Keep Your Friends Close: Cell-Cell Contact and Skeletal Myogenesis

Polysaccharide of Atractylodes macrocephala Koidz alleviates LPS-induced proliferation, differentiation inhibition and excessive apoptosis in chicken embryonic myogenic cells

BACKGROUND

Lipopolysaccharide (LPS) can induce systemic inflammation and affect the growth and development of poultry. As a kind of traditional Chinese medicine, polysaccharide of Atractylodes macrocephala Koidz (PAMK) can effectively improve the growth performance of animals and improve the immunity of animal bodies.

OBJECTIVES

The purpose of this study was to investigate the effects of PAMK on LPS-induced inflammatory response, proliferation, differentiation and apoptosis of chicken embryonic myogenic cells.

METHODS

We used chicken embryonic myogenic cells as a model by detecting EdU/MYHC immunofluorescence, the expression of inflammation, proliferation, differentiation-related genes and proteins and the number of apoptotic cells in the condition of adding LPS, PAMK, belnacasan (an inhibitor of Caspase1) or their combinations.

RESULTS

The results showed that LPS stimulation increased the expression of inflammatory factors, inhibited proliferation and differentiation, and excessive apoptosis in chicken embryonic myogenic cells, and PAMK alleviated these adverse effects induced by LPS. After the addition of belnacasan (inhibitor of Caspase1), apoptosis in myogenic cells was inhibited, and therefore, the number of apoptotic cells and the expression of pro-apoptotic genes Caspase1 and Caspase3 were increased. In addition, belnacasan inhibited the increased expression of inflammatory factors, inhibited proliferation, differentiation and excessive apoptosis in chicken embryonic myogenic cells induced by LPS.

CONCLUSIONS

This study provides a theoretical basis for further exploring the mechanism of action of PAMK and exogenous LPS on chicken embryonic myogenic cells and lays the foundation for the development and application of green feed additives in animal husbandry industry.

Tissue-specific compensatory mechanisms maintain tissue architecture and body size independent of cell size in polyploid zebrafish

Genome duplications and ploidy transitions have occurred in nearly every major taxon of eukaryotes, but they are far more common in plants than in animals. Due to the conservation of the nuclear:cytoplasmic volume ratio increased DNA content results in larger cells. In plants, polyploid organisms are larger than diploids as cell number remains relatively constant. Conversely, vertebrate body size does not correlate with cell size and ploidy as vertebrates compensate for increased cell size to maintain tissue architecture and body size. This has historically been explained by a simple reduction in cell number that matches the increase in cell size maintaining body size as ploidy increases, but here we show that the compensatory mechanisms that maintain body size in triploid zebrafish are tissue-specific: A) erythrocytes respond in the classical pattern with a reduced number of larger erythrocytes in circulation, B) muscle, a tissue comprised of polynucleated muscle fibers, compensates by reducing the number of larger nuclei such that myofiber and myotome size in unaffected by ploidy, and C) vascular tissue compensates by thickening blood vessel walls, possibly at the expense of luminal diameter. Understanding the physiological implications of ploidy on tissue function requires a detailed description of the specific mechanisms of morphological compensation occurring in each tissue to understand how ploidy changes affect development and physiology.

Cadherin-dependent adhesion is required for muscle stem cell niche anchorage and maintenance

Adhesion between stem cells and their niche provides stable anchorage and signaling cues to sustain properties such as quiescence. Skeletal muscle stem cells (MuSCs) adhere to an adjacent myofiber via cadherin-catenin complexes. Previous studies on N- and M-cadherin in MuSCs revealed that although N-cadherin is required for quiescence, they are collectively dispensable for MuSC niche localization and regenerative activity. Although additional cadherins are expressed at low levels, these findings raise the possibility that cadherins are unnecessary for MuSC anchorage to the niche. To address this question, we conditionally removed from MuSCs β- and γ-catenin, and, separately, αE- and αT-catenin, factors that are essential for cadherin-dependent adhesion. Catenin-deficient MuSCs break quiescence similarly to N-/M-cadherin-deficient MuSCs, but exit the niche and are depleted. Combined in vivo, ex vivo and single cell RNA-sequencing approaches reveal that MuSC attrition occurs via precocious differentiation, re-entry to the niche and fusion to myofibers. These findings indicate that cadherin-catenin-dependent adhesion is required for anchorage of MuSCs to their niche and for preservation of the stem cell compartment. Furthermore, separable cadherin-regulated functions govern niche localization, quiescence and MuSC maintenance.

Epigenetics of Genes Preferentially Expressed in Dissimilar Cell Populations: Myoblasts and Cerebellum

While studying myoblast methylomes and transcriptomes, we found that CDH15 had a remarkable preference for expression in both myoblasts and cerebellum. To understand how widespread such a relationship was and its epigenetic and biological correlates, we systematically looked for genes with similar transcription profiles and analyzed their DNA methylation and chromatin state and accessibility profiles in many different cell populations. Twenty genes were expressed preferentially in myoblasts and cerebellum (Myob/Cbl genes). Some shared DNA hypo- or hypermethylated regions in myoblasts and cerebellum. Particularly striking was ZNF556, whose promoter is hypomethylated in expressing cells but highly methylated in the many cell populations that do not express the gene. In reporter gene assays, we demonstrated that its promoter's activity is methylation sensitive. The atypical epigenetics of ZNF556 may have originated from its promoter's hypomethylation and selective activation in sperm progenitors and oocytes. Five of the Myob/Cbl genes (KCNJ12, ST8SIA5, ZIC1, VAX2, and EN2) have much higher RNA levels in cerebellum than in myoblasts and displayed myoblast-specific hypermethylation upstream and/or downstream of their promoters that may downmodulate expression. Differential DNA methylation was associated with alternative promoter usage for Myob/Cbl genes MCF2L, DOK7, CNPY1, and ANK1. Myob/Cbl genes PAX3, LBX1, ZNF556, ZIC1, EN2, and VAX2 encode sequence-specific transcription factors, which likely help drive the myoblast and cerebellum specificity of other Myob/Cbl genes. This study extends our understanding of epigenetic/transcription associations related to differentiation and may help elucidate relationships between epigenetic signatures and muscular dystrophies or cerebellar-linked neuropathologies.

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.

Regenerating Myofibers after an Acute Muscle Injury: What Do We Really Know about Them?

Injury to skeletal muscle through trauma, physical activity, or disease initiates a process called muscle regeneration. When injured myofibers undergo necrosis, muscle regeneration gives rise to myofibers that have myonuclei in a central position, which contrasts the normal, peripheral position of myonuclei. Myofibers with central myonuclei are called regenerating myofibers and are the hallmark feature of muscle regeneration. An important and underappreciated aspect of muscle regeneration is the maturation of regenerating myofibers into a normal sized myofiber with peripheral myonuclei. Strikingly, very little is known about processes that govern regenerating myofiber maturation after muscle injury. As knowledge of myofiber formation and maturation during embryonic, fetal, and postnatal development has served as a foundation for understanding muscle regeneration, this narrative review discusses similarities and differences in myofiber maturation during muscle development and regeneration. Specifically, we compare and contrast myonuclear positioning, myonuclear accretion, myofiber hypertrophy, and myofiber morphology during muscle development and regeneration. We also discuss regenerating myofibers in the context of different types of myofiber necrosis (complete and segmental) after muscle trauma and injurious contractions. The overall goal of the review is to provide a framework for identifying cellular and molecular processes of myofiber maturation that are unique to muscle regeneration.

Genome-Wide Gene-Set Analysis Identifies Molecular Mechanisms Associated with ALS

Amyotrophic lateral sclerosis (ALS) is a fatal late-onset motor neuron disease characterized by the loss of the upper and lower motor neurons. Our understanding of the molecular basis of ALS pathology remains elusive, complicating the development of efficient treatment. Gene-set analyses of genome-wide data have offered insight into the biological processes and pathways of complex diseases and can suggest new hypotheses regarding causal mechanisms. Our aim in this study was to identify and explore biological pathways and other gene sets having genomic association to ALS. Two cohorts of genomic data from the dbGaP repository were combined: (a) the largest available ALS individual-level genotype dataset (N = 12,319), and (b) a similarly sized control cohort (N = 13,210). Following comprehensive quality control pipelines, imputation and meta-analysis, we assembled a large European descent ALS-control cohort of 9244 ALS cases and 12,795 healthy controls represented by genetic variants of 19,242 genes. Multi-marker analysis of genomic annotation (MAGMA) gene-set analysis was applied to an extensive collection of 31,454 gene sets from the molecular signatures database (MSigDB). Statistically significant associations were observed for gene sets related to immune response, apoptosis, lipid metabolism, neuron differentiation, muscle cell function, synaptic plasticity and development. We also report novel interactions between gene sets, suggestive of mechanistic overlaps. A manual meta-categorization and enrichment mapping approach is used to explore the overlap of gene membership between significant gene sets, revealing a number of shared mechanisms.

Lipopolysaccharide Promotes the Proliferation and Differentiation of Goose Embryonic Myoblasts by Promoting Cytokine Expression and Appropriate Apoptosis Processes

Lipopolysaccharide (LPS) can trigger a series of immune reactions, leading to the occurrence of disease and a decrease in the growth performance of geese. However, the mechanisms of LPS in geese muscle development have not been reported. This study aimed to investigate the effects and mechanisms of LPS on proliferation and differentiation of goose embryonic myoblasts. Embelin and belnacasan combined with LPS were used to explore these effects. Our results demonstrated that LPS significantly induced inflammatory cytokine production in both proliferation and differentiation stages. LPS and embelin treatment significantly improved the proliferation ability (p < 0.05), while LPS reduced the differentiation ability of goose embryonic myoblasts. By adding embelin, the differentiation ability of myoblasts was enhanced, while by adding belnacasan, LPS treatment led to a lower differentiation ability. Combined with the correlation of the expression levels of myogenic, cell cycle, and inflammatory-related genes and proteins, it is speculated that one of the reason for the decrease of differentiation ability of goose embryo myoblasts induced by LPS is the increase of the expression levels of pro-inflammatory factors. Moreover, LPS, embelin and belnacasan, and LPS treatments could significantly increase the apoptosis rate of goose embryonic myoblasts. Taken together, these findings suggest that LPS promotes the proliferation and differentiation of goose embryonic myoblasts by promoting cytokine expression and appropriate apoptosis processes. These findings lay a foundation for the study of the mechanisms of LPS in goose muscle development.

The Effect of Heat Shock on Myogenic Differentiation of Human Skeletal-Muscle-Derived Mesenchymal Stem/Stromal Cells

Muscle injuries, degenerative diseases and other lesions negatively affect functioning of human skeletomuscular system and thus quality of life. Therefore, the investigation of molecular mechanisms, stimulating myogenic differentiation of primary skeletal-muscle-derived mesenchymal stem/stromal cells (SM-MSCs), is actual and needed. The aim of the present study was to investigate the myogenic differentiation of CD56 (neural cell adhesion molecule, NCAM)-positive and -negative SM-MSCs and their response to the non-cytotoxic heat stimulus. The SM-MSCs were isolated from the post operation muscle tissue, sorted by flow cytometer according to the CD56 biomarker and morphology, surface profile, proliferation and myogenic differentiation has been investigated. Data show that CD56(+) cells were smaller in size, better proliferated and had significantly higher levels of CD146 (MCAM) and CD318 (CDCP1) compared with the CD56(-) cells. At control level, CD56(+) cells significantly more expressed myogenic differentiation markers MYOD1 and myogenin (MYOG) and better differentiated to the myogenic direction. The non-cytotoxic heat stimulus significantly stronger stimulated expression of myogenic markers in CD56(+) than in CD56(-) cells that correlated with the multinucleated cell formation. Data show that regenerative properties of CD56(+) SM-MSCs can be stimulated by an extracellular stimulus and be used as a promising skeletal muscle regenerating tool in vivo.

Loss of Myomixer Results in Defective Myoblast Fusion, Impaired Muscle Growth, and Severe Myopathy in Zebrafish

The development and growth of fish skeletal muscles require myoblast fusion to generate multinucleated myofibers. While zebrafish fast-twitch muscle can fuse to generate multinucleated fibers, the slow-twitch muscle fibers remain mononucleated in zebrafish embryos and larvae. The mechanism underlying the fiber-type-specific control of fusion remains elusive. Recent genetic studies using mice identified a long-sought fusion factor named Myomixer. To understand whether Myomixer is involved in the fiber-type specific fusion, we analyzed the transcriptional regulation of myomixer expression and characterized the muscle growth phenotype upon genetic deletion of myomixer in zebrafish. The data revealed that overexpression of Sonic Hedgehog (Shh) drastically inhibited myomixer expression and blocked myoblast fusion, recapitulating the phenotype upon direct genetic deletion of myomixer from zebrafish. The fusion defect in myomixer mutant embryos could be faithfully rescued upon re-expression of zebrafish myomixer gene or its orthologs from shark or human. Interestingly, myomixer mutant fish survived to adult stage though were notably smaller than wildtype siblings. Severe myopathy accompanied by the uncontrolled adipose infiltration was observed in both fast and slow muscle tissues of adult myomixer mutants. Collectively, our data highlight an indispensable role of myomixer gene for cell fusion during both embryonic muscle development and post-larval muscle growth.

Polycystin-2 (PC2) is a key determinant of in vitro myogenesis

The development of skeletal muscle (myogenesis) is a well-orchestrated process where myoblasts withdraw from the cell cycle and differentiate into myotubes. Signaling by fluxes in intracellular calcium (Ca²⁺) is known to contribute to myogenesis, and increased mitochondrial biogenesis is required to meet the metabolic demand of mature myotubes. However, gaps remain in the understanding of how intracellular Ca²⁺ signals can govern myogenesis. Polycystin-2 (PC2 or TRPP1) is a nonselective cation channel permeable to Ca²⁺. It can interact with intracellular calcium channels to control Ca²⁺ release and concurrently modulates mitochondrial function and remodeling. Due to these features, we hypothesized that PC2 is a central protein in mediating both the intracellular Ca²⁺ responses and mitochondrial changes seen in myogenesis. To test this hypothesis, we created CRISPR/Cas9 knockout (KO) C2C12 murine myoblast cell lines. PC2 KO cells were unable to differentiate into myotubes, had impaired spontaneous Ca²⁺ oscillations, and did not develop depolarization-evoked Ca²⁺ transients. The autophagic-associated pathway beclin-1 was downregulated in PC2 KO cells, and direct activation of the autophagic pathway resulted in decreased mitochondrial remodeling. Re-expression of full-length PC2, but not a calcium channel dead pathologic mutant, restored the differentiation phenotype and increased the expression of mitochondrial proteins. Our results establish that PC2 is a novel regulator of in vitro myogenesis by integrating PC2-dependent Ca²⁺ signals and metabolic pathways.

MET Inhibition Sensitizes Rhabdomyosarcoma Cells to NOTCH Signaling Suppression

Rhabdomyosarcoma (RMS) is a pediatric myogenic soft tissue sarcoma. The Fusion-Positive (FP) subtype expresses the chimeric protein PAX3-FOXO1 (P3F) while the Fusion-Negative (FN) is devoid of any gene translocation. FP-RMS and metastatic FN-RMS are often unresponsive to conventional therapy. Therefore, novel therapeutic approaches are needed to halt tumor progression. NOTCH signaling has oncogenic functions in RMS and its pharmacologic inhibition through γ-secretase inhibitors blocks tumor growth in vitro and in vivo. Here, we show that NOTCH signaling blockade resulted in the up-regulation and phosphorylation of the MET oncogene in both RH30 (FP-RMS) and RD (FN-RMS) cell lines. Pharmacologic inhibition of either NOTCH or MET signaling slowed proliferation and restrained cell survival compared to control cells partly by increasing Annexin V and CASP3/7 activation. Co-treatment with NOTCH and MET inhibitors significantly amplified these effects and enhanced PARP1 cleavage in both cell lines. Moreover, it severely hampered cell migration, colony formation, and anchorage-independent growth compared to single-agent treatments in both cell lines and significantly prevented the growth of FN-RMS cells grown as spheroids. Collectively, our results unveil the overexpression of the MET oncogene by NOTCH signaling targeting in RMS cells and show that MET pathway blockade sensitizes them to NOTCH inhibition.

Regulation of the myoblast fusion reaction for muscle development, regeneration, and adaptations

Fusion of plasma membranes is essential for skeletal muscle development, regeneration, exercise-induced adaptations, and results in a cell that contains hundreds to thousands of nuclei within a shared cytoplasm. The differentiation process in myocytes culminates in their fusion to form a new myofiber or fusion to an existing myofiber thereby contributing more synthetic material to the syncytium. The choice for two cells to fuse and become one could be a dangerous event if the two cells are not committed to an allied function. Thus, fusion events are highly regulated with positive and negative factors to fine-tune the process, and requires muscle-specific fusogens (Myomaker and Myomerger) as well as general cellular machinery to achieve the union of membranes. While a unified vertebrate myoblast fusion pathway is not yet established, recent discoveries should make this pursuit attainable. Not only does myocyte fusion impact the normal biology of skeletal muscle, but new evidence indicates dysregulation of the process impacts pathologies of skeletal muscle. Here, I will highlight the molecular players and biochemical mechanisms that drive fusion events in muscle, and discuss how this key myogenic process impacts skeletal muscle diseases.

Transcriptomic characterization of the molecular mechanisms induced by RGMa during skeletal muscle nuclei accretion and hypertrophy

Background

The repulsive guidance molecule a (RGMa) is a GPI-anchor axon guidance molecule first found to play important roles during neuronal development. RGMa expression patterns and signaling pathways via Neogenin and/or as BMP coreceptors indicated that this axon guidance molecule could also be working in other processes and diseases, including during myogenesis. Previous works from our research group have consistently shown that RGMa is expressed in skeletal muscle cells and that its overexpression induces both nuclei accretion and hypertrophy in muscle cell lineages. However, the cellular components and molecular mechanisms induced by RGMa during the differentiation of skeletal muscle cells are poorly understood. In this work, the global transcription expression profile of RGMa-treated C2C12 myoblasts during the differentiation stage, obtained by RNA-seq, were reported.

Results

RGMa treatment could modulate the expression pattern of 2,195 transcripts in C2C12 skeletal muscle, with 943 upregulated and 1,252 downregulated. Among them, RGMa interfered with the expression of several RNA types, including categories related to the regulation of RNA splicing and degradation. The data also suggested that nuclei accretion induced by RGMa could be due to their capacity to induce the expression of transcripts related to ‘adherens junsctions’ and ‘extracellular-cell adhesion’, while RGMa effects on muscle hypertrophy might be due to (i) the activation of the mTOR-Akt independent axis and (ii) the regulation of the expression of transcripts related to atrophy. Finally, RGMa induced the expression of transcripts that encode skeletal muscle structural proteins, especially from sarcolemma and also those associated with striated muscle cell differentiation.

Conclusions

These results provide comprehensive knowledge of skeletal muscle transcript changes and pathways in response to RGMa.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08396-w.

Bioinks and Bioprinting Strategies for Skeletal Muscle Tissue Engineering

Skeletal muscles play important roles in critical body functions and their injury or disease can lead to limitation of mobility and loss of independence. Current treatments result in variable functional recovery, while reconstructive surgery, as the gold-standard approach, is limited due to donor shortage, donor-site morbidity, and limited functional recovery. Skeletal muscle tissue engineering (SMTE) has generated enthusiasm as an alternative solution for treatment of injured tissue and serves as a functional disease model. Recently, bioprinting has emerged as a promising tool for recapitulating the complex and highly organized architecture of skeletal muscles at clinically relevant sizes. Here, skeletal muscle physiology, muscle regeneration following injury, and current treatments following muscle loss are discussed, and then bioprinting strategies implemented for SMTE are critically reviewed. Subsequently, recent advancements that have led to improvement of bioprinting strategies to construct large muscle structures, boost myogenesis in vitro and in vivo, and enhance tissue integration are discussed. Bioinks for muscle bioprinting, as an essential part of any bioprinting strategy, are discussed, and their benefits, limitations, and areas to be improved are highlighted. Finally, the directions the field should expand to make bioprinting strategies more translational and overcome the clinical unmet needs are discussed.

Cell-cell contact and signaling in the muscle stem cell niche

Muscle stem cells (also called satellite cells or SCs) rely on their local niche for regulatory signals during homeostasis and regeneration. While a number of cell types communicate indirectly through secreted factors, here we focus on the significance of direct contact between SCs and their neighbors. During quiescence, SCs reside under a basal lamina and receive quiescence-promoting signals from their adjacent skeletal myofibers. Upon injury, the composition of the niche changes substantially, enabling the formation of new contacts that mediate proliferation, self-renewal, and differentiation. In this review, we summarize the latest work in understanding cell-cell contact within the satellite cell niche and highlight areas of open questions for future studies.

Electrical Stimulation of Cultured Myotubes in vitro as a Model of Skeletal Muscle Activity: Current State and Future Prospects

Skeletal muscles comprise more than a third of human body mass and critically contribute to regulation of body metabolism. Chronic inactivity reduces metabolic activity and functional capacity of muscles, leading to metabolic and other disorders, reduced life quality and duration. Cellular models based on progenitor cells isolated from human muscle biopsies and then differentiated into mature fibers in vitro can be used to solve a wide range of experimental tasks. The review discusses the aspects of myogenesis dynamics and regulation, which might be important in the development of an adequate cell model. The main function of skeletal muscle is contraction; therefore, electrical stimulation is important for both successful completion of myogenesis and in vitro modeling of major processes induced in the skeletal muscle by acute or regular physical exercise. The review analyzes the drawbacks of such cellular model and possibilities for its optimization, as well as the prospects for its further application to address fundamental aspects of muscle physiology and biochemistry and explore cellular and molecular mechanisms of metabolic diseases.

Nr4a1 promotes cell adhesion and fusion by regulating Zeb1 transcript levels in myoblasts

Nuclear receptor subfamily 4 group A member 1 (NR4A1) acts as a myogenic factor in muscle development and regeneration; however, it remains unclear how Nr4a1 regulates myoblast physiology. In this study, report a role for Nr4a1-mediated regulation of cell adhesion in myoblast and muscle tissue. Nr4a1-overexpression myoblast, Nr4a1-konckdown myoblast and mice gastrocnemius muscle following an injection with an adenovirus vector expression Nr4a1 (Nr4a1-AAV) were used to observe the changes in cell adhesion. Nr4a1 was found to enhance cell-cell contact and adhesion molecule expression in myoblasts. In contrast, the deletion of Nr4a1 expression inhibited junction and adhesion between myoblasts. Moreover, Nr4a1 increased myoblast adhesion via directly binding to an upstream site of zinc finger E-box binding homeobox 1 (Zeb1), which is required for myogenesis in myoblasts. In mice, Zeb1 induced increased cadherin and integrin expression in the gastrocnemius muscle following an injection with an adenovirus vector expressing Nr4a1(Nr4a1-AAV). These data indicate that Nr4a1 regulates myoblast adhesion via Zeb1 expression.

Formation of Aberrant Myotubes by Myoblasts Lacking Myosin VI Is Associated with Alterations in the Cytoskeleton Organization, Myoblast Adhesion and Fusion

We have previously postulated that unconventional myosin VI (MVI) could be involved in myoblast differentiation. Here, we addressed the mechanism(s) of its involvement using primary myoblast culture derived from the hindlimb muscles of Snell’s waltzer mice, the natural MVI knockouts (MVI-KO). We observed that MVI-KO myotubes were formed faster than control heterozygous myoblasts (MVI-WT), with a three-fold increase in the number of myosac-like myotubes with centrally positioned nuclei. There were also changes in the levels of the myogenic transcription factors Pax7, MyoD and myogenin. This was accompanied by changes in the actin cytoskeleton and adhesive structure organization. We observed significant decreases in the levels of proteins involved in focal contact formation, such as talin and focal adhesion kinase (FAK). Interestingly, the levels of proteins involved in intercellular communication, M-cadherin and drebrin, were also affected. Furthermore, time-dependent alterations in the levels of the key proteins for myoblast membrane fusion, myomaker and myomerger, without effect on their cellular localization, were observed. Our data indicate that in the absence of MVI, the mechanisms controlling cytoskeleton organization, as well as myoblast adhesion and fusion, are dysregulated, leading to the formation of aberrant myotubes.

EphA7 promotes myogenic differentiation via cell-cell contact

The conversion of proliferating skeletal muscle precursors (myoblasts) to terminally-differentiated myocytes is a critical step in skeletal muscle development and repair. We show that EphA7, a juxtacrine signaling receptor, is expressed on myocytes during embryonic and fetal myogenesis and on nascent myofibers during muscle regeneration in vivo. In EphA7^-/- mice, hindlimb muscles possess fewer myofibers at birth, and those myofibers are reduced in size and have fewer myonuclei and reduced overall numbers of precursor cells throughout postnatal life. Adult EphA7^-/- mice have reduced numbers of satellite cells and exhibit delayed and protracted muscle regeneration, and satellite cell-derived myogenic cells from EphA7^-/- mice are delayed in their expression of differentiation markers in vitro. Exogenous EphA7 extracellular domain will rescue the null phenotype in vitro, and will also enhance commitment to differentiation in WT cells. We propose a model in which EphA7 expression on differentiated myocytes promotes commitment of adjacent myoblasts to terminal differentiation.

Glucose Uptake and Insulin Response in Tissue-engineered Human Skeletal Muscle

BACKGROUND

Tissue-engineered muscles ("myobundles") offer a promising platform for developing a human in vitro model of healthy and diseased muscle for drug development and testing. Compared to traditional monolayer cultures, myobundles better model the three-dimensional structure of native skeletal muscle and are amenable to diverse functional measures to monitor the muscle health and drug response. Characterizing the metabolic function of human myobundles is of particular interest to enable their utilization in mechanistic studies of human metabolic diseases, identification of related drug targets, and systematic studies of drug safety and efficacy.

METHODS

To this end, we studied glucose uptake and insulin responsiveness in human tissue-engineered skeletal muscle myobundles in the basal state and in response to drug treatments.

RESULTS

In the human skeletal muscle myobundle system, insulin stimulates a 50% increase in 2-deoxyglucose (2-DG) uptake with a compiled EC₅₀ of 0.27 ± 0.03 nM. Treatment of myobundles with 400 µM metformin increased basal 2-DG uptake 1.7-fold and caused a significant drop in twitch and tetanus contractile force along with decreased fatigue resistance. Treatment with the histone deacetylase inhibitor 4-phenylbutyrate (4-PBA) increased the magnitude of insulin response from a 1.2-fold increase in glucose uptake in the untreated state to a 1.4-fold increase after 4-PBA treatment. 4-PBA treated myobundles also exhibited increased fatigue resistance and increased twitch half-relaxation time.

CONCLUSION

Although tissue-engineered human myobundles exhibit a modest increase in glucose uptake in response to insulin, they recapitulate key features of in vivo insulin sensitivity and exhibit relevant drug-mediated perturbations in contractile function and glucose metabolism.

Mechanisms regulating myoblast fusion: A multilevel interplay

Myoblast fusion into myotubes is one of the crucial steps of skeletal muscle development (myogenesis). The fusion is preceded by specification of a myogenic lineage (mesodermal progenitors) differentiating into myoblasts and is followed by myofiber-type specification and neuromuscular junction formation. Similarly to other processes of myogenesis, the fusion requires a very precise spatial and temporal regulation occuring both during embryonic development as well as regeneration and repair of the muscle. A plethora of genes and their products is involved in regulation of myoblast fusion and a precise multilevel interplay between them is crucial for myogenic cells to fuse. In this review, we describe both cellular events taking place during myoblast fusion (migration, adhesion, elongation, cell-cell recognition, alignment, and fusion of myoblast membranes enabling formation of myotubes) as well as recent findings on mechanisms regulating this process. Also, we present muscle disorders in humans that have been associated with defects in genes involved in regulation of myoblast fusion.

Stable Knockdown of Genes Encoding Extracellular Matrix Proteins in the C2C12 Myoblast Cell Line Using Small-Hairpin (sh)RNA

Extracellular matrix (ECM) proteins are crucial for skeletal muscle development and homeostasis. The stable knockdown of genes coding for ECM proteins in C2C12 myoblasts can be applied to study the role of these proteins in skeletal muscle development. Here, we describe a protocol to deplete the ECM protein ADAMTSL2 as an example, using small-hairpin (sh) RNA in C2C12 cells. Following transfection of shRNA plasmids, stable cells were batch-selected using puromycin. We further describe the maintenance of these cell lines and the phenotypic analysis via mRNA expression, protein expression, and C2C12 differentiation. The advantages of the method are the relatively fast generation of stable C2C12 knockdown cells and the reliable differentiation of C2C12 cells into multinucleated myotubes upon depletion of serum in the cell culture medium. Differentiation of C2C12 cells can be monitored by bright field microscopy and by measuring the expression levels of canonical marker genes, such as MyoD, myogenin, or myosin heavy chain (MyHC) indicating the progression of C2C12 myoblast differentiation into myotubes. In contrast to the transient knockdown of genes with small-interfering (si) RNA, genes that are expressed later during C2C12 differentiation or during myotube maturation can be targeted more efficiently by generating C2C12 cells that stably express shRNA. Limitations of the method are a variability in the knockdown efficiencies, depending on the specific shRNA that may be overcome by using gene knockout strategies based on CRISPR/Cas9, as well as potential off-target effects of the shRNA that should be considered.

Roles of lncRNAs and circRNAs in regulating skeletal muscle development

The multistep biological process of myogenesis is regulated by a variety of myoblast regulators, such as myogenic differentiation antigen, myogenin, myogenic regulatory factor, myocyte enhancer factor2A-D and myosin heavy chain. Proliferation and differentiation during skeletal muscle myogenesis contribute to the physiological function of muscles. Certain non-coding RNAs, including long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), are involved in the regulation of muscle development, and the aberrant expressions of lncRNAs and circRNAs are associated with muscular diseases. In this review, we summarize the recent advances concerning the roles of lncRNAs and circRNAs in regulating the developmental aspects of myogenesis. These findings have remarkably broadened our understanding of the gene regulation mechanisms governing muscle proliferation and differentiation, which makes it more feasible to design novel preventive, diagnostic and therapeutic strategies for muscle disorders.

Cell Fusion: Merging Membranes and Making Muscle

Cell fusion is essential for the development of multicellular organisms, and plays a key role in the formation of various cell types and tissues. Recent findings have highlighted the varied protein machinery that drives plasma-membrane merger in different systems, which is characterized by diverse structural and functional elements. We highlight the discovery and activities of several key sets of fusion proteins that together offer an evolving perspective on cell membrane fusion. We also emphasize recent discoveries in vertebrate myoblast fusion in skeletal muscle, which is composed of numerous multinucleated myofibers formed by the fusion of progenitor cells during development.

Zfp422 promotes skeletal muscle differentiation by regulating EphA7 to induce appropriate myoblast apoptosis

Zinc finger protein 422 (Zfp422) is a widely expressed zinc finger protein that serves as a transcriptional factor to regulate downstream gene expression, but until now, little is known about its roles in myogenesis. We found here that Zfp422 plays a critical role in skeletal muscle development and regeneration. It highly expresses in mouse skeletal muscle during embryonic development. Specific knockout of Zfp422 in skeletal muscle impaired embryonic muscle formation. Satellite cell-specific Zfp422 deletion severely inhibited muscle regeneration. Myoblast differentiation and myotube formation were suppressed in Zfp422-deleted C2C12 cells, isolated primary myoblasts, and satellite cells. Chromatin Immunoprecipitation Sequencing (ChIP-Seq) revealed that Zfp422 regulated ephrin type-A receptor 7 (EphA7) expression by binding an upstream 169-bp DNA sequence, which was proved to be an enhancer of EphA7. Knocking EphA7 down in C2C12 cells or deleting Zfp422 in myoblasts will inhibit cell apoptosis which is required for myoblast differentiation. These results indicate that Zfp422 is essential for skeletal muscle differentiation and fusion, through regulating EphA7 expression to maintain proper apoptosis.

Muscle tissue engineering in fibrous gelatin: implications for meat analogs

Bioprocessing applications that derive meat products from animal cell cultures require food-safe culture substrates that support volumetric expansion and maturation of adherent muscle cells. Here we demonstrate scalable production of microfibrous gelatin that supports cultured adherent muscle cells derived from cow and rabbit. As gelatin is a natural component of meat, resulting from collagen denaturation during processing and cooking, our extruded gelatin microfibers recapitulated structural and biochemical features of natural muscle tissues. Using immersion rotary jet spinning, a dry-jet wet-spinning process, we produced gelatin fibers at high rates (~ 100 g/h, dry weight) and, depending on process conditions, we tuned fiber diameters between ~ 1.3 ± 0.1 μm (mean ± SEM) and 8.7 ± 1.4 μm (mean ± SEM), which are comparable to natural collagen fibers. To inhibit fiber degradation during cell culture, we crosslinked them either chemically or by co-spinning gelatin with a microbial crosslinking enzyme. To produce meat analogs, we cultured bovine aortic smooth muscle cells and rabbit skeletal muscle myoblasts in gelatin fiber scaffolds, then used immunohistochemical staining to verify that both cell types attached to gelatin fibers and proliferated in scaffold volumes. Short-length gelatin fibers promoted cell aggregation, whereas long fibers promoted aligned muscle tissue formation. Histology, scanning electron microscopy, and mechanical testing demonstrated that cultured muscle lacked the mature contractile architecture observed in natural muscle but recapitulated some of the structural and mechanical features measured in meat products.

Multiplexed RNAscope and immunofluorescence on whole-mount skeletal myofibers and their associated stem cells

Skeletal muscle myofibers are large syncytial cells comprising hundreds of myonuclei, and in situ hybridization experiments have reported a range of transcript localization patterns within them. Although some transcripts are uniformly distributed throughout myofibers, proximity to specialized regions can affect the programming of myonuclei and functional compartmentalization of transcripts. Established techniques are limited by a lack of both sensitivity and spatial resolution, restricting the ability to identify different patterns of gene expression. In this study, we adapted RNAscope fluorescent in situ hybridization technology for use on whole-mount mouse primary myofibers, a preparation that isolates single myofibers with their associated muscle stem cells remaining in their niche. This method can be combined with immunofluorescence, enabling an unparalleled ability to visualize and quantify transcripts and proteins across the length and depth of skeletal myofibers and their associated stem cells. Using this approach, we demonstrate a range of potential uses, including the visualization of specialized transcriptional programming within myofibers, tracking activation-induced transcriptional changes, quantification of stem cell heterogeneity and evaluation of stem cell niche factor transcription patterns.

Downregulation of POFUT1 Impairs Secondary Myogenic Fusion Through a Reduced NFATc2/IL-4 Signaling Pathway

Past work has shown that the protein O-fucosyltransferase 1 (POFUT1) is involved in mammal myogenic differentiation program. Pofut1 knockdown (Po –) in murine C2C12 cells leads to numerous elongated and thin myotubes, suggesting significant defects in secondary fusion. Among the few pathways involved in this process, NFATc2/IL-4 is described as the major one. To unravel the impact of POFUT1 on secondary fusion, we used wild-type (WT) C2C12 and Po – cell lines to follow Myf6, Nfatc2, Il-4 and Il-4rα expressions during a 120 h myogenic differentiation time course. Secreted IL-4 was quantified by ELISA. IL-4Rα expression and its labeling on myogenic cell types were investigated by Western blot and immunofluorescence, respectively. Phenotypic observations of cells treated with IL-4Rα blocking antibody were performed. In Po –, we found a decrease in nuclei number per myotube and a downexpression of Myf6. The observed downregulation of Nfatc2 is correlated to a diminution of secreted IL-4 and to the low level of IL-4Rα for reserve cells. Neutralization of IL-4Rα on WT C2C12 promotes myonuclear accretion defects, similarly to those identified in Po –. Thus, POFUT1 could be a new controller of myotube growth during myogenesis, especially through NFATc2/IL-4 signaling pathway.

Epigenomic mechanisms of alcohol-induced impaired differentiation of skeletal muscle stem cells; role of Class IIA histone deacetylases

Loss of functional metabolic muscle mass remains a strong and consistent predictor of mortality among people living with human immunodeficiency virus (PLWH). PLWH have a higher incidence of alcohol use disorder (AUD), and myopathy is a significant clinical comorbidity due to AUD. One mechanism of skeletal muscle (SKM) mass maintenance and repair is by differentiation and fusion of satellite cells (SCs) to existing myofibers. Previous studies demonstrated that chronic binge alcohol (CBA) administration decreases SC differentiation potential, myogenic gene expression, and miR-206 expression in simian immunodeficiency virus (SIV)-infected male rhesus macaques and that miR-206 targets the Class IIA histone deacetylase, HDAC4. The aim of this study was to determine whether alcohol-induced increases in Class IIA HDACs mediate the observed decrease in differentiation potential of SCs. Data show that CBA dysregulated HDAC gene expression in SKM and myoblasts of SIV-infected macaques. CBA and antiretroviral therapy increased HDAC activity in SKM and this was positively correlated with HDAC4 gene expression. In vitro ethanol (ETOH) treatment increased HDAC expression during differentiation and decreased differentiation potential of myoblasts. HDAC expression was negatively correlated with fusion index and myotube formation, indicators of differentiation potential. Treatment with a Class II HDAC inhibitor, TMP195, restored differentiation in ETOH-treated myoblasts. MEF2C expression at day 3 of differentiation was positively correlated with fusion index and myotube formation. These findings suggest that an alcohol-mediated increase in Class IIA HDAC expression contributes to decreased myoblast differentiation by downregulating MEF2C, a transcription factor critical for myogenesis.

Tumor Suppressors RB1 and CDKN2a Cooperatively Regulate Cell-Cycle Progression and Differentiation During Cardiomyocyte Development and Repair

RATIONALE

Although rare cardiomyogenesis is reported in the adult mammalian heart, whether this results from differentiation or proliferation of cardiomyogenic cells remains controversial. The tumor suppressor genes RB1 (retinoblastoma) and CDKN2a (cyclin-dependent kinase inhibitor 2a) are critical cell-cycle regulators, but their roles in human cardiomyogenesis remains unclear.

OBJECTIVE

We hypothesized that developmental activation of RB1 and CDKN2a cooperatively cause permanent cell-cycle withdrawal of human cardiac precursors (CPCs) driving terminal differentiation into mature cardiomyocytes, and that dual inactivation of these tumor suppressor genes promotes myocyte cell-cycle reentry.

METHODS AND RESULTS

Directed differentiation of human pluripotent stem cells (hPSCs) into cardiomyocytes revealed that RB1 and CDKN2a are upregulated at the onset of cardiac precursor specification, simultaneously with GATA4 (GATA-binding protein 4) homeobox genes PBX1 (pre-B-cell leukemia transcription factor 1) and MEIS1 (myeloid ecotropic viral integration site 1 homolog), and remain so until terminal cardiomyocyte differentiation. In both GATA4+ hPSC cardiac precursors and postmitotic hPSC-cardiomyocytes, RB1 is hyperphosphorylated and inactivated. Transient, stage-specific, depletion of RB1 during hPSC differentiation enhances cardiomyogenesis at the cardiac precursors stage, but not in terminally differentiated hPSC-cardiomyocytes, by transiently upregulating GATA4 expression through a cell-cycle regulatory pathway involving CDKN2a. Importantly, cytokinesis in postmitotic hPSC-cardiomyocytes can be induced with transient, dual RB1, and CDKN2a silencing. The relevance of this pathway in vivo was suggested by findings in a porcine model of cardiac cell therapy post-MI, whereby dual RB1 and CDKN2a inactivation in adult GATA4+ cells correlates with the degree of scar size reduction and endogenous cardiomyocyte mitosis, particularly in response to combined transendocardial injection of adult human hMSCs (bone marrow-derived mesenchymal stromal cells) and cKit+ cardiac cells.

CONCLUSIONS

Together these findings reveal an important and coordinated role for RB1 and CDKN2a in regulating cell-cycle progression and differentiation during human cardiomyogenesis. Moreover, transient, dual inactivation of RB1 and CDKN2a in endogenous adult GATA4+ cells and cardiomyocytes mediates, at least in part, the beneficial effects of cell-based therapy in a post-MI large mammalian model, a finding with potential clinical implications.

Development of myofibres and associated connective tissues in fish axial muscle: Recent insights and future perspectives

Fish axial muscle consists of a series of W-shaped muscle blocks, called myomeres, that are composed primarily of multinucleated contractile muscle cells (myofibres) gathered together by an intricate network of connective tissue that transmits forces generated by myofibre contraction to the axial skeleton. This review summarises current knowledge on the successive and overlapping myogenic waves contributing to axial musculature formation and growth in fish. Additionally, this review presents recent insights into muscle connective tissue development in fish, focusing on the early formation of collagenous myosepta separating adjacent myomeres and the late formation of intramuscular connective sheaths (i.e. endomysium and perimysium) that is completed only at the fry stage when connective fibroblasts expressing collagens arise inside myomeres. Finally, this review considers the possibility that somites produce not only myogenic, chondrogenic and myoseptal progenitor cells as previously reported, but also mesenchymal cells giving rise to muscle resident fibroblasts.

Genetic Mutations in jamb, jamc, and myomaker Revealed Different Roles on Myoblast Fusion and Muscle Growth

Myoblast fusion is a vital step for skeletal muscle development, growth, and regeneration. Loss of Jamb, Jamc, or Myomaker (Mymk) function impaired myoblast fusion in zebrafish embryos. In addition, mymk mutation hampered fish muscle growth. However, the effect of Jamb and Jamc deficiency on fish muscle growth is not clear. Moreover, whether jamb;jamc and jamb;mymk double mutations have stronger effects on myoblast fusion and muscle growth remains to be investigated. Here, we characterized the muscle development and growth in jamb, jamc, and mymk single and double mutants in zebrafish. We found that although myoblast fusion was compromised in jamb and jamc single or jamb;jamc double mutants, these mutant fish showed no defect in muscle cell fusion during muscle growth. The mutant fish were able to grow into adults that were indistinguishable from the wild-type sibling. In contrast, the jamb;mymk double mutants exhibited a stronger muscle phenotype compared to the jamb and jamc single and double mutants. The jamb;mymk double mutant showed reduced growth and partial lethality, similar to a mymk single mutant. Single fiber analysis of adult skeletal myofibers revealed that jamb, jamc, or jamb;jamc mutants contained mainly multinucleated myofibers, whereas jamb;mymk double mutants contained mostly mononucleated fibers. Significant intramuscular adipocyte infiltration was found in skeletal muscles of the jamb;mymk mutant. Collectively, these studies demonstrate that although Jamb, Jamc, and Mymk are all involved in myoblast fusion during early myogenesis, they have distinct roles in myoblast fusion during muscle growth. While Mymk is essential for myoblast fusion during both muscle development and growth, Jamb and Jamc are dispensable for myoblast fusion during muscle growth.

The Trithorax protein Ash1L promotes myoblast fusion by activating Cdon expression

Myoblast fusion (MF) is required for muscle growth and repair, and its alteration contributes to muscle diseases. The mechanisms governing this process are incompletely understood, and no epigenetic regulator has been previously described. Ash1L is an epigenetic activator belonging to the Trithorax group of proteins and is involved in FSHD muscular dystrophy, autism and cancer. Its physiological role in skeletal muscle is unknown. Here we report that Ash1L expression is positively correlated with MF and reduced in Duchenne muscular dystrophy. In vivo, ex vivo and in vitro experiments support a selective and evolutionary conserved requirement for Ash1L in MF. RNA- and ChIP-sequencing indicate that Ash1L is required to counteract Polycomb repressive activity to allow activation of selected myogenesis genes, in particular the key MF gene Cdon. Our results promote Ash1L as an important epigenetic regulator of MF and suggest that its activity could be targeted to improve cell therapy for muscle diseases.

Myoblast fusion in skeletal muscle is a complex process but how this is regulated is unclear. Here, the authors identify Ash1L, a histone methyltransferase, as modulating myoblast fusion via activation of the myogenesis gene Cdon, and observe decreased Ash1L expression in Duchenne muscular dystrophy.

Muscle Satellite Cell Cross-Talk with a Vascular Niche Maintains Quiescence via VEGF and Notch Signaling

Skeletal muscle is a complex tissue containing tissue resident muscle stem cells (satellite cells) (MuSCs) important for postnatal muscle growth and regeneration. Quantitative analysis of the biological function of MuSCs and the molecular pathways responsible for a potential juxtavascular niche for MuSCs is currently lacking. We utilized fluorescent reporter mice and muscle tissue clearing to investigate the proximity of MuSCs to capillaries in 3 dimensions. We show that MuSCs express abundant VEGFA, which recruits endothelial cells (ECs) in vitro, whereas blocking VEGFA using both a vascular endothelial growth factor (VEGF) inhibitor and MuSC-specific VEGFA gene deletion reduces the proximity of MuSCs to capillaries. Importantly, this proximity to the blood vessels was associated with MuSC self-renewal in which the EC-derived Notch ligand Dll4 induces quiescence in MuSCs. We hypothesize that MuSCs recruit capillary ECs via VEGFA, and in return, ECs maintain MuSC quiescence though Dll4.

Global transcriptional differences in myokine and inflammatory genes in muscle of mature steer progeny are related to maternal lactation diet and muscle composition

Steer progeny suckled by cows fed a dried distillers grains and solubles (DDGS) diet the first 3 mo of lactation were heavier during feedlot finishing and had significantly lower marbling and larger longissimus muscles than steers suckled by cows fed a control diet (CON). These differences were profound in that progeny were managed and fed identically from weaning until finishing, and findings suggest that the suckling period established the developmental program of muscle composition. Here transcriptomes of longissimus muscle were measured by next-generation sequencing to investigate whether there were any developmental clues to the differences in marbling scores and muscle content between steers suckled by DDGS ( n = 5) vs. control (CON; n = 5) diet-fed cows during lactation. There were 809 genes differentially expressed ( P-adj<0.1) between CON and DDGS muscle. Of these 636 were upregulated and 173 downregulated in DDGS relative to CON. Overall the DDGS vs. CON muscle transcriptomic signature was promyogenic and antiadipogenic. In particular, myokines/satellite cell maintenance factors were found among upregulated (LIF, CNTF, FGFB1, EPHB1) genes. The antiadipogenic signature was typified by the upregulation of anti-inflammatory cytokines and receptors (IL1RAP, IL1RL2, IL13RA2, IL1F10), and downregulation of expression of inflammation/inflammatory cytokines and receptor (TNF, IL6R, CXCL9), which suggests a selection of differentiation pathways away from adipogenic line. The upregulation of TGFB, SPP1, and INHBA supports selection of fibroblast lineage of cells. Thus, the lactation phase of production can effect meat quality by affecting transcriptional signatures that favor myogenesis and depress inflammation.

HDAC4 regulates satellite cell proliferation and differentiation by targeting P21 and Sharp1 genes

Skeletal muscle exhibits a high regenerative capacity, mainly due to the ability of satellite cells to replicate and differentiate in response to appropriate stimuli. Epigenetic control is effective at different stages of this process. It has been shown that the chromatin-remodeling factor HDAC4 is able to regulate satellite cell proliferation and commitment. However, its molecular targets are still uncovered. To explain the signaling pathways regulated by HDAC4 in satellite cells, we generated tamoxifen-inducible mice with conditional inactivation of HDAC4 in Pax7⁺ cells (HDAC4 KO mice). We found that the proliferation and differentiation of HDAC4 KO satellite cells were compromised, although similar amounts of satellite cells were found in mice. Moreover, we found that the inhibition of HDAC4 in satellite cells was sufficient to block the differentiation process. By RNA-sequencing analysis we identified P21 and Sharp1 as HDAC4 target genes. Reducing the expression of these target genes in HDAC4 KO satellite cells, we also defined the molecular pathways regulated by HDAC4 in the epigenetic control of satellite cell expansion and fusion.

Myoblast fusion confusion: the resolution begins

The fusion of muscle precursor cells is a required event for proper skeletal muscle development and regeneration. Numerous proteins have been implicated to function in myoblast fusion; however, the majority are expressed in diverse tissues and regulate numerous cellular processes. How myoblast fusion is triggered and coordinated in a muscle-specific manner has remained a mystery for decades. Through the discovery of two muscle-specific fusion proteins, Myomaker and Myomerger–Minion, we are now primed to make significant advances in our knowledge of myoblast fusion. This article reviews the latest findings regarding the biology of Myomaker and Minion–Myomerger, places these findings in the context of known pathways in mammalian myoblast fusion, and highlights areas that require further investigation. As our understanding of myoblast fusion matures so does our potential ability to manipulate cell fusion for therapeutic purposes.

Ex Vivo Visualization and Analysis of the Muscle Stem Cell Niche

Adult skeletal muscle stem cells, termed satellite cells, are essential for regenerating muscle after tissue damage. Satellite cells are located in a specialized microenvironment between muscle fibers and their surrounding basal lamina. This local niche serves as a compartment to preserve satellite cell function and provides signals that facilitate the rapid response to injury. Visualization of this local niche enables the elucidation of such niche-derived signals. Here, we describe techniques for isolating single myofibers with their associated satellite cells for ex vivo visualization and analysis of an intact muscle stem cell niche.

Control of muscle formation by the fusogenic micropeptide myomixer

Skeletal muscle formation occurs through fusion of myoblasts to form multinucleated myofibers. From a genome-wide clustered regularly interspaced short palindromic repeats (CRISPR) loss-of-function screen for genes required for myoblast fusion and myogenesis, we discovered an 84-amino acid muscle-specific peptide that we call Myomixer. Myomixer expression coincides with myoblast differentiation and is essential for fusion and skeletal muscle formation during embryogenesis. Myomixer localizes to the plasma membrane, where it promotes myoblast fusion and associates with Myomaker, a fusogenic membrane protein. Myomixer together with Myomaker can also induce fibroblast-fibroblast fusion and fibroblast-myoblast fusion. We conclude that the Myomixer-Myomaker pair controls the critical step in myofiber formation during muscle development.