Pax7 and myogenic progression in skeletal muscle satellite cells

Impact of preventive and therapeutic vascular photobiomodulation on collagen deposition and distribution in experimental model of acute muscle injury

Photobiomodulation (PBM) improves collagen distribution and organization in muscles during regeneration, with most data focusing on localized irradiation. Vascular PBM (VPBM) may promote overall muscle recovery and reduce downtime. The aim of the present study was to investigate the effects of VPBM on collagen deposition in muscle tissue after acute injury, comparing preventive (before injury) and therapeutic (after injury) applications. Wistar rats (n = 85) were randomly distributed into groups: Control; Injury; Non-injury + VPBM; Prior-VPBM + Injury; and Injury + Post-injury VPBM. Muscle injury was induced by cryoinjury. VPBM (780 nm, 40 mW, 10 J/cm², 3.2 J) was applied to the caudal artery/vein. Euthanasia occurred on Days 1, 2, 5, and 7 post-injury. Samples were stained with Picrosirius Red to determine areas of collagen and extracellular matrix (ECM). The results revealed increased collagen in the VPBM-treated groups. In the five-day experimental period, the ECM area was significantly lower in the VPBM groups. In conclusion, VPBM modulated collagen deposition and ECM area, depending on the timing of irradiation.

Exercise-induced MyoD mRNA Expression in Young and Older Human Skeletal Muscle: A Systematic Review and Meta-Analysis

BACKGROUND

Myoblast determination protein 1 (MyoD) is a master transcription factor that triggers myogenesis and drives muscle growth.

OBJECTIVE

The aim was to assess acute exercise-induced MyoD mRNA expression in skeletal muscle for young and older (age > 50) adults.

DESIGN

A meta-analysis and systematic review was conducted.

METHODS

A literature search was conducted for studies reporting MyoD mRNA changes in biopsied human muscle taken within 48 h after exercise. Fifty eligible studies with 822 participants (young 20-35 years; older 53-85 years) were included for meta-analysis.

RESULTS

Significant increases in MyoD mRNA expression in human skeletal muscle were observed 3-12 h post-exercise (standardized mean difference [SMD] = 1.39, p < 0.001), subsiding within 24-48 h (SMD = 0.47, p < 0.001). Older individuals showed a similar time pattern in MyoD mRNA expression post-exercise, but the response is weaker than in younger individuals. Intriguingly, resting levels of MyoD mRNA were higher in older individuals compared to younger individuals in most age-paired studies (SMD = 0.56, p < 0.01). Considering the decline in anabolic hormones during later life, this systematic review highlights age- and sex-related impacts on exercise-induced MyoD mRNA expression in human skeletal muscle, emphasizing the roles of sex hormones and insulin.

CONCLUSION

Pooled results from the eligible studies suggest a blunted exercise-induced increase in MyoD mRNA in skeletal muscle after age 50, likely due to elevated basal MyoD expression as a compensatory mechanism against persistent catabolic conditions in aging muscle.

PROTOCOL REGISTRATION

Registration number: CRD42023471840 (PROSPERO).

Mitochondrial fatty acid oxidation regulates adult muscle stem cell function through modulating metabolic flux and protein acetylation

During homeostasis and regeneration, satellite cells, the resident stem cells of skeletal muscle, have distinct metabolic requirements for fate transitions between quiescence, proliferation and differentiation. However, the contribution of distinct energy sources to satellite cell metabolism and function remains largely unexplored. Here, we uncover a role of mitochondrial fatty acid oxidation (FAO) in satellite cell integrity and function. Single-cell RNA sequencing revealed progressive enrichment of mitochondrial FAO and downstream pathways during activation, proliferation and myogenic commitment of satellite cells. Deletion of Carnitine palmitoyltransferase 2 (Cpt2), the rate-limiting enzyme in FAO, hampered muscle stem cell expansion and differentiation upon acute muscle injury, markedly delaying regeneration. Cpt2 deficiency reduces acetyl-CoA levels in satellite cells, impeding the metabolic flux and acetylation of selective proteins including Pax7, the central transcriptional regulator of satellite cells. Notably, acetate supplementation restored cellular metabolic flux and partially rescued the regenerative defects of Cpt2-null satellite cells. These findings highlight an essential role of fatty acid oxidation in controlling satellite cell function and suggest an integration of lipid metabolism and protein acetylation in adult stem cells.

Molecular responses to clove and oregano essential oils are associated with reduced inflammation and improved gut barrier function in broiler chickens

In vitro tests were conducted to characterize the host-mediated responses of chickens to Clove Essential Oil (CEO) and Oregano Essential Oil (OEO). Chicken macrophage cells (CMCs), chicken intestinal epithelial cells (IECs), quail muscle cells (QMCs), and chicken embryonic muscle cells (EMCs) were utilized in these assays. EMCs were collected from the 13-day-old embryo during egg incubation and all cell lines were seeded at 2 × 105/mL in a 24-well plate. In CMCs, an inflammatory response was induced by stimulating with 1.0 µg/mL of Lipopolysaccharide (LPS). To induce muscle cell differentiation, 0.5 % FBS was used in QMCs and 2.0 % FBS in EMCs. Three different concentrations (1.0, 10.0, and 100 µg/mL) of CEO and OEO were administered. qRT-PCR was used to measure gene expression levels of IL-1β and IL-8 from CMCs, occludin, ZO-1, and MUC2 from IECs, and Pax7 and MyoG from QMCs and EMCs. Cytotoxic effects of CEO and OEO were determined using an MTT assay; CEO and OEO did not show cytotoxicity at concentrations below 0.1 mg/mL in CMCs, IECs, QMCs, and EMCs. CEO reduced (P < 0.05) the LPS-induced increase of IL-1β and IL-8 in CMCs and increased (P < 0.05) ZO-1 and MUC2 in IECs. OEO suppressed (P < 0.05) the release of IL-8, increased ZO-1, and Pax7. Both CEO and OEO demonstrated microbicidal activity against sporozoite of E. tenella and C. perfringens bacteria, but only at doses 10-100 × higher than those that would be used in feed. These findings support our previous findings on other phytochemicals; both CEO and OEO are promising candidates for improved resilience in chickens not due to their direct antimicrobial effects, but due to gut physiological responses that take place at the level of the host.

Tokay gecko tail regeneration involves temporally collinear expression of HOXC genes and early expression of satellite cell markers

BACKGROUND

Regeneration is the replacement of lost or damaged tissue with a functional copy. In axolotls and zebrafish, regeneration involves stem cells produced by de-differentiation. These cells form a growth zone which expresses developmental patterning genes at its apex. This system resembles an embryonic developmental field where cells undergo pattern formation. Some lizards, including geckos, can regenerate their tails, but it is unclear whether they show a "development-like" regeneration pathway.

RESULTS

Using the tokay gecko (Gekko gecko) model species, we examined seven stages of tail regeneration, and three stages of embryonic tail bud development, using transcriptomics, single-cell sequencing, and in situ hybridization. We find no apical growth zone in the regenerating tail. The transcriptomes of the regenerating vs. embryonic tails are quite different with respect to developmental patterning genes. Posterior HOXC genes were activated in a temporally collinear sequence in the regenerating tail. The major precursor populations were stromal cells (regenerating tail) vs. pluripotent stem cells (embryonic tail). Segmented skeletal muscles were regenerated with no expression of classical segmentation genes, but with the early activation of satellite cell markers.

CONCLUSIONS

Our study suggests that tail regeneration in the tokay gecko-unlike tail development-might rely on the activation of resident stem cells, guided by pre-existing positional information.

Establishment, characterization, and sensory characteristics (taste and flavor) of an immortalized muscle cell line from the seven-band grouper Epinephelus septemfasciatus: implications for cultured seafood applications

Grouper muscle satellite cells (GMSCs) from the seven-band grouper (Epinephelus septemfasciatus) were isolated, and their growth conditions were optimized (10% fetal bovine serum, 24°C, 10 ng/mL bFGF). The cells were immortalized at passage 14 and designated as grouper immortalized muscle satellite cells (GIMSCs). DNA barcoding confirmed the grouper origin of both GMSC and GIMSC lines. GIMSCs exhibited enhanced proliferation, accelerated differentiation, and robust myotube formation compared to pre-crisis GMSCs. Western blot analysis showed upregulation of key myogenic factors (Pax7, MyoD, MyoG) and structural proteins (Desmin) in GIMSC, indicating the differentiation potential. The immortalized GIMSC line maintained consistent morphology, growth rates, and viability across multiple passages. Biocompatibility studies showed GIMSCs were compatible with bio-inks (sodium alginate, gelatin, κ-carrageenan) at 250 to 10,000 µg/mL, retaining ~ 80% viability at the highest concentration. Taste sensory analysis revealed GMSCs had the highest umami and lowest saltiness and sourness, contrasting with the muscle of the seven-band grouper, which had higher saltiness and sourness. Flavor analysis identified pronounced fishy, hot fat, and ethereal flavors in the cells at higher level than in the muscle. These findings suggest GMSCs and GIMSCs are promising for producing cultured meat with enhanced umami taste and flavors, advancing cellular agriculture and sustainable food production.

Myoblast-derived ADAMTS-like 2 promotes skeletal muscle regeneration after injury

Skeletal muscle regeneration and functional recovery after minor injuries requires the activation of muscle-resident myogenic muscle stem cells (i.e. satellite cells) and their subsequent differentiation into myoblasts, myocytes, and ultimately myofibers. We recently identified secreted ADAMTS-like 2 (ADAMTSL2) as a pro-myogenic regulator of muscle development, where it promoted myoblast differentiation. Since myoblast differentiation is a key process in skeletal muscle regeneration, we here examined the role of ADAMTSL2 during muscle regeneration after BaCl2 injury. Specifically, we found that muscle regeneration was delayed after ablation of ADAMTSL2 in myogenic precursor cells and accelerated following injection of pro-myogenic ADAMTSL2 protein domains. Mechanistically, ADAMTSL2 regulated the number of committed myoblasts, which are the precursors for myocytes and regenerating myofibers. Collectively, our data support a role for myoblast-derived ADAMTSL2 as a positive regulator of muscle regeneration and provide a proof-of-concept for potential therapeutic applications.

A diarylheptanoid derivative mediates glycogen synthase kinase 3β to promote the porcine muscle satellite cell proliferation: Implications for cultured meat production

Skeletal muscle stem cells, or satellite cells, are vital for cultured meat production, driving proliferation and differentiation to form muscle fibers in vitro. However, these abilities are often compromised after long-term in vitro culturing due to a loss of their stemness characteristics. Therefore, effective pharmacological agents that enhance satellite cell proliferation and maintain stemness ability are needed for optimal cell growth for cultured meat production. In this study, the effects of the identified glycogen synthase kinase 3β (GSK3β) inhibitors, ASPP 049, a diarylheptanoid isolated from Curcuma comosa rhizomes, and CHIR 99021 on porcine muscle satellite cell (PMSC) proliferation and Wnt/β-catenin signaling pathway were investigated. We found that both compounds enhanced cell viability and proliferation while preserving the stemness marker, as evidenced by increased expression of the skeletal muscle stem cell marker, Pax7 protein. Molecular dynamics simulations showed that ASPP 049 and CHIR 99021 exhibited differing binding affinities, primarily through hydrophobic interactions, suggesting potential for the design of more potent inhibitors in the future. Despite its weaker binding, ASPP 049 still showed significant effects on the regulation of the Wnt/β-catenin signaling pathway via increased phosphorylation of GSK3β at Ser9 and decreased the phosphorylation of β-catenin at Ser33, Ser37, and Thr41, thereby subsequently activating Wnt transcriptional activity. This study highlights the potential of ASPP 049 and CHIR 99021 to enhance PMSC proliferation and maintain stemness ability, offering a promising avenue for improving cultured meat production.

Effects and mechanisms of APP and its cleavage product Aβ in the comorbidity of sarcopenia and Alzheimer's disease

Sarcopenia and AD are both classic degenerative diseases, and there is growing epidemiological evidence of their comorbidity with aging; however, the mechanisms underlying the biology of their commonality have not yet been thoroughly investigated. APP is a membrane protein that is expressed in tissues and is expressed not only in the nervous system but also in the NMJ and muscle. Deposition of its proteolytic cleavage product, Aβ, has been described as a central component of AD pathogenesis. Recent studies have shown that excessive accumulation and aberrant expression of APP in muscle lead to pathological muscle lesions, but the pathogenic mechanism by which APP and its proteolytic cleavage products act in skeletal muscle is less well understood. By summarizing and analyzing the literature concerning the role, pathogenicity and pathological mechanisms of APP and its cleavage products in the nervous system and muscles, we aimed to explore the intrinsic pathological mechanisms of myocerebral comorbidities and to provide new perspectives and theoretical foundations for the prevention and treatment of AD and sarcopenia comorbidities.

Effect of IGF1 on Myogenic Proliferation and Differentiation of Bovine Skeletal Muscle Satellite Cells Through PI3K/AKT Signaling Pathway

Background: Cultivated meat, an alternative to conventional meat, has substantial potential for alleviating environmental and ethical concerns. This method of manufacturing meat involves the isolation of skeletal muscle satellite cells (SMSCs) from donor animals, after which they proliferate in vitro and differentiate into primitive muscle fibers. The aim of this research was to evaluate how the insulin-like growth factor 1 (IGF1) gene regulates the myogenic differentiation of bovine skeletal muscle satellite cells (bSMSCs). Methods: bSMSCs isolated from newborn calves were cultured to the third generation in vitro and differentiated into myoblasts via the serum withdrawal method. An overexpression lentivirus and siRNA targeting the IGF1 gene were constructed and transduced into bSMSCs, which were subsequently analyzed via real-time fluorescence quantitative PCR(qRT-PCR) and Western blots. The mRNA and protein levels of the myogenic differentiation markers myosin heavy chain (MyHC) and myogenin (MyoG) were determined. Results: The results revealed that the lentivirus overexpressing the IGF1 gene significantly increased the expression of MyHC and MyoG, whereas the expression of both the MyHC and MyoG mRNAs and proteins was strongly reduced by si-IGF1. Conclusions: IGF1 positively regulates the myogenic differentiation of bSMSCs. This study provides a reference for further elucidating the molecular mechanism by which the IGF1 gene regulates the myogenic differentiation of bSMSCs via the PI3K/Akt signaling pathway and lays a foundation for establishing a regulatory network of bovine muscle growth and development.

Effect of NR1D1 on the proliferation and differentiation of yak skeletal muscle satellite cells

The severe conditions at high altitudes, where yaks inhabit, contribute to delayed muscular growth and compromised tenderness of their muscle tissue. Myosatellite cells are responsible for the growth and regeneration of skeletal muscle after birth and have the potential to proliferate and differentiate, its development is closely related to meat quality, and the nuclear receptor gene NR1D1 is involved in muscle formation and skeletal muscle regulation. Therefore, in order to understand the effect of NR1D1 on muscle satellite cells, we identified the mRNA expression levels of marker genes specifically expressed in muscle satellite cells at different stages to determine the type of cells isolated. Eventually, we successfully constructed a primary cell line of yak muscle satellite cells. Then we constructed NR1D1 overexpression vector and interference RNA, and introduced them into isolated yak skeletal muscle satellite cells. We performed qPCR, CCK8, and fluorescence-specific to detect the expression of genes or abundance of proteins as markers of cell proliferation and differentiation. Compared with those in the control group, the expression levels of proliferation marker genes KI-67, CYCLIND1, and CYCLINA were significantly inhibited after NR1D1 overexpression, which was also supported by the CCK-8 test, whereas differentiation marker genes MYOD, MYOG, and MYF5 were significantly inhibited. Fluorescence-specific staining showed that KI-67 protein abundance and the number of microfilaments both decreased, while the opposite trend was observed after NR1D1 interference. In conclusion, we confirmed that NR1D1 inhibited the proliferation and differentiation of yak skeletal muscle satellite cells, which provides a theoretical basis for further research on the effect of NR1D1 on improving meat quality traits and meat production performance of yaks.

Direct actions of growth hormone in rainbow trout, Oncorhynchus mykiss, skeletal muscle cells in vitro

The growth hormone (GH)-insulin-like growth factor-1 (IGF-1) system regulates skeletal muscle growth and function. GH has a major function of targeting the liver to regulate IGF-1 production and release, and IGF-1 mediates the primary anabolic action of GH on growth. However, skeletal muscle is a target tissue of GH as evidenced by dynamic GH receptor expression, but it is unclear if GH elicits any direct actions on extrahepatic tissues as it is difficult to distinguish the effects of IGF-1 from GH. Fish growth regulation is complex compared to mammals, as genome duplication events have resulted in multiple isoforms of GHs, GHRs, IGFs, and IGFRs expressed in most fish tissues. This study investigated the potential for GH direct actions on fish skeletal muscle using an in vitro system, where rainbow trout myogenic precursor cells (MPCs) were cultured in normal and serum-deprived media, to mimic in vivo fasting conditions. Fasting reduces IGF-1 signaling in the muscle, which is critical for disentangling the roles of GH from IGF-1. The direct effects of GH were analyzed by measuring changes in myogenic proliferation and differentiation genes, as well as genes regulating muscle growth and proteolysis. This study provides the first in-depth analysis of the direct actions of GH on serum-deprived fish muscle cells in vitro. Data suggest that GH induces the expression of markers for proliferation and muscle growth in the presence of serum, but all observed GH action was blocked in serum-deprived conditions. Additionally, serum deprivation alone reduced the expression of several proliferation and differentiation markers, while increasing growth and proteolysis markers. Results also demonstrate dynamic gene expression response in the presence of GH and a JAK inhibitor in serum-provided but not serum-deprived conditions. These data provide a better understanding of GH signaling in relation to serum in trout muscle cells in vitro.

Characteristics of bovine muscle satellite cell from different breeds for efficient production of cultured meat

The purpose of this study was comparing in vitro performances of three breeds of donor satellite cells for cultured meat and selecting the optimal donor and providing insight into the selection of donors for cultured meat production. Cattle muscle satellite cells were isolated from the muscle tissue of Hanwoo, Holstein, and Jeju black cattle, and then sorted by fluorescence activated cell sorting (FACS). Regarding proliferation of satellite cells, all three breeds showed similar trends. The myogenic potential, based on PAX7 and MYOD mRNA expression levels, was similar or significantly higher for Holstein than other breeds. When the area, width, and fusion index of the myotube were calculated through immunofluorescence staining of myosin, it was expressed upward for Holstein in all experiments except myotube area at passage 8. In addition, it was confirmed that Holstein's muscle satellite cells showed an upward expression in the amount of gene and protein expression related to myogenic. In the case of gene expression of MYOG, DES, and MYH4 known to play a key role in differentiation into muscles, it was confirmed that Holstein's muscle satellite cells expressed higher levels. CAV3, IGF1 and TNNT1, which contribute to hypertrophy and differentiation of muscle cells, showed high expression in Holstein. Our results suggest using cells from Holstein cattle can increase the efficiency of cultured meat production, compared to Hanwoo and Jeju breeds, because the cells exhibit superior differentiation behavior which would lead to greater yields during the maturation phase of bioprocessing.

The Effect of Geranylgeraniol and Ginger on Satellite Cells Myogenic State in Type 2 Diabetic Rats

Type 2 diabetes (T2D) is associated with increased inflammation and reactive oxygen species (ROS) in muscles, leading to basal satellite cell (SC) myogenic impairment (i.e., reduction in SC pool), which is critical for maintaining skeletal muscle mass. T2D may contribute to muscle atrophy, possibly due to reductions in the SC pool. Geranylgeraniol (GGOH) and ginger can reduce inflammation and enhance SC myogenesis in damaged muscles, thereby alleviating muscle atrophy; however, their effect on basal SC myogenic state and muscle mass in T2D rats is limited. Rats consumed a control diet (CON), high-fat diet with 35 mg/kg of streptozotocin (HFD), a HFD with 800 mg/kg body weight of GGOH (GG), or a HFD with 0.75% ginger root extract (GRE). In the eighth week, their soleus muscles were analyzed for Pax7, MyoD, and MSTN gene and protein expression, SC myogenic state, and muscle cross-sectional area (CSA). The HFD group had a significantly lower number of Pax7+/MyoD- and Pax7+/MSTN+ cells, less Pax7 and MyoD gene expression, and less MyoD and MSTN protein expression, with a smaller CSA than the CON group. Compared to the GG and GRE groups, the HFD group had a significantly lower number of Pax7+/MSTN+ cells, less MyoD protein expression, and smaller CSA. The GRE group also had a significantly lower number of Pax7-/MyoD+ and greater MSTN protein expression than the HFD group. Nevertheless, the CON group had a significantly greater number of Pax7+/MyoD- than the GG and GRE groups, and a greater number of Pax7-/MyoD+ cells than the GRE group with a larger CSA than the GG group. GGOH and ginger persevered muscle CSA, possibly through increased MyoD and the ability to maintain the SC pool in T2D rats.

Chicken Embryo Fibroblast Viability and Trans-Differentiation Potential for Cultured Meat Production Across Passages

This study was conducted to analyze the viability of primary chicken embryo fibroblasts and the efficiency of adipogenic trans-differentiation for cultured meat production. In isolating chicken embryo fibroblasts (CEFs) from a heterogeneous cell pool containing chicken satellite cells (CSCs), over 90% of CEFs expressed CD29 and vimentin. The analysis of the proliferative capabilities of CEFs revealed no significant differences in EdU-positive cells (%), cumulative cell number, doubling time, and growth rate from passage 1 to passage 9 (p > 0.05). This indicates that CEFs can be isolated by 2 h of pre-plating and survive stably up to passage 9, and that primary fibroblasts can serve as a valuable cell source for the cultured meat industry. Adipogenic trans-differentiation was induced up to passage 9 of CEFs. As passages increased, lipid accumulation and adipocyte size significantly decreased (p < 0.05). The reduced differentiation rate of primary CEFs with increasing passages poses a major challenge to the cost and efficiency of cultured meat production. Thus, effective cell management and the maintenance of cellular characteristics for a long time are crucial for ensuring stable and efficient cultured fat production in the cultured meat industry.

Exogenous hydrogen sulfide enhances myogenic differentiation of C2C12 myoblasts under high palmitate stress

Skeletal muscle atrophy was one of main complications of type 2 diabetes mellitus. Hydrogen sulfide (H2S) is involved in various physiological functions, such as anti-hypertension and anti-oxidant. Skeletal muscle atrophy caused by type 2 diabetes could lead to the regeneration of muscle fibers. Wnt signaling pathway plays a crucial important role in this process. H2S maybe regulate the Wnt signaling pathway to alleviate skeletal muscle atrophy, however, this role has not been clarified. The aim of this study is to investigate the potential regulatory role of H2S in the Wnt signaling pathway. C2C12 myoblasts treated with 500 μmol palmitate as an in vitro model. Western blot was used to detect the levels of CSE, PKM1, β-catenin, MuRF1, MYOG, MYF6 and MYOD1. In addition, MuRF1 was mutated at Cys44 and MuRF1 S-sulfhydration was detected by biotin switch assay. The interaction between PKM1 and MuRF1 was assessed via Co-immunoprecipitation. Differentiation of C2C12 myoblasts was evaluated using LAMININ staining. These data showed the levels of CSE, β-catenin, PKM1, MYOG, MYF6 and MYOD1 were decreased in pal group, compared with control and pal + NaHS groups. MuRF1 Cys44 mutants increased the protein levels of β-catenin, MYOG, MYF6 and MYOD1 in pal group. Our results suggest that H2S regulates the S-sulfhydration levels of MuRF1 at Cys44, influencing the ubiquitination levels of PKM1 and ultimately promoting myoblast differentiation.

Crusting-fabricated three-dimensional soy-based scaffolds for cultured meat production: A preliminary study

Crusting has been developed as a non-chemical and non-machine intensive scaffold fabrication method. This method is based on the self-assembling ability of soy biomolecules, allowing the fabrication of a three-dimensional network for cell growth. Preliminary characterization revealed differences in pore size, water absorption, and degradation between pure soy-based scaffold (Y2R) and with added glycerol (Y2G). The Fourier-transform infrared spectrum absorbance peaks of functional groups related to proteins, carbohydrates, and lipids hinted the integration of soy biomolecules potentially via the Maillard reaction, as supported by the visible browning of the scaffold surface. Microscopic images revealed aligned myotubes in both scaffolds, with Y2G myotubes having greater proximity after 72 h of proliferation. Both spontaneous and electro-stimulated contractions were recorded as early as 72 h in proliferation medium. Crusting-fabricated soy-based scaffolds can further be explored for its application in cultured meat production.

Inhibition of MAT2A Impairs Skeletal Muscle Repair Function

The regenerative capacity of muscle, which primarily relies on anabolic processes, diminishes with age, thereby reducing the effectiveness of therapeutic interventions aimed at treating age-related muscle atrophy. In this study, we observed a decline in the expression of methionine adenosine transferase 2A (MAT2A), which synthesizes S-adenosylmethionine (SAM), in the muscle tissues of both aged humans and mice. Considering MAT2A's critical role in anabolism, we hypothesized that its reduced expression contributes to the impaired regenerative capacity of aging skeletal muscle. Mimicking this age-related reduction in the MAT2A level, either by reducing gene expression or inhibiting enzymatic activity, led to inhibiting their differentiation into myotubes. In vivo, inhibiting MAT2A activity aggravated BaCl2-induced skeletal muscle damage and decreased the number of satellite cells, whereas supplementation with SAM improved these effects. RNA-sequencing analysis further revealed that the Fas cell surface death receptor (Fas) gene was upregulated in Mat2a-knockdown C2C12 cells. Suppressing MAT2A expression or activity elevated Fas protein levels and increased the proportion of apoptotic cells. Additionally, inhibition of MAT2A expression or activity increased p53 expression. In conclusion, our findings demonstrated that impaired MAT2A expression or activity compromised the regeneration and repair capabilities of skeletal muscle, partially through p53-Fas-mediated apoptosis.

Loss of Tob1 promotes muscle regeneration through muscle stem cell expansion

Muscle stem cells (MuSCs) play an indispensable role in postnatal muscle growth and hypertrophy in adults. MuSCs also retain a highly regenerative capacity and are therefore considered a promising stem cell source for regenerative therapy for muscle diseases. In this study, we identify tumor-suppressor protein Tob1 as a Pax7 target protein that negatively controls the population expansion of MuSCs. Tob1 protein is undetectable in the quiescent state but is upregulated during activation in MuSCs. Tob1 ablation in mice accelerates MuSC population expansion and boosts muscle regeneration. Moreover, inactivation of Tob1 in MuSCs ameliorates the efficiency of MuSC transplantation in a murine muscular dystrophy model. Collectively, selective targeting of Tob1 might be a therapeutic option for the treatment of muscular diseases, including muscular dystrophy and age-related sarcopenia.

Injection of Adipose-Derived Mesenchymal Stem/Stromal Cells Suppresses Muscle Atrophy Markers and Adipogenic Markers in a Rat Fatty Muscle Degeneration Model

Fatty muscle degeneration and muscle atrophy have not been successfully treated due to their irreversible pathology. This study evaluated the efficacy of rat adipose-derived mesenchymal stem/stromal cells (ADP MSCs) in treating fatty muscle degeneration (FD). A total of 36 rats were divided into three groups: the control (C) group (n = 12); FD model group, generated by sciatic nerve crushing (n = 12); and the group receiving ADP MSC treatment for FD (FD+MSCs) (n = 12). In Group FD+MSCs, ADP MSCs were injected locally into the gastrocnemius muscle one week after the FD model was created (Day 8). On Day 22 (n = 18) and Day 43 (n = 18), muscle morphology, histopathology, and molecular analyses (inflammation, muscle atrophy, adipocytes, and muscle differentiation markers) were performed. In Group FD+MSCs, the formation of immature myofibers was observed on Day 22, and mitigation of fatty degeneration and muscle atrophy progression was evident on Day 43. Gene expression of muscle atrophy markers (FBXO32, TRIM63, and FOXO1) and adipogenic markers (ADIPOQ, PPARG, FABP4, and PDGFRA) was lower in Group FD+MSCs than Group FD on Day 43. ADP MSCs induce anti-inflammatory effects, inhibit fat accumulation, and promote muscle regeneration, highlighting their potential as promising therapy for FD and atrophy.

The E3 ubiquitin ligase Nedd4L preserves skeletal muscle stem cell quiescence by inhibiting their activation

Adult stem cells play a critical role in tissue repair and maintenance. In tissues with slow turnover, including skeletal muscle, these cells are maintained in a mitotically quiescent state yet remain poised to re-enter the cell cycle to replenish themselves and regenerate the tissue. Using a panomics approach we show that the PAX7/NEDD4L axis acts against muscle stem cell activation in homeostatic skeletal muscle. Our findings suggest that PAX7 transcriptionally activates the E3 ubiquitin ligase Nedd4L and that the conditional genetic deletion of Nedd4L impairs muscle stem cell quiescence, with an upregulation of cell cycle and myogenic differentiation genes. Loss of Nedd4L in muscle stem cells results in the expression of doublecortin (DCX), which is exclusively expressed during their in vivo activation. Together, these data establish that the ubiquitin proteasome system, mediated by Nedd4L, is a key contributor to the muscle stem cell quiescent state in adult mice.

Muscle-specific pyruvate kinase isoforms, PKM1 and PKM2, regulate mammalian SWI/SNF proteins and histone 3 phosphorylation during myoblast differentiation

Pyruvate kinase is a glycolytic enzyme that converts phosphoenolpyruvate and ADP into pyruvate and ATP. There are two genes that encode pyruvate kinase in vertebrates; Pkm and Pkl encode muscle- and liver/erythrocyte-specific forms, respectively. Each gene encodes two isoenzymes due to alternative splicing. Both muscle-specific enzymes, PKM1 and PKM2, function in glycolysis, but PKM2 also has been implicated in gene regulation due to its ability to phosphorylate histone 3 threonine 11 (H3T11) in cancer cells. Here, we examined the roles of PKM1 and PKM2 during myoblast differentiation. RNA-seq analysis revealed that PKM2 promotes the expression of Dpf2/Baf45d and Baf250a/Arid1A. DPF2 and BAF250a are subunits that identify a specific sub-family of the mammalian SWI/SNF (mSWI/SNF) of chromatin remodeling enzymes that is required for the activation of myogenic gene expression during differentiation. PKM2 also mediated the incorporation of DPF2 and BAF250a into the regulatory sequences controlling myogenic gene expression. PKM1 did not affect expression but was required for nuclear localization of DPF2. Additionally, PKM2 was required not only for the incorporation of phosphorylated H3T11 in myogenic promoters but also for the incorporation of phosphorylated H3T6 and H3T45 at myogenic promoters via regulation of AKT and protein kinase C isoforms that phosphorylate those amino acids. Our results identify multiple unique roles for PKM2 and a novel function for PKM1 in gene expression and chromatin regulation during myoblast differentiation.

β-hydroxybutyrate exposure restores mitochondrial function in skeletal muscle satellite cells of critically ill patients

BACKGROUND & AIM

Dysfunction of skeletal muscle satellite cells might impair muscle regeneration and prolong ICU-acquired weakness, a condition associated with disability and delayed death. This study aimed to elucidate the distinct metabolic effects of critical illness and β-OH-butyrate on satellite cells isolated from these patients.

METHODS

Satellite cells were extracted from vastus lateralis muscle biopsies of patients with ICU-acquired weakness (n = 10) and control group of healthy volunteers or patients undergoing elective hip replacement surgery (n = 10). The cells were exposed to standard culture media supplemented with β-OH-butyrate to assess its influence on cell proliferation by ELISA, mitochondrial functions by extracellular flux analysis, electron transport chain complexes by high resolution respirometry, and ROS production by confocal microscopy.

RESULTS

Critical illness led to a decline in maximal respiratory capacity, ATP production and glycolytic capacity and increased ROS production in ICU patients' cells. Notably, the function of complex II was impaired due to critical illness but restored to normal levels upon exposure to β-OH-butyrate. While β-OH-butyrate significantly reduced ROS production in both control and ICU groups, it had no significant impact on global mitochondrial functions.

CONCLUSION

Critical illness induces measurable bioenergetic dysfunction of skeletal muscle satellite cells. β-OH-butyrate displayed a potential in rectifying complex II dysfunction caused by critical illness and this warrants further exploration.

Mutant RAS-driven Secretome Causes Skeletal Muscle Defects in Breast Cancer

Cancer-induced skeletal muscle defects differ in severity between individuals with the same cancer type. Cancer subtype-specific genomic aberrations are suggested to mediate these differences, but experimental validation studies are very limited. We utilized three different breast cancer patient-derived xenograft (PDX) models to correlate cancer subtype with skeletal muscle defects. PDXs were derived from brain metastasis of triple-negative breast cancer (TNBC), estrogen receptor-positive/progesterone receptor-positive (ER+/PR+) primary breast cancer from a BRCA2-mutation carrier, and pleural effusion from an ER+/PR- breast cancer. While impaired skeletal muscle function as measured through rotarod performance and reduced levels of circulating and/or skeletal muscle miR-486 were common across all three PDXs, only TNBC-derived PDX activated phospho-p38 in skeletal muscle. To further extend these results, we generated transformed variants of human primary breast epithelial cells from healthy donors using HRASG12V or PIK3CAH1047R mutant oncogenes. Mutations in RAS oncogene or its modulators are found in approximately 37% of metastatic breast cancers, which is often associated with skeletal muscle defects. Although cells transformed with both oncogenes generated adenocarcinomas in NSG mice, only HRASG12V-derived tumors caused skeletal muscle defects affecting rotarod performance, skeletal muscle contraction force, and miR-486, Pax7, pAKT, and p53 levels in skeletal muscle. Circulating levels of the chemokine CXCL1 were elevated only in animals with tumors containing HRASG12V mutation. Because RAS pathway aberrations are found in 19% of cancers, evaluating skeletal muscle defects in the context of genomic aberrations in cancers, particularly RAS pathway mutations, may accelerate development of therapeutic modalities to overcome cancer-induced systemic effects.

SIGNIFICANCE

Mutant RAS- and PIK3CA-driven breast cancers distinctly affect the function of skeletal muscle. Therefore, research and therapeutic targeting of cancer-induced systemic effects need to take aberrant cancer genome into consideration.

The Role of the MYL4 Gene in Porcine Muscle Development and Its Molecular Regulatory Mechanisms

Muscle growth stands as a pivotal economic trait within pig production, governed by a complex interplay of multiple genes, each playing a role in its quantitative manifestation. Understanding the intricate regulatory mechanisms of porcine muscle development is crucial for enhancing both pork yield and quality. This study used the GSE99749 dataset downloaded from the GEO database, conducting a detailed analysis of the RNA-seq results from the longissimus dorsi muscle (LD) of Tibetan pigs (TP), Wujin pigs (WJ) and large white pigs (LW) at 60 days of gestation, representing diverse body sizes and growth rates. Comparative analyses between TPvsWJ and TPvsLW, along with differential gene expression (DEG) analysis, functional enrichment analysis, and protein-protein interaction (PPI) network analysis, revealed 1048 and 1157 significantly differentially expressed genes (p < 0.001) in TPvsWJ and TPvsLW, respectively. With stricter screening criteria, 37 DEGs were found to overlap between the 2 groups. PPI analysis identified MYL5, MYL4, and ACTC1 as the three core genes. This article focuses on exploring the MYL4 gene. Molecular-level experimental validation, through overexpression and interference of the MYL4 gene combined with EDU staining experiments, demonstrated that overexpression of MYL4 significantly promoted the proliferation of porcine skeletal muscle satellite cells (PSMSC), while interference with MYL4 inhibited their proliferation. Furthermore, by examining the effects of overexpressing and interfering with the MYL4 gene on the muscle hypertrophy marker Fst gene and the muscle degradation marker FOXO3 gene, the pivotal role of the MYL4 gene in promoting muscle growth and preventing muscle degradation was further confirmed. These findings offer a new perspective on the molecular mechanisms behind porcine muscle growth and development, furnishing valuable data and insights for muscle biology research.

Photobiomodulation mitigates Bothrops jararacussu venom-induced damage in myoblast cells by enhancing myogenic factors and reducing cytokine production

BACKGROUND

Photobiomodulation has exhibited promise in mitigating the local effects induced by Bothrops snakebite envenoming; however, the mechanisms underlying this protection are not yet fully understood. Herein, the effectiveness of photobiomodulation effects on regenerative response of C2C12 myoblast cells following exposure to Bothrops jararacussu venom (BjsuV), as well as the mechanisms involved was investigated.

METHODOLOGY/PRINCIPAL FINDINGS

C2C12 myoblast cells were exposed to BjsuV (12.5 μg/mL) and irradiated once for 10 seconds with laser light of 660 nm (14.08 mW; 0.04 cm2; 352 mW/cm2) or 780 nm (17.6 mW; 0.04 cm2; 440 mW/ cm2) to provide energy densities of 3.52 and 4.4 J/cm2, and total energies of 0.1408 and 0.176 J, respectively. Cell migration was assessed through a wound-healing assay. The expression of MAPK p38-α, NF-Кβ, Myf5, Pax-7, MyoD, and myogenin proteins were assessed by western blotting analysis. In addition, interleukin IL1-β, IL-6, TNF-alfa and IL-10 levels were measured in the supernatant by ELISA. The PBM applied to C2C12 cells exposed to BjsuV promoted cell migration, increase the expression of myogenic factors (Pax7, MyF5, MyoD and myogenin), reduced the levels of proinflammatory cytokines, IL1-β, IL-6, TNF-alfa, and increased the levels of anti-inflammatory cytokine IL-10. In addition, PBM downregulates the expression of NF-kB, and had no effect on p38 MAKP.

CONCLUSION/SIGNIFICANCE

These data demonstrated that protection of the muscle cell by PBM seems to be related to the increase of myogenic factors as well as the modulation of inflammatory mediators. PBM therapy may offer a new therapeutic strategy to address the local effects of snakebite envenoming by promoting muscle regeneration and reducing the inflammatory process.

Muscle-Specific Pyruvate Kinase Isoforms, Pkm1 and Pkm2, Regulate Mammalian SWI/SNF Proteins and Histone 3 Phosphorylation During Myoblast Differentiation

Pyruvate kinase is a glycolytic enzyme that converts phosphoenolpyruvate and ADP into pyruvate and ATP. There are two genes that encode pyruvate kinase in vertebrates; Pkm and Pkl encode muscle- and liver/erythrocyte-specific forms, respectively. Each gene encodes two isoenzymes due to alternative splicing. Both muscle-specific enzymes, Pkm1 and Pkm2, function in glycolysis, but Pkm2 also has been implicated in gene regulation due to its ability to phosphorylate histone 3 threonine 11 (H3T11) in cancer cells. Here, we examined the roles of Pkm1 and Pkm2 during myoblast differentiation. RNA-seq analysis revealed that Pkm2 promotes the expression of Dpf2/Baf45d and Baf250a/Arid1A. Dpf2 and Baf250a are subunits that identify a specific sub-family of the mammalian SWI/SNF (mSWI/SNF) of chromatin remodeling enzymes that is required for activation of myogenic gene expression during differentiation. Pkm2 also mediated the incorporation of Dpf2 and Baf250a into the regulatory sequences controlling myogenic gene expression. Pkm1 did not affect expression but was required for nuclear localization of Dpf2. Additionally, Pkm2 was required not only for the incorporation of phosphorylated H3T11 in myogenic promoters, but also for the incorporation of phosphorylated H3T6 and H3T45 at myogenic promoters via regulation of AKT and protein kinase C isoforms that phosphorylate those amino acids. Our results identify multiple unique roles for Pkm2 and a novel function for Pkm1 in gene expression and chromatin regulation during myoblast differentiation.

The satellite cell in skeletal muscle: A story of heterogeneity

Skeletal muscle is a highly represented tissue in mammals and is composed of fibers that are extremely adaptable and capable of regeneration. This characteristic of muscle fibers is made possible by a cell type called satellite cells. Adjacent to the fibers, satellite cells are found in a quiescent state and located between the muscle fibers membrane and the basal lamina. These cells are required for the growth and regeneration of skeletal muscle through myogenesis. This process is known to be tightly sequenced from the activation to the differentiation/fusion of myofibers. However, for the past fifteen years, researchers have been interested in examining satellite cell heterogeneity and have identified different subpopulations displaying distinct characteristics based on localization, quiescence state, stemness capacity, cell-cycle progression or gene expression. A small subset of satellite cells appears to represent multipotent long-term self-renewing muscle stem cells (MuSC). All these distinctions led us to the hypothesis that the characteristics of myogenesis might not be linear and therefore may be more permissive based on the evidence that satellite cells are a heterogeneous population. In this review, we discuss the different subpopulations that exist within the satellite cell pool to highlight the heterogeneity and to gain further understanding of the myogenesis progress. Finally, we discuss the long term self-renewing MuSC subpopulation that is capable of dividing asymmetrically and discuss the molecular mechanisms regulating MuSC polarization during health and disease.

snRNA-seq analysis in multinucleated myogenic FSHD cells identifies heterogeneous FSHD transcriptome signatures associated with embryonic-like program activation and oxidative stress-induced apoptosis

The sporadic nature of DUX4 expression in FSHD muscle challenges comparative transcriptome analyses between FSHD and control samples. A variety of DUX4 and FSHD-associated transcriptional changes have been identified, but bulk RNA-seq strategies prohibit comprehensive analysis of their spatiotemporal relation, interdependence and role in the disease process. In this study, we used single-nucleus RNA-sequencing of nuclei isolated from patient- and control-derived multinucleated primary myotubes to investigate the cellular heterogeneity in FSHD. Taking advantage of the increased resolution in snRNA-sequencing of fully differentiated myotubes, two distinct populations of DUX4-affected nuclei could be defined by their transcriptional profiles. Our data provides insights into the differences between these two populations and suggests heterogeneity in two well-known FSHD-associated transcriptional aberrations: increased oxidative stress and inhibition of myogenic differentiation. Additionally, we provide evidence that DUX4-affected nuclei share transcriptome features with early embryonic cells beyond the well-described cleavage stage, progressing into the 8-cell and blastocyst stages. Altogether, our data suggests that the FSHD transcriptional profile is defined by a mixture of individual and sometimes mutually exclusive DUX4-induced responses and cellular state-dependent downstream effects.

Leukocyte-poor platelet-rich plasma and leukocyte-rich platelet-rich plasma promote myoblast proliferation through the upregulation of cyclin A, cdk1, and cdk2

Muscle injuries are common among athletes and often treated with platelet-rich plasma (PRP). However, whether the leukocyte concentration affects the efficacy of PRP in treating muscle injuries remains unclear. This study investigated the effects of leukocyte-poor platelet-rich plasma (LP-PRP) and leukocyte-rich platelet-rich plasma (LR-PRP) on myoblast proliferation and the molecular mechanisms underlying these effects. Myoblasts were treated with 0.5% LP-PRP, 0.5% LR-PRP, 1% LP-PRP, or 1% LR-PRP for 24 h. The gene expression of the LP-PRP- and LR-PRP-treated myoblasts was determined using RNA sequencing analysis. Cell proliferation was evaluated using an bromodeoxyuridine (BrdU) assay, and cell cycle progression was assessed through flow cytometry. The expression of cyclin A, cyclin-dependent kinase 1 (cdk1), and cdk2 was examined using Western blotting. The expression of myoblast determination protein 1 (MyoD1) was examined through Western blotting and immunofluorescence staining. The LP-PRP and LR-PRP both promoted the proliferation of myoblasts and increased differential gene expression of myoblasts. Moreover, the LP-PRP and LR-PRP substantially upregulated the expression of cyclin A, cdk1, and cdk2. MyoD1 expression was induced in the LP-PRP and LR-PRP-treated myoblasts. Our results corroborate the finding that LP-PRP and LR-PRP have similar positive effects on myoblast proliferation and MyoD1 expression.

The Role of Hypoxia and Hypoxia Signaling in Skeletal Muscle Physiology

Hypoxia and hypoxia signaling play an integral role in regulating skeletal muscle physiology. Environmental hypoxia and tissue hypoxia in muscles cue for their appropriate physiological response and adaptation, and cause an array of cellular and metabolic changes. In addition, muscle stem cells (satellite cells), exist in a hypoxic state, and this intrinsic hypoxic state correlates with their quiescence and stemness. The mechanisms of hypoxia-mediated regulation of satellite cells and myogenesis are yet to be characterized, and their seemingly contradicting effects reported leave their exact roles somewhat perplexing. This review summarizes the recent findings on the effect of hypoxia and hypoxia signaling on the key aspects of muscle physiology, namely, stem cell maintenance and myogenesis with a particular attention given to distinguish the intrinsic versus local hypoxia in an attempt to better understand their respective regulatory roles and how their relationship affects the overall response. This review further describes their mechanistic links and their possible implications on the relevant pathologies and therapeutics.

Pax7 reporter mouse models: a pocket guide for satellite cell research

Since their discovery, satellite cells have showcased their need as primary contributors to skeletal muscle maintenance and repair. Satellite cells lay dormant, but when needed, activate, differentiate, fuse to fibres and self-renew, that has bestowed satellite cells with the title of muscle stem cells. The satellite cell specific transcription factor Pax7 has enabled researchers to develop animal models against the Pax7 locus in order to isolate and characterise satellite cell-mediated events. This review focuses specifically on describing Pax7 reporter mouse models. Here we describe how each model was generated and the key findings obtained. The strengths and limitations of each model are also discussed. The aim is to provide new and current satellite cell enthusiasts with a basic understanding of the available Pax7 reporter mice and hopefully guide selection of the most appropriate Pax7 model to answer a specific research question.

In Vitro and In Vivo Evidence towards Fibronectin's Protective Effects against Prion Infection

A distinctive signature of the prion diseases is the accumulation of the pathogenic isoform of the prion protein, PrPSc, in the central nervous system of prion-affected humans and animals. PrPSc is also found in peripheral tissues, raising concerns about the potential transmission of pathogenic prions through human food supplies and posing a significant risk to public health. Although muscle tissues are considered to contain levels of low prion infectivity, it has been shown that myotubes in culture efficiently propagate PrPSc. Given the high consumption of muscle tissue, it is important to understand what factors could influence the establishment of a prion infection in muscle tissue. Here we used in vitro myotube cultures, differentiated from the C2C12 myoblast cell line (dC2C12), to identify factors affecting prion replication. A range of experimental conditions revealed that PrPSc is tightly associated with proteins found in the systemic extracellular matrix, mostly fibronectin (FN). The interaction of PrPSc with FN decreased prion infectivity, as determined by standard scrapie cell assay. Interestingly, the prion-resistant reserve cells in dC2C12 cultures displayed a FN-rich extracellular matrix while the prion-susceptible myotubes expressed FN at a low level. In agreement with the in vitro results, immunohistopathological analyses of tissues from sheep infected with natural scrapie demonstrated a prion susceptibility phenotype linked to an extracellular matrix with undetectable levels of FN. Conversely, PrPSc deposits were not observed in tissues expressing FN. These data indicate that extracellular FN may act as a natural barrier against prion replication and that the extracellular matrix composition may be a crucial feature determining prion tropism in different tissues.

Klotho regulates the myogenic response of muscle to mechanical loading and exercise

NEW FINDINGS

What is the central question of this study? Does the hormone Klotho affect the myogenic response of muscle cells to mechanical loading or exercise? What is the main finding and its importance? Klotho prevents direct, mechanical activation of genes that regulate muscle differentiation, including genes that encode the myogenic regulatory factor myogenin and proteins in the canonical Wnt signalling pathway. Similarly, elevated levels of klotho expression in vivo prevent the exercise-induced increase in myogenin-expressing cells and reduce exercise-induced activation of the Wnt pathway. These findings demonstrate a new mechanism through which the responses of muscle to the mechanical environment are regulated.

ABSTRACT

Muscle growth is influenced by changes in the mechanical environment that affect the expression of genes that regulate myogenesis. We tested whether the hormone Klotho could influence the response of muscle to mechanical loading. Applying mechanical loads to myoblasts in vitro increased RNA encoding transcription factors that are expressed in activated myoblasts (Myod) and in myogenic cells that have initiated terminal differentiation (Myog). However, application of Klotho to myoblasts prevented the loading-induced activation of Myog without affecting loading-induced activation of Myod. This indicates that elevated Klotho inhibits mechanically-induced differentiation of myogenic cells. Elevated Klotho also reduced the transcription of genes encoding proteins involved in the canonical Wnt pathway or their target genes (Wnt9a, Wnt10a, Ccnd1). Because the canonical Wnt pathway promotes differentiation of myogenic cells, these findings indicate that Klotho inhibits the differentiation of myogenic cells experiencing mechanical loading. We then tested whether these effects of Klotho occurred in muscles of mice experiencing high-intensity interval training (HIIT) by comparing wild-type mice and klotho transgenic mice. The expression of a klotho transgene combined with HIIT synergized to tremendously elevate numbers of Pax7+ satellite cells and activated MyoD+ cells. However, transgene expression prevented the increase in myogenin+ cells caused by HIIT in wild-type mice. Furthermore, transgene expression diminished the HIIT-induced activation of the canonical Wnt pathway in Pax7+ satellite cells. Collectively, these findings show that Klotho inhibits loading- or exercise-induced activation of muscle differentiation and indicate a new mechanism through which the responses of muscle to the mechanical environment are regulated.

Perish in the Attempt: Regulated Cell Death in Regenerative and Nonregenerative Tissue

Significance: A cell plays its roles throughout its life span, even during its demise. Regulated cell death (RCD) is one of the key topics in modern biomedical studies. It is considered the main approach for removing stressed and/or damaged cells. Research during the past two decades revealed more roles of RCD, such as coordinating tissue development and driving compensatory proliferation during tissue repair. Recent Advances: Compensatory proliferation, initially identified in primitive organisms during the regeneration of lost tissue, is an evolutionarily conserved process that also functions in mammals. Among various types of RCD, apoptosis is considered the top candidate to induce compensatory proliferation in damaged tissue. Critical Issues: The roles of apoptosis in the recovery of nonregenerative tissue are still vague. The roles of other types of RCD, such as necroptosis and ferroptosis, have not been well characterized in the context of tissue regeneration. Future Directions: In this review article, we attempt to summarize the recent insights on the role of RCD in tissue repair. We focus on apoptosis, with expansion to ferroptosis and necroptosis, in primitive organisms with significant regenerative capacity as well as common mammalian research models. After gathering hints from regenerative tissue, in the second half of the review, we take a notoriously nonregenerative tissue, the myocardium, as an example to discuss the role of RCD in terminally differentiated quiescent cells. Antioxid. Redox Signal. 39, 1053-1069.

Therapeutic Myogenesis Induced by Ultrasound Exposure in a Volumetric Skeletal Muscle Loss Injury Model

BACKGROUND

Low-intensity pulsed ultrasound (LIPUS) irradiation has been shown to induce various responses in different cells. It has been shown that LIPUS activates extracellular signal-regulated kinase 1/2 (ERK1/2) through integrin.

PURPOSE

To study the effects of LIPUS on myogenic regulatory factors and other related myogenesis elements in a volumetric skeletal muscle loss injury model.

STUDY DESIGN

Controlled laboratory study.

METHODS

C57BL/6J mice were subjected to full-thickness muscle defect injury of the quadriceps and treated with direct application of LIPUS 20 min/d or non-LIPUS treatment (control) for 3, 7, and 14 days. LIPUS was also applied to C2C12 cells in culture in the presence of low and high doses of lipopolysaccharides. The expression levels of myogenic regulatory factors and the expression levels of myokine-related and angiogenic-related proteins of the control and LIPUS groups were analyzed.

RESULTS

Muscle volume in the injury site was restored at day 14 with LIPUS treatment. Paired-box protein 7, myogenic factor 5, myogenin, and desmin expressions were significantly different between control and LIPUS groups at days 7 and 14. Myokine and angiogenic cytokine-related factors were significantly increased in the LIPUS group at day 3 and decreased with no significant difference between the groups by day 14. LIPUS induced different responses of myogenic regulatory factors in C2C12 cells with low and high doses of lipopolysaccharides. LIPUS promoted myogenesis through short-lived increase in interleukin-6 and heme oxygenase 1, together with activation of ERK1/2.

CONCLUSION

LIPUS had a constant effect on the variables of tissue damage, from macrotrauma to microtrauma, leading to efficient muscle regeneration.

CLINICAL RELEVANCE

The focus of therapeutic strategies with LIPUS has been not only for microvascular regeneration but also for skeletal muscle and related local tissue recovery from acute or chronic damage.

Loss of adenosine A3 receptors accelerates skeletal muscle regeneration in mice following cardiotoxin-induced injury

Skeletal muscle regeneration is a complex process orchestrated by multiple interacting steps. An increasing number of reports indicate that inflammatory responses play a central role in linking initial muscle injury responses to timely muscle regeneration following injury. The nucleoside adenosine has been known for a long time as an endogenously produced anti-inflammatory molecule that is generated in high amounts during tissue injury. It mediates its physiological effects via four types of adenosine receptors. From these, adenosine A3 receptors (A3Rs) are not expressed by the skeletal muscle but are present on the surface of various inflammatory cells. In the present paper, the effect of the loss of A3Rs was investigated on the regeneration of the tibialis anterior (TA) muscle in mice following cardiotoxin-induced injury. Here we report that regeneration of the skeletal muscle from A3R-/- mice is characterized by a stronger initial inflammatory response resulting in a larger number of transmigrating inflammatory cells to the injury site, faster clearance of cell debris, enhanced proliferation and faster differentiation of the satellite cells (the muscle stem cells), and increased fusion of the generated myoblasts. This leads to accelerated skeletal muscle tissue repair and the formation of larger myofibers. Though the infiltrating immune cells expressed A3Rs and showed an increased inflammatory profile in the injured A3R-/- muscles, bone marrow transplantation experiments revealed that the increased response of the tissue-resident cells to tissue injury is responsible for the observed phenomenon. Altogether our data indicate that A3Rs are negative regulators of injury-related regenerative inflammation and consequently also that of the muscle fiber growth in the TA muscle. Thus, inhibiting A3Rs might have a therapeutic value during skeletal muscle regeneration following injury.

Apabetalone, a Clinical-Stage, Selective BET Inhibitor, Opposes DUX4 Target Gene Expression in Primary Human FSHD Muscle Cells

Facioscapulohumeral dystrophy (FSHD) is a muscle disease caused by inappropriate expression of the double homeobox 4 (DUX4) gene in skeletal muscle, and its downstream activation of pro-apoptotic transcriptional programs. Inhibitors of DUX4 expression have the potential to treat FSHD. Apabetalone is a clinical-stage bromodomain and extra-terminal (BET) inhibitor, selective for the second bromodomain on BET proteins. Using primary human skeletal muscle cells from FSHD type 1 patients, we evaluated apabetalone for its ability to counter DUX4's deleterious effects and compared it with the pan-BET inhibitor JQ1, and the p38 MAPK inhibitor-and DUX4 transcriptional repressor-losmapimod. We applied RNA-sequencing and bioinformatic analysis to detect treatment-associated impacts on the transcriptome of these cells. Apabetalone inhibited the expression of DUX4 downstream markers, reversing hallmarks of FSHD gene expression in differentiated muscle cells. JQ1, but not apabetalone, was found to induce apoptosis. While both BET inhibitors modestly impacted differentiation marker expression, they did not affect myotube fusion. Losmapimod also reduced expression of DUX4 target genes but differed in its impact on FSHD-associated pathways. These findings demonstrate that apabetalone inhibits DUX4 target gene expression and reverses transcriptional programs that contribute to FSHD pathology, making this drug a promising candidate therapeutic for FSHD.

Early Weaning Possibly Increases the Activity of Lipogenic and Adipogenic Pathways in Intramuscular Adipose Tissue of Nellore Calves

This study aimed to evaluate by wide-expression profile analysis how early weaning at 120 days can alter the skeletal muscle metabolism of calves supplemented with a concentrated diet until the growth phase. Longissimus thoracis muscle samples were obtained by biopsy from two groups of calves, early weaned (EW; n = 8) and conventionally weaned (CW; n = 8) at two different times (120 days of age-T1 [EW] and 205 days of age-T2 [CW]). Next, differential gene expression analysis and functional enrichment of metabolic pathways and biological processes were performed. The results showed respectively 658 and 165 differentially expressed genes when T1 and T2 were contrasted in the early weaning group and when early and conventionally weaned groups were compared at T2. The FABP4, SCD1, FASN, LDLR, ADIPOQ, ACACA, PPARD, and ACOX3 genes were prospected in both comparisons described above. Given the key role of these differentially expressed genes in lipid and fatty acid metabolism, the results demonstrate the effect of diet on the modulation of energy metabolism, particularly favoring postnatal adipogenesis and lipogenesis, as well as a consequent trend in obtaining better quality cuts, as long as an environment for the maintenance of these alterations until adulthood is provided.

Intraspecific variation in muscle growth of two distinct populations of Port Jackson sharks under projected end-of-century temperatures

Although pervasive, the effects of climate change vary regionally, possibly resulting in differential behavioral, physiological, and/or phenotypic responses among populations within broadly distributed species. Juvenile Port Jackson sharks (Heterodontus portusjacksoni) from eastern and southern Australia were reared at their current (17.6 °C Adelaide, South Australia [SA]; 20.6 °C Jervis Bay, New South Wales [NSW]) or projected end-of-century (EOC) temperatures (20.6 °C Adelaide, SA; 23.6 °C Jervis Bay, NSW) and assessed for morphological features of skeletal muscle tissue. Nearly all skeletal muscle properties including cellularity, fiber size, myonuclear domain, and satellite cell density did not differ between locations and thermal regimes. However, capillary density was significantly influenced by thermal treatment, where Adelaide sharks raised at current temperatures had a lower capillarity than Jervis Bay sharks raised at ambient or projected EOC temperatures. This may indicate higher metabolic costs at elevated temperatures. However, our results suggest that regardless of the population, juvenile Port Jackson sharks may have limited acclimatory potential to alter muscle metabolic features under a temperature increase, which may make this species vulnerable to climate change.

Skeletal muscle atrophy, regeneration, and dysfunction in heart failure: Impact of exercise training

This review highlights some established and some more contemporary mechanisms responsible for heart failure (HF)-induced skeletal muscle wasting and weakness. We first describe the effects of HF on the relationship between protein synthesis and degradation rates, which determine muscle mass, the involvement of the satellite cells for continual muscle regeneration, and changes in myofiber calcium homeostasis linked to contractile dysfunction. We then highlight key mechanistic effects of both aerobic and resistance exercise training on skeletal muscle in HF and outline its application as a beneficial treatment. Overall, HF causes multiple impairments related to autophagy, anabolic-catabolic signaling, satellite cell proliferation, and calcium homeostasis, which together promote fiber atrophy, contractile dysfunction, and impaired regeneration. Although both wasting and weakness are partly rescued by aerobic and resistance exercise training in HF, the effects of satellite cell dynamics remain poorly explored.

PAX7, a Key for Myogenesis Modulation in Muscular Dystrophies through Multiple Signaling Pathways: A Systematic Review

Muscular dystrophy is a heterogenous group of hereditary muscle disorders caused by mutations in the genes responsible for muscle development, and is generally defined by a disastrous progression of muscle wasting and massive loss in muscle regeneration. Pax7 is closely associated with myogenesis, which is governed by various signaling pathways throughout a lifetime and is frequently used as an indicator in muscle research. In this review, an extensive literature search adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines was performed to identify research that examined signaling pathways in living models, while quantifying Pax7 expression in myogenesis. A total of 247 articles were retrieved from the Web of Science (WoS), PubMed and Scopus databases and were thoroughly examined and evaluated, resulting in 19 articles which met the inclusion criteria. Admittedly, we were only able to discuss the quantification of Pax7 carried out in research affecting various type of genes and signaling pathways, rather than the expression of Pax7 itself, due to the massive differences in approach, factor molecules and signaling pathways analyzed across the research. However, we highlighted the thorough evidence for the alteration of the muscle stem cell precursor Pax7 in multiple signaling pathways described in different living models, with an emphasis on the novel approach that could be taken in manipulating Pax7 expression itself in dystrophic muscle, towards the discovery of an effective treatment for muscular dystrophy. Therefore, we believe that this could be applied to the potential gap in muscle research that could be filled by tuning the well-established marker expression to improve dystrophic muscle.

Expansion of bovine skeletal muscle stem cells from spinner flasks to benchtop stirred-tank bioreactors for up to 38 days

Introduction

Successful long-term expansion of skeletal muscle satellite cells (MuSCs) on a large scale is fundamental for cultivating animal cells for protein production. Prerequisites for efficient cell expansion include maintaining essential native cell activities such as cell adhesion, migration, proliferation, and differentiation while ensuring consistent reproducibility.

Method

This study investigated the growth of bovine MuSC culture using low-volume spinner flasks and a benchtop stirred-tank bioreactor (STR).

Results and discussion

Our results showed for the first time the expansion of primary MuSCs for 38 days in a bench-top STR run with low initial seeding density and FBS reduction, supported by increased expression of the satellite cell marker PAX7 and reduced expression of differentiation-inducing genes like MYOG, even without adding p38-MAPK inhibitors. Moreover, the cells retained their ability to proliferate, migrate, and differentiate after enzymatic dissociation from the microcarriers. We also showed reproducible results in a separate biological benchtop STR run.

Bovine muscle satellite cells in calves and cattle: A comparative study of cellular and genetic characteristics for cultivated meat production

This study compared the cellular and genetic characteristics of bovine skeletal muscle satellite cells (SMSCs) from Hanwoo (a Korean native cattle breed), including calves and mature cattle. SMSCs were isolated using magnetic-activated cell sorting (MACS) from tissue samples of six Hanwoo (three calves and three mature cattle) using the CD29 antibody. Calves' SMSCs exhibited significantly faster growth rates than did those from cattle (P < 0.01), with a doubling time of 2.43 days. Genetic analysis revealed higher MyoD and Pax7 expression in SMSCs from calves during proliferation than in those from mature cattle (P < 0.001). However, FASN and PLAG1 expression levels were higher in mature cattle than in calves during both proliferation and differentiation (P < 0.001). These findings highlight the need for strategies to improve bovine muscle cell growth to produce competitive cultivated meat at a competitive price.

Aging disrupts gene expression timing during muscle regeneration

Skeletal muscle function and regenerative capacity decline during aging, yet factors driving these changes are incompletely understood. Muscle regeneration requires temporally coordinated transcriptional programs to drive myogenic stem cells to activate, proliferate, fuse to form myofibers, and to mature as myonuclei, restoring muscle function after injury. We assessed global changes in myogenic transcription programs distinguishing muscle regeneration in aged mice from young mice by comparing pseudotime trajectories from single-nucleus RNA sequencing of myogenic nuclei. Aging-specific differences in coordinating myogenic transcription programs necessary for restoring muscle function occur following muscle injury, likely contributing to compromised regeneration in aged mice. Differences in pseudotime alignment of myogenic nuclei when comparing aged with young mice via dynamic time warping revealed pseudotemporal differences becoming progressively more severe as regeneration proceeds. Disruptions in timing of myogenic gene expression programs may contribute to incomplete skeletal muscle regeneration and declines in muscle function as organisms age.

Molecular clocks, satellite cells, and skeletal muscle regeneration

Skeletal muscle comprises approximately 50% of individual body mass and plays vital roles in locomotion, heat production, and whole body metabolic homeostasis. This tissue exhibits a robust diurnal rhythm that is under control of the suprachiasmatic nucleus (SCN) region of the hypothalamus. The SCN acts as a "central" coordinator of circadian rhythms, while cell-autonomous "peripheral" clocks are located within almost all other tissues/organs in the body. Synchronization of the peripheral clocks in muscles (and other tissues) together with the central clock is crucial to ensure temporally coordinated physiology across all organ systems. By virtue of its mass, human skeletal muscle contains the largest collection of peripheral clocks, but within muscle resides a local stem cell population, satellite cells (SCs), which have their own functional molecular clock, independent of the numerous muscle clocks. Skeletal muscle has a daily turnover rate of 1%-2%, so the regenerative capacity of this tissue is important for whole body homeostasis/repair and depends on successful SC myogenic progression (i.e., proliferation, differentiation, and fusion). Emerging evidence suggests that SC-mediated muscle regeneration may, in part, be regulated by molecular clocks involved in SC-specific diurnal transcription. Here we provide insights on molecular clock regulation of muscle regeneration/repair and provide a novel perspective on the interplay between SC-specific molecular clocks, myogenic programs, and cell cycle kinetics that underpin myogenic progression.

In vitro myogenesis activation of specific muscle-derived stem cells from patients with Duchenne muscular dystrophy

BACKGROUND

Muscle-derived stem cells (MDSCs) contribute to the repair of injured muscles. However, the myogenicity of MDSCs generated from patients with Duchenne muscular dystrophy (DMD) relative to healthy individuals remains unclear.

METHODS

A human DMD model was established using the stem cells prepared from muscle derived from patients with DMD (DMD-hMDSCs). The expression of myogenic lineage-specific markers in MDSCs was examined with immunofluorescence, real-time polymerase chain reaction, and western blotting.

RESULTS

It was demonstrated that, compared with cells from healthy subjects, DMD-hMDSCs are primed to self-differentiate in growth-inducing medium (GM) and robustly differentiate into myotubes in differentiation-inducing medium(DM). This feature was termed "myogenesis activation," and it was speculated that it contributes to the depletion of myogenic progenitors. Furthermore, MDSCs consistently express pax7, but the time-course of this expression does not correlate with the expression of the myogenic lineage-specific markers.

CONCLUSIONS

The myogenesis activation in DMD-hMDSCs demonstrated in this study may provide novel mechanistic insights into DMD pathogenesis and potential therapies.

E3 ligase Deltex2 accelerates myoblast proliferation and inhibits myoblast differentiation by targeting Pax7 and MyoD, respectively

E3 ubiquitin ligases are closely related to cell division, differentiation, and survival in all eukaryotes and play crucial regulatory roles in multiple biological processes and diseases. While Deltex2, as a member of the DELTEX family ubiquitin ligases, is characterized by a RING domain followed by a C-terminal domain (DTC), its functions and underlying mechanisms in myogenesis have not been fully elucidated. Here, we report that Deltex2, which is highly expressed in muscles, positively regulates myoblast proliferation via mediating the expression of Pax7. Meanwhile, we find that Deltex2 is translocated from the nucleus into the cytoplasm during myogenic differentiation, and further disclose that Deltex2 inhibits myoblast differentiation and interacts with MyoD, resulting in the ubiquitination and degradation of MyoD. Altogether, our findings reveal the physiological function of Deltex2 in orchestrating myogenesis and delineate the novel role of Deltex2 as a negative regulator of MyoD protein stability.

Pathophysiological Aspects of Muscle Atrophy and Osteopenia Induced by Chronic Constriction Injury (CCI) of the Sciatic Nerve in Rats

Skeletal muscle atrophy is a condition characterized by a loss of muscle mass and muscle strength caused by an imbalance between protein synthesis and protein degradation. Muscle atrophy is often associated with a loss of bone mass manifesting as osteoporosis. The aim of this study was to evaluate if chronic constriction injury (CCI) of the sciatic nerve in rats can be a valid model to study muscle atrophy and consequent osteoporosis. Body weight and body composition were assessed weekly. Magnetic resonance imaging (MRI) was performed on day zero before ligation and day 28 before sacrifice. Catabolic markers were assessed via Western blot and Quantitative Real-time PCR. After the sacrifice, a morphological analysis of the gastrocnemius muscle and Micro-Computed Tomography (Micro-CT) on the tibia bone were performed. Rats that underwent CCI had a lower body weight increase on day 28 compared to the naive group of rats (p < 0.001). Increases in lean body mass and fat mass were also significantly lower in the CCI group (p < 0.001). The weight of skeletal muscles was found to be significantly lower in the ipsilateral hindlimb compared to that of contralateral muscles; furthermore, the cross-sectional area of muscle fibers decreased significantly in the ipsilateral gastrocnemius. The CCI of the sciatic nerve induced a statistically significant increase in autophagic and UPS (Ubiquitin Proteasome System) markers and a statistically significant increase in Pax-7 (Paired Box-7) expression. Micro-CT showed a statistically significant decrease in the bone parameters of the ipsilateral tibial bone. Chronic nerve constriction appeared to be a valid model for inducing the condition of muscle atrophy, also causing changes in bone microstructure and leading to osteoporosis. Therefore, sciatic nerve constriction could be a valid approach to study muscle-bone crosstalk and to identify new strategies to prevent osteosarcopenia.

Identification and Quantification of Proliferating Cells in Skeletal Muscle of Glutamine Supplemented Low- and Normal-Birth-Weight Piglets

One way to improve the growth of low-birth-weight (LBW) piglets can be stimulation of the cellular development of muscle by optimized amino acid supply. In the current study, it was investigated how glutamine (Gln) supplementation affects muscle tissue of LBW and normal-birth-weight (NBW) piglets. Longissimus and semitendinosus muscles of 96 male piglets, which were supplemented with 1 g Gln/kg body weight or alanine, were collected at slaughter on day 5 or 26 post natum (dpn), one hour after injection with Bromodeoxyuridine (BrdU, 12 mg/kg). Immunohistochemistry was applied to detect proliferating, BrdU-positive cells in muscle cross-sections. Serial stainings with cell type specific antibodies enabled detection and subsequent quantification of proliferating satellite cells and identification of further proliferating cell types, e.g., preadipocytes and immune cells. The results indicated that satellite cells and macrophages comprise the largest fractions of proliferating cells in skeletal muscle of piglets early after birth. The Gln supplementation somewhat stimulated satellite cells. We observed differences between the two muscles, but no influence of the piglets' birth weight was observed. Thus, Gln supplements may not be considered as effective treatment in piglets with low birth weight for improvement of muscle growth.