Nap1-mediated actin remodeling is essential for mammalian myoblast fusion

Machine learning inference of continuous single-cell state transitions during myoblast differentiation and fusion

Cells modify their internal organization during continuous state transitions, supporting functions from cell division to differentiation. However, tools to measure dynamic physiological states of individual transitioning cells are lacking. We combined live-cell imaging and machine learning to monitor ERK1/2-inhibited primary murine skeletal muscle precursor cells, that transition rapidly and robustly from proliferating myoblasts to post-mitotic myocytes and then fuse, forming multinucleated myotubes. Our models, trained using motility or actin intensity features from single-cell tracking data, effectively tracked real-time continuous differentiation, revealing that differentiation occurs 7.5-14.5 h post induction, followed by fusion ~3 h later. Co-inhibition of ERK1/2 and p38 led to differentiation without fusion. Our model inferred co-inhibition leads to terminal differentiation, indicating that p38 is specifically required for transitioning from terminal differentiation to fusion. Our model also predicted that co-inhibition leads to changes in actin dynamics. Mass spectrometry supported these in silico predictions and suggested novel fusion and maturation regulators downstream of differentiation. Collectively, this approach can be adapted to various biological processes to uncover novel links between dynamic single-cell states and their functional outcomes.

WAVE2 Is a Vital Regulator in Myogenic Differentiation of Progenitor Cells through the Mechanosensitive MRTFA-SRF Axis

Skeletal myogenesis is an intricate process involving the differentiation of progenitor cells into myofibers, which is regulated by actin cytoskeletal dynamics and myogenic transcription factors. Although recent studies have demonstrated the pivotal roles of actin-binding proteins (ABPs) as mechanosensors and signal transducers, the biological significance of WAVE2 (Wiskott-Aldrich syndrome protein family member 2), an ABP essential for actin polymerization, in myogenic differentiation of progenitor cells has not been investigated. Our study provides important insights into the regulatory roles played by WAVE2 in the myocardin-related transcription factor A (MRTFA)-serum response factor (SRF) signaling axis and differentiation of myoblasts. We demonstrate that WAVE2 expression is induced during myogenic differentiation and plays a pivotal role in actin cytoskeletal remodeling in C2C12 myoblasts. Knockdown of WAVE2 in C2C12 cells reduced filamentous actin levels, increased globular actin accumulation, and impaired the nuclear translocation of MRTFA. Furthermore, WAVE2 depletion in myoblasts inhibited the expression and transcriptional activity of SRF and suppressed cell proliferation in myoblasts. Consequently, WAVE2 knockdown suppressed myogenic regulatory factors (i.e., MyoD, MyoG, and SMYD1) expressions, thereby hindering the differentiation of myoblasts. Thus, this study suggests that WAVE2 is essential for myogenic differentiation of progenitor cells by modulating the mechanosensitive MRTFA-SRF axis.

Role of Actin-Binding Proteins in Skeletal Myogenesis

Maintenance of skeletal muscle quantity and quality is essential to ensure various vital functions of the body. Muscle homeostasis is regulated by multiple cytoskeletal proteins and myogenic transcriptional programs responding to endogenous and exogenous signals influencing cell structure and function. Since actin is an essential component in cytoskeleton dynamics, actin-binding proteins (ABPs) have been recognized as crucial players in skeletal muscle health and diseases. Hence, dysregulation of ABPs leads to muscle atrophy characterized by loss of mass, strength, quality, and capacity for regeneration. This comprehensive review summarizes the recent studies that have unveiled the role of ABPs in actin cytoskeletal dynamics, with a particular focus on skeletal myogenesis and diseases. This provides insight into the molecular mechanisms that regulate skeletal myogenesis via ABPs as well as research avenues to identify potential therapeutic targets. Moreover, this review explores the implications of non-coding RNAs (ncRNAs) targeting ABPs in skeletal myogenesis and disorders based on recent achievements in ncRNA research. The studies presented here will enhance our understanding of the functional significance of ABPs and mechanotransduction-derived myogenic regulatory mechanisms. Furthermore, revealing how ncRNAs regulate ABPs will allow diverse therapeutic approaches for skeletal muscle disorders to be developed.

FLII Modulates the Myogenic Differentiation of Progenitor Cells via Actin Remodeling-Mediated YAP1 Regulation

The dynamic rearrangement of the actin cytoskeleton plays an essential role in myogenesis, which is regulated by diverse mechanisms, such as mechanotransduction, modulation of the Hippo signaling pathway, control of cell proliferation, and the influence of morphological changes. Despite the recognized importance of actin-binding protein Flightless-1 (FLII) during actin remodeling, the role played by FLII in the differentiation of myogenic progenitor cells has not been explored. Here, we investigated the roles of FLII in the proliferation and differentiation of myoblasts. FLII was found to be enriched in C2C12 myoblasts, and its expression was stable during the early stages of differentiation but down-regulated in fully differentiated myotubes. Knockdown of FLII in C2C12 myoblasts resulted in filamentous actin (F-actin) accumulation and inhibited Yes-associated protein 1 (YAP1) phosphorylation, which triggers its nuclear translocation from the cytoplasm. Consequently, the expressions of YAP1 target genes, including PCNA, CCNB1, and CCND1, were induced, and the cell cycle and proliferation of myoblasts were promoted. Moreover, FLII knockdown significantly inhibited the expression of myogenic regulatory factors, i.e., MyoD and MyoG, thereby impairing myoblast differentiation, fusion, and myotube formation. Thus, our findings demonstrate that FLII is crucial for the differentiation of myoblasts via modulation of the F-actin/YAP1 axis and suggest that FLII is a putative novel therapeutic target for muscle wasting.

Reduced Expression of Septin7 Hinders Skeletal Muscle Regeneration

Septins are considered the fourth component of the cytoskeleton with the septin7 isoform playing a critical role in the formation of diffusion barriers in phospholipid bilayers and intra- and extracellular scaffolds. While its importance has already been confirmed in different intracellular processes, very little is known about its role in skeletal muscle. Muscle regeneration was studied in a Sept7 conditional knock-down mouse model to prove the possible role of septin7 in this process. Sterile inflammation in skeletal muscle was induced which was followed by regeneration resulting in the upregulation of septin7 expression. Partial knock-down of Sept7 resulted in an increased number of inflammatory cells and myofibers containing central nuclei. Taken together, our data suggest that partial knock-down of Sept7 hinders the kinetics of muscle regeneration, indicating its crucial role in skeletal muscle functions.

Myofibrillar myopathy hallmarks associated with ZAK deficiency

The ZAK gene encodes two functionally distinct kinases, ZAKα and ZAKβ. Homozygous loss of function mutations affecting both isoforms causes a congenital muscle disease. ZAKβ is the only isoform expressed in skeletal muscle and is activated by muscle contraction and cellular compression. The ZAKβ substrates in skeletal muscle or the mechanism whereby ZAKβ senses mechanical stress remains to be determined. To gain insights into the pathogenic mechanism, we exploited ZAK-deficient cell lines, zebrafish, mice and a human biopsy. ZAK-deficient mice and zebrafish show a mild phenotype. In mice, comparative histopathology data from regeneration, overloading, ageing and sex conditions indicate that while age and activity are drivers of the pathology, ZAKβ appears to have a marginal role in myoblast fusion in vitro or muscle regeneration in vivo. The presence of SYNPO2, BAG3 and Filamin C (FLNC) in a phosphoproteomics assay and extended analyses suggested a role for ZAKβ in the turnover of FLNC. Immunofluorescence analysis of muscle sections from mice and a human biopsy showed evidence of FLNC and BAG3 accumulations as well as other myofibrillar myopathy markers. Moreover, endogenous overloading of skeletal muscle exacerbated the presence of fibres with FLNC accumulations in mice, indicating that ZAKβ signalling is necessary for an adaptive turnover of FLNC that allows for the normal physiological response to sustained mechanical stress. We suggest that accumulation of mislocalized FLNC and BAG3 in highly immunoreactive fibres contributes to the pathogenic mechanism of ZAK deficiency.

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.

Identification and Functional Characterization of Metabolites for Skeletal Muscle Mass in Early Postmenopausal Chinese Women

Low skeletal muscle mass (SMM) is a crucial component of the sarcopenia phenotypes. In the present study, we aim to identify the specific metabolites associated with SMM variation and their functional mechanisms of decreased SMM in early postmenopausal women. We performed an untargeted metabolomics analysis in 430 early postmenopausal women to identify specific metabolite associated with skeletal muscle mass indexes (SMIes). Then, the potential causal effect of specific metabolite on SMM variation was accessed by one-sample Mendelian randomization (MR) analysis. Finally, in vitro experiments and transcriptomics bioinformatics analysis were conducted to explore the impact and potential functional mechanisms of specific metabolite on SMM variation. We detected 65 metabolites significantly associated with at least one SMI (variable importance in projection > 1.5 by partial least squares regression and p < .05 in multiple linear regression analysis). Remarkably, stearic acid (SA) was negatively associated with all SMIes, and subsequent MR analyses showed that increased serum SA level had a causal effect on decreased SMM (p < .05). Further in vitro experiments showed that SA could repress myoblast's differentiation at mRNA, protein, and phenotype levels. By combining transcriptome bioinformatics analysis, our study supports that SA may inhibit myoblast differentiation and myotube development by regulating the migration, adhesion, and fusion of myoblasts. This metabolomics study revealed specific metabolic profiles associated with decreased SMM in postmenopausal women, first highlighted the importance of SA in regulating SMM variation, and illustrated its potential mechanism on decreased SMM.

miRNA mediated downregulation of cyclase-associated protein 1 (CAP1) is required for myoblast fusion

Myoblast fusion is essential for the formation, growth, and regeneration of skeletal muscle, but the molecular mechanisms that govern fusion and myofiber formation remain poorly understood. Past studies have shown an important role of the actin cytoskeleton and actin regulators in myoblast fusion. The Cyclase-Associated Proteins (CAP) 1 and 2 recently emerged as critical regulators of actin treadmilling in higher eukaryotes including mammals. Whilst the role of CAP2 in skeletal muscle development and function is well characterized, involvement of CAP1 in this process remains elusive. Here we report that CAP1, plays a critical role in cytoskeletal remodeling during myoblast fusion and formation of myotubes. Cap1 mRNA and protein are expressed in both murine C2C12 and human LHCN-M2 myoblasts, but their abundance decreases during myogenic differentiation. Perturbing the temporally controlled expression of CAP1 by overexpression or CRISPR-Cas9 mediated knockout impaired actin rearrangement, myoblast alignment, expression of profusion molecules, differentiation into multinucleated myotubes, and myosin heavy chain expression. Endogenous Cap1 expression is post-transcriptionally downregulated during differentiation by canonical myomiRs miR-1, miR-133, and miR-206, which have conserved binding sites at the 3′ UTR of the Cap1 mRNA. Deletion of the endogenous 3′ UTR by CRISPR-Cas9 in C2C12 cells phenocopies overexpression of CAP1 by inhibiting myotube formation. Our findings implicates Cap1 and its myomiR-mediated downregulation in the myoblast fusion process and the generation of skeletal muscle.

The mechanism of Megalobrama amblycephala muscle injury repair based on RNA-seq

Skeletal muscle myogenesis and injury-induced muscle regeneration contribute to muscle formation. Skeletal muscle stem cells, termed satellite cells (SCs), proliferate to repair injured muscle. To identify the molecular mechanism of regeneration after muscle injury as well as the genes related to muscle development in fish, in this study, the immunohistochemistry and the high-throughput RNA sequencing (RNA-seq) analysis were performed after Megalobrama amblycephala muscle was injured by needle stab. The results showed that paired box7-positive (Pax7⁺) SCs increased, and peaked at 96 to 144 h-post injury (hpi). The 6729 differentially expressed genes (DEGs), including 2125 up-regulated and 4604 down-regulated genes were found. GO terms significantly enriched by DEGs contained intercellular connections, signaling transduction and enzyme activity. KEGG enrichment analysis showed that most of the pathways were related to immunity, metabolism and cells related molecules, including actin skeleton regulation, Epstein Barr virus infection and plaque adhesion. The WGCNA results revealed that actin cytoskeleton and lipid metabolism related genes probably played crucial roles during repair after muscle injury. Collectively, all these results will provide new insights into the molecular mechanisms underlying muscle injury repair of fish.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

CFL2 is an essential mediator for myogenic differentiation in C2C12 myoblasts

CFL2, a skeletal muscle-specific member of the actin depolymerizing factor/cofilin protein family, is known to be involved in the regulation of actin filament dynamics. Although the impact of CFL2 has been studied in human myopathy, its functional contribution to myogenic differentiation, in terms of its effects on cell proliferation, cell cycle, and myogenic factor modulation, remains largely unknown. Here, we report that CFL2 is required for the myogenic differentiation of C2C12 myoblasts by regulating proliferation and myogenic transcription factors expressions. CFL2 expression was induced during myogenic progression, and its knockdown by siRNA in myoblasts enhanced phalloidin staining, indicating increased filamentous actin formation. Interestingly, CFL2 depletion stimulated cell proliferation and induced a cell cycle shift from G0/G1 to G2/M phases, which are known to inhibit progenitor cell differentiation. CFL2 knockdown markedly downregulated the protein expressions of myogenic transcription factors (MyoD, MyoG, and MEF2C) and thereby impaired the differentiation and myotube formation of C2C12 myoblasts. Collectively, this study highlights the roles played by CFL2 on cell cycle progression and proliferation and suggests a novel regulatory mechanism of myogenic differentiation mediated by CFL2.

π-SACS: pH Induced Self-Assembled Cell Sheets Without the Need for Modified Surfaces

The ability to form tissue-like constructs that have high cell density with proper cell-cell and cell-ECM interactions is critical for many applications including tissue models for drug discovery and tissue regeneration. Newly emerging bioprinting methods sometimes lack the high cellular density needed to provide biophysical cues to orchestrate cellular behavior to recreate tissue architecture and function. Alternate methods using self-assembly can be used to create tissue-like constructs with high cellular density and well-defined microstructure in the form of spheroids, organoids, or cell sheets. Cell sheets have a particularly interesting architecture in the context of tissue regeneration and repair as they can be applied as patches to integrate with surrounding tissues. Until now, the preparation of these sheets has involved culturing on specialized substrates that can be triggered by temperature or phase change (hydrophobic to hydrophilic) to release cells growing on them and form sheets. Here a new technique is proposed that allows delamination of cells and secreted ECM and rapid self-assembly into a cell sheet using a simple pH trigger and without the need to use responsive surfaces or applying external stimuli such as electrical and magnetic fields, only with routine tissue culture plates. This technique can be used with cells that are capable of syncytialization and fusion such as skeletal muscle cells and placenta cells. Using C2C12 myoblast cells we show that the pH trigger induces a rapid delamination of the cells as a continuous layer that self-assembles into a thick dense sheet. The delamination process has little effect on cell viability and maturation and preserves the ECM components that allow sheets to adhere to each other within a short incubation time enabling formation of thicker constructs when multiple sheets are stacked (double- and quadruple-layer constructs are formed here). These thick grafts can be used for regeneration purposes or as in vitro models.

Proteomic resolution of IGFN1 complexes reveals a functional interaction with the actin nucleating protein COBL

The Igfn1 gene produces multiple proteins by alternative splicing predominantly expressed in skeletal muscle. Igfn1 deficient clones derived from C2C12 myoblasts show reduced fusion index and morphological differences compared to control myotubes. Here, we first show that G:F actin ratios are significantly higher in differentiating IGFN1-deficient C2C12 myoblasts, suggesting that fusion and differentiation defects are underpinned by deficient actin remodelling. We obtained pull-downs from skeletal muscle with IGFN1 fragments and applied a proteomics approach. The proteomic composition of IGFN1 complexes identified the cytoskeleton and an association with the proteasome as the main networks. The actin nucleating protein COBL was selected for further validation. COBL is expressed in C2C12 myoblasts from the first stages of myoblast fusion but not in proliferating cells. COBL is also expressed in adult muscle and, as IGFN1, localizes to the Z-disc. We show that IGFN1 interacts, stabilizes and colocalizes with COBL and prevents the ability of COBL to form actin ruffles in COS7 cells. COBL loss of function C2C12-derived clones are able to fuse, therefore indicating that COBL or the IGFN1/COBL interaction are not essential for myoblast fusion.

The regulatory role of Myomaker and Myomixer-Myomerger-Minion in muscle development and regeneration

Skeletal muscle plays essential roles in motor function, energy, and glucose metabolism. Skeletal muscle formation occurs through a process called myogenesis, in which a crucial step is the fusion of mononucleated myoblasts to form multinucleated myofibers. The myoblast/myocyte fusion is triggered and coordinated in a muscle-specific way that is essential for muscle development and post-natal muscle regeneration. Many molecules and proteins have been found and demonstrated to have the capacity to regulate the fusion of myoblast/myocytes. Interestingly, two newly discovered muscle-specific membrane proteins, Myomaker and Myomixer (also called Myomerger and Minion), have been identified as fusogenic regulators in vertebrates. Both Myomaker and Myomixer-Myomerger-Minion have the capacity to directly control the myogenic fusion process. Here, we review and discuss the latest studies related to these two proteins, including the discovery, structure, expression pattern, functions, and regulation of Myomaker and Myomixer-Myomerger-Minion. We also emphasize and discuss the interaction between Myomaker and Myomixer-Myomerger-Minion, as well as their cooperative regulatory roles in cell-cell fusion. Moreover, we highlight the areas for exploration of Myomaker and Myomixer-Myomerger-Minion in future studies and consider their potential application to control cell fusion for cell-therapy purposes.

Rab5a activates IRS1 to coordinate IGF-AKT-mTOR signaling and myoblast differentiation during muscle regeneration

Rab5 is a master regulator for endosome biogenesis and transport while its in vivo physiological function remains elusive. Here, we find that Rab5a is upregulated in several in vivo and in vitro myogenesis models. By generating myogenic Rab5a-deficient mice, we uncover the essential roles of Rab5a in regulating skeletal muscle regeneration. We further reveal that Rab5a promotes myoblast differentiation and directly interacts with insulin receptor substrate 1 (IRS1), an essential scaffold protein for propagating IGF signaling. Rab5a interacts with IRS1 in a GTP-dependent manner and this interaction is enhanced upon IGF-1 activation and myogenic differentiation. We subsequently identify that the arginine 207 and 222 of IRS1 and tyrosine 82, 89, and 90 of Rab5a are the critical amino acid residues for mediating the association. Mechanistically, Rab5a modulates IRS1 activation by coordinating the association between IRS1 and the IGF receptor (IGFR) and regulating the intracellular membrane targeting of IRS1. Both myogenesis-induced and IGF-evoked AKT-mTOR signaling are dependent on Rab5a. Myogenic deletion of Rab5a also reduces the activation of AKT-mTOR signaling during skeletal muscle regeneration. Taken together, our study uncovers the physiological function of Rab5a in regulating muscle regeneration and delineates the novel role of Rab5a as a critical switch controlling AKT-mTOR signaling by activating IRS1.

The fusogenic synapse at a glance

Cell-cell fusion is a fundamental process underlying fertilization, development, regeneration and physiology of metazoans. It is a multi-step process involving cell recognition and adhesion, actin cytoskeletal rearrangements, fusogen engagement, lipid mixing and fusion pore formation, ultimately resulting in the integration of two fusion partners. Here, we focus on the asymmetric actin cytoskeletal rearrangements at the site of fusion, known as the fusogenic synapse, which was first discovered during myoblast fusion in Drosophila embryos and later also found in mammalian muscle and non-muscle cells. At the asymmetric fusogenic synapse, actin-propelled invasive membrane protrusions from an attacking fusion partner trigger actomyosin-based mechanosensory responses in the receiving cell. The interplay between the invasive and resisting forces generated by the two fusion partners puts the fusogenic synapse under high mechanical tension and brings the two cell membranes into close proximity, promoting the engagement of fusogens to initiate fusion pore formation. In this Cell Science at a Glance article and the accompanying poster, we highlight the molecular, cellular and biophysical events at the asymmetric fusogenic synapse using Drosophila myoblast fusion as a model.

Dephosphorylation of HDAC4 by PP2A-Bδ unravels a new role for the HDAC4/MEF2 axis in myoblast fusion

Muscle formation is controlled by a number of key myogenic transcriptional regulators that govern stage-specific gene expression programs and act as terminal effectors of intracellular signaling pathways. To date, the role of phosphatases in the signaling cascades instructing muscle development remains poorly understood. Here, we show that a specific PP2A-B55δ holoenzyme is necessary for skeletal myogenesis. The primary role of PP2A-B55δ is to dephosphorylate histone deacetylase 4 (HDAC4) following myocyte differentiation and ensure repression of Myocyte enhancer factor 2D (MEF2D)-dependent gene expression programs during myogenic fusion. As a crucial HDAC4/MEF2D target gene that governs myocyte fusion, we identify ArgBP2, an upstream inhibitor of Abl, which itself is a repressor of CrkII signaling. Consequently, cells lacking PP2A-B55δ show upregulation of ArgBP2 and hyperactivation of CrkII downstream effectors, including Rac1 and FAK, precluding cytoskeletal and membrane rearrangements associated with myoblast fusion. Both in vitro and in zebrafish, loss-of-function of PP2A-B55δ severely impairs fusion of myocytes and formation of multinucleated muscle fibers, without affecting myoblast differentiation. Taken together, our results establish PP2A-B55δ as the first protein phosphatase to be involved in myoblast fusion and suggest that reversible phosphorylation of HDAC4 may coordinate differentiation and fusion events during myogenesis.

Adult stem cells at work: regenerating skeletal muscle

Skeletal muscle regeneration is a finely tuned process involving the activation of various cellular and molecular processes. Satellite cells, the stem cells of skeletal muscle, are indispensable for skeletal muscle regeneration. Their functionality is critically modulated by intrinsic signaling pathways as well as by interactions with the stem cell niche. Here, we discuss the properties of satellite cells, including heterogeneity regarding gene expression and/or their phenotypic traits and the contribution of satellite cells to skeletal muscle regeneration. We also summarize the process of regeneration with a specific emphasis on signaling pathways, cytoskeletal rearrangements, the importance of miRNAs, and the contribution of non-satellite cells such as immune cells, fibro-adipogenic progenitor cells, and PW1-positive/Pax7-negative interstitial cells.

Dynamic membrane proteome of adipogenic and myogenic precursors in skeletal muscle highlights EPHA2 may promote myogenic differentiation through ERK signaling

The balance of myogenic and adipogenic differentiation is crucial for skeletal muscle homeostasis. Given the vital role of membrane proteins (MBPs) in cell signal perception, membrane proteomics was conducted to delineate mechanisms regulating differentiation of adipogenic and myogenic precursors in skeletal muscle. Adipogenic and myogenic precursors with divergent differentiation potential were isolated from the longissimus dorsi muscle of neonatal pigs by the preplate method. A total of 85 differentially expressed MBPs (P < 0.05 and fold change ≥1.2 or ≤0.83) between 2 precursors were detected via isobaric tags for relative and absolute quantitation (iTRAQ) assay, including 67 up-regulated and 18 down-regulated in myogenic precursors. Functional enrichment analysis uncovered that myogenic and adipogenic precursors showed significant differences in cytoskeleton organization, syncytium formation, environmental information processing, and organismal systems. Furthermore, key MBPs in regulating cell differentiation were also characterized, including ITGB3, ITGAV, ITPR3, and EPHA2. Noteworthily, EPHA2 was required for myogenic differentiation, and it may promote myogenic differentiation through ERK signaling. Collectively, our study provided an insight into the distinct MBP profile between myogenic and adipogenic precursors in skeletal muscle and served as a solid basis for supporting the role of MBPs in regulating differentiation.—Zhang, X., Wang, L., Qiu, K., Xu, D., Yin, J. Dynamic membrane proteome of adipogenic and myogenic precursors in skeletal muscle highlights EPHA2 may promote myogenic differentiation through ERK signaling.

Myomaker and Myomerger Work Independently to Control Distinct Steps of Membrane Remodeling during Myoblast Fusion

Classic mechanisms for membrane fusion involve transmembrane proteins that assemble into complexes and dynamically alter their conformation to bend membranes, leading to mixing of membrane lipids (hemifusion) and fusion pore formation. Myomaker and Myomerger govern myoblast fusion and muscle formation but are structurally divergent from traditional fusogenic proteins. Here, we show that Myomaker and Myomerger independently mediate distinct steps in the fusion pathway, where Myomaker is involved in membrane hemifusion and Myomerger is necessary for fusion pore formation. Mechanistically, we demonstrate that Myomerger is required on the cell surface where its ectodomains stress membranes. Moreover, we show that Myomerger drives fusion completion in a heterologous system independent of Myomaker and that a Myomaker-Myomerger physical interaction is not required for function. Collectively, our data identify a stepwise cell fusion mechanism in myoblasts where different proteins are delegated to perform unique membrane functions essential for membrane coalescence.

Shisa2 regulates the fusion of muscle progenitors

Adult skeletal muscles are comprised of multinuclear muscle cells called myofibers. During skeletal muscle development and regeneration, mononuclear progenitor cells (myoblasts) fuse to form multinuclear myotubes, which mature and become myofibers. The molecular events mediating myoblast fusion are not fully understood. Here we report that Shisa2, an endoplasmic reticulum (ER) localized protein, regulates the fusion of muscle satellite cell-derived primary myoblasts. Shisa2 expression is repressed by Notch signaling, elevated in activated compared to quiescent satellite cells, and further upregulated during myogenic differentiation. Knockdown of Shisa2 inhibits the fusion of myoblasts without affecting proliferation. Conversely, Shisa2 overexpression in proliferating myoblasts inhibits their proliferation but promotes premature fusion. Interestingly, Shisa2-overexpressing nascent myotubes actively recruit myoblasts to fuse with. At the molecular level, Rac1/Cdc42-mediated cytoskeletal F-actin remodeling is required for Shisa2 to promote myoblast fusion. These results provide a novel mechanism through which an ER protein regulates myogenesis.

Impaired muscle relaxation and mitochondrial fission associated with genetic ablation of cytoplasmic actin isoforms

While α-actin isoforms predominate in adult striated muscle, skeletal muscle-specific knockouts (KOs) of nonmuscle cytoplasmic β_cyto - or γ_cyto -actin each cause a mild, but progressive myopathy effected by an unknown mechanism. Using transmission electron microscopy, we identified morphological abnormalities in both the mitochondria and the sarcoplasmic reticulum (SR) in aged muscle-specific β_cyto - and γ_cyto -actin KO mice. We found β_cyto - and γ_cyto -actin proteins to be enriched in isolated mitochondrial-associated membrane preparations, which represent the interface between mitochondria and sarco-endoplasmic reticulum important in signaling and mitochondrial dynamics. We also measured significantly elongated and interconnected mitochondrial morphologies associated with a significant decrease in mitochondrial fission events in primary mouse embryonic fibroblasts lacking β_cyto - and/or γ_cyto -actin. Interestingly, mitochondrial respiration in muscle was not measurably affected as oxygen consumption was similar in skeletal muscle fibers from 12 month-old muscle-specific β_cyto - and γ_cyto -actin KO mice. Instead, we found that the maximal rate of relaxation after isometric contraction was significantly slowed in muscles of 12-month-old β_cyto - and γ_cyto -actin muscle-specific KO mice. Our data suggest that impaired Ca²⁺ re-uptake may presage development of the observed SR morphological changes in aged mice while providing a potential pathological mechanism for the observed myopathy.

Trichinella spiralis muscle larvae excretory-secretory products induce changes in cytoskeletal and myogenic transcription factors in primary myoblast cultures

Trichinella spiralis infection in skeletal muscle culminates with nurse cell formation. The participation of excretory-secretory products of the muscle larvae has been implicated in this process through different studies performed in infected muscle and the muscle cell line C2C12. In this work, we developed primary myoblast cultures to analyse the changes induced by excretory-secretory products of the muscle larvae in muscle cells. Microarray analyses revealed expression changes in muscle cell differentiation, proliferation, cytoskeleton organisation, cell motion, transcription, cell cycle, apoptosis and signalling pathways such as MAPK, Jak-STAT, Wnt and PI3K-Akt. Some of these changes were further evaluated by other methodologies such as quantitative real-time PCR (qRT-PCR) and western blot, confirming that excretory-secretory products of the muscle larvae treated primary mouse myoblasts undergo increased proliferation, decreased expression of MHC and up-regulation of α-actin. In addition, changes in relevant muscle transcription factors (Pax7, Myf5 and Mef2c) were observed. Taken together, these results provide new information about how T. spiralis could alter the normal process of skeletal muscle repair after ML invasion to accomplish nurse cell formation.

Acting on identity: Myoblast fusion and the formation of the syncytial muscle fiber

The study of Drosophila muscle development dates back to the middle of the last century. Since that time, Drosophila has proved to be an ideal system for studying muscle development, differentiation, function, and disease. As in humans, Drosophila muscle forms via a series of conserved steps, starting with muscle specification, myoblast fusion, attachment to tendon cells, interactions with motorneurons, and sarcomere and myofibril formation. The genes and mechanisms required for these processes share striking similarities to those found in humans. The highly tractable genetic system and imaging approaches available in Drosophila allow for an efficient interrogation of muscle biology and for application of what we learn to other systems. In this article, we review our current understanding of muscle development in Drosophila, with a focus on myoblast fusion, the process responsible for the generation of syncytial muscle cells. We also compare and contrast those genes required for fusion in Drosophila and vertebrates.

Matrix Topographical Cue-Mediated Myogenic Differentiation of Human Embryonic Stem Cell Derivatives

Biomaterials varying in physical properties, chemical composition and biofunctionalities can be used as powerful tools to regulate skeletal muscle-specific cellular behaviors, including myogenic differentiation of progenitor cells. Biomaterials with defined topographical cues (e.g., patterned substrates) can mediate cellular alignment of progenitor cells and improve myogenic differentiation. In this study, we employed soft lithography techniques to create substrates with microtopographical cues and used these substrates to study the effect of matrix topographical cues on myogenic differentiation of human embryonic stem cell (hESC)-derived mesodermal progenitor cells expressing platelet-derived growth factor receptor alpha (PDGFRA). Our results show that the majority (>80%) of PDGFRA+ cells on micropatterned polydimethylsiloxane (PDMS) substrates were aligned along the direction of the microgrooves and underwent robust myogenic differentiation compared to those on non-patterned surfaces. Matrix topography-mediated alignment of the mononucleated cells promoted their fusion resulting in mainly (~86%⁻93%) multinucleated myotube formation. Furthermore, when implanted, the cells on the micropatterned substrates showed enhanced in vivo survival (>5⁻7 times) and engraftment (>4⁻6 times) in cardiotoxin-injured tibialis anterior (TA) muscles of NOD/SCID mice compared to cells cultured on corresponding non-patterned substrates.

Intercellular adhesion molecule-1 augments myoblast adhesion and fusion through homophilic trans-interactions

The overall objective of the study was to identify mechanisms through which intercellular adhesion molecule-1 (ICAM-1) augments the adhesive and fusogenic properties of myogenic cells. Hypotheses were tested using cultured myoblasts and fibroblasts, which do not constitutively express ICAM-1, and myoblasts and fibroblasts forced to express full length ICAM-1 or a truncated form lacking the cytoplasmic domain of ICAM-1. ICAM-1 mediated myoblast adhesion and fusion were quantified using novel assays and cell mixing experiments. We report that ICAM-1 augments myoblast adhesion to myoblasts and myotubes through homophilic trans-interactions. Such adhesive interactions enhanced levels of active Rac in adherent and fusing myoblasts, as well as triggered lamellipodia, spreading, and fusion of myoblasts through the signaling function of the cytoplasmic domain of ICAM-1. Rac inhibition negated ICAM-1 mediated lamellipodia, spreading, and fusion of myoblasts. The fusogenic property of ICAM-1-ICAM-1 interactions was restricted to myogenic cells, as forced expression of ICAM-1 by fibroblasts did not augment their fusion to ICAM-1+ myoblasts/myotubes. We conclude that ICAM-1 augments myoblast adhesion and fusion through its ability to self-associate and initiate Rac-mediated remodeling of the actin cytoskeleton.

The microprotein Minion controls cell fusion and muscle formation

Although recent evidence has pointed to the existence of small open reading frame (smORF)-encoded microproteins in mammals, their function remains to be determined. Skeletal muscle development requires fusion of mononuclear progenitors to form multinucleated myotubes, a critical but poorly understood process. Here we report the identification of Minion (microprotein inducer of fusion), a smORF encoding an essential skeletal muscle specific microprotein. Myogenic progenitors lacking Minion differentiate normally but fail to form syncytial myotubes, and Minion-deficient mice die perinatally and demonstrate a marked reduction in fused muscle fibres. The fusogenic activity of Minion is conserved in the human orthologue, and co-expression of Minion and the transmembrane protein Myomaker is sufficient to induce cellular fusion accompanied by rapid cytoskeletal rearrangement, even in non-muscle cells. These findings establish Minion as a novel microprotein required for muscle development, and define a two-component programme for the induction of mammalian cell fusion. Moreover, these data also significantly expand the known functions of smORF-encoded microproteins.

Myomerger induces fusion of non-fusogenic cells and is required for skeletal muscle development

Despite the importance of cell fusion for mammalian development and physiology, the factors critical for this process remain to be fully defined, which has severely limited our ability to reconstitute cell fusion. Myomaker (Tmem8c) is a muscle-specific protein required for myoblast fusion. Expression of myomaker in fibroblasts drives their fusion with myoblasts, but not with other myomaker-expressing fibroblasts, highlighting the requirement of additional myoblast-derived factors for fusion. Here we show that Gm7325, which we name myomerger, induces the fusion of myomaker-expressing fibroblasts. Thus, myomaker and myomerger together confer fusogenic activity to otherwise non-fusogenic cells. Myomerger is skeletal muscle-specific and genetic deletion in mice results in a paucity of muscle fibres demonstrating its requirement for normal muscle formation. Myomerger deficient myocytes differentiate and harbour organized sarcomeres but are fusion-incompetent. Our findings identify myomerger as a fundamental myoblast fusion protein and establish a system that begins to reconstitute mammalian cell fusion.

Cellular fusion is fundamental for skeletal muscle development. Here the authors show that myomerger is expressed in myoblasts, is essential for myoblast fusion in mice, and in co-operation with myomaker confers fusogenic ability to non-fusogenic cells.

A versatile cell-penetrating peptide-adaptor system for efficient delivery of molecular cargos to subcellular destinations

Cell penetrating peptides have long held great potential for delivery of biomolecular cargos for research, therapeutic and diagnostic purposes. They allow rapid, relatively nontoxic passage of a wide variety of biomolecules through the plasma membranes of living cells. However, CPP-based research tools and therapeutics have been stymied by poor efficiency in release from endosomes and a great deal of effort has been made to solve this ‘endosomal escape problem.’ Previously, we showed that use of a reversible, noncovalent coupling between CPP and cargo using calmodulin and a calmodulin binding motif allowed efficient delivery of cargo proteins to the cytoplasm in baby hamster kidney and other mammalian cell lines. The present report demonstrates the efficacy of our CPP-adaptor scheme for efficient delivery of model cargos to the cytoplasm using a variety of CPPs and adaptors. Effective overcoming of the endosomal escape problem is further demonstrated by the delivery of cargo to the nucleus, endoplasmic reticulum and peroxisomes by addition of appropriate subcellular localization signals to the cargos. CPP-adaptors were also used to deliver cargo to myotubes, demonstrating the feasibility of the system as an alternative to transfection for the manipulation of hard-to-transfect cells.

Myogenic differentiation depends on the interplay of Grb2 and N-WASP

Myogenesis requires a well-coordinated withdrawal from cell cycle, morphological changes and cell fusion mediated by actin cytoskeleton. Grb2 is an adaptor protein whose central SH2 domain binds to phosphorylated tyrosine residues of activated receptors and activates intracellular signaling pathway, while its N-terminal and C-terminal SH3 domains bind to proline rich proteins such as N-WASP (Neural-Wiskott Aldrich Syndrome Protein). We found that the expression of Grb2 was increased at the beginning of differentiation and remained constant during differentiation in C2C12 myoblasts. Knocking down endogenous Grb2 expression caused a significant increase in the fusion index and expression of MyHC, a terminal differentiation marker when compared with the control. Over expression of Grb2 in C2C12 (C2C12^Grb2-Myc) reduced myotube formation and expression of MyHC. Similarly over expression of Grb2^P49L-Myc (N-terminal SH3 domain mutant) or Grb2^R86K-Myc (SH2 domain mutant) inhibited myogenic differentiation of C2C12 cells. However, the expression of Grb2^P206L-Myc (C-terminal SH3 domain mutant) did not inhibit myotube formation and expression of MyHC. This suggests that the C-terminal SH3 domain of Grb2 is critical for the inhibition of myogenic differentiation. The C2C12^Grb2-Myc cells have reduced phalloidin staining at late stages of differentiation. Expression of N-WASP in C2C12^Grb2-Myc cells rescued the myogenic defect and increased phalloidin staining (increased F-actin) in these cells. Thus our results suggest that Grb2 is a negative regulator of myogenesis and reduces myogenic differentiation by inhibiting actin polymerization/remodeling through its C-terminal SH3 domain.

Hybrid Randomly Electrospun Poly(lactic-co-glycolic acid):Poly(ethylene oxide) (PLGA:PEO) Fibrous Scaffolds Enhancing Myoblast Differentiation and Alignment

Cellular responses are regulated by their microenvironments, and engineered synthetic scaffolds can offer control over different microenvironment properties. This important relationship can be used as a tool to manipulate cell fate and cell responses for different biomedical applications. We show for the first time in this study how blending of poly(ethylene oxide) (PEO) to poly(lactic-co-glycolic acid) (PLGA) fibers to yield hybrid scaffolds changes the physical and mechanical properties of PLGA fibrous scaffolds and in turn affects cellular response. For this purpose we employed electrospinning to create fibrous scaffolds mimicking the basic structural properties of the native extracellular matrix. We introduced PEO to PLGA electrospun fibers by spinning a blend of PLGA:PEO polymer solutions in different ratios. PEO served as a sacrificial component within the fibers upon hydration, leading to pore formation in the fibers, fiber twisting, increased scaffold disintegration, and hydrophilicity, decreased Young's modulus, and significantly improved strain at break of initially electrospun scaffolds. We observed that the blended PLGA:PEO fibrous scaffolds supported myoblast adhesion and proliferation and resulted in increased myotube formation and self-alignment, when compared to PLGA-only scaffolds, even though the scaffolds were randomly oriented. The 50:50 PLGA:PEO blended scaffold showed the most promising results in terms of mechanical properties, myotube formation, and alignment, suggesting an optimal microenvironment for myoblast differentiation from the PLGA:PEO blends tested. The explored approach for tuning fiber properties can easily extend to other polymeric scaffolds and provides a valuable tool to engineer fibrillar microenvironments for several biomedical applications.

A human in vitro model of Duchenne muscular dystrophy muscle formation and contractility

Tongue weakness, like all weakness in Duchenne muscular dystrophy (DMD), occurs as a result of contraction-induced muscle damage and deficient muscular repair. Although membrane fragility is known to potentiate injury in DMD, whether muscle stem cells are implicated in deficient muscular repair remains unclear. We hypothesized that DMD myoblasts are less sensitive to cues in the extracellular matrix designed to potentiate structure-function relationships of healthy muscle. To test this hypothesis, we drew inspiration from the tongue and engineered contractile human muscle tissues on thin films. On this platform, DMD myoblasts formed fewer and smaller myotubes and exhibited impaired polarization of the cell nucleus and contractile cytoskeleton when compared with healthy cells. These structural aberrations were reflected in their functional behavior, as engineered tongues from DMD myoblasts failed to achieve the same contractile strength as healthy tongue structures. These data suggest that dystrophic muscle may fail to organize with respect to extracellular cues necessary to potentiate adaptive growth and remodeling.

Role and organization of the actin cytoskeleton during cell-cell fusion

Cell-cell fusion is a ubiquitous process that underlies fertilization and development of eukaryotes. This process requires fusogenic machineries to promote plasma membrane merging, and also relies on the organization of dedicated sub-cortical cytoskeletal assemblies. This review describes the role of actin structures, so called actin fusion foci, essential for the fusion of two distinct cell types: Drosophila myoblast cells, which fuse to form myotubes, and sexually differentiated cells of the fission yeast Schizosaccharomyces pombe, which fuse to form a zygote. I describe the respective composition and organization of the two structures, discuss their proposed role in promoting plasma membrane apposition, and consider the universality of similar structures for cell-cell fusion.

Diaphanous regulates SCAR complex localization during Drosophila myoblast fusion

From Drosophila to man, multinucleated muscle cells form through cell-cell fusion. Using Drosophila as a model system, researchers first identified, and then demonstrated, the importance of actin cytoskeletal rearrangements at the site of fusion. These actin rearrangements at the fusion site are regulated by SCAR and WASp mediated Arp2/3 activation, which nucleates branched actin networks. Loss of SCAR, WASp or both leads to defects in myoblast fusion. Recently, we have found that the actin regulator Diaphanous (Dia) also plays a role both in organizing actin and in regulating Arp2/3 activity at the fusion site. In this Extra View article, we provide additional data showing that the Abi-SCAR complex accumulates at the fusion site and that excessive SCAR activity impairs myoblast fusion. Using constitutively active Dia constructs, we provide additional evidence that Dia functions upstream of SCAR activity to regulate actin dynamics at the fusion site and to localize the Abi-SCAR complex.

Spatial Geometries of Self-Assembled Chitohexaose Monolayers Regulate Myoblast Fusion

Myoblast fusion into functionally-distinct myotubes to form in vitro skeletal muscle constructs under differentiation serum-free conditions still remains a challenge. Herein, we report that our microtopographical carbohydrate substrates composed of bioactive hexa-N-acetyl-d-glucosamine (GlcNAc6) modulated the efficiency of myoblast fusion without requiring horse serum or any differentiation medium during cell culture. Promotion of the differentiation of dissociated mononucleated skeletal myoblasts (C2C12; a mouse myoblast cell line) into robust myotubes was found only on GlcNAc6 micropatterns, whereas the myoblasts on control, non-patterned GlcNAc6 substrates or GlcNAc6-free patterns exhibited an undifferentiated form. We also examined the possible role of GlcNAc6 micropatterns with various widths in the behavior of C2C12 cells in early and late stages of myogenesis through mRNA expression of myosin heavy chain (MyHC) isoforms. The spontaneous contraction of myotubes was investigated via the regulation of glucose transporter type 4 (GLUT4), which is involved in stimulating glucose uptake during cellular contraction. Narrow patterns demonstrated enhanced glucose uptake rate and generated a fast-twitch muscle fiber type, whereas the slow-twitch muscle fiber type was dominant on wider patterns. Our findings indicated that GlcNAc6-mediated integrin interactions are responsible for guiding myoblast fusion forward along with myotube formation.

Surface apposition and multiple cell contacts promote myoblast fusion in Drosophila flight muscles

Transmission EM methods reveal that cell–cell fusion of individual myoblasts with growing Drosophila flight muscles is a stepwise process in which the cell adhesion and branched actin machineries mediate tight apposition and formation of multiple contacts and pores between the surfaces of the fusing cells.

Fusion of individual myoblasts to form multinucleated myofibers constitutes a widely conserved program for growth of the somatic musculature. We have used electron microscopy methods to study this key form of cell–cell fusion during development of the indirect flight muscles (IFMs) of Drosophila melanogaster. We find that IFM myoblast–myotube fusion proceeds in a stepwise fashion and is governed by apparent cross talk between transmembrane and cytoskeletal elements. Our analysis suggests that cell adhesion is necessary for bringing myoblasts to within a minimal distance from the myotubes. The branched actin polymerization machinery acts subsequently to promote tight apposition between the surfaces of the two cell types and formation of multiple sites of cell–cell contact, giving rise to nascent fusion pores whose expansion establishes full cytoplasmic continuity. Given the conserved features of IFM myogenesis, this sequence of cell interactions and membrane events and the mechanistic significance of cell adhesion elements and the actin-based cytoskeleton are likely to represent general principles of the myoblast fusion process.

Tracing myoblast fusion in Drosophila embryos by fluorescent actin probes

Myoblast fusion in the Drosophila embryo is a highly elaborate process that is initiated by Founder Cells and Fusion-Competent Myoblasts (FCMs). It occurs through an asymmetric event in which actin foci assemble in the FCMs at points of cell-cell contact and direct the formation of membrane protrusions that drive fusion. Herein, we describe the approach that we have used to image in living embryos the highly dynamic actin foci and actin-rich projections that precede myoblast fusion. We discuss resources currently available for imaging actin and myogenesis, and our experience with these resources if available. This technical report is not intended to be comprehensive on providing instruction on standard microscopy practices or software utilization. However, we discuss microscope parameters that we have used in data collection, and our experience with image processing tools in data analysis.

Dual roles of palladin protein in in vitro myogenesis: inhibition of early induction but promotion of myotube maturation

Palladin is a microfilament-associated phosphoprotein whose function in skeletal muscle has rarely been studied. Therefore, we investigate whether myogenesis is influenced by the depletion of palladin expression known to interfere with the actin cytoskeleton dynamic required for skeletal muscle differentiation. The inhibition of palladin in C2C12 myoblasts leads to precocious myogenic differentiation with a concomitant reduction in cell apoptosis. This premature myogenesis is caused, in part, by an accelerated induction of p21, myogenin, and myosin heavy chain, suggesting that palladin acts as a negative regulator in early differentiation phases. Paradoxically, palladin-knockdown myoblasts are unable to differentiate terminally, despite their ability to perform some initial steps of differentiation. Cells with attenuated palladin expression form thinner myotubes with fewer myonuclei compared to those of the control. It is noteworthy that a negative regulator of myogenesis, myostatin, is activated in palladin-deficient myotubes, suggesting the palladin-mediated impairment of late-stage myogenesis. Additionally, overexpression of 140-kDa palladin inhibits myoblast differentiation while 200-kDa and 90-kDa palladin-overexpressed cells display an enhanced differentiation rate. Together, our data suggest that palladin might have both positive and negative roles in maintaining the proper skeletal myogenic differentiation in vitro.

Creatine kinase B is necessary to limit myoblast fusion during myogenesis

Myoblast fusion is critical for proper muscle growth and regeneration. During myoblast fusion, the localization of some molecules is spatially restricted; however, the exact reason for such localization is unknown. Creatine kinase B (CKB), which replenishes local ATP pools, localizes near the ends of cultured primary mouse myotubes. To gain insights into the function of CKB, we performed a yeast two-hybrid screen to identify CKB-interacting proteins. We identified molecules with a broad diversity of roles, including actin polymerization, intracellular protein trafficking, and alternative splicing, as well as sarcomeric components. In-depth studies of α-skeletal actin and α-cardiac actin, two predominant muscle actin isoforms, demonstrated their biochemical interaction and partial colocalization with CKB near the ends of myotubes in vitro. In contrast to other cell types, specific knockdown of CKB did not grossly affect actin polymerization in myotubes, suggesting other muscle-specific roles for CKB. Interestingly, knockdown of CKB resulted in significantly increased myoblast fusion and myotube size in vitro, whereas knockdown of creatine kinase M had no effect on these myogenic parameters. Our results suggest that localized CKB plays a key role in myotube formation by limiting myoblast fusion during myogenesis.

Intercellular adhesion molecule-1 expression by skeletal muscle cells augments myogenesis

We previously demonstrated that the expression of intercellular adhesion molecule-1 (ICAM-1) by skeletal muscle cells after muscle overload contributes to ensuing regenerative and hypertrophic processes in skeletal muscle. The objective of the present study is to reveal mechanisms through which skeletal muscle cell expression of ICAM-1 augments regenerative and hypertrophic processes of myogenesis. This was accomplished by genetically engineering C2C12 myoblasts to stably express ICAM-1, and by inhibiting the adhesive and signaling functions of ICAM-1 through the use of a neutralizing antibody or cell penetrating peptide, respectively. Expression of ICAM-1 by cultured skeletal muscle cells augmented myoblast-myoblast adhesion, myotube formation, myonuclear number, myotube alignment, myotube-myotube fusion, and myotube size without influencing the ability of myoblasts to proliferate or differentiate. ICAM-1 augmented myotube formation, myonuclear accretion, and myotube alignment through a mechanism involving adhesion-induced activation of ICAM-1 signaling, as these dependent measures were reduced via antibody and peptide inhibition of ICAM-1. The adhesive and signaling functions of ICAM-1 also facilitated myotube hypertrophy through a mechanism involving myotube-myotube fusion, protein synthesis, and Akt/p70s6k signaling. Our findings demonstrate that ICAM-1 expression by skeletal muscle cells augments myogenesis, and establish a novel mechanism through which the inflammatory response facilitates growth processes in skeletal muscle.

Myogenesis defect due to Toca-1 knockdown can be suppressed by expression of N-WASP

Skeletal muscle formation is a multistep process involving proliferation, differentiation, alignment and fusion of myoblasts to form myotubes which fuse with additional myoblast to form myofibers. Toca-1 (Transducer of Cdc42-dependent actin assembly), is an adaptor protein which activates N-WASP in conjunction with Cdc42 to facilitate membrane invagination, endocytosis and actin cytoskeleton remodeling. Expression of Toca-1 in mouse primary myoblasts and C2C12 myoblasts was up-regulated on day 1 of differentiation and subsequently down-regulated during differentiation. Knocking down Toca-1 expression in C2C12 cells (Toca-1(KD) cells) resulted in a significant decrease in myotube formation and expression of shRNA-resistant Toca-1 in Toca-1(KD) cells rescued the myogenic defect, suggesting that the knockdown was specific and Toca-1 is essential for myotube formation. Toca-1(KD) cells exhibited elongated spindle-like morphology, expressed myogenic markers (MyoD and MyHC) and localized N-Cadherin at cell periphery similar to control cells suggesting that Toca-1 is not essential for morphological changes or expression of proteins critical for differentiation. Toca-1(KD) cells displayed prominent actin fibers suggesting a defect in actin cytoskeleton turnover necessary for cell-cell fusion. Toca-1(KD) cells migrated faster than control cells and had a reduced number of vinculin patches similar to N-WASP(KO) MEF cells. Transfection of N-WASP-expressing plasmid into Toca-1(KD) cells restored myotube formation of Toca-1(KD) cells. Thus, our results suggest that Toca-1(KD) cells have defects in formation of myotubes probably due to reduced activity of actin cytoskeleton regulators such as N-WASP. This is the first study to identify and characterize the role of Toca-1 in myogenesis.

PI(4,5)P2 regulates myoblast fusion through Arp2/3 regulator localization at the fusion site

Cell-cell fusion is a regulated process that requires merging of the opposing membranes and underlying cytoskeletons. However, the integration between membrane and cytoskeleton signaling during fusion is not known. Using Drosophila, we demonstrate that the membrane phosphoinositide PI(4,5)P2 is a crucial regulator of F-actin dynamics during myoblast fusion. PI(4,5)P2 is locally enriched and colocalizes spatially and temporally with the F-actin focus that defines the fusion site. PI(4,5)P2 enrichment depends on receptor engagement but is upstream or parallel to actin remodeling. Regulators of actin branching via Arp2/3 colocalize with PI(4,5)P2 in vivo and bind PI(4,5)P2 in vitro. Manipulation of PI(4,5)P2 availability leads to impaired fusion, with a reduction in the F-actin focus size and altered focus morphology. Mechanistically, the changes in the actin focus are due to a failure in the enrichment of actin regulators at the fusion site. Moreover, improper localization of these regulators hinders expansion of the fusion interface. Thus, PI(4,5)P2 enrichment at the fusion site encodes spatial and temporal information that regulates fusion progression through the localization of activators of actin polymerization.

Self-contact elimination by membrane fusion

Mutual, homophilic cell-cell adhesion between epithelial cells is required for proper maintenance of epithelial barrier function. Whereas opposing membranes from neighboring cells rapidly assemble junctional complexes, self-contacting membranes curiously do not, suggesting that cells have the ability to prevent the maturation of self-junctions. Using a self-contact-inducing microfabricated substrate, we show that self-contacts of normal epithelial cells are rapidly eliminated by membrane fusion between two opposing plasma membranes of a single cell. This membrane fusion is most frequently observed in E-cadherin-expressing epithelial cells, but not in fibroblasts. The efficiency of self-contact elimination depends on extracellular calcium concentration and the level of E-cadherin, suggesting that E-cadherin, although not required, enhances membrane fusion efficiency by bringing opposing membranes into close apposition to one another. Additionally, Rho-associated protein kinase inhibition decreases self-contact-induced membrane fusion of epithelial cells, suggesting that this fusion may be mechanically regulated through the actin-myosin network. This self-contact-induced membrane fusion is a key elimination mechanism for unwanted self-junctions and may be a feature of cell self-recognition.

Normal muscle regeneration requires tight control of muscle cell fusion by tetraspanins CD9 and CD81

Skeletal muscle regeneration after injury follows a remarkable sequence of synchronized events. However, the mechanisms regulating the typical organization of the regenerating muscle at different stages remain largely unknown. Here we show that muscle regeneration in mice lacking either CD9 or CD81 is abnormal and characterized by the formation of discrete giant dystrophic myofibres, which form more quickly in the absence of both tetraspanins. We also show that, in myoblasts, these two tetraspanins associate with the immunoglobulin domain molecule CD9P-1 (EWI-F/FPRP), and that grafting of CD9P-1-depleted myoblasts in regenerating muscles also leads to abnormal regeneration. In vitro myotubes lacking CD9P-1 or both CD9 and CD81 fuse with a higher frequency than normal myotubes. Our study unveils a mechanism preventing inappropriate fusion of myotubes that has an important role in the restitution of normal muscle architecture during muscle regeneration.