Differentially expressed fibroblast growth factors regulate skeletal muscle development through autocrine and paracrine mechanisms

Gene expression differences associated with intrinsic hindfoot muscle loss in the jerboa, Jaculus jaculus

Vertebrate animals that run or jump across sparsely vegetated habitats, such as horses and jerboas, have reduced the number of distal limb bones, and many have lost most or all distal limb muscle. We previously showed that nascent muscles are present in the jerboa hindfoot at birth and that these myofibers are rapidly and completely lost soon after by a process that shares features with pathological skeletal muscle atrophy. Here, we apply an intra- and interspecies differential RNA-Seq approach, comparing jerboa and mouse muscles, to identify gene expression differences associated with the initiation and progression of jerboa hindfoot muscle loss. We show evidence for reduced hepatocyte growth factor and fibroblast growth factor signaling and an imbalance in nitric oxide signaling; all are pathways that are necessary for skeletal muscle development and regeneration. We also find evidence for phagosome formation, which hints at how myofibers may be removed by autophagy or by nonprofessional phagocytes without evidence for cell death or immune cell activation. Last, we show significant overlap between genes associated with jerboa hindfoot muscle loss and genes that are differentially expressed in a variety of human muscle pathologies and rodent models of muscle loss disorders. All together, these data provide molecular insight into the process of evolutionary and developmental muscle loss in jerboa hindfeet.

MyoD Over-Expression Rescues GST-bFGF Repressed Myogenesis

During embryogenesis, basic fibroblast growth factor (bFGF) is released from neural tube and myotome to promote myogenic fate in the somite, and is routinely used for the culture of adult skeletal muscle (SKM) stem cells (MuSC, called satellite cells). However, the mechanism employed by bFGF to promote SKM lineage and MuSC proliferation has not been analyzed in detail. Furthermore, the question of if the post-translational modification (PTM) of bFGF is important to its stemness-promoting effect has not been answered. In this study, GST-bFGF was expressed and purified from E.coli, which lacks the PTM system in eukaryotes. We found that both GST-bFGF and commercially available bFGF activated the Akt-Erk pathway and had strong cell proliferation effect on C2C12 myoblasts and MuSC. GST-bFGF reversibly compromised the myogenesis of C2C12 myoblasts and MuSC, and it increased the expression of Myf5, Pax3/7, and Cyclin D1 but strongly repressed that of MyoD, suggesting the maintenance of myogenic stemness amid repressed MyoD expression. The proliferation effect of GST-bFGF was conserved in C2C12 over-expressed with MyoD (C2C12-tTA-MyoD), implying its independence of the down-regulation of MyoD. In addition, the repressive effect of GST-bFGF on myogenic differentiation was almost totally rescued by the over-expression of MyoD. Together, these evidences suggest that (1) GST-bFGF and bFGF have similar effects on myogenic cell proliferation and differentiation, and (2) GST-bFGF can promote MuSC stemness and proliferation by differentially regulating MRFs and Pax3/7, (3) MyoD repression by GST-bFGF is reversible and independent of the proliferation effect, and (4) GST-bFGF can be a good substitute for bFGF in sustaining MuSC stemness and proliferation.

Production of Proliferation- and Differentiation-Competent Porcine Myoblasts for Preclinical Studies in a Porcine Large Animal Model of Muscular Insufficiency

Muscular insufficiency is observed in many conditions after injury, chronic inflammation, and especially in elderly populations. Causative cell therapies for muscle deficiencies are not state of the art. Animal models to study the therapy efficacy are, therefore, needed. We developed an improved protocol to produce myoblasts suitable for pre-clinical muscle therapy studies in a large animal model. Myoblasts were isolated from the striated muscle, expanded by employing five different protocols, and characterized on transcript and protein expression levels to determine procedures that yielded optimized regeneration-competent myoblasts and multi-nucleated myotubes. We report that swine skeletal myoblasts proliferated well under improved conditions without signs of cellular senescence, and expressed significant levels of myogenic markers including Pax7, MyoD1, Myf5, MyoG, Des, Myf6, CD56 (p ≤ 0.05 each). Upon terminal differentiation, myoblasts ceased proliferation and generated multi-nucleated myotubes. Injection of such myoblasts into the urethral sphincter complex of pigs with sphincter muscle insufficiency yielded an enhanced functional regeneration of this muscle (81.54% of initial level) when compared to the spontaneous regeneration in the sham controls without myoblast injection (67.03% of initial level). We conclude that the optimized production of porcine myoblasts yields cells that seem suitable for preclinical studies of cell therapy in a porcine large animal model of muscle insufficiency.

Distinct human stem cell subpopulations drive adipogenesis and fibrosis in musculoskeletal injury

Fibroadipogenic progenitors (FAPs) maintain healthy skeletal muscle in homeostasis but drive muscle degeneration in chronic injuries by promoting adipogenesis and fibrosis. To uncover how these stem cells switch from a pro-regenerative to pro-degenerative role we perform single-cell mRNA sequencing of human FAPs from healthy and injured human muscles across a spectrum of injury, focusing on rotator cuff tears. We identify multiple subpopulations with progenitor, adipogenic, or fibrogenic gene signatures. We utilize full spectrum flow cytometry to identify distinct FAP subpopulations based on highly multiplexed protein expression. Injury severity increases adipogenic commitment of FAP subpopulations and is driven by the downregulation of DLK1. Treatment of FAPs both in vitro and in vivo with DLK1 reduces adipogenesis and fatty infiltration, suggesting that during injury, reduced DLK1 within a subpopulation of FAPs may drive degeneration. This work highlights how stem cells perform varied functions depending on tissue context, by dynamically regulating subpopulation fate commitment, which can be targeted improve patient outcomes after injury.

The Comparison of Commercial Serum-Free Media for Hanwoo Satellite Cell Proliferation and the Role of Fibroblast Growth Factor 2

Fetal bovine serum (FBS), which contains various nutrients, comprises 20% of the growth medium for cell-cultivated meat. However, ethical, cost, and scientific issues, necesitates identification of alternatives. In this study, we investigated commercially manufactured serum-free media capable of culturing Hanwoo satellite cells (HWSCs) to identify constituent proliferation enhancing factors. Six different serum-free media were selected, and the HWSC proliferation rates in these serum-free media were compared with that of control medium supplemented with 20% FBS. Among the six media, cell proliferation rates were higher only in StemFlexTM Medium (SF) and Mesenchymal Stem Cell Growth Medium DXF (MS) than in the control medium. SF and MS contain high fibroblast growth factor 2 (FGF2) concentrations, and we found upregulated FGF2 protein expression in cells cultured in SF or MS. Activation of the fibroblast growth factor receptor 1 (FGFR1)-mediated signaling pathway and stimulation of muscle satellite cell proliferation-related factors were confirmed by the presence of related biomarkers (FGFR1, FRS2, Raf1, ERK, p38, Pax7, and MyoD) as indicated by quantitative polymerase chain reaction, western blotting, and immunocytochemistry. Moreover, PD173074, an FGFR1 inhibitor suppressed cell proliferation in SF and MS and downregulated related biomarkers (FGFR1, FRS2, Raf1, and ERK). The promotion of cell proliferation in SF and MS was therefore attributed to FGF2, which indicates that FGFR1 activation in muscle satellite cells may be a target for improving the efficiency of cell-cultivated meat production.

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.

The emergence of the stem cell niche

Stem cell niches are composed of dynamic microenvironments that support stem cells over a lifetime. The emerging niche is distinct from the adult because its main role is to support the progenitors that build organ systems in development. Emerging niches mature through distinct stages to form the adult niche and enable proper stem cell support. As a model of emerging niches, this review highlights how differences in the skeletal muscle microenvironment influence emerging versus satellite cell (SC) niche formation in skeletal muscle, which is among the most regenerative tissue systems. We contrast how stem cell niches regulate intrinsic properties between progenitor and stem cells throughout development to adulthood. We describe new applications for generating emerging niches from human pluripotent stem cells (hPSCs) using developmental principles and highlight potential applications for regeneration and therapeutics.

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.

Consumer Acceptance and Production of In Vitro Meat: A Review

In vitro meat (IVM) is a recent development in the production of sustainable food. The consumer perception of IVM has a strong impact on the commercial success of IVM. Hence this review examines existing studies related to consumer concerns, acceptance and uncertainty of IVM. This will help create better marketing strategies for IVM-producing companies in the future. In addition, IVM production is described in terms of the types of cells and culture conditions employed. The applications of self-organising, scaffolding, and 3D printing techniques to produce IVM are also discussed. As the conditions for IVM production are controlled and can be manipulated, it will be feasible to produce a chemically safe and disease-free meat with improved consumer acceptance on a sustainable basis.

Fibroblast growth factor 6 regulates sizing of the muscle stem cell pool

Skeletal muscle stem cells, i.e., satellite cells (SCs), are the essential source of new myonuclei for skeletal muscle regeneration following injury or chronic degenerative myopathies. Both SC number and regenerative capacity diminish during aging. However, molecular regulators that govern sizing of the initial SC pool are unknown. We demonstrate that fibroblast growth factor 6 (FGF6) is critical for SC pool scaling. Mice lacking FGF6 have reduced SCs of early postnatal origin and impaired regeneration. By contrast, increasing FGF6 during the early postnatal period is sufficient for SC expansion. Together, these data support that FGF6 is necessary and sufficient to modulate SC numbers during a critical postnatal period to establish the quiescent adult muscle stem cell pool. Our work highlights postnatal development as a time window receptive for scaling a somatic stem cell population via growth factor signaling, which might be relevant for designing new biomedical strategies to enhance tissue regeneration.

The Essential Role of Stathmin in Myoblast C2C12 for Vertical Vibration-Induced Myotube Formation

Vertical vibration (VV) is a type of whole body vibration, which induces muscle contraction through vibration to improve muscle strength and bone density. However, the mechanism of VV on muscle cell myotube formation is still unclear. In the current study, we aim to clarify the mechanism involved in VV's stimulation of myotube formation. In order to identify the molecules regulated by VV, we performed proteomics analysis including 2D electrophoresis combined with MALDI-TOF/TOF Mass. Stathmin was identified as a high potential molecule responding to VV stimulation, and we found that under VV stimulation, the expression of stathmin gene and protein increased in a time-dependent manner. In addition, we also confirmed that the increase of stathmin stimulated by VV is mediated through the PI3K/Akt pathway. Furthermore, stathmin siRNA significantly down-regulated the expression of myogenic regulatory factor (MRF) MyoD, decorin, and type I collagen (Col-I), and down-regulated the cellular process regulators such as FGF7, TGFBr1 and PAK3. Taken together, our results confirm that under the stimulation of VV, PI3K/Akt and stathmin would be activated, as well as the up-regulation of MRFs, such as FGF7, TGFBr1 and PAK3 to initiate myogenesis. It also showed that the response of MRF to VV stimulation was significantly related to stathmin expression, which also confirmed the importance of stathmin in the entire myotube formation process. This study may provide evidence of stathmin as a biological indicator of VV to increase muscle strength.

FGF-2-dependent signaling activated in aged human skeletal muscle promotes intramuscular adipogenesis

Aged skeletal muscle is markedly affected by fatty muscle infiltration, and strategies to reduce the occurrence of intramuscular adipocytes are urgently needed. Here, we show that fibroblast growth factor-2 (FGF-2) not only stimulates muscle growth but also promotes intramuscular adipogenesis. Using multiple screening assays upstream and downstream of microRNA (miR)-29a signaling, we located the secreted protein and adipogenic inhibitor SPARC to an FGF-2 signaling pathway that is conserved between skeletal muscle cells from mice and humans and that is activated in skeletal muscle of aged mice and humans. FGF-2 induces the miR-29a/SPARC axis through transcriptional activation of FRA-1, which binds and activates an evolutionary conserved AP-1 site element proximal in the miR-29a promoter. Genetic deletions in muscle cells and adeno-associated virus-mediated overexpression of FGF-2 or SPARC in mouse skeletal muscle revealed that this axis regulates differentiation of fibro/adipogenic progenitors in vitro and intramuscular adipose tissue (IMAT) formation in vivo. Skeletal muscle from human donors aged >75 y versus <55 y showed activation of FGF-2-dependent signaling and increased IMAT. Thus, our data highlights a disparate role of FGF-2 in adult skeletal muscle and reveals a pathway to combat fat accumulation in aged human skeletal muscle.

Heparin-Mimicking Polymer-Based In Vitro Platform Recapitulates In Vivo Muscle Atrophy Phenotypes

The cell–cell/cell–matrix interactions between myoblasts and their extracellular microenvironment have been shown to play a crucial role in the regulation of in vitro myogenic differentiation and in vivo skeletal muscle regeneration. In this study, by harnessing the heparin-mimicking polymer, poly(sodium-4-styrenesulfonate) (PSS), which has a negatively charged surface, we engineered an in vitro cell culture platform for the purpose of recapitulating in vivo muscle atrophy-like phenotypes. Our initial findings showed that heparin-mimicking moieties inhibited the fusion of mononucleated myoblasts into multinucleated myotubes, as indicated by the decreased gene and protein expression levels of myogenic factors, myotube fusion-related markers, and focal adhesion kinase (FAK). We further elucidated the underlying molecular mechanism via transcriptome analyses, observing that the insulin/PI3K/mTOR and Wnt signaling pathways were significantly downregulated by heparin-mimicking moieties through the inhibition of FAK/Cav3. Taken together, the easy-to-adapt heparin-mimicking polymer-based in vitro cell culture platform could be an attractive platform for potential applications in drug screening, providing clear readouts of changes in insulin/PI3K/mTOR and Wnt signaling pathways.

Vitamin D Restores Skeletal Muscle Cell Remodeling and Myogenic Program: Potential Impact on Human Health

Skeletal muscle cells, albeit classified as vitamin D receptor (VDR)-poor cells, are finely controlled by vitamin D through genomic and non-genomic mechanisms. Skeletal muscle constantly undergoes cell remodeling, a complex system under multilevel regulation, mainly orchestrated by the satellite niche in response to a variety of stimuli. Cell remodeling is not limited to satisfy reparative and hypertrophic needs, but, through myocyte transcriptome/proteome renewal, it warrants the adaptations necessary to maintain tissue integrity. While vitamin D insufficiency promotes cell maladaptation, restoring vitamin D levels can correct/enhance the myogenic program. Hence, vitamin D fortified foods or supplementation potentially represents the desired approach to limit or avoid muscle wasting and ameliorate health. Nevertheless, consensus on protocols for vitamin D measurement and supplementation is still lacking, due to the high variability of lab tests and of the levels required in different contexts (i.e., age, sex, heath status, lifestyle). This review aims to describe how vitamin D can orchestrate skeletal muscle cell remodeling and myogenic programming, after reviewing the main processes and cell populations involved in this important process, whose correct progress highly impacts on human health. Topics on vitamin D optimal levels, supplementation and blood determination, which are still under debate, will be addressed.

Targeting IRES-dependent translation as a novel approach for treating Duchenne muscular dystrophy

Internal-ribosomal entry sites (IRES) are translational elements that allow the initiation machinery to start protein synthesis via internal initiation. IRESs promote tissue-specific translation in stress conditions when conventional cap-dependent translation is inhibited. Since many IRES-containing mRNAs are relevant to diseases, this cellular mechanism is emerging as an attractive therapeutic target for pharmacological and genetic modulations. Indeed, there has been growing interest over the past years in determining the therapeutic potential of IRESs for several disease conditions such as cancer, neurodegeneration and neuromuscular diseases including Duchenne muscular dystrophy (DMD). IRESs relevant for DMD have been identified in several transcripts whose protein product results in functional improvements in dystrophic muscles. Together, these converging lines of evidence indicate that activation of IRES-mediated translation of relevant transcripts in DMD muscle represents a novel and appropriate therapeutic strategy for DMD that warrants further investigation, particularly to identify agents that can modulate their activity.

Tempora: Cell trajectory inference using time-series single-cell RNA sequencing data

Single-cell RNA sequencing (scRNA-seq) can map cell types, states and transitions during dynamic biological processes such as tissue development and regeneration. Many trajectory inference methods have been developed to order cells by their progression through a dynamic process. However, when time series data is available, most of these methods do not consider the available time information when ordering cells and are instead designed to work only on a single scRNA-seq data snapshot. We present Tempora, a novel cell trajectory inference method that orders cells using time information from time-series scRNA-seq data. In performance comparison tests, Tempora inferred known developmental lineages from three diverse tissue development time series data sets, beating state of the art methods in accuracy and speed. Tempora works at the level of cell clusters (types) and uses biological pathway information to help identify cell type relationships. This approach increases gene expression signal from single cells, processing speed, and interpretability of the inferred trajectory. Our results demonstrate the utility of a combination of time and pathway information to supervise trajectory inference for scRNA-seq based analysis.

Single-cell RNA sequencing (scRNA-seq) enables an unparalleled ability to map the heterogeneity of dynamic multicellular processes, such as tissue development, tumor growth, wound response and repair, and inflammation. Multiple methods have been developed to order cells along a pseudotime axis that represents a trajectory through such processes using the concept that cells that are closely related in a lineage will have similar transcriptomes. However, time series experiments provide another useful information source to order cells, from earlier to later time point. By introducing a novel use of biological pathway prior information, our Tempora algorithm improves the accuracy and speed of cell trajectory inference from time-series scRNA-seq data as measured by reconstructing known developmental trajectories from three diverse data sets. By analyzing scRNA-seq data at the cluster (cell type) level instead of at the single-cell level and by using known pathway information, Tempora amplifies gene expression signals from one cell using similar cells in a cluster and similar genes within a pathway. This approach also reduces computational time and resources needed to analyze large data sets because it works with a relatively small number of clusters instead of a potentially large number of cells. Finally, it eases interpretation, via operating on a relatively small number of clusters which usually represent known cell types, as well as by identifying time-dependent pathways. Tempora is useful for finding novel insights in dynamic processes.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

Fibrocyte accumulation in the lungs of cystic fibrosis patients

BACKGROUND

Cystic fibrosis (CF) patients develop severe lung disease including chronic airway infections, neutrophilic inflammation, and progressive fibrotic remodeling in airways. However, cellular and molecular processes that regulate excessive collagen deposition in airways in these patients remain unclear. Fibrocytes are bone marrow (BM)-derived mesenchymal cells that express the hematopoietic cell marker CD45, and mesenchymal cell markers and implicated in collagen deposition in several fibrotic diseases. It is unknown whether fibrocytes accumulate in the lungs of CF patients, so the current study evaluates the presence of fibrocytes in the fibrotic lesions of airways in explanted CF lungs compared to non-CF unused donor lungs (control).

METHODS

We used immunofluorescence staining to determine if fibrocytes accumulate in explanted CF lungs compared to healthy donor lungs. Simultaneously, we evaluated cells collected by bronchoalveolar lavage (BAL) in CF patients using multi-color flow cytometry. Finally, we analyzed transcripts differentially expressed in fibrocytes isolated from the explanted CF lungs compared to control to assess fibrocyte-specific pro-fibrotic gene networks.

RESULTS

Our findings demonstrate fibrocyte accumulation in CF lungs compared to non-CF lungs. Additionally, fibrocytes were detected in the BAL of all CF children. Transcriptomic analysis of fibrocytes identified dysregulated genes associated with fibrotic remodeling in CF lungs.

CONCLUSIONS

With significantly increased fibrocytes that show increased expression of pro-fibrotic gene transcripts compared to control, our findings suggest an intervention for fibrotic remodeling as a potential therapeutic target in CF.

FGF binding proteins (FGFBPs): Modulators of FGF signaling in the developing, adult, and stressed nervous system

Members of the fibroblast growth factor (FGF) family are involved in a variety of cellular processes. In the nervous system, they affect the differentiation and migration of neurons, the formation and maturation of synapses, and the repair of neuronal circuits following insults. Because of the varied yet critical functions of FGF ligands, their availability and activity must be tightly regulated for the nervous system, as well as other tissues, to properly develop and function in adulthood. In this regard, FGF binding proteins (FGFBPs) have emerged as strong candidates for modulating the actions of secreted FGFs in neural and non-neural tissues. Here, we will review the roles of FGFBPs in the peripheral and central nervous systems.

The Muscle Stem Cell Niche in Health and Disease

The regulation of stem cells that maintain and regenerate postnatal tissues depends on extrinsic signals originating from their microenvironment, commonly referred to as the stem cell niche. Complex higher-order regulatory interrelationships with the tissue and factors in the systemic circulation are integrated and propagated to the stem cells through the niche. The stem cell niche in skeletal muscle tissue is both a paradigm for a structurally and functionally relatively static niche that maintains stem cell quiescence during tissue homeostasis, and a highly dynamic regenerative niche that is subject to extensive structural remodeling and a flux of different support cell populations. Conditions ranging from aging to chronically degenerative skeletal muscle diseases affect the composition of the niche and thereby impair the regenerative potential of muscle stem cells. A holistic and integrative understanding of the extrinsic mechanisms regulating muscle stem cells in health and disease in a broad systemic context will be imperative for the identification of regulatory hubs in the niche interactome that can be targeted to maintain, restore, or enhance the regenerative capacity of muscle tissue. Here, we review the microenvironmental regulation of muscle stem cells, summarize how niche dysfunction can contribute to disease, and discuss emerging therapeutic implications.

The Dlx5-FGF10 signaling cascade controls cranial neural crest and myoblast interaction during oropharyngeal patterning and development

Craniofacial development depends on cell-cell interactions, coordinated cellular movement and differentiation under the control of regulatory gene networks, which include the distal-less (Dlx) gene family. However, the functional significance of Dlx5 in patterning the oropharyngeal region has remained unknown. Here, we show that loss of Dlx5 leads to a shortened soft palate and an absence of the levator veli palatini, palatopharyngeus and palatoglossus muscles that are derived from the 4th pharyngeal arch (PA); however, the tensor veli palatini, derived from the 1st PA, is unaffected. Dlx5-positive cranial neural crest (CNC) cells are in direct contact with myoblasts derived from the pharyngeal mesoderm, and Dlx5 disruption leads to altered proliferation and apoptosis of CNC and muscle progenitor cells. Moreover, the FGF10 pathway is downregulated in Dlx5^-/- mice, and activation of FGF10 signaling rescues CNC cell proliferation and myogenic differentiation in these mutant mice. Collectively, our results indicate that Dlx5 plays crucial roles in the patterning of the oropharyngeal region and development of muscles derived from the 4th PA mesoderm in the soft palate, likely via interactions between CNC-derived and myogenic progenitor cells.

Muscle Fibers Secrete FGFBP1 to Slow Degeneration of Neuromuscular Synapses during Aging and Progression of ALS

The identity of muscle secreted factors critical for the development and maintenance of neuromuscular junctions (NMJs) remains largely unknown. Here, we show that muscle fibers secrete and concentrate the fibroblast growth factor binding protein 1 (FGFBP1) at NMJs. Although FGFBP1 expression increases during development, its expression decreases before NMJ degeneration during aging and in SOD1^G93A mice, a mouse model for amyotrophic lateral sclerosis (ALS). Based on these findings, we examined the impact of deleting FGFBP1 on NMJs. In the absence of FGFBP1, NMJs exhibit structural abnormalities in developing and middle age mice. Deletion of FGFBP1 from SOD1^G93A mice also accelerates NMJ degeneration and death. Based on these findings, we sought to identify the mechanism responsible for decreased FGFBP1 in stressed skeletal muscles. We show that FGFBP1 expression is inhibited by increased accumulation of the transforming growth factor-β1 (TGF-β1) in skeletal muscles and at their NMJs. These findings suggest that targeting the FGFBP1 and TGF-β1 signaling axis holds promise for slowing age- and disease-related degeneration of NMJs.

SIGNIFICANCE STATEMENT

The neuromuscular junction (NMJ) is critical for all voluntary movement. Its malformation during development and degeneration in adulthood impairs motor function. Therefore, it is important to identify factors that function to maintain the structural integrity of NMJs. We show that muscle fibers secrete and concentrate the fibroblast growth factor binding protein 1 (FGFBP1) at NMJs. However, FGFBP1 expression decreases in skeletal muscles during aging and before NMJ degeneration in SOD1^G93A mice, a mouse model for amyotrophic lateral sclerosis. We show that transforming growth factor-β1 is responsible for the decreased levels of FGFBP1. Importantly, we demonstrate critical roles for FGFBP1 at NMJs in developing, aging and SOD1^G93A mice.

Effect of different levels of copper nanoparticles and copper sulphate on performance, metabolism and blood biochemical profiles in broiler chicken

A study was conducted to investigate the influence of copper administration in ovo to chicken embryos and/or supplied in drinking water to growing chickens in the form copper nanoparticles (Cu-NP) or copper sulphate (CuSO₄ ). The fertilised eggs were assigned to three groups (n = 50 per group): control (not injected), injected with 50 mg/kg Cu-NP or with 50 mg/kg CuSO₄ at day 1 of incubation. Thereafter, 126 one-day-old broiler chickens were randomly assigned to seven post-hatched groups: control not injected and not provided with Cu in the drinking water, injected with 50 mg/kg Cu-NP + 20 mg/kg in water, not injected + 20 mg/kg Cu-NP in water, injected with 50 mg/kg CuSO₄  + 20 mg/kg in water, not injected + 20 mg/kg CuSO₄ in water, injected with 50 mg/kg Cu-NP and injected with 50 mg/kg CuSO₄ . The experiment was carried out from day 1 to 35 post-hatching. The in ovo injection of Cu improved the final body weight, average daily gain and feed conversion ratio in relation to the control group. Conversely, the provision of Cu in the drinking water had less of an effect on growth performance in comparison with the injected groups. A significant improvement was shown in energy and nitrogen utilisation, being better for Cu-NP than CuSO₄ . The cholesterol, urea and glucose levels in the blood were reduced by Cu-NP treatment in relation to the other groups. The relative weight of the liver was decreased, while bursa of Fabricius was increased in Cu groups in relation to the control group. Cu excretion was only reduced in chickens injected with 50 mg/kg Cu-NP + 20 mg/kg in water. The immune-related genes were not affected by the treatments. The in ovo injection of Cu-NP might improve broiler performance more efficiently than the injection of CuSO₄ or the provision of Cu-NP and/or CuSO₄ in drinking water.

Vitamin D induces myogenic differentiation in skeletal muscle derived stem cells

Skeletal muscle wasting is a serious disorder associated with health conditions such as aging, chronic kidney disease and AIDS. Vitamin D is most widely recognized for its regulation of calcium and phosphate homeostasis in relation to bone development and maintenance. Recently, vitamin D supplementation has been shown to improve muscle performance and reduce the risk of falls in vitamin D deficient older adults. However, little is known of the underlying molecular mechanism(s) or the role it plays in myogenic differentiation. We examined the effect of 1,25-D₃ on myogenic cell differentiation in skeletal muscle derived stem cells. Primary cultures of skeletal muscle satellite cells were isolated from the tibialis anterior, soleus and gastrocnemius muscles of 8-week-old C57/BL6 male mice and then treated with 1,25-D₃ The efficiency of satellite cells isolation determined by PAX7+ cells was 81%, and they expressed VDR. Incubation of satellite cells with 1,25-D₃ induces increased expression of: (i) MYOD, (ii) MYOG, (iii) MYC2, (iv) skeletal muscle fast troponin I and T, (v) MYH1, (vi) IGF1 and 2, (vii) FGF1 and 2, (viii) BMP4, (ix) MMP9 and (x) FST. It also promotes myotube formation and decreases the expression of MSTN. In conclusion, 1,25-D₃ promoted a robust myogenic effect on satellite cells responsible for the regeneration of muscle after injury or muscle waste. This study provides a mechanistic justification for vitamin D supplementation in conditions characterized by loss of muscle mass and also in vitamin D deficient older adults with reduced muscle mass and strength, and increased risk of falls.

Regulation of skeletal muscle stem cells by fibroblast growth factors

Fibroblast growth factors (FGFs) are essential for self-renewal of skeletal muscle stem cells (satellite cells) and required for maintenance and repair of skeletal muscle. Satellite cells express high levels of FGF receptors 1 and 4, low levels of FGF receptor 3, and little or no detectable FGF receptor 2. Of the multiple FGFs that influence satellite cell function in culture, FGF2 and FGF6 are the only members that regulate satellite cell function in vivo by activating ERK MAPK, p38α/β MAPKs, PI3 kinase, PLCγ and STATs. Regulation of FGF signaling is complex in satellite cells, requiring Syndecan-4, a heparan sulfate proteoglycan, as well as ß1-integrin and fibronectin. During aging, reduced responsiveness to FGF diminishes satellite cell self-renewal, leading to impaired skeletal muscle regeneration and depletion of satellite cells. Mislocalization of ß1-integrin, reductions in fibronectin, and alterations in heparan sulfate content all contribute to reduced FGF responsiveness in satellite cells. How these cell surface proteins regulate satellite cell self-renewal is incompletely understood. Here we summarize the current knowledge, highlighting the role(s) for FGF signaling in skeletal muscle regeneration, satellite cell behavior, and age-induced muscle wasting. Developmental Dynamics 246:359-367, 2017. © 2017 Wiley Periodicals, Inc.

Fgf regulates dedifferentiation during skeletal muscle regeneration in adult zebrafish

Fibroblast growth factors (Fgfs) regulate critical biological processes such as embryonic development, tissue homeostasis, wound healing, and tissue regeneration. In zebrafish, Fgf signaling plays an important role in the regeneration of the spinal cord, liver, heart, fin, and photoreceptors, although its exact mechanism of action is not fully understood. Utilizing an adult zebrafish extraocular muscle (EOM) regeneration model, we demonstrate that blocking Fgf receptor function using either a chemical inhibitor (SU5402) or a dominant-negative transgenic construct (dnFGFR1a:EGFP) impairs muscle regeneration. Adult zebrafish EOMs regenerate through a myocyte dedifferentiation process, which involves a muscle-to-mesenchyme transition and cell cycle reentry by differentiated myocytes. Blocking Fgf signaling reduced cell proliferation and active caspase 3 levels in the regenerating muscle with no detectable levels of apoptosis, supporting the hypothesis that Fgf signaling is involved in the early steps of dedifferentiation. Fgf signaling in regenerating myocytes involves the MAPK/ERK pathway: inhibition of MEK activity with U0126 mimicked the phenotype of the Fgf receptor inhibition on both muscle regeneration and cell proliferation, and activated ERK (p-ERK) was detected in injured muscles by immunofluorescence and western blot. Interestingly, following injury, ERK2 expression is specifically induced and activated by phosphorylation, suggesting a key role in muscle regeneration. We conclude that the critical early steps of myocyte dedifferentiation in EOM regeneration are dependent on Fgf signaling.

Feedback, Lineages and Self-Organizing Morphogenesis

Feedback regulation of cell lineage progression plays an important role in tissue size homeostasis, but whether such feedback also plays an important role in tissue morphogenesis has yet to be explored. Here we use mathematical modeling to show that a particular feedback architecture in which both positive and negative diffusible signals act on stem and/or progenitor cells leads to the appearance of bistable or bi-modal growth behaviors, ultrasensitivity to external growth cues, local growth-driven budding, self-sustaining elongation, and the triggering of self-organization in the form of lamellar fingers. Such behaviors arise not through regulation of cell cycle speeds, but through the control of stem or progenitor self-renewal. Even though the spatial patterns that arise in this setting are the result of interactions between diffusible factors with antagonistic effects, morphogenesis is not the consequence of Turing-type instabilities.

Functional myotube formation from adult rat satellite cells in a defined serum-free system

This manuscript describes the development of a culture system whereby mature contracting myotubes were formed from adult rat derived satellite cells. Satellite cells, extracted from the Tibialis Anterior of adult rats, were grown in defined serum-free growth and differentiation media, on a nonbiological substrate, N-1[3-trimethoxysilyl propyl] diethylenetriamine. Myotubes were evaluated morphologically and immunocytochemically, using MyHC specific antibodies, as well as functionally using patch clamp electrophysiology to measure ion channel activity. Results indicated the establishment of the rapid expression of adult myosin isoforms that contrasts to their slow development in embryonic cultures. This culture system has applications in the understanding and treatment of age-related muscle myopathy, muscular dystrophy, and for skeletal muscle engineering by providing a more relevant phenotype for both in vitro and in vivo applications.

Muscle stem cells at a glance

Muscle stem cells facilitate the long-term regenerative capacity of skeletal muscle. This self-renewing population of satellite cells has only recently been defined through genetic and transplantation experiments. Although muscle stem cells remain in a dormant quiescent state in uninjured muscle, they are poised to activate and produce committed progeny. Unlike committed myogenic progenitor cells, the self-renewal capacity gives muscle stem cells the ability to engraft as satellite cells and capitulate long-term regeneration. Similar to other adult stem cells, understanding the molecular regulation of muscle stem cells has significant implications towards the development of pharmacological or cell-based therapies for muscle disorders. This Cell Science at a Glance article and accompanying poster will review satellite cell characteristics and therapeutic potential, and provide an overview of the muscle stem cell hallmarks: quiescence, self-renewal and commitment.

ALK5-mediated transforming growth factor β signaling in neural crest cells controls craniofacial muscle development via tissue-tissue interactions

The development of the craniofacial muscles requires reciprocal interactions with surrounding craniofacial tissues that originate from cranial neural crest cells (CNCCs). However, the molecular mechanism involved in the tissue-tissue interactions between CNCCs and muscle progenitors during craniofacial muscle development is largely unknown. In the current study, we address how CNCCs regulate the development of the tongue and other craniofacial muscles using Wnt1-Cre; Alk5(fl/fl) mice, in which loss of Alk5 in CNCCs results in severely disrupted muscle formation. We found that Bmp4 is responsible for reduced proliferation of the myogenic progenitor cells in Wnt1-Cre; Alk5(fl/fl) mice during early myogenesis. In addition, Fgf4 and Fgf6 ligands were reduced in Wnt1-Cre; Alk5(fl/fl) mice and are critical for differentiation of the myogenic cells. Addition of Bmp4 or Fgf ligands rescues the proliferation and differentiation defects in the craniofacial muscles of Alk5 mutant mice in vitro. Taken together, our results indicate that CNCCs play critical roles in controlling craniofacial myogenic proliferation and differentiation through tissue-tissue interactions.

Midkine-deficient mice delayed degeneration and regeneration after skeletal muscle injury

Midkine (MK), a heparin-binding growth factor, was previously found to be expressed in the rat myotube-forming stage. We investigated MK gene-deficient (Mdk(-/-)) mice in terms of skeletal muscle degeneration and regeneration after injury by bupivacaine injection into the tibialis anterior muscle. Injured muscles showed intense inflammatory cell infiltration. Myotubes, myofibers with centrally located nuclei in their cytoplasm, were significantly smaller in Mdk(-/-) mice than in wild type (Mdk(+/+)) mice 7 days after injury (p=0.02). The distribution of myotube sizes showed quantitative differences between the two groups at 5 and 7 days, but not at 14 days. Many small myotubes were found in the regenerative area of Mdk(-/-) mice compared with that of Mdk(+/+)mice 5 and 7 days after injury. The expression of Iba1, a macrophage marker, was significantly lower in Mdk(-/-) mice 3 days after injury (p=0.01). The number of desmin-positive cells like myoblasts in Mdk(-/-) mice was significantly fewer than that in Mdk(+/+) mice 3 days after injury. Our results suggested that deletion of MK results in a delay in regeneration, preceded by decelerated migration of macrophages to the damaged area, and that MK has a role in cell differentiation and maturation after skeletal muscle injury.

p38 MAPK signaling underlies a cell-autonomous loss of stem cell self-renewal in skeletal muscle of aged mice

Skeletal muscle aging results in a gradual loss of skeletal muscle mass, skeletal muscle function and decreased regenerative capacity, which can lead to sarcopenia and increased mortality. While the mechanisms underlying sarcopenia remain unclear, the skeletal muscle stem cell, or satellite cell, is required for muscle regeneration. Therefore, identification of signaling pathways affecting satellite cell function during aging may provide insights into therapeutic targets for combating sarcopenia. Here, we show that a cell-autonomous loss in self-renewal occurs via alterations in FGF Receptor 1 and p38αβ MAPK signaling in aged satellite cells. We further demonstrate that pharmacological manipulation of these pathways can ameliorate age-associated self-renewal defects. Thus, our data highlight an age-associated deregulation of a satellite cell homeostatic network and reveal potential therapeutic opportunities for the treatment of progressive muscle wasting.

Adeno-associated viral (AAV) vectors do not efficiently target muscle satellite cells

Adeno-associated viral (AAV) vectors are becoming an important tool for gene therapy of numerous genetic and other disorders. Several recombinant AAV vectors (rAAV) have the ability to transduce striated muscles in a variety of animals following intramuscular and intravascular administration, and have attracted widespread interest for therapy of muscle disorders such as the muscular dystrophies. However, most studies have focused on the ability to transduce mature muscle cells, and have not examined the ability to target myogenic stem cells such as skeletal muscle satellite cells. Here we examined the relative ability of rAAV vectors derived from AAV6 to target myoblasts, myocytes, and myotubes in culture and satellite cells and myofibers in vivo. AAV vectors are able to transduce proliferating myoblasts in culture, albeit with reduced efficiency relative to postmitotic myocytes and myotubes. In contrast, quiescent satellite cells are refractory to transduction in adult mice. These results suggest that while muscle disorders characterized by myofiber regeneration can be slowed or halted by AAV transduction, little if any vector transduction can be obtained in myogenic stems cells that might other wise support ongoing muscle regeneration.

Expression and methylation of FGF2, TGF-β and their downstream mediators during different developmental stages of leg muscles in chicken

A number of growth factors determine the proliferation of myoblasts and therefore the number of ultimate myofibers. The members of transforming growth factor-beta (TGF-β) family and the fibroblast growth factor 2 (FGF2) have profound effects on skeletal myoblasts proliferation in various animal systems. To investigate their involvement in different stages of avian skeletal muscle development in vivo, we detected the mRNA expression and DNA methylation profiles of TGF-β2, TGF-β3, FGF2 and their downstream mediators in leg muscles at embryonic day 10, day of hatch and day 45 posthatch, using both Arbor Acres meat-type and White Leghorn egg-type chickens. By real-time PCR, we found that the expression levels of TGF-β2, TGF-β3, Smad3 and FGF2 were significantly (P≤0.01) higher at embryonic day 10, a developmental window of abundant fetal myoblasts expansion, by comparison to day of hatch and day 45 posthatch. The methylation status of the 5' end region of these four genes was examined subsequently. A section of a CpG island in the 5' end region of FGF2 was significantly hypomethylated (P≤0.01) at embryonic day 10, compared with neonatal and postnatal stages in both stocks. Our results suggested that TGF-β2, TGF-β3, Smad3 and FGF2 may play important roles in fetal myoblasts proliferation in chicken hindlimb, and the transcription of FGF2 in this wave of myogenesis could be affected by DNA methylation in 5' flanking region. These outcomes contribute to our knowledge of the growth factors in avian myogenesis. Further investigation is needed to confirm and fully understand their functions in fetal limb myogenesis in birds.

Promotion of muscle regeneration by myoblast transplantation combined with the controlled and sustained release of bFGFcpr

Although myoblast transplantation is an attractive method for muscle regeneration, its efficiency remains limited. The efficacy of myoblast transplantation in combination with the controlled and sustained delivery of basic fibroblast growth factor (bFGF) was investigated. Defects of thigh muscle in Sprague-Dawley (SD) rats were created, and GFP-positive myoblasts were subsequently transplanted. The rats were divided into three groups. In control group 1 (C1) only myoblasts were transplanted, while in control group 2 (C2) myoblasts were introduced along with empty gelatin hydrogel microspheres. In the experimental group (Ex), myoblasts were transplanted along with bFGF incorporated into gelatin hydrogel microspheres. Four weeks after transplantation, GFP-positive myoblasts were found to be integrated into the recipient muscle and to contribute to muscle fibre regeneration in all groups. A significantly higher expression level of GFP in the Ex group demonstrated that the survival rate of transplanted myoblasts in Ex was remarkably improved compared with that in C1 and C2. Furthermore, myofibre regeneration, characterized by centralization of the nuclei, was markedly accelerated in Ex. The expression level of CD31 in Ex was higher than that in both C1 and C2, but the differences were not statistically significant. A significantly higher expression level of Myogenin and a lower expression level of MyoD1 were both observed in Ex after 4 weeks, suggesting the promotion of differentiation to myotubes. Our findings suggest that the controlled and sustained release of bFGF from gelatin hydrogel microspheres improves the survival rate of transplanted myoblasts and promotes muscle regeneration by facilitating myogenesis rather than angiogenesis.

Improved tissue culture conditions for engineered skeletal muscle sheets

The potential clinical utility of engineered muscle is currently restricted by limited in vitro capacity of expanded muscle precursor cells to fuse and form mature myofibers. The purpose of this study was to use isotropic skeletal muscle sheets to explore the impact of (1) fibroblast coculture and (2) fibroblast-conditioned media (fCM) on in vitro myogenesis. Muscle sheets were prepared by seeding varying ratios of skeletal myoblasts and fibroblasts on a biomimetic substrate and culturing the resulting tissue in either control media or fCM. Muscle sheets were prepared from two cell subpopulations, (1) C2C12 and NOR-10 and (2) primary neonatal rat skeletal muscle cells (nSKM). In C2C12/Nor-10 muscle sheets fCM conferred a myogenic advantage early in culture; at D1 a statistically significant 3.12 ± 0.8-fold increase in myofiber density was observed with fCM. A high purity satellite cell population was collected from an initially mixed population of nSKMs via cell sorting for positive α 7-integrin expression. On D6, tissue sheets with low fibroblast concentrations (0 & 10%) cultured in fCM had increased average myofiber density (4.8 ± 0.2 myofibers/field) compared to tissue sheets with high fibroblast concentrations (50%) cultured in control media (1.0 ± 0.1 myofibers/field). Additionally, fCM promoted longer, thicker myofibers with a mature phenotype.

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.

Fibroblasts influence muscle progenitor differentiation and alignment in contact independent and dependent manners in organized co-culture devices

Myoblasts are precursor muscle cells that lie nascent to mature skeletal muscle. Once muscle is damaged, these cells migrate, fuse, and regenerate the muscle tissue. It is known that skeletal muscle can partially regenerate in vivo after muscle tissue damage. However, this regeneration does not always occur, especially in more severe injuries. Cellular therapy using tissue-engineering approaches has been shown to improve organ repair and function. To exploit potential benefits of using cell therapy as an avenue for skeletal muscle repair, it is important to understand the cellular dynamics underlying skeletal myocyte formation and growth. Cardiac fibroblasts have been shown to have a major influence on cardiomyocyte function, repair, and overall spatial distribution. However, little is known regarding fibroblasts' role on skeletal myocyte function. In this study, we utilized a reconfigurable co-culture device to understand the contact and paracrine effects of fibroblasts on skeletal myocyte alignment and differentiation using murine myoblast and fibroblast cell lines. We demonstrate that myotube alignment is increased by direct contact with fibroblasts, while myotube differentiation is reduced both in the gap and contact configurations with fibroblasts after 6 days of co-culture. Furthermore, neutralizing antibodies to FGF-2 can block these effects of fibroblasts on myotube differentiation and alignment. Finally, bi-directional signaling is critical to the observed myoblast-fibroblast interactions, since conditioned media could not reproduce the same effects observed in the gap configuration. These findings could have direct implications on cell therapies for repairing skeletal muscle, which have only utilized skeletal myoblasts or stem cell populations alone.

Heparin mimicking polymer promotes myogenic differentiation of muscle progenitor cells

Heparin and heparan sulfate mediated basic fibroblast growth factor (bFGF) signaling plays an important role in skeletal muscle homeostasis by maintaining a balance between proliferation and differentiation of muscle progenitor cells. In this study we investigate the role of a synthetic mimic of heparin, poly(sodium-4-styrenesulfonate) (PSS), on myogenic differentiation of C2C12 cells. Exogenous supplementation of PSS increased the differentiation of C2C12 cells in a dose-dependent manner, while the formation of multinucleated myotubes exhibited a nonmonotonic dependence with the concentration of PSS. Our results further suggest that one possible mechanism by which PSS promotes myogenic differentiation is by downregulating the mitogen activated extracellular regulated signaling kinase (MAPK/ERK) pathway. The binding ability of PSS to bFGF was found to be comparable to heparin through molecular docking calculations and by native PAGE. Such synthetic heparin mimics could offer a cost-effective alternative to heparin and also reduce the risk associated with batch-to-batch variation and contamination of heparin.

Developing a novel serum-free cell culture model of skeletal muscle differentiation by systematically studying the role of different growth factors in myotube formation

This work describes the step-by-step development of a novel, serum-free, in vitro cell culture system resulting in the formation of robust, contracting, multinucleate myotubes from dissociated skeletal muscle cells obtained from the hind limbs of fetal rats. This defined system consisted of a serum-free medium formulation developed by the systematic addition of different growth factors as well as a nonbiological cell growth promoting substrate, N-1[3-(trimethoxysilyl) propyl] diethylenetriamine. Each growth factor in the medium was experimentally evaluated for its effect on myotube formation. The resulting myotubes were evaluated immunocytochemically using embryonic skeletal muscle, specifically the myosin heavy chain antibody. Based upon this analysis, we propose a new skeletal muscle differentiation protocol that reflects the roles of the various growth factors which promote robust myotube formation. Further observation noted that the proposed skeletal muscle differentiation technique also supported muscle-nerve coculture. Immunocytochemical evidence of nerve-muscle coculture has also been documented. Applications for this novel culture system include biocompatibility and skeletal muscle differentiation studies, understanding myopathies, neuromuscular disorders, and skeletal muscle tissue engineering.

Extrinsic regulation of domestic animal-derived myogenic satellite cells II

The existence of myogenic satellite cells was reported some 47 years ago, and, since that time, satellite cell research has flourished. So much new information is generated (daily) on these cells that it can be difficult for individuals to keep abreast of important issues related to their activation and proliferation, the modulation of the activity of other cell types, the differentiation of the cells to facilitate normal skeletal muscle growth and development, or to the repair of damaged myofibers. The intent of this review is to summarize new information about the extrinsic regulation of myogenic satellite cells and to provide specific mechanisms involved in altering satellite cell physiology. Where possible, examples from agriculturally important animals are used for illustrative purposes.

Expression of mRNA for specific fibroblast growth factors associates with that of the myogenic markers MyoD and proliferating cell nuclear antigen in regenerating and overloaded rat plantaris muscle

AIM

To examine the relations between specific fibroblast growth factors (FGFs) and satellite cell activation during muscle regeneration and hypertrophy in vivo, we measured mRNA expression of FGFs and myogenic markers in rat plantaris muscle after bupivacaine administration and synergist ablation.

METHODS

mRNA levels for MyoD, myogenin, proliferating cell nuclear antigen (PCNA), p21, M-cadherin, Pax7, FGF-1, FGF-2, FGF-3, FGF-4, FGF-5, FGF-6, FGF-7, FGF-8 and hepatocyte growth factor (HGF) were measured continually for up to 72 h after bupivacaine administration and synergist ablation. FGF-5, FGF-7 and HGF proteins were immunostained at 72 h after bupivacaine administration.

RESULTS

MyoD and PCNA mRNAs started increasing 24 h after bupivacaine administration. Myogenin, p21, M-cadherin and Pax7 mRNAs started to increase after 48 and 72 h. After synergist ablation, MyoD, PCNA, M-cadherin and Pax7 mRNAs had increased at 24 and 48 h, and myogenin and p21 mRNAs at 12 and 24 h. FGF-1, FGF-7 and HGF mRNAs after the treatments started to increase at the same time as MyoD and PCNA mRNAs. FGF-5 was expressed at the same time as MyoD and PCNA mRNAs after bupivacaine administration but did not after the ablation. FGF-2, FGF-3, FGF-4, FGF-6 and FGF-8 mRNAs were not associated with the expression of the myogenic markers. FGF-7 and HGF proteins were expressed in immature muscle fibre nuclei and the extracellular matrix, but FGF-5 protein was preferentially expressed in extracellular matrix.

CONCLUSION

These results indicate that FGF-1, FGF-7 and HGF are associated with specific myogenic marker expression during muscle regeneration and hypertrophy.

FGFR1 over-expression in primary rhabdomyosarcoma tumors is associated with hypomethylation of a 5' CpG island and abnormal expression of the AKT1, NOG, and BMP4 genes

Rhabdomyosarcoma (RMS), the most common pediatric soft tissue sarcoma likely results from abnormal proliferation and differentiation during skeletal myogenesis. Multiple genetic alterations are associated with the three RMS histopathological subtypes, embryonal, alveolar, and pleomorphic adult variant. Recently, we reported the novel amplification of the FGFR1 gene in a RMS tumor. The involvement of FGFR1 in RMS was now further studied in primary tumors and RMS cell lines by mutation screening, quantitative RNA expression, and methylation analyses. No mutation was found by DHPLC and sequencing of the entire FGFR1 coding sequence and exon-intron boundaries. However, FGFR1 over-expression was detected in all primary RMS tumors and cell lines tested. A hypomethylation of a CpG island upstream to FGFR1 exon 1 was identified in the primary RMS tumors, using sodium bisulfite modification method, suggesting a molecular mechanism to FGFR1 over-expression. Expression analysis of additional genes, AKT1, NOG and its antagonist BMP4, which interact downstream to FGFR1, demonstrated expression differences between primary RMS tumors and normal skeletal muscles. Our data suggest an important role for FGFR1 and FGFR1-downstream genes in RMS tumorigenesis and a possible association with the deregulation of proliferation and differentiation of skeletal myoblasts in RMS.

FGFR1 inhibits skeletal muscle atrophy associated with hindlimb suspension

BACKGROUND

Skeletal muscle atrophy can occur under many different conditions, including prolonged disuse or immobilization, cachexia, cushingoid conditions, secondary to surgery, or with advanced age. The mechanisms by which unloading of muscle is sensed and translated into signals controlling tissue reduction remains a major question in the field of musculoskeletal research. While the fibroblast growth factors (FGFs) and their receptors are synthesized by, and intimately involved in, embryonic skeletal muscle growth and repair, their role maintaining adult muscle status has not been examined.

METHODS

We examined the effects of ectopic expression of FGFR1 during disuse-mediated skeletal muscle atrophy, utilizing hindlimb suspension and DNA electroporation in mice.

RESULTS

We found skeletal muscle FGF4 and FGFR1 mRNA expression to be modified by hind limb suspension,. In addition, we found FGFR1 protein localized in muscle fibers within atrophying mouse muscle which appeared to be resistant to atrophy. Electroporation and ectopic expression of FGFR1 significantly inhibited the decrease in muscle fiber area within skeletal muscles of mice undergoing suspension induced muscle atrophy. Ectopic FGFR1 expression in muscle also significantly stimulated protein synthesis in muscle fibers, and increased protein degradation in weight bearing muscle fibers.

CONCLUSION

These results support the theory that FGF signaling can play a role in regulation of postnatal skeletal muscle maintenance, and could offer potentially novel and efficient therapeutic options for attenuating muscle atrophy during aging, illness and spaceflight.

Fibroblast growth factor-2 enhances functional recovery of reinnervated muscle

Long-term denervation of muscles results in fibrosis and fat replacement, which prevent muscles from regaining contractile function despite reinnervation. Therefore, prevention of muscle atrophy between nerve repair and muscle reinnervation may improve the functional outcome. A variety of growth factors play significant roles in muscle mass modulation and muscle regeneration. The purpose of the present study was to investigate the effect of fibroblast growth factor-2 (FGF2) and nerve growth factor (NGF) on muscle mass modulation after denervation and reinnervation using a nerve-to-muscle neurotization model. Growth factors were injected into the anterior tibial muscle after direct neurotization of muscles every 7 days up to 4 weeks after surgery. FGF2 significantly increased the amplitude of compound muscle action potentials (CMAPs), wet muscle weight, and the number of motor endplates, especially at higher concentration, compared to the vehicle. In contrast, NGF did not increase either the amplitude of CMAPs or muscle weight, although it significantly increased the number of motor endplates. These findings indicate that both growth factors enhance reinnervation of muscles; however, only FGF2 is involved in muscle mass modulation.

A defined system to allow skeletal muscle differentiation and subsequent integration with silicon microstructures

This work documents the development of an in vitro cell culture model consisting of a novel serum-free medium and a non-biological growth substrate, N-1[3 (trimethoxysilyl) propyl] diethylenetriamine (DETA), to enable functional myotube integration with cantilevers fabricated using MEMS technology. This newly developed, defined in vitro model was used to study the differentiation of fetal rat skeletal muscle and it promoted the formation of myotubes from the dissociated rat fetal muscle cells. The myotubes were characterized by morphological analysis, immunocytochemistry and electrophysiology. Further, it was demonstrated that when the dissociated muscle cells were plated on fabricated microcantilevers, the muscle cells aligned along the major axis of the cantilever and formed robust myotubes. This novel system could not only find applications in skeletal muscle differentiation and biocompatibility studies but also in bioartificial muscle engineering, hybrid actuation system development, biorobotics and for a better understanding of myopathies and neuromuscular disorders.