Hepatocyte growth factor plays a dual role in regulating skeletal muscle satellite cell proliferation and differentiation

Agent-based model demonstrates the impact of nonlinear, complex interactions between cytokinces on muscle regeneration

Muscle regeneration is a complex process due to dynamic and multiscale biochemical and cellular interactions, making it difficult to identify microenvironmental conditions that are beneficial to muscle recovery from injury using experimental approaches alone. To understand the degree to which individual cellular behaviors impact endogenous mechanisms of muscle recovery, we developed an agent-based model (ABM) using the Cellular-Potts framework to simulate the dynamic microenvironment of a cross-section of murine skeletal muscle tissue. We referenced more than 100 published studies to define over 100 parameters and rules that dictate the behavior of muscle fibers, satellite stem cells (SSCs), fibroblasts, neutrophils, macrophages, microvessels, and lymphatic vessels, as well as their interactions with each other and the microenvironment. We utilized parameter density estimation to calibrate the model to temporal biological datasets describing cross-sectional area (CSA) recovery, SSC, and fibroblast cell counts at multiple timepoints following injury. The calibrated model was validated by comparison of other model outputs (macrophage, neutrophil, and capillaries counts) to experimental observations. Predictions for eight model perturbations that varied cell or cytokine input conditions were compared to published experimental studies to validate model predictive capabilities. We used Latin hypercube sampling and partial rank correlation coefficient to identify in silico perturbations of cytokine diffusion coefficients and decay rates to enhance CSA recovery. This analysis suggests that combined alterations of specific cytokine decay and diffusion parameters result in greater fibroblast and SSC proliferation compared to individual perturbations with a 13% increase in CSA recovery compared to unaltered regeneration at 28 days. These results enable guided development of therapeutic strategies that similarly alter muscle physiology (i.e. converting extracellular matrix [ECM]-bound cytokines into freely diffusible forms as studied in cancer therapeutics or delivery of exogenous cytokines) during regeneration to enhance muscle recovery after injury.

Control of muscle satellite cell function by specific exercise-induced cytokines and their applications in muscle maintenance

Exercise is recognized to play an observable role in improving human health, especially in promoting muscle hypertrophy and intervening in muscle mass loss-related diseases, including sarcopenia. Recent rapid advances have demonstrated that exercise induces the release of abundant cytokines from several tissues (e.g., liver, muscle, and adipose tissue), and multiple cytokines improve the functions or expand the numbers of adult stem cells, providing candidate cytokines for alleviating a wide range of diseases. Muscle satellite cells (SCs) are a population of muscle stem cells that are mitotically quiescent but exit from the dormancy state to become activated in response to physical stimuli, after which SCs undergo asymmetric divisions to generate new SCs (stem cell pool maintenance) and commit to later differentiation into myocytes (skeletal muscle replenishment). SCs are essential for the postnatal growth, maintenance, and regeneration of skeletal muscle. Emerging evidence reveals that exercise regulates muscle function largely via the exercise-induced cytokines that govern SC potential, but this phenomenon is complicated and confusing. This review provides a comprehensive integrative overview of the identified exercise-induced cytokines and the roles of these cytokines in SC function, providing a more complete picture regarding the mechanism of SC homeostasis and rejuvenation therapies for skeletal muscle.

Agent-based model demonstrates the impact of nonlinear, complex interactions between cytokines on muscle regeneration

Muscle regeneration is a complex process due to dynamic and multiscale biochemical and cellular interactions, making it difficult to identify microenvironmental conditions that are beneficial to muscle recovery from injury using experimental approaches alone. To understand the degree to which individual cellular behaviors impact endogenous mechanisms of muscle recovery, we developed an agent-based model (ABM) using the Cellular Potts framework to simulate the dynamic microenvironment of a cross-section of murine skeletal muscle tissue. We referenced more than 100 published studies to define over 100 parameters and rules that dictate the behavior of muscle fibers, satellite stem cells (SSC), fibroblasts, neutrophils, macrophages, microvessels, and lymphatic vessels, as well as their interactions with each other and the microenvironment. We utilized parameter density estimation to calibrate the model to temporal biological datasets describing cross-sectional area (CSA) recovery, SSC, and fibroblast cell counts at multiple time points following injury. The calibrated model was validated by comparison of other model outputs (macrophage, neutrophil, and capillaries counts) to experimental observations. Predictions for eight model perturbations that varied cell or cytokine input conditions were compared to published experimental studies to validate model predictive capabilities. We used Latin hypercube sampling and partial rank correlation coefficient to identify in silico perturbations of cytokine diffusion coefficients and decay rates to enhance CSA recovery. This analysis suggests that combined alterations of specific cytokine decay and diffusion parameters result in greater fibroblast and SSC proliferation compared to individual perturbations with a 13% increase in CSA recovery compared to unaltered regeneration at 28 days. These results enable guided development of therapeutic strategies that similarly alter muscle physiology (i.e. converting ECM-bound cytokines into freely diffusible forms as studied in cancer therapeutics or delivery of exogenous cytokines) during regeneration to enhance muscle recovery after injury.

Candidate Regulatory Genes for Hindlimb Development in the Embryos of the Chinese Alligator (Alligator sinensis)

Crocodilians, which are a kind of animal secondary adaptation to an aquatic environment, their hindlimb can provide the power needed to engage in various life activities, even in low-oxygen water environments. The development of limbs is an important aspect of animal growth and development, as it is closely linked to body movement, support, heat production, and other critical functions. For the Chinese alligator, the hindlimb is one of the main sources of power, and its development and differentiation will directly influence the survival ability in the wild. Furthermore, a better understanding of the hindlimb developmental process will provide data support for the comparative evolutionary and functional genomics of crocodilians. In this study, the expression levels of genes related to hindlimb development in the Chinese alligator embryos during fetal development (on days 29, 35, 41, and 46) were investigated through transcriptome analysis. A total of 1675 differentially expressed genes (DEGs) at different stages were identified by using limma software. These DEGs were then analyzed using weighted correlation network analysis (WGCNA), and 4 gene expression modules and 20 hub genes were identified that were associated with the development of hindlimbs in the Chinese alligator at different periods. The results of GO enrichment and hub gene expression showed that the hindlimb development of the Chinese alligator embryos involves the development of the embryonic structure, nervous system, and hindlimb muscle in the early stage (H29) and the development of metabolic capacity occurs in the later stage (H46). Additionally, the enrichment results showed that the AMPK signaling pathway, calcium signaling pathway, HIF-1 signaling pathway, and neuroactive ligand-receptor interaction are involved in the development of the hindlimb of the Chinese alligator. Among these, the HIF-1 signaling pathway and neuroactive ligand-receptor interaction may be related to the adaptation of Chinese alligators to low-oxygen environments. Additionally, five DEGs (CAV1, IRS2, LDHA, LDB3, and MYL3) were randomly selected for qRT-PCR to verify the transcriptome results. It is expected that further research on these genes will help us to better understand the process of embryonic hindlimb development in the Chinese alligator.

Single-cell analysis of bovine muscle-derived cell types for cultured meat production

Cultured meat technologies leverage the proliferation and differentiation of animal-derived stem cells ex vivo to produce edible tissues for human consumption in a sustainable fashion. However, skeletal muscle is a dynamic and highly complex tissue, involving the interplay of numerous mono- and multinucleated cells, including muscle fibers, satellite cells (SCs) and fibro-adipogenic progenitors (FAPs), and recreation of the tissue in vitro thus requires the characterization and manipulation of a broad range of cell types. Here, we use a single-cell RNA sequencing approach to characterize cellular heterogeneity within bovine muscle and muscle-derived cell cultures over time. Using this data, we identify numerous distinct cell types, and develop robust protocols for the easy purification and proliferation of several of these populations. We note overgrowth of undesirable cell types within heterogeneous proliferative cultures as a barrier to efficient cultured meat production, and use transcriptomics to identify conditions that favor the growth of SCs in the context of serum-free medium. Combining RNA velocities computed in silico with time-resolved flow cytometric analysis, we characterize dynamic subpopulations and transitions between active, quiescent, and committed states of SCs, and demonstrate methods for modulation of these states during long-term proliferative cultures. This work provides an important reference for advancing our knowledge of bovine skeletal muscle biology, and its application in the development of cultured meat technologies.

The Functions and Mechanisms of Tendon Stem/Progenitor Cells in Tendon Healing

Tendon injury is one of the prevalent disorders of the musculoskeletal system in orthopedics and is characterized by pain and limitation of joint function. Due to the difficulty of spontaneous tendon healing, and the scar tissue and low mechanical properties that usually develops after healing. Therefore, the healing of tendon injury remains a clinical challenge. Although there are a multitude of approaches to treating tendon injury, the therapeutic effects have not been satisfactory to date. Recent studies have shown that stem cell therapy has a facilitative effect on tendon healing. In particular, tendon stem/progenitor cells (TSPCs), a type of stem cell from tendon tissue, play an important role not only in tendon development and tendon homeostasis, but also in tendon healing. Compared to other stem cells, TSPCs have the potential to spontaneously differentiate into tenocytes and express higher levels of tendon-related genes. TSPCs promote tendon healing by three mechanisms: modulating the inflammatory response, promoting tenocyte proliferation, and accelerating collagen production and balancing extracellular matrix remodeling. However, current investigations have shown that TSPCs also have a negative effect on tendon healing. For example, misdifferentiation of TSPCs leads to a "failed healing response," which in turn leads to the development of chronic tendon injury (tendinopathy). The focus of this paper is to describe the characteristics of TSPCs and tenocytes, to demonstrate the roles of TSPCs in tendon healing, while discussing the approaches used to culture and differentiate TSPCs. In addition, the limitations of TSPCs in clinical application and their potential therapeutic strategies are elucidated.

Molecular regulation of satellite cells via intercellular signaling

Somatic stem cells are tissue-specific reserve cells tasked to sustain tissue homeostasis in adulthood and/or effect tissue regeneration after traumatic injury. The stem cells of skeletal muscle tissue are the satellite cells, which were originally described and named after their localization beneath the muscle fiber lamina and attached to the multi-nucleated muscle fibers. During adult homeostasis, satellite cells are maintained in quiescence, a state of reversible cell cycle arrest. Yet, upon injury, satellite cells are rapidly activated, becoming highly mitotically active to generate large numbers of myoblasts that differentiate and fuse to regenerate the injured muscle fibers. A subset self-renews to replenish the pool of muscle stem cells.Complex intrinsic gene regulatory networks maintain the quiescent state of satellite cells, or upon injury, direct their activation, proliferation, differentiation and self-renewal. Molecular cues from the satellite cells' environment provide the essential information as to when and where satellite cells are to stay quiescent or break quiescence and effect regenerative myogenesis. Predominantly, these cues are secreted, diffusible or membrane-bound ligands that bind to and activate their specific cognate receptors on the satellite cell to activate downstream signaling cascades and elicit context-specific cell behavior. This review aims to offer a concise overview of major intercellular signaling pathways regulating satellite cells during quiescence and in injury-induced skeletal muscle regeneration.

Muscle regeneration in gilthead sea bream: Implications of endocrine and local regulatory factors and the crosstalk with bone

Fish muscle regeneration is still a poorly known process. In the present study, an injury was done into the left anterior epaxial skeletal muscle of seventy 15 g gilthead sea bream (Sparus aurata) juveniles to evaluate at days 0, 1, 2, 4, 8, 16 and 30 post-wound, the expression of several muscle genes. Moreover, transcripts' expression in the bone (uninjured tissue) was also analyzed. Histology of the muscle showed the presence of dead tissue the first day after injury and how the damaged fibers were removed and replaced by new muscle fibers by day 16 that kept growing up to day 30. Gene expression results showed in muscle an early upregulation of igf-2 and a downregulation of ghr-1 and igf-1. Proteolytic systems expression increased with capn2 and ctsl peaking at 1 and 2 days post-injury, respectively and mafbx at day 8. A pattern of expression that fitted well with active myogenesis progression 16 days after the injury was then observed, with the recovery of igf-1, pax7, cmet, and cav1 expression; and later on, that of cav3 as well. Furthermore, the first days post-injury, the cytokines il-6 and il-15 were also upregulated confirming the tissue inflammation, while tnfα was only upregulated at days 16 and 30 to induce satellite cells recruitment; overall suggesting a possible role for these molecules as myokines. The results of the bone transcripts showed an upregulation first, of bmp2 and ctsk at days 1 and 2, respectively; then, ogn1 and ocn peaked at day 4 in parallel to mstn2 downregulation, and runx2 and ogn2 increased after 8 days of muscle injury, suggesting a possible tissue crosstalk during the regenerative process. Overall, the present model allows studying the sequential involvement of different regulatory molecules during muscle regeneration, as well as the potential relationship between muscle and other tissues such as bone to control musculoskeletal development and growth, pointing out an interesting new line of research in this group of vertebrates.

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.

Neuronal Agrin Promotes Proliferation of Primary Human Myoblasts in an Age-Dependent Manner

Neuronal agrin, a heparan sulphate proteoglycan secreted by the α-motor neurons, promotes the formation and maintenance of the neuromuscular junction by binding to Lrp4 and activating muscle-specific kinase (MuSK). Neuronal agrin also promotes myogenesis by enhancing differentiation and maturation of myotubes, but its effect on proliferating human myoblasts, which are often considered to be unresponsive to agrin, remains unclear. Using primary human myoblasts, we determined that neuronal agrin induced transient dephosphorylation of ERK1/2, while c-Abl, STAT3, and focal adhesion kinase were unresponsive. Gene silencing of Lrp4 and MuSK markedly reduced the BrdU incorporation, suggesting the functional importance of the Lrp4/MuSK complex for myoblast proliferation. Acute and chronic treatments with neuronal agrin increased the proliferation of human myoblasts in old donors, but they did not affect the proliferation of myoblasts in young donors. The C-terminal fragment of agrin which lacks the Lrp4-binding site and cannot activate MuSK had a similar age-dependent effect, indicating that the age-dependent signalling pathways activated by neuronal agrin involve the Lrp4/MuSK receptor complex as well as an Lrp4/MuSK-independent pathway which remained unknown. Collectively, our results highlight an age-dependent role for neuronal agrin in promoting the proliferation of human myoblasts.

Quantifying the inflammatory secretome of human intermuscular adipose tissue

Adipose tissue secretes an abundance of lipid and protein mediators, and this secretome is depot-specific, with local and systemic effects on metabolic regulation. Intermuscular adipose tissue (IMAT) accumulates within the skeletal muscle compartment in obesity, and is associated with insulin resistance and metabolic disease. While the human IMAT secretome decreases insulin sensitivity in vitro, its composition is entirely unknown. The current study was conducted to investigate the composition of the human IMAT secretome, compared to that of the subcutaneous (SAT) and visceral adipose tissue (VAT) depots. IMAT, SAT, and VAT explants from individuals with obesity were used to generate conditioned media. Proteomics analysis of conditioned media was performed using multiplex proximity extension assays, and eicosanoid analysis using liquid chromatography-tandem mass spectrometry. Compared to SAT and/or VAT, IMAT secreted significantly more cytokines (IL2, IL5, IL10, IL13, IL27, FGF23, IFNγ and CSF1) and chemokines (MCP1, IL8, CCL11, CCL20, CCL25 and CCL27). Adipokines hepatocyte growth factor and resistin were secreted significantly more by IMAT than SAT or VAT. IMAT secreted significantly more eicosanoids (PGE2, TXB2 , 5-HETE, and 12-HETE) compared to SAT and/or VAT. In the context of obesity, IMAT is a distinct adipose tissue with a highly immunogenic and inflammatory secretome, and given its proximity to skeletal muscle, may be critical to glucose regulation and insulin resistance.

Disturbance of calcium homeostasis and myogenesis caused by TET2 deletion in muscle stem cells

Skeletal muscle myogenesis is a sophisticated process controlled by genetic and epigenetic regulators. In animals, one of the key enzymes for the DNA demethylation of 5-methylcytosine is TET2. Although TET2 is essential for muscle development, the mechanisms by which TET2 regulates myogenesis, particularly the implication for muscle stem cells, remains unclear. In the present study, we employed the TET2 knockout mouse model to investigate the function of TET2 in muscle development and regeneration. We observed that TET2 deficiency caused impaired muscle stem cell proliferation and differentiation, resulting in the reduction in both myofiber number and muscle tissue size. Specifically, TET2 maintains calcium homeostasis in muscle stem cells by controlling the DNA methylation levels of the calcium pathway genes. Forced expression of the sodium/calcium exchanger protein SLC8A3 could rescue the myogenic defects in TET2 knockout cells. Our data not only illustrated the vital function of TET2 during myogenesis but also identified novel targets that contribute to calcium homeostasis for enhancing muscle function.

Lack of Tgfbr1 and Acvr1b synergistically stimulates myofibre hypertrophy and accelerates muscle regeneration

In skeletal muscle, transforming growth factor-β (TGF-β) family growth factors, TGF-β1 and myostatin, are involved in atrophy and muscle wasting disorders. Simultaneous interference with their signalling pathways may improve muscle function; however, little is known about their individual and combined receptor signalling. Here, we show that inhibition of TGF-β signalling by simultaneous muscle-specific knockout of TGF-β type I receptors Tgfbr1 and Acvr1b in mice, induces substantial hypertrophy, while such effect does not occur by single receptor knockout. Hypertrophy is induced by increased phosphorylation of Akt and p70S6K and reduced E3 ligases expression, while myonuclear number remains unaltered. Combined knockout of both TGF-β type I receptors increases the number of satellite cells, macrophages and improves regeneration post cardiotoxin-induced injury by stimulating myogenic differentiation. Extra cellular matrix gene expression is exclusively elevated in muscle with combined receptor knockout. Tgfbr1 and Acvr1b are synergistically involved in regulation of myofibre size, regeneration, and collagen deposition.

Transcriptional analysis of muscle tissue and isolated satellite cells in spastic cerebral palsy

AIM

To analyze transcriptomes from muscle tissue and cells to improve our understanding of differences in gene expression and molecular function in cerebral palsy (CP) muscle.

METHOD

In this case-control study, eight participants with CP (five males, three females; mean [SD] age 14y 2mo [1y 8mo]) and 11 comparison individuals (eight males, three females; mean [SD] age 14y 0mo [2y 6mo]) were enrolled after informed consent/assent and skeletal muscle was obtained during surgery. RNA was extracted from tissue and from primary satellite cells grown to form myotubes in vitro. RNA sequencing data were analyzed using validated informatics pipelines.

RESULTS

Analysis identified expression of 6308 genes in the tissue samples and 7459 in the cultured cells. Significant differential expression between CP and control was identified in 87 genes in the tissue and 90 genes in isolated satellite cell-derived myotube cultures.

INTERPRETATION

Both tissue and cell analyses identified differential expression of genes associated with muscle development and multiple pathways of interest. What this paper adds Expression differences were found in muscle tissue and in isolated muscle cells. There was low variability in expression among cells isolated from different muscles. Expression differences suggest complex functional alterations in spastic cerebral palsy.

Met and Cxcr4 cooperate to protect skeletal muscle stem cells against inflammation-induced damage during regeneration

Acute skeletal muscle injury is followed by an inflammatory response, removal of damaged tissue, and the generation of new muscle fibers by resident muscle stem cells, a process well characterized in murine injury models. Inflammatory cells are needed to remove the debris at the site of injury and provide signals that are beneficial for repair. However, they also release chemokines, reactive oxygen species, as well as enzymes for clearance of damaged cells and fibers, which muscle stem cells have to withstand in order to regenerate the muscle. We show here that MET and CXCR4 cooperate to protect muscle stem cells against the adverse environment encountered during muscle repair. This powerful cyto-protective role was revealed by the genetic ablation of Met and Cxcr4 in muscle stem cells of mice, which resulted in severe apoptosis during early stages of regeneration. TNFα neutralizing antibodies rescued the apoptosis, indicating that TNFα provides crucial cell-death signals during muscle repair that are counteracted by MET and CXCR4. We conclude that muscle stem cells require MET and CXCR4 to protect them against the harsh inflammatory environment encountered in an acute muscle injury.

Promoting endogenous repair of skeletal muscle using regenerative biomaterials

Skeletal muscles normally have a remarkable ability to repair themselves; however, large muscle injuries and several myopathies diminish this ability leading to permanent loss of function. No clinical therapy yet exists that reliably restores muscle integrity and function following severe injury. Consequently, numerous tissue engineering techniques, both acellular and with cells, are being investigated to enhance muscle regeneration. Biomaterials are an essential part of these techniques as they can present physical and biochemical signals that augment the repair process. Successful tissue engineering strategies require regenerative biomaterials that either actively promote endogenous muscle repair or create an environment supportive of regeneration. This review will discuss several acellular biomaterial strategies for skeletal muscle regeneration with a focus on those under investigation in vivo. This includes materials that release bioactive molecules, biomimetic materials and immunomodulatory materials.

The Satellite Cell at 60: The Foundation Years

The resident stem cell for skeletal muscle is the satellite cell. On the 50th anniversary of its discovery in 1961, we described the history of skeletal muscle research and the seminal findings made during the first 20 years in the life of the satellite cell (Scharner and Zammit 2011, doi: 10.1186/2044-5040-1-28). These studies established the satellite cell as the source of myoblasts for growth and regeneration of skeletal muscle. Now on the 60th anniversary, we highlight breakthroughs in the second phase of satellite cell research from 1980 to 2000. These include technical innovations such as isolation of primary satellite cells and viable muscle fibres complete with satellite cells in their niche, together with generation of many useful reagents including genetically modified organisms and antibodies still in use today. New methodologies were combined with description of endogenous satellite cells markers, notably Pax7. Discovery of the muscle regulatory factors Myf5, MyoD, myogenin, and MRF4 in the late 1980s revolutionized understanding of the control of both developmental and regerenative myogenesis. Emergence of genetic lineage markers facilitated identification of satellite cells in situ, and also empowered transplantation studies to examine satellite cell function. Finally, satellite cell heterogeneity and the supportive role of non-satellite cell types in muscle regeneration were described. These major advances in methodology and in understanding satellite cell biology provided further foundations for the dramatic escalation of work on muscle stem cells in the 21st century.

Skeletal Muscle Tissue Engineering: Biomaterials-Based Strategies for the Treatment of Volumetric Muscle Loss

Millions of Americans suffer from skeletal muscle injuries annually that can result in volumetric muscle loss (VML), where extensive musculoskeletal damage and tissue loss result in permanent functional deficits. In the case of small-scale injury skeletal muscle is capable of endogenous regeneration through activation of resident satellite cells (SCs). However, this is greatly reduced in VML injuries, which remove native biophysical and biochemical signaling cues and hinder the damaged tissue's ability to direct regeneration. The current clinical treatment for VML is autologous tissue transfer, but graft failure and scar tissue formation leave patients with limited functional recovery. Tissue engineering of instructive biomaterial scaffolds offers a promising approach for treating VML injuries. Herein, we review the strategic engineering of biophysical and biochemical cues in current scaffold designs that aid in restoring function to these preclinical VML injuries. We also discuss the successes and limitations of the three main biomaterial-based strategies to treat VML injuries: acellular scaffolds, cell-delivery scaffolds, and in vitro tissue engineered constructs. Finally, we examine several innovative approaches to enhancing the design of the next generation of engineered scaffolds to improve the functional regeneration of skeletal muscle following VML injuries.

Hepatocyte growth factor plays a dual role in tendon-derived stem cell proliferation, migration, and differentiation

Heterotopic ossification is common in tendon healing after trauma, but the detailed mechanisms remain unknown. Tendon-derived stem cells (TDSCs) are a type of progenitor cell found in the tendon niche, and their incorrect differentiation after trauma may lead to tendon calcification. The expression of hepatocyte growth factor (HGF) presents drastic fluctuations in serum/tissue after trauma and was found to activate quiescent stellate cells and contribute to wound healing; however, its potential role in TDSCs remains elusive. In this study, TDSCs isolated from rats were cultured in media containing HGF with or without a signaling inhibitor, and the proliferation, migration, and differentiation ability of TDSCs were measured to determine the role and mechanism of HGF in TDSCs. We showed that HGF promotes TDSC proliferation and migration but inhibits TDSC osteogenic differentiation ability. HGF activated-HGF/c-Met, mitogen-activated protein kinase (MAPK)/extracellular signal-regulated protein kinases 1 and 2 (ERK1/2), and phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) signaling, which was positively correlated with TDSCs proliferation and migration but negatively related to TDSC osteogenic differentiation ability. The phosphorylation of Smad1/5/8 was also negatively related to HGF/c-Met, MAPK/ERK1/2, and PI3K/AKT signaling, which demonstrated that the inhibition of osteogenic differentiation was dependent on BMP/Smad1/5/8 signaling. Overall, we showed that HGF could promote TDSCs proliferation and migration and inhibit osteogenic differentiation in vitro, suggesting a potential role for HGF as a cytokine treatment of tendon trauma.

The adipose tissue stromal vascular fraction secretome enhances the proliferation but inhibits the differentiation of myoblasts

Background

Adipose tissue is an excellent source for isolation of stem cells for treating various clinical conditions including injuries to the neuromuscular system. Many previous studies have focused on differentiating these adipose stem cells (ASCs) towards a Schwann cell-like phenotype (dASCs), which can enhance axon regeneration and reduce muscle atrophy. However, the stromal vascular fraction (SVF), from which the ASCs are derived, also exerts broad regenerative potential and might provide a faster route to clinical translation of the cell therapies for treatment of neuromuscular disorders.

Methods

The aim of this study was to establish the effects of SVF cells on the proliferation and differentiation of myoblasts using indirect co-culture experiments. A Growth Factor PCR Array was used to compare the secretomes of SVF and dASCs, and the downstream signaling pathways were investigated.

Results

SVF cells, unlike culture-expanded dASCs, expressed and secreted hepatocyte growth factor (HGF) at concentrations sufficient to enhance the proliferation of myoblasts. Pharmacological inhibitor studies revealed that the signal is mediated via ERK1/2 phosphorylation and that the effect is significantly reduced by the addition of 100 pM Norleual, a specific HGF inhibitor. When myoblasts were differentiated into multinucleated myotubes, the SVF cells reduced the expression levels of fast-type myosin heavy chain (MyHC2) suggesting an inhibition of the differentiation process.

Conclusions

In summary, this study shows the importance of HGF as a mediator of the SVF effects on myoblasts and provides further evidence for the importance of the secretome in cell therapy and regenerative medicine applications.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1096-6) contains supplementary material, which is available to authorized users.

Association of hemoglobin concentration with handgrip strength in relation to hepatocyte growth factor levels among elderly Japanese men aged 60-69 years: a cross-sectional study

Background

Hemoglobin concentration reportedly is positively associated with muscle strength, for example, handgrip strength. However, hemoglobin cannot repair muscle directly, but is beneficial only in a supportive role. Since hepatocyte growth factor (HGF) regulates muscle satellite cell production and differentiation, which is stimulated by organ injury, the supportive effect of hemoglobin should thus be stronger for participants with high HGF than for those with low HGF. However, the association between hemoglobin concentration and handgrip strength in relation to HGF levels remains unknown.

Methods

We conducted a cross-sectional study of 255 Japanese elderly men aged 60–69 years who participated in annual health check-ups in 2014–2015. The study population was categorized on the basis of a median value of HGF of 300.6 pg/mL.

Results

Among present study population, 128 participants showed low HGF. For participants with low HGF, hemoglobin concentration showed no significant association with handgrip strength (standardized parameter estimate (β) = 0.03, p = 0.767), but for those with high HGF, hemoglobin concentration was significantly positively associated with handgrip strength (β = 0.23, p = 0.014).

Conclusions

A significant positive association between hemoglobin level and handgrip strength was established for elderly Japanese men aged 60–69 years with high HGF but not for participants with low HGF. Our finding indicates that HGF levels could determine the relationship of hemoglobin concentration with handgrip strength in elderly Japanese men aged 60–69 years. This result can be expected to serve as an effective tool for the clarification of the roles played by HGF and hemoglobin concentration in maintenance of muscle strength.

Molecular Regulation of Cellular Quiescence: A Perspective from Adult Stem Cells and Its Niches

Cellular quiescence is a reversible growth arrest state. In response to extracellular environment, quiescent cells are capable of resuming proliferation for tissue homeostasis and tissue regeneration. Subpopulations of adult stem cells remain quiescent and reside in their specialized stem cell niches. Within the niche, they interact with a repertoire of niche components. Niche integrates signals to maintain quiescence or gear stem cells toward regeneration. Recent studies provide insights into the regulatory components of stem cell niche and their influence on residing stem cells. Aberrant niche activities perturb stem cell quiescence and activation, compromise stem cell functions, and contribute to tissue aging and disease pathogenesis. This review covers current knowledge regarding cellular quiescence with a focus on original and emerging concepts of how niches influence stem cell quiescence.

The physiopathologic interplay between stem cells and tissue niche in muscle regeneration and the role of IL-6 on muscle homeostasis and diseases

Skeletal muscle is a complex, dynamic tissue characterized by an elevated plasticity. Although the adult muscle is mainly composed of multinucleated fibers with post mitotic nuclei, it retains a remarkable ability to regenerate in response to traumatic events. The regenerative potential of the adult skeletal muscle relies in the activity of satellite cells, mononucleated cells residing within the muscle in intimate association with myofibers. Satellite cells normally remain quiescent in their sublaminar position, sporadically entering the cell cycle to guarantee an efficient cellular turnover, by fusing with pre-existing myofibers, and to maintain the stem cell pool. However, after muscle injury satellite cells undergo an extensive increase of their activity in response to environmental stimuli, thereby participating to the regeneration of a functional muscle tissue. Nevertheless, regeneration is affected in several pathologic conditions and by a wide range of environmental signals that are highly variable, not only through time, but also depending on the physiological or pathological conditions of the musculature. Among these factors, the interleukin-6 (IL-6) plays a critical physiopathologic role on muscle homeostasis and diseases. The basis of muscle regeneration and the impact of IL-6 on the physiopathology of skeletal muscle will be discussed.

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.

HGF potentiates extracellular matrix-driven migration of human myoblasts: involvement of matrix metalloproteinases and MAPK/ERK pathway

Background

The hepatocyte growth factor (HGF) is required for the activation of muscle progenitor cells called satellite cells (SC), plays a role in the migration of proliferating SC (myoblasts), and is present as a soluble factor during muscle regeneration, along with extracellular matrix (ECM) molecules. In this study, we aimed at determining whether HGF is able to interact with ECM proteins, particularly laminin 111 and fibronectin, and to modulate human myoblast migration.

Methods

We evaluated the expression of the HGF-receptor c-Met, laminin, and fibronectin receptors by immunoblotting, flow cytometry, or immunofluorescence and used Transwell assays to analyze myoblast migration on laminin 111 and fibronectin in the absence or presence of HGF. Zymography was used to check whether HGF could modulate the production of matrix metalloproteinases by human myoblasts, and the activation of MAPK/ERK pathways was evaluated by immunoblotting.

Results

We demonstrated that human myoblasts express c-Met, together with laminin and fibronectin receptors. We observed that human laminin 111 and fibronectin have a chemotactic effect on myoblast migration, and this was synergistically increased when low doses of HGF were added. We detected an increase in MMP-2 activity in myoblasts treated with HGF. Conversely, MMP-2 inhibition decreased the HGF-associated stimulation of cell migration triggered by laminin or fibronectin. HGF treatment also induced in human myoblasts activation of MAPK/ERK pathways, whose specific inhibition decreased the HGF-associated stimulus of cell migration triggered by laminin 111 or fibronectin.

Conclusions

We demonstrate that HGF induces ERK phosphorylation and MMP production, thus stimulating human myoblast migration on ECM molecules. Conceptually, these data state that the mechanisms involved in the migration of human myoblasts comprise both soluble and insoluble moieties. This should be taken into account to optimize the design of therapeutic cell transplantation strategies by improving the migration of donor cells within the host tissue, a main issue regarding this approach.

Electronic supplementary material

The online version of this article (10.1186/s13395-017-0138-6) contains supplementary material, which is available to authorized users.

Genetic biomarkers in non-contact muscle injuries in elite soccer players

PURPOSE

Damage to skeletal muscle necessitates regeneration to maintain proper muscle form and function. Interindividual differences in injury severity, recovery time, and injury rate could be explained by the presence of single nucleotide polymorphisms (SNPs) in genes involved in the reparation and regeneration of connective tissue . We wished to identify new genetic biomarkers that could help to prevent or minimize the risk of non-contact muscle injuries and are associated with a predisposition to developing muscle injuries.

METHODS

Using allelic discrimination techniques, we analysed 12 SNPs in selected genes from the genomic DNA of 74 elite soccer players.

RESULTS

SNPs in the hepatocyte growth factor (HGF) gene showed evidence of a statistically significant association with injury incidence, severity, and recovery time. SNPs in the SOX15 gene showed evidence of a statistically significant association with injury incidence. SNPs in the GEFT and LIF genes showed evidence of a statistically significant association with recovery time.

CONCLUSIONS

Genetic profile could explain why some elite soccer players are predisposed to suffer more injuries than others and why they need more time to recover from a particular injury. SNPs in HGF genes have an important role as biomarkers of biological processes fragility within muscle injuries related to injury rate, severity, and long recovery time.

Vascular Endothelial Growth Factor Up-regulation in Human Amniotic Fluid Stem Cell Enhances Nephroprotection After Ischemia-Reperfusion Injury in the Rat

OBJECTIVE

To evaluate if the up-regulation of vascular endothelial growth factor strengthens the protective effect of amniotic fluid stem cells in a renal ischemia-reperfusion injury model.

DESIGN

Randomized animal study.

SETTINGS

University research laboratory.

SUBJECTS

A total of 40 males 12-week-old Wistar rats were subjected to ischemia-reperfusion and assigned to four groups: amniotic fluid stem cells, vascular endothelial growth factor-amniotic fluid stem cells in two different doses, and vehicle. Ten animals were used as sham-controls.

INTERVENTION

Six hours after induction of renal ischemia-reperfusion injury, amniotic fluid stem cells, vascular endothelial growth factor-amniotic fluid stem cells in two different doses, or vehicle were injected intraarterially.

MEASUREMENTS AND MAIN RESULTS

Analyses were performed at 24 hours, 48 hours, and 2 months after treatment. Outcome measures included serum creatinine, urine microprotenuira, and immunohistomorphometric analyses. Vascular endothelial growth factor-amniotic fluid stem cells induced a significantly higher nephroprotection than amniotic fluid stem cells. This effect was mediated mainly by immunomodulation, which led to lower macrophage infiltration and higher presence of regulatory T cell after ischemia-reperfusion injury. At medium term, it inhibited the progression toward chronic kidney disease. Vascular endothelial growth factor-amniotic fluid stem cells can worsen the ischemia-reperfusion injury when delivered in a high dose.

CONCLUSIONS

Up-regulation of vascular endothelial growth factor enhances the therapeutic effect of human amniotic fluid stem cells in rats with renal ischemia-reperfusion injury, mainly by mitogenic, angiogenic, and anti-inflammatory mechanisms.

* Design of an In Vitro Model of Cell Recruitment for Skeletal Muscle Regeneration Using Hepatocyte Growth Factor-Loaded Fibrin Microthreads

Large skeletal muscle defects that result in volumetric muscle loss (VML) result in the destruction of the basal lamina, which removes key signaling molecules such as hepatocyte growth factor (HGF) from the wound site, eliminating the endogenous capacity of these injuries to regenerate. We recently showed that HGF-loaded fibrin microthreads increased the force production in muscle tissues after 60 days in a mouse VML model. In this study, we created an in vitro, three-dimensional (3D) microscale outgrowth assay system designed to mimic cell recruitment in vivo, and investigated the effect of HGF-loaded, cross-linked fibrin microthreads on myoblast recruitment to predict the results observed in vivo. This outgrowth assay discretely separated the cellular and molecular functions (migration, proliferation, and chemotaxis) that direct outgrowth from the wound margin, creating a powerful platform to model cell recruitment in axially aligned tissues, such as skeletal muscle. The degree of cross-linking was controlled by pH and microthreads cross-linked using physiologically neutral pH (EDCn) facilitated the release of active HGF; increasing the two-dimensional migration and 3D outgrowth of myoblasts twofold. While HGF adsorbed to uncross-linked microthreads, it did not enhance myoblast migration, possibly due to the low concentrations that were adsorbed. Regardless of the amount of HGF adsorbed on the microthreads, myoblast proliferation increased significantly on stiffer, cross-linked microthreads. Together, the results of these studies show that HGF loaded onto EDCn microthreads supported enhanced myoblast migration and recruitment and suggest that our novel outgrowth assay system is a robust in vitro screening tool that predicts the performance of fibrin microthreads in vivo.

Mesenchymal stem cells and myoblast differentiation under HGF and IGF-1 stimulation for 3D skeletal muscle tissue engineering

Background

Volumetric muscle loss caused by trauma or after tumour surgery exceeds the natural regeneration capacity of skeletal muscle. Hence, the future goal of tissue engineering (TE) is the replacement and repair of lost muscle tissue by newly generating skeletal muscle combining different cell sources, such as myoblasts and mesenchymal stem cells (MSCs), within a three-dimensional matrix. Latest research showed that seeding skeletal muscle cells on aligned constructs enhance the formation of myotubes as well as cell alignment and may provide a further step towards the clinical application of engineered skeletal muscle.

In this study the myogenic differentiation potential of MSCs upon co-cultivation with myoblasts and under stimulation with hepatocyte growth factor (HGF) and insulin-like growth factor-1 (IGF-1) was evaluated. We further analysed the behaviour of MSC-myoblast co-cultures in different 3D matrices.

Results

Primary rat myoblasts and rat MSCs were mono- and co-cultivated for 2, 7 or 14 days. The effect of different concentrations of HGF and IGF-1 alone, as well as in combination, on myogenic differentiation was analysed using microscopy, multicolour flow cytometry and real-time PCR. Furthermore, the influence of different three-dimensional culture models, such as fibrin, fibrin-collagen-I gels and parallel aligned electrospun poly-ε-caprolacton collagen-I nanofibers, on myogenic differentiation was analysed. MSCs could be successfully differentiated into the myogenic lineage both in mono- and in co-cultures independent of HGF and IGF-1 stimulation by expressing desmin, myocyte enhancer factor 2, myosin heavy chain 2 and alpha-sarcomeric actinin. An increased expression of different myogenic key markers could be observed under HGF and IGF-1 stimulation. Even though, stimulation with HGF/IGF-1 does not seem essential for sufficient myogenic differentiation. Three-dimensional cultivation in fibrin-collagen-I gels induced higher levels of myogenic differentiation compared with two-dimensional experiments. Cultivation on poly-ε-caprolacton-collagen-I nanofibers induced parallel alignment of cells and positive expression of desmin.

Conclusions

In this study, we were able to myogenically differentiate MSC upon mono- and co-cultivation with myoblasts. The addition of HGF/IGF-1 might not be essential for achieving successful myogenic differentiation. Furthermore, with the development of a biocompatible nanofiber scaffold we established the basis for further experiments aiming at the generation of functional muscle tissue.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-017-0131-2) contains supplementary material, which is available to authorized users.

Designing Biopolymer Microthreads for Tissue Engineering and Regenerative Medicine

Native tissue structures possess elaborate extracellular matrix (ECM) architectures that inspire the design of fibrous structures in the field of regenerative medicine. We review the literature with respect to the successes and failures, as well as the future promise of biopolymer microthreads as scaffolds to promote endogenous and exogenous tissue regeneration. Biomimetic microthread tissue constructs have been proposed for the functional regeneration of tendon, ligament, skeletal muscle, and ventricular myocardial tissues. To date, biopolymer microthreads have demonstrated promising results as materials to recapitulate the hierarchical structure of simple and complex tissues and well as biochemical signaling cues to direct cell-mediated tissue regeneration. Biopolymer microthreads have also demonstrated exciting potential as a platform technology for the targeted delivery of stem cells and therapeutic molecules. Future studies will focus on the design of microthread-based tissue analogs that strategically integrate growth factors and progenitor cells to temporally direct cell-mediated processes that promote enhanced functional tissue regeneration.

Hepatocyte Growth Factor and Satellite Cell Activation

Satellite cells are the "currency" for the muscle growth that is critical to meat production in many species, as well as to phenotypic distinctions in development at the level of species or taxa, and for human muscle growth, function and regeneration. Careful research on the activation and behaviour of satellite cells, the stem cells in skeletal muscle, including cross-species comparisons, has potential to reveal the mechanisms underlying pathological conditions in animals and humans, and to anticipate implications of development, evolution and environmental change on muscle function and animal performance.

Satellite cells in human skeletal muscle plasticity

Skeletal muscle satellite cells are considered to play a crucial role in muscle fiber maintenance, repair and remodeling. Our knowledge of the role of satellite cells in muscle fiber adaptation has traditionally relied on in vitro cell and in vivo animal models. Over the past decade, a genuine effort has been made to translate these results to humans under physiological conditions. Findings from in vivo human studies suggest that satellite cells play a key role in skeletal muscle fiber repair/remodeling in response to exercise. Mounting evidence indicates that aging has a profound impact on the regulation of satellite cells in human skeletal muscle. Yet, the precise role of satellite cells in the development of muscle fiber atrophy with age remains unresolved. This review seeks to integrate recent results from in vivo human studies on satellite cell function in muscle fiber repair/remodeling in the wider context of satellite cell biology whose literature is largely based on animal and cell models.

Molecular targets of androgen signaling that characterize skeletal muscle recovery and regeneration

The high regenerative capacity of adult skeletal muscle relies on a self-renewing depot of adult stem cells, termed muscle satellite cells (MSCs). Androgens, known mediators of overall body composition and specifically skeletal muscle mass, have been shown to regulate MSCs. The possible overlapping function of androgen regulation of muscle growth and MSC activation has not been carefully investigated with regards to muscle regeneration.Therefore, the aim of this study was to examine coinciding androgen-mediated genetic changes in an in vitro MSC model and clinically relevant in vivo models. A gene signature was established via microarray analysis for androgen-mediated MSC engagement and highlighted several markers including follistatin (FST), IGF-1, C-X-C chemokine receptor 4 (CXCR4), hepatocyte growth factor (HGF) and glucocorticoid receptor (GR). In an in vivo muscle atrophy model, androgen re-supplementation significantly increased muscle size and expression of IGF-1, FST, and HGF, while significantly decreasing expression of GR. Biphasic gene expression profiles over the 7-day re-supplementation period identified temporal androgen regulation of molecular targets involved in satellite cell engagement into myogenesis. In a muscle injury model, removal of androgens resulted in delayed muscle recovery and regeneration. Modifications in the androgen signaling gene signature, along with reduced Pax7 and MyoD expression, suggested that limited MSC activation and increased inflammation contributed to the delayed regeneration. However, enhanced MSC activation in the androgen-deplete mouse injury model was driven by an androgen receptor (AR) agonist. These results provide novel in vitro and in vivo evidence describing molecular targets of androgen signaling, while also increasing support for translational use of AR agonists in skeletal muscle recovery and regeneration.

Biomimetic scaffolds for regeneration of volumetric muscle loss in skeletal muscle injuries

Skeletal muscle injuries typically result from traumatic incidents such as combat injuries where soft-tissue extremity injuries are present in one of four cases. Further, about 4.5 million reconstructive surgical procedures are performed annually as a result of car accidents, cancer ablation, or cosmetic procedures. These combat- and trauma-induced skeletal muscle injuries are characterized by volumetric muscle loss (VML), which significantly reduces the functionality of the injured muscle. While skeletal muscle has an innate repair mechanism, it is unable to compensate for VML injuries because large amounts of tissue including connective tissue and basement membrane are removed or destroyed. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. Here, the structure and organization of native skeletal muscle tissue is described in order to reveal clear design parameters that are necessary for scaffolds to mimic in order to successfully regenerate muscular tissue. We review the literature with respect to the materials and methodologies used to develop scaffolds for skeletal muscle tissue regeneration as well as the limitations of these materials. We further discuss the variety of cell sources and different injury models to provide some context for the multiple approaches used to evaluate these scaffold materials. Recent findings are highlighted to address the state of the field and directions are outlined for future strategies, both in scaffold design and in the use of different injury models to evaluate these materials, for regenerating functional skeletal muscle.

STATEMENT OF SIGNIFICANCE

Volumetric muscle loss (VML) injuries result from traumatic incidents such as those presented from combat missions, where soft-tissue extremity injuries are represented in one of four cases. These injuries remove or destroy large amounts of skeletal muscle including the basement membrane and connective tissue, removing the structural, mechanical, and biochemical cues that usually direct its repair. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. In this review, we examine current strategies for the development of scaffold materials designed for skeletal muscle regeneration, highlighting advances and limitations associated with these methodologies. Finally, we identify future approaches to enhance skeletal muscle regeneration.

Rapid release of growth factors regenerates force output in volumetric muscle loss injuries

A significant challenge in the design and development of biomaterial scaffolds is to incorporate mechanical and biochemical cues to direct organized tissue growth. In this study, we investigated the effect of hepatocyte growth factor (HGF) loaded, crosslinked fibrin (EDCn-HGF) microthread scaffolds on skeletal muscle regeneration in a mouse model of volumetric muscle loss (VML). The rapid, sustained release of HGF significantly enhanced the force production of muscle tissue 60 days after injury, recovering more than 200% of the force output relative to measurements recorded immediately after injury. HGF delivery increased the number of differentiating myoblasts 14 days after injury, and supported an enhanced angiogenic response. The architectural morphology of microthread scaffolds supported the ingrowth of nascent myofibers into the wound site, in contrast to fibrin gel implants which did not support functional regeneration. Together, these data suggest that EDCn-HGF microthreads recapitulate several of the regenerative cues lost in VML injuries, promote remodeling of functional muscle tissue, and enhance the functional regeneration of skeletal muscle. Further, by strategically incorporating specific biochemical factors and precisely tuning the structural and mechanical properties of fibrin microthreads, we have developed a powerful platform technology that may enhance regeneration in other axially aligned tissues.

Myogenic induction of adult and pluripotent stem cells using recombinant proteins

Met Activating Genetically Improved Chimeric Factor 1 (Magic-F1) is a human recombinant protein, derived from dimerization of the receptor-binding domain of hepatocyte growth factor. Previous experiments demonstrate that in transgenic mice, the skeletal muscle specific expression of Magic-F1 can induce a constitutive muscular hypertrophy, improving running performance and accelerating muscle regeneration after injury. In order to evaluate the therapeutic potential of Magic-F1, we tested its effect on multipotent and pluripotent stem cells. In murine mesoangioblasts (adult vessel-associated stem cells), the presence of Magic-F1 did not alter their osteogenic, adipogenic or smooth muscle differentiation ability. However, when analyzing their myogenic potential, mesoangioblasts expressing Magic-F1 differentiated spontaneously into myotubes. Finally, Magic-F1 inducible cassette was inserted into a murine embryonic stem cell line by homologous recombination. When embryonic stem cells were subjected to myogenic differentiation, the presence of Magic-F1 resulted in the upregulation of Pax3 and Pax7 that enhanced the myogenic commitment of transgenic pluripotent stem cells. Taken together our results candidate Magic-F1 as a potent myogenic stimulator, able to enhance muscular differentiation from both adult and pluripotent stem cells.

Messenger RNA sequencing and pathway analysis provide novel insights into the biological basis of chickens' feed efficiency

Background

Advanced selection technologies have been developed and continually optimized to improve traits of agricultural importance; however, these methods have been primarily applied without knowledge of underlying biological changes that may be induced by selection. This study aims to characterize the biological basis of differences between chickens with low and high feed efficiency (FE) with a long-term goal of improving the ability to select for FE.

Results

High-throughput RNA sequencing was performed on 23 breast muscle samples from commercial broiler chickens with extremely high (n = 10) and low (n = 13) FE. An average of 34 million paired-end reads (75 bp) were produced for each sample, 80% of which were properly mapped to the chicken reference genome (Ensembl Galgal4). Differential expression analysis identified 1,059 genes (FDR < 0.05) that significantly divergently expressed in breast muscle between the high- and low-FE chickens. Gene function analysis revealed that genes involved in muscle remodeling, inflammatory response and free radical scavenging were mostly up-regulated in the high-FE birds. Additionally, growth hormone and IGFs/PI3K/Akt signaling pathways were enriched in differentially expressed genes, which might contribute to the high breast muscle yield in high-FE birds and partly explain the FE advantage of high-FE chickens.

Conclusions

This study provides novel insights into transcriptional differences in breast muscle between high- and low-FE broiler chickens. Our results show that feed efficiency is associated with breast muscle growth in these birds; furthermore, some physiological changes, e.g., inflammatory response and oxidative stress, may occur in the breast muscle of the high-FE chickens, which may be of concern for continued selection for both of these traits together in modern broiler chickens.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1364-0) contains supplementary material, which is available to authorized users.

Anti-Differentiation Effect of Oncogenic Met Receptor in Terminally-Differentiated Myotubes

Activation of the hepatocyte growth factor/Met receptor is involved in muscle regeneration, through promotion of proliferation and inhibition of differentiation in myogenic stem cells (MSCs). We previously described that the specific expression of an oncogenic version of the Met receptor (Tpr-Met) in terminally-differentiated skeletal muscle causes muscle wasting in vivo. Here, we induced Tpr-Met in differentiated myotube cultures derived from the transgenic mouse. These cultures showed a reduced protein level of myosin heavy chain (MyHC), increased phosphorylation of Erk1,2 MAPK, the formation of giant sacs of myonuclei and the collapse of elongated myotubes. Treatment of the cultures with an inhibitor of the MAPK kinase pathway or with an inhibitor of the proteasome increased the expression levels of MyHC. In addition, the inhibition of the MAPK kinase pathway prevented the formation of myosacs and myotube collapse. Finally, we showed that induction of Tpr-Met in primary myotubes was unable to produce endoreplication in their nuclei. In conclusion, our data indicate that multinucleated, fused myotubes may be forced to disassemble their contractile apparatus by the Tpr-Met oncogenic factor, but they resist the stimulus toward the reactivation of the cell cycle.

Dose-dependent modulation of myogenesis by HGF: implications for c-Met expression and downstream signalling pathways

Hepatocyte growth factor (HGF) regulates satellite cell activation, proliferation, and differentiation. We analyzed the dose-dependent effects of HGF on myogenesis. Murine C2C12 and human donor-derived skeletal muscle myoblasts were treated with 0, 2, or 10 ng/ml HGF followed by assessment of proliferation and differentiation. HGF (2 ng/ml) significantly promoted cell division, but reduced myogenic commitment and fusion. Conversely, 10 ng/ml HGF reduced proliferative capability, but increased differentiation. c-Met expression analysis revealed significantly decreased expression in differentiating cells cultured with 2 ng/ml HGF, but increased expression in proliferating cells with 10 ng/ml HGF. Mitogen-activated protein kinase (MAPKs: ERK, JNK, or p38K) and phosphatidylinositol-3-kinase (PI3K) inhibition abrogated the HGF-stimulated increase in cell number. Interestingly, PI3K and p38 kinase facilitated the negative effect of HGF on proliferation, while ERK inhibition abrogated the HGF-mediated decrease in differentiation. Dose-dependent effects of HGF are mediated by changes in c-Met expression and downstream MAPK and PI3K signalling.

A novel isoform of met receptor tyrosine kinase blocks hepatocyte growth factor/Met signaling and stimulates skeletal muscle cell differentiation

Hepatocyte growth factor (HGF) and its receptor, Met, regulate skeletal muscle differentiation. In the present study, we identified a novel alternatively spliced isoform of Met lacking exon 13 (designated Δ13Met), which is expressed mainly in human skeletal muscle. Alternative splicing yielded a truncated Met having extracellular domain only, suggesting an inhibitory role. Indeed, Δ13Met expression led to a decrease in HGF-induced tyrosine phosphorylation of Met and ERK phosphorylation, as well as cell proliferation and migration via sequestration of HGF. Interestingly, in human primary myoblasts undergoing differentiation, Δ13Met mRNA and protein levels were rapidly increased, concomitantly with a decrease in wild type Met mRNA and protein. Inhibition of Δ13Met with siRNA led to a decreased differentiation, whereas its overexpression potentiated differentiation of human primary myoblasts. Furthermore, in notexin-induced mouse injury model, exogenous Δ13Met expression enhanced regeneration of skeletal muscle, further confirming a stimulatory role of the isoform in muscle cell differentiation. In summary, we identified a novel alternatively spliced inhibitory isoform of Met that stimulates muscle cell differentiation, which confers a new means to control muscle differentiation and/or regeneration.

In situ regeneration of skeletal muscle tissue through host cell recruitment

Standard reconstructive procedures for restoring normal function after skeletal muscle defects involve the use of existing host tissues such as muscular flaps. In many instances, this approach is not feasible and delays the rehabilitation process and restoration of tissue function. Currently, cell-based tissue engineering strategies have been used for reconstruction; however, donor tissue biopsy and ex vivo cell manipulation are required prior to implantation. The present study aimed to overcome these limitations by demonstrating mobilization of muscle cells into a target-specific site for in situ muscle regeneration. First, we investigated whether host muscle cells could be mobilized into an implanted scaffold. Poly(l-lactic acid) (PLLA) scaffolds were implanted in the tibialis anterior (TA) muscle of rats, and the retrieved scaffolds were characterized by examining host cell infiltration in the scaffolds. The host cell infiltrates, including Pax7+ cells, gradually increased with time. Second, we demonstrated that host muscle cells could be enriched by a myogenic factor released from the scaffolds. Gelatin-based scaffolds containing a myogenic factor were implanted in the TA muscle of rats, and the Pax7+ cell infiltration and newly formed muscle fibers were examined. By the second week after implantation, the Pax7+ cell infiltrates and muscle formation were significantly accelerated within the scaffolds containing insulin-like growth factor 1 (IGF-1). Our data suggest an ability of host stem cells to be recruited into the scaffolds with the capability of differentiating to muscle cells. In addition, the myogenic factor effectively promoted host cell recruitment, which resulted in accelerating muscle regeneration in situ.

Glypican-1 regulates myoblast response to HGF via Met in a lipid raft-dependent mechanism: effect on migration of skeletal muscle precursor cells

BACKGROUND

Via the hepatocyte growth factor receptor (Met), hepatocyte growth factor (HGF) exerts key roles involving skeletal muscle development and regeneration. Heparan sulfate proteoglycans (HSPGs) are critical modulators of HGF activity, but the role of specific HSPGs in HGF regulation is poorly understood. Glypican-1 is the only HSPG expressed in myoblasts that localize in lipid raft membrane domains, controlling cell responses to extracellular stimuli. We determined if glypican-1 in these domains is necessary to stabilize the HGF-Met signaling complex and myoblast response to HGF.

METHODS

C2C12 myoblasts and a derived clone (C6) with low glypican-1 expression were used as an experimental model. The activation of Met, ERK1/2 and AKT in response to HGF was evaluated. The distribution of Met and its activated form in lipid raft domains, as well as its dependence on glypican-1, were characterized by sucrose density gradient fractionation in both cell types. Rescue experiments reexpressing glypican-1 or a chimeric glypican-1 fused to the transmembrane and cytoplasmic domains of mouse syndecan-1 or myoblast pretreatment with MβCD were conducted. In vitro and in vivo myoblast migration assays in response to HGF were also performed.

RESULTS

Glypican-1 localization in membrane raft domains was required for a maximum cell response to HGF. It stabilized Met and HGF in lipid raft domains, forming a signaling complex where the active phospho-Met receptor was concentrated. Glypican-1 also stabilized CD44 in a HGF-dependent manner. In addition, glypican-1 was required for in vitro and in vivo HGF-dependent myoblast migration.

CONCLUSIONS

Glypican-1 is a regulator of HGF-dependent signaling via Met in lipid raft domains.

c-MET regulates myoblast motility and myocyte fusion during adult skeletal muscle regeneration

Adult muscle stem cells, satellite cells (SCs), endow skeletal muscle with tremendous regenerative capacity. Upon injury, SCs activate, proliferate, and migrate as myoblasts to the injury site where they become myocytes that fuse to form new muscle. How migration is regulated, though, remains largely unknown. Additionally, how migration and fusion, which both require dynamic rearrangement of the cytoskeleton, might be related is not well understood. c-MET, a receptor tyrosine kinase, is required for myogenic precursor cell migration into the limb for muscle development during embryogenesis. Using a genetic system to eliminate c-MET function specifically in adult mouse SCs, we found that c-MET was required for muscle regeneration in response to acute muscle injury. c-MET mutant myoblasts were defective in lamellipodia formation, had shorter ranges of migration, and migrated slower compared to control myoblasts. Surprisingly, c-MET was also required for efficient myocyte fusion, implicating c-MET in dual functions of regulating myoblast migration and myocyte fusion.

The methyltransferase SMYD3 mediates the recruitment of transcriptional cofactors at the myostatin and c-Met genes and regulates skeletal muscle atrophy

Elucidating the epigenetic mechanisms underlying muscle mass determination and skeletal muscle wasting holds the potential of identifying molecular pathways that constitute possible drug targets. Here, we report that the methyltransferase SMYD3 modulates myostatin and c-Met transcription in primary skeletal muscle cells and C2C12 myogenic cells. SMYD3 targets the myostatin and c-Met genes and participates in the recruitment of the bromodomain protein BRD4 to their regulatory regions through protein-protein interaction. By recruiting BRD4, SMYD3 favors chromatin engagement of the pause-release factor p-TEFb (positive transcription elongation factor) and elongation of Ser2-phosphorylated RNA polymerase II (PolIISer2P). Reducing SMYD3 decreases myostatin and c-Met transcription, thus protecting from glucocorticoid-induced myotube atrophy. Supporting functional relevance of the SMYD3/BRD4 interaction, BRD4 pharmacological blockade by the small molecule JQ1 prevents dexamethasone-induced myostatin and atrogene up-regulation and spares myotube atrophy. Importantly, in a mouse model of dexamethasone-induced skeletal muscle atrophy, SMYD3 depletion prevents muscle loss and fiber size decrease. These findings reveal a mechanistic link between SMYD3/BRD4-dependent transcriptional regulation, muscle mass determination, and skeletal muscle atrophy and further encourage testing of small molecules targeting specific epigenetic regulators in animal models of muscle wasting.

Satellite cells and the muscle stem cell niche

Adult skeletal muscle in mammals is a stable tissue under normal circumstances but has remarkable ability to repair after injury. Skeletal muscle regeneration is a highly orchestrated process involving the activation of various cellular and molecular responses. As skeletal muscle stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation of satellite cells not only maintains the stem cell population but also provides numerous myogenic cells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitution of a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscle regeneration is tightly regulated through the dynamic interplay between intrinsic factors within satellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. For the last half century, the advance of molecular biology, cell biology, and genetics has greatly improved our understanding of skeletal muscle biology. Here, we review some recent advances, with focuses on functions of satellite cells and their niche during the process of skeletal muscle regeneration.

Effects of sequential injections of hepatocyte growth factor and insulin-like growth factor-I on adult rabbit extraocular muscle

PURPOSE

To determine whether hepatocyte growth factor (HGF) and insulin-like growth factor-I (IGF-I) have synergistic effects in promoting extraocular muscle fiber growth and force generation.

METHODS

A superior rectus muscle of adult rabbits was treated with either a single injection of HGF or sequential injections of HGF followed 1 week later by IGF-I. One week after HGF alone and 1 week after the IGF-I injection, the superior rectus muscles from treated and control orbits were examined for alterations in force generation as well as changes in myofiber size.

RESULTS

Injection of HGF alone did not result in changes to muscle force, specific tension, or myofiber cross-sectional area; however, it did result in a significant increase in numbers of satellite cells. Sequential injection of HGF and IGF-I resulted in significantly increased force, specific tension, and myofiber cross-sectional areas as well as increased numbers of satellite cells.

CONCLUSIONS

Preinjection with HGF augments the treatment effect of IGF-I. This synergistic effect is likely a result of HGF-induced activation of satellite cells and should allow a reduction in IGF-I dosing required to produce a given increase in extraocular muscle force generation.