Mechano Growth Factor E peptide (MGF-E), derived from an isoform of IGF-1, activates human muscle progenitor cells and induces an increase in their fusion potential at different ages

Anti-PEc: Development of a novel monoclonal antibody against prostate cancer

BACKGROUND

The Ec peptide (PEc) that defines the IGF-1Ec isoform, is associated with prostate cancer progression by inducing proliferation, metastases, and tumour repair. On these grounds, an anti-PEc monoclonal antibody (MAb) was developed. Our objective is to examine the effects of this antibody on prostate cancer and its possible side effects.

METHODS

The effects of the obtained MAb were examined in cancer and non-cancerous cell lines (unmodified and modified either to overexpress or silence PEc) and in tumours in SCID mice injected with unmodified prostate cancer cells. The investigation was obtained with respect to cellular proliferation, migration, invasion, toxicity to tumours, effects on the cell cycle, immune response activation, effects on mesenchymal stem cell mobilisation leading to tumour repair, tissue distribution, and toxicity to mice.

RESULTS

Anti-PEc MAb treatment led to a significant decrease in cellular proliferation, migration, and invasion compared to the untreated cell lines (p < 0.0005 in every case). Mechanistically, these effects were associated with the downregulation of pERK1/2 and vimentin and the upregulation of E-Cadherin. In vivo, anti-PEc MAb treatment was associated with a significant decrease in tumour size and metastases rate (p < 0.0005 in every case) by reversing the tumours mesenchymal phenotype. It also inhibited host stem cell mobilisation towards the tumour, leading to apoptosis. Anti-PEc MAb assessment in respect to distribution and toxicity, indicated its tumour specificity and lack of toxicity.

CONCLUSIONS

These data indicate that the therapeutic targeting of PEc with the anti-PEc MAb may have considerable clinical benefit for prostate cancer patients.

IGF1 promotes human myotube differentiation toward a mature metabolic and contractile phenotype

Skeletal muscle mediates the beneficial effects of exercise, thereby improving insulin sensitivity and reducing the risk for type 2 diabetes. Current human skeletal muscle models in vitro are incapable of fully recapitulating its physiological functions especially muscle contractility. By supplementation of insulin-like growth factor 1 (IGF1), a growth factor secreted by myofibers in vivo, we aimed to overcome these limitations. We monitored the differentiation process starting from primary human CD56-positive myoblasts in the presence/absence of IGF1 in serum-free medium in daily collected samples for 10 days. IGF1-supported differentiation formed thicker multinucleated myotubes showing physiological contraction upon electrical pulse stimulation (EPS) following day 6. Myotubes without IGF1 were almost incapable of contraction. IGF1 treatment shifted the proteome toward skeletal muscle-specific proteins that contribute to myofibril and sarcomere assembly, striated muscle contraction, and ATP production. Elevated PPARGC1A, MYH7, and reduced MYH1/2 suggest a more oxidative phenotype further demonstrated by higher abundance of proteins of the respiratory chain and elevated mitochondrial respiration. IGF1-treatment also upregulated glucose transporter (GLUT)4 and increased insulin-dependent glucose uptake compared with myotubes differentiated without IGF1. To conclude, addition of IGF1 to serum-free medium significantly improves the differentiation of human myotubes that showed enhanced myofibril formation, response to electrical pulse stimulation, oxidative respiratory capacity, and glucose metabolism overcoming limitations of previous standards. This novel protocol enables investigation of muscular exercise on a molecular level.NEW & NOTEWORTHY Human skeletal muscle models are highly valuable to study how exercise prevents type 2 diabetes without invasive biopsies. Current models did not fully recapitulate the function of skeletal muscle especially during exercise. By supplementing insulin-like growth factor 1 (IGF1), the authors developed a functional human skeletal muscle model characterized by inducible contractility and increased oxidative and insulin-sensitive metabolism. The novel protocol overcomes the limitations of previous standards and enables investigation of exercise on a molecular level.

Age-Related Alternative Splicing: Driver or Passenger in the Aging Process?

Alternative splicing changes are closely linked to aging, though it remains unclear if they are drivers or effects. As organisms age, splicing patterns change, varying gene isoform levels and functions. These changes may contribute to aging alterations rather than just reflect declining RNA quality control. Three main splicing types-intron retention, cassette exons, and cryptic exons-play key roles in age-related complexity. These events modify protein domains and increase nonsense-mediated decay, shifting protein isoform levels and functions. This may potentially drive aging or serve as a biomarker. Fluctuations in splicing factor expression also occur with aging. Somatic mutations in splicing genes can also promote aging and age-related disease. The interplay between splicing and aging has major implications for aging biology, though differentiating correlation and causation remains challenging. Declaring a splicing factor or event as a driver requires comprehensive evaluation of the associated molecular and physiological changes. A greater understanding of how RNA splicing machinery and downstream targets are impacted by aging is essential to conclusively establish the role of splicing in driving aging, representing a promising area with key implications for understanding aging, developing novel therapeutical options, and ultimately leading to an increase in the healthy human lifespan.

Anabolic Resistance in the Pathogenesis of Sarcopenia in the Elderly: Role of Nutrition and Exercise in Young and Old People

The development of sarcopenia in the elderly is associated with many potential factors and/or processes that impair the renovation and maintenance of skeletal muscle mass and strength as ageing progresses. Among them, a defect by skeletal muscle to respond to anabolic stimuli is to be considered. Common anabolic stimuli/signals in skeletal muscle are hormones (insulin, growth hormones, IGF-1, androgens, and β-agonists such epinephrine), substrates (amino acids such as protein precursors on top, but also glucose and fat, as source of energy), metabolites (such as β-agonists and HMB), various biochemical/intracellular mediators), physical exercise, neurogenic and immune-modulating factors, etc. Each of them may exhibit a reduced effect upon skeletal muscle in ageing. In this article, we overview the role of anabolic signals on muscle metabolism, as well as currently available evidence of resistance, at the skeletal muscle level, to anabolic factors, from both in vitro and in vivo studies. Some indications on how to augment the effects of anabolic signals on skeletal muscle are provided.

The role of mechano growth factor in chondrocytes and cartilage defects: a concise review

Mechano growth factor (MGF), an isoform of insulin-like growth factor 1 (IGF-1), is recognized as a typical mechanically sensitive growth factor and has been shown to play an indispensable role in the skeletal system. In the joint cavity, MGF is highly expressed in chondrocytes, especially in the damaged cartilage tissue caused by trauma or degenerative diseases such as osteoarthritis (OA). Cartilage is an extremely important component of joints because it functions as a shock absorber and load distributer at the weight-bearing interfaces in the joint cavity, but it can hardly be repaired once injured due to its lack of blood vessels, lymphatic vessels, and nerves. MGF has been proven to play an important role in chondrocyte cell behaviors, including cell proliferation, migration, differentiation, inflammatory reactions and apoptosis, in and around the injury site. Moreover, under the normalized mechanical microenvironment in the joint cavity, MGF can sense and respond to mechanical stimuli, regulate chondrocyte activity, and maintain the homeostasis of cartilage tissue. Recent reports continue to explain its effects on various cell types and sport-related tissues, but its role in cartilage development, homeostasis and disease occurrence is still controversial, and its internal biological mechanism is still elusive. In this review, we summarize recent discoveries in the role of MGF in chondrocytes and cartilage defects, including tissue repair at the macroscopic level and chondrocyte activities at the microcosmic level, and discuss the current state of research and potential gaps in knowledge.

The initial delay to mitotic activity in primary cultures of equine satellite cells is reduced by combinations of growth factors

Satellite cell (SC) activation is defined as the time frame during which the stem cell becomes poised to reenter G1 of the cell cycle. The growth factors and events leading to full mitotic activation in equine SCs remain largely unknown. Insulin-like growth factor I (IGF-I), hepatocyte growth factor (HGF), and fibroblast growth factor 2 (FGF2) are sequentially transcribed during the muscle repair and recovery period following strenuous exercise in adult horses. Expression of IGF-I occurs within 24 h of the postexercise recovery period suggesting it may affect early SC actions. As a first step, gluteus medius muscle cryosections from adult horses (n = 9) were examined for the presence of central nuclei (CN), a marker of SC addition to the fiber. Results demonstrate few CN fibers prior to exercise with a 3-fold increase (P = 0.05) 24 h postexercise. Cultures of SC (n = 4 isolates) were treated with 100 ng/mL IGF-I for varying times prior to measurement of myogenic events. Results demonstrate that IGF-I does not affect the initial lag period, proliferation, or subsequent differentiation of equine SC in vitro (P > 0.05). However, media containing a combination of IGF-I and 10 ng/mL FGF2 and 25 ng/mL HGF hastens (P < 0.05) the time to S-phase entry in fresh isolates of SCs. Media supplementation with optimal concentrations of FGF2, HGF, or a combination of HGF and FGF2 suppresses (P < 0.05) the percentage of myogenin immunopositive SCs to levels below that found in control- or IGF-I-treated SCs. These results provide new insight into the combinatorial roles growth factors play during equine SC myogenesis.

In vitro investigation of growth factors including MGF and IGF-1 in neural stem cell activation, proliferation, and migration

Neurogenesis is mainly activated after damage in adult tissues. This destruction activates the neural stem cells (NSCs) by exiting from a quiescent state and initiating proliferation, differentiation, and migration towards the damaged area. Although studies have investigated to clarify the process of NSC biology and neurogenesis, there are still significant artifacts in understanding the primary mechanism. It is known that only a small percentage of NSC become neurons and integrate into the brain tissue after this process. The significant proportion differentiates to become either astrocytes or oligodendrocytes. Furthermore, the quiescent stem cells in the niche are mainly activated by the stimuli affect. In recent years, many studies have been conducted with varying hormones, some of which might provide neuro-stimulation effect and/or involved in the regeneration of the brain tissue and/or neuroprotection from traumatic or ischemic pathologies, including Insulin-like growth factor 1 (IGF-1), Mechano Growth Factor (MGF), Basic Fibroblast Growth Factor (FGF-2), Erythropoietin (EPO), Epidermal Growth Factor (EGF), Nerve Growth Factor (NGF) and Brain-Derived Neurotrophic Factor (BDNF). In this study, we examined the effects of FGF-2, MGF, IGF-1, EPO, EGF, NGF, and BDNF alone or with various combinations on rat hippocampal NSC by tracking the changes in the expression of Nestin, GFAP, TUBB3, and DCX genes during 24 h (h), 72 h and 168 h time frame. The apoptosis analysis revealed that FGF-2 and FGF-2 coupled growth factors effectively protect NSCs against apoptosis, whereas MGF coupled growth factors failed in this protection. The cell cycle analysis demonstrated that these growth factors had accumulated the NSCs exit from the quiescent phase to the Mitosis phase, mostly without being long in the Synthesis Phase. Neurosphere sizes were increased with MGF, signifying MGF being effective in neural progenitor cells. The combined use of MGF with FGF-2 was more effective in postmitotic neurons than MGF alone. We have comparatively demonstrated the effect of cytokines alone and combined administration on activation, proliferation, and migration of NSCs. Although many issues are still waiting to be investigated in adult neurogenesis, neural regeneration, and adult neural stem cell biology, the results provide vital resources to the researchers that are interested in the varying effect of growth factor on NSC.

Insulin-Like Growth Factor-1 (IGF-1) and Its Monitoring in Medical Diagnostic and in Sports

Insulin-like growth factor-1 (IGF-1) is the principal mediator of growth hormone (GH), plays a crucial role in promoting cell growth and differentiation in childhood and continues to have an anabolic effect in adults. IGF-1 is part of a wide network of growth factors, receptors and binding proteins involved in mediating cellular proliferation, differentiation and apoptosis. Bioavailability of IGF-1 is affected by insulin-like growth factor binding proteins (IGFBPs) which bind IGF-1 in circulation with an affinity equal to or greater than that of the IGF-1 receptor (IGF-1R). The six IGFBPs serve as carrier proteins and bind approximately 98% of all circulating IGF-1. Other proteins known to bind IGF-1 include ten IGFBP-related proteins (IGFBP-rPs), albeit with lower affinities than the IGFBPs. IGF-1 expression levels vary in a number of clinical conditions suggesting it has the potential to provide crucial information as to the state of an individual’s health. IGF-1 is also a popular doping agent in sport and has featured in many high-profile doping cases in recent years. However, the existence of IGFBPs significantly reduces the levels of immunoreactive IGF-1 in samples, requiring multiple pre-treatment steps that reduce reproducibility and complicates interpretation of IGF-1 assay results. Here we provide an overview of the IGF network of growth factors, their receptors and the entirety of the extended family of IGFBPs, IGFBP-rPs, E peptides as well as recombinant IGF-1 and their derivatives. We also discuss issues related to the detection and quantification of bioavailable IGF-1.

Longitudinal Muscle and Myocellular Changes in Community-Dwelling Men Over Two Decades of Successful Aging-The ULSAM Cohort Revisited

Participants of the population-based Uppsala longitudinal study of adult men (ULSAM) cohort reaching more than 88 years of age (survivors, S) were investigated at age 70, 82, and 88-90 and compared at 70 years with non-survivors (NS) not reaching 82 years. Body composition, muscle mass and muscle histology were remarkably stable over 18 years of advanced aging in S. Analysis of genes involved in muscle remodeling showed that S had higher mRNA levels of myogenic differentiation factors (Myogenin, MyoD), embryonic myosin (eMyHC), enzymes involved in regulated breakdown of myofibrillar proteins (Smad2, Trim32, MuRF1,) and NCAM compared with healthy adult men (n = 8). S also had higher mRNA levels of eMyHC, Smad 2, MuRF1 compared with NS. At 88 years, S expressed decreased levels of Myogenin, MyoD, eMyHC, NCAM and Smad2 towards those seen in NS at 70 years. The gene expression pattern of S at 70 years was likely beneficial since they maintained muscle fiber histology and appendicular lean body mass until advanced age. The expression pattern at 88 years may indicate a diminished muscle remodeling coherent with a decline of reinnervation capacity and/or plasticity at advanced age.

Role of Alternatively Spliced Messenger RNA (mRNA) Isoforms of the Insulin-Like Growth Factor 1 (IGF1) in Selected Human Tumors

Insulin-like growth factor 1 (IGF1) is a key regulator of tissue growth and development that is also implicated in the initiation and progression of various cancers. The human IGF1 gene contains six exons and five long introns, the transcription of which is controlled by two promoters (P1 and P2). Alternate promoter usage, as well as alternative splicing (AS) of IGF1, results in the expression of six various variants (isoforms) of mRNA, i.e., IA, IB, IC, IIA, IIB, and IIC. A mature 70-kDa IGF1 protein is coded only by exons 3 and 4, while exons 5 and 6 are alternatively spliced code for the three C-terminal E peptides: Ea (exon 6), Eb (exon 5), and Ec (fragments of exons 5 and 6). The most abundant of those transcripts is IGF1Ea, followed by IGF1Eb and IGF1Ec (also known as mechano-growth factor, MGF). The presence of different IGF1 transcripts suggests tissue-specific auto- and/or paracrine action, as well as separate regulation of both of these gene promoters. In physiology, the role of different IGF1 mRNA isoforms and pro-peptides is best recognized in skeletal muscle tissue. Their functions include the development and regeneration of muscles, as well as maintenance of proper muscle mass. In turn, in nervous tissue, a neuroprotective function of short peptides, produced as a result of IGF1 expression and characterized by significant blood-brain barrier penetrance, has been described and could be a potential therapeutic target. When it comes to the regulation of carcinogenesis, the potential biological role of different var iants of IGF1 mRNAs and pro-peptides is also intensively studied. This review highlights the role of IGF1 isoform expression (mRNAs, proteins) in physiology and different types of human tumors (e.g., breast cancer, cervical cancer, colorectal cancer, osteosarcoma, prostate and thyroid cancers), as well as mechanisms of IGF1 spliced variants involvement in tumor biology.

DNA methylation across the genome in aged human skeletal muscle tissue and muscle-derived cells: the role of HOX genes and physical activity

Skeletal muscle tissue demonstrates global hypermethylation with age. However, methylome changes across the time-course of differentiation in aged human muscle derived cells, and larger coverage arrays in aged muscle tissue have not been undertaken. Using 850K DNA methylation arrays we compared the methylomes of young (27 ± 4.4 years) and aged (83 ± 4 years) human skeletal muscle and that of young/aged heterogenous muscle-derived human primary cells (HDMCs) over several time points of differentiation (0, 72 h, 7, 10 days). Aged muscle tissue was hypermethylated compared with young tissue, enriched for; pathways-in-cancer (including; focal adhesion, MAPK signaling, PI3K-Akt-mTOR signaling, p53 signaling, Jak-STAT signaling, TGF-beta and notch signaling), rap1-signaling, axon-guidance and hippo-signalling. Aged cells also demonstrated a hypermethylated profile in pathways; axon-guidance, adherens-junction and calcium-signaling, particularly at later timepoints of myotube formation, corresponding with reduced morphological differentiation and reductions in MyoD/Myogenin gene expression compared with young cells. While young cells showed little alterations in DNA methylation during differentiation, aged cells demonstrated extensive and significantly altered DNA methylation, particularly at 7 days of differentiation and most notably in focal adhesion and PI3K-AKT signalling pathways. While the methylomes were vastly different between muscle tissue and HDMCs, we identified a small number of CpG sites showing a hypermethylated state with age, in both muscle tissue and cells on genes KIF15, DYRK2, FHL2, MRPS33, ABCA17P. Most notably, differential methylation analysis of chromosomal regions identified three locations containing enrichment of 6–8 CpGs in the HOX family of genes altered with age. With HOXD10, HOXD9, HOXD8, HOXA3, HOXC9, HOXB1, HOXB3, HOXC-AS2 and HOXC10 all hypermethylated in aged tissue. In aged cells the same HOX genes (and additionally HOXC-AS3) displayed the most variable methylation at 7 days of differentiation versus young cells, with HOXD8, HOXC9, HOXB1 and HOXC-AS3 hypermethylated and HOXC10 and HOXC-AS2 hypomethylated. We also determined that there was an inverse relationship between DNA methylation and gene expression for HOXB1, HOXA3 and HOXC-AS3. Finally, increased physical activity in young adults was associated with oppositely regulating HOXB1 and HOXA3 methylation compared with age. Overall, we demonstrate that a considerable number of HOX genes are differentially epigenetically regulated in aged human skeletal muscle and HDMCs and increased physical activity may help prevent age-related epigenetic changes in these HOX genes.

Mechano growth factor interacts with nucleolin to protect against cisplatin-induced neurotoxicity

Mechano growth factor (MGF) is an alternatively spliced form of insulin-like growth factor-1 (IGF-1) that has shown to be neuroprotective against 6-hydroxydopamine toxicity and ischemic injury in the brain. MGF also induces neural stem cell proliferation in the hippocampus and preserves olfactory function in aging mice. Cisplatin is a chemotherapy drug that induces peripheral neuropathy in 30-40% of treated patients. Our studies were designed to see if MGF would protect dorsal root ganglion (DRG) neurons from cisplatin-induced neurotoxicity and to identify potential mechanisms that may be involved. Expression of endogenous MGF in adult DRG neurons in vivo ameliorated cisplatin-induced thermal hyperalgesia. Exogenous MGF and MGF with a cysteine added to the N-terminus (CMGF) also protected embryonic DRG neurons from cisplatin-induced cell death in vitro. Mass spectroscopy analysis of proteins bound to MGF showed that nucleolin is a key-binding partner. Antibodies against nucleolin prevented the neuroprotective effect of MGF and CMGF in culture. Both nucleolin and MGF are located in the nucleolus of DRG neurons. RNAseq of RNA associated with MGF indicated that MGF may be involved in RNA processing, protein targeting and transcription/translation. Nucleolin is an RNA binding protein that is readily shuttled between the nucleus, cytoplasm and plasma membrane. Nucleolin and MGF may work together to prevent cisplatin-induced neurotoxicity. Exploring the known mechanisms of nucleolin may help us better understand the mechanisms of cisplatin toxicity and how MGF protects DRG neurons.

Mechanisms of IGF-1-Mediated Regulation of Skeletal Muscle Hypertrophy and Atrophy

Insulin-like growth factor-1 (IGF-1) is a key growth factor that regulates both anabolic and catabolic pathways in skeletal muscle. IGF-1 increases skeletal muscle protein synthesis via PI3K/Akt/mTOR and PI3K/Akt/GSK3β pathways. PI3K/Akt can also inhibit FoxOs and suppress transcription of E3 ubiquitin ligases that regulate ubiquitin proteasome system (UPS)-mediated protein degradation. Autophagy is likely inhibited by IGF-1 via mTOR and FoxO signaling, although the contribution of autophagy regulation in IGF-1-mediated inhibition of skeletal muscle atrophy remains to be determined. Evidence has suggested that IGF-1/Akt can inhibit muscle atrophy-inducing cytokine and myostatin signaling via inhibition of the NF-κΒ and Smad pathways, respectively. Several miRNAs have been found to regulate IGF-1 signaling in skeletal muscle, and these miRs are likely regulated in different pathological conditions and contribute to the development of muscle atrophy. IGF-1 also potentiates skeletal muscle regeneration via activation of skeletal muscle stem (satellite) cells, which may contribute to muscle hypertrophy and/or inhibit atrophy. Importantly, IGF-1 levels and IGF-1R downstream signaling are suppressed in many chronic disease conditions and likely result in muscle atrophy via the combined effects of altered protein synthesis, UPS activity, autophagy, and muscle regeneration.

Graphene oxide nanofilm and the addition of L-glutamine can promote development of embryonic muscle cells

Background

Formation of muscular pseudo-tissue depends on muscle precursor cells, the extracellular matrix (ECM)-mimicking structure and factors stimulating cell differentiation. These three things cooperate and can create a tissue-like structure, however, their interrelationships are relatively unknown. The objective was to study the interaction between surface properties, culture medium composition and heterogeneous cell culture. We would like to demonstrate that changing the surface properties by coating with graphene oxide nanofilm (nGO) can affect cell behaviour and especially their need for the key amino acid l-glutamine (L-Glu).

Results

Chicken embryo muscle cells and their precursors, cultured in vitro, were used as the experimental model. The mesenchymal stem cell, collected from the hind limb of the chicken embryo at day 8 were divided into 4 groups; the control group and groups treated with nGO, L-Glu and nGO supplied with L-Glu (nGOxL-Glu). The roughness of the surface of the plastic plate covered with nGO was much lower than a standard plate. The test of nGO biocompatibility demonstrated that the cells were willing to settle on the nGO without any toxic effects. Moreover, nGO by increasing hydrophilicity and reducing roughness and presumably through chemical bonds available on the GO surface stimulated the colonisation of primary stromal cells that promote embryonic satellite cells. The viability significantly increased in cells cultured on nGOxL-Glu. Observations of cell morphology showed that the most mature state of myogenesis was characteristic for the group nGOxL-Glu. This result was confirmed by increasing the expression of MYF5 genes at mRNA and protein levels. nGO also increased the expression of MYF5 and also very strongly the expression of PAX7 at mRNA and protein levels. However, when analysing the expression of PAX7, a positive link was observed between the nGO surface and the addition of L-Glu.

Conclusions

The use of nGO and L-Glu supplement may improve myogenesis and also the myogenic potential of myocytes and their precursors by promoting the formation of satellite cells. Studies have, for the first time, demonstrated positive cooperation between surface properties nGO and L-Glu supplementation to the culture medium regarding the myogenic potential of cells involved in muscle formation.

Association between sarcopenia and levels of growth hormone and insulin-like growth factor-1 in the elderly

Background

Age-related sarcopenia is a serious global health issue in elderly individuals and for the community as it induces disability and significant economic burden. The purpose of the study is to understand the factors associated with sarcopenia and the role of growth hormone (GH) and insulin-like growth factor (IGF-1) in the occurrence of sarcopenia.

Methods

Elderly patients (n = 3276) were included in this cross-sectional study. Survey and measurement of body composition (bioelectrical impedance), grip strength, and step speed were performed according to the Asian Working Group on Sarcopenia (AWGS) diagnostic criteria. Hematological and hormonal indicators were compared between patients with and without sarcopenia in order to identify the associated factors.

Results

There were significant differences in the demographic parameters between the sarcopenia and non-sarcopenia groups (all P < 0.05). There were significant differences between the two groups regarding the blood levels of GH, IGF-1, testosterone (T), and mechanical growth factor (MGF) (all P < 0.001). Correlation analyses showed that the appendicular skeletal muscle mass (ASMI) was positively associated with gender and BMI, and with GH, T, IGF-1, MGF, BUN, Cr, and Hb levels, but negatively associated with HDL-C (all P < 0.05). Logistic multivariable regression analysis showed that IGF-1, MGF, BMI, and gender were independently associated with appendicular skeletal muscle mass (ASMI) (all P < 0.05).

Conclusions

GH and IGF-1 are associated with sarcopenia in the elderly. IGF-1 and MGF are independently associated with the reduction of skeletal muscle mass, along with BMI and gender.

A focused review of myokines as a potential contributor to muscle hypertrophy from resistance-based exercise

PURPOSE

Resistance exercise induces muscle growth and is an important treatment for age-related losses in muscle mass and strength. Myokines are hypothesized as a signal conveying physiological information to skeletal muscle, possibly to "fine-tune" other regulatory pathways. While myokines are released from skeletal muscle following contraction, their role in increasing muscle mass and strength in response to resistance exercise or training is not established. Recent research identified both local and systemic release of myokines after an acute bout of resistance exercise. However, it is not known whether myokines with putative anabolic function are mechanistically involved in producing muscle hypertrophy after resistance exercise. Further, nitric oxide (NO), an important mediator of muscle stem cell activation, upregulates the expression of certain myokine genes in skeletal muscle.

METHOD

In the systemic context of complex hypertrophic signaling, this review: (1) summarizes literature on several well-recognized, representative myokines with anabolic potential; (2) explores the potential mechanistic role of myokines in skeletal muscle hypertrophy; and (3) identifies future research required to advance our understanding of myokine anabolism specifically in skeletal muscle.

RESULT

This review establishes a link between myokines and NO production, and emphasizes the importance of considering systemic release of potential anabolic myokines during resistance exercise as complementary to other signals that promote hypertrophy.

CONCLUSION

Investigating adaptations to resistance exercise in aging opens a novel avenue of interdisciplinary research into myokines and NO metabolites during resistance exercise, with the longer-term goal to improve muscle health in daily living, aging, and rehabilitation.

Featured characteristics and pivotal roles of satellite cells in skeletal muscle regeneration

Skeletal muscle, the essential organ for locomotion, as well as energy reservoir and expenditure, has robust regenerative capacity in response to mechanical stress and injury. As muscle-specific stem cells, satellite cells are responsible for providing new myoblasts during the process of muscle growth and regeneration. Self-renewal capacity and the fate of satellite cells are highly regulated and influenced by their surrounding factors, such as extracellular matrix and soluble proteins. The strong myogenic potential of satellite cells makes them a potential resource for stem cell therapy to cure genetic muscle disease and repair injured muscle. Here, we both review key features of satellite cells during skeletal muscle development and regeneration and summarize recent outcomes of satellite cell transplantation studies.

Engineered skeletal muscles for disease modeling and drug discovery

Skeletal muscle is the largest organ of human body with several important roles in everyday movement and metabolic homeostasis. The limited ability of small animal models of muscle disease to accurately predict drug efficacy and toxicity in humans has prompted the development in vitro models of human skeletal muscle that fatefully recapitulate cell and tissue level functions and drug responses. We first review methods for development of three-dimensional engineered muscle tissues and organ-on-a-chip microphysiological systems and discuss their potential utility in drug discovery research and development of new regenerative therapies. Furthermore, we describe strategies to increase the functional maturation of engineered muscle, and motivate the importance of incorporating multiple tissue types on the same chip to model organ cross-talk and generate more predictive drug development platforms. Finally, we review the ability of available in vitro systems to model diseases such as type II diabetes, Duchenne muscular dystrophy, Pompe disease, and dysferlinopathy.

Local Vibration Stimuli Induce Mechanical Stress-Induced Factors and Facilitate Recovery From Immobilization-Induced Oxidative Myofiber Atrophy in Rats

Muscle atrophy can be caused by unloading stress such as microgravity environments or cast immobilization. Therapies for such disuse muscle atrophy and their underlying mechanisms are incompletely understood. Here, we investigated the therapeutic effects of local vibration stimulation on immobilization-induced skeletal muscle atrophy. A rat model was made by placing the left hindlimb in a cast for 1 week, leading to oxidative myofiber atrophy without myopathic changes in soleus skeletal muscle. Vibration stimulus (90 Hz, 15 min) to the plantar fascia of the atrophic hindlimb was performed once a day using a hand-held vibration massager after removal of a cast at the end of the immobilization period. After 2 weeks, rats were dissected, and quantitative analysis of the cross-sectional areas of soleus myofibers was performed. The results revealed that vibration induced significant recovery from disuse muscle atrophy, compared with untreated immobilized samples. Furthermore, vibration treatment suppressed the fiber transition from slow to fast fiber types compared with vibration-untreated immobilized samples. Western blotting analyses of mechanical stress-induced factors revealed that the expression of mechano-growth factor (MGF), systemic insulin-like growth factor I, and the mechanotransduction protein, Yes-associated protein 1 (YAP1), was decreased in untreated immobilized soleus muscle, whereas vibration stimulation restored their expression. No change in the level of phosphorylation of YAP1^Ser127 was observed, leading to no change in p-YAP1/YAP1 ratio in vibration-treated immobilized soleus muscle. The results indicate that vibration stimulus is effective to restore immobilization-induced inactivation of YAP1 pathway. Phosphorylation of ERK 1/2, but not AKT, was enhanced in vibration-treated immobilized soleus muscle. Furthermore, vibration stimuli restored immobilization-induced downregulation of the paired box transcription factor, PAX7, a critical factor for regenerative myogenesis in muscle satellite cells. Our results indicate that cyclic vibration stimuli are effective in activating satellite cells and facilitate recovery from immobilization-induced oxidative myofiber atrophy through upregulation of MGF and YAP1.

Insulin-Like Growth Factor I Regulation and Its Actions in Skeletal Muscle

The insulin-like growth factor (IGF) pathway is essential for promoting growth and survival of virtually all tissues. It bears high homology to its related protein insulin, and as such, there is an interplay between these molecules with regard to their anabolic and metabolic functions. Skeletal muscle produces a significant proportion of IGF-1, and is highly responsive to its actions, including increased muscle mass and improved regenerative capacity. In this overview, the regulation of IGF-1 production, stability, and activity in skeletal muscle will be described. Second, the physiological significance of the forms of IGF-1 produced will be discussed. Last, the interaction of IGF-1 with other pathways will be addressed. © 2019 American Physiological Society. Compr Physiol 9:413-438, 2019.

Effects of different types of exercise on skeletal muscle atrophy, antioxidant capacity and growth factors expression following myocardial infarction

AIMS

Myocardial infarction (MI) is accompanied with skeletal muscle abnormalities. The aims are to explore an optimal exercise mode to improve cardiac function and prevent skeletal muscle atrophy, and detect the possible mechanisms of exercise-induced inhibition of muscle atrophy.

MAIN METHODS

Rats were subjected to four weeks of different types of exercise after MI surgery (resistance training, RT; moderated-intensity continuous aerobic exercise, MCE and high-intensity intermittent aerobic exercise, HIA). Cardiac function, histological changes of heart and skeletal muscle, oxidative stress, antioxidant capacity and the expression of muscle atrophy-related factors were detected in skeletal muscle.

KEY FINDINGS

The three types of exercise improved heart function, reduced cardiac fibrosis and increased muscle weight and cross-section area (CSA) of muscle fibers in different degrees. The survival rates of MI rats intervened by RT and MCE were higher than HIA. Exercise down-regulated the mRNA levels of murf1 and atrogin-1, decreased reactive oxygen species level, increased antioxidant capacity, regulated the expression of insulin-like growth factor 1 (IGF1), mechano growth factor (MGF), Neuregulin1 (NRG1) and Myostatin (MSTN), and activated Akt and Erk1/2 signalings in soleus muscle. Furthermore, CSA of muscle fibers and the expression of IGF1, MGF, NRG1 in skeletal muscle had correlations with cardiac function.

SIGNIFICANCE

RT and MCE are the first two choices for the early exercise rehabilitation following MI. All types of exercise can effectively inhibit skeletal muscle atrophy through increasing the antioxidant capacity, reducing oxidative stress and protein degradation, and regulating the growth factors expression in skeletal muscle.

Hepatocyte growth factor acts as a mitogen for equine satellite cells via protein kinase C δ-directed signaling

Hepatocyte growth factor (HGF) signals mediate mouse skeletal muscle stem cell, or satellite cell (SC), reentry into the cell cycle and myoblast proliferation. Because the athletic horse experiences exercise-induced muscle damage, the objective of the experiment was to determine the effect of HGF on equine SC (eqSC) bioactivity. Fresh isolates of adult eqSC were incubated with increasing concentrations of HGF and the initial time to DNA synthesis was measured. Media supplementation with HGF did not shorten (P > 0.05) the duration of G0/G1 transition suggesting the growth factor does not affect activation. Treatment with 25 ng/mL HGF increased (P < 0.05) eqSC proliferation that was coincident with phosphorylation of extracellular signal-regulated kinase (ERK)1/2 and AKT serine/threonine kinase 1 (AKT1). Chemical inhibition of the upstream effectors of ERK1/2 or AKT1 elicited no effect (P > 0.05) on HGF-mediated 5-ethynyl-2'-deoxyuridine (EdU) incorporation. By contrast, treatment of eqSC with 2 µm Gö6983, a pan-protein kinase C (PKC) inhibitor, blocked (P < 0.05) HGF-initiated mitotic activity. Gene-expression analysis revealed that eqSC express PKCα, PKCδ, and PKCε isoforms. Knockdown of PKCδ with a small interfering RNA (siRNA) prevented (P > 0.05) HGF-mediated EdU incorporation. The siPKCδ was specific to the kinase and did not affect (P > 0.05) expression of either PKCα or PKCε. Treatment of confluent eqSC with 25 ng/mL HGF suppressed (P < 0.05) nuclear myogenin expression during the early stages of differentiation. These results demonstrate that HGF may not affect activation but can act as a mitogen and modest suppressor of differentiation.

IL-6 is associated to IGF-1Ec upregulation and Ec peptide secretion, from prostate tumors

Background

Ec peptide (PEc), resulting from the proteolytic cleavage of the IGF-1Ec isoform, is involved in prostate cancer progression and metastasis, whereas in muscle tissue, it is associated with the mobilization of satellite cells prior to repair. Our aim is to determine the physiological conditions associated to the IGF-1Ec upregulation and PEc secretion in prostate tumors, as well as, the effect of tumor PEc on tumor repair.

Methods

IGF-1 (mature and isoforms) expression was examined by qRT-PCR, both in prostate cancer cells co-incubated with cells of the immune response (IR) and in tumors. PEc secretion was determined by Multiple Reaction Monitoring.

The effect of PEc, on mesenchymal stem cell (MSC) mobilization and repair, was examined using migration and invasion assays, FISH and immunohistochemistry (IHC). The JAK/STAT signaling pathway leading to the IGF1-Ec expression was examined by western blot analysis. Determination of the expression and localization of IL-6 and IGF-1Ec in prostate tumors was examined by qRT-PCR and by IHC.

Results

We documented that IL-6 secreted by IR cells activates the JAK2 and STAT3 pathway through IL-6 receptor in cancer cells, leading to the IGF-1Ec upregulation and PEc secretion, as well as to the IL-6 expression and secretion. The resulting PEc, apart from its oncogenic role, also mobilizes MSCs towards the tumor, thus promoting tumor repair.

Conclusions

IL-6 leads to the PEc secretion from prostate cancer cells. Apart from its oncogenic role, PEc is also involved in the mobilization of MSCs resulting in tumor repair.

Electronic supplementary material

The online version of this article (10.1186/s10020-018-0003-z) contains supplementary material, which is available to authorized users.

The Role of IGF-1 Signaling in Skeletal Muscle Atrophy

Insulin-like growth factor 1 (IGF-1) is a key anabolic growth factor stimulating phosphatidylinositol 3-kinase (PI3K)/Akt signaling which is well known for regulating muscle hypertrophy. However, the role of IGF-1 in muscle atrophy is less clear. This review provides an overview of the mechanisms via which IGF-1 signaling is implicated in several conditions of muscle atrophy and via which mechanisms protein turnover is altered. IGF-1/PI3K/Akt signaling stimulates the rate of protein synthesis via p70S6Kinase and p90 ribosomal S6 kinase and negatively regulates protein degradation, predominantly by its inhibiting effect on proteasomal and lysosomal protein degradation. Caspase-dependent protein degradation is also attenuated by IGF/PI3K/Akt signaling, whereas evidence for an effect on calpain-dependent protein degradation is inconclusive. IGF-1/PI3K/Akt signaling reduces during denervation-, unloading-, and joint immobilization-induced muscle atrophy, whereas IGF-1/PI3K/Akt signaling seems unaltered during aging-associated muscle atrophy. During denervation and aging, IGF-1 overexpression or injection counteracts denervation- and aging-associated muscle atrophy, despite enhanced anabolic resistance with regard to IGF-1 signaling with aging. It remains unclear whether pharmacological stimulation of IGF-1/PI3K/Akt signaling attenuates immobilization- or unloading-induced muscle atrophy. Exploration of the possibilities to interfere with IGF-1/PI3K/Akt signaling reveals that microRNAs targeting IGF-1 signaling components are promising targets to counterbalance muscle atrophy. Overall, the findings summarized in this review show that in disuse conditions, but not with aging, IGF-1/PI3K/Akt signaling is attenuated and that in some conditions stimulation of this pathway may alleviate skeletal muscle atrophy.

Specific titin and myomesin domains stimulate myoblast proliferation

Myofibrillar proteins titin and myomesin stimulated myoblast proliferation as determined by MTT-test and labelled thymidine incorporation in the DNA. Specific Fn type III and Ig-like domains of these proteins were able to exert mitogenic effects as well. Proliferative effect of Fn type III domains was highly sensitive to inhibition of Ca²⁺/calmodulin dependent protein kinase, whereas the effect of Ig-like domains showed greater sensitivity to the inhibition of adenylyl cyclase – cAMP – PKA pathway. IGF-1 autocrine signalling inhibition partially suppressed mitogenic effects revealed by both domain types.

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Titin and myomesin as well as their domains stimulate myoblast proliferation.

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Ig-like and Fn type III domains activate proliferation via different pathways.

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IGF-1 autocrine signalling is partially involved in observed proliferative effects.

MUSCLE STRENGTH, GH AND IGF-1 IN OLDER WOMEN SUBMITTED TO LAND AND AQUATIC RESISTANCE TRAINING

ABSTRACT Introduction: Aging leads to decline in multiple organ systems associated to hormone alterations and loss of muscle mass and strength. Objective: To evaluate the effects of land and water resistance training on muscle strength, basal serum levels of GH, IGF-1, and IGFBP3 in elderly women. Methods: Subjects were distributed in three groups: land resistance training (LRT; n=10; age: 66.10±2.77 years), water resistance training (WRT; n=10; age: 67.10±3.54 years) and control (CG; n=10; age: 68.80±5.41 years). Muscle strength was assessed with the 1-RM test in bench press (BP) and leg press (LP) exercises. Serum levels of GH, IGF-1, and IGFBP3 were analyzed by chemiluminescence. Results: Repeated measures ANOVA showed elevated muscle strength in the BP only in LRT (D%=60.2%; p=0.0001) when compared to the CG (D%=46.73%, p=0.0001) and WRT (D%=32.49%, p=0.004). LRT and WRT increased muscle strength in LP (D%LRT=57.14%; p=0.004; D%WRT=42.3%; p=0.033). There was an increase in LRT and WRT in the LP when compared to CG (D%=45.59%, p=0.019; D%=43.97%, p=0.026, respectively). IGF-1 was elevated only in LRT (D%=49.72%, p=0.004) from pre- to post-test and when compared to WRT (D%=56.76%, p=0.002) and CG (D%=74.63%, p=0.0001). IGFBP3 levels showed that only WRT increased (D%=26.79%, p=0.044) from pre- to post-test. The IGF-1/IGFBP3 ratio demonstrated that LRT reached intragroup elevation (D%=52.66%, p=0.002) and when compared to WRT (D%=94.11%, p=0.0001) and CG (D%=73.45%, p=0.0001). There was no significant difference in GH. Conclusion: Land resistance training may provide better anabolic effects in elderly women.

Age-related effect of cell death on fiber morphology and number in tongue muscle

INTRODUCTION

Multiple pathways may exist for age-related tongue muscle degeneration. Cell death is one mechanism contributing to muscle atrophy and decreased function. We hypothesized with aging, apoptosis, and apoptotic regulators would be increased, and muscle fiber size and number would be reduced in extrinsic tongue muscles.

METHODS

Cell death indices, expression of caspase-3 and Bcl-2, and measures of muscle morphology and number were determined in extrinsic tongue muscles of young and old rats.

RESULTS

Significant increases in cell death, caspase-3, and Bcl-2 were observed in all extrinsic tongue muscles along with reductions in muscle fiber number in old rats.

DISCUSSION

We demonstrated that apoptosis indices increase with age in lingual muscles and that alterations in apoptotic regulators may be associated with age-related degeneration in muscle fiber size and number. These observed apoptotic processes may be detrimental to muscle function, and may contribute to degradation of cranial functions with age. Muscle Nerve 57: E29-E37, 2018.

Intramuscular injection of mechano growth factor E domain peptide regulated expression of memory-related sod, miR-134 and miR-125b-3p in rat hippocampus under simulated weightlessness

OBJECTIVE

To investigate the expression of memory-related antioxidant genes and miRNAs under simulated weightlessness and the regulation of mechano growth factor (MGF) E domain, the peptide preventing nerve damage.

RESULTS

Igf-iea and mgf mRNA levels, expression of antioxidant genes sod1 and sod2 and levels of miR-134 and miR-125b-3p increased in rat hippocampus after 14 days tail suspension to simulate weightlessness which was inhibited with intramuscular injection of E domain peptide. Therefore, administration of MGF E domain peptide could reverse increased expressions of memory-related igf-iea, mgf, sod1, sod2, miR-134 and miR-125b-3p in rat hippocampus under simulated weightlessness.

CONCLUSIONS

MGF may regulate the redox state and miRNA-targeted NR-CREB signaling, and intramuscular injection may be the alternative administration because of its safety, convenience and ability to pass through the blood brain barrier.

Does skeletal muscle have an 'epi'-memory? The role of epigenetics in nutritional programming, metabolic disease, aging and exercise

Skeletal muscle mass, quality and adaptability are fundamental in promoting muscle performance, maintaining metabolic function and supporting longevity and healthspan. Skeletal muscle is programmable and can ‘remember’ early‐life metabolic stimuli affecting its function in adult life. In this review, the authors pose the question as to whether skeletal muscle has an ‘epi’‐memory? Following an initial encounter with an environmental stimulus, we discuss the underlying molecular and epigenetic mechanisms enabling skeletal muscle to adapt, should it re‐encounter the stimulus in later life. We also define skeletal muscle memory and outline the scientific literature contributing to this field. Furthermore, we review the evidence for early‐life nutrient stress and low birth weight in animals and human cohort studies, respectively, and discuss the underlying molecular mechanisms culminating in skeletal muscle dysfunction, metabolic disease and loss of skeletal muscle mass across the lifespan. We also summarize and discuss studies that isolate muscle stem cells from different environmental niches in vivo (physically active, diabetic, cachectic, aged) and how they reportedly remember this environment once isolated in vitro. Finally, we will outline the molecular and epigenetic mechanisms underlying skeletal muscle memory and review the epigenetic regulation of exercise‐induced skeletal muscle adaptation, highlighting exercise interventions as suitable models to investigate skeletal muscle memory in humans. We believe that understanding the ‘epi’‐memory of skeletal muscle will enable the next generation of targeted therapies to promote muscle growth and reduce muscle loss to enable healthy aging.

Adapted physical exercise enhances activation and differentiation potential of satellite cells in the skeletal muscle of old mice

During ageing, a progressive loss of skeletal muscle mass and a decrease in muscle strength and endurance take place, in the condition termed sarcopenia. The mechanisms of sarcopenia are complex and still unclear; however, it is known that muscle atrophy is associated with a decline in the number and/or efficiency of satellite cells, the main contributors to muscle regeneration. Physical exercise proved beneficial in sarcopenia; however, knowledge of the effect of adapted physical exercise on the myogenic properties of satellite cells in aged muscles is limited. In this study the amount and activation state of satellite cells as well as their proliferation and differentiation potential were assessed in situ by morphology, morphometry and immunocytochemistry at light and transmission electron microscopy on 28-month-old mice submitted to adapted aerobic physical exercise on a treadmill. Sedentary age-matched mice served as controls, and sedentary adult mice were used as a reference for an unperturbed control at an age when the capability of muscle regeneration is still high. The effect of physical exercise in aged muscles was further analysed by comparing the myogenic potential of satellite cells isolated from old running and old sedentary mice using an in vitro system that allows observation of the differentiation process under controlled experimental conditions. The results of this ex vivo and in vitro study demonstrated that adapted physical exercise increases the number and activation of satellite cells as well as their capability to differentiate into structurally and functionally correct myotubes (even though the age-related impairment in myotube formation is not fully reversed): this evidence further supports adapted physical exercise as a powerful, non-pharmacological approach to counteract sarcopenia and the age-related deterioration of satellite cell capabilities even at very advanced age.

Satellite cell response to erythropoietin treatment and endurance training in healthy young men

KEY POINT

Erythropoietin (Epo) treatment may induce myogenic differentiation factor (MyoD) expression and prevent apoptosis in satellite cells (SCs) in murine and in vitro models. Endurance training stimulates SC proliferation in vivo in murine and human skeletal muscle. In the present study, we show, in human skeletal muscle, that treatment with an Epo-stimulating agent (darbepoetin-α) in vivo increases the content of MyoD(+) SCs in healthy young men. Moreover, we report that Epo receptor mRNA is expressed in adult human SCs, suggesting that Epo may directly target SCs through ligand-receptor interaction. Moreover, endurance training, but not Epo treatment, increases the SC content in type II myofibres, as well as the content of MyoD(+) SCs. Collectively, our results suggest that Epo treatment can regulate human SCs in vivo, supported by Epo receptor mRNA expression in human SCs. In effect, long-term Epo treatment during disease conditions involving anaemia may impact SCs and warrants further investigation. Satellite cell (SC) proliferation is observed following erythropoitin treatment in vitro in murine myoblasts and endurance training in vivo in human skeletal muscle. The present study aimed to investigate the effects of prolonged erythropoiesis-stimulating agent (ESA; darbepoetin-α) treatment and endurance training, separately and combined, on SC quantity and commitment in human skeletal muscle. Thirty-five healthy, untrained men were randomized into four groups: sedentary-placebo (SP, n = 9), sedentary-ESA (SE, n = 9), training-placebo (TP, n = 9) or training-ESA (TE, n = 8). ESA/placebo was injected once weekly and training consisted of ergometer cycling three times a week for 10 weeks. Prior to and following the intervention period, blood samples and muscle biopsies were obtained and maximal oxygen uptake (V̇O2, max) was measured. Immunohistochemical analyses were used to quantify fibre type specific SCs (Pax7(+)), myonuclei and active SCs (Pax7(+)/MyoD(+)). ESA treatment led to elevated haematocrit, whereas endurance training increased V̇O2, max. Endurance training led to an increase in SCs associated with type II fibres (P < 0.05), whereas type I fibres showed no changes. Both ESA treatment and endurance training increased Pax7(+)/MyoD(+) cells, whereas only ESA treatment increased the total content of MyoD(+) cells. Epo-R mRNA presence in adult SC was tested with real-time RT-PCR using fluorescence-activated cell sorting (CD56(+)/CD45(-)/CD31(-)) to isolate cells from a human rectus abdominis muscle and was found to be considerably higher than in whole muscle. In conclusion, endurance training and ESA treatment may separately stimulate SC commitment to the myogenic program. Furthermore, ESA-treatment may alter SC activity by direct interaction with the Epo-R expressed on SCs.

MicroRNA-29 induces cellular senescence in aging muscle through multiple signaling pathways

The mechanisms underlying the development of aging-induced muscle atrophy are unclear. By microRNA array and individual qPCR analyses, we found significant up-regulation of miR-29 in muscles of aged rodents vs. results in young. With aging, p85α, IGF-1 and B-myb muscle levels were lower while the expression of certain cell arrest proteins (p53, p16 and pRB) increased. When miR-29 was expressed in muscle progenitor cells (MPC), their proliferation was impaired while SA-βgal expression increased signifying the development of senescence. Impaired MPC proliferation resulted from interactions between miR-29 and the 3'-UTR of p85a, IGF-1 and B-myb, suppressing the translation of these mediators of myoblast proliferation. In vivo, electroporation of miR-29 into muscles of young mice suppressed the proliferation and increased levels of cellular arrest proteins, recapitulating aging-induced responses in muscle. A potential stimulus of miR-29 expression is Wnt-3a since we found that exogenous Wnt-3a stimulated miR-29 expression 2.7-fold in primary cultures of MPCs. Thus, aging-induced muscle senescence results from activation of miR-29 by Wnt-3a leading to suppressed expression of several signaling proteins (p85α, IGF-1 and B-myb) that act coordinately to impair the proliferation of MPCs contributing to muscle atrophy. The increase in miR-29 provides a potential mechanism for aging-induced sarcopenia.

Induction of insulin-like growth factor 1 splice forms by subfragments of myofibrillar proteins

Expression of insulin-like growth factor 1 (IGF-1) mRNAs splice forms was recently shown to be stimulated by myofibrillar proteins released from the damaged muscle. In this study, we report that individual subfragments of titin and myomesin composed of Fn type III and Ig-like domains can activate expression of two IGF-1 splice forms in cultured myoblasts, both at protein and mRNA levels. Competition studies showed that each of the domain-types interacts with its own receptor. Induction of IGF-1 expression caused by domains of different types showed dissimilar sensitivity to inhibitors of regulatory cascades. The effect of Fn type III domains was more sensitive to inhibition of Ca(2+)/calmodulin dependent protein kinase, whereas the effect of Ig-like domains showed greater sensitivity to the inhibition of the adenylyl cyclase-cAMP-PKA pathway.

Sustained delivery of MGF peptide from microrods attracts stem cells and reduces apoptosis of myocytes

Local release of drugs may have many advantages for tissue repair but also presents major challenges. Bioengineering approaches allow microstructures to be fabricated that contain bioactive peptides for sustained local delivery. Heart tissue damage is associated with local increases in mechano growth factor (MGF), a member of the IGF-1 family. The E domain of MGF peptide is anti-apoptotic and a stem cell homing factor. The objectives of this study were to fabricate a microrod delivery device of poly (ethylene glycol) dimethacrylate (PEGDMA) hydrogel loaded with MGF peptide and to determine the elution profile and bioactivity of MGF. The injectable microrods are 30 kPa stiffness and 15 μm widths by 100 μm lengths, chosen to match heart stiffness and myocyte size. Successful encapsulation of native MGF peptide within microrods was achieved with delivery of MGF for 2 weeks, as measured by HPLC. Migration of human mesenchymal stem cells (hMSCs) increased with MGF microrod treatment (1.72 ± 0.23, p < 0.05). Inhibition of the apoptotic pathway in neonatal rat ventricular myocytes was induced by 8 h of hypoxia (1 % O2). Protection from apoptosis by MGF microrod treatment was shown by the TUNEL assay and increased Bcl-2 expression (2 ± 0.19, p < 0.05). Microrods without MGF regulated the cytoskeleton, adhesion, and proliferation of hMSCs, and MGF had no effect on these properties. Therefore, the combination microdevice provided both the mechanical cues and 2-week MGF bioactivity to reduce apoptosis and recruit stem cells, suggesting potential use of MGF microrods for cardiac regeneration therapy in vivo.

Pretreatment with mechano-growth factor E peptide protects bone marrow mesenchymal cells against damage by fluid shear stress

Improper fluid shear stress (FSS) can cause serious damages to bone marrow mesenchymal stem cells (MSCs). Mechano-growth factor (MGF) E peptide pretreatment was proposed to protect MSCs against FSS damage in this study. MSCs were exposed to FSS for 30 min after they were pretreated with MGF E peptide for 24 h. Then, the effects of MGF E peptide on the viability, proliferation and cell apoptosis of MSCs were investigated. MGF E peptide pretreatment could recover the cellular metabolic activity of MSCs reduced by 72 dyne cm(-2) FSS and had a synergistic effect with FSS on the cellular metabolic viability of MSCs under 24 and 72 dyne cm(-2) FSS. These results suggested that MGF E peptide pretreatment could be an effective method for the protection of FSS damage in bone tissue engineering.

Mechano growth factor E peptide regulates migration and differentiation of bone marrow mesenchymal stem cells

IGF1Ec in humans or IGF1Eb in rodents (known as mechano growth factor (MGF)) has a unique E domain, and the C-terminal end of the E domain (MGF E peptide) plays important roles in proliferation, migration and differentiation of many cell types. Bone marrow mesenchymal stem cells (BMSCs) have multiple differentiation potentials and are considered as perfect seed cells for tissue repair. But the role of MGF E peptide on BMSCs is seldom investigated and the mechanism is still unclear. In this study, we investigated the effects of MGF E peptide on rat BMSCs (rBMSCs). Our results revealed that treatment with MGF E peptide had no effect on BMSC proliferation. However, both wound-healing and transwell assays indicated that MGF E peptide could significantly enhance rBMSCs migration ability. Further analysis indicated that MGF E peptide also reduced the expression levels of osteogenic genes, but increased the expression levels of adipogenic genes. Analysis of molecular mechanism showed that phosphorylation-Erk1/2 was activated by MGF E peptide and blockage of either Erk1/2 or IGF1 receptor could repress the migration effect of MGF E peptide. In conclusion, MGF E peptide is able to inhibit osteogenic differentiation but promote adipogenic differentiation. In addition, the migration effect of MGF E peptide on rBMSCs depends on IGF1 receptor via Erk1/2 signal pathway.

Biological activity of the e domain of the IGF-1Ec as addressed by synthetic peptides

Insulin-like growth factor-1 (IGF-1) is a multipotent growth factor involved in the growth, development and regulation of homeostasis in a tissue-specific manner. Alternative splicing, multiple transcription initiation sites and different polyadelynation signals give rise to diverse mRNA isoforms, such as IGF-1Ea, IGF-1Eb and IGF-1Ec transcripts. There is increasing interest in the expression of the IGF-1 isoforms and their potential distinct biological role. IGF-1Ec results from alternative splicing of exons 4-5-6 and its expression is upregulated in various conditions and pathologies. Recent studies have shown that IGF-1Ec is preferentially increased after injury in skeletal muscle during post-infarctal myocardium remodelling and in cancer tissues and cell lines. A synthetic analogue corresponding to the last 24 aa of the E domain of the IGF-1Ec isoform has been used to elucidate its potential biological role. The aim of the present review is to describe and discuss the putative bioactivity of the E domain of the IGF-1Ec isoform.

Mechano-growth factor peptide, the COOH terminus of unprocessed insulin-like growth factor 1, has no apparent effect on myoblasts or primary muscle stem cells

A splice form of IGF-1, IGF-1Eb, is upregulated after exercise or injury. Physiological responses have been ascribed to the 24-amino acid COOH-terminal peptide that is cleaved from the NH3-terminal 70-amino acid mature IGF-1 protein. This COOH-terminal peptide was termed "mechano-growth factor" (MGF). Activities claimed for the MGF peptide included enhancing muscle satellite cell proliferation and delaying myoblast fusion. As such, MGF could represent a promising strategy to improve muscle regeneration. Thus, at our two pharmaceutical companies, we attempted to reproduce the claimed effect of MGF peptides on human and mouse muscle myoblast proliferation and differentiation in vitro. Concentrations of peptide up to 500 ng/ml failed to increase the proliferation of C2C12 cells or primary human skeletal muscle myoblasts. In contrast, all cell types exhibited a proliferative response to mature IGF-1 or full-length IGF-1Eb. MGF also failed to inhibit the differentiation of myoblasts into myotubes. To address whether the response to MGF was lost in these tissue culture lines, we measured proliferation and differentiation of primary mouse skeletal muscle stem cells exposed to MGF. This, too, failed to demonstrate a significant effect. Finally, we tested whether MGF could alter a separate documented in vitro effect of the peptide, activation of p-ERK, but not p-Akt, in cardiac myocytes. Although a robust response to IGF-1 was observed, there were no demonstrated activating responses from the native or a stabilized MGF peptide. These results call in to question whether there is a physiological role for MGF.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

Posttranscriptional regulation of insulin family ligands and receptors

Insulin system including ligands (insulin and IGFs) and their shared receptors (IR and IGFR) are critical regulators of insulin signaling and glucose homeostasis. Altered insulin system is associated with major pathological conditions like diabetes and cancer. The mRNAs encoding for these ligands and their receptors are posttranscriptionally controlled by three major groups of regulators; (i) alternative splicing regulatory factors; (ii) turnover and translation regulator RNA-binding proteins (TTR-RBPs); and (iii) non-coding RNAs including miRNAs and long non-coding RNAs (lncRNAs). In this review, we discuss the influence of these regulators on alternative splicing, mRNA stability and translation. Due to the pathological impacts of insulin system, we also discussed the possibilities of discovering new potential regulators which will improve understanding of insulin system and associated diseases.

The therapeutic potential of IGF-I in skeletal muscle repair

Skeletal muscle loss due to aging, motor-neuron degeneration, cancer, heart failure, and ischemia is a serious condition for which currently there is no effective treatment. Insulin-like growth factor 1 (IGF-I) plays an important role in muscle maintenance and repair. Preclinical studies have shown that IGF-I is involved in increasing muscle mass and strength, reducing degeneration, inhibiting the prolonged and excessive inflammatory process due to toxin injury, and increasing the proliferation potential of satellite cells. However, clinical trials have not been successful due to ineffective delivery methods. Choosing the appropriate isoforms or peptides and developing targeted delivery techniques can resolve this issue. Here we discuss the latest development in the field with special emphasis on novel therapeutic approaches.

Human Eb peptide: not just a by-product of pre-pro-IGF1b processing?

Several physiological activities have been assigned to E-peptides derived from pre-pro-insulin-like growth factor (IGF1) processing; however, the whole range of the E-peptides' functions is still unknown. The objective of this study was to investigate human Eb peptide (hEb) in terms of its bioactivity, cellular localization, and intracellular trafficking using human cancer cells. Human Eb fused with red fluorescence protein (RFP) or green fluorescence protein (GFP) localizes strongly to nucleoli and to a lesser extent to nuclei of HeLa and U2-OS cells. Mutagenesis of hEb nucleolus localization sequence (NoLS) leads to its partial delocalization from nuclei and nucleoli to cytoplasm of transfected cells. Thus, NoLS is not sufficient for the hEb to be localized in nucleoli of the cells and a different mechanism may be involved in hEb targeting. A BrdU ELISA showed that the proliferation index of cells expressing hEb hybrid proteins increased up to 28%. For comparison, the same assay was performed using HeLa cells treated extracellularly with synthetic hEb. A significant increase in the proliferation index was observed (41-58% for concentrations ranging from 10-100 nM, respectively). Additionally, a cell migration assay was performed using stable U2-OS cell lines expressing hEb fused with RFP or RFP alone as a negative control. The migration index of hEb expressing cells was 38.3% greater. The increase in cell proliferation index and in motile properties of hEb expressing cells demonstrate that hEb is more than a pre-pro-IGF1b processing product, and has intrinsic activity of biological significance.

New Modulators for IGF-I Activity within IGF-I Processing Products

Insulin-like growth factor I (IGF-I) is a key regulator of muscle development and growth. The pre-pro-peptide produced by the Igf1 gene undergoes several post-translational processing steps to result in a secreted mature protein, which is thought to be the obligate ligand for the IGF-I receptor (IGF-IR). However, the significance of the additional forms and peptides produced from Igf1 is not clear. For instance, the C-terminal extensions called the E-peptides that are part of pro-IGF-I, have been implicated in playing roles in cell growth, including cell proliferation and migration and muscle hypertrophy in an IGF-IR independent manner. However, the activity of these peptides has been controversial. IGF-IR independent actions suggest the existence of an E-peptide receptor, yet such a protein has not been discovered. We propose a new concept: there is no E-peptide receptor, rather the E-peptides coordinate with IGF-I to modulate activity of the IGF-IR. Growing evidence reveals that the presence of an E-peptide alters IGF-I activity, whether as part of pro-IGF-I, or as a separate peptide. In this review, we will examine the past literature on IGF-I processing and E-peptide actions in skeletal muscle, address the previous attempts to separate IGF-I and E-peptide effects, propose a new model for IGF-I/E-peptide synergy, and suggest future experiments to test if the E-peptides truly modulate IGF-I activity.

E-peptides control bioavailability of IGF-1

Insulin-like growth factor 1 (IGF-1) is a potent cytoprotective growth factor that has attracted considerable attention as a promising therapeutic agent. Transgenic over-expression of IGF-1 propeptides facilitates protection and repair in a broad range of tissues, although transgenic mice over-expressing IGF-1 propeptides display little or no increase in IGF-1 serum levels, even with high levels of transgene expression. IGF-1 propeptides are encoded by multiple alternatively spliced transcripts including C-terminal extension (E) peptides, which are highly positively charged. In the present study, we use decellularized mouse tissue to show that the E-peptides facilitate in vitro binding of murine IGF-1 to the extracellular matrix (ECM) with varying affinities. This property is independent of IGF-1, since proteins consisting of the E-peptides fused to relaxin, a related member of the insulin superfamily, bound equally avidly to decellularized ECM. Thus, the E-peptides control IGF-1 bioavailability by preventing systemic circulation, offering a potentially powerful way to tether IGF-1 and other therapeutic proteins to the site of synthesis and/or administration.