Oxidative stress-mediated senescence in mesenchymal progenitor cells causes the loss of their fibro/adipogenic potential and abrogates myoblast fusion

Senolytic treatment does not mitigate oxidative stress-induced muscle atrophy but improves muscle force generation in CuZn superoxide dismutase knockout mice

Oxidative stress is associated with tissue dysfunctions that can lead to reduced health. Prior work has shown that oxidative stress contributes to both muscle atrophy and cellular senescence, which is a hallmark of aging that may drive in muscle atrophy and muscle contractile dysfunction. The purpose of the study was to test the hypothesis that cellular senescence contributes to muscle atrophy or weakness. To increase potential senescence in skeletal muscle, we used a model of oxidative stress-induced muscle frailty, the CuZn superoxide dismutase knockout (Sod1KO) mouse. We treated 6-month-old wildtype (WT) and Sod1KO mice with either vehicle or a senolytic treatment of combined dasatinib (5 mg/kg) + quercetin (50 mg/kg) (D + Q) for 3 consecutive days every 15 days. We continued treatment for 7 months and sacrificed the mice at 13 months of age. Treatment with D + Q did not preserve muscle mass, reduce NMJ fragmentation, or alter muscle protein synthesis in Sod1KO mice when compared to the vehicle-treated group. However, we observed an improvement in muscle-specific force generation in Sod1KO mice treated with D + Q when compared to Sod1KO-vehicle mice. Overall, these data suggest that reducing cellular senescence via D + Q is not sufficient to mitigate loss of muscle mass in a mouse model of oxidative stress-induced muscle frailty but may mitigate some aspects of oxidative stress-induced muscle dysfunction.

A novel biomarker of fibrofatty replacement in dystrophinopathies identified by integrating transcriptome, magnetic resonance imaging, and pathology data

BACKGROUND

We aimed to analyse genome-wide transcriptome differences between Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) patients and identify biomarkers that correlate well with muscle magnetic resonance imaging (MRI) and histological fibrofatty replacement in both patients, which have not been reported.

METHODS

One hundred and one male patients with dystrophinopathies (55 DMD and 46 BMD) were enrolled. Muscle-derived genome-wide RNA-sequencing was performed in 31 DMD patients, 29 BMD patients, and 11 normal controls. Fibrofatty replacement was scored on muscle MRI and histological levels in all patients. A unique pipeline, single-sample gene set enrichment analysis combined with Spearman's rank correlations (ssGSEA-Cor) was developed to identify the most correlated gene signature for fibrofatty replacement. Quantitative real-time PCR (qRT-PCR) analysis, western blot analysis, and single-nucleus RNA-sequencing (snRNA-seq) were performed in the remaining patients to validate the most correlated gene signature.

RESULTS

Comparative transcriptomic analysis revealed that 31 DMD muscles were characterized by a significant increase of inflammation/immune response and extracellular matrix remodelling compared with 29 BMD muscles (P < 0.05). The ssGSEA-Cor pipeline revealed that the gene set of CDKN2A and CDKN2B was the most correlated gene signature for fibrofatty replacement (histological rs  = 0.744, P < 0.001; MRI rs  = 0.718, P < 0.001). Muscle qRT-PCR confirmed that CDKN2A mRNA expression in both 15 DMD (median = 25.007, P < 0.001) and 12 BMD (median = 5.654, P < 0.001) patients were significantly higher than that in controls (median = 1.101), while no significant difference in CDKN2B mRNA expression was found among DMD, BMD, and control groups. In the 27 patients, muscle CDKN2A mRNA expression respectively correlated with muscle MRI (rs  = 0.883, P < 0.001) and histological fibrofatty replacement (rs  = 0.804, P < 0.001) and disease duration (rs  = 0.645, P < 0.001) and North Star Ambulatory Assessment total scores (rs  = -0.698, P < 0.001). Muscle western blot analysis confirmed that both four DMD (median = 2.958, P < 0.05) and four BMD (median = 1.959, P < 0.01) patients had a significantly higher level of CDKN2A protein expression than controls (median = 1.068). The snRNA-seq analysis of two DMD muscles revealed that CDKN2A was mainly expressed in fibro-adipogenic progenitors, satellite cells, and myoblasts.

CONCLUSIONS

We identify CDKN2A expression as a novel biomarker of fibrofatty replacement, which might be a new target for antifibrotic therapy in dystrophinopathies.

Enhancement of mitochondrial energy metabolism by melatonin promotes vascularized skeletal muscle regeneration in a volumetric muscle loss model

Volumetric muscle loss (VML) is a condition that results in the extensive loss of 20 % or more of skeletal muscle due to trauma or tumor ablation, leading to severe functional impairment and permanent disability. The current surgical interventions have limited functional regeneration of skeletal muscle due to the compromised self-repair mechanism. Melatonin has been reported to protect skeletal muscle from exercise-induced oxidative damage and holds great potential to treat muscle diseases. In this study, we hypothesize that melatonin can enhance myoblast differentiation and promote effective recovery of skeletal muscle following VML. In vitro administration of melatonin resulted in a significant enhancement of myogenesis in C2C12 myoblast cells, as evidenced by the up-regulation of myogenic marker genes in a dose-dependent manner. Further experiments revealed that silent information of regulator type 3 (SIRT3) played a critical role in the melatonin-enhanced myoblast differentiation through enhancement of mitochondrial energy metabolism and activation of mitochondrial antioxidant enzymes such as superoxide dismutase 2 (SOD2). Silencing of Sirt3 completely abrogated the protective effect of melatonin on the mitochondrial function of myoblasts, evidenced by the increased reactive oxygen species, decreased adenosine triphosphate production, and down-regulated myoblast-specific marker gene expression. In order to attain a protracted and consistent release, liposome-encapsuled melatonin was integrated into gelatin methacryloyl hydrogel (GelMA-Lipo@MT). The implantation of GelMA-Lipo@MT into a tibialis anterior muscle defect in a VML model effectively stimulated the formation of myofibers and new blood vessels in situ, while concurrently inhibiting fibrotic collagen deposition. The findings of this study indicate that the incorporation of melatonin with GelMA hydrogel has facilitated the de novo vascularized skeletal muscle regeneration by augmenting mitochondrial energy metabolism. This represents a promising approach for the development of skeletal muscle tissue engineering, which could be utilized for the treatment of VML and other severe muscle injuries.

Autophagy in sarcopenia: Possible mechanisms and novel therapies

With global population aging, age-related diseases, especially sarcopenia, have attracted much attention in recent years. Characterized by low muscle strength, low muscle quantity or quality and low physical performance, sarcopenia is one of the major factors associated with an increased risk of falls and disability. Much effort has been made to understand the cellular biological and physiological mechanisms underlying sarcopenia. Autophagy is an important cellular self-protection mechanism that relies on lysosomes to degrade misfolded proteins and damaged organelles. Research designed to obtain new insight into human diseases from the autophagic aspect has been carried out and has made new progress, which encourages relevant studies on the relationship between autophagy and sarcopenia. Autophagy plays a protective role in sarcopenia by modulating the regenerative capability of satellite cells, relieving oxidative stress and suppressing the inflammatory response. This review aims to reveal the specific interaction between sarcopenia and autophagy and explore possible therapies in hopes of encouraging more specific research in need and unlocking novel promising therapies to ameliorate sarcopenia.

Clearance of defective muscle stem cells by senolytics restores myogenesis in myotonic dystrophy type 1

Muscle stem cells, the engine of muscle repair, are affected in myotonic dystrophy type 1 (DM1); however, the underlying molecular mechanism and the impact on the disease severity are still elusive. Here, we show using patients' samples that muscle stem cells/myoblasts exhibit signs of cellular senescence in vitro and in situ. Single cell RNAseq uncovers a subset of senescent myoblasts expressing high levels of genes related to the senescence-associated secretory phenotype (SASP). We show that the levels of interleukin-6, a prominent SASP cytokine, in the serum of DM1 patients correlate with muscle weakness and functional capacity limitations. Drug screening revealed that the senolytic BCL-XL inhibitor (A1155463) can specifically remove senescent DM1 myoblasts by inducing their apoptosis. Clearance of senescent cells reduced the expression of SASP, which rescued the proliferation and differentiation capacity of DM1 myoblasts in vitro and enhanced their engraftment following transplantation in vivo. Altogether, this study identifies the pathogenic mechanism associated with muscle stem cell defects in DM1 and opens a therapeutic avenue that targets these defective cells to restore myogenesis.

The Contribution of Muscle Innate Immunity to Uremic Cachexia

Protein energy wasting (PEW) is a common complication both in chronic kidney disease (CKD) and end-stage kidney disease (ESKD). Of note, PEW is one of the stronger predictors of morbidity and mortality in this patient population. The pathogenesis of PEW involves several mechanisms, including anorexia, insulin resistance, acidosis and low-grade inflammation. In addition, "sterile" muscle inflammation contributes to PEW at an advanced CKD stage. Both immune and resident muscle cells can activate innate immunity; thus, they have critical roles in triggering "sterile" tissue inflammation. Toll-like receptor 4 (TLR4) can detect endogenous danger-associated molecular patterns generated or retained in blood in uremia and induce a sterile muscle inflammatory response via NF-κB in myocytes. In addition, TLR4, though the activation of the NLRP3 inflammasome, links the sensing of metabolic uremic stress in muscle to the activation of pro-inflammatory cascades, which lead to the production of IL-1β and IL-18. Finally, uremia-induced accelerated cell senescence is associated with a secretory phenotype that favors fibrosis in muscle. Targeting these innate immune pathways could lead to novel therapies for CKD-related PEW.

An unbiased proteomics approach to identify the senescence-associated secretory phenotype of human bone marrow-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) derived from bone marrow can support skeletal tissue repair and regeneration owing to their self-renewing capacity, differentiation ability, and trophic functions. Bone marrow-derived MSCs undergo dramatic changes with aging, including the senescence-associated secretory phenotype (SASP) which may largely contribute to age-related changes in bone tissue leading to osteoporosis. A mass spectrometry-based proteomics approach was used to investigate the MSC SASP. Replicative senescence was achieved by exhaustive in vitro sub-cultivation and confirmed by standard proliferation criteria. Conditioned media from non-senescent and senescent MSCs underwent mass spectrometry. Proteomics and bioinformatics analyses enabled the identification of 95 proteins expressed uniquely in senescent MSCs. Protein ontology analysis revealed the enrichment of proteins linked to the extracellular matrix, exosomes, cell adhesion, and calcium ion binding. The proteomic analysis was independently validated by taking ten identified proteins with relevance to bone aging and confirming their increased abundance in conditioned media from replicatively senescent versus non-senescent MSCs (ACTα2, LTF, SOD1, IL-6, LTBP2, PXDN, SERPINE 1, COL1α1, THBS1, OPG). These target proteins were used to further investigate changes in the MSC SASP profile in response to other inducers of senescence, ionizing radiation (IR) and H₂O₂. Similar secreted protein expression profiles with replicatively senescent cells were seen with H₂O₂ treatment except for LTF and PXDN, which were increased by IR treatment. With both IR and H₂O₂ treatment there was a decrease in THBS1. In vivo investigation of these secreted proteins with aging was shown by significant changes in the abundance of OPG, COL1α1, IL-6, ACTα2, SERPINE 1, and THBS1 in the plasma of aged rats. This unbiased, comprehensive analysis of the changes in the MSC secretome with senescence defines the unique protein signature of the SASP in these cells and provides a better understanding of the aging bone microenvironment.

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Identified the senescence-associated secretory phenotype of mesenchymal stem cells.

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Investigated protein expression under different senescence induction conditions.

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Showed significant changes in in vivo abundance of target proteins in aging rats.

The impact of senescence on muscle wasting in chronic kidney disease

BACKGROUND

Muscle wasting is a common complication of chronic kidney disease (CKD) that is associated with higher mortality. Although the mechanisms of myofibre loss in CKD has been widely studied, the contribution of muscle precursor cell (MPC) senescence remains poorly understood. Senescent MPCs no longer proliferate and can produce proinflammatory factors or cytokines. In this study, we tested the hypothesis that the senescence associated secretory phenotype (SASP) of MPCs contributes to CKD-induced muscle atrophy and weakness.

METHODS

CKD was induced in mice by 5/6th nephrectomy. Kidney function, muscle size, and function were measured, and markers of atrophy, inflammation, and senescence were evaluated using immunohistochemistry, immunoblots, or qPCR. To study the impact of senescence, a senolytics cocktail of dasatinib + quercetin (D&Q) was given orally to mice for 8 weeks. To investigate CKD-induced senescence at the cellular level, primary MPCs were incubated with serum from CKD or control subjects. The roles of specific proteins in MPC senescence were studied using adenoviral transduction, siRNA, and plasmid transfection.

RESULTS

In the hindlimb muscles of CKD mice, (i) the senescence biomarker SA-β-gal was sharply increased (~30-fold); (ii) the DNA damage response marker γ-H2AX was increased 1.9-fold; and (iii) the senescence pathway markers p21 and p16INK4a were increased 1.99-fold and 2.82-fold, respectively (all values, P < 0.05), whereas p53 was unchanged. γ-H2AX, p21, and p16INK4A were negatively correlated at P < 0.05 with gastrocnemius weight, suggesting a causal relationship with muscle atrophy. Administration of the senolytics cocktail to CKD mice for 8 weeks eliminated the disease-related elevation of p21, p16INK4a , and γ-H2AX, abolished positive SA-β-gal, and depressed the high levels of the SASP cytokines, TNF-α, IL-6, IL-1β, and IFN (all values, P < 0.05). Skeletal muscle weight, myofibre cross-sectional area, and grip function were improved in CKD mice receiving D&Q. Markers of protein degradation, inflammation, and MPCs dysfunction were also attenuated by D&Q treatment compared with the vehicle treatment in 5/6th nephrectomy mice (all values, P < 0.05). Uraemic serum induced senescence in cultured MPCs. Overexpression of FoxO1a in MPCs increased the number of p21+ senescent cells, and p21 siRNA prevented uraemic serum-induced senescence (P < 0.05).

CONCLUSIONS

Senescent MPCs are likely to contribute to the development of muscle wasting during CKD by producing inflammatory cytokines. Limiting senescence with senolytics ameliorated muscle wasting and improved muscle strength in vivo and restored cultured MPC functions. These results suggest potential new therapeutic targets to improve muscle health and function in CKD.

Identification and Functional Characterization of Metabolites for Skeletal Muscle Mass in Early Postmenopausal Chinese Women

Low skeletal muscle mass (SMM) is a crucial component of the sarcopenia phenotypes. In the present study, we aim to identify the specific metabolites associated with SMM variation and their functional mechanisms of decreased SMM in early postmenopausal women. We performed an untargeted metabolomics analysis in 430 early postmenopausal women to identify specific metabolite associated with skeletal muscle mass indexes (SMIes). Then, the potential causal effect of specific metabolite on SMM variation was accessed by one-sample Mendelian randomization (MR) analysis. Finally, in vitro experiments and transcriptomics bioinformatics analysis were conducted to explore the impact and potential functional mechanisms of specific metabolite on SMM variation. We detected 65 metabolites significantly associated with at least one SMI (variable importance in projection > 1.5 by partial least squares regression and p < .05 in multiple linear regression analysis). Remarkably, stearic acid (SA) was negatively associated with all SMIes, and subsequent MR analyses showed that increased serum SA level had a causal effect on decreased SMM (p < .05). Further in vitro experiments showed that SA could repress myoblast's differentiation at mRNA, protein, and phenotype levels. By combining transcriptome bioinformatics analysis, our study supports that SA may inhibit myoblast differentiation and myotube development by regulating the migration, adhesion, and fusion of myoblasts. This metabolomics study revealed specific metabolic profiles associated with decreased SMM in postmenopausal women, first highlighted the importance of SA in regulating SMM variation, and illustrated its potential mechanism on decreased SMM.

Clearance of Senescent Cells From Injured Muscle Abrogates Heterotopic Ossification in Mouse Models of Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease caused by mutations in activin A receptor type I/activin-like kinase 2 (ACVR1/ALK2), a bone morphogenetic protein (BMP) type I receptor, resulting in the formation of extraskeletal or heterotopic ossification (HO) and other features consistent with premature aging. During the first decade of life, episodic bouts of inflammatory swellings (flare-ups) occur, which are typically triggered by soft tissue trauma. Through an endochondral process, these exacerbations ultimately result in skeletal muscles, tendons, ligaments, fascia, and aponeuroses transforming into ectopic bone, rendering movement impossible. We have previously shown that soft tissue injury causes early FOP lesions characterized by cellular hypoxia, cellular damage, and local inflammation. Here we show that muscle injury in FOP also results in senescent cell accumulation, and that senescence promotes tissue reprogramming toward a chondrogenic fate in FOP muscle but not wild-type (WT) muscle. Using a combination of senolytic drugs we show that senescent cell clearance and reduction in the senescence associated secretory phenotype (SASP) ameliorate HO in mouse models of FOP. We conclude that injury-induced senescent cell burden and the SASP contribute to FOP lesion formation and that tissue reprogramming in FOP is mediated by cellular senescence, altering myogenic cell fate toward a chondrogenic cell fate. Furthermore, pharmacological removal of senescent cells abrogates tissue reprogramming and HO formation. Here we provide proof-of-principle evidence for senolytic drugs as a future therapeutic strategy in FOP. © 2021 American Society for Bone and Mineral Research (ASBMR).

Fibro-adipogenic progenitors in skeletal muscle homeostasis, regeneration and diseases

Skeletal muscle possesses a remarkable regenerative capacity that relies on the activity of muscle stem cells, also known as satellite cells. The presence of non-myogenic cells also plays a key role in the coordination of skeletal muscle regeneration. Particularly, fibro-adipogenic progenitors (FAPs) emerged as master regulators of muscle stem cell function and skeletal muscle regeneration. This population of muscle resident mesenchymal stromal cells has been initially characterized based on its bi-potent ability to differentiate into fibroblasts or adipocytes. New technologies such as single-cell RNAseq revealed the cellular heterogeneity of FAPs and their complex regulatory network during muscle regeneration. In acute injury, FAPs rapidly enter the cell cycle and secrete trophic factors that support the myogenic activity of muscle stem cells. Conversely, deregulation of FAP cell activity is associated with the accumulation of fibrofatty tissue in pathological conditions such as muscular dystrophies and ageing. Considering their central role in skeletal muscle pathophysiology, the regulatory mechanisms of FAPs and their cellular and molecular crosstalk with muscle stem cells are highly investigated in the field. In this review, we summarize the current knowledge on FAP cell characteristics, heterogeneity and the cellular crosstalk during skeletal muscle homeostasis and regeneration. We further describe their role in muscular disorders, as well as different therapeutic strategies targeting these cells to restore muscle regeneration.

Hepatic stellate cell senescence in liver fibrosis: Characteristics, mechanisms and perspectives

Myofibroblasts play an important role in fibrogenesis. Hepatic stellate cells are the main precursors of myofibroblasts. Cellular senescence is the terminal cell fate in which proliferating cells undergo irreversible cell cycle arrest. Senescent hepatic stellate cells were identified in liver fibrosis. Senescent hepatic stellate cells display decreased collagen production and proliferation. Therefore, induction of senescence could be a protective mechanism against progression of liver fibrosis and the concept of therapy-induced senescence has been proposed to treat liver fibrosis. In this review, characteristics of senescent hepatic stellate cells and the essential signaling pathways involved in senescence are reviewed. Furthermore, the potential impact of senescent hepatic stellate cells on other liver cell types are discussed. Senescent cells are cleared by the immune system. The persistence of senescent cells can remodel the microenvironment and interact with inflammatory cells to induce aging-related dysfunction. Therefore, senolytics, a class of compounds that selectively induce death of senescent cells, were introduced as treatment to remove senescent cells and consequently decrease the disadvantageous effects of persisting senescent cells. The effects of senescent hepatic stellate cells in liver fibrosis need further investigation.

Cellular senescence in musculoskeletal homeostasis, diseases, and regeneration

Emerging insights into cellular senescence highlight the relevance of senescence in musculoskeletal disorders, which represent the leading global cause of disability. Cellular senescence was initially described by Hayflick et al. in 1961 as an irreversible nondividing state in in vitro cell culture studies. We now know that cellular senescence can occur in vivo in response to various stressors as a heterogeneous and tissue-specific cell state with a secretome phenotype acquired after the initial growth arrest. In the past two decades, compelling evidence from preclinical models and human data show an accumulation of senescent cells in many components of the musculoskeletal system. Cellular senescence is therefore a defining feature of age-related musculoskeletal disorders, and targeted elimination of these cells has emerged recently as a promising therapeutic approach to ameliorate tissue damage and promote repair and regeneration of the skeleton and skeletal muscles. In this review, we summarize evidence of the role of senescent cells in the maintenance of bone homeostasis during childhood and their contribution to the pathogenesis of chronic musculoskeletal disorders, including osteoporosis, osteoarthritis, and sarcopenia. We highlight the diversity of the senescent cells in the microenvironment of bone, joint, and skeletal muscle tissue, as well as the mechanisms by which these senescent cells are involved in musculoskeletal diseases. In addition, we discuss how identifying and targeting senescent cells might positively affect pathologic progression and musculoskeletal system regeneration.

TNF-α and IFN-γ Participate in Improving the Immunoregulatory Capacity of Mesenchymal Stem/Stromal Cells: Importance of Cell-Cell Contact and Extracellular Vesicles

Mesenchymal stem/stromal cells (MSCs) have an immunoregulatory capacity and have been used in different clinical protocols requiring control of the immune response. However, variable results have been obtained, mainly due to the effect of the microenvironment on the induction, increase, and maintenance of MSC immunoregulatory mechanisms. In addition, the importance of cell–cell contact for MSCs to efficiently modulate the immune response has recently been highlighted. Because these interactions would be difficult to achieve in the physiological context, the release of extracellular vesicles (EVs) and their participation as intermediaries of communication between MSCs and immune cells becomes relevant. Therefore, this article focuses on analyzing immunoregulatory mechanisms mediated by cell contact, highlighting the importance of intercellular adhesion molecule-1 (ICAM-1) and the participation of EVs. Moreover, the effects of tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ), the main cytokines involved in MSC activation, are examined. These cytokines, when used at the appropriate concentrations and times, would promote increases in the expression of immunoregulatory molecules in the cell and allow the acquisition of EVs enriched with these molecules. The establishment of certain in vitro activation guidelines will facilitate the design of conditioning protocols to obtain functional MSCs or EVs in different pathophysiological conditions.

Suppression of MyoD induces spontaneous adipogenesis in skeletal muscle progenitor cell culture

The degree of intramuscular adipose tissue accumulation is one of the factors affecting meat quality. Accumulation of adipocytes is also observed under the pathological condition of skeletal muscle such as muscular dystrophy and sarcopenia. The origin of adipocytes seen in skeletal muscle is mesenchymal progenitor cells that can give rise to both adipocytes and fibroblasts. In the present study, we demonstrated that siRNA-mediated suppression of MyoD expression in rat skeletal muscle progenitor cell culture, which comprises both myogenic satellite cells and mesenchymal progenitor cells, resulted in diminished myotube formation and an unexpected spontaneous appearance of white adipocytes. Suppressing myomaker expression also resulted in complete absence of myotube formation without reducing MyoD expression, but no adipogenesis was seen in this scenario, indicating that decline in MyoD expression rather than decreased myotube formation is necessary to induce adipogenesis. In addition, spontaneous adipogenesis induced by suppressing MyoD expression in culture was inhibited by the conditioned medium from control culture, indicating that anti-adipogenic factor(s) are secreted from MyoD-positive myogenic cells. These results indicate the presence of regulatory mechanism on adipogenesis by myogenic cells.

Role of Hyperglycemia in the Senescence of Mesenchymal Stem Cells

The regenerative and immunomodulatory properties of mesenchymal stem cells (MSCs) have laid a sound foundation for their clinical application in various diseases. However, the clinical efficiency of MSC treatments varies depending on certain cell characteristics. Among these, the roles of cell aging or senescence cannot be excluded. Despite their stemness, evidence of senescence in MSCs has recently gained attention. Many factors may contribute to the senescence of MSCs, including MSC origin (biological niche), donor conditions (age, obesity, diseases, or unknown factors), and culture conditions in vitro. With the rapidly increasing prevalence of diabetes mellitus (DM) and gestational diabetes mellitus (GDM), the effects of hyperglycemia on the senescence of MSCs should be evaluated to improve the application of autologous MSCs. This review aims to present the available data on the senescence of MSCs, its relationship with hyperglycemia, and the strategies to suppress the senescence of MSCs in a hyperglycemic environment.

Sirt3 Attenuates Oxidative Stress Damage and Rescues Cellular Senescence in Rat Bone Marrow Mesenchymal Stem Cells by Targeting Superoxide Dismutase 2

Oxidative stress is one of the main causes of aging. The process of physiological aging is always accompanied by increased levels of endogenous oxidative stress. Exogenous oxidants have contributed to premature cellular senescence. As a deacetylase located in mitochondrial matrix, Sirt3 plays critical roles in mitochondrial energy metabolism, oxidative stress regulation, and cellular senescence. However, it remains unknown whether Sirt3 exerts the analogous role in cellular senescence caused by two different oxidation pathways. In this study, the function of Sirt3 was investigated in age-related natural senescence and H₂O₂-induced premature senescence of rat bone marrow mesenchymal stem cells (MSCs). Our results showed that Sirt3 expression was significantly decreased in both senescent MSCs, which was concerned with reduced cellular reactive oxygen species (ROS) and aggravated DNA injury. Sirt3 repletion could partly reverse the senescence-associated phenotypic features in natural and premature senescent MSCs. Moreover, Sirt3 replenishment led to the reduction in the levels of cellular ROS by enhancing the expression and activity of superoxide dismutase 2 (SOD2), thus maintaining the balance of intracellular oxidation and antioxidation and ameliorating oxidative stress damage. Altogether, Sirt3 inhibits MSC natural senescence and H₂O₂-induced premature senescence through alleviating ROS-induced injury and upregulating SOD2 expression and activity. Our research indicates that Sirt3 might contribute to uncovering the novel mechanisms underlying MSC senescence and provide new insights to aging and oxidative stress-related diseases.

Cellular senescence-mediated exacerbation of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease characterised by chronic muscle degeneration and inflammation. Our previously established DMD model rats (DMD rats) have a more severe disease phenotype than the broadly used mouse model. We aimed to investigate the role of senescence in DMD using DMD rats and patients. Senescence was induced in satellite cells and mesenchymal progenitor cells, owing to the increased expression of CDKN2A, p16- and p19-encoding gene. Genetic ablation of p16 in DMD rats dramatically restored body weight and muscle strength. Histological analysis showed a reduction of fibrotic and adipose tissues invading skeletal muscle, with increased muscle regeneration. Senolytic drug ABT263 prevented loss of body weight and muscle strength, and increased muscle regeneration in rats even at 8 months—the late stage of DMD. Moreover, senescence markers were highly expressed in the skeletal muscle of DMD patients. In situ hybridization of CDKN2A confirmed the expression of it in satellite cells and mesenchymal progenitor cells in patients with DMD. Collectively, these data provide new insights into the integral role of senescence in DMD progression.

Factors inducing transdifferentiation of myoblasts into adipocytes

Fat infiltration in skeletal muscle is observed in several myopathies, is associated with muscular dysfunction, and is strongly correlated with insulin resistance, diabetes, obesity, and aging. In animal production, skeletal muscle fat (also known as intermuscular and intramuscular fat) is positively related to meat quality including tenderness, flavor, and juiciness. Thus, understanding the cell origin and regulation mechanism of skeletal muscle fat infiltration is important for developing therapies against human myopathies as well as for improving meat quality. Notably, age, sarcopenia, oxidative stress, injury, and regeneration can activate adipogenic differentiation potential in myoblasts and affect fat accumulation in skeletal muscle. In addition, several transcriptional and nutritional factors can directly induce transdifferentiation of myoblasts into adipocytes. In this review, we focused on the recent progress in understanding the muscle-to-adipocyte differentiation and summarized and discussed the genetic, nutritional, and physiological factors that can induce transdifferentiation of myoblasts into adipocytes. Moreover, the regulatory roles and mechanisms of these factors during the transdifferentiation process were also discussed.

Theaflavin Promotes Myogenic Differentiation by Regulating the Cell Cycle and Surface Mechanical Properties of C2C12 Cells

Aging and muscle diseases often lead to a decline in the differentiation capacity of myoblasts, which in turn results in the deterioration of skeletal muscle (SkM) function and impairment of regeneration ability after injury. Theaflavins, the "gold molecules" found in black tea, have been reported to possess various biological activities and have a positive effect on maintaining human health. In this study, we found that among the four theaflavins (theaflavin (TF1), theaflavin-3-gallate (TF2A), theaflavin-3'-gallate (TF2B), and theaflavin-3,3'-digallate (TF3) monomers), TF1 (20 μM) significantly promoted the fusion index of myoblasts, number of mature myotubes, and degree of myotube development. By combining transcriptomics, bioinformatics, and molecular biology experiments, we showed that TF1 may promote myoblast differentiation by (1) regulating the withdrawal of myoblasts from the cell cycle, inducing the release of myogenic factors (MyoD, MyoG, and MyHC) and accelerating myogenic differentiation and (2) regulating the adhesion force of myoblasts and mechanical properties of mature myotubes and promoting the migration, fusion, and development of myoblasts. In conclusion, our study outcomes show that TF1 can promote myoblast differentiation and regulate myotube mechanical properties. It is a potential dietary supplement for the elderly. Our findings provide a new scientific basis for the relationship between tea drinking and aging.

Sarcopenia and Its Implications for Metabolic Health

Sarcopenia not only affects the ability to lead an active lifestyle but also contributes to increased obesity, reduced quality of life, osteoporosis, and metabolic health, in part due to reduced locomotion economy and ease. On the other hand, increased obesity, decreased quality of life, and reduced metabolic health also contribute to sarcopenia. The purpose of this mini-review is to discuss the implications sarcopenia has for the development of obesity and comorbidities that occur with aging.