FoxO transcription factors: their roles in the maintenance of skeletal muscle homeostasis

Primary myxoid and epithelioid mesenchymal tumor of the kidney with a novel GLI1-FOXO4 fusion

To our knowledge, we describe the first mesenchymal tumor with a novel GLI1-FOXO4 fusion gene. This well-circumscribed kidney tumor displayed variably myxoid and epithelioid histologic features with a focally nodular growth pattern. The tumor cells showed bland, round to ovoid nuclei, with no overt high-grade features. The tumor showed focal immunopositivity for smooth muscle actin and Melan-A, which raised the possibility of a relationship with a perivascular epithelioid cell tumor. The clinical and morphologic features appear distinct from other reported neoplasms harboring GLI1 or FOXO4 gene rearrangements. The patient underwent radical nephrectomy and is without evidence of disease during a relatively short clinical follow-up period. However, the features of this tumor likely warrant long-term follow-up to monitor for the possibility of a late recurrence or metastasis. In addition to reporting this novel fusion-positive tumor, we also provide a brief review of GLI1 and FOXO4 gene functions in both normal and neoplastic contexts.

Upregulation of Hallmark Muscle Genes Protects GneM743T/M743T Mutated Knock-In Mice From Kidney and Muscle Phenotype

Background:

Mutations in GNE cause a recessive, adult onset myopathy characterized by slowly progressive distal and proximal muscle weakness. Knock-in mice carrying the most frequent mutation in GNE myopathy patients, Gne^M743T/M743T, usually die few days after birth from severe renal failure, with no muscle phenotype. However, a spontaneous sub-colony remains healthy throughout a normal lifespan without any kidney or muscle pathology.

Objective:

We attempted to decipher the molecular mechanisms behind these phenotypic differences and to determine the mechanisms preventing the kidney and muscles from disease.

Methods:

We analyzed the transcriptome and proteome of kidneys and muscles of sick and healthy Gne^M743T/M743T mice.

Results:

The sick Gne^M743T/M743T kidney was characterized by up-regulation of extra-cellular matrix degradation related processes and by down-regulation of oxidative phosphorylation and respiratory electron chain pathway, that was also observed in the asymptomatic muscles. Surprisingly, the healthy kidneys of the Gne^M743T/M743T mice were characterized by up-regulation of hallmark muscle genes. In addition the asymptomatic muscles of the sick Gne^M743T/M743T mice showed upregulation of transcription and translation processes.

Conclusions:

Overexpression of muscle physiology genes in healthy Gne^M743T/M743T mice seems to define the protecting mechanism in these mice. Furthermore, the strong involvement of muscle related genes in kidney may bridge the apparent phenotypic gap between GNE myopathy and the knock-in Gne^M743T/M743T mouse model and provide new directions in the study of GNE function in health and disease.

Estrogen Regulates the Satellite Cell Compartment in Females

Skeletal muscle mass, strength, and regenerative capacity decline with age, with many measures showing a greater deterioration in females around the time estrogen levels decrease at menopause. Here, we show that estrogen deficiency severely compromises the maintenance of muscle stem cells (i.e., satellite cells) as well as impairs self-renewal and differentiation into muscle fibers. Mechanistically, by hormone replacement, use of a selective estrogen-receptor modulator (bazedoxifene), and conditional estrogen receptor knockout, we implicate 17β-estradiol and satellite cell expression of estrogen receptor α and show that estrogen signaling through this receptor is necessary to prevent apoptosis of satellite cells. Early data from a biopsy study of women who transitioned from peri- to post-menopause are consistent with the loss of satellite cells coincident with the decline in estradiol in humans. Together, these results demonstrate an important role for estrogen in satellite cell maintenance and muscle regeneration in females.

Collins et al. show the loss of estrogen in female mice and post-menopausal women leads to a decrease in skeletal muscle stem cells. Using muscle stem cell-specific mutants, it was demonstrated that ERα is necessary for satellite cell maintenance, self-renewal, and protection from apoptosis, thereby promoting optimal muscle regeneration.

Myostatin and muscle atrophy during chronic kidney disease

Chronic kidney disease (CKD) patients often exhibit a low muscle mass and strength, leading to physical impairment and an increased mortality. Two major signalling pathways control protein synthesis, the insulin-like growth factor-1/Akt (IGF-1/Akt) pathway, acting as a positive regulator, and the myostatin (Mstn) pathway, acting as a negative regulator. Mstn, also known as the growth development factor-8 (GDF-8), is a member of the transforming growth factor-β superfamily, which is secreted by mature muscle cells. Mstn inhibits satellite muscle cell proliferation and differentiation and induces a proteolytic phenotype of muscle cells by activating the ubiquitin-proteasome system. Recent advances have been made in the comprehension of the Mstn pathway disturbance and its role in muscle wasting during CKD. Most studies report higher Mstn concentrations in CKD and dialysis patients than in healthy subjects. Several factors increase Mstn production in uraemic conditions: low physical activity, chronic or acute inflammation and oxidative stress, uraemic toxins, angiotensin II, metabolic acidosis and glucocorticoids. Mstn seems to be only scarcely removed during haemodialysis or peritoneal dialysis, maybe because of its large molecule size in plasma where it is linked to its prodomain. In dialysis patients, Mstn has been proposed as a biomarker of muscle mass, muscle strength or physical performances, but more studies are needed in this field. This review outlines the interconnection between Mstn activation, muscle dysfunction and CKD. We discuss mechanisms of action and efficacy of pharmacological Mstn pathway inhibition that represents a promising treatment approach of striated muscle dysfunction. Many approaches and molecules are in development but until now, no study has proved a benefit in CKD.

Identification of potential specific biomarkers and key signaling pathways between osteogenic and adipogenic differentiation of hBMSCs for osteoporosis therapy

Background

The differentiation of bone mesenchymal stem cells (BMSCs) into adipogenesis (AD) rather than osteogenesis (OS) is an important pathological feature of osteoporosis. Illuminating the detailed mechanisms of the differentiation of BMSCs into OS and AD would contribute to the interpretation of osteoporosis pathology.

Methods

To identify the regulated mechanism in lineage commitment of the BMSCs into OS and AD in the early stages, the gene expression profiles with temporal series were downloaded to reveal the distinct fates when BMSCs adopt a committed lineage. For both OS and AD lineages, the profiles of days 2–4 were compared with day 0 to screen the differentially expressed genes (DEGs), respectively. Next, the functional enrichment analysis was utilized to find out the biological function, and protein-protein interaction network to predict the central genes. Finally, experiments were performed to verify our finding.

Results

FoxO signaling pathway with central genes like FoxO3, IL6, and CAT is the crucial mechanism of OS, while Rap1 signaling pathway of VEGFA and FGF2 enrichment is more significant for AD. Besides, PI3K-Akt signaling pathway might serve as the latent mechanism about the initiation of differentiation of BMSCs into multiple lineages.

Conclusion

Above hub genes and early-responder signaling pathways control osteogenic and adipogenic fates of BMSCs, which maybe mechanistic models clarifying the changes of bone metabolism in the clinical progress of osteoporosis. The findings provide a crucial reference for the prevention and therapy of osteoporosis.

The bright and the dark sides of L-carnitine supplementation: a systematic review

Background

L-carnitine (LC) is used as a supplement by recreationally-active, competitive and highly trained athletes. This systematic review aims to evaluate the effect of prolonged LC supplementation on metabolism and metabolic modifications.

Methods

A literature search was conducted in the MEDLINE (via PubMed) and Web of Science databases from the inception up February 2020. Eligibility criteria included studies on healthy human subjects, treated for at least 12 weeks with LC administered orally, with no drugs or any other multi-ingredient supplements co-ingestion.

Results

The initial search retrieved 1024 articles, and a total of 11 studies were finally included after applying inclusion and exclusion criteria. All the selected studies were conducted with healthy human subjects, with supplemented dose ranging from 1 g to 4 g per day for either 12 or 24 weeks. LC supplementation, in combination with carbohydrates (CHO) effectively elevated total carnitine content in skeletal muscle. Twenty-four-weeks of LC supplementation did not affect muscle strength in healthy aged women, but significantly increased muscle mass, improved physical effort tolerance and cognitive function in centenarians. LC supplementation was also noted to induce an increase of fasting plasma trimethylamine-N-oxide (TMAO) levels, which was not associated with modification of determined inflammatory nor oxidative stress markers.

Conclusion

Prolonged LC supplementation in specific conditions may affect physical performance. On the other hand, LC supplementation elevates fasting plasma TMAO, compound supposed to be pro-atherogenic. Therefore, additional studies focusing on long-term supplementation and its longitudinal effect on the cardiovascular system are needed.

FOXO transcription factor family in cancer and metastasis

Forkhead box O (FOXO) transcription factors regulate diverse biological processes, affecting development, metabolism, stem cell maintenance and longevity. They have also been increasingly recognised as tumour suppressors through their ability to regulate genes essential for cell proliferation, cell death, senescence, angiogenesis, cell migration and metastasis. Mechanistically, FOXO proteins serve as key connection points to allow diverse proliferative, nutrient and stress signals to converge and integrate with distinct gene networks to control cell fate, metabolism and cancer development. In consequence, deregulation of FOXO expression and function can promote genetic disorders, metabolic diseases, deregulated ageing and cancer. Metastasis is the process by which cancer cells spread from the primary tumour often via the bloodstream or the lymphatic system and is the major cause of cancer death. The regulation and deregulation of FOXO transcription factors occur predominantly at the post-transcriptional and post-translational levels mediated by regulatory non-coding RNAs, their interactions with other protein partners and co-factors and a combination of post-translational modifications (PTMs), including phosphorylation, acetylation, methylation and ubiquitination. This review discusses the role and regulation of FOXO proteins in tumour initiation and progression, with a particular emphasis on cancer metastasis. An understanding of how signalling networks integrate with the FOXO transcription factors to modulate their developmental, metabolic and tumour-suppressive functions in normal tissues and in cancer will offer a new perspective on tumorigenesis and metastasis, and open up therapeutic opportunities for malignant diseases.

The role of mitochondria in redox signaling of muscle homeostasis

In the past, contraction-induced production of reactive oxygen species (ROS) has been implicated in oxidative stress to skeletal muscle. As research advances, clear evidence has revealed a more complete role of ROS under both physiologic and pathologic conditions. Central to the role of ROS is the redox signaling pathways that control exercise-induced major physiologic and cellular responses and adaptations, such as mitochondrial biogenesis, mitophagy, mitochondrial morphologic dynamics, antioxidant defense, and inflammation. The current review focuses on how muscle contraction and immobilization may activate or inhibit redox signalings and their impact on muscle mitochondrial homeostasis and physiologic implications.

Alternative signaling pathways from IGF1 or insulin to AKT activation and FOXO1 nuclear efflux in adult skeletal muscle fibers

Muscle atrophy is regulated by the balance between protein degradation and synthesis. FOXO1, a transcription factor, helps to determine this balance by activating pro-atrophic gene transcription when present in muscle fiber nuclei. Foxo1 nuclear efflux is promoted by AKT-mediated Foxo1 phosphorylation, eliminating FOXO1's atrophy-promoting effect. AKT activation can be promoted by insulin-like growth factor 1 (IGF1) or insulin via a pathway including IGF1 or insulin, phosphatidylinositol 3-kinase, and AKT. We used confocal fluorescence time-lapse imaging of FOXO1-GFP in adult isolated living muscle fibers maintained in culture to explore the effects of IGF1 and insulin on FOXO1-GFP nuclear efflux with and without pharmacological inhibitors. We observed that although AKT inhibitor blocks the IGF1- or insulin-induced effect on FOXO1 nuclear efflux, phosphatidylinositol 3-kinase inhibitors, which we show to be effective in these fibers, do not. We also found that inhibition of the protein kinase ACK1 or ATM contributes to the suppression of FOXO1 nuclear efflux after IGF1. These results indicate a novel pathway that has been unexplored in the IGF1- or insulin-induced regulation of FOXO1 and present information useful both for therapeutic interventions for muscle atrophy and for further investigative areas into insulin insensitivity and type 2 diabetes.

Transcriptional Changes Involved in Atrophying Muscles during Prolonged Fasting in Rats

Food deprivation resulting in muscle atrophy may be detrimental to health. To better understand how muscle mass is regulated during such a nutritional challenge, the current study deciphered muscle responses during phase 2 (P2, protein sparing) and phase 3 (P3, protein mobilization) of prolonged fasting in rats. This was done using transcriptomics analysis and a series of biochemistry measurements. The main findings highlight changes for plasma catabolic and anabolic stimuli, as well as for muscle transcriptome, energy metabolism, and oxidative stress. Changes were generally consistent with the intense use of lipids as fuels during P2. They also reflected increased muscle protein degradation and repressed synthesis, in a more marked manner during P3 than P2 compared to the fed state. Nevertheless, several unexpected changes appeared to be in favor of muscle protein synthesis during fasting, notably at the level of the phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway, transcription and translation processes, and the response to oxidative stress. Such mechanisms might promote protein sparing during P2 and prepare the restoration of the protein compartment during P3 in anticipation of food intake for optimizing the effects of an upcoming refeeding, thereby promoting body maintenance and survival. Future studies should examine relevance of such targets for improving nitrogen balance during catabolic diseases.

Serum Uric Acid Is Positively Associated with Muscle Mass and Strength, but Not with Functional Capacity, in Kidney Transplant Patients

Background: Our aim was to associate serum uric acid (UA) with muscle mass, strength and functional capacity in kidney transplant patients (KTPs). Methods: A cross-sectional study was performed on 113 KTPs. The fat-free mass and total and appendicular muscle mass were estimated by performing a bioelectrical impedance analysis. The strength was evaluated using the handgrip strength test (HGS) and the five times sit to stand test (5STS). The functional capacity was evaluated using the 4 m walk test and the short physical performance battery (SPPB). Results: Linear regression showed that the UA levels were positively associated with the muscle mass, fat-free mass, appendicular muscle mass, muscle mass index and appendicular muscle mass index. The 5STS results (seconds) were inversely associated with the UA levels, showing that individuals with higher UA were more likely to have more strength. However, UA was not associated with the HGS, 4 m walk test and SPPB results. Conclusion: UA levels were positively associated with muscle mass and strength, but not with functional capacity, in KTPs.

Effects of astaxanthin on the protection of muscle health (Review)

Sarcopenia refers to the involuntary and generalized deterioration of skeletal muscle mass and strength, which may lead to falls, frailty, physical disability, loss of independence, morbidity and mortality. The majority of molecular and cellular changes involved in the degeneration of muscle tissues are mediated by oxidative stress. Therefore, astaxanthin may act as a potential adjunct therapy for sarcopenia owing to its antioxidant activity. The present review examines the effects of astaxanthin on the promotion of skeletal muscle performance and prevention of muscle atrophy and the potential mechanisms underlying these effects. The available evidence till date was retrieved from PubMed and Medline electronic databases. The present review reported the beneficial effects of astaxanthin in preventing muscle degeneration in various animal models of sarcopenia. In humans, the effects of astaxanthin in combination with other antioxidants on muscle health are mixed, wherein positive and negligible effects were reported. Mechanistic studies revealed that astaxanthin promotes muscle health by reducing oxidative stress, myoblast apoptosis and proteolytic pathways while promoting mitochondria regeneration and formation of blood vessels. Thus, astaxanthin is a potential therapeutic agent for sarcopenia but its effects in humans require further validation.

Cardiotoxin-induced skeletal muscle injury elicits profound changes in anabolic and stress signaling, and muscle fiber type composition

To improve muscle healing upon injury, it is of importance to understand the interplay of key signaling pathways during muscle regeneration. To study this, mice were injected with cardiotoxin (CTX) or PBS in the Tibialis Anterior muscle and were sacrificed 2, 5 and 12 days upon injection. The time points represent different phases of the regeneration process, i.e. destruction, repair and remodeling, respectively. Two days upon CTX-injection, p-mTORC1 signaling and stress markers such as BiP and p-ERK1/2 were upregulated. Phospho-ERK1/2 and p-mTORC1 peaked at d5, while BiP expression decreased towards PBS levels. Phospho-FOXO decreased 2 and 5 days following CTX-injection, indicative of an increase in catabolic signaling. Furthermore, CTX-injection induced a shift in the fiber type composition, characterized by an initial loss in type IIa fibers at d2 and at d5. At d5, new type IIb fibers appeared, whereas type IIa fibers were recovered at d12. To conclude, CTX-injection severely affected key modulators of muscle metabolism and histology. These data provide useful information for the development of strategies that aim to improve muscle molecular signaling and thereby recovery.

Signaling Pathways That Control Muscle Mass

The loss of skeletal muscle mass under a wide range of acute and chronic maladies is associated with poor prognosis, reduced quality of life, and increased mortality. Decades of research indicate the importance of skeletal muscle for whole body metabolism, glucose homeostasis, as well as overall health and wellbeing. This tissue’s remarkable ability to rapidly and effectively adapt to changing environmental cues is a double-edged sword. Physiological adaptations that are beneficial throughout life become maladaptive during atrophic conditions. The atrophic program can be activated by mechanical, oxidative, and energetic distress, and is influenced by the availability of nutrients, growth factors, and cytokines. Largely governed by a transcription-dependent mechanism, this program impinges on multiple protein networks including various organelles as well as biosynthetic and quality control systems. Although modulating muscle function to prevent and treat disease is an enticing concept that has intrigued research teams for decades, a lack of thorough understanding of the molecular mechanisms and signaling pathways that control muscle mass, in addition to poor transferability of findings from rodents to humans, has obstructed efforts to develop effective treatments. Here, we review the progress made in unraveling the molecular mechanisms responsible for the regulation of muscle mass, as this continues to be an intensive area of research.

The Effects of Cold Water Immersion and Active Recovery on Molecular Factors That Regulate Growth and Remodeling of Skeletal Muscle After Resistance Exercise

Regular postexercise cooling attenuates muscle hypertrophy, yet its effects on the key molecular factors that regulate muscle growth and remodeling are not well characterized. In the present study, nine men completed two sessions of single-leg resistance exercise on separate days. On 1 day, they sat in cold water (10°C) up to their waist for 10 min after exercise. On the other day, they exercised at a low intensity for 10 min after exercise. Muscle biopsies were collected from the exercised leg before, 2, 24, and 48 h after exercise in both trials. These muscle samples were analyzed to evaluate changes in genes and proteins involved in muscle growth and remodeling. Muscle-specific RING finger 1 mRNA increased at 2 h after both trials (P < 0.05), while insulin-like growth factor (IGF)-1 Ec, IGF-1 receptor, growth arrest and DNA damage-inducible protein 45, collagen type I alpha chain A, collagen type III alpha chain 1, laminin and tissue inhibitor of metallopeptidase 1 mRNA increased 24−48 h after both trials (P < 0.05). By contrast, atrogin-1 mRNA decreased at all time points after both trials (P < 0.05). Protein expression of tenascin C increased 2 h after the active recovery trial (P < 0.05), whereas FoxO3a protein expression decreased after both trials (P < 0.05). Myostatin mRNA and ubiquitin protein expression did not change after either trial. These responses were not significantly different between the trials. The present findings suggest that regular cold water immersion attenuates muscle hypertrophy independently of changes in factors that regulate myogenesis, proteolysis and extracellular matrix remodeling in muscle after exercise.

Yin Yang 1 is required for PHD finger protein 20-mediated myogenic differentiation in vitro and in vivo

The development of skeletal muscle requires progression of a highly ordered cascade of events comprising myogenic lineage commitment, myoblast proliferation, and terminal differentiation. The process of myogenesis is controlled by several myogenic transcription factors that act as terminal effectors of signaling cascades and produce appropriate developmental stage-specific transcripts. PHD finger protein 20 (PHF20) is a multidomain protein and subunit of a lysine acetyltransferase complex that acetylates histone H4 and p53, but its function is unclear. Notably, it has been reported that PHF20 knockout mice die shortly after birth and display a wide variety of phenotypes within the skeletal and hematopoietic systems. Therefore, the putative role of PHF20 in myogenic differentiation was further investigated. In the present study, we found that protein and mRNA expression levels of PHF20 were decreased during myogenic differentiation in C₂C₁₂ cells. At the same time, Yin Yang 1 (YY1) was also decreased during myogenic differentiation. PHF20 overexpression increased YY1 expression during myogenic differentiation, together with a delay in MyoD expression. PHF20 expression enhanced the transcriptional activity of YY1 while shRNA-mediated depletion of PHF20 resulted in the reduction of YY1 promoter activity in C₂C₁₂ cells. In addition, PHF20 directly bounds to the YY1 promoter in C₂C₁₂ cells. In a similar manner, YY1 expression was elevated while myosin heavy chain expression was decreased in PHF20 transgenic (TG) mice. Histological analysis revealed abnormalities in the shape and length of muscles in PHF20-TG mice. Furthermore, PHF20-TG muscles slowly regenerated after cardiotoxin injection, indicating that PHF20 affected muscle differentiation and regeneration after injury in vivo. Taken together, these results suggested that PHF20 plays an important role in myogenic differentiation by regulating YY1.

Autophagy Functions to Prevent Methylglyoxal-Induced Apoptosis in HK-2 Cells

Methylglyoxal (MGO), a reactive carbonyl species, causes cellular damage and is closely related to kidney disease, particularly diabetic nephropathy. Although MGO has been reported to induce autophagy and apoptosis, the relationships between the two pathways are unclear. Here, we evaluated whether autophagy may be the underlying mechanism inhibiting MGO-induced apoptosis. MGO treatment induced concentration- and time-dependent apoptosis in HK-2 cells. Moreover, MGO upregulated the autophagy markers p62 and LC3-II. Apoptosis caused by MGO was increased in ATG5-knockdown cells compared to that in wild-type cells. In contrast, autophagy activation by 5-aminoimidazole-4-carboxamide ribonucleotide resulted in reduced apoptosis, suggesting that autophagy played a role in protecting against MGO-induced cell death. To examine the mechanisms through which autophagy occurred following MGO stimulation, we investigated changes in AKT/mammalian target of rapamycin (mTOR) signaling. Autophagy induction by MGO treatment was not related to AKT/mTOR signaling; however, it did involve autophagy-related gene expression promoted by AMP-activated protein kinase-mediated transcription factors, such as forkhead box 1. Overall, our findings indicate that MGO-induced cellular damage can be mitigated by autophagy, suggesting that autophagy may be a potential therapeutic target for diseases such as diabetic nephropathy.

Aging alters acetylation status in skeletal and cardiac muscles

During aging, organs such as skeletal muscle and heart require sufficient NAD+ both as a coenzyme for oxidative-reductive electron transfer and as a substrate for multiple signaling pathways. Sirtuins (SIRTs), a family of NAD+-dependent deacetylase, play an important role in regulating mitochondrial homeostasis and antioxidant defense by deacetylating transcription factors and enzymes such as PGC-1α, p65, GCN5, and SOD2. However, age-related DNA damage and increased SASP activate PARP-1 and CD38, the enzymes competing with SIRTs for NAD+. Thus, it is important to know how aging alters intracellular NAD+ status and NAD+-depending enzyme expression in muscles. In this study, we report that the acetylation level of muscle protein pool, as well as major SIRTs target proteins (PGC-1α, GCN5, p65, and SOD2), was significantly increased in hindlimb and cardiac muscles of 24-month old mice compared with their 6-month old counterparts, despite the fact that most members of the SIRT family were upregulated with aging. Aging increased the protein content of PARP-1 and CD38, whereas decreased NAD+ levels in both skeletal and heart muscles. Aged muscles demonstrated clear signs of mitochondrial dysfunction, oxidative stress, and inflammation. Taken together, our data suggest that despite the upregulation of SIRTs, aged muscles suffered from NAD+ deficit partly due to the competition of elevated CD38 and PARP-1. The enhanced acetylation of several key proteins involved in broad cellular functions may contribute to the age-related muscle deterioration.

A Human Skeletal Muscle Atlas Identifies the Trajectories of Stem and Progenitor Cells across Development and from Human Pluripotent Stem Cells

The developmental trajectory of human skeletal myogenesis and the transition between progenitor and stem cell states are unclear. We used single-cell RNA sequencing to profile human skeletal muscle tissues from embryonic, fetal, and postnatal stages. In silico, we identified myogenic as well as other cell types and constructed a "roadmap" of human skeletal muscle ontogeny across development. In a similar fashion, we also profiled the heterogeneous cell cultures generated from multiple human pluripotent stem cell (hPSC) myogenic differentiation protocols and mapped hPSC-derived myogenic progenitors to an embryonic-to-fetal transition period. We found differentially enriched biological processes and discovered co-regulated gene networks and transcription factors present at distinct myogenic stages. This work serves as a resource for advancing our knowledge of human myogenesis. It also provides a tool for a better understanding of hPSC-derived myogenic progenitors for translational applications in skeletal muscle-based regenerative medicine.

Muscle Disuse Atrophy Caused by Discord of Intracellular Signaling

Significance: Regular contractile activity plays a critical role in maintaining skeletal muscle morphological integrity and physiological function. If the muscle is forced to stop contraction, such as during limb immobilization (IM), the IGF/Akt/mTOR signaling pathway that normally stimulates protein synthesis and inhibits proteolysis will be suppressed, whereas the FoxO-controlled catabolic pathways such as ubiquitin-proteolysis and autophagy/mitophagy will be activated and dominate, resulting in muscle fiber atrophy. Recent Advances: Mitochondria occupy a central position in the regulation of both protein synthesis and degradation through several redox-sensitive pathways, including peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), mitochondrial fusion and fission proteins, mitophagy, and sirtuins. Prolonged IM downregulates PGC-1α due to AMPK (5'-AMP-activated protein kinase) and FoxO activation, thus decreasing mitochondrial biogenesis and causing oxidative damage. Decrease of mitochondrial inner membrane potential and increase of mitochondrial fission can trigger cascades of mitophagy leading to loss of mitochondrial homeostasis (mitostasis), inflammation, and apoptosis. The phenotypic outcomes of these disorders are compromised muscle function and fiber atrophy. Critical Issues: Given the molecular mechanism of the pathogenesis, it is imperative that the integrity of intracellular signaling be restored to prevent the deterioration. So far, overexpression of PGC-1α via transgene and in vivo DNA transfection has been found to be effective in ameliorating mitostasis and reduces IM-induced muscle atrophy. Nutritional supplementation of select amino acids and phytochemicals also provides mechanistic and practical insights into the prevention of muscle disuse atrophy. Future Directions: In light of the importance of mitochondria in regulating the various critical signaling pathways, future work should focus on exploring new epigenetic strategies to restore mitostasis and redox balance.

Prdx6 Plays a Main Role in the Crosstalk Between Aging and Metabolic Sarcopenia

With the increase in average life expectancy, several individuals are affected by age-associated non-communicable chronic diseases (NCDs). The presence of NCDs, such as type 2 diabetes mellitus (T2DM), leads to the reduction in skeletal muscle mass, a pathological condition defined as sarcopenia. A key factor linking sarcopenia with cellular senescence and diabetes mellitus (DM) is oxidative stress. We previously reported as the absence of Peroxiredoxin 6 (Prdx6), an antioxidant enzyme implicated in maintaining intracellular redox homeostasis, induces an early-stage of T2DM. In the present study we sought to understand the role of Prdx6 in the crosstalk between aging and diabetic sarcopenia, by using Prdx6 knockout (Prdx6-/-) mice. Absence of Prdx6 reduced telomeres length and Sirtuin1 (SIRT1) nuclear localization. An increase in Sa-β-Gal activity and p53-p21 pro-aging pathway were also evident. An impairment in IGF-1 (Insulin-like Groth Factor-1)/Akt-1/mTOR pathway leading to a relative increase in Forkhead Box O1 (FOXO1) nuclear localization and in a decrease of muscle differentiation as per lower levels of myoblast determination protein 1 (MyoD) was observed. Muscle atrophy was also present in Prdx6-/- mice by the increase in Muscle RING finger 1 (MuRF1) levels and proteins ubiquitination associated to a reduction in muscle strength. The present study, innovatively, highlights a fundamental role of Prdx6, in the crosstalk between aging, sarcopenia, and DM.

Potential Biomarkers for Feed Efficiency-Related Traits in Nelore Cattle Identified by Co-expression Network and Integrative Genomics Analyses

Feed efficiency helps to reduce environmental impacts from livestock production, improving beef cattle profitability. We identified potential biomarkers (hub genes) for feed efficiency, by applying co-expression analysis in Longissimus thoracis RNA-Seq data from 180 Nelore steers. Six co-expression modules were associated with six feed efficiency-related traits (p-value ≤ 0.05). Within these modules, 391 hub genes were enriched for pathways as protein synthesis, muscle growth, and immune response. Trait-associated transcription factors (TFs) ELF1, ELK3, ETS1, FLI1, and TCF4, were identified with binding sites in at least one hub gene. Gene expression of CCDC80, FBLN5, SERPINF1, and OGN was associated with multiple feed efficiency-related traits (FDR ≤ 0.05) and were previously related to glucose homeostasis, oxidative stress, fat mass, and osteoblastogenesis, respectively. Potential regulatory elements were identified, integrating the hub genes with previous studies from our research group, such as the putative cis-regulatory elements (eQTLs) inferred as affecting the PCDH18 and SPARCL1 hub genes related to immune system and adipogenesis, respectively. Therefore, our analyses contribute to a better understanding of the biological mechanisms underlying feed efficiency in bovine and the hub genes disclosed can be used as biomarkers for feed efficiency-related traits in Nelore cattle.

Triptolide induces atrophy of myotubes by triggering IRS-1 degradation and activating the FoxO3 pathway

Triptolide is an active ingredient isolated from an ancient Chinese herb (Tripterygium wilfordii Hook. f) for inflammatory and immune disorders. It has been shown to inhibit the proliferation of skeletal muscle; however, mechanisms of this effect remain unclear. We used mouse C2C12 myotubes as an in vitro model to investigate the effects of triptolide on skeletal muscle. Triptolide markedly inhibited the expression of myosin heavy chain and upregulated the expression of muscle atrophy-related proteins, leading to atrophy of the myotubes. Triptolide dose-dependently decreased the phosphorylation of Forkhead box O3 (FoxO3) and activated FoxO3 transcription activity, which regulates the expression of muscle atrophy-related proteins. Furthermore, triptolide inhibited the phosphorylation of Akt on the site of S473 and T308, and decreased the phosphorylation of insulin receptor substrate-1 (IRS-1) on the site of S302. In addition, triptolide reduced the protein level, but not mRNA level of IRS-1, whereas other upstream regulators of the Akt signaling pathway were not affected. Finally, a time-course experiment showed that the triptolide-induced degradation of IRS-1 in myotubes occurred 12 h prior to both inhibition of Akt activity and the activation of FoxO3. These data indicate that triptolide triggers IRS-1 degradation to promote FoxO3 activation, which subsequently led to atrophy of myotubes, providing us a potential target to prevent triptolide-induced skeletal muscle atrophy.

The glucocorticoid-induced leucine zipper mediates statin-induced muscle damage

Statins, the most prescribed class of drugs for the treatment of hypercholesterolemia, can cause muscle-related adverse effects. It has been shown that the glucocorticoid-induced leucine zipper (GILZ) plays a key role in the anti-myogenic action of dexamethasone. In the present study, we aimed to evaluate the role of GILZ in statin-induced myopathy. Statins induced GILZ expression in C2C12 cells, primary murine myoblasts/myotubes, primary human myoblasts, and in vivo in zebrafish embryos and human quadriceps femoris muscle. Gilz induction was mediated by FOXO3 activation and binding to the Gilz promoter, and could be reversed by the addition of geranylgeranyl, but not farnesyl, pyrophosphate. Atorvastatin decreased Akt phosphorylation and increased cleaved caspase-3 levels in myoblasts. This effect was reversed in myoblasts from GILZ knockout mice. Similarly, myofibers isolated from knockout animals were more resistant toward statin-induced cell death than their wild-type counterparts. Statins also impaired myoblast differentiation, and this effect was accompanied by GILZ induction. The in vivo relevance of our findings was supported by the observation that gilz overexpression in zebrafish embryos led to impaired embryonic muscle development. Taken together, our data point toward GILZ as an essential mediator of the molecular mechanisms leading to statin-induced muscle damage.

FOXO1 inhibition prevents renal ischemia-reperfusion injury via cAMP-response element binding protein/PPAR-γ coactivator-1α-mediated mitochondrial biogenesis

BACKGROUND AND PURPOSE

Growing evidence indicates targeting mitochondrial dynamics and biogenesis could accelerate recovery from renal ischemia-reperfusion (I/R) injury, but the underlying mechanisms remain elusive. Transcription factor forkhead box O1 (FOXO1) is a key regulator of mitochondrial homeostasis and plays a pathological role in the progression of renal disease.

EXPERIMENTAL APPROACH

A mouse model of renal I/R injury and a hypoxia/reoxygenation (H/R) injury model for human renal tubular epithelial cells were used.

KEY RESULTS

I/R injury up-regulated renal expression of FOXO1 and treatment with FOXO1-selective inhibitor AS1842856 prior to I/R injury decreased serum urea nitrogen, serum creatinine and the tubular damage score after injury. Post-I/R injury AS1842856 treatment could also ameliorate renal function and improve the survival rate of mice following injury. AS1842856 administration reduced mitochondrial-mediated apoptosis, suppressed the overproduction of mitochondrial ROS and accelerated recovery of ATP both in vivo and in vitro. Additionally, FOXO1 inhibition improved mitochondrial biogenesis and suppressed mitophagy. Expression of PPAR-γ coactivator 1α (PGC-1α), a master regulator of mitochondrial biogenesis, was down-regulated in both I/R and H/R injury, which could be abrogated by FOXO1 inhibition. Experiments using integrated bioinformatics analysis and coimmunoprecipitation established that FOXO1 inhibited PGC-1α transcription by competing with cAMP-response element binding protein (CREB) for its binding to transcriptional coactivators CREBBP/EP300 (CBP/P300).

CONCLUSION AND IMPLICATIONS

These findings suggested that FOXO1 was critical to maintain mitochondrial function in renal tubular epithelial cells and FOXO1 may serve as a therapeutic target for pharmacological intervention in renal I/R injury.

Cellular mechanism of immobilization-induced muscle atrophy: A mini review

It is well-established that regular contraction maintains morphological and functional integrity of skeletal muscle, whereas rigorous exercise training can upregulate muscle metabolic and contractile function. However, when muscles stop contraction, such as during immobilization (IM) and denervation, withdrawal of IGF/Akt/mTOR signaling allows FoxO-controlled protein degradation pathways to dominate. Mitochondria play an important role in regulating both protein synthesis and degradation via several redox sensitive signaling pathways such as mitochondrial biogenesis, fusion and fission dynamics, ubiquitin-proteolysis, autophagy/mitophagy, and apoptosis. During prolonged IM, downregulation of PGC-1α and increased mitochondrial oxidative damage facilitate fission protein and inflammatory cytokine production and activate mitophagic process, leading to a vicious cycle of protein degradation. This “mitostasis theory of muscle atrophy” is the opposite pathway of hormesis, which defines enhanced muscle function with contractile overload. The demonstration that PGC-1α overexpression via transgene or in vivo DNA transfection can successfully restore mitochondrial homeostasis and reverse myocyte atrophy supports such a proposition. Understanding the mechanism governing mitostasis can be instrumental to the treatment of muscle atrophy associated with bedrest, cancer cachexia and sarcopenia.

Exercise Mitigates the Loss of Muscle Mass by Attenuating the Activation of Autophagy during Severe Energy Deficit

The loss of skeletal muscle mass with energy deficit is thought to be due to protein breakdown by the autophagy-lysosome and the ubiquitin-proteasome systems. We studied the main signaling pathways through which exercise can attenuate the loss of muscle mass during severe energy deficit (5500 kcal/day). Overweight men followed four days of caloric restriction (3.2 kcal/kg body weight day) and prolonged exercise (45 min of one-arm cranking and 8 h walking/day), and three days of control diet and restricted exercise, with an intra-subject design including biopsies from muscles submitted to distinct exercise volumes. Gene expression and signaling data indicate that the main catabolic pathway activated during severe energy deficit in skeletal muscle is the autophagy-lysosome pathway, without apparent activation of the ubiquitin-proteasome pathway. Markers of autophagy induction and flux were reduced by exercise primarily in the muscle submitted to an exceptional exercise volume. Changes in signaling are associated with those in circulating cortisol, testosterone, cortisol/testosterone ratio, insulin, BCAA, and leucine. We conclude that exercise mitigates the loss of muscle mass by attenuating autophagy activation, blunting the phosphorylation of AMPK/ULK1/Beclin1, and leading to p62/SQSTM1 accumulation. This includes the possibility of inhibiting autophagy as a mechanism to counteract muscle loss in humans under severe energy deficit.

Age-dependent effects of caloric restriction on mTOR and ubiquitin-proteasome pathways in skeletal muscles

In skeletal muscles, calorie restriction (CR) preserves muscle mass in middle-aged rats but not younger rats. The underlying mechanisms for this age-specific response are unknown. Skeletal muscle mass depends on several factors, with protein synthesis and degradation playing major roles. Therefore, the purpose of this study was to investigate whether CR affects younger and older animals differently on mTOR signaling and ubiquitin-proteasome pathway (UPP). Four-, 8-, and 16-month-old rats, with or without 40% CR for a duration of 14 weeks, were sacrificed after an overnight fasting. Total protein content and the phosphorylation level of AKT, mTOR, S6K, and 4EBP1 and protein content of key markers in the UPP (FOXO3a, atrogin, MuRF1, ubiquitinated proteins, proteasome subunits alpha 7 and beta 5) were determined. Unlike younger rats, CR decreased the content of phosphorylated mTOR, S6K, phosphorylated S6K, FOXO3a, and ubiquitinated proteins in middle-aged rats. In conclusion, CR-induced reduction of content/ phosphorylation levels of key proteins in mTOR signaling and the UPP occurred in the middle-aged rats but not younger rats. The age-dependent effects of CR on mTOR signaling and the UPP indirectly explained the age-related effects of CR on muscle mass of animals.

Dysregulation of 1-carbon metabolism and muscle atrophy: potential roles of forkhead box O proteins and PPARγ co-activator-1α

Homocysteine, a non-proteinogenic amino acid but an important metabolic intermediate is generated as an integral component for the “1-carbon metabolism” during normal physiology. It is catabolized to cysteine via the transulfuration pathway resulting in the generation of hydrogen sulfide, a naturally endogenous byproduct. Genetics or metabolic derangement can alter homocysteine concentration leading to hyperhomocysteinemia (HHcy), a physiologically unfavorable condition that causes serious medical conditions including muscle wasting. HHcy environment can derail physiological processes by targeting biomolecules such as Akt; however, not much is known regarding the effects of HHcy on regulation of transcription factors such as forkhead box O (FOXO) proteins. Recently, hydrogen sulfide has been shown to be highly effective in alleviating the effects of HHcy by serving as an antiapoptotic factor, but role of FOXO and its interaction with hydrogen sulfide are yet to be established. In this review, we discuss role(s) of HHcy in skeletal muscle atrophy and how HHcy interact with FOXO and peroxisome proliferator-activated receptor gamma coactivator 1-alpha expressions that are relevant in musculoskeletal atrophy. Further, therapeutic intervention with hydrogen sulfide for harnessing its beneficial effects might help mitigate the dysregulated 1-carbon metabolism that happens to be the hallmark of HHcy-induced pathologies such as muscle atrophy.

Cold water immersion attenuates anabolic signaling and skeletal muscle fiber hypertrophy, but not strength gain, following whole-body resistance training

We determined the effects of cold water immersion (CWI) on long-term adaptations and post-exercise molecular responses in skeletal muscle before and after resistance training. Sixteen men (22.9 ± 4.6 y; 85.1 ± 17.9 kg; mean ± SD) performed resistance training (3 day/wk) for 7 wk, with each session followed by either CWI [15 min at 10°C, CWI (COLD) group, n = 8] or passive recovery (15 min at 23°C, control group, n = 8). Exercise performance [one-repetition maximum (1-RM) leg press and bench press, countermovement jump, squat jump, and ballistic push-up], body composition (dual X-ray absorptiometry), and post-exercise (i.e., +1 and +48 h) molecular responses were assessed before and after training. Improvements in 1-RM leg press were similar between groups [130 ± 69 kg, pooled effect size (ES): 1.53 ± 90% confidence interval (CI) 0.49], whereas increases in type II muscle fiber cross-sectional area were attenuated with CWI (-1,959 ± 1,675 µM² ; ES: -1.37 ± 0.99). Post-exercise mechanistic target of rapamycin complex 1 signaling (rps6 phosphorylation) was blunted for COLD at post-training (POST) +1 h (-0.4-fold, ES: -0.69 ± 0.86) and POST +48 h (-0.2-fold, ES: -1.33 ± 0.82), whereas basal protein degradation markers (FOX-O1 protein content) were increased (1.3-fold, ES: 2.17 ± 2.22). Training-induced increases in heat shock protein (HSP) 27 protein content were attenuated for COLD (-0.8-fold, ES: -0.94 ± 0.82), which also reduced total HSP72 protein content (-0.7-fold, ES: -0.79 ± 0.57). CWI blunted resistance training-induced muscle fiber hypertrophy, but not maximal strength, potentially via reduced skeletal muscle protein anabolism and increased catabolism. Post-exercise CWI should therefore be avoided if muscle hypertrophy is desired.NEW & NOTEWORTHY This study adds to existing evidence that post-exercise cold water immersion attenuates muscle fiber growth with resistance training, which is potentially mediated by attenuated post-exercise increases in markers of skeletal muscle anabolism coupled with increased catabolism and suggests that blunted muscle fiber growth with cold water immersion does not necessarily translate to impaired strength development.

Mitochondrial dysregulation and muscle disuse atrophy

It is well established that mitochondria play a critical role in the metabolic and physiological adaptation of skeletal muscle to enhanced contractile activity. Several redox-sensitive signaling pathways such as PGC-1α, AMPK, IGF/Akt/mTOR, SIRT, NFκB, and FoxO are involved with extensive crosstalk to regulate vital cellular functions such as mitochondrial biogenesis, mitochondrial fusion and fission dynamics, autophagy/mitophagy, and apoptosis under altered demand and stress. However, when muscles cease contraction, such as during immobilization and denervation, mitochondria undergo a series of detrimental changes characterized by downregulation of PGC-1α and antioxidant defense, increased ROS generation, activated FoxO, NFκB, and inflammation, enhanced ubiquitination, and finally mitophagy and apoptotic cascades. The phenotypic outcome of the discord of mitochondrial homeostasis is elevated proteolysis and muscle atrophy. The demonstration that PGC-1α overexpression via transgene or in vivo DNA transfection can restore mitochondrial homeostasis and reverse myocyte atrophy supports the "mitostasis theory of muscle atrophy".

The FoxO-Autophagy Axis in Health and Disease

Autophagy controls cellular remodeling and quality control. Dysregulated autophagy has been implicated in several human diseases including obesity, diabetes, cardiovascular disease, neurodegenerative diseases, and cancer. Current evidence has revealed that FoxO (forkhead box class O) transcription factors have a multifaceted role in autophagy regulation and dysregulation. Nuclear FoxOs transactivate genes that control the formation of autophagosomes and their fusion with lysosomes. Independently of transactivation, cytosolic FoxO proteins induce autophagy by directly interacting with autophagy proteins. Autophagy is also controlled by FoxOs through epigenetic mechanisms. Moreover, FoxO proteins can be degraded directly or indirectly by autophagy. Cutting-edge evidence is reviewed that the FoxO-autophagy axis plays a crucial role in health and disease.

Beta-hydroxy beta-methyl butyrate decreases muscle protein degradation via increased Akt/FoxO3a signaling and mitochondrial biogenesis in weanling piglets after lipopolysaccharide challenge

The aim of this study was to investigate the effects of dietary β-hydroxy-β-methylbutyrate (HMB) on lipopolysaccharide (LPS)-induced muscle atrophy and to investigate the mechanisms involved. Sixty pigs (21 ± 2 days old, 5.86 ± 0.18 kg body weight) were used in a 2 × 3 factorial design and the main factors included diet (0, 0.60%, or 1.20% HMB) and immunological challenge (LPS or saline). After 15 d of treatment with LPS and/or HMB, growth performance, blood parameters, and muscle protein degradation rate were measured. The results showed that in LPS-injected pigs, 0.60% HMB supplementation increased the average daily gain and average daily feed intake and decreased the feed : gain ratio (P < 0.05), with a concurrent increase of lean percentage. Moreover, 0.60% HMB supplementation decreased the serum concentrations of blood urea nitrogen, IL-1β, and TNF-α and the rate of protein degradation as well as cell apoptosis in selected muscles (P < 0.05). In addition, dietary HMB supplementation (0.60%) regulated the expression of genes involved in mitochondrial biogenesis and increased the phosphorylation of Akt and Forkhead Box O3a (FoxO3a) in selected muscles, accompanied by decreased protein expression of muscle RING finger 1 and muscle atrophy F-box. These results indicate that HMB may exert protective effects against LPS-induced muscle atrophy by normalizing the Akt/FoxO3a axis that regulates ubiquitin proteolysis and by improving mitochondrial biogenesis.

Myostatin Increases Smad2 Phosphorylation and Atrogin-1 Expression in Chick Embryonic Myotubes

Skeletal muscle mass is an important trait in poultry meat production. In mammals, myostatin, a negative regulator of skeletal muscle growth, activates Smad transcription factors and induces the expression of atrogin-1 by regulating the Akt/FOXO pathway. Although the amino acid sequence of chicken myostatin is known to be completely identical to its mammalian counterpart, previous studies in chicken skeletal muscles have implied that the physiological roles of chicken myostatin are different from those of mammals. Furthermore, it remains to be elucidated whether myostatin affects cellular signaling factors and atrogin-1 expression. In this study, using chick embryonic myotubes, we found that myostatin significantly increased the phosphorylation rate of Smad2 and mRNA levels of atrogin-1. No significant change was observed in the phosphorylation of Akt and FOXO1. These in vitro results suggest that the molecular mechanisms underlying myostatin-induced expression of atrogin-1 might be different between chickens and mammals.

Skeletal muscle atrogenes: From rodent models to human pathologies

Skeletal muscle atrophy is a common side effect of most human diseases. Muscle loss is not only detrimental for the quality of life but it also dramatically impairs physiological processes of the organism and decreases the efficiency of medical treatments. While hypothesized for years, the existence of an atrophying programme common to all pathologies is still incompletely solved despite the discovery of several actors and key regulators of muscle atrophy. More than a decade ago, the discovery of a set of genes, whose expression at the mRNA levels were similarly altered in different catabolic situations, opened the way of a new concept: the presence of atrogenes, i.e. atrophy-related genes. Importantly, the atrogenes are referred as such on the basis of their mRNA content in atrophying muscles, the regulation at the protein level being sometimes more complicate to elucidate. It should be noticed that the atrogenes are markers of atrophy and that their implication as active inducers of atrophy is still an open question for most of them. While the atrogene family has grown over the years, it has mostly been incremented based on data coming from rodent models. Whether the rodent atrogenes are valid for humans still remain to be established. An "atrogene" was originally defined as a gene systematically up- or down-regulated in several catabolic situations. Even if recent works often restrict this notion to the up-regulation of a limited number of proteolytic enzymes, it is important to keep in mind the big picture view. In this review, we provide an update of the validated and potential rodent atrogenes and the metabolic pathways they belong, and based on recent work, their relevance in human physio-pathological situations. We also propose a more precise definition of the atrogenes that integrates rapid recovery when catabolic stimuli are stopped or replaced by anabolic ones.

Effects of acute resistance exercise on proteolytic and myogenic markers in skeletal muscles of former weightlifters and age-matched sedentary controls

BACKGROUND

Former athletes who continue a regular, performance-oriented training throughout life provide a unique model for studying successful aging. With this in mind, the current study aimed to compare the effects of an acute resistance exercise on proteolytic and myogenic markers in older weightlifters and untrained participants.

METHODS

Sixteen older men (8 former weightlifters, 8 age-matched untrained controls) with an age of 61.2±8.2 years volunteered to participate in the study. Two days after assessing 1-RM, an acute exercise protocol (3 sets, 70-75% of one-repetition maximum until voluntary fatigue) was applied unilaterally on the dominant leg while the other leg served as control. Three hours after termination of the exercise, skeletal muscle tissue was obtained from m. vastus lateralis of both legs.

RESULTS

Acute resistance exercise led to an up-regulation (>1.5-fold) of 14 genes in controls and of 13 genes in weightlifters. The transcription factors FOS and early growth response 1 (EGR1), as well as the E3 protein ligase TRIM63 comprised the most responsive genes to resistance exercise (EGR1:15.7-fold increase, P=0.003, FOS: 36.3-fold increase, P<0.001; TRIM63: 2.9-fold increase, P<0.001). In addition, myostatin levels were decreased in the exercised leg (0.6-fold, P<0.001). FOXO3 gene expression was significantly higher in weightlifters than in untrained controls (1.5-fold, P=0.042).

CONCLUSIONS

Trained and untrained older adults respond to an acute bout of resistance exercise in a very similar way irrespective of training status, although some differences exist in FOXO3, potentially reflecting the superior capacity of trained persons in regulating cellular homeostasis.

Circular RNA circ-FoxO3 Inhibits Myoblast Cells Differentiation

CircRNA is a type of closed circular non-coding RNA formed by reverse splicing and plays an important role in regulating the growth and development of plants and animals. To investigate the function of circ-FoxO3 in mouse myoblast cells' (C2C12) differentiation and proliferation, we used RT-qPCR to detect the expression level of circ-FoxO3 in mouse myoblast cells at different densities and different differentiation stages, and the specific interference fragment was used to inhibit the expression level of circ-FoxO3 in myoblast cells to observe its effect on myoblast cells proliferation and differentiation. We found that the expression level of circ-FoxO3 in myoblast cells increased with the prolongation of myoblast cells differentiation time, and its expression level decreased with the proliferation of myoblast cells. At the same time, we found that the differentiation ability of the cells was significantly increased (p < 0.05), but the cell proliferation was unchanged (p > 0.05) after inhibiting the expression of circ-FoxO3 in myoblast cells. Combining the results of bioinformatics analysis and the dual luciferase reporter experiment, we found that circ-FoxO3 is a sponge of miR-138-5p, which regulates muscle differentiation. Our study shows that circ-FoxO3 can inhibit the differentiation of C2C12 myoblast cells and lay a scientific foundation for further study of skeletal muscle development at circRNA levels.

A novel oleanolic acid derivative HA-19 ameliorates muscle atrophy via promoting protein synthesis and preventing protein degradation

Muscle atrophy refers to a decrease in the size of muscles in the body, occurs in certain muscles with inactivity in many diseases and lacks effective therapies up to date. Natural products still play an important role in drug discovery. In the present study, derivatives of a natural product, oleanolic acid, were screened with myoblast differentiation and myotube atrophy assays, respectively. Results revealed that one of the derivatives, HA-19 showed the most potent anti-muscle atrophy activity, and was used for further studies. We demonstrated that HA-19 led to the increase of the protein synthesis by activating mechanistic target of rapamycin complex 1 (mTORC1)/p70 S6K pathways, and also enhanced myoblast proliferation and terminal differentiation via up-regulating of the myogenic transcription factors Pax7, MyoD and Myogenin. The interesting thing was that HA-19 also suppressed protein degradation to prevent myotube atrophy by down-regulating negative growth factors, FoxO1, MuRF1 and Atrogin-1. The results were also supported by puromycin labelling and protein ubiquitination assays. These data revealed that HA-19 possessed a "dual effect" on inhibition of muscle atrophy. In disuse-induced muscle atrophy mice model, HA-19 treatment significantly increased the weights of bilateral tibialis anterior (TA), gastrocnemius (Gastroc.), quadriceps (Quad.), suggesting the effectiveness of HA-19 to remit disuse-induced muscle atrophy. Our finding demonstrated that HA-19 has a great potential as an inhibitor or lead compound for the anti-muscle atrophy drug discovery.

Recent Data on Cellular Component Turnover: Focus on Adaptations to Physical Exercise

Significant progress has expanded our knowledge of the signaling pathways coordinating muscle protein turnover during various conditions including exercise. In this manuscript, the multiple mechanisms that govern the turnover of cellular components are reviewed, and their overall roles in adaptations to exercise training are discussed. Recent studies have highlighted the central role of the energy sensor (AMP)-activated protein kinase (AMPK), forkhead box class O subfamily protein (FOXO) transcription factors and the kinase mechanistic (or mammalian) target of rapamycin complex (MTOR) in the regulation of autophagy for organelle maintenance during exercise. A new cellular trafficking involving the lysosome was also revealed for full activation of MTOR and protein synthesis during recovery. Other emerging candidates have been found to be relevant in organelle turnover, especially Parkin and the mitochondrial E3 ubiquitin protein ligase (Mul1) pathways for mitochondrial turnover, and the glycerolipids diacylglycerol (DAG) for protein translation and FOXO regulation. Recent experiments with autophagy and mitophagy flux assessment have also provided important insights concerning mitochondrial turnover during ageing and chronic exercise. However, data in humans are often controversial and further investigations are needed to clarify the involvement of autophagy in exercise performed with additional stresses, such as hypoxia, and to understand the influence of exercise modality. Improving our knowledge of these pathways should help develop therapeutic ways to counteract muscle disorders in pathological conditions.

Effect of dietary fish oil intake on ubiquitin ligase expression during muscle atrophy induced by sciatic nerve denervation in mice

Dietary fish oil intake improves muscle atrophy in several atrophy models however the effect on denervation-induced muscle atrophy is not clear. Thus, the aim of this study was to investigate the effects of dietary fish oil intake on muscle atrophy and the expression of muscle atrophy markers induced by sciatic nerve denervation in mice. We performed histological and quantitative mRNA expression analysis of muscle atrophy markers in mice fed with fish oil with sciatic nerve denervation. Histological analysis indicated that dietary fish oil intake slightly prevented the decrease of muscle fiber diameter induced by denervation treatment. In addition, dietary fish oil intake suppressed the MuRF1 (tripartite motif-containing 63) expression up-regulated by denervation treatment, and this was due to decreased tumor necrosis factor-alpha (TNF-α) production in skeletal muscle. We concluded that dietary fish oil intake suppressed MuRF1 expression by decreasing TNF-α production during muscle atrophy induced by sciatic nerve denervation in mice.

Functional wiring of proteostatic and mitostatic modules ensures transient organismal survival during imbalanced mitochondrial dynamics

Being an assembly of protein machines, cells depend on adequate supply of energetic molecules for retaining their homeodynamics. Consequently, mitochondria functionality is ensured by quality control systems and mitochondrial dynamics (fusion/fission). Similarly, proteome stability is maintained by the machineries of the proteostasis network. We report here that reduced mitochondrial fusion rates in Drosophila caused developmental lethality or if induced in the adult accelerated aging. Imbalanced mitochondrial dynamics were tolerable for various periods in young flies, where they caused oxidative stress and proteome instability that mobilized Nrf2 and foxo to upregulate cytoprotective antioxidant/proteostatic modules. Consistently, proteasome inhibition or Nrf2, foxo knock down in young flies exaggerated perturbed mitochondrial dynamics toxicity. Neither Nrf2 overexpression (with concomitant proteasome activation) nor Atg8a upregulation suppressed the deregulated mitochondrial dynamics toxicity, which was mildly mitigated by antioxidants. Thus, despite extensive functional wiring of mitostatic and antioxidant/proteostatic modules, sustained loss-of mitostasis exhausts adaptation responses triggering premature aging.

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Reduced mitochondrial fusion rates cause severe organismal toxicity and progeria.

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Perturbed mitostasis activates cytoprotective antioxidant and proteostatic modules.

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Nrf2 or Foxo KD exaggerates the imbalanced mitochondrial dynamics induced toxicity.

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Antioxidants mildly alleviate loss-of mitochondrial dynamics-mediated progeria.

The secret messages between mitochondria and nucleus in muscle cell biology

Over two thousand proteins are found in the mitochondrial compartment but the mitochondrial genome codes for only 13 proteins. The majority of mitochondrial proteins are products of nuclear genes and are synthesized in the cytosol, then translocated into the mitochondria. Most of the subunits of the five respiratory chain complexes in the inner mitochondrial membrane, which generate a proton gradient across the membrane and produce ATP, are encoded by nuclear genes. Therefore, it is quite clear that import of nuclear-encoded proteins into the mitochondria is essential for mitochondrial function. Nuclear to mitochondrial communication is well studied. However, there is another arm to this communication, mitochondria to nucleus retrograde signaling. This plays an important role in the maintenance of cellular homeostasis, and is less well studied. Several transcription factors, including Sp1, SIRT3 and GSP2, are activated by altered mitochondrial function. These activated transcription factors then translocate to the nucleus. Based on the mitochondrially generated molecular signal, nuclear genes are targeted, which alters transcription of nuclear genes that code for mitochondrial proteins. This review article will mainly focus on this interactive and bi-directional communication between mitochondria and nucleus, and how this communication plays a significant role in muscle cell biology.

LncRNA-SULT1C2A regulates Foxo4 in congenital scoliosis by targeting rno-miR-466c-5p through PI3K-ATK signalling

Congenital scoliosis (CS) is the result of anomalous vertebrae development, but the pathogenesis of CS remains unclear. Long non‐coding RNAs (lncRNAs) have been implicated in embryo development, but their role in CS remains unknown. In this study, we investigated the role and mechanisms of a specific lncRNA, SULT1C2A, in somitogenesis in a rat model of vitamin A deficiency (VAD)‐induced CS. Bioinformatics analysis and quantitative real‐time PCR (qRT‐PCR) indicated that SULT1C2A expression was down‐regulated in VAD group, accompanied by increased expression of rno‐miR‐466c‐5p but decreased expression of Foxo4 and somitogenesis‐related genes such as Pax1, Nkx3‐2 and Sox9 on gestational day (GD) 9. Luciferase reporter and small interfering RNA (siRNA) assays showed that SULT1C2A functioned as a competing endogenous RNA to inhibit rno‐miR‐466c‐5p expression by direct binding, and rno‐miR‐466c‐5p inhibited Foxo4 expression by binding to its 3′ untranslated region (UTR). The spatiotemporal expression of SULT1C2A, rno‐miR‐466c‐5p and Foxo4 axis was dynamically altered on GDs 3, 8, 11, 15 and 21 as detected by qRT‐PCR and northern blot analyses, with parallel changes in Protein kinase B (AKT) phosphorylation and PI3K expression. Taken together, our findings indicate that SULT1C2A enhanced Foxo4 expression by negatively modulating rno‐miR‐466c‐5p expression via the PI3K‐ATK signalling pathway in the rat model of VAD‐CS. Thus, SULT1C2A may be a potential target for treating CS.

Cellular mechanisms promoting cachexia and how they are opposed by sirtuins 1

Many chronic diseases are associated with unintentional loss of body weight, which is termed "cachexia". Cachexia is a complex multifactorial syndrome associated with the underlying primary disease, and characterized by loss of skeletal muscle with or without loss of fat tissue. Patients with cachexia face dire symptoms like dyspnea, fatigue, edema, exercise intolerance, and low responsiveness to medical therapy, which worsen quality of life. Because cachexia is not a stand-alone disorder, treating primary disease - such as cancer - takes precedence for the physician, and it remains mostly a neglected illness. Existing clinical trials have demonstrated limited success mostly because of their monotherapeutic approach and late detection of the syndrome. To conquer cachexia, it is essential to identify as many molecular targets as possible using the latest technologies we have at our disposal. In this review, we have discussed different aspects of cachexia, which include various disease settings, active molecular pathways, and recent novel advances made in this field to understand consequences of this illness. We also discuss roles of the sirtuins, the NAD+-dependent lysine deacetylases, microRNAs, certain dietary options, and epigenetic drugs as potential approaches, which can be used to tackle cachexia as early as possible in its course.

Lactate Overload Inhibits Myogenic Activity in C2C12 Myotubes

Lactate (LA), an endogenous metabolite produced from pyruvate, can accumulate in skeletal muscle in certain conditions including major diseases, as well as during intensive exercise. Using differentiated C2C12 myotubes, we evaluated the early (1-h) and delayed (24-h) effects of LA (8 mM) on mechanisms involved in myogenesis or muscle atrophy, including 5'-adenosine monophosphate-activated protein kinase (AMPK)-mediated inhibition of protein synthesis through the mTOR/P70-S6K pathway, Akt-mediated inhibition of expression of the MAFbx atrophic factor by FOXO3a and expression of the myogenic transcription factors, MyoD, myogenin and myosin heavy chain. Although the early effects of LA overload were not significant on myogenic or atrophic mechanisms, LA treatment for 24 h significantly activated atrophic mechanisms but suppressed myogenesis in myotubes. In addition, LA overload for 24 h significantly suppressed the expression of Sirtuin 1 and peroxisome proliferator-activated receptor gamma coactivator-1 alpha. Consistent with LA-induced activation of atrophic mechanisms, the diameter of C2C12 myotubes treated with LA for 24 h, but not for 1 h, was significantly lower than in control myotubes. Thus, a sustained, but not a transient, LA overload could induce muscle atrophy through the regulation of AMPK- and Akt-mediated pathways, although further in vivo studies are needed to confirm this.

FoxO Transcription Factors Are Critical Regulators of Diabetes-Related Muscle Atrophy

Insulin deficiency and uncontrolled diabetes lead to a catabolic state with decreased muscle strength, contributing to disease-related morbidity. FoxO transcription factors are suppressed by insulin and thus are key mediators of insulin action. To study their role in diabetic muscle wasting, we created mice with muscle-specific triple knockout of FoxO1/3/4 and induced diabetes in these M-FoxO-TKO mice with streptozotocin (STZ). Muscle mass and myofiber area were decreased 20-30% in STZ-Diabetes mice due to increased ubiquitin-proteasome degradation and autophagy alterations, characterized by increased LC3-containing vesicles, and elevated levels of phosphorylated ULK1 and LC3-II. Both the muscle loss and markers of increased degradation/autophagy were completely prevented in STZ FoxO-TKO mice. Transcriptomic analyses revealed FoxO-dependent increases in ubiquitin-mediated proteolysis pathways in STZ-Diabetes, including regulation of Fbxo32 (Atrogin1), Trim63 (MuRF1), Bnip3L, and Gabarapl. These same genes were increased 1.4- to 3.3-fold in muscle from humans with type 1 diabetes after short-term insulin deprivation. Thus, FoxO-regulated genes play a rate-limiting role in increased protein degradation and muscle atrophy in insulin-deficient diabetes.

Protein recycling and limb muscle recovery after critical illness in slow- and fast-twitch limb muscle

An impaired capacity of muscle to regenerate after critical illness results in long-term functional disability. We previously described in a long-term rat peritonitis model that gastrocnemius displays near-normal histology whereas soleus demonstrates a necrotizing phenotype. We thus investigated the link between the necrotizing phenotype of critical illness myopathy and proteasome activity in these two limb muscles. We studied male Wistar rats that underwent an intraperitoneal injection of the fungal cell wall constituent zymosan or n-saline as a sham-treated control. Rats (n = 74) were killed at 2, 7, and 14 days postintervention with gastrocnemius and soleus muscle removed and studied ex vivo. Zymosan-treated animals displayed an initial reduction of body weight but a persistent decrease in mass of both lower hindlimb muscles. Zymosan increased chymotrypsin- and trypsin-like proteasome activities in gastrocnemius at days 2 and 7 but in soleus at day 2 only. Activated caspases-3 and -9, polyubiquitin proteins, and 14-kDa fragments of myofibrillar actin (proteasome substrates) remained persistently increased from day 2 to day 14 in soleus but not in gastrocnemius. These results suggest that a relative proteasome deficiency in soleus is associated with a necrotizing phenotype during long-term critical illness. Rescuing proteasome clearance may offer a potential therapeutic option to prevent long-term functional disability in critically ill patients.

Other Molecular Mechanisms Regulating Autophagy

Autophagy is a catabolic process in eukaryotic cells that delivers cytoplasmic components and organelles to the lysosomes for digestion. It is thought that various environmental signaling pathways are somehow integrated with autophagy signaling, such as the lack of nutrients (amino acids or glucose or others), changes in pH or osmotic pressure. Autophagy has been seen as an adaptive response to stress. In the extreme cases of starvation, the breakdown of cellular components promotes survival by maintaining cellular energy levels. A series of studies have found that the signaling pathway regulating autophagy is very complex. In addition to 40 autophagy-related genes (ATG) involved in the formation of autophagosomes, there are many other transcription factors that participated in the regulation of autophagy. This chapter focuses on the role of FoxO, NFκB, E2F and TFEB in autophagy.

Intensified mitophagy in skeletal muscle with aging is downregulated by PGC-1alpha overexpression in vivo

Mitochondrial dysfunction plays an important role in the etiology of age-related muscle atrophy known as sarcopenia. PGC-1α is positioned at the center of crosstalk in regulating mitochondrial quality control, but its role in mitophagy in aged skeletal muscle is currently unclear. The present study investigated the effects of aging and PGC-1α overexpression via in vivo DNA transfection on key mitophagy protein markers, as well as mitochondrial dynamics related proteins, metabolic function and antioxidant capacity in mouse muscle. C57BL/6J mice at the age of 2 mo (young, Y; N = 14) and 24 mo (old, O; N = 14) were transfected in vivo with either PGC-1α DNA (OE, N = 7) or GFP (N = 7) into the tibialis anterior (TA) muscle followed by electroporation. PINK1 and Parkin protein contents were 3.6 and 1.4-fold higher (P < 0.01), whereas mitochondrial ubiquitination (Ub) increased 1.5-fold (P < 0.05), in O vs. Y mice. PGC-1 OE suppressed PINK and Parkin protein levels by 50-60% (P < 0.01), and decreased Ub by 20% (P < 0.05) in old mice. Aging significantly increased the protein content of LC3II (30%, P < 0.05), p62 (42%, P < 0.05), RheB (5.5-fold, P < 0.01), Beclin-1 (3-fold, P < 0.01) and Mfn2 (~4-fold, P < 0.01) in the TA muscle. However, these age-related increases in mitophagy markers were attenuated by PGC-1α OE. Furthermore, aging dramatically increased Fis-1 protein content by 14-fold (P < 0.01), along with a severe reduction of citrate synthase activity (64%, P < 0.01) and cytochrome c oxidase subunit IV (COXIV) protein content (85%, P < 0.01). PGC-1α OE mitigated the age effects on Fis-1 and Drp-1 (P < 0.05). Moreover, PGC-1α OE enhanced mitochondrial oxidative function and antioxidant enzyme activities, and decreased lipid peroxidation and inner membrane damage found in old mice (P < 0.01). In summary, our data demonstrate that mitophagy protein expression in skeletal muscle was enhanced at old age driven possibly by increased mitochondrial dysfunction, damage, and fission. PGC-1α OE was effective in ameliorating mitochondrial deficits but did not restore muscle fiber atrophy.