muscle protein mass, is a significant problem in the aging population and in numerous disease states. Unraveling molecular signals that trigger and promote atrophy may lead to a better understanding of treatment options; however, there is no single cause of atrophy identified to date. To gain insight into this problem, we chose to investigate changes in protein profiles during muscle atrophy in Manduca sexta and Drosophila melanogaster. The use of insect models provides an interesting parallel to probe atrophic mechanisms as these organisms undergo a normal developmental atrophy process during the pupal transition stage. Leveraging the inherent advantages of each model organism, we first defined protein signature changes during M. sexta intersegmental muscle (ISM) atrophy and then used genetic approaches to confirm their functional importance in the D. melanogaster dorsal internal oblique muscles (DIOMs). Our data reveal an upregulation of proteasome and peptidase components and a general downregulation of proteins that regulate actin filament formation. Surprisingly, thick filament proteins that comprise the A-band are increased in abundance, providing support for the ordered destruction of myofibrillar components during developmental atrophy. We also uncovered the actin filament regulator ciboulot (Cib) as a novel regulator of muscle atrophy. These insights provide a framework towards a better understanding of global changes that occur during atrophy and may eventually lead to therapeutic targets.

Transcriptome Analysis of Immune Receptor Activation and Energy Metabolism Reduction as the Underlying Mechanisms in Interleukin-6-Induced Skeletal Muscle Atrophy

Background

Inflammation may trigger skeletal muscle atrophy induced by cancer cachexia. As a pro-inflammatory factor, interleukin-6 may cause skeletal muscle atrophy, but the underlying molecular mechanisms have not been explored.

Methods

In this experimental study, we used adult male ICR mice, weighing 25 ± 2 g, and the continuous infusion of interleukin-6 into the tibialis anterior muscle to construct a skeletal muscle atrophy model (experimental group). A control group received a saline infusion. RNA-sequencing was used to analyze the differentially expressed genes in tissue samples after one and three days. Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes analysis were applied to define the function of these genes, and protein-protein interaction analysis was performed to identify potential transcription factors. Fluorescence microscopy was used to determine the muscle fiber cross-sectional area after 14 days.

Results

Continuous infusion of interleukin-6 for 14 days caused significant muscle atrophy. RNA-sequencing found 359 differentially expressed genes in the 1- and 3-day tissue samples and 1748 differentially expressed genes only in the 3-day samples. Functional analysis showed that the differentially expressed genes found in both the 1- and 3-day samples were associated with immune receptor activation, whereas the differentially expressed genes found only in the 3-day sample were associated with reduced energy metabolism. The expression of multiple genes in the oxidative phosphorylation and tricarboxylic acid cycle pathways was down-regulated. Furthermore, differentially expressed transcription factors were identified, and their interaction with interleukin-6 and the differentially expressed genes was predicted, which indicated that STAT3, NF-κB, TP53 and MyoG may play an important role in the process of interleukin-6-induced muscle atrophy.

Conclusions

This study found that interleukin-6 caused skeletal muscle atrophy through immune receptor activation and a reduction of the energy metabolism. Several transcription factors downstream of IL-6 have the potential to become new regulators of skeletal muscle atrophy. This study not only enriches the molecular regulation mechanism of muscle atrophy, but also provides a potential target for targeted therapy of muscle atrophy.

Genome-wide association studies of preweaning growth and in vivo carcass composition traits in Esme sheep

Sheep are considered as a major contributor of global food security. Moreover, sheep preweaning growth traits as well as in vivo carcass composition traits such as ultrasonic measurements of Longissimus dorsi muscle depth (UMD) and back-fat thickness (UFD) are crucially important indicators of meat yield and hot carcass composition. Despite their relative importance for productivity and profitability of a sheep production system, detected QTL for these traits are quite scarce. Therefore, we implemented GWAS for these traits using animal mixed model-based association approach provided by GenABEL in Esme sheep. Three genome-wide and 14 individual chromosome-wide associated SNPs were discovered. As a result, ESRP1, LOC105613082, ZNF641, DUSP5, TEAD1, SMOX, PTPRT, RALYL, POM121C, PHIP, LOC101106051, ZIM3, PEG3, TRPC7, FBXL4, LOC105610397, LOC105616489 and DNAAF2 were suggested as candidates. Some of the discovered genes and involved pathways were already annotated to contribute growth and development in various species including human, mice and cattle. All in all, the results of this study are expected to strongly contribute to shed a light on the underlying molecular mechanisms behind growth and carcass composition traits, with potential implications on studies aiming faster genetic improvement, targeted low-resolution SNP panel designs and genome-editing studies.

Hypertrophy of Rat Skeletal Muscle Is Associated with Increased SIRT1/Akt/mTOR/S6 and Suppressed Sestrin2/SIRT3/FOXO1 Levels

Despite the intensive investigation of the molecular mechanism of skeletal muscle hypertrophy, the underlying signaling processes are not completely understood. Therefore, we used an overload model, in which the main synergist muscles (gastrocnemius, soleus) of the plantaris muscle were surgically removed, to cause a significant overload in the remaining plantaris muscle of 8-month-old Wistar male rats. SIRT1-associated pro-anabolic, pro-catabolic molecular signaling pathways, NAD and H₂S levels of this overload-induced hypertrophy were studied. Fourteen days of overload resulted in a significant 43% (p < 0.01) increase in the mass of plantaris muscle compared to sham operated animals. Cystathionine-β-synthase (CBS) activities and bioavailable H₂S levels were not modified by overload. On the other hand, overload-induced hypertrophy of skeletal muscle was associated with increased SIRT1 (p < 0.01), Akt (p < 0.01), mTOR, S6 (p < 0.01) and suppressed sestrin 2 levels (p < 0.01), which are mostly responsible for anabolic signaling. Decreased FOXO1 and SIRT3 signaling (p < 0.01) suggest downregulation of protein breakdown and mitophagy. Decreased levels of NAD⁺, sestrin2, OGG1 (p < 0.01) indicate that the redox milieu of skeletal muscle after 14 days of overloading is reduced. The present investigation revealed novel cellular interactions that regulate anabolic and catabolic processes in the hypertrophy of skeletal muscle.

Magnesium Deficiency Alters Expression of Genes Critical for Muscle Magnesium Homeostasis and Physiology in Mice

Chronic Mg²⁺ deficiency is the underlying cause of a broad range of health dysfunctions. As 25% of body Mg²⁺ is located in the skeletal muscle, Mg²⁺ transport and homeostasis systems (MgTHs) in the muscle are critical for whole-body Mg²⁺ homeostasis. In the present study, we assessed whether Mg²⁺ deficiency alters muscle fiber characteristics and major pathways regulating muscle physiology. C57BL/6J mice received either a control, mildly, or severely Mg²⁺-deficient diet (0.1%; 0.01%; and 0.003% Mg²⁺ wt/wt, respectively) for 14 days. Mg²⁺ deficiency slightly decreased body weight gain and muscle Mg²⁺ concentrations but was not associated with detectable variations in gastrocnemius muscle weight, fiber morphometry, and capillarization. Nonetheless, muscles exhibited decreased expression of several MgTHs (MagT1, CNNM2, CNNM4, and TRPM6). Moreover, TaqMan low-density array (TLDA) analyses further revealed that, before the emergence of major muscle dysfunctions, even a mild Mg²⁺ deficiency was sufficient to alter the expression of genes critical for muscle physiology, including energy metabolism, muscle regeneration, proteostasis, mitochondrial dynamics, and excitation–contraction coupling.

Nutraceuticals in the Prevention and Treatment of the Muscle Atrophy

Imbalance of protein homeostasis, with excessive protein degradation compared with protein synthesis, leads to the development of muscle atrophy resulting in a decrease in muscle mass and consequent muscle weakness and disability. Potential triggers of muscle atrophy include inflammation, malnutrition, aging, cancer, and an unhealthy lifestyle such as sedentariness and high fat diet. Nutraceuticals with preventive and therapeutic effects against muscle atrophy have recently received increasing attention since they are potentially more suitable for long-term use. The implementation of nutraceutical intervention might aid in the development and design of precision medicine strategies to reduce the burden of muscle atrophy. In this review, we will summarize the current knowledge on the importance of nutraceuticals in the prevention of skeletal muscle mass loss and recovery of muscle function. We also highlight the cellular and molecular mechanisms of these nutraceuticals and their possible pharmacological use, which is of great importance for the prevention and treatment of muscle atrophy.

Muscle Proteomic and Transcriptomic Profiling of Healthy Aging and Metabolic Syndrome in Men

(1) Background: Aging is associated with a progressive decline in muscle mass and function. Aging is also a primary risk factor for metabolic syndrome, which further alters muscle metabolism. However, the molecular mechanisms involved remain to be clarified. Herein we performed omic profiling to decipher in muscle which dominating processes are associated with healthy aging and metabolic syndrome in old men. (2) Methods: This study included 15 healthy young, 15 healthy old, and 9 old men with metabolic syndrome. Old men were selected from a well-characterized cohort, and each vastus lateralis biopsy was used to combine global transcriptomic and proteomic analyses. (3) Results: Over-representation analysis of differentially expressed genes (ORA) and functional class scoring of pathways (FCS) indicated that healthy aging was mainly associated with upregulations of apoptosis and immune function and downregulations of glycolysis and protein catabolism. ORA and FCS indicated that with metabolic syndrome the dominating biological processes were upregulation of proteolysis and downregulation of oxidative phosphorylation. Proteomic profiling matched 586 muscle proteins between individuals. The proteome of healthy aging revealed modifications consistent with a fast-to-slow transition and downregulation of glycolysis. These transitions were reduced with metabolic syndrome, which was more associated with alterations in NADH/NAD+ shuttle and β-oxidation. Proteomic profiling further showed that all old muscles overexpressed protein chaperones to preserve proteostasis and myofiber integrity. There was also evidence of aging-related increases in reactive oxygen species but better detoxifications of cytotoxic aldehydes and membrane protection in healthy than in metabolic syndrome muscles. (4) Conclusions: Most candidate proteins and mRNAs identified herein constitute putative muscle biomarkers of healthy aging and metabolic syndrome in old men.

Global alternative splicing landscape of skeletal muscle atrophy induced by hindlimb unloading

Background

Long-term exposure to microgravity will cause skeletal muscle atrophy, which can cause serious harm to astronauts in space travel. Therefore, it is important to explore skeletal muscle atrophy’s molecular mechanism for its prevention and treatment. However, as an important regulatory approach of skeletal muscle physiology, the role of alternative splicing in skeletal muscle atrophy, especially skeletal muscle atrophy caused by disuse, is unclear.

Methods

We established a rat hindlimb unloading model and performed RNA sequencing on soleus muscle, which was seriously atrophied during unloading. Several bioinformatics methods were used to identify alternative splicing events and determine their gene functions.

Results

Many alternative splicing events were found to occur at different time points (12 h, 24 h, 36 h, 3 days, and 7 days) after hindlimb unloading. These differential alternative splicing events mainly occurred in the gene's coding domain sequence region, and 59% of the alternative splicing events caused open reading frameshift. Bioinformatics analysis results showed that genes with different alternative splicing events were enriched in multiple pathways related to muscle atrophy, including the insulin signaling pathway, endocytosis, mitophagy, and ubiquitin-proteasome pathway. Moreover, alternative splicing of several deubiquitinase genes persisted during skeletal muscle atrophy induced by unloading. Additionally, we identified 10 differentially expressed RNA binding proteins during skeletal muscle atrophy induced by unloading, mainly containing Xpo4, Eif4e2, P4ha1, Lrrfip1, Zc3h14, Emg1, Hnrnp h1, Mbnl2, RBfox1, and Mbnl1. Hnrnp h1 and Mbnl2 were significantly downregulated, and RBfox1 and Mbnl1 were significantly upregulated during skeletal muscle atrophy caused by unloading.

Conclusions

To the best of our knowledge, the present study is the first to propose alternative splicing alterations related to disuse-induced muscle atrophy, emphasizing that alternative splicing is a new focus of attention in the occurrence of muscle atrophy.

TRIM32: A Multifunctional Protein Involved in Muscle Homeostasis, Glucose Metabolism, and Tumorigenesis

Human tripartite motif family of proteins 32 (TRIM32) is a ubiquitous multifunctional protein that has demonstrated roles in differentiation, muscle physiology and regeneration, and tumor suppression. Mutations in TRIM32 result in two clinically diverse diseases. A mutation in the B-box domain gives rise to Bardet–Biedl syndrome (BBS), a disease whose clinical presentation shares no muscle pathology, while mutations in the NHL (NCL-1, HT2A, LIN-41) repeats of TRIM32 causes limb-girdle muscular dystrophy type 2H (LGMD2H). TRIM32 also functions as a tumor suppressor, but paradoxically is overexpressed in certain types of cancer. Recent evidence supports a role for TRIM32 in glycolytic-mediated cell growth, thus providing a possible mechanism for TRIM32 in the accumulation of cellular biomass during regeneration and tumorigenesis, including in vitro and in vivo approaches, to understand the broad spectrum of TRIM32 functions. A special emphasis is placed on the utility of the Drosophila model, a unique system to study glycolysis and anabolic pathways that contribute to the growth and homeostasis of both normal and tumor tissues.

Cerebrum, liver, and muscle regulatory networks uncover maternal nutrition effects in developmental programming of beef cattle during early pregnancy

The molecular basis underlying fetal programming in response to maternal nutrition remains unclear. Herein, we investigated the regulatory relationships between genes in fetal cerebrum, liver, and muscle tissues to shed light on the putative mechanisms that underlie the effects of early maternal nutrient restriction on bovine developmental programming. To this end, cerebrum, liver, and muscle gene expression were measured with RNA-Seq in 14 fetuses collected on day 50 of gestation from dams fed a diet initiated at breeding to either achieve 60% (RES, n = 7) or 100% (CON, n = 7) of energy requirements. To build a tissue-to-tissue gene network, we prioritized tissue-specific genes, transcription factors, and differentially expressed genes. Furthermore, we built condition-specific networks to identify differentially co-expressed or connected genes. Nutrient restriction led to differential tissue regulation between the treatments. Myogenic factors differentially regulated by ZBTB33 and ZNF131 may negatively affect myogenesis. Additionally, nutrient-sensing pathways, such as mTOR and PI3K/Akt, were affected by gene expression changes in response to nutrient restriction. By unveiling the network properties, we identified major regulators driving gene expression. However, further research is still needed to determine the impact of early maternal nutrition and strategic supplementation on pre- and post-natal performance.

Silencing COX-2 blocks PDK1/TRAF4-induced AKT activation to inhibit fibrogenesis during skeletal muscle atrophy

Skeletal muscle atrophy with high prevalence can induce weakness and fatigability and place huge burden on both health and quality of life. During skeletal muscle degeneration, excessive fibroblasts and extracellular matrix (ECM) accumulated to replace and impair the resident muscle fiber and led to loss of muscle mass. Cyclooxygenase-2 (COX-2), the rate-limiting enzyme in synthesis of prostaglandin, has been identified as a positive regulator in pathophysiological process like inflammation and oxidative stress. In our study, we found injured muscles of human subjects and mouse model overexpressed COX-2 compared to the non-damaged region and COX-2 was also upregulated in fibroblasts following TGF-β stimulation. Then we detected the effect of selective COX-2 inhibitor celecoxib on fibrogenesis. Celecoxib mediated anti-fibrotic effect by inhibiting fibroblast differentiation, proliferation and migration as well as inactivating TGF-β-dependent signaling pathway, non-canonical TGF-β pathways and suppressing generation of reactive oxygen species (ROS) and oxidative stress. In vivo pharmacological inhibition of COX-2 by celecoxib decreased tissue fibrosis and increased skeletal muscle fiber preservation reflected by less ECM formation and myofibroblast accumulation with decreased p-ERK1/2, p-Smad2/3, TGF-βR1, VEGF, NOX2 and NOX4 expression. Expression profiling further found that celecoxib could suppress PDK1 expression. The interaction between COX-2 and PDK1/AKT signaling remained unclear, here we found that COX-2 could bind to PDK1/AKT to form compound. Knockdown of COX-2 in fibroblasts by pharmacological inactivation or by siRNA restrained PDK1 expression and AKT phosphorylation induced by TGF-β treatment. Besides, si-COX-2 prevented TGF-β-induced K63-ubiquitination of AKT by blocking the interaction between AKT and E3 ubiquitin ligase TRAF4. In summary, we found blocking COX-2 inhibited fibrogenesis after muscle atrophy induced by injury and suppressed AKT signaling pathway by inhibiting upstream PDK1 expression and preventing the recruitment of TRAF4 to AKT, indicating that COX-2/PDK1/AKT signaling pathway promised to be target for treating muscle atrophy in the future.

Quercetin Modulates IGF-I and IGF-II Levels After Eccentric Exercise-Induced Muscle-Damage: A Placebo-Controlled Study

BACKGROUND

Prolonged or unaccustomed eccentric exercise may cause muscle damage and depending from its extent, this event negatively affects physical performance.

OBJECTIVES

The aim of the present investigation was to evaluate, in humans, the effect of the flavonoid quercetin on circulating levels of the anabolic insulin-like growth factor 1 (IGF-I) and insulin-like growth factor 2 (IGF-II), produced during the recovery period after an eccentric-induced muscle damage (EIMD).

METHODS

A randomized, double-blind, crossover study has been performed; twelve young men ingested quercetin (1 g/day) or placebo for 14 days and then underwent an eccentric-induced muscle damaging protocol. Blood samples were collected, and cell damage markers [creatine kinase (CK), lactate dehydrogenase (LDH) and myoglobin (Mb)], the inflammatory responsive interleukin 6 (IL-6), IGF-I and IGF-II levels were evaluated before the exercise and at different recovery times from 24 hours to 7 days after EIMD.

RESULTS

We found that, in placebo treatment the increase in IGF-I (72 h) preceded IGF-II increase (7 d). After Q supplementation there was a more marked increase in IGF-I levels and notably, the IGF-II peak was found earlier, compared to placebo, at the same time of IGF-I (72 h). Quercetin significantly reduced plasma markers of cell damage [CK (p<0.005), LDH (p<0.001) and Mb (p<0.05)] and the interleukin 6 level [IL-6 (p<0.05)] during recovery period following EIMD compared to placebo.

CONCLUSIONS

Our data are encouraging about the use of quercetin as dietary supplementation strategy to adopt in order to mitigate and promote a faster recovery after eccentric exercise as suggested by the increase in plasma levels of the anabolic factors IGF-I and IGF-II.

The Dunkin Hartley Guinea Pig Is a Model of Primary Osteoarthritis That Also Exhibits Early Onset Myofiber Remodeling That Resembles Human Musculoskeletal Aging

Skeletal muscle dysfunction, articular cartilage degeneration, and bone loss occur essentially in parallel during aging. Mechanisms contributing to this systemic musculoskeletal decline remain incompletely understood, limiting progress toward developing effective therapeutics. Because the progression of human musculoskeletal aging is slow, researchers rely on rodent models to identify mechanisms and test interventions. The Dunkin Hartley guinea pig is an outbred strain that begins developing primary osteoarthritis by 4 months of age with a progression and pathology similar to aging humans. The purpose of this study was to determine if skeletal muscle remodeling during the progression of osteoarthritis in these guinea pigs resembles musculoskeletal aging in humans. We compared Dunkin Hartley guinea pigs to Strain 13 guinea pigs, which develop osteoarthritis much later in the lifespan. We measured myofiber type and size, muscle density, and long-term fractional protein synthesis rates of the gastrocnemius and soleus muscles in 5, 9, and 15-month-old guinea pigs. There was an age-related decline in skeletal muscle density, a greater proportion of smaller myofibers, and a decline in type II concomitant with a rise in type I myofibers in the gastrocnemius muscles from Dunkin Hartley guinea pigs only. These changes were accompanied by age-related declines in myofibrillar and mitochondrial protein synthesis in the gastrocnemius and soleus. Collectively, these findings suggest Dunkin Hartley guinea pigs experience myofiber remodeling alongside the progression of osteoarthritis, consistent with human musculoskeletal aging. Thus, Dunkin Hartley guinea pigs may be a model to advance discovery and therapeutic development for human musculoskeletal aging.

Skeletal Muscle Recovery from Disuse Atrophy: Protein Turnover Signaling and Strategies for Accelerating Muscle Regrowth

Skeletal muscle fibers have a unique capacity to adjust their metabolism and phenotype in response to alternations in mechanical loading. Indeed, chronic mechanical loading leads to an increase in skeletal muscle mass, while prolonged mechanical unloading results in a significant decrease in muscle mass (muscle atrophy). The maintenance of skeletal muscle mass is dependent on the balance between rates of muscle protein synthesis and breakdown. While molecular mechanisms regulating protein synthesis during mechanical unloading have been relatively well studied, signaling events implicated in protein turnover during skeletal muscle recovery from unloading are poorly defined. A better understanding of the molecular events that underpin muscle mass recovery following disuse-induced atrophy is of significant importance for both clinical and space medicine. This review focuses on the molecular mechanisms that may be involved in the activation of protein synthesis and subsequent restoration of muscle mass after a period of mechanical unloading. In addition, the efficiency of strategies proposed to improve muscle protein gain during recovery is also discussed.

Essential amino acid supplementation alters the p53 transcriptional response and cytokine gene expression following total knee arthroplasty

Reducing muscle atrophy following orthopedic surgery is critical during the postoperative period. Our previous work in patients who underwent total knee arthroplasty (TKA) showed that the vast majority of atrophy occurs within 2 wk following surgery and that essential amino acid (EAA) supplementation attenuates this atrophy. We used RNA-sequencing (RNA-seq) to identify genes associated with atrophy after TKA with and without EAAs. Analysis of overrepresented gene-ontology terms revealed that p53 signaling and the cytokine-cytokine receptor pathways were highly upregulated after TKA. Relative to the placebo group, the EAA group had altered expression of p53 regulators such as MDM2. This altered expression may account for differences between groups in timing of upregulation of some p53 targets such as apoptosis genes, and may account for the reduction in muscle loss in the subjects receiving EAAs. Furthermore, we observed altered expression of a large number of cytokine-signaling genes including TNFRSF12A, which plays a critical role in muscle atrophy, myogenesis, fibrosis, and the noncanonical NF-κB pathway.NEW & NOTEWORTHY Total knee arthroplasty is the most frequently performed inpatient surgical procedure for those over 45 yr in the United States. Following surgery, patients lose a large amount of muscle, which impacts functional mobility. Previously, our laboratory found that supplementing patients' diets with essential amino acids (EAAs) reduces postsurgical muscle loss. Here, our goal was to characterize the transcriptional changes associated with surgery with and without EAA supplementation to uncover the underlying mechanisms by which EAAs attenuate this muscle loss.

C2C12 cell model: its role in understanding of insulin resistance at the molecular level and pharmaceutical development at the preclinical stage

OBJECTIVES

The myoblast cell line, C2C12, has been utilised extensively in vitro as an examination model in understanding metabolic disease progression. Although it is indispensable in both preclinical and pharmaceutical research, a comprehensive review of its use in the investigation of insulin resistance progression and pharmaceutical development is not available.

KEY FINDINGS

C2C12 is a well-documented model, which can facilitate our understanding in glucose metabolism, insulin signalling mechanism, insulin resistance, oxidative stress, reactive oxygen species and glucose transporters at cellular and molecular levels. With the aid of the C2C12 model, recent studies revealed that insulin resistance has close relationship with various metabolic diseases in terms of disease progression, pathogenesis and therapeutic management. A holistic, safe and effective disease management is highly of interest. Therefore, significant efforts have been paid to explore novel drug compounds and natural herbs that can elicit therapeutic effects in the targeted sites at both cellular (e.g. mitochondria, glucose transporter) and molecular level (e.g. genes, signalling pathway).

SUMMARY

The use of C2C12 myoblast cell line is meaningful in pharmaceutical and biomedical research due to their expression of GLUT-4 and other features that are representative to human skeletal muscle cells. With the use of the C2C12 cell model, the impact of drug delivery systems (nanoparticles and quantum dots) on skeletal muscle, as well as the relationship between exercise, pancreatic β-cells and endothelial cells, was discovered.

High-methionine diet in skeletal muscle remodeling: epigenetic mechanism of homocysteine-mediated growth retardation

Epigenetic DNA methylation (1-carbon metabolism) is crucial for gene imprinting/off-printing that ensures epigenetic memory but also generates a copious amount of homocysteine (Hcy), unequivocally. That is why during pregnancy, expectant mothers are recommended "folic acid" preemptively to avoid birth defects in the young ones because of elevated Hcy levels (i.e., hyperhomocysteinemia (HHcy)). As we know, children born with HHcy have several musculoskeletal abnormalities, including growth retardation. Here, we focus on the gut-dysbiotic microbiome implication(s) that we believe instigates the "1-carbon metabolism" and HHcy causing growth retardation along with skeletal muscle abnormalities. We test our hypothesis whether high-methionine diet (HMD) (an amino acid that is high in red meat), a substrate for Hcy, can cause skeletal muscle and growth retardation, and treatment with probiotics (PB) to mitigate skeletal muscle dysfunction. To test this, we employed cystathionine β-synthase, CBS deficient mouse (CBS^+/-) fed with/without HMD and with/without a probiotic (Lactobacillus rhamnosus) in drinking water for 16 weeks. Matrix metalloproteinase (MMP) activity, a hallmark of remodeling, was measured by zymography. Muscle functions were scored via electric stimulation. Our results suggest that compared to the wild-type, CBS^+/- mice exhibited reduced growth phenotype. MMP-2 activity was robust in CBS^+/- and HMD effects were successfully attenuated by PB intervention. Electrical stimulation magnitude was decreased in CBS^+/- and CBS^+/- treated with HMD. Interestingly; PB mitigated skeletal muscle growth retardation and atrophy. Collectively, results imply that individuals with mild/moderate HHcy seem more prone to skeletal muscle injury and its dysfunction.

Identification and characterization of Fbxl22, a novel skeletal muscle atrophy-promoting E3 ubiquitin ligase

Muscle-specific E3 ubiquitin ligases have been identified in muscle atrophy-inducing conditions. The purpose of the current study was to explore the functional role of F-box and leucine-rich protein 22 (Fbxl22), and a newly identified splice variant (Fbxl22-193), in skeletal muscle homeostasis and neurogenic muscle atrophy. In mouse C₂C₁₂ muscle cells, promoter fragments of the Fbxl22 gene were cloned and fused with the secreted alkaline phosphatase reporter gene to assess the transcriptional regulation of Fbxl22. The tibialis anterior muscles of male C57/BL6 mice (12-16 wk old) were electroporated with expression plasmids containing the cDNA of two Fbxl22 splice variants and tissues collected after 7, 14, and 28 days. Gastrocnemius muscles of wild-type and muscle-specific RING finger 1 knockout (MuRF1 KO) mice were electroporated with an Fbxl22 RNAi or empty plasmid and denervated 3 days posttransfection, and tissues were collected 7 days postdenervation. The full-length gene and novel splice variant are transcriptionally induced early (after 3 days) during neurogenic muscle atrophy. In vivo overexpression of Fbxl22 isoforms in mouse skeletal muscle leads to evidence of myopathy/atrophy, suggesting that both are involved in the process of neurogenic muscle atrophy. Knockdown of Fbxl22 in the muscles of MuRF1 KO mice resulted in significant additive muscle sparing 7 days after denervation. Targeting two E3 ubiquitin ligases appears to have a strong additive effect on protecting muscle mass loss with denervation, and these findings have important implications in the development of therapeutic strategies to treat muscle atrophy.

Effectiveness of Resistive Vibration Exercise and Whey Protein Supplementation Plus Alkaline Salt on the Skeletal Muscle Proteome Following 21 Days of Bed Rest in Healthy Males

Muscle atrophy is a deleterious consequence of physical inactivity and is associated with increased morbidity and mortality. The aim of this study was to decipher the mechanisms involved in disuse muscle atrophy in eight healthy men using a 21 day bed rest with a cross-over design (control, with resistive vibration exercise (RVE), or RVE combined with whey protein supplementation and an alkaline salt (NEX)). The main physiological findings show a significant reduction in whole-body fat-free mass (CON -4.1%, RVE -4.3%, NEX -2.7%, p < 0.05), maximal oxygen consumption (CON -20.5%, RVE -6.46%, NEX -7.9%, p < 0.05), and maximal voluntary contraction (CON -15%, RVE -12%, and NEX -9.5%, p < 0.05) and a reduction in mitochondrial enzyme activity (CON -30.7%, RVE -31.3%, NEX -17%, p < 0.05). The benefits of nutrition and exercise countermeasure were evident with an increase in leg lean mass (CON -1.7%, RVE +8.9%, NEX +15%, p < 0.05). Changes to the vastus lateralis muscle proteome were characterized using mass spectrometry-based label-free quantitative proteomics, the findings of which suggest alterations to cell metabolism, mitochondrial metabolism, protein synthesis, and degradation pathways during bed rest. The observed changes were partially mitigated during RVE, but there were no significant pathway changes during the NEX trial. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the dataset identifier PXD006882. In conclusion, resistive vibration exercise, when combined with whey/alkalizing salt supplementation, could be an effective strategy to prevent skeletal muscle protein changes, muscle atrophy, and insulin sensitivity during medium duration bed rest.

Edward F. Adolph Distinguished Lecture. Skeletal muscle atrophy: Multiple pathways leading to a common outcome

Skeletal muscle atrophy continues to be a serious consequence of many diseases and conditions for which there is no treatment. Our understanding of the mechanisms regulating skeletal muscle mass has improved considerably over the past two decades. For many years it was known that skeletal muscle atrophy resulted from an imbalance between protein synthesis and protein breakdown, with the net balance shifting toward protein breakdown. However, the molecular and cellular mechanisms underlying the increased breakdown of myofibrils was unknown. Over the past two decades, numerous reports have identified novel genes and signaling pathways that are upregulated and activated in response to stimuli such as disuse, inflammation, metabolic stress, starvation and others that induce muscle atrophy. This review summarizes the discovery efforts performed in the identification of several pathways involved in the regulation of skeletal muscle mass: the mammalian target of rapamycin (mTORC1) and the ubiquitin proteasome pathway and the E3 ligases, MuRF1 and MAFbx. While muscle atrophy is a common outcome of many diseases, it is doubtful that a single gene or pathway initiates or mediates the breakdown of myofibrils. Interestingly, however, is the observation that upregulation of the E3 ligases, MuRF1 and MAFbx, is a common feature of many divergent atrophy conditions. The challenge for the field of muscle biology is to understand how all of the various molecules, transcription factors, and signaling pathways interact to produce muscle atrophy and to identify the critical factors for intervention.

Activating transcription factor 4 (ATF4) promotes skeletal muscle atrophy by forming a heterodimer with the transcriptional regulator C/EBPβ

Skeletal muscle atrophy is a highly-prevalent and debilitating condition that remains poorly understood at the molecular level. Previous work found that aging, fasting, and immobilization promote skeletal muscle atrophy via expression of activating transcription factor 4 (ATF4) in skeletal muscle fibers. However, the direct biochemical mechanism by which ATF4 promotes muscle atrophy is unknown. ATF4 is a member of the basic leucine zipper transcription factor (bZIP) superfamily. Because bZIP transcription factors are obligate dimers, and because ATF4 is unable to form highly-stable homodimers, we hypothesized that ATF4 may promote muscle atrophy by forming a heterodimer with another bZIP family member. To test this hypothesis, we biochemically isolated skeletal muscle proteins that associate with the dimerization- and DNA-binding domain of ATF4 (the bZIP domain) in mouse skeletal muscle fibers in vivo Interestingly, we found that ATF4 forms at least five distinct heterodimeric bZIP transcription factors in skeletal muscle fibers. Furthermore, one of these heterodimers, composed of ATF4 and CCAAT enhancer-binding protein β (C/EBPβ), mediates muscle atrophy. Within skeletal muscle fibers, the ATF4-C/EBPβ heterodimer interacts with a previously unrecognized and evolutionarily conserved ATF-C/EBP composite site in exon 4 of the Gadd45a gene. This three-way interaction between ATF4, C/EBPβ, and the ATF-C/EBP composite site activates the Gadd45a gene, which encodes a critical mediator of muscle atrophy. Together, these results identify a biochemical mechanism by which ATF4 induces skeletal muscle atrophy, providing molecular-level insights into the etiology of skeletal muscle atrophy.

Increased Adenine Nucleotide Degradation in Skeletal Muscle Atrophy

Adenine nucleotides (AdNs: ATP, ADP, AMP) are essential biological compounds that facilitate many necessary cellular processes by providing chemical energy, mediating intracellular signaling, and regulating protein metabolism and solubilization. A dramatic reduction in total AdNs is observed in atrophic skeletal muscle across numerous disease states and conditions, such as cancer, diabetes, chronic kidney disease, heart failure, COPD, sepsis, muscular dystrophy, denervation, disuse, and sarcopenia. The reduced AdNs in atrophic skeletal muscle are accompanied by increased expression/activities of AdN degrading enzymes and the accumulation of degradation products (IMP, hypoxanthine, xanthine, uric acid), suggesting that the lower AdN content is largely the result of increased nucleotide degradation. Furthermore, this characteristic decrease of AdNs suggests that increased nucleotide degradation contributes to the general pathophysiology of skeletal muscle atrophy. In view of the numerous energetic, and non-energetic, roles of AdNs in skeletal muscle, investigations into the physiological consequences of AdN degradation may provide valuable insight into the mechanisms of muscle atrophy.

Muscular Atrophy and Sarcopenia in the Elderly: Is There a Role for Creatine Supplementation?

Sarcopenia is characterized by a loss of muscle mass, quality, and function, and negatively impacts health, functionality, and quality of life for numerous populations, particularly older adults. Creatine is an endogenously produced metabolite, which has the theoretical potential to counteract many of the morphological and metabolic parameters underpinning sarcopenia. This can occur through a range of direct and indirect mechanisms, including temporal and spatial functions that accelerate ATP regeneration during times of high energy demand, direct anabolic and anti-catabolic functions, and enhanced muscle regenerating capacity through positively impacting muscle stem cell availability. Studies conducted in older adults show little benefit of creatine supplementation alone on muscle function or mass. In contrast, creatine supplementation as an adjunct to exercise training seems to augment the muscle adaptive response to the training stimulus, potentially through increasing capacity for higher intensity exercise, and/or by enhancing post-exercise recovery and adaptation. As such, creatine may be an effective dietary strategy to combat age-related muscle atrophy and sarcopenia when used to complement the benefits of exercise training.

An investigation of p53 in skeletal muscle aging

Age-related skeletal muscle atrophy is a very common and serious condition that remains poorly understood at the molecular level. Several lines of evidence have suggested that the tumor suppressor p53 may play a central, causative role in skeletal muscle aging, whereas other, apparently contradictory lines of evidence have suggested that p53 may be critical for normal skeletal muscle function. To help address these issues, we performed an aging study in male muscle-specific p53-knockout mice (p53 mKO mice), which have a lifelong absence of p53 expression in skeletal muscle fibers. We found that the absence of p53 expression in skeletal muscle fibers had no apparent deleterious or beneficial effects on skeletal muscle mass or function under basal conditions up to 6 mo of age, when mice are fully grown and exhibit peak muscle mass and function. Furthermore, at 22 and 25 mo of age, when age-related muscle weakness and atrophy are clearly evident in mice, p53 mKO mice demonstrated no improvement or worsening of skeletal muscle mass or function relative to littermate control mice. At advanced ages, p53 mKO mice began to die prematurely and had an increased incidence of osteosarcoma, precluding analyses of muscle mass and function in very old p53 mKO mice. In light of these results, we conclude that p53 expression in skeletal muscle fibers has minimal if any direct, cell autonomous effect on basal or age-related changes in skeletal muscle mass and function up to at least 22 mo of age.NEW & NOTEWORTHY Previous studies implicated the transcriptional regulator p53 as a potential mediator of age-related skeletal muscle weakness and atrophy. We tested this hypothesis by investigating the effect of aging in muscle-specific p53-knockout mice. Our results strongly suggest that p53 activity within skeletal muscle fibers is not required for age-related skeletal muscle atrophy or weakness.