Myomaker is a membrane activator of myoblast fusion and muscle formation

A change in cis-regulatory logic underlying obligate versus facultative muscle multinucleation in chordates

Vertebrates and tunicates are sister groups that share a common fusogenic factor, Myomaker (Mymk), that drives myoblast fusion and muscle multinucleation. Yet they are divergent in when and where they express Mymk. In vertebrates, all developing skeletal muscles express Mymk and are obligately multinucleated. In tunicates, Mymk is expressed only in post-metamorphic multinucleated muscles, but is absent from mononucleated larval muscles. In this study, we demonstrate that cis-regulatory sequence differences in the promoter region of Mymk underlie the different spatiotemporal patterns of its transcriptional activation in tunicates and vertebrates. Although in vertebrates myogenic regulatory factors (MRFs) such as MyoD1 alone are required and sufficient for Mymk transcription in all skeletal muscles, we show that transcription of Mymk in post-metamorphic muscles of the tunicate Ciona requires the combinatorial activity of MRF, MyoD and Early B-cell Factor (Ebf). This macroevolutionary difference appears to be encoded in cis, likely due to the presence of a putative Ebf-binding site adjacent to predicted MRF binding sites in the Ciona Mymk promoter. We further discuss how Mymk and myoblast fusion might have been regulated in the last common ancestor of tunicates and vertebrates, for which we propose two models.

The Innovative Role of Nuclear Receptor Interaction Protein in Orchestrating Invadosome Formation for Myoblast Fusion

BACKGROUND

Nuclear receptor interaction protein (NRIP) is versatile and engages with various proteins to execute its diverse biological function. NRIP deficiency was reported to cause small myofibre size in adult muscle regeneration, indicating a crucial role of NRIP in myoblast fusion.

METHODS

The colocalization and interaction of NRIP with actin were investigated by immunofluorescence and immunoprecipitation assay, respectively. The participation of NRIP in myoblast fusion was demonstrated by cell fusion assay and time-lapse microscopy. The NRIP mutants were generated for mechanism study in NRIP-null C2C12 (termed KO19) cells and muscle-specific NRIP knockout (NRIP cKO) mice. A GEO profile database was used to analyse NRIP expression in Duchenne muscular dystrophy (DMD) patients.

RESULTS

In this study, we found that NRIP directly and reciprocally interacted with actin both in vitro and in cells. Immunofluorescence microscopy showed that the endogenous NRIP colocalized with components of invadosome, such as actin, Tks5, and cortactin, at the tips of cells during C2C12 differentiation. The KO19 cells were generated and exhibited a significant deficit in myoblast fusion compared with wild-type C2C12 cells (3.16% vs. 33.67%, p < 0.005). Overexpressed NRIP in KO19 cells could rescue myotube formation compared with control (3.37% vs. 1.00%, p < 0.01). We further confirmed that NRIP directly participated in cell fusion by using a cell-cell fusion assay. We investigated the mechanism of invadosome formation for myoblast fusion, which depends on NRIP-actin interaction, by analysing NRIP mutants in NRIP-null cells. Loss of actin-binding of NRIP reduced invadosome (enrichment ratio, 1.00 vs. 2.54, p < 0.01) and myotube formation (21.82% vs. 35.71%, p < 0.05) in KO19 cells and forced NRIP expression in KO19 cells and muscle-specific NRIP knockout (NRIP cKO) mice increased myofibre size compared with controls (over 1500 μm2, 61.01% vs. 20.57%, p < 0.001). We also found that the NRIP mRNA level was decreased in DMD patients compared with healthy controls (18 072 vs. 28 289, p < 0.001, N = 10 for both groups).

CONCLUSIONS

NRIP is a novel actin-binding protein for invadosome formation to induce myoblast fusion.

Pleiotropic Gene HMGA2 Regulates Myoblast Proliferation and Affects Body Size of Sheep

Uncovering genes associated with muscle growth and body size will benefit the molecular breeding of meat Hu sheep. HMGA2 has proven to be an important gene in mouse muscle growth and is associated with the body size of various species. However, its roles in sheep are still limited. Using sheep myoblast as a cell model, the overexpression of HMGA2 significantly promoted sheep myoblast proliferation, while interference with HMGA2 expression inhibited proliferation, indicated by qPCR, EdU, and CCK-8 assays. Furthermore, the dual-luciferase reporter system indicated that the region NC_056056.1: 154134300-154134882 (-618 to -1200 bp upstream of the HMGA2 transcription start site) was one of the habitats of the HMGA2 core promoter, given the observation that this fragment led to a ~3-fold increase in luciferase activity. Interestingly, SNP rs428001129 (NC_056056.1:g.154134315 C>A) was detected in this fragment by Sanger sequencing of the PCR product of pooled DNA from 458 crossbred sheep. This SNP was found to affect the promoter activity and was significantly associated with chest width at birth and two months old, as well as chest depth at two and six months old. The data obtained in this study demonstrated the phenotypic regulatory role of the HMGA2 gene in sheep production traits and the potential of rs428001129 in marker-assisted selection for sheep breeding in terms of chest width and chest depth.

Functional study of the ST6GAL2 gene regulating skeletal muscle growth and development

ST6GAL2, a member of the sialoglycosyltransferase family, primarily localizes within the cellular Golgi apparatus. However, the role of the ST6GAL2 gene in skeletal muscle growth and development remains elusive. In this study, the impact of the ST6GAL2 gene on the proliferation, differentiation, and apoptosis of primary chicken myoblasts at the cellular level was investigated. Quantitative fluorescent PCR was used to measure the expression levels of genes. Subsequently, using gene knockout mice, we assessed its effects on skeletal muscle growth and development in vivo. Our findings reveal that the ST6GAL2 gene promotes the expression of cell cycle and proliferation-related genes, including CCNB2 and PCNA, and apoptosis-related genes, such as Fas and Caspase-9. At the individual level, double knockout of ST6GAL2 inhibited the formation of both fast and slow muscle fibers in the quadriceps, extensor digitorum longus, and tibial anterior muscle, while promoting their formation in the gastrocnemius and soleus. These results collectively demonstrate that the ST6GAL2 gene facilitates the proliferation, apoptosis, and fusion processes of primary chicken myoblasts. Additionally, it promotes the enlargement of cross-sectional muscle fiber areas and regulates the formation of fast and slow muscle fibers at the individual level, albeit inhibiting muscle fusion. This study provides valuable insights into the role of the ST6GAL2 gene in promoting proliferation of skeletal muscle.

Satellite cell-derived TRIM28 is pivotal for mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discover that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Interestingly, different from the role reported in a previous study based on C2C12 myoblasts, multiple lines of both in vitro and in vivo evidence reveal that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 are not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discover that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.

Ectopic expression of Myomaker and Myomixer in slow muscle cells induces slow muscle fusion and myofiber death

Zebrafish embryos possess two major types of myofibers, the slow and fast fibers, with distinct patterns of cell fusion. The fast muscle cells can fuse, while the slow muscle cells cannot. Here, we show that myomaker is expressed in both slow and fast muscle precursors, whereas myomixer is exclusive to fast muscle cells. The loss of Prdm1a, a regulator of slow muscle differentiation, results in strong myomaker and myomixer expression and slow muscle cell fusion. This abnormal fusion is further confirmed by the direct ectopic expression of myomaker or myomixer in slow muscle cells of transgenic models. Using the transgenic models, we show that the heterologous fusion between slow and fast muscle cells can alter slow muscle cell migration and gene expression. Furthermore, the overexpression of myomaker and myomixer also disrupts membrane integrity, resulting in muscle cell death. Collectively, this study identifies that the fiber-type-specific expression of fusogenic proteins is critical for preventing inappropriate fusion between slow and fast fibers in fish embryos, highlighting the need for precise regulation of fusogenic gene expression to maintain muscle fiber integrity and specificity.

The Regulatory Role of Myomaker in the Muscle Growth of the Chinese Perch (Siniperca chuatsi)

The fusion of myoblasts is a crucial stage in the growth and development of skeletal muscle. Myomaker is an important myoblast fusion factor that plays a crucial role in regulating myoblast fusion. However, the function of Myomaker in economic fish during posthatching has been poorly studied. In this study, we found that the expression of Myomaker in the fast muscle of Chinese perch (Siniperca chuatsi) was higher than that in other tissues. To determine the function of Myomaker in fast muscle, Myomaker-siRNA was used to knockdown Myomaker in Chinese perch and the effect on muscle growth was determined. The results showed that the growth of Chinese perch was significantly decreased in the Myomaker-siRNA group. Furthermore, both the diameter of muscle fibers and the number of nuclei in single muscle fibers were significantly reduced in the Myomaker-siRNA group, whereas there was no significant difference in the number of BrdU-positive cells (proliferating cells) between the control and the Myomaker-siRNA groups. Together, these findings indicate that Myomaker may regulate growth of fast muscle in Chinese perch juveniles by promoting myoblast fusion rather than proliferation.

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

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

Sporadic inclusion body myositis-derived myotube culture revealed muscle cell-autonomous expression profiles

Sporadic inclusion body myositis (sIBM) is a muscle disease in older people and is characterized by inflammatory cell invasion into intact muscle fibers and rimmed vacuoles. The pathomechanism of sIBM is not fully elucidated yet, and controversy exists as to whether sIBM is a primary autoimmune disease or a degenerative muscle disease with secondary inflammation. Previously, we established a method of collecting CD56-positive myoblasts from human skeletal muscle biopsy samples. We hypothesized that the myoblasts derived from these patients are useful to see the cell-autonomous pathomechanism of sIBM. With these resources, myoblasts were differentiated into myotubes, and the expression profiles of cell-autonomous pathology of sIBM were analyzed. Myoblasts from three sIBM cases and six controls were differentiated into myotubes. In the RNA-sequencing analysis of these "myotube" samples, 104 differentially expressed genes (DEGs) were found to be significantly upregulated by more than twofold in sIBM, and 13 DEGs were downregulated by less than twofold. For muscle biopsy samples, a comparative analysis was conducted to determine the extent to which "biopsy" and "myotube" samples differed. Fifty-three DEGs were extracted of which 32 (60%) had opposite directions of expression change (e.g., increased in biopsy vs decreased in myotube). Apolipoprotein E (apoE) and transmembrane protein 8C (TMEM8C or MYMK) were commonly upregulated in muscle biopsies and myotubes from sIBM. ApoE and myogenin protein levels were upregulated in sIBM. Given that enrichment analysis also captured changes in muscle contraction and development, the triggering of muscle atrophy signaling and abnormal muscle differentiation via MYMK or myogenin may be involved in the pathogenesis of sIBM. The presence of DEGs in sIBM suggests that the myotubes formed from sIBM-derived myoblasts revealed the existence of muscle cell-autonomous degeneration in sIBM. The catalog of DEGs will be an important resource for future studies on the pathogenesis of sIBM focusing on primary muscle degeneration.

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

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

Potential regulator of meat quality in geese: C1QTNF1 implications on cell proliferation and muscle growth

Goose creates important economic value depending on their enrich nutrients of meat. Our previous study investigates potential candidate genes associated with variations in meat quality between Xianghai Flying (XHF) Goose and Zi Goose through genomic and transcriptome integrated analysis. Screening of 5 differential expression candidate genes related to muscle development identified by the FST, XP-EHH and RNA-seq in breast muscle from various geese. Among them, C1QTNF1 (C1q and TNF related protein 1), a gene of unknown function in goose, which observed mutations in coding sequence regions in sequencing data. Its function was explored after overexpression and knockdown which designed depending on the genetic sequence of the goose, respectively. Results showed that over-expression of C1QTNF1 significantly enhances cell proliferation and viability. In addition, the expression levels of the fusion marker gene Myomaker and the differentiation marker gene MyoD are significantly upregulated in cells. Knock-down C1QTNF1 leads to down regulated Myomaker and MyoD which involved muscle formation. But, the expression level of muscle atrophy marker MuRF is not significantly changed among different transfection groups. Since protein structures and interactions are closely related to their functions, we further analyzed the C1QTNF1 for physicochemical properties, structural predictions, protein interactions and homology. It can be reasonably inferred that C1QTNF1 has a similar effect to collagen, which may affect muscle development. In summary, we first speculate that C1QTNF1 may play an important regulatory role in muscle growth and development and thereby contributes to the further understanding of the genetic mechanisms that underlie meat quality traits of goose.

Transcriptomic and epigenomic insights into pectoral muscle fiber formation at the late embryonic development in pure chicken lines

Long-term intensive genetic selection has led to significant differences between broiler and layer chickens, which are evident during the embryonic period. Despite this, there is a paucity of research on the genetic regulation of the initial formation of muscle fiber morphology in chick embryos. Embryonic d 17 (E17) is the key time point for myoblast fusion completion and muscle fiber morphology formation in chickens. This study aimed to explore the genetic regulatory mechanisms underlying the early muscle fiber morphology establishment in broiler chickens of Cornish (CC) and White Plymouth Rock (RR) and layer chickens of White Leghorn (WW) at E17 using the transcriptomic and chromatin accessibility sequencing of pectoral major muscles. The results showed that broiler chickens exhibited significant higher embryo weight and pectoral major muscle weight at E17 compared to layer chickens (P = 0.000). A total of 1,278, 1,248, and 892 differentially expressed genes (DEGs) of RNA-seq data were identified between CC vs. WW, RR vs. WW, and CC vs. RR, separately. All DEGs were combined for cluster analysis and they were divided into 6 clusters, including cluster 1 with higher expression in broilers and cluster 6 with higher expression in layers. DEGs in cluster 1 were enriched in terms related to macrophage activation (P = 0.002) and defense response to bacteria (P = 0.002), while DEGs in cluster 6 showed enrichment in protein-DNA complex (P = 0.003) and monooxygenase activity (P = 0.000). ATAC-seq data analysis identified a total of 38,603 peaks, with 13,051 peaks for CC, 18,780 peaks for RR, and 6,772 peaks for WW. Integrative analysis of transcriptomic and chromatin accessibility data revealed GOLM1, ISLR2, and TOPAZ1 were commonly upregulated genes in CC and RR. Furthermore, screening of all upregulated DEGs in cluster 1 from CC and RR identified GOLM1, ISLR2, and HNMT genes associated with neuroimmune functions and MYOM3 linked to muscle morphology development, showing significantly elevated expression in broiler chickens compared to layer chickens. These findings suggest active neural system connectivity during the initial formation of muscle fiber morphology in embryonic period, highlighting the early interaction between muscle fiber formation morphology and the nervous system. This study provides novel insights into late chick embryo development and lays a deeper foundation for further research.

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

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

We need to talk-how muscle stem cells communicate

Skeletal muscle is one of the tissues with the highest ability to regenerate, a finely controlled process which is critically depending on muscle stem cells. Muscle stem cell functionality depends on intrinsic signaling pathways and interaction with their immediate niche. Upon injury quiescent muscle stem cells get activated, proliferate and fuse to form new myofibers, a process involving the interaction of multiple cell types in regenerating skeletal muscle. Receptors in muscle stem cells receive the respective signals through direct cell-cell interaction, signaling via secreted factors or cell-matrix interactions thereby regulating responses of muscle stem cells to external stimuli. Here, we discuss how muscle stem cells interact with their immediate niche focusing on how this controls their quiescence, activation and self-renewal and how these processes are altered in age and disease.

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

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

Transcriptomic gene signatures measure satellite cell activity in muscular dystrophies

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

MTM1-mediated production of phosphatidylinositol 5-phosphate fuels the formation of podosome-like protrusions regulating myoblast fusion

Myogenesis is a multistep process that requires a spatiotemporal regulation of cell events resulting finally in myoblast fusion into multinucleated myotubes. Most major insights into the mechanisms underlying fusion seem to be conserved from insects to mammals and include the formation of podosome-like protrusions (PLPs) that exert a driving force toward the founder cell. However, the machinery that governs this process remains poorly understood. In this study, we demonstrate that MTM1 is the main enzyme responsible for the production of phosphatidylinositol 5-phosphate, which in turn fuels PI5P 4-kinase α to produce a minor and functional pool of phosphatidylinositol 4,5-bisphosphate that concentrates in PLPs containing the scaffolding protein Tks5, Dynamin-2, and the fusogenic protein Myomaker. Collectively, our data reveal a functional crosstalk between a PI-phosphatase and a PI-kinase in the regulation of PLP formation.

Antioxidant and anti-ageing effects of oleuropein aglycone in canine skeletal muscle cells

Reactive oxygen species (ROS) are normally produced in skeletal muscle. However, an imbalance in their regulatory systems can lead to their accumulation and ultimately to oxidative stress, which is one of the causes of the ageing process. Companion dogs share the same environment and lifestyle as humans, making them an excellent comparative model for the study of ageing, as well as they constitute a growing market for bioactive molecules that improve the quality of life of pets. The anti-ageing properties of oleuropein aglycone (OLE), a bioactive compound from olive leaves known for its antioxidant properties, were investigated in Myok9 canine muscle cell model. After incubation with OLE, senescence was induced in the canine cellular model by hydrogen peroxide (H2O2). Analyses were performed on cells after seven days of differentiation. The oxidative stress induced by H2O2 treatment on differentiated canine muscle cells led to a significant increase in ROS formation, which was reduced by OLE pretreatment alone or in combination with H2O2 by about 34% and 32%, respectively. Cells treated with H2O2 showed a 48% increase the area of senescent cells stained by SA-β-gal, while OLE significantly reduced the coloured area by 52%. OLE, alone or in combination with H2O2, showed a significant antioxidant activity, possibly through autophagy activation, as indicated by the expression of autophagic markers.

Dasatinib promotes muscle differentiation and disrupts normal muscle regeneration

Dasatinib is one of the second-generation tyrosine kinase inhibitors used to treat chronic myeloid leukemia and has a broad target spectrum, including KIT, PDGFR, and SRC family kinases. Due to its broad drug spectrum, dasatinib has been reported at the basic research level to improve athletic performance by eliminating senescent cell removal and to have an effect on muscle diseases such as Duchenne muscular dystrophy, but its effect on myoblasts has not been investigated. In this study, we evaluated the effects of dasatinib on skeletal muscle both under normal conditions and in the regenerating state. Dasatinib suppressed the proliferation and promoted the fusion of C2C12 myoblasts. During muscle regeneration, dasatinib increased the gene expressions of myogenic-related genes (Myod, Myog, and Mymx), and caused abnormally thin muscle fibers on the CTX-induced muscle injury mouse model. From these results, dasatinib changes the closely regulated gene expression pattern of myogenic regulatory factors during muscle differentiation and disrupts normal muscle regeneration. Our data suggest that when using dasatinib, its effects on skeletal muscle should be considered, particularly at regenerating stages.

Aberrant myonuclear domains and impaired myofiber contractility despite marked hypertrophy in MYMK-related, Carey-Fineman-Ziter Syndrome

Carey Fineman Ziter Syndrome (CFZS) is a rare autosomal recessive disease caused by mutations in the MYMK locus which encodes the protein, myomaker. Myomaker is essential for fusion and concurrent myonuclei donation of muscle progenitors during growth and development. Strikingly, in humans, MYMK mutations appear to prompt myofiber hypertrophy but paradoxically, induce generalised muscle weakness. As the underlying cellular mechanisms remain unexplored, the present study aimed to gain insights by combining myofiber deep-phenotyping and proteomic profiling. Hence, we isolated individual muscle fibers from CFZS patients and performed mechanical, 3D morphological and proteomic analyses. Myofibers from CFZS patients were ~ 4x larger than controls and possessed ~ 2x more myonuclei than those from healthy subjects, leading to disproportionally larger myonuclear domain volumes. These greater myonuclear domain sizes were accompanied by smaller intrinsic cellular force generating-capacities in myofibers from CFZS patients than in control muscle cells. Our complementary proteomic analyses indicated remodelling in 233 proteins particularly those associated with cellular respiration. Overall, our findings suggest that myomaker is somewhat functional in CFZS patients, but the associated nuclear accretion may ultimately lead to non-functional hypertrophy and altered energy-related mechanisms in CFZS patients. All of these are likely contributors of the muscle weakness experienced by CFZS patients.

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

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

Whole transcriptome profiling reveals a lncMDP1 that regulates myogenesis by adsorbing miR-301a-5p targeting CHAC1

Myoblast proliferation and differentiation are essential for skeletal muscle development. In this study, we generated the expression profiles of mRNAs, long noncoding RNAs (lncRNAs), and microRNAs (miRNAs) in different developmental stages of chicken primary myoblasts (CPMs) using RNA sequencing (RNA-seq) technology. The dual luciferase reporter system was performed using chicken embryonic fibroblast cells (DF-1), and functional studies quantitative real-time polymerase chain reaction (qPCR), cell counting kit-8 (CCK-8), 5-Ethynyl-2'-deoxyuridine (EdU), flow cytometry cycle, RNA fluorescence in situ hybridization (RNA-FISH), immunofluorescence, and western blotting assay. Our research demonstrated that miR-301a-5p had a targeted binding ability to lncMDP1 and ChaC glutathione-specific gamma-glutamylcyclotransferase 1 (CHAC1). The results revealed that lncMDP1 regulated the proliferation and differentiation of myoblasts via regulating the miR-301a-5p/CHAC1 axis, and CHAC1 promotes muscle regeneration. This study fulfilled the molecular regulatory network of skeletal muscle development and providing an important theoretical reference for the future improvement of chicken meat performance and meat quality.

Uremia Impedes Skeletal Myocyte Myomixer Expression and Fusogenic Activity: Implication for Uremic Sarcopenia

In patients with chronic kidney disease (CKD), skeletal muscle mass and function are known to occasionally decline. However, the muscle regeneration and differentiation process in uremia has not been extensively studied. In mice with CKD induced by adenine-containing diet, the tibialis anterior muscle injured using a barium chloride injection method recovered poorly as compared to control mice. In the cultured murine skeletal myocytes, stimulation with indoxyl sulfate (IS), a representative uremic toxin, morphologically jeopardized the differentiation, which was counteracted by L-ascorbic acid (L-AsA) treatment. Transcriptome analysis of cultured myocytes identified a set of genes whose expression was down-regulated by IS stimulation but up-regulated by L-AsA treatment. Gene silencing of myomixer, one of the genes in the set, impaired myocyte fusion during differentiation. By contrast, lentiviral overexpression of myomixer compensated for a hypomorphic phenotype caused by IS treatment. The split-luciferase technique demonstrated that IS stimulation negatively affected early myofusion activity that was rescued by L-AsA treatment. Lastly, in mice with CKD compared with control mice, myomixer expression in the muscle tissue in addition to the muscle weight after the injury was reduced, both of which were restored with L-AsA treatment. Collectively, data showed that the uremic milieu impairs the expression of myomixer and impedes the myofusion process. Considering frequent musculoskeletal injuries in uremic patients, defective myocyte fusion followed by delayed muscle damage recovery could underlie their muscle loss and weakness.

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.

Integrating miRNA and full-length transcriptome profiling to elucidate the mechanism of muscle growth in Muscovy ducks reveals key roles for miR-301a-3p/ANKRD1

BACKGROUND

The popularity of Muscovy ducks is attributed not only to their conformation traits but also to their slightly higher content of breast and leg meat, as well as their stronger-tasting meat compared to that of typical domestic ducks. However, there is a lack of comprehensive systematic research on the development of breast muscle in Muscovy ducks. In addition, since the number of skeletal muscle myofibers is established during the embryonic period, this study conducted a full-length transcriptome sequencing and microRNA sequencing of the breast muscle. Muscovy ducks at four developmental stages, namely Embryonic Day 21 (E21), Embryonic Day 27 (E27), Hatching Day (D0), and Post-hatching Day 7 (D7), were used to isolate total RNA for analysis.

RESULTS

A total of 68,161 genes and 472 mature microRNAs were identified. In order to uncover deeper insights into the regulation of mRNA by miRNAs, we conducted an integration of the differentially expressed miRNAs (known as DEMs) with the differentially expressed genes (referred to as DEGs) across various developmental stages. This integration allowed us to make predictions regarding the interactions between miRNAs and mRNA. Through this analysis, we identified a total of 274 DEGs that may serve as potential targets for the 68 DEMs. In the predicted miRNA‒mRNA interaction networks, let-7b, miR-133a-3p, miR-301a-3p, and miR-338-3p were the hub miRNAs. In addition, multiple DEMs also showed predicted target relationships with the DEGs associated with skeletal system development. These identified DEGs and DEMs as well as their predicted interaction networks involved in the regulation of energy homeostasis and muscle development were most likely to play critical roles in facilitating the embryo-to-hatchling transition. A candidate miRNA, miR-301a-3p, exhibited increased expression during the differentiation of satellite cells and was downregulated in the breast muscle tissues of Muscovy ducks at E21 compared to E27. A dual-luciferase reporter assay suggested that the ANKRD1 gene, which encodes a transcription factor, is a direct target of miR-301a-3p.

CONCLUSIONS

miR-301a-3p suppressed the posttranscriptional activity of ANKRD1, which is an activator of satellite cell proliferation, as determined with gain- and loss-of-function experiments. miR-301a-3p functions as an inducer of myogenesis by targeting the ANKRD1 gene in Muscovy ducks. These results provide novel insights into the early developmental process of black Muscovy breast muscles and will improve understanding of the underlying molecular mechanisms.

Mustn1 is a smooth muscle cell-secreted microprotein that modulates skeletal muscle extracellular matrix composition

OBJECTIVE

Skeletal muscle plasticity and remodeling are critical for adapting tissue function to use, disuse, and regeneration. The aim of this study was to identify genes and molecular pathways that regulate the transition from atrophy to compensatory hypertrophy or recovery from injury. Here, we have used a mouse model of hindlimb unloading and reloading, which causes skeletal muscle atrophy, and compensatory regeneration and hypertrophy, respectively.

METHODS

We analyzed mouse skeletal muscle at the transition from hindlimb unloading to reloading for changes in transcriptome and extracellular fluid proteome. We then used qRT-PCR, immunohistochemistry, and bulk and single-cell RNA sequencing data to determine Mustn1 gene and protein expression, including changes in gene expression in mouse and human skeletal muscle with different challenges such as exercise and muscle injury. We generated Mustn1-deficient genetic mouse models and characterized them in vivo and ex vivo with regard to muscle function and whole-body metabolism. We isolated smooth muscle cells and functionally characterized them, and performed transcriptomics and proteomics analysis of skeletal muscle and aorta of Mustn1-deficient mice.

RESULTS

We show that Mustn1 (Musculoskeletal embryonic nuclear protein 1, also known as Mustang) is highly expressed in skeletal muscle during the early stages of hindlimb reloading. Mustn1 expression is transiently elevated in mouse and human skeletal muscle in response to intense exercise, resistance exercise, or injury. We find that Mustn1 expression is highest in smooth muscle-rich tissues, followed by skeletal muscle fibers. Muscle from heterozygous Mustn1-deficient mice exhibit differences in gene expression related to extracellular matrix and cell adhesion, compared to wild-type littermates. Mustn1-deficient mice have normal muscle and aorta function and whole-body glucose metabolism. We show that Mustn1 is secreted from smooth muscle cells, and that it is present in arterioles of the muscle microvasculature and in muscle extracellular fluid, particularly during the hindlimb reloading phase. Proteomics analysis of muscle from Mustn1-deficient mice confirms differences in extracellular matrix composition, and female mice display higher collagen content after chemically induced muscle injury compared to wild-type littermates.

CONCLUSIONS

We show that, in addition to its previously reported intracellular localization, Mustn1 is a microprotein secreted from smooth muscle cells into the muscle extracellular space. We explore its role in muscle ECM deposition and remodeling in homeostasis and upon muscle injury. The role of Mustn1 in fibrosis and immune infiltration upon muscle injury and dystrophies remains to be investigated, as does its potential for therapeutic interventions.

Challenges and Considerations of Preclinical Development for iPSC-Based Myogenic Cell Therapy

Cell therapies derived from induced pluripotent stem cells (iPSCs) offer a promising avenue in the field of regenerative medicine due to iPSCs' expandability, immune compatibility, and pluripotent potential. An increasing number of preclinical and clinical trials have been carried out, exploring the application of iPSC-based therapies for challenging diseases, such as muscular dystrophies. The unique syncytial nature of skeletal muscle allows stem/progenitor cells to integrate, forming new myonuclei and restoring the expression of genes affected by myopathies. This characteristic makes genome-editing techniques especially attractive in these therapies. With genetic modification and iPSC lineage specification methodologies, immune-compatible healthy iPSC-derived muscle cells can be manufactured to reverse the progression of muscle diseases or facilitate tissue regeneration. Despite this exciting advancement, much of the development of iPSC-based therapies for muscle diseases and tissue regeneration is limited to academic settings, with no successful clinical translation reported. The unknown differentiation process in vivo, potential tumorigenicity, and epigenetic abnormality of transplanted cells are preventing their clinical application. In this review, we give an overview on preclinical development of iPSC-derived myogenic cell transplantation therapies including processes related to iPSC-derived myogenic cells such as differentiation, scaling-up, delivery, and cGMP compliance. And we discuss the potential challenges of each step of clinical translation. Additionally, preclinical model systems for testing myogenic cells intended for clinical applications are described.

Myomerger Induces Membrane Hemifusion and Regulates Fusion Pore Expansion

Membrane fusion is a crucial mechanism in a wide variety of important events in cell biology from viral infection to exocytosis. However, despite many efforts and much progress, cell-cell fusion has remained elusive to our understanding. Along the life of the fusion pore, large conformational changes take place from the initial lipid bilayer bending, passing through the hemifusion intermediates, and ending with the formation of the first nascent fusion pore. In this sense, computer simulations are an ideal technique for describing such complex lipid remodeling at the molecular level. In this work, we studied the role played by the muscle-specific membrane protein Myomerger during the formation of the fusion pore. We have conducted μs length atomistic and coarse-grained molecular dynamics, together with free-energy calculations using ad hoc collective variables. Our results show that Myomerger favors the hemifusion diaphragm-stalk transition, reduces the nucleation-expansion energy difference, and promotes the formation of nonenlarging fusion pores.

Molecular regulation of myocyte fusion

Myocyte fusion is a pivotal process in the development and regeneration of skeletal muscle. Failure during fusion can lead to a range of developmental as well as pathological consequences. This review aims to comprehensively explore the intricate processes underlying myocyte fusion, from the molecular to tissue scale. We shed light on key players, such as the muscle-specific fusogens - Myomaker and Myomixer, in addition to some lesser studied molecules contributing to myocyte fusion. Conserved across vertebrates, Myomaker and Myomixer play a crucial role in driving the merger of plasma membranes of fusing myocytes, ensuring the formation of functional muscle syncytia. Our multiscale approach also delves into broader cell and tissue dynamics that orchestrate the timing and positioning of fusion events. In addition, we explore the relevance of muscle fusogens to human health and disease. Mutations in fusogen genes have been linked to congenital myopathies, providing unique insights into the molecular basis of muscle diseases. We conclude with a discussion on potential therapeutic avenues that may emerge from manipulating the myocyte fusion process to remediate skeletal muscle disorders.

Chromatin profiling reveals TFAP4 as a critical transcriptional regulator of bovine satellite cell differentiation

BACKGROUND

Satellite cells are myogenic precursor cells in adult skeletal muscle and play a crucial role in skeletal muscle regeneration, maintenance, and growth. Like embryonic myoblasts, satellite cells have the ability to proliferate, differentiate, and fuse to form multinucleated myofibers. In this study, we aimed to identify additional transcription factors that control gene expression during bovine satellite cell proliferation and differentiation.

RESULTS

Using chromatin immunoprecipitation followed by sequencing, we identified 56,973 and 54,470 genomic regions marked with both the histone modifications H3K4me1 and H3K27ac, which were considered active enhancers, and 50,956 and 59,174 genomic regions marked with H3K27me3, which were considered repressed enhancers, in proliferating and differentiating bovine satellite cells, respectively. In addition, we identified 1,216 and 1,171 super-enhancers in proliferating and differentiating bovine satellite cells, respectively. Analyzing these enhancers showed that in proliferating bovine satellite cells, active enhancers were associated with genes stimulating cell proliferation or inhibiting myoblast differentiation whereas repressed enhancers were associated with genes essential for myoblast differentiation, and that in differentiating satellite cells, active enhancers were associated with genes essential for myoblast differentiation or muscle contraction whereas repressed enhancers were associated with genes stimulating cell proliferation or inhibiting myoblast differentiation. Active enhancers in proliferating bovine satellite cells were enriched with binding sites for many transcription factors such as MYF5 and the AP-1 family transcription factors; active enhancers in differentiating bovine satellite cells were enriched with binding sites for many transcription factors such as MYOG and TFAP4; and repressed enhancers in both proliferating and differentiating bovine satellite cells were enriched with binding sites for NF-kB, ZEB-1, and several other transcription factors. The role of TFAP4 in satellite cell or myoblast differentiation was previously unknown, and through gene knockdown and overexpression, we experimentally validated a critical role for TFAP4 in the differentiation and fusion of bovine satellite cells into myofibers.

CONCLUSIONS

Satellite cell proliferation and differentiation are controlled by many transcription factors such as AP-1, TFAP4, NF-kB, and ZEB-1 whose roles in these processes were previously unknown in addition to those transcription factors such as MYF5 and MYOG whose roles in these processes are widely known.

A change in cis-regulatory logic underlying obligate versus facultative muscle multinucleation in chordates

Vertebrates and tunicates are sister groups that share a common fusogenic factor, Myomaker (Mymk), that drives myoblast fusion and muscle multinucleation. Yet they are divergent in when and where they express Mymk. In vertebrates, all developing skeletal muscles express Mymk and are obligately multinucleated. In tunicates, Mymk is only expressed in post-metamorphic multinucleated muscles, but is absent from mononucleated larval muscles. In this study, we demonstrate that cis-regulatory sequence differences in the promoter region of Mymk underlie the different spatiotemporal patterns of its transcriptional activation in tunicates and vertebrates. While in vertebrates Myogenic Regulatory Factors (MRFs) like MyoD1 alone are required and sufficient for Mymk transcription in all skeletal muscles, we show that transcription of Mymk in post-metamorphic muscles of the tunicate Ciona requires the combinatorial activity of MRF/MyoD and Early B-Cell Factor (Ebf). This macroevolutionary difference appears to be encoded in cis, likely due to the presence of a putative Ebf binding site adjacent to predicted MRF binding sites in the Ciona Mymk promoter. We further discuss how Mymk and myoblast fusion might have been regulated in the last common ancestor of tunicates and vertebrates, for which we propose two models.

RRM2 promotes the proliferation of chicken myoblasts, inhibits their differentiation and muscle regeneration

During myogenesis and regeneration, the proliferation and differentiation of myoblasts play key regulatory roles and may be regulated by many genes. In this study, we analyzed the transcriptomic data of chicken primary myoblasts at different periods of proliferation and differentiation with protein‒protein interaction network, and the results indicated that there was an interaction between cyclin-dependent kinase 1 (CDK1) and ribonucleotide reductase regulatory subunit M2 (RRM2). Previous studies in mammals have a role for RRM2 in skeletal muscle development as well as cell growth, but the role of RRM2 in chicken is unclear. In this study, we investigated the effects of RRM2 on skeletal muscle development and regeneration in chickens in vitro and in vivo. The interaction between RRM2 and CDK1 was initially identified by co-immunoprecipitation and mass spectrometry. Through a dual luciferase reporter assay and quantitative real-time PCR, we identified the core promoter region of RRM2, which is regulated by the SP1 transcription factor. In this study, through cell counting kit-8 assays, 5-ethynyl-2'-deoxyuridine incorporation assays, flow cytometry, immunofluorescence staining, and Western blot analysis, we demonstrated that RRM2 promoted the proliferation and inhibited the differentiation of myoblasts. In vivo studies showed that RRM2 reduced the diameter of muscle fibers and slowed skeletal muscle regeneration. In conclusion, these data provide preliminary insights into the biological functions of RRM2 in chicken muscle development and skeletal muscle regeneration.

Muscle stem cell dysfunction in rhabdomyosarcoma and muscular dystrophy

Muscle stem cells (MuSCs) are crucial to the repair and homeostasis of mature skeletal muscle. MuSC dysfunction and dysregulation of the myogenic program can contribute to the development of pathology ranging from cancers like rhabdomyosarcoma (RMS) or muscle degenerative diseases such as Duchenne muscular dystrophy (DMD). Both diseases exhibit dysregulation at nearly all steps of myogenesis. For instance, MuSC self-renewal processes are altered. In RMS, this leads to the creation of tumor propagating cells. In DMD, impaired asymmetric stem cell division creates a bias towards producing self-renewing stem cells instead of committing to differentiation. Hyperproliferation of these cells contribute to tumorigenesis in RMS and symmetric expansion of the self-renewing MuSC population in DMD. Both diseases also exhibit a repression of factors involved in terminal differentiation, halting RMS cells in the proliferative stage and thus driving tumor growth. Conversely, the MuSCs in DMD exhibit impaired differentiation and fuse prematurely, affecting myonuclei maturation and the integrity of the dystrophic muscle fiber. Finally, both disease states cause alterations to the MuSC niche. Various elements of the niche such as inflammatory and migratory signaling that impact MuSC behavior are dysregulated. Here we show how these seemingly distantly related diseases indeed have similarities in MuSC dysfunction, underlying the importance of considering MuSCs when studying the pathophysiology of muscle diseases.

Satellite cells in the growth and maintenance of muscle

Embryonic skeletal muscle growth is contingent upon a population of somite derived satellite cells, however, the contribution of these cells to early postnatal skeletal muscle growth remains relatively high. As prepubertal postnatal development proceeds, the activity and contribution of satellite cells to skeletal muscle growth diminishes. Eventually, at around puberty, a population of satellite cells escapes terminal commitment, continues to express the paired box transcription factor Pax7, and reside in a quiescent state orbiting the myofiber periphery adjacent to the basal lamina. After adolescence, some satellite cell contributions to muscle maintenance and adaptation occur, however, their necessity is reduced relative to embryonic, early postnatal, and prepubertal growth.

Cell Fusion and Syncytia Formation in Cancer

The natural phenomenon of cell-cell fusion does not only take place in physiological processes, such as placentation, myogenesis, or osteoclastogenesis, but also in pathophysiological processes, such as cancer. More than a century ago postulated, today the hypothesis that the fusion of cancer cells with normal cells leads to the formation of cancer hybrid cells with altered properties is in scientific consensus. Some studies that have investigated the mechanisms and conditions for the fusion of cancer cells with other cells, as well as studies that have characterized the resulting cancer hybrid cells, are presented in this review. Hypoxia and the cytokine TNFα, for example, have been found to promote cell fusion. In addition, it has been found that both the protein Syncytin-1, which normally plays a role in placentation, and phosphatidylserine signaling on the cell membrane are involved in the fusion of cancer cells with other cells. In human cancer, cancer hybrid cells were detected not only in the primary tumor, but also in the circulation of patients as so-called circulating hybrid cells, where they often correlated with a worse outcome. Although some data are available, the questions of how and especially why cancer cells fuse with other cells are still not fully answered.

Muscle Progenitor Cell Fusion in the Maintenance of Skeletal Muscle

Skeletal muscle possesses a resident, multipotent stem cell population that is essential for its repair and maintenance throughout life. Here I highlight the role of this stem cell population in muscle repair and regeneration and review the genetic control of the process; the mechanistic steps of activation, migration, recognition, adhesion, and fusion of these cells; and discuss the novel recognition of the membrane signaling that coordinates myogenic cell-cell fusion, as well as the identification of a two-part fusogen system that facilitates it.

Development of a Cre-recombination-based color-switching reporter system for cell fusion detection

Cell fusion plays a key role in the development and formation of tissues and organs in several organisms. Skeletal myogenesis is assessed in vitro by cell shape and gene and protein expression using immunofluorescence and immunoblotting assays. However, these conventional methods are complex and do not allow for easy time-course observation in living cells. Therefore, this study aimed to develop a Cre recombination-based fluorescent reporter system to monitor cell-cell fusion. We combined green and red fluorescent proteins with a Cre-loxP system to detect syncytium formation using a fluorescent binary switch. This allowed us to visualize mononucleated cells with green fluorescence before fusion and multinucleated syncytia with red fluorescence by conditional expression after cell fusion. The formation of multinuclear myotubes during myogenic differentiation was detected by the change in fluorescence from green to red after Cre-mediated recombination. The distribution of the fluorescence signal correlated with the expression of myogenic differentiation markers. Moreover, red reporter fluorescence intensity was correlated with the number of nuclei contained in the red fluorescent-positive myotubes. We also successfully demonstrated that our fusion monitoring system is applicable to the formation of skeletal muscle myotube and placental syncytiotrophoblast. These results suggest that the color-switching fluorescent reporter system, using Cre-mediated recombination, could be a robust tool used to facilitate the study of cell-to-cell fusion.

The role of satellite cell-derived TRIM28 in mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discovered that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Unexpectedly, multiple lines of both in vitro and in vivo evidence revealed that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 were not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discovered that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.

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.

Fn14 promotes myoblast fusion during regenerative myogenesis

Skeletal muscle regeneration involves coordinated activation of an array of signaling pathways. Fibroblast growth factor-inducible 14 (Fn14) is a bona fide receptor for the TWEAK cytokine. Levels of Fn14 are increased in the skeletal muscle of mice after injury. However, the cell-autonomous role of Fn14 in muscle regeneration remains unknown. Here, we demonstrate that global deletion of the Fn14 receptor in mice attenuates muscle regeneration. Conditional ablation of Fn14 in myoblasts but not in differentiated myofibers of mice inhibits skeletal muscle regeneration. Fn14 promotes myoblast fusion without affecting the levels of myogenic regulatory factors in the regenerating muscle. Fn14 deletion in myoblasts hastens initial differentiation but impairs their fusion. The overexpression of Fn14 in myoblasts results in the formation of myotubes having an increased diameter after induction of differentiation. Ablation of Fn14 also reduces the levels of various components of canonical Wnt and calcium signaling both in vitro and in vivo. Forced activation of Wnt signaling rescues fusion defects in Fn14-deficient myoblast cultures. Collectively, our results demonstrate that Fn14-mediated signaling positively regulates myoblast fusion and skeletal muscle regeneration.

Ryanodine receptor type 1 content decrease-induced endoplasmic reticulum stress is a hallmark of myopathies

BACKGROUND

Decreased ryanodine receptor type 1 (RyR1) protein levels are a well-described feature of recessive RYR1-related myopathies. The aim of the present study was twofold: (1) to determine whether RyR1 content is also decreased in other myopathies and (2) to investigate the mechanisms by which decreased RyR1 protein triggers muscular disorders.

METHODS

We used publicly available datasets, muscles from human inflammatory and mitochondrial myopathies, an inducible muscle-specific RYR1 recessive mouse model and RyR1 knockdown in C2C12 muscle cells to measure RyR1 content and endoplasmic reticulum (ER) stress markers. Proteomics, lipidomics, molecular biology and transmission electron microscopy approaches were used to decipher the alterations associated with the reduction of RyR1 protein levels.

RESULTS

RYR1 transcripts were reduced in muscle samples of patients suffering from necrotizing myopathy (P = 0.026), inclusion body myopathy (P = 0.003), polymyositis (P < 0.001) and juvenile dermatomyositis (P < 0.001) and in muscle samples of myotonic dystrophy type 2 (P < 0.001), presymptomatic (P < 0.001) and symptomatic (P < 0.001) Duchenne muscular dystrophy, Becker muscular dystrophy (P = 0.004) and limb-girdle muscular dystrophy type 2A (P = 0.004). RyR1 protein content was also significantly decreased in inflammatory myopathy (-75%, P < 0.001) and mitochondrial myopathy (-71%, P < 0.001) muscles. Proteomics data showed that depletion of RyR1 protein in C2C12 myoblasts leads to myotubes recapitulating the common molecular alterations observed in myopathies. Mechanistically, RyR1 protein depletion reduces ER-mitochondria contact length (-26%, P < 0.001), Ca2+ transfer to mitochondria (-48%, P = 0.002) and the mitophagy gene Parkinson protein 2 transcripts (P = 0.037) and induces mitochondrial accumulation (+99%, P = 0.005) and dysfunction (P < 0.001). This was associated to the accumulation of deleterious sphingolipid species. Our data showed increased levels of the ER stress marker chaperone-binding protein/glucose regulated protein 78, GRP78-Bip, in RyR1 knockdown myotubes (+45%, P = 0.046), in mouse RyR1 recessive muscles (+58%, P = 0.001) and in human inflammatory (+96%, P = 0.006) and mitochondrial (+64%, P = 0.049) myopathy muscles. This was accompanied by increased protein levels of the pro-apoptotic protein CCAAT-enhancer-binding protein homologous protein, CHOP-DDIT3, in RyR1 knockdown myotubes (+27%, P < 0.001), mouse RyR1 recessive muscles (+63%, P = 0.009), human inflammatory (+50%, P = 0.038) and mitochondrial (+51%, P = 0.035) myopathy muscles. In publicly available datasets, the decrease in RYR1 content in myopathies was also associated to increased ER stress markers and RYR1 transcript levels are inversely correlated with ER stress markers in the control population.

CONCLUSIONS

Decreased RyR1 is commonly observed in myopathies and associated to ER stress in vitro, in mouse muscle and in human myopathy muscles, suggesting a potent role of RyR1 depletion-induced ER stress in the pathogenesis of myopathies.

Regenerating human skeletal muscle forms an emerging niche in vivo to support PAX7 cells

Skeletal muscle stem and progenitor cells including those derived from human pluripotent stem cells (hPSCs) offer an avenue towards personalized therapies and readily fuse to form human-mouse myofibres in vivo. However, skeletal muscle progenitor cells (SMPCs) inefficiently colonize chimeric stem cell niches and instead associate with human myofibres resembling foetal niches. We hypothesized competition with mouse satellite cells (SCs) prevented SMPC engraftment into the SC niche and thus generated an SC ablation mouse compatible with human engraftment. Single-nucleus RNA sequencing of SC-ablated mice identified the absence of a transient myofibre subtype during regeneration expressing Actc1. Similarly, ACTC1+ human myofibres supporting PAX7+ SMPCs increased in SC-ablated mice, and after re-injury we found SMPCs could now repopulate into chimeric niches. To demonstrate ACTC1+ myofibres are essential to supporting PAX7 SMPCs, we generated caspase-inducible ACTC1 depletion human pluripotent stem cells, and upon SMPC engraftment we found a 90% reduction in ACTC1+ myofibres and a 100-fold decrease in PAX7 cell numbers compared with non-induced controls. We used spatial RNA sequencing to identify key factors driving emerging human niche formation between ACTC1+ myofibres and PAX7+ SMPCs in vivo. This revealed that transient regenerating human myofibres are essential for emerging niche formation in vivo to support PAX7 SMPCs.

Regulation of myogenesis and adipogenesis by the electromagnetic perceptive gene

Obesity has been increasing in many regions of the world, including Europe, USA, and Korea. To manage obesity, we should consider it as a disease and apply therapeutic methods for its treatment. Molecular and therapeutic approaches for obesity management involve regulating biomolecules such as DNA, RNA, and protein in adipose-derived stem cells to prevent to be fat cells. Multiple factors are believed to play a role in fat differentiation, with one of the most effective factor is Ca2+. We recently reported that the electromagnetic perceptive gene (EPG) regulated intracellular Ca2+ levels under various electromagnetic fields. This study aimed to investigate whether EPG could serve as a therapeutic method against obesity. We confirmed that EPG serves as a modulator of Ca2+ levels in primary adipose cells, thereby regulating several genes such as CasR, PPARγ, GLU4, GAPDH during the adipogenesis. In addition, this study also identified EPG-mediated regulation of myogenesis that myocyte transcription factors (CasR, MyoG, MyoD, Myomaker) were changed in C2C12 cells and satellite cells. In vivo experiments carried out in this study confirmed that total weight/ fat/fat accumulation were decreased and lean mass was increased by EPG with magnetic field depending on age of mice. The EPG could serve as a potent therapeutic agent against obesity.

RhoA Is a Crucial Regulator of Myoblast Fusion

Satellite cells (SCs) are adult muscle stem cells that are mobilized when muscle homeostasis is perturbed. Here we show that RhoA in SCs is indispensable to have correct muscle regeneration and hypertrophy. In particular, the absence of RhoA in SCs prevents a correct SC fusion both to other RhoA-deleted SCs (regeneration context) and to growing control myofibers (hypertrophy context). We demonstrated that RhoA is dispensable for SCs proliferation and differentiation; however, RhoA-deleted SCs have an inefficient movement even if their cytoskeleton assembly is not altered. Proliferative myoblast and differentiated myotubes without RhoA display a decreased expression of Chordin, suggesting a crosstalk between these genes for myoblast fusion regulation. These findings demonstrate the importance of RhoA in SC fusion regulation and its requirement to achieve an efficient skeletal muscle homeostasis restoration.

Postnatal skeletal muscle myogenesis governed by signal transduction networks: MAPKs and PI3K-Akt control multiple steps

Skeletal muscle myogenesis represents one of the most intensively and extensively examined systems of cell differentiation, tissue formation, and regeneration. Muscle regeneration provides an in vivo model system of postnatal myogenesis. It comprises multiple steps including muscle stem cell (or satellite cell) quiescence, activation, migration, myogenic determination, myoblast proliferation, myocyte differentiation, myofiber maturation, and hypertrophy. A variety of extracellular signaling and subsequent intracellular signal transduction pathways or networks govern the individual steps of postnatal myogenesis. Among them, MAPK pathways (the ERK, JNK, p38 MAPK, and ERK5 pathways) and PI3K-Akt signaling regulate multiple steps of myogenesis. Ca2+, cytokine, and Wnt signaling also participate in several myogenesis steps. These signaling pathways often control cell cycle regulatory proteins or the muscle-specific MyoD family and the MEF2 family of transcription factors. This article comprehensively reviews molecular mechanisms of the individual steps of postnatal skeletal muscle myogenesis by focusing on signal transduction pathways or networks. Nevertheless, no or only a partial signaling molecules or pathways have been identified in some responses during myogenesis. The elucidation of these unidentified signaling molecules and pathways leads to an extensive understanding of the molecular mechanisms of myogenesis.

Cryo-EM structures of Myomaker reveal a molecular basis for myoblast fusion

The fusion of mononucleated myoblasts produces multinucleated muscle fibers leading to the formation of skeletal muscle. Myomaker, a skeletal muscle-specific membrane protein, is essential for myoblast fusion. Here we report the cryo-EM structures of mouse Myomaker (mMymk) and Ciona robusta Myomaker (cMymk). Myomaker contains seven transmembrane helices (TMs) that adopt a G-protein-coupled receptor-like fold. TMs 2-4 form a dimeric interface, while TMs 3 and 5-7 create a lipid-binding site that holds the polar head of a phospholipid and allows the alkyl tails to insert into Myomaker. The similarity of cMymk and mMymk suggests a conserved Myomaker-mediated cell fusion mechanism across evolutionarily distant species. Functional analyses demonstrate the essentiality of the dimeric interface and the lipid-binding site for fusogenic activity, and heterologous cell-cell fusion assays show the importance of transcellular interactions of Myomaker protomers for myoblast fusion. Together, our findings provide structural and functional insights into the process of myoblast fusion.

LncRNA-encoded peptides: unveiling their significance in cardiovascular physiology and pathology-current research insights

Long non-coding RNAs (lncRNAs), which are RNA transcripts exceeding 200 nucleotides were believed to lack any protein-coding capacity. But advancements in -omics technology have revealed that some lncRNAs have small open reading frames (sORFs) that can be translated by ribosomes to encode peptides, some of which have important biological functions. These encoded peptides subserve important biological functions by interacting with their targets to modulate transcriptional or signalling axes, thereby enhancing or suppressing cardiovascular disease (CVD) occurrence and progression. In this review, we summarize what is known about the research strategy of lncRNA-encoded peptides, mainly comprising predictive websites/tools and experimental methods that have been widely used for prediction, identification, and validation. More importantly, we have compiled a list of lncRNA- encoded peptides, with a focus on those that play significant roles in cardiovascular physiology and pathology, including ENSRNOT (RNO)-sORF6/RNO-sORF7/RNO-sORF8, dwarf open reading frame (DOWRF), myoregulin (NLN), etc. Additionally, we have outlined the functions and mechanisms of these peptides in cardiovascular physiology and pathology, such as cardiomyocyte hypertrophy, myocardial contraction, myocardial infarction, and vascular remodelling. Finally, an overview of the existing challenges and potential future developments in the realm of lncRNA-encoded peptides was provided, with consideration given to prospective avenues for further research. Given that many lncRNA-encoded peptides have not been functionally annotated yet, their application in CVD diagnosis and treatment still requires further research.

Hexavalent chromium inhibits myogenic differentiation and induces myotube atrophy

Hexavalent chromium [Cr(VI)] is extensively used in many industrial processes. Previous studies reported that Cr(VI) exposures during early embryonic development reduced body weight with musculoskeletal malformations in rodents while exposures in adult mice increased serum creatine kinase activity, a marker of muscle damage. However, the impacts of Cr(VI) on muscle differentiation remain largely unknown. Here, we report that acute exposures to Cr(VI) in mouse C2C12 myoblasts inhibit myogenic differentiation in a dose-dependent manner. Exposure to 2 μM of Cr(VI) resulted in delayed myotube formation, as evidenced by a significant decrease in myotube formation and expression of muscle-specific markers, such as muscle creatine kinase (Mck), Myocyte enhancer factor 2 (Mef2), Myomaker (Mymk) and Myomixer (Mymx). Interestingly, exposure to 5 μM of Cr(VI) completely abolished myotube formation in differentiating C2C12 cells. Moreover, the expression of key myogenic regulatory factors (MRFs) including myoblast determination protein 1 (MyoD), myogenin (MyoG), myogenic factor 5 (Myf5), and myogenic factor 6 (Myf6) were significantly altered in Cr(VI)-treated cells. The inhibitory effect of Cr(VI) on myogenic differentiation was further confirmed in freshly isolated mouse satellite cells, a stem cell population essential for adult skeletal muscle regeneration. Furthermore, Cr(VI) exposure to fully differentiated C2C12 myotubes resulted in a decrease in myotube diameter, which was exacerbated upon co-treatment with dexamethasone. Together, our results demonstrate that Cr(VI) inhibits myogenic differentiation and induces myotube atrophy in vitro.

Rab44 negatively regulates myoblast differentiation by controlling fusogenic protein transport and mTORC1 signaling

Skeletal muscle is composed of multinucleated myotubes formed by the fusion of mononucleated myoblasts. Skeletal muscle differentiation, termed as myogenesis, have been investigated using the mouse skeletal myoblast cell line C2C12. It has been reported that several "small" Rab proteins, major membrane-trafficking regulators, possibly regulate membrane protein transport in C2C12 cells; however, the role of Rab proteins in myogenesis remains unexplored. Rab44, a member of "large" Rab GTPases, has recently been identified as a negative regulator of osteoclast differentiation. In this study, using C2C12 cells, we found that Rab44 expression was upregulated during myoblast differentiation into myotubes. Knockdown of Rab44 enhanced myoblast differentiation and myotube formation. Consistent with these results, Rab44 knockdown in myoblasts increased expression levels of several myogenic marker genes. Rab44 knockdown increased the surface accumulation of myomaker and myomixer, two fusogenic proteins required for multinucleation, implying enhanced cell fusion. Conversely, Rab44 overexpression inhibited myoblast differentiation and tube formation, accompanied by decreased expression of some myogenic markers. Furthermore, Rab44 was found to be predominantly localized in lysosomes, and Rab44 overexpression altered the number and size of lysosomes. Considering the underlying molecular mechanism, Rab44 overexpression impaired the signaling pathway of the mechanistic target of rapamycin complex1 (mTORC1) in C2C12 cells. Namely, phosphorylation levels of mTORC1 and downstream mTORC1 substrates, such as S6 and P70-S6K, were notably lower in Rab44 overexpressing cells than those in control cells. These results indicate that Rab44 negatively regulates myoblast differentiation into myotubes by controlling fusogenic protein transport and mTORC1 signaling.

Rab44 deficiency accelerates recovery from muscle damage by regulating mTORC1 signaling and transport of fusogenic regulators

The skeletal muscle is a tissue that shows remarkable plasticity to adapt to various stimuli. The development and regeneration of skeletal muscles are regulated by numerous molecules. Among these, we focused on Rab44, a large Rab GTPase, that has been recently identified in immune cells and osteoclasts. Recently, bioinformatics data has revealed that Rab44 is upregulated during the myogenic differentiation of myoblasts into myotubes in C2C12 cells. Thus, Rab44 may be involved in myogenesis. Here, we have investigated the effects of Rab44 deficiency on the development and regeneration of skeletal muscle in Rab44 knockout (KO) mice. Although KO mice exhibited body and muscle weights similar to those of wild-type (WT) mice, the histochemical analysis showed that the myofiber cross-sectional area (CSA) of KO mice was significantly smaller than that of WT mice. Importantly, the results of muscle regeneration experiments using cardiotoxin revealed that the CSA of KO mice was significantly larger than that of WT mice, suggesting that Rab44 deficiency promotes muscle regeneration. Consistent with the in vivo results, in vitro experiments indicated that satellite cells derived from KO mice displayed enhanced proliferation and differentiation. Mechanistically, KO satellite cells exhibited an increased mechanistic target of rapamycin complex 1 (mTORC1) signaling compared to WT cells. Additionally, enhanced cell surface transport of myomaker and myomixer, which are essential membrane proteins for myoblast fusion, was observed in KO satellite cells compared to WT cells. Therefore, Rab44 deficiency enhances muscle regeneration by modulating the mTORC1 signaling pathway and transport of fusogenic regulators.