Control of muscle formation by the fusogenic micropeptide myomixer

Integrated lncRNA function upon genomic and epigenomic regulation

Although some long noncoding (lnc)RNAs are known since the 1950s, the past 25 years have uncovered myriad lncRNAs with diverse sequences, structures, and functions. The advent of high-throughput and sensitive technologies has further uncovered the vast heterogeneity of lncRNA-interacting molecules and patterns of expressed lncRNAs. We propose a unifying functional theme for the expansive family of lncRNAs. At an elementary level, the genomic program of gene expression is elicited via canonical transcription and post-transcriptional mRNA assembly, turnover, and translation. Building upon this regulation, an epigenomic program refines the basic genomic control by modifying chromatin architecture as well as DNA and RNA chemistry. Superimposed over the genomic and epigenomic programs, lncRNAs create an additional regulatory dimension: by interacting with the proteins and nucleic acids that regulate gene expression in the nucleus and cytoplasm, lncRNAs help establish robust, nimble, and specific transcriptional and post-transcriptional control. We describe our present understanding of lncRNA-coordinated control of protein programs and cell fate and discuss challenges and opportunities as we embark on the next 25 years of lncRNA discovery.

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.

Impaired activity of the fusogenic micropeptide Myomixer causes myopathy resembling Carey-Fineman-Ziter syndrome

Skeletal muscle fibers contain hundreds of nuclei, which increase the overall transcriptional activity of the tissue and perform specialized functions. Multinucleation occurs through myoblast fusion, mediated by the muscle fusogens Myomaker (MYMK) and Myomixer (MYMX). We describe a human pedigree harboring a recessive truncating variant of the MYMX gene that eliminates an evolutionarily conserved extracellular hydrophobic domain of MYMX, thereby impairing fusogenic activity. Homozygosity of this human variant resulted in a spectrum of abnormalities that mimicked the clinical presentation of Carey-Fineman-Ziter syndrome (CFZS), caused by hypomorphic MYMK variants. Myoblasts generated from patient-derived induced pluripotent stem cells displayed defective fusion, and mice bearing the human MYMX variant died perinatally due to muscle abnormalities. In vitro assays showed that the human MYMX variant conferred minimal cell-cell fusogenicity, which could be restored with CRISPR/Cas9-mediated base editing, thus providing therapeutic potential for this disorder. Our findings identify MYMX as a recessive, monogenic human disease gene involved in CFZS, and provide new insights into the contribution of myoblast fusion to neuromuscular diseases.

Implication of N-glycolylneuraminic acid in regulation of cell adhesiveness of C2C12 myoblast cells during differentiation into myotube cells

A transition of sialic acid (Sia) species on GM3 ganglioside from N-acetylneuraminic acid (Neu5Ac) to N-glycolylneuraminic acid (Neu5Gc) takes place in mouse C2C12 myoblast cells during their differentiation into myotube cells. However, the meaning of this Sia transition remains unclear. This study thus aims to gain a functional insight into this phenomenon. The following lines of evidence show that the increased de novo synthesis of Neu5Gc residues in differentiating myoblast cells promotes adhesiveness of the cells, which is beneficial for promotion of differentiation. First, the Sia transition occurred even in the C2C12 cells cultured in serum-free medium, indicating that it happens through de novo synthesis of Neu5Gc. Second, GM3(Neu5Gc) was localized in myoblast cells, but not in myotube cells, and related to expression of the CMP-Neu5Ac hydroxylase (CMAH) gene. Notably, expression of CMAH precedes myotube formation not only in differentiating C2C12 cells, but also in mouse developing embryos. Since the myoblast cells were attached on the dish surface more strongly than the myotube cells, expression of GM3(Neu5Gc) may be related to the surface attachment of the myoblast cells. Third, exogenous Neu5Gc, but not Neu5Ac, promoted differentiation of C2C12 cells, thus increasing the number of cells committed to fuse with each other. Fourth, the CMAH-transfected C2C12 cells were attached on the gelatin-coated surface much more rapidly than the mock-cells, suggesting that the expression of CMAH promotes cell adhesiveness through the expression of Neu5Gc.

Mitochondrial micropeptide STMP1 enhances mitochondrial fission to promote tumor metastasis

Micropeptides are a recently discovered class of molecules that play vital roles in various cellular processes, including differentiation, proliferation, and apoptosis. Here, we sought to identify cancer-associated micropeptides and to uncover their mechanistic functions. A micropeptide named short trans-membrane protein 1 (STMP1) that localizes at the inner mitochondrial membrane was identified to be upregulated in various cancer types and associated with metastasis and recurrence of hepatocellular carcinoma. Both gain- and loss-of-function studies revealed that STMP1 increased dynamin-related protein 1 (DRP1) activation to promote mitochondrial fission and enhanced migration of tumor cells. STMP1 silencing inhibited in vivo tumor metastasis in xenograft mouse models. Overexpression of STMP1 led to redistribution of mitochondria to the leading edge of cells and enhanced lamellipodia formation. Treatment with a DRP1 inhibitor abrogated the promotive effect of STMP1 on mitochondrial fission, lamellipodia formation, and tumor cell migration in vitro and metastasis in vivo. Furthermore, STMP1 interacted with myosin heavy chain 9 (MYH9), the subunit of non-muscle myosin II, and silencing MYH9 abrogated STMP1-induced DRP1 activation, mitochondrial fission, and cell migration. Collectively, this study identifies STMP1 as a critical regulator of metastasis and a novel unit of the mitochondrial fission protein machinery, providing a potential therapeutic target for treating metastases.

Generation of Cancer Stem/Initiating Cells by Cell-Cell Fusion

CS/ICs have raised great expectations in cancer research and therapy, as eradication of this key cancer cell type is expected to lead to a complete cure. Unfortunately, the biology of CS/ICs is rather complex, since no common CS/IC marker has yet been identified. Certain surface markers or ALDH1 expression can be used for detection, but some studies indicated that cancer cells exhibit a certain plasticity, so CS/ICs can also arise from non-CS/ICs. Another problem is intratumoral heterogeneity, from which it can be inferred that different CS/IC subclones must be present in the tumor. Cell–cell fusion between cancer cells and normal cells, such as macrophages and stem cells, has been associated with the generation of tumor hybrids that can exhibit novel properties, such as an enhanced metastatic capacity and even CS/IC properties. Moreover, cell–cell fusion is a complex process in which parental chromosomes are mixed and randomly distributed among daughter cells, resulting in multiple, unique tumor hybrids. These, if they have CS/IC properties, may contribute to the heterogeneity of the CS/IC pool. In this review, we will discuss whether cell–cell fusion could also lead to the origin of different CS/ICs that may expand the overall CS/IC pool in a primary tumor.

Proteomic-driven identification of short open reading frame-encoded peptides

Accumulating evidence has shown that a large number of short open reading frames (sORFs) also have the ability to encode proteins. The discovery of sORFs opens up a new research area, leading to the identification and functional study of sORF encoded peptides (SEPs) at the omics level. Besides bioinformatics prediction and ribosomal profiling, mass spectrometry (MS) has become a significant tool as it directly detects the sequence of SEPs. Though MS-based proteomics methods have proved to be effective for qualitative and quantitative analysis of SEPs, the detection of SEPs is still a great challenge due to their low abundance and short sequence. To illustrate the progress in method development, we described and discussed the main steps of large-scale proteomics identification of SEPs, including SEP extraction and enrichment, MS detection, data processing and quality control, quantification, and function prediction and validation methods. This article is protected by copyright. All rights reserved.

Long Noncoding RNAs in CNS Myelination and Disease

Myelination by oligodendrocytes is crucial for neuronal survival and function, and defects in myelination or failure in myelin repair can lead to axonal degeneration and various neurological diseases. At present, the factors that promote myelination and overcome the remyelination block in demyelinating diseases are poorly defined. Although the roles of protein-coding genes in oligodendrocyte differentiation have been extensively studied, the majority of the mammalian genome is transcribed into noncoding RNAs, and the functions of these molecules in myelination are poorly characterized. Long noncoding RNAs (lncRNAs) regulate transcription at multiple levels, providing spatiotemporal control and robustness for cell type-specific gene expression and physiological functions. lncRNAs have been shown to regulate neural cell-type specification, differentiation, and maintenance of cell identity, and dysregulation of lncRNA function has been shown to contribute to neurological diseases. In this review, we discuss recent advances in our understanding of the functions of lncRNAs in oligodendrocyte development and myelination as well their roles in neurological diseases and brain tumorigenesis. A more systematic characterization of lncRNA functional networks will be instrumental for a better understanding of CNS myelination, myelin disorders, and myelin repair.

The dark proteome: translation from noncanonical open reading frames

Omics-based technologies have revolutionized our understanding of the coding potential of the genome. In particular, these studies revealed widespread unannotated open reading frames (ORFs) throughout genomes and that these regions have the potential to encode novel functional (micro-)proteins and/or hold regulatory roles. However, despite their genomic prevalence, relatively few of these noncanonical ORFs have been functionally characterized, likely in part due to their under-recognition by the broader scientific community. The few that have been investigated in detail have demonstrated their essentiality in critical and divergent biological processes. As such, here we aim to discuss recent advances in understanding the diversity of noncanonical ORFs and their roles, as well as detail biologically important examples within the context of the mammalian genome.

Cell Fusion-Related Proteins and Signaling Pathways, and Their Roles in the Development and Progression of Cancer

Cell fusion is involved in many physiological and pathological processes, including gamete binding, and cancer development. The basic processes of cell fusion include membrane fusion, cytoplasmic mixing, and nuclear fusion. Cell fusion is regulated by different proteins and signaling pathways. Syncytin-1, syncytin-2, glial cell missing 1, galectin-1 and other proteins (annexins, myomaker, myomerger etc.) involved in cell fusion via the cyclic adenosine-dependent protein kinase A, mitogen-activated protein kinase, wingless/integrase-1, and c-Jun N-terminal kinase signaling pathways. In the progression of malignant tumors, cell fusion is essential during the organ-specific metastasis, epithelial-mesenchymal transformation, the formation of cancer stem cells (CSCs), cancer angiogenesis and cancer immunity. In addition, diploid cells can be induced to form polyploid giant cancer cells (PGCCs) via cell fusion under many kinds of stimuli, including cobalt chloride, chemotherapy, radiotherapy, and traditional Chinese medicine. PGCCs have CSC-like properties, and the daughter cells derived from PGCCs have a mesenchymal phenotype and exhibit strong migration, invasion, and proliferation abilities. Therefore, exploring the molecular mechanisms of cell fusion can enable us better understand the development of malignant tumors. In this review, the basic process of cell fusion and its significance in cancer is discussed.

Chondroitin sulfate E downregulates N-cadherin and suppresses myotube formation

Myogenesis, the formation of muscle fibers, is affected by certain glycoproteins, including chondroitin sulfate (CS), which are involved in various cellular processes. We aimed to investigate the mechanism underlying CS-E-induced suppression of myotube formation using the myoblast cell line C2C12. Differentiated cells treated with 0.1 mg/ml CS-E for nine days showed multinucleated and rounded myotubes with myosin heavy chain positivity. No difference was found between the CS-E-treated group with rounded myotubes and CS (−) controls with elongated myotubes in the levels of phospho-cofilin, a protein involved in the dynamics of actin cytoskeleton. Interestingly, N-cadherin, which is involved in the gene expression of myoblast fusion factors (myomaker and myomixer), was significantly downregulated at both the mRNA and protein levels following CS-E treatment. These results suggest that N-cadherin downregulation is one of the mechanisms underlying the CS-E-induced suppression of myotube formation.

Microproteins in skeletal muscle: hidden keys in muscle physiology

Recent advances in the transcriptomics, translatomics, and proteomics have led us to the exciting new world of functional endogenous microproteins. These microproteins have a small size and are derived from small open reading frames (smORFs) of RNAs previously annotated as non-coding (e.g. lncRNAs and circRNAs) as well as from untranslated regions and canonical mRNAs. The presence of these microproteins reveals a much larger translatable portion of the genome, shifting previously defined dogmas and paradigms. These findings affect our view of organisms as a whole, including skeletal muscle tissue. Emerging evidence demonstrates that several smORF-derived microproteins play crucial roles during muscle development (myogenesis), maintenance, and regeneration, as well as lipid and glucose metabolism and skeletal muscle bioenergetics. These microproteins are also involved in processes including physical activity capacity, cellular stress, and muscular-related diseases (i.e. myopathy, cachexia, atrophy, and muscle wasting). Given the role of these small proteins as important key regulators of several skeletal muscle processes, there are rich prospects for the discovery of new microproteins and possible therapies using synthetic microproteins.

Defining and identifying satellite cell-opathies within muscular dystrophies and myopathies

Muscular dystrophies and congenital myopathies arise from specific genetic mutations causing skeletal muscle weakness that reduces quality of life. Muscle health relies on resident muscle stem cells called satellite cells, which enable life-course muscle growth, maintenance, repair and regeneration. Such tuned plasticity gradually diminishes in muscle diseases, suggesting compromised satellite cell function. A central issue however, is whether the pathogenic mutation perturbs satellite cell function directly and/or indirectly via an increasingly hostile microenvironment as disease progresses. Here, we explore the effects on satellite cell function of pathogenic mutations in genes (myopathogenes) that associate with muscle disorders, to evaluate clinical and muscle pathological hallmarks that define dysfunctional satellite cells. We deploy transcriptomic analysis and comparison between muscular dystrophies and myopathies to determine the contribution of satellite cell dysfunction using literature, expression dynamics of myopathogenes and their response to the satellite cell regulator PAX7. Our multimodal approach extends current pathological classifications to define Satellite Cell-opathies: muscle disorders in which satellite cell dysfunction contributes to pathology. Primary Satellite Cell-opathies are conditions where mutations in a myopathogene directly affect satellite cell function, such as in Progressive Congenital Myopathy with Scoliosis (MYOSCO) and Carey-Fineman-Ziter Syndrome (CFZS). Primary satellite cell-opathies are generally characterised as being congenital with general hypotonia, and specific involvement of respiratory, trunk and facial muscles, although serum CK levels are usually within the normal range. Secondary Satellite Cell-opathies have mutations in myopathogenes that affect both satellite cells and muscle fibres. Such classification aids diagnosis and predicting probable disease course, as well as informing on treatment and therapeutic development.

Amphioxus muscle transcriptomes reveal vertebrate-like myoblast fusion genes and a highly conserved role of insulin signalling in the metabolism of muscle

BACKGROUND

The formation and functioning of muscles are fundamental aspects of animal biology, and the evolution of 'muscle genes' is central to our understanding of this tissue. Feeding-fasting-refeeding experiments have been widely used to assess muscle cellular and metabolic responses to nutrition. Though these studies have focused on vertebrate models and only a few invertebrate systems, they have found similar processes are involved in muscle degradation and maintenance. Motivation for these studies stems from interest in diseases whose pathologies involve muscle atrophy, a symptom also triggered by fasting, as well as commercial interest in the muscle mass of animals kept for consumption. Experimentally modelling atrophy by manipulating nutritional state causes muscle mass to be depleted during starvation and replenished with refeeding so that the genetic mechanisms controlling muscle growth and degradation can be understood.

RESULTS

Using amphioxus, the earliest branching chordate lineage, we address the gap in previous work stemming from comparisons between distantly related vertebrate and invertebrate models. Our amphioxus feeding-fasting-refeeding muscle transcriptomes reveal a highly conserved myogenic program and that the pro-orthologues of many vertebrate myoblast fusion genes were present in the ancestral chordate, despite these invertebrate chordates having unfused mononucleate myocytes. We found that genes differentially expressed between fed and fasted amphioxus were orthologous to the genes that respond to nutritional state in vertebrates. This response is driven in a large part by the highly conserved IGF/Akt/FOXO pathway, where depleted nutrient levels result in activation of FOXO, a transcription factor with many autophagy-related gene targets.

CONCLUSION

Reconstruction of these gene networks and pathways in amphioxus muscle provides a key point of comparison between the distantly related groups assessed thus far, significantly refining the reconstruction of the ancestral state for chordate myoblast fusion genes and identifying the extensive role of duplicated genes in the IGF/Akt/FOXO pathway across animals. Our study elucidates the evolutionary trajectory of muscle genes as they relate to the increased complexity of vertebrate muscles and muscle development.

The cardiac-enriched microprotein mitolamban regulates mitochondrial respiratory complex assembly and function in mice

Microproteins are a growing class of versatile small proteins previously overlooked by standard gene annotation methods due to their small size. Here we characterize mitolamban as a cardiac-enriched inner mitochondrial membrane–localized microprotein, which interacts with complex III of the electron transport chain and contributes to complex III assembly and function. Mitolamban gene deletion in mice leads to a reduction in complex III activity and metabolic perturbations in the heart that are consistent with complex III deficiency, as well as altered complex III assembly into respiratory supercomplexes. These findings define a functional role for mitolamban in the heart and highlight the importance of microproteins in regulating mitochondrial function and cardiomyocyte biology.

Emerging evidence indicates that a subset of RNA molecules annotated as noncoding contain short open reading frames that code for small functional proteins called microproteins, which have largely been overlooked due to their small size. To search for cardiac-expressed microproteins, we used a comparative genomics approach and identified mitolamban (Mtlbn) as a highly conserved 47-amino acid transmembrane protein that is abundantly expressed in the heart. Mtlbn localizes specifically to the inner mitochondrial membrane where it interacts with subunits of complex III of the electron transport chain and with mitochondrial respiratory supercomplexes. Genetic deletion of Mtlbn in mice altered complex III assembly dynamics and reduced complex III activity. Unbiased metabolomic analysis of heart tissue from Mtlbn knockout mice further revealed an altered metabolite profile consistent with deficiencies in complex III activity. Cardiac-specific Mtlbn overexpression in transgenic (TG) mice induced cardiomyopathy with histological, biochemical, and ultrastructural pathologic features that contributed to premature death. Metabolomic analysis and biochemical studies indicated that hearts from Mtlbn TG mice exhibited increased oxidative stress and mitochondrial dysfunction. These findings reveal Mtlbn as a cardiac-expressed inner mitochondrial membrane microprotein that contributes to mitochondrial electron transport chain activity through direct association with complex III and the regulation of its assembly and function.

Magnesium Homeostasis in Myogenic Differentiation-A Focus on the Regulation of TRPM7, MagT1 and SLC41A1 Transporters

Magnesium (Mg) is essential for skeletal muscle health, but little is known about the modulation of Mg and its transporters in myogenic differentiation. Here, we show in C2C12 murine myoblasts that Mg concentration fluctuates during their differentiation to myotubes, declining early in the process and reverting to basal levels once the cells are differentiated. The level of the Mg transporter MagT1 decreases at early time points and is restored at the end of the process, suggesting a possible role in the regulation of intracellular Mg concentration. In contrast, TRPM7 is rapidly downregulated and remains undetectable in myotubes. The reduced amounts of TRPM7 and MagT1 are due to autophagy, one of the proteolytic systems activated during myogenesis and essential for the membrane fusion process. Moreover, we investigated the levels of SLC41A1, which increase once cells are differentiated, mainly through transcriptional regulation. In conclusion, myogenesis is associated with alterations of Mg homeostasis finely tuned through the modulation of MagT1, TRPM7 and SLC41A1.

Drosophila melanogaster: A Model System to Study Distinct Genetic Programs in Myoblast Fusion

Muscle fibers are multinucleated cells that arise during embryogenesis through the fusion of mononucleated myoblasts. Myoblast fusion is a lifelong process that is crucial for the growth and regeneration of muscles. Understanding the molecular mechanism of myoblast fusion may open the way for novel therapies in muscle wasting and weakness. Recent reports in Drosophila and mammals have provided new mechanistic insights into myoblast fusion. In Drosophila, muscle formation occurs twice: during embryogenesis and metamorphosis. A fundamental feature is the formation of a cell–cell communication structure that brings the apposing membranes into close proximity and recruits possible fusogenic proteins. However, genetic studies suggest that myoblast fusion in Drosophila is not a uniform process. The complexity of the players involved in myoblast fusion can be modulated depending on the type of muscle that is formed. In this review, we introduce the different types of multinucleated muscles that form during Drosophila development and provide an overview in advances that have been made to understand the mechanism of myoblast fusion. Finally, we will discuss conceptual frameworks in cell–cell fusion in Drosophila and mammals.

TMEM8C-mediated fusion is regionalized and regulated by NOTCH signalling during foetal myogenesis

The location and regulation of fusion events within skeletal muscles during development remain unknown. Using the fusion marker myomaker (Mymk), named TMEM8C in chicken, as a readout of fusion, we identified a co-segregation of TMEM8C-positive cells and MYOG-positive cells in single-cell RNA-sequencing datasets of limbs from chicken embryos. We found that TMEM8C transcripts, MYOG transcripts and the fusion-competent MYOG-positive cells were preferentially regionalized in central regions of foetal muscles. We also identified a similar regionalization for the gene encoding the NOTCH ligand JAG2 along with an absence of NOTCH activity in TMEM8C+ fusion-competent myocytes. NOTCH function in myoblast fusion had not been addressed so far. We analysed the consequences of NOTCH inhibition for TMEM8C expression and myoblast fusion during foetal myogenesis in chicken embryos. NOTCH inhibition increased myoblast fusion and TMEM8C expression and released the transcriptional repressor HEYL from the TMEM8C regulatory regions. These results identify a regionalization of TMEM8C-dependent fusion and a molecular mechanism underlying the fusion-inhibiting effect of NOTCH in foetal myogenesis. The modulation of NOTCH activity in the fusion zone could regulate the flux of fusion events.

Nano-sized graphene oxide coated nanopillars on microgroove polymer arrays that enhance skeletal muscle cell differentiation

The degeneration or loss of skeletal muscles, which can be caused by traumatic injury or disease, impacts most aspects of human activity. Among various techniques reported to regenerate skeletal muscle tissue, controlling the external cellular environment has been proven effective in guiding muscle differentiation. In this study, we report a nano-sized graphene oxide (sGO)-modified nanopillars on microgroove hybrid polymer array (NMPA) that effectively controls skeletal muscle cell differentiation. sGO-coated NMPA (sG-NMPA) were first fabricated by sequential laser interference lithography and microcontact printing methods. To compensate for the low adhesion property of polydimethylsiloxane (PDMS) used in this study, graphene oxide (GO), a proven cytophilic nanomaterial, was further modified. Among various sizes of GO, sGO (< 10 nm) was found to be the most effective not only for coating the surface of the NM structure but also for enhancing the cell adhesion and spreading on the fabricated substrates. Remarkably, owing to the micro-sized line patterns that guide cellular morphology to an elongated shape and because of the presence of sGO-modified nanostructures, mouse myoblast cells (C2C12) were efficiently differentiated into skeletal muscle cells on the hybrid patterns, based on the myosin heavy chain expression levels. Therefore, the developed sGO coated polymeric hybrid pattern arrays can serve as a potential platform for rapid and highly efficient in vitro muscle cell generation.

The Small Open Reading Frame-Encoded Peptides: Advances in Methodologies and Functional Studies

Small open reading frames (sORFs) are an important class of genes with less than 100 codons. They were historically annotated as noncoding or even junk sequences. In recent years, accumulating evidence suggests that sORFs could encode a considerable number of polypeptides, many of which play important roles in both physiology and disease pathology. However, it has been technically challenging to directly detect sORF-encoded peptides (SEPs). Here, we discuss the latest advances in methodologies for identifying SEPs with mass spectrometry, as well as the progress on functional studies of SEPs.

Cell-Cell Fusion Mediated by Viruses and HERV-Derived Fusogens in Cancer Initiation and Progression

Cell fusion is a well-known, but still scarcely understood biological phenomenon, which might play a role in cancer initiation, progression and formation of metastases. Although the merging of two (cancer) cells appears simple, the entire process is highly complex, energy-dependent and tightly regulated. Among cell fusion-inducing and -regulating factors, so-called fusogens have been identified as a specific type of proteins that are indispensable for overcoming fusion-associated energetic barriers and final merging of plasma membranes. About 8% of the human genome is of retroviral origin and some well-known fusogens, such as syncytin-1, are expressed by human (cancer) cells. Likewise, enveloped viruses can enable and facilitate cell fusion due to evolutionarily optimized fusogens, and are also capable to induce bi- and multinucleation underlining their fusion capacity. Moreover, multinucleated giant cancer cells have been found in tumors derived from oncogenic viruses. Accordingly, a potential correlation between viruses and fusogens of human endogenous retroviral origin in cancer cell fusion will be summarized in this review.

Control of satellite cell function in muscle regeneration and its disruption in ageing

Skeletal muscle contains a designated population of adult stem cells, called satellite cells, which are generally quiescent. In homeostasis, satellite cells proliferate only sporadically and usually by asymmetric cell division to replace myofibres damaged by daily activity and maintain the stem cell pool. However, satellite cells can also be robustly activated upon tissue injury, after which they undergo symmetric divisions to generate new stem cells and numerous proliferating myoblasts that later differentiate to muscle cells (myocytes) to rebuild the muscle fibre, thereby supporting skeletal muscle regeneration. Recent discoveries show that satellite cells have a great degree of population heterogeneity, and that their cell fate choices during the regeneration process are dictated by both intrinsic and extrinsic mechanisms. Extrinsic cues come largely from communication with the numerous distinct stromal cell types in their niche, creating a dynamically interactive microenvironment. This Review discusses the role and regulation of satellite cells in skeletal muscle homeostasis and regeneration. In particular, we highlight the cell-intrinsic control of quiescence versus activation, the importance of satellite cell-niche communication, and deregulation of these mechanisms associated with ageing. The increasing understanding of how satellite cells are regulated will help to advance muscle regeneration and rejuvenation therapies.

Filopodia powered by class x myosin promote fusion of mammalian myoblasts

Skeletal muscle fibers are multinucleated cellular giants formed by the fusion of mononuclear myoblasts. Several molecules involved in myoblast fusion have been discovered, and finger-like projections coincident with myoblast fusion have also been implicated in the fusion process. The role of these cellular projections in muscle cell fusion was investigated herein. We demonstrate that these projections are filopodia generated by class X myosin (Myo10), an unconventional myosin motor protein specialized for filopodia. We further show that Myo10 is highly expressed by differentiating myoblasts, and Myo10 ablation inhibits both filopodia formation and myoblast fusion in vitro. In vivo, Myo10 labels regenerating muscle fibers associated with Duchenne muscular dystrophy and acute muscle injury. In mice, conditional loss of Myo10 from muscle-resident stem cells, known as satellite cells, severely impairs postnatal muscle regeneration. Furthermore, the muscle fusion proteins Myomaker and Myomixer are detected in myoblast filopodia. These data demonstrate that Myo10-driven filopodia facilitate multinucleated mammalian muscle formation.

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

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

Lnc-GD2H Promotes Proliferation by Forming a Feedback Loop With c-Myc and Enhances Differentiation Through Interacting With NACA to Upregulate Myog in C2C12 Myoblasts

In the present study, the roles of a novel long non-coding RNA (lncRNA), lnc-GD2H, in promoting C2C12 myoblast proliferation and differentiation and muscle regeneration were investigated by quantitative polymerase chain reaction, western blotting, Cell Counting Kit-8, 5-ethynyl-2′-deoxyuridine (EdU), immunofluorescence staining, luciferase reporter, mass spectrometry, pulldown, chromatin immunoprecipitation, RNA immunoprecipitation assay, wound healing assays, and cardiotoxin (CTX)-induced muscle injury assays. It was observed that lnc-GD2H promoted myoblast proliferation as evidenced by the enhancement of the proliferation markers c-Myc, CDK2, CDK4, and CDK6, percentage of EdU-positive cells, and rate of cell survival during C2C12 myoblast proliferation. Additional experiments confirmed that c-Myc bound to the lnc-GD2H promoter and regulated its transcription. lnc-GD2H promoted cell differentiation with enhanced MyHC immunostaining as well as increased expression of the myogenic marker genes myogenin (Myog), Mef2a, and Mef2c during myoblast differentiation. Additional assays indicated that lnc-GD2H interacted with NACA which plays a role of transcriptional regulation in myoblast differentiation, and the enrichment of NACA at the Myog promoter was impaired by lnc-GD2H. Furthermore, inhibition of lnc-GD2H impaired muscle regeneration after CTX-induced injury in mice. lnc-GD2H facilitated the expression of proliferating marker genes and formed a feedback loop with c-Myc during myoblast proliferation. In differentiating myoblasts, lnc-GD2H interacted with NACA to relieve the inhibitory effect of NACA on Myog, facilitating Myog expression to promote differentiation. The results provide evidence for the role of lncRNAs in muscle regeneration and are useful for developing novel therapeutic targets for muscle disorders.

Hybrid Formation and Fusion of Cancer Cells In Vitro and In Vivo

Simple Summary

Cell fusion as a fundamental biological process is required for various physiological processes, including fertilization, placentation, myogenesis, osteoclastogenesis, and wound healing/tissue regeneration. However, cell fusion is also observed during pathophysiological processes like tumor development. Mesenchymal stroma/stem-like cells (MSC) which play an important role within the tumor microenvironment like other cell types such as macrophages can closely interact and hybridize with cancer cells. The formation of cancer hybrid cells can involve various different mechanisms whereby the genomic parts of the hybrid cells require rearrangement to form a new functional hybrid cell. The fusion of cancer cells with neighboring cell types may represent an important mechanism during tumor development since cancer hybrid cells are detectable in various tumor tissues. During this rare event with resulting genomic instability the cancer hybrid cells undergo a post-hybrid selection process (PHSP) to reorganize chromosomes of the two parental nuclei whereby the majority of the hybrid population undergoes cell death. The remaining cancer hybrid cells survive by displaying altered properties within the tumor tissue.

Abstract

The generation of cancer hybrid cells by intra-tumoral cell fusion opens new avenues for tumor plasticity to develop cancer stem cells with altered properties, to escape from immune surveillance, to change metastatic behavior, and to broaden drug responsiveness/resistance. Genomic instability and chromosomal rearrangements in bi- or multinucleated aneuploid cancer hybrid cells contribute to these new functions. However, the significance of cell fusion in tumorigenesis is controversial with respect to the low frequency of cancer cell fusion events and a clonal advantage of surviving cancer hybrid cells following a post-hybrid selection process. This review highlights alternative processes of cancer hybrid cell development such as entosis, emperipolesis, cannibalism, therapy-induced polyploidization/endoreduplication, horizontal or lateral gene transfer, and focusses on the predominant mechanisms of cell fusion. Based upon new properties of cancer hybrid cells the arising clinical consequences of the subsequent tumor heterogeneity after cancer cell fusion represent a major therapeutic challenge.

Cilia, Centrosomes and Skeletal Muscle

Primary cilia are non-motile, cell cycle-associated organelles that can be found on most vertebrate cell types. Comprised of microtubule bundles organised into an axoneme and anchored by a mature centriole or basal body, primary cilia are dynamic signalling platforms that are intimately involved in cellular responses to their extracellular milieu. Defects in ciliogenesis or dysfunction in cilia signalling underlie a host of developmental disorders collectively referred to as ciliopathies, reinforcing important roles for cilia in human health. Whilst primary cilia have long been recognised to be present in striated muscle, their role in muscle is not well understood. However, recent studies indicate important contributions, particularly in skeletal muscle, that have to date remained underappreciated. Here, we explore recent revelations that the sensory and signalling functions of cilia on muscle progenitors regulate cell cycle progression, trigger differentiation and maintain a commitment to myogenesis. Cilia disassembly is initiated during myoblast fusion. However, the remnants of primary cilia persist in multi-nucleated myotubes, and we discuss their potential role in late-stage differentiation and myofiber formation. Reciprocal interactions between cilia and the extracellular matrix (ECM) microenvironment described for other tissues may also inform on parallel interactions in skeletal muscle. We also discuss emerging evidence that cilia on fibroblasts/fibro-adipogenic progenitors and myofibroblasts may influence cell fate in both a cell autonomous and non-autonomous manner with critical consequences for skeletal muscle ageing and repair in response to injury and disease. This review addresses the enigmatic but emerging role of primary cilia in satellite cells in myoblasts and myofibers during myogenesis, as well as the wider tissue microenvironment required for skeletal muscle formation and homeostasis.

Genome-Wide Association Study Based on Random Regression Model Reveals Candidate Genes Associated with Longitudinal Data in Chinese Simmental Beef Cattle

Simple Summary

Genome-wide association study (GWAS) has become the main approach for detecting functional genes that affects complex traits. For growth traits, the conventional GWAS method can only deal with the single-record traits observed at specific time points, rather than the longitudinal traits measured at multiple time points. Previous studies have reported the random regression model (RRM) for longitudinal data could overcome the limitation of the traditional GWAS model. Here, we present an association analysis based on RRM (GWAS-RRM) for 808 Chinese Simmental beef cattle at four stages of age. Ultimately, 37 significant single-nucleotide polymorphisms (SNPs) and several important candidate genes were screened to be associated with the body weight. Enrichment analysis showed these genes were significantly enriched in the signaling transduction pathway and lipid metabolism. This study not only offers a further understanding of the genetic basis for growth traits in beef cattle, but also provides a robust analytics tool for longitudinal traits in various species.

Abstract

Body weight (BW) is an important longitudinal trait that directly described the growth gain of bovine in production. However, previous genome-wide association study (GWAS) mainly focused on the single-record traits, with less attention paid to longitudinal traits. Compared with traditional GWAS models, the association studies based on the random regression model (GWAS-RRM) have better performance in the control of the false positive rate through considering time-stage effects. In this study, the BW trait data were collected from 808 Chinese Simmental beef cattle aged 0, 6, 12, and 18 months, then we performed a GWAS-RRM to fit the time-varied SNP effect. The results showed a total of 37 significant SNPs were associated with BW. Gene functional annotation and enrichment analysis indicated FGF4, ANGPT4, PLA2G4A, and ITGA5 were promising candidate genes for BW. Moreover, these genes were significantly enriched in the signaling transduction pathway and lipid metabolism. These findings will provide prior molecular information for bovine gene-based selection, as well as facilitate the extensive application of GWAS-RRM in domestic animals.

Feedback regulation of Notch signaling and myogenesis connected by MyoD-Dll1 axis

Muscle precursor cells known as myoblasts are essential for muscle development and regeneration. Notch signaling is an ancient intercellular communication mechanism that plays prominent roles in controlling the myogenic program of myoblasts. Currently whether and how the myogenic cues feedback to refine Notch activities in these cells are largely unknown. Here, by mouse and human gene gain/loss-of-function studies, we report that MyoD directly turns on the expression of Notch-ligand gene Dll1 which activates Notch pathway to prevent precautious differentiation in neighboring myoblasts, while autonomously inhibits Notch to facilitate a myogenic program in Dll1 expressing cells. Mechanistically, we studied cis-regulatory DNA motifs underlying the MyoD-Dll1-Notch axis in vivo by characterizing myogenesis of a novel E-box deficient mouse model, as well as in human cells through CRISPR-mediated interference. These results uncovered the crucial transcriptional mechanism that mediates the reciprocal controls of Notch and myogenesis.

A systematic evaluation of the computational tools for lncRNA identification

The computational identification of long non-coding RNAs (lncRNAs) is important to study lncRNAs and their functions. Despite the existence of many computation tools for lncRNA identification, to our knowledge, there is no systematic evaluation of these tools on common datasets and no consensus regarding their performance and the importance of the features used. To fill this gap, in this study, we assessed the performance of 17 tools on several common datasets. We also investigated the importance of the features used by the tools. We found that the deep learning-based tools have the best performance in terms of identifying lncRNAs, and the peptide features do not contribute much to the tool accuracy. Moreover, when the transcripts in a cell type were considered, the performance of all tools significantly dropped, and the deep learning-based tools were no longer as good as other tools. Our study will serve as an excellent starting point for selecting tools and features for lncRNA identification.

Establishment of myoblast cell line and identification of key genes regulating myoblast differentiation in a marine teleost, Sebastes schlegelii

Skeletal myoblasts are activated satellite cells capable of proliferation and differentiation. Studies on mammalian myoblast differentiation and myogenesis could be carried out in vitro thanks to the availability of mouse myoblast cell line C2C12. Lacking of muscle cell line hinders the studies of teleost fish myogenesis. Here, we established a continuous skeletal muscle cell line from juvenile rockfish (Sebastes schlegelii) muscle using explant method and subcultured more than 50 passages for over 150 days. Stable expression of myoblast-specific marker, MyoD (myoblast determination protein) and the potential of differentiation into multi-nucleated skeletal myotubes upon induction suggested the cell line were predominately composed of myoblasts. Transcriptome analysis revealed a total of 4375 genes differentially expressed at four time points after the switch to differentiation medium, which were mainly involved in proliferation and differentiation of myoblasts. KIF22 (kinesin family member 22) and POLA1 (DNA polymerase alpha 1) were identified as the key genes involved in fish myoblast proliferation whereas MYL3 (myosin light chain 3) and TNNT2 (troponin T2) were determined as the crucial genes responsible for differentiation. In all, the continuous myoblasts cultured in this study provided a cell platform for future studies on marine fish myoblast differentiation and myogenesis. The molecular process of myoblast differentiation revealed in this study will open a window into the understanding of indeterminate muscle growth of large teleost.

Survival motor neuron deficiency slows myoblast fusion through reduced myomaker and myomixer expression

BACKGROUND

Spinal muscular atrophy is an inherited neurodegenerative disease caused by insufficient levels of the survival motor neuron (SMN) protein. Recently approved treatments aimed at increasing SMN protein levels have dramatically improved patient survival and have altered the disease landscape. While restoring SMN levels slows motor neuron loss, many patients continue to have smaller muscles and do not achieve normal motor milestones. While timing of treatment is important, it remains unclear why SMN restoration is insufficient to fully restore muscle size and function. We and others have shown that SMN-deficient muscle precursor cells fail to efficiently fuse into myotubes. However, the role of SMN in myoblast fusion is not known.

METHODS

In this study, we show that SMN-deficient myoblasts readily fuse with wild-type myoblasts, demonstrating fusion competency. Conditioned media from wild type differentiating myoblasts do not rescue the fusion deficit of SMN-deficient cells, suggesting that compromised fusion may primarily be a result of altered membrane dynamics at the cell surface. Transcriptome profiling of skeletal muscle from SMN-deficient mice revealed altered expression of cell surface fusion molecules. Finally, using cell and mouse models, we investigate if myoblast fusion can be rescued in SMN-deficient myoblast and improve the muscle pathology in SMA mice.

RESULTS

We found reduced expression of the muscle fusion proteins myomaker (P = 0.0060) and myomixer (P = 0.0051) in the muscle of SMA mice. Suppressing SMN expression in C2C12 myoblast cells reduces expression of myomaker (35% reduction; P < 0.0001) and myomixer, also known as myomerger and minion, (30% reduction; P < 0.0001) and restoring SMN levels only partially restores myomaker and myomixer expression. Ectopic expression of myomixer improves myofibre number (55% increase; P = 0.0006) and motor function (35% decrease in righting time; P = 0.0089) in SMA model mice and enhances motor function (82% decrease in righting time; P < 0.0001) and extends survival (28% increase; P < 0.01) when administered in combination with an antisense oligonucleotide that increases SMN protein levels.

CONCLUSIONS

Here, we identified reduced expression of muscle fusion proteins as a key factor in the fusion deficits of SMN-deficient myoblasts. This discovery provides a novel target to improve SMA muscle pathology and motor function, which in combination with SMN increasing therapy could enhance clinical outcomes for SMA patients.

From Yeast to Mammals, the Nonsense-Mediated mRNA Decay as a Master Regulator of Long Non-Coding RNAs Functional Trajectory

The Nonsense-Mediated mRNA Decay (NMD) has been classically viewed as a translation-dependent RNA surveillance pathway degrading aberrant mRNAs containing premature stop codons. However, it is now clear that mRNA quality control represents only one face of the multiple functions of NMD. Indeed, NMD also regulates the physiological expression of normal mRNAs, and more surprisingly, of long non-coding (lnc)RNAs. Here, we review the different mechanisms of NMD activation in yeast and mammals, and we discuss the molecular bases of the NMD sensitivity of lncRNAs, considering the functional roles of NMD and of translation in the metabolism of these transcripts. In this regard, we describe several examples of functional micropeptides produced from lncRNAs. We propose that translation and NMD provide potent means to regulate the expression of lncRNAs, which might be critical for the cell to respond to environmental changes.

LncRNA-Encoded Short Peptides Identification Using Feature Subset Recombination and Ensemble Learning

Long non-coding RNA (lncRNA), which is a type of non-coding RNA, was reported to contain short open reading frames (sORFs). SORFs-encoded short peptides (SEPs) have been demonstrated to play a crucial role in regulating the biological processes such as growth, development, and resistance response. The identification of SEPs is vital to further understanding their function. However, there is still a lack of methods for identifying SEPs effectively and rapidly. In this study, a novel method for lncRNA-encoded short peptides identification based on feature subset recombination and ensemble learning, lncPepid, is developed. lncPepid transforms the data of Zea mays and Arabidopsis thaliana into hybrid features from two aspects including sequence composition and physicochemical properties separately. It optimizes hybrid features by proposing a novel weighted iteration-based feature selection method to recombine a stable subset that characterizes SEPs effectively. Different classification models with different optimized features are constructed and tested separately. The outputs of the optimal models are integrated for ensemble classification to improve efficiency. Experimental results manifest that the geometric mean of sensitivity and specificity of lncPepid is about 70% on the identification of functional SEPs derived from multiple species. It is an effective and rapid method for the identification of lncRNA-encoded short peptides. This study can be extended to the research on SEPs from other species and have crucial implications for further findings and studies of functional genomics.

The Bromodomains of the mammalian SWI/SNF (mSWI/SNF) ATPases Brahma (BRM) and Brahma Related Gene 1 (BRG1) promote chromatin interaction and are critical for skeletal muscle differentiation

Skeletal muscle regeneration is mediated by myoblasts that undergo epigenomic changes to establish the gene expression program of differentiated myofibers. mSWI/SNF chromatin remodeling enzymes coordinate with lineage-determining transcription factors to establish the epigenome of differentiated myofibers. Bromodomains bind to acetylated lysines on histone N-terminal tails and other proteins. The mutually exclusive ATPases of mSWI/SNF complexes, BRG1 and BRM, contain bromodomains with undefined functional importance in skeletal muscle differentiation. Pharmacological inhibition of mSWI/SNF bromodomain function using the small molecule PFI-3 reduced differentiation in cell culture and in vivo through decreased myogenic gene expression, while increasing cell cycle-related gene expression and the number of cells remaining in the cell cycle. Comparative gene expression analysis with data from myoblasts depleted of BRG1 or BRM showed that bromodomain function was required for a subset of BRG1- and BRM-dependent gene expression. Reduced binding of BRG1 and BRM after PFI-3 treatment showed that the bromodomain is required for stable chromatin binding at target gene promoters to alter gene expression. Our findings demonstrate that mSWI/SNF ATPase bromodomains permit stable binding of the mSWI/SNF ATPases to promoters required for cell cycle exit and establishment of muscle-specific gene expression.

The Fertilization Enigma: How Sperm and Egg Fuse

Fertilization is a multistep process that culminates in the fusion of sperm and egg, thus marking the beginning of a new organism in sexually reproducing species. Despite its importance for reproduction, the molecular mechanisms that regulate this singular event, particularly sperm-egg fusion, have remained mysterious for many decades. Here, we summarize our current molecular understanding of sperm-egg interaction, focusing mainly on mammalian fertilization. Given the fundamental importance of sperm-egg fusion yet the lack of knowledge of this process in vertebrates, we discuss hallmarks and emerging themes of cell fusion by drawing from well-studied examples such as viral entry, placenta formation, and muscle development. We conclude by identifying open questions and exciting avenues for future studies in gamete fusion. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Understanding small ORF diversity through a comprehensive transcription feature classification

Small open reading frames (small ORFs/sORFs/smORFs) are potentially coding sequences smaller than 100 codons that have historically been considered junk DNA by gene prediction software and in annotation screening; however, the advent of next-generation sequencing has contributed to the deeper investigation of junk DNA regions and their transcription products, resulting in the emergence of smORFs as a new focus of interest in systems biology. Several smORF peptides were recently reported in noncanonical mRNAs as new players in numerous biological contexts; however, their relevance is still overlooked in coding potential analysis. Hence, this review proposes a smORF classification based on transcriptional features, discussing the most promising approaches to investigate smORFs based on their different characteristics. First, smORFs were divided into nonexpressed (intergenic) and expressed (genic) smORFs. Second, genic smORFs were classified as smORFs located in noncoding RNAs (ncRNAs) or canonical mRNAs. Finally, smORFs in ncRNAs were further subdivided into sequences located in small or long RNAs, whereas smORFs located in canonical mRNAs were subdivided into several specific classes depending on their localization along the gene. We hope that this review provides new insights into large-scale annotations and reinforces the role of smORFs as essential components of a hidden coding DNA world.

LncRNA-encoded microproteins: A new form of cargo in cell culture-derived and circulating extracellular vesicles

Advancements in omics-based technologies over the past few years have led to the discovery of numerous biologically relevant peptides encoded by small open reading frames (smORFs) embedded in long noncoding RNA (lncRNA) transcripts (referred to as microproteins here) in a variety of species. However, the mechanisms and modes of action that underlie the roles of microproteins have yet to be fully characterized. Herein, we provide the first experimental evidence of abundant microproteins in extracellular vesicles (EVs) derived from glioma cancer cells, indicating that the EV-mediated transfer of microproteins may represent a novel mechanism for intercellular communication. Intriguingly, when examining human plasma, 48, 11 and 3 microproteins were identified from purified EVs, whole plasma and EV-free plasma, respectively, suggesting that circulating microproteins are primarily enriched in EVs. Most importantly, the preliminary data showed that the expression profile of EV microproteins in glioma patient diverged from the health donors, suggesting that the circulating microproteins in EVs might have potential diagnostic application in identifying patients with glioma.

Nigella Sativa-Coated Hydroxyapatite Scaffolds: Synergetic Cues to Stimulate Myoblasts Differentiation and Offset Infections

BACKGROUND

At present osteoporosis has come into view as a major health concern. Skeletal diseases typified by weak and fragile bones have imposed threats of fissure. Hydroxyapatite (HAP) is known to induce osteoblast like differentiation and provide mechanical strength, hence, used in bone tissue engineering; whereas, Nigella sativa has also demonstrated potential to treat bone and muscle diseases. This study was aimed to develop potential orthopedic scaffold exploiting natural resources of Saudi Arabia which can be used as prospective tissue engineering implant.

METHODS

The bone scaffold was developed by grafting biogenic HAP with N. sativa essential oil. N. sativa was applied for boosting osteogenesis and to stimulate antimicrobial potential. Antimicrobial potential was investigated utilizing S. aureus bacteria. Spectroscopic and surface characters of N. sativa grafted HAP scaffolds were analyzed using Fourier-transform infrared spectroscopy, X-ray crystallography and Scanning electron microscopy. To ensure biocompatibility of scaffolds; we selected C2C12 cell-lines; best model to study mechanistic pathways related to osteoblasts and myoblasts differentiation.

RESULTS

Grafting of HAP with N. sativa did not affect typical spherical silhouette of nanoparticles. Characteristically; protein loaded polynucleated myotubes are result of in vitro myogenesis of C2C12 myoblasts in squat serum environment.

CONCLUSION

It is first study of unique combination of N. sativa and HAP scaffold as a possible candidate of implantation for skeletal muscles regeneration. Outcome of this finding revealed N. sativa grafted HAP enhance differentiation significantly over that of HAP. The proposed scaffold will be an economical natural material for hard and soft tissue engineering and will aid in curing skeletal muscle diseases. Our findings have implications for treatment of muscular/bone diseases.

Deeply Mining a Universe of Peptides Encoded by Long Noncoding RNAs

Many small ORFs embedded in long noncoding RNA (lncRNA) transcripts have been shown to encode biologically functional polypeptides (small ORF-encoded polypeptides [SEPs]) in different organisms. Despite some novel SEPs have been found, the identification is still hampered by their poor predictability, diminutive size, and low relative abundance. Here, we take advantage of NONCODE, a repository containing the most complete collection and annotation of lncRNA transcripts from different species, to build a novel database that attempts to maximize a collection of SEPs from human and mouse lncRNA transcripts. In order to further improve SEP discovery, we implemented two effective and complementary polypeptide enrichment strategies using 30-kDa molecular weight cutoff filter and C8 solid-phase extraction column. These combined strategies enabled us to discover 353 SEPs from eight human cell lines and 409 SEPs from three mouse cell lines and eight mouse tissues. Importantly, 19 of them were then verified through in vitro expression, immunoblotting, parallel reaction monitoring, and synthetic peptides. Subsequent bioinformatics analysis revealed that some of the physical and chemical properties of these novel SEPs, including amino acid composition and codon usage, are different from those commonly found in canonical proteins. Intriguingly, nearly 65% of the identified SEPs were found to be initiated with non-AUG start codons. The 762 novel SEPs probably represent the largest number of SEPs detected by MS reported to date. These novel SEPs might not only provide new clues for the annotation of noncoding elements in the genome but also serve as a valuable resource for functional study.

The Role of Embryonic Chick Muscle Cell Culture in the Study of Skeletal Myogenesis

The mechanisms involved in the development of skeletal muscle fibers have been studied in the last 70 years and yet many aspects of this process are still not completely understood. A myriad of in vivo and in vitro invertebrate and vertebrate animal models has been used for dissecting the molecular and cellular events involved in muscle formation. Among the most used animal models for the study of myogenesis are the rodents rat and mouse, the fruit fly Drosophila, and the birds chicken and quail. Here, we describe the robustness and advantages of the chick primary muscle culture model for the study of skeletal myogenesis. In the myoblast culture obtained from embryonic chick pectoralis muscle it is possible to analyze all the steps involved in skeletal myogenesis, such as myoblast proliferation, withdrawal from cell cycle, cell elongation and migration, myoblast alignment and fusion, the assembly of striated myofibrils, and the formation of multinucleated myotubes. The fact that in vitro chick myotubes can harbor hundreds of nuclei, whereas myotubes from cell lines have only a dozen nuclei demonstrates the high level of differentiation of the autonomous chick myogenic program. This striking differentiation is independent of serum withdrawal, which points to the power of the model. We also review the major pro-myogenic and anti-myogenic molecules and signaling pathways involved in chick myogenesis, in addition to providing a detailed protocol for the preparation of embryonic chick myogenic cultures. Moreover, we performed a bibliometric analysis of the articles that used this model to evaluate which were the main explored topics of interest and their contributors. We expect that by describing the major findings, and their advantages, of the studies using the embryonic chick myogenic model we will foster new studies on the molecular and cellular process involved in muscle proliferation and differentiation that are more similar to the actual in vivo condition than the muscle cell lines.

Skeletal muscle fibers count on nuclear numbers for growth

Skeletal muscle cells are noteworthy for their syncytial nature, with each myofiber accumulating hundreds or thousands of nuclei derived from resident muscle stem cells (MuSCs). These nuclei are accrued through cell fusion, which is controlled by the two essential fusogens Myomaker and Myomerger that are transiently expressed within the myogenic lineage. While the absolute requirement of fusion for muscle development has been known for decades, the underlying need for the magnitude of multinucleation in muscle remains mysterious. Possible advantages of multinucleation include the potential it affords for transcriptional diversity within these massive cells, and as a means of increasing DNA content to support optimal cell size and function. In this article, we review recent advances that elucidate the relationship between myonuclear numbers and establishment of myofiber size, and discuss how this new information refines our understanding of the concept of myonuclear domains (MND), the cytoplasmic volumes that each resident myonucleus can support. Finally, we explore the potential consequences and costs of multinucleation and its impacts on myonuclear transcriptional reserve capacity, growth potential, myofiber size regulation, and muscle adaptability. We anticipate this report will not only serve to highlight the latest advances in the basic biology of syncytial muscle cells but also provide information to help design the next generation of therapeutic strategies to maintain muscle mass and function.

Evolutionarily related small viral fusogens hijack distinct but modular actin nucleation pathways to drive cell-cell fusion

Fusion-associated small transmembrane (FAST) proteins are a diverse family of nonstructural viral proteins. Once expressed on the plasma membrane of infected cells, they drive fusion with neighboring cells, increasing viral spread and pathogenicity. Unlike viral fusogens with tall ectodomains that pull two membranes together through conformational changes, FAST proteins have short fusogenic ectodomains that cannot bridge the intermembrane gap between neighboring cells. One orthoreovirus FAST protein, p14, has been shown to hijack the actin cytoskeleton to drive cell-cell fusion, but the actin adaptor-binding motif identified in p14 is not found in any other FAST protein. Here, we report that an evolutionarily divergent FAST protein, p22 from aquareovirus, also hijacks the actin cytoskeleton but does so through different adaptor proteins, Intersectin-1 and Cdc42, that trigger N-WASP-mediated branched actin assembly. We show that despite using different pathways, the cytoplasmic tail of p22 can replace that of p14 to create a potent chimeric fusogen, suggesting they are modular and play similar functional roles. When we directly couple p22 with the parallel filament nucleator formin instead of the branched actin nucleation promoting factor N-WASP, its ability to drive fusion is maintained, suggesting that localized mechanical pressure on the plasma membrane coupled to a membrane-disruptive ectodomain is sufficient to drive cell-cell fusion. This work points to a common biophysical strategy used by FAST proteins to push rather than pull membranes together to drive fusion, one that may be harnessed by other short fusogens responsible for physiological cell-cell fusion.

SARS-CoV-2 requires cholesterol for viral entry and pathological syncytia formation

Many enveloped viruses induce multinucleated cells (syncytia), reflective of membrane fusion events caused by the same machinery that underlies viral entry. These syncytia are thought to facilitate replication and evasion of the host immune response. Here, we report that co-culture of human cells expressing the receptor ACE2 with cells expressing SARS-CoV-2 spike, results in synapse-like intercellular contacts that initiate cell-cell fusion, producing syncytia resembling those we identify in lungs of COVID-19 patients. To assess the mechanism of spike/ACE2-driven membrane fusion, we developed a microscopy-based, cell-cell fusion assay to screen ~6000 drugs and >30 spike variants. Together with quantitative cell biology approaches, the screen reveals an essential role for biophysical aspects of the membrane, particularly cholesterol-rich regions, in spike-mediated fusion, which extends to replication-competent SARS-CoV-2 isolates. Our findings potentially provide a molecular basis for positive outcomes reported in COVID-19 patients taking statins and suggest new strategies for therapeutics targeting the membrane of SARS-CoV-2 and other fusogenic viruses.

The role of micropeptides in biology

Micropeptides are small polypeptides coded by small open-reading frames. Progress in computational biology and the analyses of large-scale transcriptomes and proteomes have revealed that mammalian genomes produce a large number of transcripts encoding micropeptides. Many of these have been previously annotated as long noncoding RNAs. The role of micropeptides in cellular homeostasis maintenance has been demonstrated. This review discusses different types of micropeptides as well as methods to identify them, such as computational approaches, ribosome profiling, and mass spectrometry.

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

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

Cell-cell fusions and cell-in-cell phenomena in healthy cells and cancer: Lessons from protists and invertebrates

Herein we analyze two special routes of the multinucleated cells' formation - the fusion of mononuclear cells and the formation of cell-in-cell structures - in the healthy tissues and in tumorigenesis. There are many theories of tumorigenesis based on the phenomenon of emergence of the hybrid cancer cells. We consider the phenomena, which are rarely mentioned in those theories: namely, cellularization of syncytium or coenocytes, and the reversible or irreversible somatogamy. The latter includes the short-term and the long-term vegetative (somatic) cells' fusions in the life cycles of unicellular organisms. The somatogamy and multinuclearity have repeatedly and independently emerged in various groups of unicellular eukaryotes. These phenomena are among dominant survival and biodiversity sustaining strategies in protists and we admit that they can likely play an analogous role in cancer cells.

The cardiac translational landscape reveals that micropeptides are new players involved in cardiomyocyte hypertrophy

Hypertrophic growth of cardiomyocytes is one of the major compensatory responses in the heart after physiological or pathological stimulation. Protein synthesis enhancement, which is mediated by the translation of messenger RNAs, is one of the main features of cardiomyocyte hypertrophy. Although the transcriptome shift caused by cardiac hypertrophy induced by different stimuli has been extensively investigated, translatome dynamics in this cellular process has been less studied. Here, we generated a nucleotide-resolution translatome as well as transcriptome data from isolated primary cardiomyocytes undergoing hypertrophy. More than 10,000 open reading frames (ORFs) were detected from the deep sequencing of ribosome-protected fragments (Ribo-seq), which orchestrated the shift of the translatome in hypertrophied cardiomyocytes. Our data suggest that rather than increase the translational rate of ribosomes, the increased efficiency of protein synthesis in cardiomyocyte hypertrophy was attributable to an increased quantity of ribosomes. In addition, more than 100 uncharacterized short ORFs (sORFs) were detected in long noncoding RNA genes from Ribo-seq with potential of micropeptide coding. In a random test of 15 candidates, the coding potential of 11 sORFs was experimentally supported. Three micropeptides were identified to regulate cardiomyocyte hypertrophy by modulating the activities of oxidative phosphorylation, the calcium signaling pathway, and the mitogen-activated protein kinase (MAPK) pathway. Our study provides a genome-wide overview of the translational controls behind cardiomyocyte hypertrophy and demonstrates an unrecognized role of micropeptides in cardiomyocyte biology.

Skeletal Muscle Transcriptome Analysis of Hanzhong Ma Duck at Different Growth Stages Using RNA-Seq

As one of the most important poultry worldwide, ducks (Anas platyrhynchos) are raised mainly for meat and egg products, and muscle development in ducks is important for meat production. Therefore, an investigation of gene expression in duck skeletal muscle would significantly contribute to our understanding of muscle development. In this study, twenty-four cDNA libraries were constructed from breast and leg muscles of Hanzhong Ma ducks at day 17, 21, 27 of the embryo and postnatal at 6-month-old. High-throughput sequencing and bioinformatics were used to determine the abundances and characteristics of transcripts. A total of 632,172,628 (average 52,681,052) and 637,213,938 (average 53,101,162) reads were obtained from the sequencing data of breast and leg muscles, respectively. Over 71.63% and 77.36% of the reads could be mapped to the Anas platyrhynchos genome. In the skeletal muscle of Hanzhong duck, intron variant (INTRON), synonymous variant (SYNONYMOUS_CODING), and prime 3' UTR variant (UTR_3_PRIME) were the main single nucleotide polymorphisms (SNP) annotation information, and "INTRON", "UTR_3_PRIME", and downstream-gene variant (DOWNSTREAM) were the main insertion-deletion (InDel) annotation information. The predicted number of alternative splicing (AS) in all samples were mainly alternative 5' first exon (transcription start site)-the first exon splicing (TSS) and alternative 3' last exon (transcription terminal site)-the last exon splicing (TTS). Besides, there were 292 to 2801 annotated differentially expressed genes (DEGs) in breast muscle and 304 to 1950 annotated DEGs in leg muscle from different databases. It is worth noting that 75 DEGs in breast muscle and 49 DEGs in leg muscle were co-expressed at all developmental points of comparison, respectively. The RNA-Seq data were confirmed to be reliable by qPCR. The identified DEGs, such as CREBL2, RHEB, GDF6, SHISA2, MYLK2, ACTN3, RYR3, and STMN1, were specially highlighted, indicating their strong associations with muscle development in the Hanzhong Ma duck. KEGG pathway analysis suggested that regulation of actin cytoskeleton, oxidative phosphorylation, and focal adhesion were involved in the development of skeletal muscle. The findings from this study can contribute to future investigations of the growth and development mechanism in duck skeletal muscle.