PPARγ and MyoD are differentially regulated by myostatin in adipose-derived stem cells and muscle satellite cells

From inflammation to bone formation: the intricate role of neutrophils in skeletal muscle injury and traumatic heterotopic ossification

Neutrophils are emerging as an important player in skeletal muscle injury and repair. Neutrophils accumulate in injured tissue, thus releasing inflammatory factors, proteases and neutrophil extracellular traps (NETs) to clear muscle debris and pathogens when skeletal muscle is damaged. During the process of muscle repair, neutrophils can promote self-renewal and angiogenesis in satellite cells. When neutrophils are abnormally overactivated, neutrophils cause collagen deposition, functional impairment of satellite cells, and damage to the skeletal muscle vascular endothelium. Heterotopic ossification (HO) refers to abnormal bone formation in soft tissue. Skeletal muscle injury is one of the main causes of traumatic HO (tHO). Neutrophils play a pivotal role in activating BMPs and TGF-β signals, thus promoting the differentiation of mesenchymal stem cells and progenitor cells into osteoblasts or osteoclasts to facilitate HO. Furthermore, NETs are specifically localized at the site of HO, thereby accelerating the formation of HO. Additionally, the overactivation of neutrophils contributes to the disruption of immune homeostasis to trigger HO. An understanding of the diverse roles of neutrophils will not only provide more information on the pathogenesis of skeletal muscle injury for repair and HO but also provides a foundation for the development of more efficacious treatment modalities for HO.

The role of TGF-β signaling in muscle atrophy, sarcopenia and cancer cachexia

Skeletal muscle, comprising a significant proportion (40 to 50 percent) of total body weight in humans, plays a critical role in maintaining normal physiological conditions. Muscle atrophy occurs when the rate of protein degradation exceeds protein synthesis. Sarcopenia refers to age-related muscle atrophy, while cachexia represents a more complex form of muscle wasting associated with various diseases such as cancer, heart failure, and AIDS. Recent research has highlighted the involvement of signaling pathways, including IGF1-Akt-mTOR, MuRF1-MAFbx, and FOXO, in regulating the delicate balance between muscle protein synthesis and breakdown. Myostatin, a member of the TGF-β superfamily, negatively regulates muscle growth and promotes muscle atrophy by activating Smad2 and Smad3. It also interacts with other signaling pathways in cachexia and sarcopenia. Inhibition of myostatin has emerged as a promising therapeutic approach for sarcopenia and cachexia. Additionally, other TGF-β family members, such as TGF-β1, activin A, and GDF11, have been implicated in the regulation of skeletal muscle mass. Furthermore, myostatin cooperates with these family members to impair muscle differentiation and contribute to muscle loss. This review provides an overview of the significance of myostatin and other TGF-β signaling pathway members in muscular dystrophy, sarcopenia, and cachexia. It also discusses potential novel therapeutic strategies targeting myostatin and TGF-β signaling for the treatment of muscle atrophy.

Intermuscular adipose tissue in obesity and related disorders: cellular origins, biological characteristics and regulatory mechanisms

Intermuscular adipose tissue (IMAT) is a unique adipose depot interspersed between muscle fibers (myofibers) or muscle groups. Numerous studies have shown that IMAT is strongly associated with insulin resistance and muscular dysfunction in people with metabolic disease, such as obesity and type 2 diabetes. Moreover, IMAT aggravates obesity-related muscle metabolism disorders via secretory factors. Interestingly, researchers have discovered that intermuscular brown adipocytes in rodent models provide new hope for obesity treatment by acting on energy dissipation, which inspired researchers to explore the underlying regulation of IMAT formation. However, the molecular and cellular properties and regulatory processes of IMAT remain debated. Previous studies have suggested that muscle-derived stem/progenitor cells and other adipose tissue progenitors contribute to the development of IMAT. Adipocytes within IMAT exhibit features that are similar to either white adipocytes or uncoupling protein 1 (UCP1)-positive brown adipocytes. Additionally, given the heterogeneity of skeletal muscle, which comprises myofibers, satellite cells, and resident mesenchymal progenitors, it is plausible that interplay between these cellular components actively participate in the regulation of intermuscular adipogenesis. In this context, we review recent studies associated with IMAT to offer insights into the cellular origins, biological properties, and regulatory mechanisms of IMAT. Our aim is to provide novel ideas for the therapeutic strategy of IMAT and the development of new drugs targeting IMAT-related metabolic diseases.

Identification of key pathways and hub genes in the myogenic differentiation of pluripotent stem cell: a bioinformatics and experimental study

Background

The regeneration of muscle cells from stem cells is an intricate process, and various genes are included in the process such as myoD, mf5, mf6, etc. The key genes and pathways in the differentiating stages are various. Therefore, the differential expression of key genes after 4 weeks of differentiation were investigated in our study.

Method

Three published gene expression profiles, GSE131125, GSE148994, and GSE149055, about the comparisons of pluripotent stem cells to differentiated cells after 4 weeks were obtained from the Gene Expression Omnibus (GEO) database. Common differentially expressed genes (DEGs) were obtained for further analysis such as protein-protein interaction (PPI) network, Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and GSEA analysis. After hub genes and key pathways were obtained, we manipulated in vitro cell research for substantiation such as immunohistochemical staining and semi-quantitative analysis and quantitative real-time PCR.

Results

A total of 824 DEGs including 350 upregulated genes and 474 downregulated genes were identified in the three GSEs. Nineteen hub genes were identified from the PPI network. The GO and KEGG pathway analyses confirmed that myogenic differentiation at 4 weeks was strongly associated with pathway in cancer, PI3K pathway, actin cytoskeleton regulation and metabolic pathway, biosynthesis of antibodies, and cell cycle. GSEA analysis indicated the differentiated cells were enriched in muscle cell development and myogenesis. Meanwhile, the core genes in each pathway were identified from the GSEA analysis. The in vitro cell research revealed that actin cytoskeleton and myoD were upregulated after 4-week differentiation.

Conclusions

The research revealed the potential hub genes and key pathways after 4-week differentiation of stem cells which contribute to further study about the molecular mechanism of myogenesis regeneration, paving a way for more accurate treatment for muscle dysfunction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13018-020-01979-x.

Long Non-coding RNA H19 Regulates Porcine Satellite Cell Differentiation Through miR-140-5p/SOX4 and DBN1

The H19 gene promotes skeletal muscle differentiation in mice, but the regulatory models and mechanisms of myogenesis regulated by H19 are largely unknown in pigs. Therefore, the regulatory modes of H19 in the differentiation of porcine skeletal muscle satellite cells (PSCs) need to be determined. We observed that H19 gene silencing could decrease the expressions of the myogenin (MYOG) gene, myogenic differentiation (MYOD), and myosin heavy chain (MYHC) in PSCs. Therefore, we constructed and sequenced 12 cDNA libraries of PSCs after knockdown of H19 at two differentiation time points to analyze the transcriptome differences. A total of 11,419 differentially expressed genes (DEGs) were identified. Among these DEGs, we found through bioinformatics analysis and protein interaction experiment that SRY-box transcription factor 4 (SOX4) and Drebrin 1 (DBN1) were the key genes in H19-regulated PSC differentiation. Functional analysis shows that SOX4 and DBN1 promote PSC differentiation. Mechanistically, H19 regulates PSC differentiation through two different pathways. On the one hand, H19 functions as a molecular sponge of miR-140-5p, which inhibits the differentiation of PSCs, thereby modulating the derepression of SOX4. On the other hand, H19 regulates PSC differentiation through directly binding with DBN1. Furthermore, MYOD binds to the promoters of H19 and DBN1. The knockdown of MYOD inhibits the expression of H19 and DBN1. We determined the function of H19 and provided a molecular model to elucidate H19’s role in regulating PSC differentiation.

Inefficient ATP synthesis by inhibiting mitochondrial respiration causes lipids to decrease in MSTN-lacking muscles of loach Misgurnus anguillicaudatus

Myostatin (MSTN) lacking could lead to enhanced muscle growth and lipid metabolism disorder in animals. Plenty of researches have been performed to warrant a better understanding of the mechanisms underlying the enhanced muscle growth; however, mechanisms for lipid metabolic changes are poorly understood. In this study, MSTN-depletion loaches Misgurnus anguillicaudatus (MU for short) were firstly generated by CRISPR/Cas9 technique. Based on histological observation, we found that skeletal muscle fat accumulation in MU sharply reduced compared with wild-type loaches (WT for short). To further investigate the fat change, muscle lipidomic analysis was performed. There were no significant differences in three membrane phospholipid contents between WT and MU. The contents of six other major lipid species in MU muscles were all significantly lower than those in WT muscles, indicating that MSTN deficiency could obviously decrease muscle lipid production in the loach. Meanwhile, it was also supported by results of three lipogenesis-related genes' expressions. And then combined with muscle ATP determination and gene expression profiles of the five mitochondrial respiration chain complexes, we speculated that MSTN lacking may cause the weak of mitochondrial respiration functions in the loach muscles, leading to ATP synthesis decreasing and finally reducing the production of lipids.

Myostatin/SMAD4 signaling-mediated regulation of miR-124-3p represses glucocorticoid receptor expression and inhibits adipocyte differentiation

The mechanism of adipocyte regulation specifically in muscle and the influence of muscle tissue on intramuscular fat deposition are unknown. Our previous studies have shown that myostatin, a myokine, is involved in inhibiting the differentiation of preadipocytes and may be a potential regulator that affects the deposition of intramuscular fat. Myostatin inhibited adipogenesis by downregulating the expression of glucocorticoid receptor (GR) in porcine preadipocytes. However, the mechanism of regulation is not yet clear. In this study, we demonstrate microRNA (miR-124-3p) mediates regulation of GR by myostatin. We found that miR-124-3p can target GR 3'-UTR and negatively regulate GR expression. We demonstrate that overexpression of miR-124-3p can reduce differentiation of 3T3-L1 cells by inhibiting GR, and vice versa. The expression of miR-124-3p was upregulated in 3T3-L1 cells treated with myostatin. Further study revealed that myostatin also promotes the expression of SMAD4 and its transfer and localization to the nucleus. The activated myostatin/SMAD4 signal promotes the expression of miR-124-3p by SMAD4 binding to the promoter region of miR-124-3p. When myostatin or SMAD4 activity is inhibited, the upregulation of miR-124-3p is also inhibited. All of these findings suggested that myostatin could inhibit adipogenic differentiation of 3T3-L1 cells by activating miR-124-3p to inhibit GR. These data may provide an explanation for how myostatin signaling affects intramuscular fat deposition in a tissue-specific manner.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

Fibromodulin and regulation of the intricate balance between myoblast differentiation to myocytes or adipocyte-like cells

Interactions between myoblasts and the surrounding microenvironment led us to explore the role of fibromodulin (FMOD), an extracellular matrix protein, in the maintenance of myoblast stemness and function. Microarray analysis of FMOD_kd myoblasts and in silico studies were used to identify the top most differentially expressed genes in FMOD_kd, and helped establish that FMOD-based regulations of integral membrane protein 2a and clusterin are essential components of the myogenic program. Studies in knockout, obese, and diabetic mouse models helped characterize the operation of a novel FMOD-based regulatory circuit that controls myoblast switching from a myogenic to a lipid accumulation fate. FMOD regulation of myoblasts is an essential part of the myogenic program, and it offers opportunities for the development of therapeutics for the treatment of different muscle diseases.-Lee, E. J., Jan, A. T., Baig, M. H., Ahmad, K., Malik, A., Rabbani, G., Kim, T., Lee, I.-K., Lee, Y. H., Park, S.-Y., Choi, I. Fibromodulin and regulation of the intricate balance between myoblast differentiation to myocytes or adipocyte-like cells.

Function analysis of Mef2c promoter in muscle differentiation

Regeneration of adult skeletal muscle following injury occurs through the activation of satellite cells that proliferates, differentiates, and fuses with injured myofibers. Myocyte enhancer factor 2 (MEF2) proteins are reported to have the potential contributions to adult muscle regeneration. To further understand Mef2c gene, the promoter of pig Mef2c gene was analyzed in this paper. Quantitative real-time PCR (qRT-PCR) revealed the expression pattern of Mef2c gene in muscle of eight tissues. The Mef2c promoter had the higher transcriptional activity in differentiated C2C12 cells than that in proliferating C2C12 cells, which was accompanied by the upregulation of mRNA expression of Mef2c gene. Function deletion and mutation analyses showed that MyoD and MEF2 binding sites within the Mef2c promoter were responsible for the regulation of Mef2c transcription. MEF2C could upregulate the transcriptional activities of Mef2c promoter constructs, which contained a 3'-end nucleotide sequence with p300 binding site. The electrophoretic mobility shift assays and chromatin immunoprecipitation assays determined the MyoD binding site in Mef2c promoter. These results advanced our knowledge of the promoter of the pig Mef2c gene, and the study of Mef2c promoter regulator elements helped to elucidate the regulation mechanisms of Mef2c in muscle differentiation or muscle repair and regeneration.

Muscle-specific downregulation of GR levels inhibits adipogenesis in porcine intramuscular adipocyte tissue

Intramuscular adipose is conducive to good pork quality, whereas subcutaneous adipose is considered as waste in pig production. So uncovering the regulation differences between these two adiposes is helpful to tissue-specific control of fat deposition. In this study, we found the sensitivity to glucocorticoids (GCs) was lower in intramuscular adipocytes (IMA) compared with subcutaneous adipocytes (SA). Comparison of glucocorticoid receptor (GR) revealed that IMA had lower GR level which contributed to its reduced GCs sensitivity. Higher methylation levels of GR promotor 1-C and 1-H were detected in IMA compared with SA. GR expression decrease was also found in adipocytes when treated with muscle conditioned medium (MCM) in vitro, which resulted in significant inhibition of adipocytes proliferation and differentiation. Since abundant myostatin (MSTN) was detected in MCM by ELISA assay, we further investigated the effect of this myokine on adipocytes. MSTN treatment suppressed adipocytes GR expression, cell proliferation and differentiation, which mimicked the effects of MCM. The methylation levels of GR promotor 1-C and 1-H were also elevated after MSTN treatment. Our study reveals the role of GR in muscle fiber inhibition on intramuscular adipocytes, and identifies myostatin as a muscle-derived modulator for adipose GR level.

The function of myostatin in the regulation of fat mass in mammals

Myostatin (MSTN), also referred to as growth and differentiation factor-8, is a protein secreted in muscle tissues. Researchers believe that its primary function is in negatively regulating muscle because a mutation in its coding region can lead to the famous double muscle trait in cattle. Muscle and adipose tissue develop from the same mesenchymal stem cells, and researchers have found that MSTN is expressed in fat tissues and plays a key role in adipogenesis. Interestingly, MSTN can exert a dual function, either inhibiting or promoting adipogenesis, according to the situation. Due to its potential function in controlling body fat mass, MSTN has attracted the interest of researchers. In this review, we explore its function in regulating adipogenesis in mammals, including preadipocytes, multipotent stem cells and fat mass.

MyoD promotes porcine PPARγ gene expression through an E-box and a MyoD-binding site in the PPARγ promoter region

Peroxisome proliferator-activated receptor γ (PPARγ) is a key transcription factor in adipogenesis and can be regulated by adipogenesis-related factors. However, little information is available regarding its regulation by myogenic factors. In this study, we found that over-expression of MyoD enhanced porcine adipocyte differentiation and up-regulated PPARγ expression, whereas small interfering RNA against MyoD significantly attenuated porcine adipocyte differentiation and inhibited PPARγ expression. The MyoD-binding sites in the PPARγ promoter region at -412 to -396 and -155 to -150 were identified by promoter deletion analysis and site-directed mutagenesis. Electrophoretic mobility shift assays and chromatin immunoprecipitation further showed that these two regions are MyoD-binding sites, both in vitro and in vivo, indicating that MyoD directly interacts with the porcine PPARγ promoter. Thus, our results demonstrate that an Enhancer box and a binding site for a cooperative co-activator of MyoD are present in the promoter region of porcine PPARγ; furthermore, MyoD up-regulates PPARγ expression and promotes porcine adipocyte differentiation.

C2C12 myotubes inhibit the proliferation and differentiation of 3T3-L1 preadipocytes by reducing the expression of glucocorticoid receptor gene

Obesity is a well-established risk factor to health for its relationship with insulin resistance, diabetes and metabolic syndrome. Myocyte-adipocyte crosstalk model plays a significant role in studying the interaction of muscle and adipose development. Previous related studies mainly focus on the effects of adipocytes on the myocytes activity, however, the influence of myotubes on the preadipocytes development remains unclear. The present study was carried out to settle this issue. Firstly, the co-culture experiment showed that the proliferation, cell cycle, and differentiation of 3T3-L1 preadipocytes were arrested, and the apoptosis was induced, by differentiated C2C12 myotubes. Next, the sensitivity of 3T3-L1 preadipocytes to glucocorticoids (GCs), which was well known as cell proliferation, differentiation, apoptosis factor, was decreased after co-cultured with C2C12 myotubes. What's more, our results showed that C2C12 myotubes suppressed the mRNA and protein expression of glucocorticoid receptor (GR) in 3T3-L1 preadipocytes, indicating the potential mechanism of GCs sensitivity reduction. Taken together, we conclude that C2C12 myotubes inhibited 3T3-L1 preadipocytes proliferation and differentiation by reducing the expression of GR. These data suggest that decreasing GR by administration of myokines may be a promising therapy for treating patients with obesity or diabetes.

Elastic hydrogel substrate supports robust expansion of murine myoblasts and enhances their engraftment

The application of satellite cell-derived myoblasts in regenerative medicine has been restricted by the rapid loss of stemness during in vitro cell expansion using traditional culture systems. However, studies published in the past decade have highlighted the influence of substrate elasticity on stem cell fate and revealed that culture on a soft hydrogel substrate can promote self-renewal and prolong the regenerative potential of muscle stem cells. Whether hydrogel substrates have similar effects after long-term robust expansion remains to be determined. Herein we prepared an elastic chitosan/beta-glycerophosphate/collagen hydrogel mimicking the soft microenvironment of muscle tissues for use as the substrate for satellite cell culture and investigated its influence on long-term cell expansion. After 20 passages in culture, satellite cell-derived myoblasts cultured on our hydrogel substrate exhibited significant improvements in proliferation capability, cell viability, colony forming frequency, and potential for myogenic differentiation compared to those cultured on a routine rigid culture surface. Immunochemical staining and western blot analysis both confirmed that myoblasts cultured on the hydrogel substrate expressed higher levels of several differentiation-related markers, including Pax7, Pax3, and SSEA-1, and a lower level of MyoD compared to myoblasts cultured on rigid culture plates (all p<0.05). After transplantation into the tibialis anterior of nude mice, myoblasts that had been cultured on the hydrogel substrate demonstrated a significantly greater engraftment efficacy than those cultured on the traditional surface. Collectively, these results indicate that the elastic hydrogel substrate supported robust expansion of murine myoblasts and enhanced their engraftment in vivo.