Myocyte-derived Tnfsf14 is a survival factor necessary for myoblast differentiation and skeletal muscle regeneration

Live Cell Sorting of Differentiated Primary Human Osteoclasts Allows Generation of Transcriptomic Signature Matrix

Osteoclasts are specialized cells that degrade the bone matrix to create space for bone regeneration. During tumorigenesis, cancer cells metastasize to bone by disrupting bone's natural remodeling cycle. However, the mechanisms underlying critical bone-tumor interactions are poorly understood due to challenges in isolating osteoclasts from human bone. Thus, the conventional method to obtain osteoclasts for in vitro studies is via the differentiation of peripheral blood monocytes, which results in mixed cultures containing progenitor cells and osteoclasts of varying maturity and nuclearity. Presently, we hypothesized that the transcriptomic signatures of mature, multinucleated osteoclasts are distinct from osteoclasts with fewer nuclei. We established a live cell biomarker expression-based sorting protocol to allow purification of mature osteoclasts while maintaining viability and function. We observed that mature, multinucleated osteoclasts were transcriptomically distinct from those with fewer nuclei and that mature osteoclasts showed higher expression of genes that are associated with osteoclast fusion and function.

A non-invasive mouse model that recapitulates disuse-induced muscle atrophy in immobilized patients

Disuse muscle atrophy occurs consequent to prolonged limb immobility or bed rest, which represents an unmet medical need. As existing animal models of limb immobilization often cause skin erosion, edema, and other untoward effects, we here report an alternative method via thermoplastic immobilization of hindlimbs in mice. While significant decreases in the weight and fiber size were noted after 7 days of immobilization, no apparent skin erosion or edema was found. To shed light onto the molecular mechanism underlying this muscle wasting, we performed the next-generation sequencing analysis of gastrocnemius muscles from immobilized versus non-mobilized legs. Among a total of 55,487 genes analyzed, 787 genes were differentially expressed (> fourfold; 454 and 333 genes up- and down-regulated, respectively), which included genes associated with muscle tissue development, muscle system process, protein digestion and absorption, and inflammation-related signaling. From a clinical perspective, this model may help understand the molecular/cellular mechanism that drives muscle disuse and identify therapeutic strategies for this debilitating disease.

Nonsense-mediated mRNA decay suppresses injury-induced muscle regeneration via inhibiting MyoD transcriptional activity

Skeletal muscle regeneration is a crucial physiological process that occurs in response to injury or disease. As an important transcriptome surveillance system that regulates tissue development, the role of nonsense-mediated mRNA decay (NMD) in muscle regeneration remains unclear. Here, we found that NMD inhibits myoblast differentiation by targeting the phosphoinositide-3-kinase regulatory subunit 5 gene, which leads to the suppression of the transcriptional activity of myogenic differentiation (MyoD), a key regulator of myoblast differentiation. This disruption of MyoD transcriptional activity subsequently affects the expression levels of myogenin and myosin heavy chain, crucial markers of myoblast differentiation. Additionally, through up-frameshift protein 1 knockdown experiments, we observed that inhibiting NMD can accelerate muscle regeneration in vivo. These findings highlight the potential of NMD as a novel therapeutic target for the treatment of muscle-related injuries and diseases.

Nonsense-mediated mRNA decay promote C2C12 cell proliferation by targeting PIK3R5

Nonsense mediated mRNA decay (NMD) is a highly conserved RNA quality control system, which can specifically clear abnormal mRNA and play an important role in tumorigenesis. Myoblast proliferation plays an important role in the repair of skeletal muscle injury and the development of myosarcoma, and is controlled by a variety of transcription factors and signals. The molecular mechanism by which NMD regulates the proliferation of myoblast cells is not completely clear. In this study, we found that the NMD activity of skeletal muscle is high in 1-week-old mice but decreases gradually with age, corresponding to a weakening capacity for muscle growth and regeneration. Here, we provide evidence that NMD plays an important role in myoblast proliferation and apoptosis. In addition, we found that PIK3R5 is an NMD substrate gene which can inhibit AKT activity and C2C12 cell proliferation. Therefore, NMD can target PIK3R5 to enhance AKT activity, which in turn promotes C2C12 cell proliferation. This study provides new insights into NMD regulatory mechanisms in muscular development and into potential novel therapeutic strategies for muscle atrophy.

Muscle cell-derived cytokines in skeletal muscle regeneration

Regeneration of the mammalian adult skeletal muscle is a well-orchestrated process regulated by multiple proteins and signalling pathways. Cytokines constitute a major class of regulators of skeletal myogenesis. It is well established that infiltrating immune cells at the site of muscle injury secrete cytokines, which play critical roles in the myofibre repair and regeneration process. In the past 10-15 years, skeletal muscle itself has emerged as a prolific producer of cytokines. Much attention in the field has been focused on the endocrine effects of muscle-secreted cytokines (myokines) on metabolic regulation. However, ample evidence suggests that muscle-derived cytokines also regulate myogenic differentiation and muscle regeneration in an autocrine manner. In this review, we survey cytokines that meet two criteria: (a) evidence of expression by muscle cells; (b) evidence demonstrating a myogenic function. Dozens of cytokines representing several major classes make up this group, and together they regulate all steps of the myogenic process. How such a large array of cytokines coordinate their signalling to form a regulatory network is a fascinating, pressing question. Functional studies that can distinguish the source of the cytokines in vivo are also much needed in order to facilitate exploration of their full therapeutic potential.

TNFSF14-Derived Molecules as a Novel Treatment for Obesity and Type 2 Diabetes

Obesity is one of the most prevalent metabolic diseases in the Western world and correlates directly with glucose intolerance and insulin resistance, often culminating in Type 2 Diabetes (T2D). Importantly, our team has recently shown that the TNF superfamily (TNFSF) member protein, TNFSF14, has been reported to protect against high fat diet induced obesity and pre-diabetes. We hypothesized that mimics of TNFSF14 may therefore be valuable as anti-diabetic agents. In this study, we use in silico approaches to identify key regions of TNFSF14 responsible for binding to the Herpes virus entry mediator and Lymphotoxin β receptor. In vitro evaluation of a selection of optimised peptides identified six potentially therapeutic TNFSF14 peptides. We report that these peptides increased insulin and fatty acid oxidation signalling in skeletal muscle cells. We then selected one of these promising peptides to determine the efficacy to promote metabolic benefits in vivo. Importantly, the TNFSF14 peptide 7 reduced high fat diet-induced glucose intolerance, insulin resistance and hyperinsulinemia in a mouse model of obesity. In addition, we highlight that the TNFSF14 peptide 7 resulted in a marked reduction in liver steatosis and a concomitant increase in phospho-AMPK signalling. We conclude that TNFSF14-derived molecules positively regulate glucose homeostasis and lipid metabolism and may therefore open a completely novel therapeutic pathway for treating obesity and T2D.

GSI Treatment Preserves Protein Synthesis in C2C12 Myotubes

It has been demonstrated that inhibiting Notch signaling through γ-secretase inhibitor (GSI) treatment increases myogenesis, AKT/mTOR signaling, and muscle protein synthesis (MPS) in C2C12 myotubes. The purpose of this study was to determine if GSI-mediated effects on myogenesis and MPS are dependent on AKT/mTOR signaling. C2C12 cells were assessed for indices of myotube formation, anabolic signaling, and MPS following GSI treatment in combination with rapamycin and API-1, inhibitors of mTOR and AKT, respectively. GSI treatment increased several indices of myotube fusion and MPS in C2C12 myotubes. GSI-mediated effects on myotube formation and fusion were completely negated by treatment with rapamycin and API-1. Meanwhile, GSI treatment was able to rescue MPS in C2C12 myotubes exposed to rapamycin or rapamycin combined with API-1. Examination of protein expression revealed that GSI treatment was able to rescue pGSK3β Ser9 despite AKT inhibition by API-1. These findings demonstrate that GSI treatment is able to rescue MPS independent of AKT/mTOR signaling, possibly via GSK3β modulation.

Lentivirus-Mediated RNAi in Skeletal Myogenesis

RNA interference (RNAi) has greatly facilitated investigation of gene functions in vitro as well as in vivo. Recombinant lentivirus is widely used to deliver small hairpin RNA (shRNA) because of its high transduction capacity into diverse cell types and tissues. Here, we describe methods of lentivirus-mediated delivery of shRNA for the study of skeletal muscle cell differentiation in vitro and injury-induced muscle regeneration in mice.

The Role of TNF Family Molecules Light in Cellular Interaction Between Airway Smooth Muscle Cells and T Cells During Chronic Allergic Inflammation

Interaction between T cells and airway smooth muscle (ASM) cells has been identified as an important factor in the development of asthma. LIGHT (known as TNFSF14) -mediated signaling likely contributes to various inflammatory disorders and airway remodeling. The objective of this study was to investigate the roles of LIGHT-mediated pathways in the interaction between ASM cells and T cells during chronic allergic inflammation. Mice were sensitized and challenged by ovalbumin (OVA) to induce chronic airway allergic inflammation. The control group received PBS only. The histological features and LIGHT expressions in lungs were assessed in vivo. Furthermore, T cells and ASM cells derived from the model mice were co-cultured both in the presence and absence of anti-LIGHT Ab for 72 h. The effects of LIGHT blockade on expressions of downstream signaling molecules, proliferation, and apoptosis of ASM cells, differentiation of T cells, and inflammatory cytokines release were evaluated. We demonstrated that LIGHT blockade strikingly inhibited the mRNA and protein expressions of HVEM, c-JUN, and NFκB. Additionally, LIGHT blockade resulted in decreased proliferation and increased apoptosis of ASM cells. Moreover, depletion of LIGHT dramatically reduced the differentiation of CD4⁺ T cells into Th1, Th2, and Th17 cells, as well as inhibited inflammatory cytokines release including IL-13, TGF-β, and IFN-γ, which are associated with CD4⁺ T cell differentiation and ASM cell proliferation. LIGHT plays an important role in the interaction between T cells and ASM cells in chronic allergic asthma. Blockade of LIGHT markedly suppressed ASM hyperplasia and inflammatory responses, which might be modulated through HVEM-NFκB or c-JUN pathways. Therefore, targeting LIGHT is a promising therapeutic strategy for airway inflammation and remodeling in chronic allergic asthma.

Myogenic regulatory factors: The orchestrators of myogenesis after 30 years of discovery

Prenatal and postnatal myogenesis share many cellular and molecular aspects. Myogenic regulatory factors are basic Helix-Loop-Helix transcription factors that indispensably regulate both processes. These factors (Myf5, MyoD, Myogenin, and MRF4) function as an orchestrating cascade, with some overlapped actions. Prenatally, myogenic regulatory factors are restrictedly expressed in somite-derived myogenic progenitor cells and their derived myoblasts. Postnatally, myogenic regulatory factors are important in regulating the myogenesis process via satellite cells. Many positive and negative regulatory mechanisms exist either between myogenic regulatory factors themselves or between myogenic regulatory factors and other proteins. Upstream factors and signals are also involved in the control of myogenic regulatory factors expression within different prenatal and postnatal myogenic cells. Here, the authors have conducted a thorough and an up-to-date review of the myogenic regulatory factors since their discovery 30 years ago. This review discusses the myogenic regulatory factors structure, mechanism of action, and roles and regulations during prenatal and postnatal myogenesis. Impact statement Myogenic regulatory factors (MRFs) are key players in the process of myogenesis. Despite a considerable amount of literature regarding these factors, their exact mechanisms of actions are still incompletely understood with several overlapped functions. Herein, we revised what has hitherto been reported in the literature regarding MRF structures, molecular pathways that regulate their activities, and their roles during pre- and post-natal myogenesis. The work submitted in this review article is considered of great importance for researchers in the field of skeletal muscle formation and regeneration, as it provides a comprehensive summary of all the biological aspects of MRFs and advances a better understanding of the cellular and molecular mechanisms regulating myogenesis. Indeed, attaining a better understanding of MRFs could be utilized in developing novel therapeutic protocols for multiple myopathies.

NDRG4 promotes myogenesis via Akt/CREB activation

N-Myc downstream-regulated gene 4 (NDRG4) plays an important role in biological processes and pathogenesis, but its function in muscle development is unclear. In this study, we investigated the function of the NDRG4 gene in the regulation of myogenic differentiation. NDRG4 expression is upregulated during muscle regeneration and C2C12 myoblast differentiation. Gain and loss of function studies revealed that NDRG4 dramatically promotes expression of myogenic differentiation factor (MyoD), myogenin (MyoG), and myosin heavy chain (MyHC) genes and myotube formation. Mechanistically, the binding of NDRG4 to carboxyl-terminal modulator protein (CTMP) abates the interaction of CTMP and protein kinase B (Akt) and increases the phosphorylation of Akt and cAMP response element binding protein (CREB), which leads to increased expression of myogenic genes. Our results reveal that NDRG4 promotes myogenic differentiation via Akt/CREB activation.

Xk-related protein 8 regulates myoblast differentiation and survival

Xk-related protein 8 (Xkr8) is a scramblase and responsible for phosphatidylserine (PS) exposure on the cell surface in a caspase-dependent manner. Although PS exposure is found to be important for myotube formation during myoblast differentiation, the role of Xkr8 during myogenesis has not been elucidated. Here we show that Xkr8 contributes to myoblast differentiation. Xkr8 overexpression induced the formation of large myotubes during early differentiation, but this phenotype was not related to caspase-dependent cleavage of Xkr8. Furthermore, forced Xkr8 expression accelerated myoblast differentiation and conferred cell-death resistance after the induction of differentiation. Consistent with these results, Xkr8-knocked-down myoblasts exhibited impaired differentiation and more apoptotic cells during differentiation, implying the involvements of Xkr8 in the survival and proliferation of myoblasts. Taken together, the study shows Xkr8 influences myogenesis by acting as a positive regulator of terminal differentiation and myoblast survival.

Cxcl14 depletion accelerates skeletal myogenesis by promoting cell cycle withdrawal

Skeletal muscle in adults retains a robust ability to regenerate after injury, which progressively declines with age. Many of the regulators of skeletal myogenesis are unknown or incompletely understood. Intriguingly, muscle cells secrete a wide variety of factors, such as cytokines, which can influence muscle development and regeneration in an autocrine or paracrine manner. Here we describe chemokine (C-X-C motif) ligand 14 (Cxcl14) as a novel negative regulator of skeletal myogenesis. We found that Cxcl14 expression in myoblasts prevented cell cycle withdrawal, thereby inhibiting subsequent differentiation. Knockdown of Cxcl14 in vitro enhanced myogenic differentiation through promoting cell cycle withdrawal in an ERK1/2-dependent manner. Recapitulating these in vitro observations, the process of muscle regeneration following injury in young adult mice was accelerated by Cxcl14 depletion, accompanied by reduced cell proliferation. Furthermore, impaired capacity for muscle regeneration in aging mice was fully restored by Cxcl14 depletion. Our results indicate that Cxcl14 may be a promising target for development of therapeutics to treat muscle disease, especially aging-related muscle wasting.