Pax7 is required for the specification of myogenic satellite cells

Prehatch thermal manipulation of embryos and posthatch baicalein supplementation increased liver metabolism, and muscle proliferation in broiler chickens

The exposure of broiler chickens to high ambient temperatures causes heat stress (HS), negatively affecting their health and production performance. To mitigate heat stress in broilers, various strategies, including dietary, managerial, and genetic interventions, have been extensively tested with varying degrees of efficacy. For sustainable broiler production, it is imperative to develop an innovative approach that effectively mitigates the adverse effects of HS. Our previous studies have provided valuable insights into the effects of prehatch embryonic thermal manipulation (TM) and posthatch baicalein supplementation on embryonic thermotolerance, metabolism, and posthatch growth performance. This follow-up study investigated the effect of these interventions on gluconeogenesis and lipid metabolism in the liver, as well as muscle proliferation and regeneration capacity in heat-stressed broiler chickens. A total of six-hundred fertile Cobb 500 eggs were incubated for 21 d. After candling, 238 eggs were subjected to TM at 38.5°C with 55% relative humidity (RH) from embryonic day (ED) 12 to 18. These eggs were transferred to the hatcher and kept at a standard temperature (37.5°C) from ED 19 to 21, while 236 eggs were incubated at a controlled temperature (37.5°C) till hatch. After hatching, 180 day-old chicks from both groups were raised in 36 pens treatment (n = 10 birds/pen, 6 replicates per treatment). The treatments were: 1) Control, 2) TM, 3) Control heat stress (CHS), 4) Thermal manipulation heat stress (TMHS), 5) Control heat stress supplement (CHSS), and 6) Thermal manipulation heat stress supplement (TMHSS). Baicalein was added to the treatment group diets starting from d 1. All birds were raised under the standard environment for 21 d, followed by chronic heat stress from d 22 to 35 (32-33 ⁰C for 8 h) in the CHS, TMHS, CHSS, and TMHSS groups. A thermoneutral (22-24⁰C) environment was maintained in the Control and TM groups. RH was constant (50 ± 5%) throughout the trial. In the liver, TM significantly increased (P < 0.05) IGF2 expression. Baicalein supplementation significantly increased (P < 0.05) HSF3, HSP70, SOD1, SOD2, TXN, PRARα, and GHR expression. Moreover, the combination of TM and baicalein supplementation significantly increased (P < 0.05) the expression of HSPH1, HSPB1, HSP90, LPL, and GHR. In the muscle, TM significantly increased (P < 0.05) HSF3 and Myf5 gene expression. TM and baicalein supplementation significantly increased (P < 0.05) the expression of MyoG and significantly (P < 0.05) decreased mTOR and PAX7. In conclusion, the prehatch TM of embryos and posthatch baicalein supplementation mitigated the deleterious effects of HS on broiler chickens by upregulating genes related to liver gluconeogenesis, lipid metabolism, and muscle proliferation.

Histone modifications in Duchenne muscular dystrophy: pathogenesis insights and therapeutic implications

Duchenne muscular dystrophy (DMD) is a commonly encountered genetic ailment marked by loss-of-function mutations in the Dystrophin gene, ultimately resulting in progressive debilitation of skeletal muscle. The investigation into the pathogenesis of DMD has increasingly converged on the role of histone modifications within the broader context of epigenetic regulation. These modifications, including histone acetylation, methylation and phosphorylation, are catalysed by specific enzymes and play a critical role in gene expression. This article provides an overview of the histone modifications occurring in DMD and analyses the research progress and potential of different types of histone modifications in DMD due to changes in cellular signalling for muscle regeneration, to provide new insights into diagnostic and therapeutic options for DMD.

Preservation of masseter muscle until the end stage in the SOD1G93A mouse model for ALS

Amyotrophic lateral sclerosis (ALS) progressively impairs motor neurons, leading to muscle weakness and loss of voluntary muscle control. This study compared the effects of SOD1 mutation on masticatory and limb muscles from disease onset to death in ALS model mice. Notably, limb muscles begin to atrophy soon after ALS-like phenotype appear, whereas masticatory muscles maintain their volume and function in later stages. Our analysis showed that, unlike limb muscles, masticatory muscles retain their normal structure and cell makeup throughout most of the disease course. We found an increase in the number of muscle satellite cells (SCs), which are essential for muscle repair, in masticatory muscles. In addition, we observed no reduction in the number of muscle nuclei and no muscle fibre-type switching in masticatory muscles. This indicates that masticatory muscles have a higher resistance to ALS-related damage than limb muscles, likely because of differences in cell composition and repair mechanisms. Understanding why masticatory muscles are less affected by ALS could lead to the development of new treatments. This study highlights the importance of studying different muscle groups in ALS to clarify disease aetiology and mechanisms.

Branched actin polymerization drives invasive protrusion formation to promote myoblast fusion during skeletal muscle regeneration

Skeletal muscle regeneration is a multistep process involving the activation, proliferation, differentiation, and fusion of muscle stem cells, known as satellite cells. The fusion of satellite cell-derived mononucleated muscle cells (SCMs) is indispensable for the generation of multinucleated, contractile myofibers during muscle repair. However, the molecular and cellular mechanisms underlying SCM fusion during muscle regeneration remain poorly understood. In this study, we uncovered an essential role for branched actin polymerization in SCM fusion. Using conditional knockouts of the Arp2/3 complex and its actin nucleation-promoting factors, N-WASP and WAVE, we demonstrated that branched actin polymerization is required for the SCM fusion, but not for satellite cell proliferation, differentiation, and migration. We showed that the N-WASP and WAVE complexes have partially redundant functions in regulating SCM fusion. Furthermore, we showed that branched actin polymerization is essential for generating invasive protrusions at the fusogenic synapses in SCMs. Taken together, our study has identified new components of the myoblast fusion machinery in skeletal muscle regeneration and demonstrated a critical role for branched actin-propelled invasive protrusions in this process.

N-Acetylcysteine assists muscle development in offspring of mice subjected to maternal heat stress during pregnancy

BACKGROUND

Heat stress (HS) has been shown to affect reproductive performance and muscle development negatively in animals. N-Acetylcysteine (NAC) plays a pivotal role in enhancing the antioxidant performance in animals as a recognized antioxidant. The present study assesses the potential of NAC to modulate the reproductive performance and antioxidant function in pregnant mice exposed to HS. The role of NAC in muscle development of offspring mice was also explored.

RESULTS

The results showed that NAC supplementation from day 12 to day 18 of gestation increased the number of litters and enhanced the antioxidant function in pregnant mice under HS exposure. It improved the weight and body condition significantly in the offspring mice (P < 0.05). The alleviation of HS-induced muscle impairment with NAC was consistent with the alleviation of apoptosis, the enrichment of the proliferation and differentiation in the offspring mice muscle. N-Acetylcysteine also reversed HS-induced reduction in the cross-sectional area of the leg muscle and increased the proportion of myosin heavy chain IIx (MYHCIIx) in the muscle fiber.

CONCLUSION

The results of the present study support the use of NAC at a dose of 100 mg kg-1 body weight as supplement for protecting the offspring derived from pregnant mice exposed to HS from muscle impairment by accelerating proliferation and differentiation. © 2024 Society of Chemical Industry.

Macrophages in the Context of Muscle Regeneration and Duchenne Muscular Dystrophy

Macrophages are essential to muscle regeneration, as they regulate inflammation, carry out phagocytosis, and facilitate tissue repair. These cells exhibit phenotypic switching from pro-inflammatory (M1) to anti-inflammatory (M2) states during muscle repair, influencing myoblast proliferation, differentiation, and myofiber formation. In Duchenne Muscular Dystrophy (DMD), asynchronous muscle injuries disrupt the normal temporal stages of regeneration, leading to fibrosis and failed regeneration. Altered macrophage activity is associated with DMD progression and physiopathology. Gaining insight into the intricate relationship between macrophages and muscle cells is crucial for creating effective therapies aimed at treating this muscle disorder. This review explores the dynamic functions of macrophages in muscle regeneration and their implications in DMD.

Caspase-Activated DNase localizes to cancer causing translocation breakpoints during cell differentiation

Caspase activated DNase (CAD) induced DNA breaks promote cell differentiation and therapy-induced cancer cell resistance. CAD targeting activity is assumed to be unique to each condition, as differentiation and cancer genesis are divergent cell fates. Here, we made the surprising discovery that a subset of CAD-bound targets in differentiating muscle cells are the same genes involved in the genesis of cancer-causing translocations. In muscle cells, a prominent CAD-bound gene pair is Pax7 and Foxo1a, the mismatched reciprocal loci that give rise to alveolar rhabdomyosarcoma. We show that CAD-targeted breaks in the Pax7 gene are physiologic to reduce Pax7 expression, a prerequisite for muscle cell differentiation. A cohort of these CAD gene targets are also conserved in early differentiating T cells and include genes that spur leukemia/lymphoma translocations. Our results suggest the CAD targeting of translocation prone oncogenic genes is non-pathologic biology and aligns with initiation of cell fate transitions.

ADAR1 Promotes Myogenic Proliferation and Differentiation of Goat Skeletal Muscle Satellite Cells

As one of the most important economic traits for domestic animal husbandry, skeletal muscle is regulated by an intricate molecular network. Adenosine deaminase acting on RNA (ADAR1) involves various physiological processes and diseases, such as innate immunity and the development of lung adenocarcinoma, breast cancer, gastric cancer, etc. However, its role in skeletal muscle growth requires further clarification. Here, we explored the functions of ADAR1 in the myogenic process of goat skeletal muscle satellite cells (MuSCs). The ADAR1 transcripts were noticeably enriched in goat visceral tissues compared to skeletal muscle. Additionally, its levels in slow oxidative muscles like the psoas major and minor muscles were higher than in the fast oxidative glycolytic and fast glycolytic muscles. Among the two common isoforms from ADAR1, p110 is more abundant than p150. Moreover, overexpressing ADAR1 enhanced the proliferation and myogenic differentiation of MuSCs. The mRNA-seq performed on MuSCs' knockdown of ADAR1 obtained 146 differentially expressed genes (DEGs), 87 upregulated and 59 downregulated. These DEGs were concentrated in muscle development and process pathways, such as the MAPK and cAMP signaling pathways. Furthermore, many DEGs as the key nodes defined by protein-protein interaction networks (PPI), including STAT3, MYH3/8, TGFβ2, and ACTN4, were closely related to the myogenic process. Finally, RNA immunoprecipitation combined with qPCR (RIP-qPCR) showed that ADAR1 binds to PAX7 and MyoD mRNA. This study indicates that ADAR1 promotes the myogenic development of goat MuSCs, which provides a useful scientific reference for further exploring the ADAR1-related regulatory networks underlying mammal skeletal muscle growth.

The androgen receptor in mesenchymal progenitors regulates skeletal muscle mass via Igf1 expression in male mice

Androgens exert their effects primarily by binding to the androgen receptor (AR), a ligand-dependent nuclear receptor. While androgens have anabolic effects on skeletal muscle, previous studies reported that AR functions in myofibers to regulate skeletal muscle quality, rather than skeletal muscle mass. Therefore, the anabolic effects of androgens are exerted via nonmyofiber cells. In this context, the cellular and molecular mechanisms of AR in mesenchymal progenitors, which play a crucial role in maintaining skeletal muscle homeostasis, remain largely unknown. In this study, we demonstrated expression of AR in mesenchymal progenitors and found that targeted AR ablation in mesenchymal progenitors reduced limb muscle mass in mature adult, but not young or aged, male mice, although fatty infiltration of muscle was not affected. The absence of AR in mesenchymal progenitors led to remarkable perineal muscle hypotrophy, regardless of age, due to abnormal regulation of transcripts associated with cell death and extracellular matrix organization. Additionally, we revealed that AR in mesenchymal progenitors regulates the expression of insulin-like growth factor 1 (Igf1) and that IGF1 administration prevents perineal muscle atrophy in a paracrine manner. These findings indicate that the anabolic effects of androgens regulate skeletal muscle mass via, at least in part, AR signaling in mesenchymal progenitors.

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.

Organoid culture promotes dedifferentiation of mouse myoblasts into stem cells capable of complete muscle regeneration

Experimental cell therapies for skeletal muscle conditions have shown little success, primarily because they use committed myogenic progenitors rather than true muscle stem cells, known as satellite cells. Here we present a method to generate in vitro-derived satellite cells (idSCs) from skeletal muscle tissue. When transplanted in small numbers into mouse muscle, mouse idSCs fuse into myofibers, repopulate the satellite cell niche, self-renew, support multiple rounds of muscle regeneration and improve force production on par with freshly isolated satellite cells in damaged skeletal muscle. We compared the epigenomic and transcriptional signatures between idSCs, myoblasts and satellite cells and used these signatures to identify core signaling pathways and genes that confer idSC functionality. Finally, from human muscle biopsies, we successfully generated satellite cell-like cells in vitro. After further development, idSCs may provide a scalable source of cells for the treatment of genetic muscle disorders, trauma-induced muscle damage and age-related muscle weakness.

Dual-specificity phosphatases 13 and 27 as key switches in muscle stem cell transition from proliferation to differentiation

Muscle regeneration depends on muscle stem cell (MuSC) activity. Myogenic regulatory factors, including myoblast determination protein 1 (MyoD), regulate the fate transition of MuSCs. However, the direct target of MYOD in the process is not completely clear. Using previously established MyoD knock-in (MyoD-KI) mice, we revealed that MyoD targets dual-specificity phosphatase (Dusp) 13 and Dusp27. In Dusp13:Dusp27 double knock-out mice, the ability for muscle regeneration after injury was reduced. Moreover, single-cell RNA sequencing of MyoD-high expressing MuSCs from MyoD-KI mice revealed that Dusp13 and Dusp27 are expressed only in specific populations within MyoD-high MuSCs, which also express Myogenin. Overexpressing Dusp13 in MuSCs causes premature muscle differentiation. Thus, we propose a model where DUSP13 and DUSP27 contribute to the fate transition of MuSCs from proliferation to differentiation during myogenesis.

A PRDM16-driven signal regulates body composition in testosterone-treated hypogonadal men

Background

Testosterone (T) therapy increases lean mass and reduces total body and truncal fat mass in hypogonadal men. However, the underlying molecular mechanisms for the reciprocal changes in fat and lean mass in humans are not entirely clear.

Methods

Secondary analysis of specimens obtained from a single-arm, open-label clinical trial on pharmacogenetics of response to T therapy in men with late-onset hypogonadism, conducted between 2011 and 2016 involving 105 men (40-74 years old), who were given intramuscular T cypionate 200 mg every 2 weeks for 18 months. Subcutaneous fat (SCF), peripheral blood mononuclear cells (PBMC) and serum were obtained from the participants at different time points of the study. We measured transcription factors for adipogenesis and myogenesis in the SCF, and PBMC, respectively, by real-time quantitative PCR at baseline and 6 months. Serum levels of FOLLISTATIN, PAX7, MYOSTATIN, ADIPSIN, and PRDM16 were measured by ELISA.

Results

As expected, there was a significant increase in T and estradiol levels after 6 months of T therapy. There was also a reduction in fat mass and an increase in lean mass after 6 months of T therapy. Gene-protein studies showed a significant reduction in the expression of the adipogenic markers PPARγ in SCF and ADIPSIN levels in the serum, together with a concomitant significant increase in the expression of myogenic markers, MYOD in PBMC and PAX7 and FOLLISTATIN levels in the serum after 6 months of T therapy compared to baseline. Interestingly, there was a significant increase in the adipo-myogenic switch, PRDM16, expression in SCF and PBMC, and in circulating protein levels in the serum after 6 months of T therapy, which is likely from increased estradiol.

Conclusion

Our study supports that molecular shift from the adipogenic to the myogenic pathway in men with hypogonadism treated with T could be mediated directly or indirectly by enhanced PRDM16 activity, in turn a result from increased estradiol level. This might have led to the reduction in body fat and increase in lean mass commonly seen in hypogonadal men treated with T.

Actin-organizing protein palladin modulates C2C12 cell fate determination

Background

Cell confluency and serum deprivation promote the transition of C2C12 myoblasts into myocytes and subsequence fusion into myotubes. However, despite all myoblasts undergoing the same serum deprivation trigger, their responses vary: whether they become founder myocytes, remain proliferative, or evolve into fusion-competent myocytes remains unclear. We have previously shown that depletion of the scaffolding protein palladin in myoblasts inhibits cell migration and promotes premature muscle differentiation, pointing to its potential significance in muscle development and the necessity for a more in-depth examination of its function in cellular heterogeneity.

Methods and results

Here, we showed that the subcellular localization of palladin might contribute to founder-fate cell decision in the early differentiation process. Depleting palladin in C2C12 myoblasts depleted integrin-β3 plasma membrane localization of and focal adhesion formation at the early stage of myogenesis, decreased kindlin-2 and metavinculin expression during the myotube maturation process, leading to the inability of myocytes to fuse into preexisting mature myotubes. This aligns with previous findings where early differentiation into nascent myotubes occurred but compromised maturation. In contrast, wildtype C2C12 overexpressing the 140-kDa palladin isoform developed a polarized morphology with star-like structures toward other myoblasts. However, this behaviour was not observed in palladin-depleted cells, where the 140-kDa palladin overexpression could not recover cell migration capacity, suggesting other palladin isoforms are also needed to establish cell polarity.

Conclusion

Our study identifies a counter-intuitive role for palladin in regulating myoblast-to-myocyte cell fate decisions and impacting their ability to form mature multinucleated myotubes by influencing cell signalling pathways and cytoskeletal organization, necessary for skeletal muscle regeneration and repair studies.

Dietary fishmeal replacement by Clostridium autoethanogenum protein meal influences the nutritional and sensory quality of turbot (Scophthalmus maximus) via the TOR/AAR/AMPK pathways

Clostridium autoethanogenum protein (CAP) is a promising protein source for aquaculture; however, how CAP influences fish quality is worth extensive research. We randomly allocated 630 turbot with initial body weights of about 180 g into 6 groups, with fishmeal-based control diet or diet with CAP replacing 15% (CAP15), 30% (CAP30), 45% (CAP45), 60% (CAP60), or 75% (CAP75) of fishmeal protein. After a 70-d feeding trial, the fillet yield (P = 0.015) and content of protein (P = 0.017), collagen (P < 0.001), hydroxyproline (P < 0.001), C20:5n-3 (P = 0.007), and ∑n-3/∑n-6 polyunsaturated fatty acids ratio (P < 0.001) in turbot muscle was found to decrease linearly with increasing CAP. However, turbot fed CAP15 diet maintained these parameters (P > 0.05). By contrast, the muscle hardness increased linearly with increasing CAP (P = 0.004), accompanied by linear reduction of muscle fiber area (P = 0.003) and expression of myogenesis-related genes, including cathepsin D (ctsd P < 0.001) and muscle ring finger protein 1 (murf 1, P < 0.001). Phosphorylation of protein kinase B (Akt, P < 0.001), target of rapamycin (TOR, P = 0.001), eukaryotic initiation factor 4E-binding protein 1 (4E-BP1, P < 0.001), and ribosomal protein S6 (S6, P < 0.001) decreased linearly; however, phosphorylation of AMP-activated protein kinase (AMPK, P < 0.001), eukaryotic initiation factor 2α (eIF2α, P < 0.001), and the abundance of activating transcription factor 4 (ATF4, P < 0.001) increased with increasing CAP, suggesting that the TOR signaling pathway was inhibited, and the amino acid response (AAR) and AMPK pathways were activated. Additionally, expression of genes related to protein degradation, including myogenic factor 5 (myf 5, P < 0.001), myogenic differentiation (myod, P < 0.001), paired box 7 (pax 7, P < 0.001), and ctsd (P < 0.001), decreased linearly with increasing CAP. In conclusion, CAP could be used to replace up to 15% of fishmeal without negatively impacting turbot quality. However, higher levels of CAP decreased fillet yield, muscle protein content, and muscle fiber diameter while increasing muscle hardness, which could be attributed to the inhibition of the TOR pathway and activation of the AAR and AMPK pathways.

GLUD1 determines murine muscle stem cell fate by controlling mitochondrial glutamate levels

Muscle stem cells (MuSCs) enable muscle growth and regeneration after exercise or injury, but how metabolism controls their regenerative potential is poorly understood. We describe that primary metabolic changes can determine murine MuSC fate decisions. We found that glutamine anaplerosis into the tricarboxylic acid (TCA) cycle decreases during MuSC differentiation and coincides with decreased expression of the mitochondrial glutamate deaminase GLUD1. Deletion of Glud1 in proliferating MuSCs resulted in precocious differentiation and fusion, combined with loss of self-renewal in vitro and in vivo. Mechanistically, deleting Glud1 caused mitochondrial glutamate accumulation and inhibited the malate-aspartate shuttle (MAS). The resulting defect in transporting NADH-reducing equivalents into the mitochondria induced compartment-specific NAD+/NADH ratio shifts. MAS activity restoration or directly altering NAD+/NADH ratios normalized myogenesis. In conclusion, GLUD1 prevents deleterious mitochondrial glutamate accumulation and inactivation of the MAS in proliferating MuSCs. It thereby acts as a compartment-specific metabolic brake on MuSC differentiation.

Isolation of a persistently quiescent muscle satellite cell population

Although studies have identified characteristics of quiescent satellite cells (SCs), their isolation has been hampered by the fact that the isolation procedures result in the activation of these cells into their rapidly proliferating progeny (myoblasts). Thus, the use of myoblasts for therapeutic (regenerative medicine) or industrial applications (cellular agriculture) has been impeded by the limited proliferative and differentiative capacity of these myogenic progenitors. Here we identify a subpopulation of satellite cells isolated from mouse skeletal muscle using flow cytometry that is highly Pax7-positive, exhibit a very slow proliferation rate (7.7 ± 1.2 days/doubling), and are capable of being maintained in culture for at least 3 mo without a change in phenotype. These cells can be activated from quiescence using a p38 inhibitor or by exposure to freeze-thaw cycles. Once activated, these cells proliferate rapidly (22.7 ± 0.2 h/doubling), have reduced Pax7 expression (threefold decrease in Pax7 fluorescence vs. quiescence), and differentiate into myotubes with a high efficiency. Furthermore, these cells withstand freeze-thawing readily without a significant loss of viability (83.1 ± 2.1% live). The results presented here provide researchers with a method to isolate quiescent satellite cells, allowing for more detailed examinations of the factors affecting satellite cell quiescence/activation and providing a cell source that has a unique potential in the regenerative medicine and cellular agriculture fields.NEW & NOTEWORTHY We provide a method to isolate quiescent satellite cells from skeletal muscle. These cells are highly Pax7-positive, exhibit a very slow proliferation rate, and are capable of being maintained in culture for months without a change in phenotype. The use of these cells by muscle researchers will allow for more detailed examinations of the factors affecting satellite cell quiescence/activation and provide a novel cell source for the regenerative medicine and cellular agriculture fields.

GSK3 regulation Wnt/β-catenin signaling affects adipogenesis in bovine skeletal muscle fibro/adipogenic progenitors

Clarifying the cellular origin and regulatory mechanisms of intramuscular fat (IMF) deposition is crucial for improving beef quality. Here, we used single-nucleus RNA sequencing to analyze the structure and heterogeneity of skeletal muscle cell populations in different developmental stages of Yanbian cattle and identified eight cell types in two developmental stages of calves and adults. Among them, fibro/adipogenic progenitors (FAPs) expressing CD29 (ITGA7)pos and CD56 (NCAM1)neg surface markers were committed to IMF deposition in beef cattle and expressed major Wnt ligands and receptors. LY2090314/XAV-939 was used to activate/inhibit Wnt/β-catenin signal. The results showed that the blockade of Glycogen Synthase Kinase 3 (GSK3) by LY2090314 promoted the stabilization of β-catenin and reduced the expression of genes related adipogenic differentiation (e.g., PPARγ and C/EBPα) in bovine FAPs, confirming the anti-adipogenic effect of GSK3. XAV-939 inhibition of the Wnt/β-catenin pathway promoted the lipid accumulation capacity of FAPs. Furthermore, we found that blocking GSK3 enhanced the paracrine effects of FAPs-MuSCs and increased myotube formation in muscle satellite cells (MuSCs). Overall, our results outline a single-cell atlas of skeletal muscle development in Yanbian cattle, revealed the role of Wnt/GSK3/β-catenin signaling in FAPs adipogenesis, and provide a theoretical basis for further regulation of bovine IMF deposition.

The IRE1α/XBP1 signaling axis drives myoblast fusion in adult skeletal muscle

Skeletal muscle regeneration involves a signaling network that regulates the proliferation, differentiation, and fusion of muscle precursor cells to injured myofibers. IRE1α, one of the arms of the unfolded protein response, regulates cellular proteostasis in response to ER stress. Here, we demonstrate that inducible deletion of IRE1α in satellite cells of mice impairs skeletal muscle regeneration through inhibiting myoblast fusion. Knockdown of IRE1α or its downstream target, X-box protein 1 (XBP1), also inhibits myoblast fusion during myogenesis. Transcriptome analysis revealed that knockdown of IRE1α or XBP1 dysregulates the gene expression of molecules involved in myoblast fusion. The IRE1α-XBP1 axis mediates the gene expression of multiple profusion molecules, including myomaker (Mymk). Spliced XBP1 (sXBP1) transcription factor binds to the promoter of Mymk gene during myogenesis. Overexpression of myomaker in IRE1α-knockdown cultures rescues fusion defects. Inducible deletion of IRE1α in satellite cells also inhibits myoblast fusion and myofiber hypertrophy in response to functional overload. Collectively, our study demonstrates that IRE1α promotes myoblast fusion through sXBP1-mediated up-regulation of the gene expression of multiple profusion molecules, including myomaker.

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.

Discovery of fibroblast growth factor 2-derived peptides for enhancing mice skeletal muscle satellite cell proliferation

Skeletal muscle satellite cells (SCs) are essential for muscle regeneration. Their proliferation and differentiation are influenced by fibroblast growth factor (FGF)-2. In this study, we screened for FGF-2-derived peptides that promote SC proliferation. Utilizing photocleavable peptide array technology, a library of 7-residue peptides was synthesized, and its effect on SC proliferation was examined using a mixture of five peptides. The results showed that peptides 1-5 (136%), 21-25 (136%), 26-30 (141%), 31-35 (159%), 71-75 (135%), 76-80 (144%), and 126-130 (137%) significantly increased SC proliferation. Further experiments revealed that peptide 33, CKNGGFF, enhanced SC proliferation. Furthermore, its extended form, peptide 33-13, CKNGGFFLRIHPD, promoted SC proliferation and increased the percentage of Pax7-positive cells, indicating that SCs were maintained in an undifferentiated state. The addition of FGF-2 and peptide 33-13 further induced cell proliferation but did not increase the percentage of Pax7-positive cells. A proliferation assay using an FGF receptor (FGFR) inhibitor suggested that peptide 33-13 acts through the FGFR-mediated and other pathways. Although further research is necessary to explore the mechanisms of action of these peptides and their potential for in vivo and in vitro use, the high sequence conservation of peptides 33 and 33-13 in FGF-2 across multiple species suggests their broad application prospects in biomedical engineering and biotechnology.

Lipidomics and single-cell RNA sequencing reveal lipid and cell dynamics of porcine glycerol-injured skeletal muscle regeneration model

AIMS

Intramuscular fat (IMF) infiltration and extracellular matrix (ECM) deposition are characteristic features of muscle dysfunction, such as muscular dystrophy and severe muscle injuries. However, the underlying mechanisms of cellular origin, adipocyte formation and fibrosis in skeletal muscle are still unclear.

MAIN METHODS

Pigs were injected with 50 % glycerol (GLY) to induce skeletal muscle injury and regeneration. The acyl chain composition was analyzed by lipidomics, and the cell atlas and molecular signatures were revealed via single-cell RNA sequencing (scRNA-seq). Adipogenesis analysis was performed on fibroblast/fibro-adipogenic progenitors (FAPs) isolated from pigs.

KEY FINDINGS

The porcine GLY-injured skeletal muscle regeneration model was characterized by IMF infiltration and ECM deposition. Skeletal muscle stem cells (MuSCs) and FAP clusters were analyzed to explore the potential mechanisms of adipogenesis and fibrosis, and it was found that the TGF-β signaling pathway might be a key switch that regulates differentiation. Consistently, activation of the TGF-β signaling pathway increased SMAD2/3 phosphorylation and inhibited adipogenesis in FAPs, while inhibition of the TGF-β signaling pathway increased the expression of PPARγ and promoted adipogenesis.

SIGNIFICANCE

GLY-induced muscle injury and regeneration provides comprehensive insights for the development of therapies for human skeletal muscle dysfunction and fatty infiltration-related diseases in which the TGF-β/SMAD signaling pathway might play a primary regulatory role.

In Silico Functional and Structural Analysis of Non-synonymous Single Nucleotide Polymorphisms (nsSNPs) in Human Paired Box 4 Gene

In human genome, members of Paired box (PAX) transcription factor family are highly sequence-specific DNA-binding proteins. Among PAX gene family members, PAX4 gene has significant role in growth, proliferation, differentiation, and insulin secretion of pancreatic β-cells. Single nucleotide polymorphisms (SNPs) in PAX4 gene progress in the pathogenesis of various human diseases. Hence, the molecular mechanism of how these SNPs in PAX4 gene significantly progress diseases pathogenesis needs to be elucidated. For the reason, a series of bioinformatic analyzes were done to identify the SNPs of PAX4 gene that contribute in diseases pathogenesis. From the analyzes, 4145 SNPs (rsIDs) in PAX4 gene were obtained, where, 362 missense (8.73%), 169 synonymous (4.08%), and 2323 intron variants (56.04%). The rest SNPs were unspecified. Among the 362 missense variants, 118 nsSNPs were found as deleterious in SIFT analysis. Among those, 25 nsSNPs were most probably damaging and 23 were deleterious as observed in PolyPhen-2 and PROVEAN analyzes, respectively. Following all analyzes, 14 nsSNPs (rs149708455, rs115887120, rs147279315, rs35155575, rs370095957, rs373939873, rs145468905, rs121917718, rs2233580, rs3824004, rs372751660, rs369459316, rs375472849, rs372497946) were common and observed as deleterious, probably damaging, affective and diseases associated. Following structural analyzes, 11 nsSNPs guided proteins were found as most unstable and highly conserved. Among these, R20W, R39Q, R45Q, R60H, G65D, and A223D mutated proteins were highly harmful. Hence, the results from above-mentioned integrated comprehensive bioinformatic analyzes guide how different nsSNPs in PAX4 gene alter structural and functional characteristics of the protein that might progress diseases pathogenesis in human including type 2 diabetes.

Keratinocytes drive the epithelial hyperplasia key to sea lice resistance in coho salmon

BACKGROUND

Salmonid species have followed markedly divergent evolutionary trajectories in their interactions with sea lice. While sea lice parasitism poses significant economic, environmental, and animal welfare challenges for Atlantic salmon (Salmo salar) aquaculture, coho salmon (Oncorhynchus kisutch) exhibit near-complete resistance to sea lice, achieved through a potent epithelial hyperplasia response leading to rapid louse detachment. The molecular mechanisms underlying these divergent responses to sea lice are unknown.

RESULTS

We characterized the cellular and molecular responses of Atlantic salmon and coho salmon to sea lice using single-nuclei RNA sequencing. Juvenile fish were exposed to copepodid sea lice (Lepeophtheirus salmonis), and lice-attached pelvic fin and skin samples were collected 12 h, 24 h, 36 h, 48 h, and 60 h after exposure, along with control samples. Comparative analysis of control and treatment samples revealed an immune and wound-healing response that was common to both species, but attenuated in Atlantic salmon, potentially reflecting greater sea louse immunomodulation. Our results revealed unique but complementary roles of three layers of keratinocytes in the epithelial hyperplasia response leading to rapid sea lice rejection in coho salmon. Our results suggest that basal keratinocytes direct the expansion and mobility of intermediate and, especially, superficial keratinocytes, which eventually encapsulate the parasite.

CONCLUSIONS

Our results highlight the key role of keratinocytes in coho salmon's sea lice resistance and the diverged biological response of the two salmonid host species when interacting with this parasite. This study has identified key pathways and candidate genes that could be manipulated using various biotechnological solutions to improve Atlantic salmon sea lice resistance.

Muscle Fibrosis, NF-κB, and TGF-β Are Differentially Altered in Two Models of Paralysis (Botox Versus Neurectomy)

Objective: Volumetric muscle loss results in intramuscular axotomy, denervating muscle distal to the injury and leading to paralysis, denervation, and loss of muscle function. Once the nerve is damaged, paralyzed skeletal muscle will atrophy and accumulate noncontractile connective tissue. The objective of this study was to determine differences in connective tissue, atrophy, and inflammatory signaling between two paralysis models, botulinum toxin (Botox), which blocks acetylcholine transmission while keeping nerves intact, and neurectomy, which eliminates all nerve-to-muscle signaling. Approach: Twenty male Sprague Dawley rats were randomized and received a sciatic-femoral neurectomy (SFN), Botox-induced muscle paralysis of the proximal femur muscles, quadriceps femoris, hamstrings, and calf muscles (BTX), or sham. Muscle force was measured 52 days postsurgery, and samples were collected for histology, protein, and mRNA assays. Results: SFN and BTX decreased twitch and tetanic force, decreased fiber size by twofold, and increased myogenic expression compared with controls. SFN increased the levels of all major extracellular matrix proteins correlating with fibrosis [e.g., laminin, fibronectin, and collagen type(s) I, III, VI]. SFN also increased profibrotic and proinflammatory mRNA compared with BTX and controls. Innovation: SFN and BTX were similar in gross morphology and functional deficiencies. However, SFN exhibited a higher amount of fibrosis in histological sections and immunoblotting. The present study shows evidence that nerve signaling changes NF-κB and TGF-β signaling, warranting future studies to determine the mechanisms involved. Conclusion: These data indicate that nerve signaling may influence fibrogenesis following denervation, but the mechanisms involved may differ as a function of the method of paralysis.

Therapeutics Targeting Skeletal Muscle in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a complex neuromuscular disease characterized by progressive motor neuron degeneration, neuromuscular junction dismantling, and muscle wasting. The pathological and therapeutic studies of ALS have long been neurocentric. However, recent insights have highlighted the significance of peripheral tissue, particularly skeletal muscle, in disease pathology and treatment. This is evidenced by restricted ALS-like muscle atrophy, which can retrogradely induce neuromuscular junction and motor neuron degeneration. Moreover, therapeutics targeting skeletal muscles can effectively decelerate disease progression by modulating muscle satellite cells for muscle repair, suppressing inflammation, and promoting the recovery or regeneration of the neuromuscular junction. This review summarizes and discusses therapeutic strategies targeting skeletal muscles for ALS treatment. It aims to provide a comprehensive reference for the development of novel therapeutics targeting skeletal muscles, potentially ameliorating the progression of ALS.

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.

Delta-like noncanonical notch ligand 2 regulates the proliferation and differentiation of sheep myoblasts through the Wnt/β-catenin signaling pathway

This study delved into the role of delta-like noncanonical notch ligand 2 (DLK2) in the cell cycle, proliferation, apoptosis, and differentiation of myoblasts, as well as its interaction with the classical Wnt/β-catenin signaling pathway in regulating myoblast function. The research revealed that upregulation of DLK2 in myoblasts during the proliferation phase enhanced myoblast proliferation, facilitated cell cycle progression, and reduced apoptosis. Conversely, downregulation of DLK2 expression using siRNA during the differentiation phase promoted myoblast hypertrophy and fusion, suppressed the expression of muscle fiber degradation factors, and expedited the differentiation process. DLK2 regulates myoblasts function by influencing the expression of various factors associated with the Wnt/β-catenin signaling pathway, including CTNNB1, FZD1, FZD6, RSPO1, RSPO4, WNT4, WNT5A, and adenomatous polyposis coli. In essence, DLK2, with the involvement of the Wnt/β-catenin signaling pathway, plays a crucial regulatory role in the cell cycle, proliferation, apoptosis, and differentiation of myoblasts.

A dual-color PAX7 and MYF5 in vivo reporter to investigate muscle stem cell heterogeneity in regeneration and aging

Increasing evidence suggests that the muscle stem cell (MuSC) pool is heterogeneous. In particular, a rare subset of PAX7-positive MuSCs that has never expressed the myogenic regulatory factor MYF5 displays unique self-renewal and engraftment characteristics. However, the scarcity and limited availability of protein markers make the characterization of these cells challenging. Here, we describe the generation of StemRep reporter mice enabling the monitoring of PAX7 and MYF5 proteins based on equimolar levels of dual nuclear fluorescence. High levels of PAX7 protein and low levels of MYF5 delineate a deeply quiescent MuSC subpopulation with an increased capacity for asymmetric division and distinct dynamics of activation, proliferation, and commitment. Aging primarily reduces the MYF5Low MuSCs and skews the stem cell pool toward MYF5High cells with lower quiescence and self-renewal potential. Altogether, we establish the StemRep model as a versatile tool to study MuSC heterogeneity and broaden our understanding of mechanisms regulating MuSC quiescence and self-renewal in homeostatic, regenerating, and aged muscles.

Protective Effects of Hydrogen Gas Inhalation for Hindlimb Ischaemia-Reperfusion Injury in a Mouse Model

OBJECTIVE

Ischaemia-reperfusion (I/R) injury is a severe post-operative complication that triggers an inflammatory response and causes severe damage. Hydrogen gas has anti-oxidant and anti-apoptotic properties and has been shown to be safe in humans. The study aimed to investigate whether hydrogen gas protects against skeletal muscle I/R injury.

METHODS

Experimental basic research using mice. A total of 160 eight to 10 week old albino laboratory bred strain of house mice (25.8 ± 0.68 g) were used in this study. The mice were cable tied to the hindlimb under anaesthesia and then placed in an anaesthesia box filled with air and 2% isoflurane (control group); 80 mice were additionally subjected to 1.3% hydrogen gas in this mix (hydrogen group). After two hours, the cable ties were removed to initiate reperfusion, and hydrogen inhalation lasted for six hours in the hydrogen group. After six hours, the mice were taken out of the box and kept in cages under standard conditions until time for observation at 16 different time points after reperfusion: zero, two, four, six, eight, and 10 hours and one, two, three, four, five, six, seven, 14, 21, and 28 days. Five mice were sacrificed using excess anaesthesia at each time point, and the bilateral hindlimb tissues were harvested. The inflammatory effects of the I/R injury were assessed by evaluating serum interleukin-6 concentrations using enzyme linked immunosorbent assay, as well as histological and immunohistochemical analyses. Untreated mice with I/R injury were used as controls.

RESULTS

Hydrogen gas showed protective effects associated with a reduction in inflammatory cell infiltration (neutrophils, macrophages, and lymphocytes), a reduced area of damaged muscle, maintenance of normal muscle cells, and replacement of damaged muscle cells with neoplastic myocytes.

CONCLUSION

Inhalation of hydrogen gas had a protective effect against hindlimb I/R injury in mice, in part by reducing inflammatory cell infiltration and in part by preserving normal muscle cells.

Skeletal muscle: molecular structure, myogenesis, biological functions, and diseases

Skeletal muscle is an important motor organ with multinucleated myofibers as its smallest cellular units. Myofibers are formed after undergoing cell differentiation, cell-cell fusion, myonuclei migration, and myofibril crosslinking among other processes and undergo morphological and functional changes or lesions after being stimulated by internal or external factors. The above processes are collectively referred to as myogenesis. After myofibers mature, the function and behavior of skeletal muscle are closely related to the voluntary movement of the body. In this review, we systematically and comprehensively discuss the physiological and pathological processes associated with skeletal muscles from five perspectives: molecule basis, myogenesis, biological function, adaptive changes, and myopathy. In the molecular structure and myogenesis sections, we gave a brief overview, focusing on skeletal muscle-specific fusogens and nuclei-related behaviors including cell-cell fusion and myonuclei localization. Subsequently, we discussed the three biological functions of skeletal muscle (muscle contraction, thermogenesis, and myokines secretion) and its response to stimulation (atrophy, hypertrophy, and regeneration), and finally settled on myopathy. In general, the integration of these contents provides a holistic perspective, which helps to further elucidate the structure, characteristics, and functions of skeletal muscle.

Effect of exercise-induced Neutrophil maturation on skeletal muscle repair in vitro

Neutrophils as first line defender initiate a cascade of healing process immediately after muscle injury. At muscle injury site, neutrophils remove damaged muscle fibers and recruit other immune cells and these functions show in mature neutrophils. In the previous study, physical exercise can mediate neutrophils' functional changes such as phagocytosis and chemotaxis, though there is no research on how exercise-induced neutrophils contribute the muscle regeneration. In this present study, we investigated the maturation of neutrophils after 4 weeks of mouse treadmill exercise and assessed wound healing assay to evaluate whether treatment with exercise-activated neutrophils is effective for skeletal muscle repair in vitro. In the exercise group, significantly higher mRNA levels of maturation markers compared to the sedentary group and exercise-activated neutrophils improved wound healing of mouse muscle cells. To confirm at the human cell level, based on the well-known fact that exercise increases circulating cortisol levels, neutrophil-like cells were treated with dexamethasone (dHL60 + dex) as exercise mimetics. dHL60 + dex had significantly higher mRNA levels of neutrophil maturation marker and improved wound healing of human skeletal muscle cells compared to the control. These findings suggest that exercise affects neutrophil maturation and that exercise-induced neutrophils contribute to skeletal muscle repair in vitro.

Transcriptomic gene signatures measure satellite cell activity in muscular dystrophies

The routine need for myonuclear turnover in skeletal muscle, together with more sporadic demands for hypertrophy and repair, are performed by resident muscle stem cells called satellite cells. Muscular dystrophies are characterized by muscle wasting, stimulating chronic repair/regeneration by satellite cells. Here, we derived and validated transcriptomic signatures for satellite cells, myoblasts/myocytes, and myonuclei using publicly available murine single cell RNA-Sequencing data. Our signatures distinguished disease from control in transcriptomic data from several muscular dystrophies including facioscapulohumeral muscular dystrophy (FSHD), Duchenne muscular dystrophy, and myotonic dystrophy type I. For FSHD, the expression of our gene signatures correlated with direct counts of satellite cells on muscle sections, as well as with increasing clinical and pathological severity. Thus, our gene signatures enable the investigation of myogenesis in bulk transcriptomic data from muscle biopsies. They also facilitate study of muscle regeneration in transcriptomic data from human muscle across health and disease.

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.

Interplay between Pitx2 and Pax7 temporally governs specification of extraocular muscle stem cells

Gene regulatory networks that act upstream of skeletal muscle fate determinants are distinct in different anatomical locations. Despite recent efforts, a clear understanding of the cascade of events underlying the emergence and maintenance of the stem cell pool in specific muscle groups remains unresolved and debated. Here, we invalidated Pitx2 with multiple Cre-driver mice prenatally, postnatally, and during lineage progression. We showed that this gene becomes progressively dispensable for specification and maintenance of the muscle stem (MuSC) cell pool in extraocular muscles (EOMs) despite being, together with Myf5, a major upstream regulator during early development. Moreover, constitutive inactivation of Pax7 postnatally led to a greater loss of MuSCs in the EOMs compared to the limb. Thus, we propose a relay between Pitx2, Myf5 and Pax7 for EOM stem cell maintenance. We demonstrate also that MuSCs in the EOMs adopt a quiescent state earlier that those in limb muscles and do not spontaneously proliferate in the adult, yet EOMs have a significantly higher content of Pax7+ MuSCs per area pre- and post-natally. Finally, while limb MuSCs proliferate in the mdx mouse model for Duchenne muscular dystrophy, significantly less MuSCs were present in the EOMs of the mdx mouse model compared to controls, and they were not proliferative. Overall, our study provides a comprehensive in vivo characterisation of MuSC heterogeneity along the body axis and brings further insights into the unusual sparing of EOMs during muscular dystrophy.

The Mediator Med23 controls a transcriptional switch for muscle stem cell proliferation and differentiation in muscle regeneration

Muscle stem cells (MuSCs) contribute to a robust muscle regeneration process after injury, which is highly orchestrated by the sequential expression of multiple key transcription factors. However, it remains unclear how key transcription factors and cofactors such as the Mediator complex cooperate to regulate myogenesis. Here, we show that the Mediator Med23 is critically important for MuSC-mediated muscle regeneration. Med23 is increasingly expressed in activated/proliferating MuSCs on isolated myofibers or in response to muscle injury. Med23 deficiency reduced MuSC proliferation and enhanced its precocious differentiation, ultimately compromising muscle regeneration. Integrative analysis revealed that Med23 oppositely impacts Ternary complex factor (TCF)-targeted MuSC proliferation genes and myocardin-related transcription factor (MRTF)-targeted myogenic differentiation genes. Consistently, Med23 deficiency decreases the ETS-like transcription factor 1 (Elk1)/serum response factor (SRF) binding at proliferation gene promoters but promotes MRTF-A/SRF binding at myogenic gene promoters. Overall, our study reveals the important transcriptional control mechanism of Med23 in balancing MuSC proliferation and differentiation in muscle regeneration.

Cell-specific functions of androgen receptor in skeletal muscles

Androgens play a vital role not only in promoting the development of male sexual characteristics but also in exerting diverse physiological effects, including the regulation of skeletal muscle growth and function. Given that the effects of androgens are mediated through androgen receptor (AR) binding, an understanding of AR functionality is crucial for comprehending the mechanisms of androgen action on skeletal muscles. Drawing from insights gained using conditional knockout mouse models facilitated by Cre/loxP technology, we review the cell-specific functions of AR in skeletal muscles. We focus on three specific cell populations expressing AR within skeletal muscles: skeletal muscle cells, responsible for muscle contraction; satellite cells, which are essential stem cells contributing to the growth and regeneration of skeletal muscles; and mesenchymal progenitors, situated in interstitial areas and playing a crucial role in muscle homeostasis. Furthermore, the indirect effects of androgens on skeletal muscle through extra-muscle tissue are essential, especially for the regulation of skeletal muscle mass. The regulation of genes by AR varies across different cell types and contexts, including homeostasis, regeneration and hypertrophy of skeletal muscles. The varied mechanisms orchestrated by AR collectively influence the physiology of skeletal muscles.

Deletion of TECRL promotes skeletal muscle repair by up-regulating EGR2

Myogenic regeneration relies on the proliferation and differentiation of satellite cells. TECRL (trans-2,3-enoyl-CoA reductase like) is an endoplasmic reticulum protein only expressed in cardiac and skeletal muscle. However, its role in myogenesis remains unknown. We show that TECRL expression is increased in response to injury. Satellite cell-specific deletion of TECRL enhances muscle repair by increasing the expression of EGR2 through the activation of the ERK1/2 signaling pathway, which in turn promotes the expression of PAX7. We further show that TECRL deletion led to the upregulation of the histone acetyltransferase general control nonderepressible 5, which enhances the transcription of EGR2 through acetylation. Importantly, we showed that AAV9-mediated TECRL silencing improved muscle repair in mice. These findings shed light on myogenic regeneration and muscle repair.

Cellular and molecular alterations to muscles and neuromuscular synapses in a mouse model of MEGF10-related myopathy

Loss-of-function mutations in MEGF10 lead to a rare and understudied neuromuscular disorder known as MEGF10-related myopathy. There are no treatments for the progressive respiratory distress, motor impairment, and structural abnormalities in muscles caused by the loss of MEGF10 function. In this study, we deployed cellular and molecular assays to obtain additional insights about MEGF10-related myopathy in juvenile, young adult, and middle-aged Megf10 knockout (KO) mice. We found fewer muscle fibers in juvenile and adult Megf10 KO mice, supporting published studies that MEGF10 regulates myogenesis by affecting satellite cell differentiation. Interestingly, muscle fibers do not exhibit morphological hallmarks of atrophy in either young adult or middle-aged Megf10 KO mice. We next examined the neuromuscular junction (NMJ), in which MEGF10 has been shown to concentrate postnatally, using light and electron microscopy. We found early and progressive degenerative features at the NMJs of Megf10 KO mice that include increased postsynaptic fragmentation and presynaptic regions not apposed by postsynaptic nicotinic acetylcholine receptors. We also found perisynaptic Schwann cells intruding into the NMJ synaptic cleft. These findings strongly suggest that the NMJ is a site of postnatal pathology in MEGF10-related myopathy. In support of these cellular observations, RNA-seq analysis revealed genes and pathways associated with myogenesis, skeletal muscle health, and NMJ stability dysregulated in Megf10 KO mice compared to wild-type mice. Altogether, these data provide new and valuable cellular and molecular insights into MEGF10-related myopathy.

Dynamics of pax7 expression during development, muscle regeneration, and in vitro differentiation of satellite cells in rainbow trout (Oncorhynchus mykiss)

Essential for muscle fiber formation and hypertrophy, muscle stem cells, also called satellite cells, reside beneath the basal lamina of the muscle fiber. Satellite cells have been commonly identified by the expression of the Paired box 7 (Pax7) due to its specificity and the availability of antibodies in tetrapods. In fish, the identification of satellite cells remains difficult due to the lack of specific antibodies in most species. Based on the development of a highly sensitive in situ hybridization (RNAScope®) for pax7, we showed that pax7+ cells were detected in the undifferentiated myogenic epithelium corresponding to the dermomyotome at day 14 post-fertilization in rainbow trout. Then, from day 24, pax7+ cells gradually migrated into the deep myotome and were localized along the muscle fibers and reach their niche in satellite position of the fibres after hatching. Our results showed that 18 days after muscle injury, a large number of pax7+ cells accumulated at the wound site compared to the uninjured area. During the in vitro differentiation of satellite cells, the percentage of pax7+ cells decreased from 44% to 18% on day 7, and some differentiated cells still expressed pax7. Taken together, these results show the dynamic expression of pax7 genes and the follow-up of these muscle stem cells during the different situations of muscle fiber formation in trout.

Validation of positional candidates Rps6ka6 and Pou3f4 for a locus associated with skeletal muscle mass variability

Genetic variability significantly contributes to individual differences in skeletal muscle mass; however, the specific genes involved in that process remain elusive. In this study, we examined the role of positional candidates, Rps6ka6 and Pou3f4, of a chromosome X locus, implicated in muscle mass variability in CFW laboratory mice. Histology of hindlimb muscles was studied in CFW male mice carrying the muscle "increasing" allele C (n = 15) or "decreasing" allele T (n = 15) at the peak marker of the locus, rs31308852, and in the Pou3f4y/- and their wild-type male littermates. To study the role of the Rps6ka6 gene, we deleted exon 7 (Rps6ka6-ΔE7) using clustered regularly interspaced palindromic repeats-Cas9 based method in H2Kb myogenic cells creating a severely truncated RSK4 protein. We then tested whether that mutation affected myoblast proliferation, migration, and/or differentiation. The extensor digitorum longus muscle was 7% larger (P < 0.0001) due to 10% more muscle fibers (P = 0.0176) in the carriers of the "increasing" compared with the "decreasing" CFW allele. The number of fibers was reduced by 15% (P = 0.0268) in the slow-twitch soleus but not in the fast-twitch extensor digitorum longus (P = 0.2947) of Pou3f4y/- mice. The proliferation and migration did not differ between the Rps6ka6-ΔE7 and wild-type H2Kb myoblasts. However, indices of differentiation (myosin expression, P < 0.0001; size of myosin-expressing cells, P < 0.0001; and fusion index, P = 0.0013) were significantly reduced in Rps6ka6-ΔE7 cells. This study suggests that the effect of the X chromosome locus on muscle fiber numbers in the fast-twitch extensor digitorum longus is mediated by the Rps6ka6 gene, whereas the Pou3f4 gene affects fiber number in slow-twitch soleus.

Generation of allogeneic and xenogeneic functional muscle stem cells for intramuscular transplantation

Satellite cells, the stem cells of skeletal muscle tissue, hold a remarkable regeneration capacity and therapeutic potential in regenerative medicine. However, low satellite cell yield from autologous or donor-derived muscles hinders the adoption of satellite cell transplantation for the treatment of muscle diseases, including Duchenne muscular dystrophy (DMD). To address this limitation, here we investigated whether satellite cells can be derived in allogeneic or xenogeneic animal hosts. First, injection of CRISPR/Cas9-corrected Dmdmdx mouse induced pluripotent stem cells (iPSCs) into mouse blastocysts carrying an ablation system of host satellite cells gave rise to intraspecies chimeras exclusively carrying iPSC-derived satellite cells. Furthermore, injection of genetically corrected DMD iPSCs into rat blastocysts resulted in the formation of interspecies rat-mouse chimeras harboring mouse satellite cells. Notably, iPSC-derived satellite cells or derivative myoblasts produced in intraspecies or interspecies chimeras restored dystrophin expression in DMD mice following intramuscular transplantation and contributed to the satellite cell pool. Collectively, this study demonstrates the feasibility of producing therapeutically competent stem cells across divergent animal species, raising the possibility of generating human muscle stem cells in large animals for regenerative medicine purposes.

Cysteine Rich Intestinal Protein 2 is a copper-responsive regulator of skeletal muscle differentiation

Copper (Cu) is an essential trace element required for respiration, neurotransmitter synthesis, oxidative stress response, and transcriptional regulation. Imbalance in Cu homeostasis can lead to several pathological conditions, affecting neuronal, cognitive, and muscular development. Mechanistically, Cu and Cu-binding proteins (Cu-BPs) have an important but underappreciated role in transcription regulation in mammalian cells. In this context, our lab investigates the contributions of novel Cu-BPs in skeletal muscle differentiation using murine primary myoblasts. Through an unbiased synchrotron X-ray fluorescence-mass spectrometry (XRF/MS) metalloproteomic approach, we identified the murine cysteine rich intestinal protein 2 (mCrip2) in a sample that showed enriched Cu signal, which was isolated from differentiating primary myoblasts derived from mouse satellite cells. Immunolocalization analyses showed that mCrip2 is abundant in both nuclear and cytosolic fractions. Thus, we hypothesized that mCrip2 might have differential roles depending on its cellular localization in the skeletal muscle lineage. mCrip2 is a LIM-family protein with 4 conserved Zn2+-binding sites. Homology and phylogenetic analyses showed that mammalian Crip2 possesses histidine residues near two of the Zn2+-binding sites (CX2C-HX2C) which are potentially implicated in Cu+-binding and competition with Zn2+. Biochemical characterization of recombinant human hsCRIP2 revealed a high Cu+-binding affinity for two and four Cu+ ions and limited redox potential. Functional characterization using CRISPR/Cas9-mediated deletion of mCrip2 in primary myoblasts did not impact proliferation, but impaired myogenesis by decreasing the expression of differentiation markers, possibly attributed to Cu accumulation. Transcriptome analyses of proliferating and differentiating mCrip2 KO myoblasts showed alterations in mRNA processing, protein translation, ribosome synthesis, and chromatin organization. CUT&RUN analyses showed that mCrip2 associates with a select set of gene promoters, including MyoD1 and metallothioneins, acting as a novel Cu-responsive or Cu-regulating protein. Our work demonstrates novel regulatory functions of mCrip2 that mediate skeletal muscle differentiation, presenting new features of the Cu-network in myoblasts.

CRISPR Screen Identifies the RNA-Binding Protein Eef1a1 as a Key Regulator of Myogenesis

Skeletal muscle myogenesis hinges on gene regulation, meticulously orchestrated by molecular mechanisms. While the roles of transcription factors and non-coding RNAs in myogenesis are widely known, the contribution of RNA-binding proteins (RBPs) has remained unclear until now. Therefore, to investigate the functions of post-transcriptional regulators in myogenesis and uncover new functional RBPs regulating myogenesis, we employed CRISPR high-throughput RBP-KO (RBP-wide knockout) library screening. Through this approach, we successfully identified Eef1a1 as a novel regulatory factor in myogenesis. Using CRISPR knockout (CRISPRko) and CRISPR interference (CRISPRi) technologies, we successfully established cellular models for both CRISPRko and CRISPRi. Our findings demonstrated that Eef1a1 plays a crucial role in promoting proliferation in C2C12 myoblasts. Through siRNA inhibition and overexpression methods, we further elucidated the involvement of Eef1a1 in promoting proliferation and suppressing differentiation processes. RIP (RNA immunoprecipitation), miRNA pull-down, and Dual-luciferase reporter assays confirmed that miR-133a-3p targets Eef1a1. Co-transfection experiments indicated that miR-133a-3p can rescue the effect of Eef1a1 on C2C12 myoblasts. In summary, our study utilized CRISPR library high-throughput screening to unveil a novel RBP, Eef1a1, involved in regulating myogenesis. Eef1a1 promotes the proliferation of myoblasts while inhibiting the differentiation process. Additionally, it acts as an antagonist to miR-133a-3p, thus modulating the process of myogenesis.

Establishment and Characterization of SV40 T-Antigen Immortalized Porcine Muscle Satellite Cell

Muscle satellite cells (MuSCs) are crucial for muscle development and regeneration. The primary pig MuSCs (pMuSCs) is an ideal in vitro cell model for studying the pig's muscle development and differentiation. However, the long-term in vitro culture of pMuSCs results in the gradual loss of their stemness, thereby limiting their application. To address this conundrum and maintain the normal function of pMuSCs during in vitro passaging, we generated an immortalized pMuSCs (SV40 T-pMuSCs) by stably expressing SV40 T-antigen (SV40 T) using a lentiviral-based vector system. The SV40 T-pMuSCs can be stably sub-cultured for over 40 generations in vitro. An evaluation of SV40 T-pMuSCs was conducted through immunofluorescence staining, quantitative real-time PCR, EdU assay, and SA-β-gal activity. Their proliferation capacity was similar to that of primary pMuSCs at passage 1, and while their differentiation potential was slightly decreased. SiRNA-mediated interference of SV40 T-antigen expression restored the differentiation capability of SV40 T-pMuSCs. Taken together, our results provide a valuable tool for studying pig skeletal muscle development and differentiation.

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.

Control of muscle satellite cell function by specific exercise-induced cytokines and their applications in muscle maintenance

Exercise is recognized to play an observable role in improving human health, especially in promoting muscle hypertrophy and intervening in muscle mass loss-related diseases, including sarcopenia. Recent rapid advances have demonstrated that exercise induces the release of abundant cytokines from several tissues (e.g., liver, muscle, and adipose tissue), and multiple cytokines improve the functions or expand the numbers of adult stem cells, providing candidate cytokines for alleviating a wide range of diseases. Muscle satellite cells (SCs) are a population of muscle stem cells that are mitotically quiescent but exit from the dormancy state to become activated in response to physical stimuli, after which SCs undergo asymmetric divisions to generate new SCs (stem cell pool maintenance) and commit to later differentiation into myocytes (skeletal muscle replenishment). SCs are essential for the postnatal growth, maintenance, and regeneration of skeletal muscle. Emerging evidence reveals that exercise regulates muscle function largely via the exercise-induced cytokines that govern SC potential, but this phenomenon is complicated and confusing. This review provides a comprehensive integrative overview of the identified exercise-induced cytokines and the roles of these cytokines in SC function, providing a more complete picture regarding the mechanism of SC homeostasis and rejuvenation therapies for skeletal muscle.

Muscle stem cell niche dynamics during muscle homeostasis and regeneration

The process of skeletal muscle regeneration involves a coordinated interplay of specific cellular and molecular interactions within the injury site. This review provides an overview of the cellular and molecular components in regenerating skeletal muscle, focusing on how these cells or molecules in the niche regulate muscle stem cell functions. Dysfunctions of muscle stem cell-to-niche cell communications during aging and disease will also be discussed. A better understanding of how niche cells coordinate with muscle stem cells for muscle repair will greatly aid the development of therapeutic strategies for treating muscle-related disorders.