Myogenin expression, cell cycle withdrawal, and phenotypic differentiation are temporally separable events that precede cell fusion upon myogenesis

Studying the impact of geometrical and cellular cues on myogenesis with a skeletal muscle-on-chip

In the skeletal muscle tissue, cells are organized following an anisotropic architecture, which is both required during myogenesis when muscle precursor cells fuse to generate myotubes and for its contractile function. To build an in vitro skeletal muscle tissue, it is therefore essential to develop methods to organize cells in an anisotropic fashion, which can be particularly challenging, especially in 3D. In this study, we present a versatile muscle-on-chip system with adjustable collagen hollow tubes that can be seeded with muscle precursor cells. The collagen acts both as a tube-shaped hollow mold and as an extracellular matrix scaffold that can house other cell types for co-culture. We found that the diameter of the channel affects the organization of the muscle cells and that proper myogenesis was obtained at a diameter of 75 μm. In these conditions, muscle precursor cells fused into long myotubes aligned along these collagen channels, resulting in a fascicle-like structure. These myotubes exhibited actin striations and upregulation of multiple myogenic genes, reflecting their maturation. Moreover, we showed that our chip allowed muscle tissue culture and maturation over a month, with the possibility of fibroblast co-culture embedding in the collagen matrix.

Combined regenerative rehabilitation improves recovery following volumetric muscle loss injury in a rat model

Volumetric muscle loss (VML) injury causes irreversible deficits in muscle mass and function, often resulting in permanent disability. The current standard of care is physical therapy, but it is limited in mitigating functional deficits. We have previously optimized a rehabilitation technique using electrically stimulated eccentric contraction training (EST) that improved muscle mass, strength, and size in VML-injured rats. A biosponge scaffold composed of extracellular matrix proteins has previously enhanced muscle function postVML. This study aimed to determine whether combining a regenerative therapy (i.e., biosponge) with a novel rehabilitation technique (i.e., EST) could enhance recovery in a rat model of VML. A VML defect was created by removing ~20% of muscle mass from the tibialis anterior muscle in adult male Lewis rats. Experimental groups included VML-injured rats treated with biosponge with EST or biosponge alone (n = 6/group). EST was implemented 2 weeks postinjury at 150 Hz and was continued for 4 weeks. A linear increase in eccentric torque over 4 weeks showed the adaptability of the VML-injured muscle to EST. Combining biosponge with EST improved peak isometric torque by ~52% compared with biosponge treatment alone at 6 weeks postinjury. Application of EST increased MyoD gene expression and the percentage of large (>2000 μm2) type 2B myofibers but reduced fibrotic tissue deposition in VML-injured muscles. Together, these changes may provide the basis for improved torque production. This study demonstrates the potential for combined regenerative and rehabilitative therapy to improve muscle recovery following VML.

The Capicua C1 Domain is Required for Full Activity of the CIC::DUX4 Fusion Oncoprotein

Rearrangements between genes can yield neomorphic fusions that drive oncogenesis. Fusion oncogenes are made up of fractional segments of the partner genes that comprise them, with each partner potentially contributing some of its own function to the nascent fusion oncoprotein. Clinically, fusion oncoproteins driving one diagnostic entity are typically clustered into a single molecular subset and are often treated a similar fashion. However, knowledge of where specific fusion breakpoints occur in partner genes, and the resulting retention of functional domains in the fusion, is an important determinant of fusion oncoprotein activity and may differ between patients. This study investigates this phenomena through the example of CIC::DUX4, a fusion between the transcriptional repressor capicua (CIC) and the double homeobox 4 gene (DUX4), which drives an aggressive subset of undifferentiated round cell sarcoma. Using a harmonized dataset of over 100 patient fusion breakpoints from the literature, we show that most bona fide CIC::DUX4 fusions retain the C1 domain, which is known to contribute to DNA binding by wild type CIC. Mechanistically, deletion or mutation of the C1 domain reduces, but does not eliminate, activation of CIC target genes by CIC::DUX4. We also find that expression of C1-deleted CIC::DUX4 is capable of exerting intermediate transformation-related phenotypes compared with those imparted by full-length CIC::DUX4, but was not sufficient for tumorigenesis in a subcutaneous mouse model. In summary, our results suggest a supercharging role for the C1 domain in the activity of CIC::DUX4.

Temporal regulation of the Mediator complex during muscle proliferation, differentiation, regeneration, aging, and disease

Genesis of skeletal muscle relies on the differentiation and fusion of mono-nucleated muscle progenitor cells into the multi-nucleated muscle fiber syncytium. The temporally-controlled cellular and morphogenetic changes underlying this process are initiated by a series of highly coordinated transcription programs. At the core, the myogenic differentiation cascade is driven by muscle-specific transcription factors, i.e., the Myogenic Regulatory Factors (MRFs). Despite extensive knowledge on the function of individual MRFs, very little is known about how they are coordinated. Ultimately, highly specific coordination of these transcription programs is critical for their masterfully timed transitions, which in turn facilitates the intricate generation of skeletal muscle fibers from a naïve pool of progenitor cells. The Mediator complex links basal transcriptional machinery and transcription factors to regulate transcription and could be the integral component that coordinates transcription factor function during muscle differentiation, growth, and maturation. In this study, we systematically deciphered the changes in Mediator complex subunit expression in skeletal muscle development, regeneration, aging, and disease. We incorporated our in vitro and in vivo experimental results with analysis of publicly available RNA-seq and single nuclei RNA-seq datasets and uncovered the regulation of Mediator subunits in different physiological and temporal contexts. Our experimental results revealed that Mediator subunit expression during myogenesis is highly dynamic. We also discovered unique temporal patterns of Mediator expression in muscle stem cells after injury and during the early regeneration period, suggesting that Mediator subunits may have unique contributions to directing muscle stem cell fate. Although we observed few changes in Mediator subunit expression in aging muscles compared to younger muscles, we uncovered extensive heterogeneity of Mediator subunit expression in dystrophic muscle nuclei, characteristic of chronic muscle degeneration and regeneration cycles. Taken together, our study provides a glimpse of the complex regulation of Mediator subunit expression in the skeletal muscle cell lineage and serves as a springboard for mechanistic studies into the function of individual Mediator subunits in skeletal muscle.

FoxP1 Represses MEF2A in Striated Muscle

Myocyte enhancer factor 2 (MEF2) proteins are involved in multiple developmental, physiological, and pathological processes in vertebrates. Protein-protein interactions underlie the plethora of biological processes impacted by MEF2A, necessitating a detailed characterization of the MEF2A interactome. A nanobody based affinity-purification/mass spectrometry strategy was employed to achieve this goal. Specifically, the MEF2A protein complexes were captured from myogenic lysates using a GFP-tagged MEF2A protein immobilized with a GBP-nanobody followed by LC-MS/MS proteomic analysis to identify MEF2A interactors. After bioinformatic analysis, we further characterized the interaction of MEF2A with a transcriptional repressor, FOXP1. FOXP1 coprecipitated with MEF2A in proliferating myogenic cells which diminished upon differentiation (myotube formation). Ectopic expression of FOXP1 inhibited MEF2A driven myogenic reporter genes (derived from the creatine kinase muscle and myogenin genes) and delayed induction of endogenous myogenin during differentiation. Conversely, FOXP1 depletion enhanced MEF2A transactivation properties and myogenin expression. The FoxP1:MEF2A interaction is also preserved in cardiomyocytes and FoxP1 depletion enhanced cardiomyocyte hypertrophy. FOXP1 prevented MEF2A phosphorylation and activation by the p38MAPK pathway. Overall, these data implicate FOXP1 in restricting MEF2A function in order to avoid premature differentiation in myogenic progenitors and also to possibly prevent re-activation of embryonic gene expression in cardiomyocyte hypertrophy.

Extraocular muscle stem cells exhibit distinct cellular properties associated with non-muscle molecular signatures

Skeletal muscle stem cells (MuSCs) are recognised as functionally heterogeneous. Cranial MuSCs are reported to have greater proliferative and regenerative capacity when compared with those in the limb. A comprehensive understanding of the mechanisms underlying this functional heterogeneity is lacking. Here, we have used clonal analysis, live imaging and single cell transcriptomic analysis to identify crucial features that distinguish extraocular muscle (EOM) from limb muscle stem cell populations. A MyogeninntdTom reporter showed that the increased proliferation capacity of EOM MuSCs correlates with deferred differentiation and lower expression of the myogenic commitment gene Myod. Unexpectedly, EOM MuSCs activated in vitro expressed a large array of extracellular matrix components typical of mesenchymal non-muscle cells. Computational analysis underscored a distinct co-regulatory module, which is absent in limb MuSCs, as driver of these features. The EOM transcription factor network, with Foxc1 as key player, appears to be hardwired to EOM identity as it persists during growth, disease and in vitro after several passages. Our findings shed light on how high-performing MuSCs regulate myogenic commitment by remodelling their local environment and adopting properties not generally associated with myogenic cells.

Cyclin D3 Colocalizes with Myogenin and p21 in Skeletal Muscle Satellite Cells during Early-Stage Functional Overload

Myogenic cell differentiation is modulated by multiple regulatory factors, such as myogenin, p21, and cyclin D3 during myogenesis in vitro. It is also recognized that myogenin and p21 play important roles in regulating muscle satellite cell (SC) differentiation during overload-induced muscle hypertrophy in vivo. However, the expression patterns and functional role of cyclin D3 in the progress of muscle hypertrophy remain unclear. Thus, the present study investigated cyclin D3 expression in skeletal muscles during early-stage functional overload. Plantaris muscles were exposed to functional overload due to ablation of the gastrocnemius and soleus muscles. As a result, cyclin D3 expression was detected in the nuclei of SCs but not in myonuclei on day 1 after surgery. Cyclin D3 expression, after functional overload, gradually increased, reaching a maximum on day 7 along with myogenin expression. Moreover, in response to the functional overload, cyclin D3 was expressed simultaneously with myogenin and p21 in SC nuclei. Therefore, the present study suggests that cyclin D3 with myogenin and p21 may interactively regulate SC differentiation during early-stage functional overload.

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.

Homer 1 genotype AA variant relates to congenital splay leg syndrome in piglets by repressing Pax7 in myogenic progenitors

Introduction

Porcine congenital splay leg syndrome (PCS) is a major birth defect in piglets, resulting in lameness and high mortality rates. The multifactorial pathogenesis of PSC is not well understood but includes a polygenic inheritance.

Methods

Here, in addition to morphological investigations, we characterized the expression of myogenic genes and functional (proliferation and differentiation) properties of myogenic precursor/satellite cells (SATCs) in 1 day-old PCS piglets, non-affected littermates (LCs), and piglets from PCS-free healthy litters (HCs). In addition, PCS phenotypes were related to the SNP Homer1_rs325197091 within the Homer1 locus, which has been identified as a potential hereditary cause of PCS.

Results and discussion

Samples from musculus semitendinosus (ST) of PCS piglets had a higher proportion of type II fibers, reflecting myofiber immaturity. In addition, myofiber atrophy, a lower number of myonuclei per fiber (ST), and a higher apoptotic activity (in ST and longissimus dorsi muscle; LD) were found in the PCS group. A higher proportion of cycling committed myoblasts (Pax7+/Ki67+ cells) occurred in samples from PCS-affected piglets, and on the other hand, the mRNA expression of genes involved in differentiation (muscle differentiation 1; MyoD, myogenin; MyoG) was repressed compared with HCs. Cultured SATCs from PCS-affected animals showed a temporal shift in peak expression of Pax7, MyoD, and MyoG toward days 3 and 4 of their 7 days differentiation regime. In vitro experiments with isolated SATCs confirmed the lower differentiation potential and the delayed progression of the myogenic processes in cells from piglets with PCS phenotype. In addition, Pax7 and desmin were differently expressed in Homer1_rs325197091 genotype variants (GG, GA, and AA). Both genes showed the lowest expression in the homozygous AA-variant, which was most frequently found in PCS-affected animals. The homozygous AA-variant was also associated with lower expression of the truncated Homer1-subtype 205. Thus, we hypothesize that in PCS, the balance between Homer1 proteins and its signaling functions is changed in a way detrimental to the myogenic differentiation program. Our results demonstrated direct negative effects of the Homer1 AA genotype on Pax7 expression, but the exact mode of action still needs to be elucidated.

Upregulation of human GD3 synthase (hST8Sia I) gene expression during serum starvation-induced osteoblastic differentiation of MG-63 cells

In this study, we have firstly elucidated that serum starvation augmented the levels of human GD3 synthase (hST8Sia I) gene and ganglioside GD3 expression as well as bone morphogenic protein-2 and osteocalcin expression during MG-63 cell differentiation using RT-PCR, qPCR, Western blot and immunofluorescence microscopy. To evaluate upregulation of hST8Sia I gene during MG-63 cell differentiation by serum starvation, promoter area of the hST8Sia I gene was functionally analyzed. Promoter analysis using luciferase reporter assay system harboring various constructs of the hST8Sia I gene proved that the cis-acting region at -1146/-646, which includes binding sites of the known transcription factors AP-1, CREB, c-Ets-1 and NF-κB, displays the highest level of promoter activity in response to serum starvation in MG-63 cells. The -731/-722 region, which contains the NF-κB binding site, was proved to be essential for expression of the hST8Sia I gene by serum starvation in MG-63 cells by site-directed mutagenesis, NF-κB inhibition, and chromatin immunoprecipitation (ChIP) assay. Knockdown of hST8Sia I using shRNA suggested that expressions of hST8Sia I and GD3 have no apparent effect on differentiation of MG-63 cells. Moreover, the transcriptional activation of hST8Sia I gene by serum starvation was strongly hindered by SB203580, a p38MAPK inhibitor in MG-63 cells. From these results, it has been suggested that transcription activity of hST8Sia I gene by serum starvation in human osteosarcoma MG-63 cells is regulated by p38MAPK/NF-κB signaling pathway.

Exploring the impact of temporal heat stress on skeletal muscle hypertrophy in bovine myocytes

The primary aim of this investigation was to explore the impact of different temporal stress conditions on the regulators associated with skeletal muscle hypertrophy in bovine myocytes. Bovine satellite cells (BSCs) were extracted from three-month-old Holstein bull calves and subjected to myogenic differentiation under three thermal treatments: 38 °C (control; CON), 39.5 °C (moderate heat stress; MHS), and 41 °C (extreme heat stress; EHS) for a duration of 3 or 48 h. Exposure to EHS resulted in elevated (P < 0.01) expression levels of heat shock protein (HSP)20, HSP27, HSP70, and HSP90, along with increased (P < 0.01) protein levels. Moreover, cells exposed to MHS and EHS exhibited enhanced (P < 0.01) gene expression of myoblast determination protein 1 (MyoD), while myogenin (MyoG) was overexpressed (P < 0.01) in cells exposed to EHS. These findings suggest that heat exposure can potentially induce myogenic differentiation through the modulation of myogenic regulatory factors. Furthermore, our investigations revealed that exposure to EHS upregulated (P < 0.01) myosin heavy chain (MHC) I expression, whereas MHC IIA (P < 0.01) and IIX (P < 0.01) expression were increased; P < 0.01) under MHS conditions. These observations suggest that the temperature of the muscle may alter the proportion of muscle fiber types. Additionally, our data indicated that EHS activated (P < 0.01) the expression of insulin-like growth factor 1 (IGF-1) and triggered the activation of the Akt/mTOR/S6KB1 pathway, a known anabolic pathway associated with cellular protein synthesis. Consequently, these altered signaling pathways contributed to enhanced protein synthesis and increased myotube size. Overall, the results obtained from our current study revealed that extreme heat exposure (41 °C) may promote skeletal muscle hypertrophy by regulating myogenic regulatory factors and IGF-1-mediated mTOR pathway in bovine myocytes.

MicroRNA-142a-3p regulates neurogenic skeletal muscle atrophy by targeting Mef2a

Peripheral nerve injury can lead to progressive muscle atrophy and poor motor function recovery, which is a difficult point of treatment, and the mechanism needs to be further explored. In previous studies, we found that miR-142a-3p was significantly upregulated and persistently highly expressed in denervated mouse skeletal muscle. Here, we show that overexpression of miR-142a-3p inhibited the growth and differentiation of C2C12 myoblast, while knockdown of miR-142a-3p had a promoting effect. In vitro, knockdown of miR-142a-3p in denervated mouse skeletal muscle effectively increased proliferating muscle satellite cells and ameliorated muscle atrophy. Mechanistically, the myoregulator Mef2a was proved to be an important downstream target of miR-142a-3p, and miR-142a-3p regulates skeletal muscle differentiation and regeneration by inhibiting the expression of Mef2a. The co-knockdown of Mef2a and miR-142a-3p effectively alleviated or offset the biological effects of miR-142a-3p knockdown. In conclusion, our data revealed that miR-142a-3p regulates neurogenic skeletal muscle atrophy by targeting Mef2a.

A dysfunctional miR-1-TRPS1-MYOG axis drives ERMS by suppressing terminal myogenic differentiation

Rhabdomyosarcoma is the most common pediatric soft tissue tumor, comprising two major subtypes: the PAX3/7-FOXO1 fusion-negative embryonal and the PAX3/7-FOXO1 fusion-positive alveolar subtype. Here, we demonstrate that the expression levels of the transcriptional repressor TRPS1 are specifically enhanced in the embryonal subtype, resulting in impaired terminal myogenic differentiation and tumor growth. During normal myogenesis, expression levels of TRPS1 have to decrease to allow myogenic progression, as demonstrated by overexpression of TRPS1 in myoblasts impairing myotube formation. Consequentially, myogenic differentiation in embryonal rhabdomyosarcoma in vitro as well as in vivo can be achieved by reducing TRPS1 levels. Furthermore, we show that TRPS1 levels in RD cells, the bona fide model cell line for embryonal rhabdomyosarcoma, are regulated by miR-1 and that TRPS1 and MYOD1 share common genomic binding sites. The myogenin (MYOG) promoter is one of the critical targets of TRPS1 and MYOD1; we demonstrate that TRPS1 restricts MYOG expression and thereby inhibits terminal myogenic differentiation. Therefore, reduction of TRPS1 levels in embryonal rhabdomyosarcoma might be a therapeutic approach to drive embryonal rhabdomyosarcoma cells into myogenic differentiation, thereby generating postmitotic myotubes.

The Functional Role of Myogenin in Cardiomyoblast H9c2 Cells Treated with High Glucose and Palmitic Acid: Insights into No-Rejection Heart Transplantation

Various pathological alterations, including lipid-deposition-induced comparative cardiac lipotoxicity, contribute to cardiac aging in the failing heart. A decline in endogenous myogenin proteins can lead to the reversal of muscle cell differentiation and the creation of mononucleated muscle cells. Myogenin may be a specific regulator of adaptive responses to avoid pathological hypertrophy in the heart. Hence, it is important to understand the regulation of myogenin expression and functions in response to exposure to varied stresses. In this study, we first examined and verified the cytotoxic effect of palmitic acid on H9c2 cells. The reduction in myogenin mRNA and protein expression by palmitic acid was independent of the effect of glucose. Meanwhile, the induction of cyclooxygenase 2 and activating transcription factor 3 mRNAs and proteins by palmitic acid was dependent on the presence of glucose. In addition, palmitic acid failed to disrupt cell cycle progression when H9c2 cells were treated with no glucose. Next, we examined the functional role of myogenin in palmitic-acid-treated H9c2 cells and found that myogenin may be involved in palmitic-acid-induced mitochondrial and cytosolic ROS generation, cellular senescence, and mitochondrial membrane potential. Finally, the GSE150059 dataset was deposited in the Gene Expression Omnibus website and the dataset was further analyzed via the molecular microscope diagnostic system (MMDx), demonstrating that many heart transplant biopsies currently diagnosed as no rejection have mild molecular-antibody-mediated rejection-related changes. Our data show that the expression levels of myogenin were lower than the average level in the studied population. Combining these results, we uncover part of the functional role of myogenin in lipid- and glucose-induced cardiac cell stresses. This finding provides valuable insight into the differential role of fatty-acid-associated gene expression in cardiovascular tissues. Additionally, the question of whether this gene expression is regulated by myogenin also highlights the usefulness of a platform such as MMDx-Heart and can help elucidate the functional role of myogenin in heart transplantation.

The ensured proliferative capacity of myoblast in serum-reduced conditions with Methyl-β-cyclodextrin

To produce muscle fibers for cultured meat on a large scale, it is important to expand myoblasts in a serum-reduced or serum-free medium to avoid cost, ethical, and environmental issues. Myoblasts such as C2C12 cells differentiate quickly into myotubes and lose their ability to proliferate when the serum-rich medium is replaced with a serum-reduced medium. This study demonstrates that Methyl-β-cyclodextrin (MβCD), a starch-derived agent that depletes cholesterol, can inhibit further differentiation of myoblasts at the MyoD-positive stage by reducing plasma membrane cholesterol on C2C12 cells and primary cultured chick muscle cells. Furthermore, MβCD efficiently blocks cholesterol-dependent apoptotic cell death of myoblasts, which is one of the mechanisms by which it inhibits the differentiation of C2C12 myoblast cells, as dead cells of myoblast are necessary for the fusion of adjacent myoblasts during the differentiation process into myotubes. Importantly, MβCD maintains the proliferative capacity of myoblasts only under differentiation conditions with a serum-reduced medium, suggesting that its mitogenic effect is due to its inhibitory effect on myoblast differentiation into myotube. In conclusion, this study provides significant insights into ensuring the proliferative capacity of myoblasts in a future serum-free condition for cultured meat production.

Sensitization to Ionizing Radiation by MEK Inhibition Is Dependent on SNAI2 in Fusion-Negative Rhabdomyosarcoma

In fusion-negative rhabdomyosarcoma (FN-RMS), a pediatric malignancy with skeletal muscle characteristics, >90% of high-risk patients have mutations that activate the RAS/MEK signaling pathway. We recently discovered that SNAI2, in addition to blocking myogenic differentiation downstream of MEK signaling in FN-RMS, represses proapoptotic BIM expression to protect RMS tumors from ionizing radiation (IR). As clinically relevant concentrations of the MEK inhibitor trametinib elicit poor responses in preclinical xenograft models, we investigated the utility of low-dose trametinib in combination with IR for the treatment of RAS-mutant FN-RMS. We hypothesized that trametinib would sensitize FN-RMS to IR through its downregulation of SNAI2 expression. While we observed little to no difference in myogenic differentiation or cell survival with trametinib treatment alone, robust differentiation and reduced survival were observed after IR. In addition, IR-induced apoptosis was significantly increased in FN-RMS cells treated concurrently with trametinib, as was increased BIM expression. SNAI2's role in these processes was established using overexpression rescue experiments, where overexpression of SNAI2 prevented IR-induced myogenic differentiation and apoptosis. Moreover, combining MEK inhibitor with IR resulted in complete tumor regression and a 2- to 4-week delay in event-free survival (EFS) in preclinical xenograft and patient-derived xenograft models. Our findings demonstrate that the combination of MEK inhibition and IR results in robust differentiation and apoptosis, due to the reduction of SNAI2, which leads to extended EFS in FN-RMS. SNAI2 thus is a potential biomarker of IR insensitivity and target for future therapies to sensitize aggressive sarcomas to IR.

Heat stress inhibits the proliferation and differentiation of myoblasts and is associated with damage to mitochondria

Introduction: Heat stress is harmful to the health of humans and animals, more and more common, as a consequence of global warming, while the mechanism that heat stress modulates skeletal development remains unknown. Hence, we conducted a model of heat stress in vitro. Methods: We used Hu sheep myoblasts as the research object, real-time quantitative PCR (RT-qPCR) and western blot (WB) were conducted to detect the expression of mRNA and protein in heat-stressed myoblasts. The would-healing assay was used to detect the migration of myoblasts. The mitochondria were observed by a transmission electron microscope. Results: mRNA and protein expression of HSP60 was significantly enriched in the heat-stressed myoblasts during proliferation and differentiation (p < 0.05). In our study, we indicated that heat stress enriched the intracellular ROS of the myoblasts (p < 0.001), leading to an increase in autophagy in the myoblasts to induce apoptosis. The results demonstrated that the protein expression of LC3B-1 and BCL-2 was significantly increased in myoblasts under heat stress during proliferation and differentiation (p < 0.05). Additionally, heat stress inhibited mitochondrial biogenesis and function and reduced the mitochondrial membrane potential and downregulated the expression of mtCo2, mtNd1 and DNM1L (p < 0.05) in myoblasts during proliferation and differentiation. Consequently, heat stress inhibited the proliferation and differentiation of the myoblasts, in accordance with the downregulation of the expression of PAX7, MYOD, MYF5, MYOG and MYHC (p < 0.05). Moreover, heat stress also inhibited the cell migration of the myoblasts. Discussion: This work demonstrates that heat stress inhibits proliferation and differentiation, and accelerates apoptosis by impairing mitochondrial function and promoting autophagy, which provides a mechanism to understand heat stress affects the development of the skeletal muscle.

Mealworm Ethanol Extract Enhances Myogenic Differentiation and Alleviates Dexamethasone-Induced Muscle Atrophy in C2C12 Cells

Aging, and other disease-related muscle disorders are serious health problems. Dexamethasone (DEX), a synthetic glucocorticoid, can trigger skeletal muscle atrophy. This study examined the effects of mealworm (Tenebrio molitor larva) ethanol extract (TME) on C2C12 myoblast differentiation and DEX-induced myotube atrophy. TME induced myotube formation compared to the differentiation medium (DM) group. TME also significantly increased the mRNA expression of muscle creatine kinase (CKm) and myogenic regulatory factors (MRFs), such as myogenin (MyoG), myogenic factor (Myf)5, and MRF4 (Myf6). TME dramatically increased the muscle-specific protein, MyoG, compared to the control, whereas the expression of myogenic differentiation 1 (MyoD) remained unchanged. It also activated the mammalian target of rapamycin (mTOR) signaling pathway. In the DEX-induced muscle atrophy C2C12 model, TME reduced the gene expression of atrogin-1, muscle RING finger protein-1 (MuRF-1), and myostatin, which are involved in protein degradation in skeletal muscles. Furthermore, TME elevated the phosphorylation of forkhead box O3 (FoxO3α) and protein kinase B (Akt). These findings suggest that TME can enhance myotube hypertrophy by regulating the mTOR signaling pathway, and can rescue DEX-induced muscle atrophy by alleviating atrophic muscle markers mediated by Akt activation. Thus, TME can be a potential therapeutic agent for treating muscle weakness and atrophy.

Effects of calcitriol on the cell cycle of rhabdomyosarcoma cells

Rhabdomyosarcoma (RMS) is a type of cancer of skeletal muscle. Calcitriol is the active form of vitamin D₃, also recognised as a steroid hormone called 1α, 25-dihydroxy vitamin D₃ (1,25D). We previously reported that 1,25D promoted cell proliferation and differentiation in non-cancerous skeletal muscle cells C2C12. The aim of this work is to evaluate some of the events triggered by 1,25D in RD cells, a human RMS cell line. In this work we reported that RD cells expressed vitamin D receptor (VDR) and treatment with 1,25D reduced VDR expression at 72 h. At the same time an acute decrease in viable cells as well as in cells in S-phase of cell cycle was also observed. Furthermore, up-regulation of p15^INK4b was accompanied in a timely manner by down-regulation of cyclin D3, p21^Waf1/Cip1 and myogenin protein levels. Simultaneously, 1,25D induced early apoptosis markers such as cyclin D1 and CDK4, and the disruption of the mitochondrial network together with a redistribution of mitochondria around the nucleus. Finally, 1,25D induced changes in the plasma membrane of RD cells associated with early and late apoptosis at 72 h, as determined by flow cytometry. Taken together, these results determine that treatment with 1,25D for 72 h triggers apoptosis in RD cells.

From cyclins to CDKIs: Cell cycle regulation of skeletal muscle stem cell quiescence and activation

After extensive proliferation during development, the adult skeletal muscle cells remain outside the cell cycle, either as post-mitotic myofibers or as quiescent muscle stem cells (MuSCs). Despite its terminally differentiated state, adult skeletal muscle has a remarkable regeneration potential, driven by MuSCs. Upon injury, MuSC quiescence is reversed to support tissue growth and repair and it is re-established after the completion of muscle regeneration. The distinct cell cycle states and transitions observed in the different myogenic populations are orchestrated by elements of the cell cycle machinery. This consists of i) complexes of cyclins and Cyclin-Dependent Kinases (CDKs) that ensure cell cycle progression and ii) their negative regulators, the Cyclin-Dependent Kinase Inhibitors (CDKIs). In this review we discuss the roles of these factors in developmental and adult myogenesis, with a focus on CDKIs that have emerging roles in stem cell functions.

Myogenic Precursor Cells Show Faster Activation and Enhanced Differentiation in a Male Mouse Model Selected for Advanced Endurance Exercise Performance

Satellite cells (SATC), the most abundant skeletal muscle stem cells, play a main role in muscle plasticity, including the adaptive response following physical activity. Thus, we investigated how long-term phenotype selection of male mice for high running performance (Dummerstorf high Treadmill Performance; DUhTP) affects abundance, creatine kinase activity, myogenic marker expression (Pax7, MyoD), and functionality (growth kinetics, differentiation) of SATC and their progeny. SATC were isolated from sedentary male DUhTP and control (Dummerstorf Control; DUC) mice at days 12, 43, and 73 of life and after voluntary wheel running for three weeks (day 73). Marked line differences occur at days 43 and 73 (after activity). At both ages, analysis of SATC growth via xCELLigence system revealed faster activation accompanied by a higher proliferation rate and lower proportion of Pax7+ cells in DUhTP mice, indicating reduced reserve cell formation and faster transition into differentiation. Cultures from sedentary DUhTP mice contain an elevated proportion of actively proliferating Pax7+/MyoD+ cells and have a higher fusion index leading to the formation of more large and very large myotubes at day 43. This robust hypertrophic response occurs without any functional load in the donor mice. Thus, our selection model seems to recruit myogenic precursor cells/SATC with a lower activation threshold that respond more rapidly to external stimuli and are more primed for differentiation at the expense of more primitive cells.

MicroRNA-24-3p promotes skeletal muscle differentiation and regeneration by regulating HMGA1

Numerous studies have established the critical roles of microRNAs in regulating post-transcriptional gene expression in diverse biological processes. Here, we report on the role and mechanism of miR-24-3p in skeletal muscle differentiation and regeneration. miR-24-3p promotes myoblast differentiation and skeletal muscle regeneration by directly targeting high mobility group AT-hook 1 (HMGA1) and regulating it and its direct downstream target, the inhibitor of differentiation 3 (ID3). miR-24-3p knockdown in neonatal mice increases PAX7-positive proliferating muscle stem cells (MuSCs) by derepressing Hmga1 and Id3. Similarly, inhibition of miR-24-3p in the tibialis anterior muscle prevents Hmga1 and Id3 downregulation and impairs regeneration. These findings provide evidence that the miR-24-3p/HMGA1/ID3 axis is required for MuSC differentiation and skeletal muscle regeneration in vivo.

Electrohydrodynamic-direct-printed cell-laden microfibrous structure using alginate-based bioink for effective myotube formation

In this study, a fully aligned microfibrous structure fabricated using fibrin-assisted alginate bioink and electrohydrodynamic direct-printing was proposed for skeletal muscle tissue engineering. To safely construct the aligned alginate/fibrin microfibrous structure laden with myoblasts or endothelial cells, various printing conditions, such as an applied electric field, distance between the nozzle and target, and nozzle moving speed, were selected appropriately. Furthermore, to accelerate the formation of myotubes more efficiently, the alginate/fibrin bioink with vascular endothelial cells was co-printed into a spatially patterned structure within a myoblast-laden structure. The myoblast-laden structure co-cultured with endothelial cells presented fully aligned myotube formation and significantly greater myogenic differentiation compared to the myoblast-laden structure without the endothelial cells owing to the more abundant secretion of angiogenic cytokines. Also, when adipose stem cell- and endothelial cell-laden fibrous structure was implanted in a mouse volumetric muscle loss model, accelerated volumetric muscle repair was observed compared to the defect model. Based on the results, this study demonstrates an alginate-based bioink and new bio-fabricating method to obtain microfibrous cell-laden alginate/fibrin structures with mechanically stable and topographical cues. The proposed method can provide a myoblast/endothelial cell-laden fibrous alginate structure to efficiently induce engineering of skeletal muscle tissue, which could be used in muscle-on-a-chip or recovering structures of volumetric muscle defects.

Porcine ZBED6 regulates growth of skeletal muscle and internal organs via multiple targets

ZBED6 (zinc finger BED domain containing protein 6) is a transcription factor unique to placental mammals and its interaction with the IGF2 (insulin-like growth factor 2) locus plays a prominent role in the regulation of postnatal skeletal muscle growth. Here, we generated lean Bama miniature pigs by generating ZBED6-knockout (ZBED6^−/−) and investigated the mechanism underlying ZBED6 in growth of muscle and internal organs of placental mammals. ZBED6^−/− pigs show markedly higher lean mass, lean mass rate, larger muscle fiber area and heavier internal organs (heart and liver) than wild-type (WT) pigs. The striking phenotypic changes of ZBED6^-/- pigs coincided with remarkable upregulation of IGF2 mRNA and protein expression across three tissues (gastrocnemius muscle, longissimus dorsi, heart). Despite a significant increase in liver weight, ZBED6^-/- pigs show comparable levels of IGF2 expression to those of WT controls. A mechanistic study revealed that elevated methylation in the liver abrogates ZBED6 binding at the IGF2 locus, explaining the unaltered hepatic IGF2 expression in ZBED6^-/- pigs. These results indicate that a ZBED6-IGF2-independent regulatory pathway exists in the liver. Transcriptome analysis and ChIP-PCR revealed new ZBED6 target genes other than IGF2, including cyclin dependent kinase inhibitor 1A (CDKN1A) and tsukushi, small leucine rich proteoglycan (TSKU), that regulates growth of muscle and liver, respectively.

The lean meat rate is an important economic trait for the swine industry and it is determined by muscle growth and development. A single base change in intron 3 of the insulin-like growth factor 2 (IGF2) gene increases meat production in pigs by disrupting a binding site for zinc finger BED domain containing protein 6 (ZBED6). Chinese indigenous pig breeds carrying the homozygous IGF2 wild-type allele produce low lean meat. We thus generate a lean pig model in Chinese Bama pig by knocking out ZBED6. In this model, we demonstrate that ZBED6 KO increases muscle and internal organ growth through ZBED6-IGF2 axis and other target genes. These results not only open new strategies for lean meat breeding in Chinese indigenous pigs, but also provide new insights to the global function of ZBED6 in organ growth and development.

Restoring the Cell Cycle and Proliferation Competence in Terminally Differentiated Skeletal Muscle Myotubes

Terminal differentiation is an ill-defined, insufficiently characterized, nonproliferation state. Although it has been classically deemed irreversible, it is now clear that at least several terminally differentiated (TD) cell types can be brought back into the cell cycle. We are striving to uncover the molecular bases of terminal differentiation, whose fundamental understanding is a goal in itself. In addition, the field has sought to acquire the ability to make TD cells proliferate. Attaining this end would probe the very molecular mechanisms we are trying to understand. Equally important, it would be invaluable in regenerative medicine, for tissues depending on TD cells and devoid of significant self-repair capabilities. The skeletal muscle has long been used as a model system to investigate the molecular foundations of terminal differentiation. Here, we summarize more than 50 years of studies in this field.

Kinetin stimulates differentiation of C2C12 myoblasts

Kinetin or N6-furfuryladenine (K) belongs to a class of plant hormones called cytokinins, which are biologically active molecules modulating many aspects of plant growth and development. However, biological activities of cytokinins are not only limited to plants; their effects on animals have been widely reported in the literature. Here, we found that Kinetin is a potent small molecule that efficiently stimulates differentiation of C2C12 myoblasts into myotubes in vitro. The highest efficacy was achieved at 1μM and 10μM Kinetin concentrations, in both mitogen-poor and rich media. More importantly, Kinetin was able to strongly stimulate the MyoD-dependent conversion of fibroblasts into myotubes. Kinetin alone did not give rise to fibroblast conversion and required MyoD; this demonstrates that Kinetin augments the molecular repertoire of necessary key regulatory factors to facilitate MyoD-mediated myogenic differentiation. This novel Kinetin pro-myogenic function may be explained by its ability to alter intracellular calcium levels and by its potential to impact on Reactive Oxygen Species (ROS) signalling. Taken together, our findings unravel the effects of a new class of small molecules with potent pro-myogenic activities. This opens up new therapeutic avenues with potential for treating skeletal muscle diseases related to muscle aging and wasting.

Inhibition of Postn Rescues Myogenesis Defects in Myotonic Dystrophy Type 1 Myoblast Model

Myotonic dystrophy type 1 (DM1) is an inherited neuromuscular disease caused by expanded CTG repeats in the 3' untranslated region (3'UTR) of the DMPK gene. The myogenesis process is defective in DM1, which is closely associated with progressive muscle weakness and wasting. Despite many proposed explanations for the myogenesis defects in DM1, the underlying mechanism and the involvement of the extracellular microenvironment remained unknown. Here, we constructed a DM1 myoblast cell model and reproduced the myogenesis defects. By RNA sequencing (RNA-seq), we discovered that periostin (Postn) was the most significantly upregulated gene in DM1 myogenesis compared with normal controls. This difference in Postn was confirmed by real-time quantitative PCR (RT-qPCR) and western blotting. Moreover, Postn was found to be significantly upregulated in skeletal muscle and myoblasts of DM1 patients. Next, we knocked down Postn using a short hairpin RNA (shRNA) in DM1 myoblast cells and found that the myogenesis defects in the DM1 group were successfully rescued, as evidenced by increases in the myotube area, the fusion index, and the expression of myogenesis regulatory genes. Similarly, Postn knockdown in normal myoblast cells enhanced myogenesis. As POSTN is a secreted protein, we treated the DM1 myoblast cells with a POSTN-neutralizing antibody and found that DM1 myogenesis defects were successfully rescued by POSTN neutralization. We also tested the myogenic ability of myoblasts in the skeletal muscle injury mouse model and found that Postn knockdown improved the myogenic ability of DM1 myoblasts. The activity of the TGF-β/Smad3 pathway was upregulated during DM1 myogenesis but repressed when inhibiting Postn with a Postn shRNA or a POSTN-neutralizing antibody, which suggested that the TGF-β/Smad3 pathway might mediate the function of Postn in DM1 myogenesis. These results suggest that Postn is a potential therapeutical target for the treatment of myogenesis defects in DM1.

An Environmental Friendly Tapioca Starch-Alginate Cultured Scaffold as Biomimetic Muscle Tissue

Natural porous scaffolds have been studied and developed for decades in biomedical science in order to support cells with a simulated extracellular matrix in natural tissue as an ideal environment. Such three-dimensional scaffolds provide many degrees of freedom to modulate cell activity, such as porosity, pore size, mechanical strength, biodegradability, and biocompatibility. In this study, a porous, three-dimensional material of alginate incorporating tapioca starch was fabricated. A particular freeze-gelation method was applied to homogenously mix starch in the alginate, and the concentration was controllable. This pure natural composite porous scaffold was characterized physically and biologically. The synergistic functions, including biocompatibility, biodegradability, cell adhesion, and cell proliferation, were also investigated. A myogenic differentiation model further verified that the composite porous scaffold provided a suitable environment, supporting the differentiation effect in the myogenic process. The positive results demonstrated that this novel material has the potential to serve as a biomedical or clean meat appliance.

YAP1 nuclear efflux and transcriptional reprograming follow membrane diminution upon VSV-G-induced cell fusion

Cells in many tissues, such as bone, muscle, and placenta, fuse into syncytia to acquire new functions and transcriptional programs. While it is known that fused cells are specialized, it is unclear whether cell-fusion itself contributes to programmatic-changes that generate the new cellular state. Here, we address this by employing a fusogen-mediated, cell-fusion system to create syncytia from undifferentiated cells. RNA-Seq analysis reveals VSV-G-induced cell fusion precedes transcriptional changes. To gain mechanistic insights, we measure the plasma membrane surface area after cell-fusion and observe it diminishes through increases in endocytosis. Consequently, glucose transporters internalize, and cytoplasmic glucose and ATP transiently decrease. This reduced energetic state activates AMPK, which inhibits YAP1, causing transcriptional-reprogramming and cell-cycle arrest. Impairing either endocytosis or AMPK activity prevents YAP1 inhibition and cell-cycle arrest after fusion. Together, these data demonstrate plasma membrane diminishment upon cell-fusion causes transient nutrient stress that may promote transcriptional-reprogramming independent from extrinsic cues.

Graphene oxide film guided skeletal muscle differentiation

Engineered muscle tissues can be used for the regeneration or substitution of irreversibly damaged or diseased muscles. Recently, graphene oxide (GO) has been shown to improve the adsorption of biomolecules through its biocompatibility and intrinsic π-π interactions. The possibility of producing various GO modifications may also provide additional functionality as substrates for cell culture. In particular, substrates fabricated from pristine GO have been shown to improve cellular functions and influence stem cell differentiation. In this study, we fabricated tunable GO substrates with various physical and chemical properties and demonstrated the ability of the substrate to support myogenic differentiation. Higher cellular adhesion affinity with unique microfilament anchorage was observed for GO substrates with increased GO concentrations. In addition, amino acid (AA)-conjugated GO (GO-AA) substrates were fabricated to modify GO chemical properties and study the effects of chemically modified GO substrates on myogenic differentiation. Our findings demonstrate that minor tuning of GO significantly influences myogenic differentiation.

MyoD induces ARTD1 and nucleoplasmic poly-ADP-ribosylation during fibroblast to myoblast transdifferentiation

While protein ADP-ribosylation was reported to regulate differentiation and dedifferentiation, it has so far not been studied during transdifferentiation. Here, we found that MyoD-induced transdifferentiation of fibroblasts to myoblasts promotes the expression of the ADP-ribosyltransferase ARTD1. Comprehensive analysis of the genome architecture by Hi-C and RNA-seq analysis during transdifferentiation indicated that ARTD1 locally contributed to A/B compartmentalization and coregulated a subset of MyoD target genes that were however not sufficient to alter transdifferentiation. Surprisingly, the expression of ARTD1 was accompanied by the continuous synthesis of nuclear ADP ribosylation that was neither dependent on the cell cycle nor induced by DNA damage. Conversely to the H₂O₂-induced ADP-ribosylation, the MyoD-dependent ADP-ribosylation was not associated to chromatin but rather localized to the nucleoplasm. Together, these data describe a MyoD-induced nucleoplasmic ADP-ribosylation that is observed particularly during transdifferentiation and thus potentially expands the plethora of cellular processes associated with ADP-ribosylation.

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MyoD-dependent transdifferentiation of IMR90 to myoblasts induces ARTD1 expression

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Transdifferentiation induces nuclear ARTD1-dependent ADP-ribosylation in myoblasts

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This ADP-ribosylation is induced independent of cell cycle and of DNA damage

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ARTD1-mediated poly-ADP-ribosylation localizes to the nucleoplasm in myoblasts

Magnesium Influences Membrane Fusion during Myogenesis by Modulating Oxidative Stress in C2C12 Myoblasts

Magnesium (Mg) is essential to skeletal muscle where it plays a key role in myofiber relaxation. Although the importance of Mg in the mature skeletal muscle is well established, little is known about the role of Mg in myogenesis. We studied the effects of low and high extracellular Mg in C2C12 myogenic differentiation. Non-physiological Mg concentrations induce oxidative stress in myoblasts. The increase of reactive oxygen species, which occurs during the early phase of the differentiation process, inhibits myoblast membrane fusion, thus impairing myogenesis. Therefore, correct Mg homeostasis, also maintained through a correct dietary intake, is essential to assure the regenerative capacity of skeletal muscle fibers.

Dynamics of myogenic differentiation using a novel Myogenin knock-in reporter mouse

Background

Myogenin is a transcription factor that is expressed during terminal myoblast differentiation in embryonic development and adult muscle regeneration. Investigation of this cell state transition has been hampered by the lack of a sensitive reporter to dynamically track cells during differentiation.

Results

Here, we report a knock-in mouse line expressing the tdTOMATO fluorescent protein from the endogenous Myogenin locus. Expression of tdTOMATO in Myog^ntdTom mice recapitulated endogenous Myogenin expression during embryonic muscle formation and adult regeneration and enabled the isolation of the MYOGENIN⁺ cell population. We also show that tdTOMATO fluorescence allows tracking of differentiating myoblasts in vitro and by intravital imaging in vivo. Lastly, we monitored by live imaging the cell division dynamics of differentiating myoblasts in vitro and showed that a fraction of the MYOGENIN⁺ population can undergo one round of cell division, albeit at a much lower frequency than MYOGENIN⁻ myoblasts.

Conclusions

We expect that this reporter mouse will be a valuable resource for researchers investigating skeletal muscle biology in developmental and adult contexts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-021-00260-x.

Comparative nuclear matrix proteome analysis of skeletal muscle cells in different cellular states

The nuclear matrix (NuMat) serves as the structural framework for organizing and maintaining nuclear architecture, however, the mechanisms by which this non-chromatin compartment is constructed and regulated are poorly understood. This study presents a proteomic analysis of the NuMat isolated from cultured skeletal muscle cells in three distinct cellular states- proliferating myoblasts (MBs), terminally differentiated myotubes (MTs), and mitotically quiescent (G0) myoblasts. About 40% of the proteins identified were found to be common in the NuMat proteome of these morphologically and functionally distinct cell states. These proteins, termed as the "core NuMat," define the stable, conserved, structural constituent of the nucleus, with functions such as RNA splicing, cytoskeletal organization, and chromatin modification, while the remaining NuMat proteins showed cell-state specificity, consistent with a more dynamic and potentially regulatory function. Specifically, myoblast NuMat was enriched in cell cycle, DNA replication and repair proteins, myotube NuMat in muscle differentiation and muscle function proteins, while G0 NuMat was enriched in metabolic, transcription, and transport proteins. These findings offer a new perspective for a cell-state-specific role of nuclear architecture and spatial organization, integrated with diverse cellular processes, and implicate NuMat proteins in the control of the cell cycle, lineage commitment, and differentiation.

Post-Transcriptional Regulation in Skeletal Muscle Development, Repair, and Disease

Skeletal muscle formation is a complex process that requires tight spatiotemporal control of key myogenic factors. Emerging evidence suggests that RNA processing is crucial for the regulation of these factors, and that multiple post-transcriptional regulatory pathways work dependently and independently of one another to enable precise control of transcripts throughout muscle development and repair. Moreover, disruption of these pathways is implicated in neuromuscular disease, and the recent development of RNA-mediated therapies shows enormous promise in the treatment of these disorders. We discuss the overlapping post-transcriptional regulatory pathways that mediate muscle development, how these pathways are disrupted in neuromuscular disorders, and advances in RNA-mediated therapies that present a novel approach to the treatment of these diseases.

Andrographolide promotes skeletal muscle regeneration after acute injury through epigenetic modulation

Myopathy is a muscle disease in which muscle fibers do not function properly, and eventually cause severe diseases, such as muscular dystrophy. The properly regeneration of skeletal muscle plays a pivotal role to maintain the muscle function after muscle injury. The aim of this study is to determine whether andrographolide plays an effect role on regulating skeletal muscle regeneration. Mouse satellite cells, C2C12 cells and Cardiotoxin (CTX) intramuscular injection induced acute skeletal muscle injury model were used to evaluate whether andrographolide is essential for skeletal muscle regeneration. The underling mechanism detected using immunohistochemistry stain, western blot, real time PCR. Andrographolide promotes mouse skeletal muscle regeneration. In cardiotoxin induced skeletal muscle injury model, andrographolide treatment enhanced myotube generation and promoted myotube fusion. Andrographolide treatment dramatically increased expression of myotube differentiation related genes, including Desmin, MyoD, MyoG, Myomaker, Tnni2, Dmd, Myoz1 and Myoz3. For the mechanism studies, we observed that andrographolide treatment significantly promoted histone modification, such as H3K4Me2, H3K4Me3 and H3K36Me2, both in vivo and in vitro. Treatment with DZNep, a Lysine methyltransferase EZH2 inhibitor, significantly attenuated andrographolide-induced expression of Myf5, Myomaker, Skeletal muscle α-actin, MyoD and MyoG. Taken together, our data in this study demonstrate andrographolide epigenetically drives differentiation and fusion of myotube, eventually promotes skeletal muscle regeneration. This should be a therapeutic treatment for skeletal muscle regeneration after muscle damage.

Dynamic Profiling of Protein Mole Synthesis Rates during C2C12 Myoblast Differentiation

Mole (MSR) and fractional (FSR) synthesis rates of proteins during C2C12 myoblast differentiation are investigated. Myoblast cultures supplemented with D₂ O during 0-24 h or 72-96 h of differentiation are analyzed by LC-MS/MS to calculate protein FSR and MSR after samples are spiked with yeast alcohol dehydrogenase (ADH1). Profiling of 153 proteins detected 70 significant (p ≤ 0.05, FDR ≤ 1%) differences in abundance between cell states. Early differentiation is enriched by clusters of ribosomal and heat shock proteins, whereas later differentiation is associated with actin filament binding. The median (first-third quartile) FSR (%/h) during early differentiation 4.1 (2.7-5.3) is approximately twofold greater than later differentiation 1.7 (1.0-2.2), equating to MSR of 0.64 (0.38-1.2) and 0.28 (0.1-0.5) fmol h^-1  µg^-1 total protein, respectively. MSR corresponds more closely with abundance data and highlights proteins associated with glycolytic processes and intermediate filament protein binding that are not evident among FSR data. Similarly, MSR during early differentiation accounts for 78% of the variation in protein abundance during later differentiation, whereas FSR accounts for 4%. Conclusively, the interpretation of protein synthesis data differs when reported in mole or fractional terms, which has consequences when studying the allocation of cellular resources.

Induced Fetal Human Muscle Stem Cells with High Therapeutic Potential in a Mouse Muscular Dystrophy Model

Duchenne muscular dystrophy (DMD) is a progressive and fatal muscle-wasting disease caused by DYSTROPHIN deficiency. Cell therapy using muscle stem cells (MuSCs) is a potential cure. Here, we report a differentiation method to generate fetal MuSCs from human induced pluripotent stem cells (iPSCs) by monitoring MYF5 expression. Gene expression profiling indicated that MYF5-positive cells in the late stage of differentiation have fetal MuSC characteristics, while MYF5-positive cells in the early stage of differentiation have early myogenic progenitor characteristics. Moreover, late-stage MYF5-positive cells demonstrated good muscle regeneration potential and produced DYSTROPHIN in vivo after transplantation into DMD model mice, resulting in muscle function recovery. The engrafted cells also generated PAX7-positive MuSC-like cells under the basal lamina of DYSTROPHIN-positive fibers. These findings suggest that MYF5-positive fetal MuSCs induced in the late stage of iPSC differentiation have cell therapy potential for DMD.

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Wnt agonists at high dose and long term induces dermomyotome cells effectively

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MYF5+ cell characteristics vary between early- and late-stage differentiation

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Late-stage MYF5+ cells acquire characteristics resembling fetal muscle stem cells

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MYF5+ cells recover dystrophin and improves muscular function

Microtubule Organization in Striated Muscle Cells

Distinctly organized microtubule networks contribute to the function of differentiated cell types such as neurons, epithelial cells, skeletal myotubes, and cardiomyocytes. In striated (i.e., skeletal and cardiac) muscle cells, the nuclear envelope acts as the dominant microtubule-organizing center (MTOC) and the function of the centrosome—the canonical MTOC of mammalian cells—is attenuated, a common feature of differentiated cell types. We summarize the mechanisms known to underlie MTOC formation at the nuclear envelope, discuss the significance of the nuclear envelope MTOC for muscle function and cell cycle progression, and outline potential mechanisms of centrosome attenuation.

Functional analyses of miRNA-146b-5p during myogenic proliferation and differentiation in chicken myoblasts

Background

In the poultry and livestock industries, precise genetic information is crucial for improving economic traits. Thus, functional genomic studies help to generate faster, healthier, and more efficient animal production. Chicken myoblast cells, which are required for muscle development and regeneration, are particularly important because chicken growth is closely related to muscle mass.

Results

In this study, we induced expression of microRNA-146b-5p mediated by the piggyBac transposon system in primary chicken myoblast (pCM) cells. Subsequently, we analyzed and compared the proliferation and differentiation capacity and also examined the expression of related genes in regular pCM (rpCM) cells and pCM cells overexpressing miRNA-146b-5p (pCM-146b OE cells). pCM-146b OE cells showed increased proliferation and upregulated gene expression related to cell proliferation. In addition, next-generation sequencing analyses were performed to compare global gene expression patterns between rpCM cells and pCM-146b OE cells. We found that the higher proliferation in pCM-146b OE cells was the result of upregulation of gene sets related to the cell cycle. Moreover, miRNA-146b-5p overexpression had inhibitory effects on myotube differentiation in pCM cells.

Conclusions

Collectively these results demonstrate that miR-146b-5p is closely related to the proliferation and differentiation of chicken myogenic cells as a modulator of post-transcription.

A New Edible Film to Produce In Vitro Meat

In vitro meat is a novel concept of food science and biotechnology. Methods to produce in vitro meat employ muscle cells cultivated on a scaffold in a serum-free medium using a bioreactor. The microstructure of the scaffold is a key factor, because muscle cells must be oriented to generate parallel alignments of fibers. This work aimed to develop a new scaffold (microstructured film) to grow muscle fibers. The microstructured edible films were made using micromolding technology. A micromold was tailor-made using a laser cutting machine to obtain parallel fibers with a diameter in the range of 70-90 µm. Edible films were made by means of solvent casting using non-mammalian biopolymers. Myoblasts were cultured on flat and microstructured films at three cell densities. Cells on the microstructured films grew with a muscle fiber morphology, but in the case of using the flat film, they only produced unorganized cell proliferation. Myogenic markers were assessed using quantitative polymerase chain reaction. After 14 days, the expression of desmin, myogenin, and myosin heavy chain were significantly higher in microstructured films compared to the flat films. The formation of fiber morphology and the high expression of myogenic markers indicated that a microstructured edible film can be used for the production of in vitro meat.

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

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

Integrative Bayesian Analysis Identifies Rhabdomyosarcoma Disease Genes

Identifying oncogenic drivers and tumor suppressors remains a challenge in many forms of cancer, including rhabdomyosarcoma. Anticipating gene expression alterations resulting from DNA copy-number variants to be particularly important, we developed a computational and experimental strategy incorporating a Bayesian algorithm and CRISPR/Cas9 "mini-pool" screen that enables both genome-scale assessment of disease genes and functional validation. The algorithm, called iExCN, identified 29 rhabdomyosarcoma drivers and suppressors enriched for cell-cycle and nucleic-acid-binding activities. Functional studies showed that many iExCN genes represent rhabdomyosarcoma line-specific or shared vulnerabilities. Complementary experiments addressed modes of action and demonstrated coordinated repression of multiple iExCN genes during skeletal muscle differentiation. Analysis of two separate cohorts revealed that the number of iExCN genes harboring copy-number alterations correlates with survival. Our findings highlight rhabdomyosarcoma as a cancer in which multiple drivers influence disease biology and demonstrate a generalizable capacity for iExCN to unmask disease genes in cancer.

Promotion of myotube differentiation and attenuation of muscle atrophy in murine C2C12 myoblast cells treated with teaghrelin

This study aimed to investigate the effects of teaghrelin, an active ingredient of Chin-shin oolong tea, on murine C2C12 myoblast cells. Under high serum conditions, teaghrelin inhibited C2C12 cell proliferation, indicating a cell cycle arrest and cessation of proliferative progression. Teaghrelin promoted pro-differentiation of C2C12 cells as evidenced by a progressively elongated morphology, as well as the induction of muscle specific myogenin, myosin heavy chain (MHC), and MyoD. The formation of multinucleated myotubes, and the increase of MHC-positive immunoreactivity within the myotubes, further reflected a complete differentiation and maturation of the contractile skeletal muscle cells induced by teaghrelin. Like ghrelin, teaghrelin attenuated dexamethasone-decreased myotube diameter, indicating its protective effects against skeletal muscle atrophy. Additionally, the expressions of Atrogin-1 and MuRF-1 ubiquitin E3 ligase were reduced. In conclusion, the results highlight a possibility of developing teaghrelin as a functional food for the prevention or therapeutic treatment of disease-associated skeletal muscle atrophy.

Methylmercury modifies temporally expressed myogenic regulatory factors to inhibit myoblast differentiation

Methylmercury (MeHg) is a pervasive environmental toxicant, with known detrimental effects on neurodevelopment. Despite a longstanding paradigm of neurotoxicity, where motor deficits are prevalent among those developmentally exposed, consideration of muscle as a MeHg target has received minimal investigation. Recent evidence has identified muscle-specific gene networks that modulate developmental sensitivity to MeHg toxicity. One such network is muscle cell differentiation. Muscle cell differentiation is a coordinated process regulated by the myogenic regulatory factors (MRFs): Myf5, MyoD, MyoG, and MRF4. A previous study demonstrated that MeHg inhibits muscle cell differentiation in vitro, concurrent with reduced MyoG expression. The potential for MeHg to modify the temporal expression of the MRFs to alter differentiation, however, has yet to be fully explored. Using the C2C12 mouse myoblast model, we examined MRF expression profiles at various stages subsequent to MeHg exposure to proliferating myoblasts. MeHg was seen to persistently alter myoblast differentiation capacity, as myod, myog, and mrf4 gene expression were all affected. Myog exhibited the most robust changes in expression across the various culture conditions, while myf5 was unaffected. Following MeHg exposure to myoblasts, where elevated p21 expression indicated departure from proliferation, cells failed to subsequently differentiate, even in the absence of MeHg, as reflected by a concurrent reduction in MRF4 and myosin heavy chain (MHC), markers of terminal differentiation. Our results indicate that within a brief window of exposure MeHg can disrupt the intrinsic myogenic differentiation program of proliferative myoblasts.

Cytoplasmic sequestration of the RhoA effector mDiaphanous1 by Prohibitin2 promotes muscle differentiation

Muscle differentiation is controlled by adhesion and growth factor-dependent signalling through common effectors that regulate muscle-specific transcriptional programs. Here we report that mDiaphanous1, an effector of adhesion-dependent RhoA-signalling, negatively regulates myogenesis at the level of Myogenin expression. In myotubes, over-expression of mDia1ΔN3, a RhoA-independent mutant, suppresses Myogenin promoter activity and expression. We investigated mDia1-interacting proteins that may counteract mDia1 to permit Myogenin expression and timely differentiation. Using yeast two-hybrid and mass-spectrometric analysis, we report that mDia1 has a stage-specific interactome, including Prohibitin2, MyoD, Akt2, and β-Catenin, along with a number of proteosomal and mitochondrial components. Of these interacting partners, Prohibitin2 colocalises with mDia1 in cytoplasmic punctae in myotubes. We mapped the interacting domains of mDia1 and Phb2, and used interacting (mDia1ΔN3/Phb2 FL or mDia1ΔN3/Phb2-Carboxy) and non-interacting pairs (mDia1H + P/Phb2 FL or mDia1ΔN3/Phb2-Amino) to dissect the functional consequences of this partnership on Myogenin promoter activity. Co-expression of full-length as well as mDia1-interacting domains of Prohibitin2 reverse the anti-myogenic effects of mDia1ΔN3, while non-interacting regions do not. Our results suggest that Prohibitin2 sequesters mDia1, dampens its anti-myogenic activity and fine-tunes RhoA-mDia1 signalling to promote differentiation. Overall, we report that mDia1 is multi-functional signalling effector whose anti-myogenic activity is modulated by a differentiation-dependent interactome. The data have been deposited to the ProteomeXchange with identifier PXD012257.

Nontranslational function of leucyl-tRNA synthetase regulates myogenic differentiation and skeletal muscle regeneration

Aside from its catalytic function in protein synthesis, leucyl-tRNA synthetase (LRS) has a nontranslational function in regulating cell growth via the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) pathway by sensing amino acid availability. mTOR also regulates skeletal myogenesis, but the signaling mechanism is distinct from that in cell growth regulation. A role of LRS in myogenesis has not been reported. Here we report that LRS negatively regulated myoblast differentiation in vitro. This function of LRS was independent of its regulation of protein synthesis, and it required leucine-binding but not tRNA charging activity of LRS. Local knock down of LRS accelerated muscle regeneration in a mouse injury model, and so did the knock down of Rag or Raptor. Further in vitro studies established a Rag-mTORC1 pathway, which inhibits the IRS1-PI3K-Akt pathway, to be the mediator of the nontranslational function of LRS in myogenesis. BC-LI-0186, an inhibitor reported to disrupt LRS-Rag interaction, promoted robust muscle regeneration with enhanced functional recovery, and this effect was abolished by cotreatment with an Akt inhibitor. Taken together, our findings revealed what we believe is a novel function for LRS in controlling the homeostasis of myogenesis, and suggested a potential therapeutic strategy to target a noncanonical function of a housekeeping protein.

Polycomb/Trithorax Antagonism: Cellular Memory in Stem Cell Fate and Function

Stem cells are continuously challenged with the decision to either self-renew or adopt a new fate. Self-renewal is regulated by a system of cellular memory, which must be bypassed for differentiation. Previous studies have identified Polycomb group (PcG) and Trithorax group (TrxG) proteins as key modulators of cellular memory. In this Perspective, we draw from embryonic and adult stem cell studies to discuss the complex roles played by PcG and TrxG in maintaining cell identity while allowing for microenvironment-mediated alterations in cell fate. Finally, we discuss the potential for targeting these proteins as a therapeutic approach in cancer.