Mechano Growth Factor

Improved Muscle Regeneration into a Joint Prosthesis with Mechano-Growth Factor Loaded within Mesoporous Silica Combined with Carbon Nanotubes on a Porous Titanium Alloy.

Xiang Wei, Qin Chen, Lingtong Bu +6 more

Total joint replacement (TJR) is widely applied as a promising treatment for the reconstruction of serious joint diseases but is usually characterized by critical loss of skeletal muscle attachment to metal joint prostheses, resulting in fibrous scar tissue formation and subsequent motor dysfunction. Tissue engineering technology may provide a potential strategy for skeletal muscle regeneration into metal joint prostheses. Here, a porous titanium (Ti) alloy scaffold coated with carbon nanotubes (CNTs) and mesoporous silica nanoparticles (MSNs) through electrophoretic deposition (EPD) was designed as a mechano-growth factor (MGF) carrier. This two-layered coating exhibits a nanostructured topology, excellent MGF loading, and prolonged release performance via covalent bonding to improve myoblast adhesion, proliferation and myogenic differentiation in porous Ti alloy scaffolds without cytotoxicity. The Akt/mTOR signaling pathway plays a key role in this process. Furthermore, in vivo studies show that the scaffold promotes the growth of muscle, rather than fibrotic tissue, into the porous Ti alloy structure and improves muscle-derived mechanical properties, the migration of satellite cells, and possibly immunomodulation. In summary, this nanomaterial-coated scaffold provides a practical biomaterial platform to regenerate periprosthetic muscle tissue and restore comparable motor function to that of the natural joint.

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

Fusako Usuki, Masatake Fujimura, Atsushi Nakamura +3 more

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 YAP1Ser127 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.

The effect of strength training volume on satellite cells, myogenic regulatory factors, and growth factors.

K E Hanssen, N H Kvamme, T S Nilsen +7 more

The aim of this work was to study the effect of training volume on activation of satellite cells. Healthy untrained men were randomly assigned into two groups. The 3L-1UB group (n = 10) performed three-set leg exercises and single-set upper body exercises, and the 1L-3UB group (n = 11) performed single-set leg exercises and three-set upper body exercises. Both groups performed three sessions (80-90 min) per week for 11 weeks. Biopsies were taken from m. vastus lateralis and m. trapezius. The number of satellite cells, satellite cells positive for myogenin and MyoD, and the number of myonuclei were counted. Homogenized muscle was analyzed for myogenin and MyoD, and extracted ribonucleic acid (RNA) was monitored for selected growth factor transcripts. Knee extensor strength increased more in the 3L-1UB group than in the 1L-3UB group (48 ± 4% vs 29 ± 4%), whereas the strength gain in shoulder press was similar in both training groups. The number of satellite cells in m. vastus lateralis increased more in the 3L-1UB group than in the 1L-3UB group. The number of myonuclei increased similarly in both groups. The messenger RNA expression of growth factors peaked after 2 weeks of training. In conclusion, increasing training volume enhanced satellite cell numbers in the leg muscle, but not in the upper body muscle.

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.

Mara Fornaro, Aaron C Hinken, Saul Needle +12 more

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.

Ageing is associated with diminished muscle re-growth and myogenic precursor cell expansion early after immobility-induced atrophy in human skeletal muscle.

C Suetta, U Frandsen, A L Mackey +9 more

Recovery of skeletal muscle mass from immobilisation-induced atrophy is faster in young than older individuals, yet the cellular mechanisms remain unknown. We examined the cellular and molecular regulation of muscle recovery in young and older human subjects subsequent to 2 weeks of immobility-induced muscle atrophy. Retraining consisted of 4 weeks of supervised resistive exercise in 9 older (OM: mean age) 67.3, range 61-74 yrs) and 11 young (YM: mean age 24.4, range 21-30 yrs) males. Measures of myofibre area (MFA), Pax7-positive satellite cells (SCs) associated with type I and type II muscle fibres, as well as gene expression analysis of key growth and transcription factors associated with local skeletal muscle milieu, were performed after 2 weeks immobility (Imm) and following 3 days (+3d) and 4 weeks (+4wks) of retraining. OM demonstrated no detectable gains in MFA (vastus lateralis muscle) and no increases in number of Pax7-positive SCs following 4wks retraining, whereas YM increased their MFA (P < 0.05), number of Pax7-positive cells, and had more Pax7-positive cells per type II fibre than OM at +3d and +4wks (P < 0.05). No age-related differences were observed in mRNA expression of IGF-1Ea, MGF, MyoD1 and HGF with retraining, whereas myostatin expression levels were more down-regulated in YM compared to OM at +3d (P < 0.05). In conclusion, the diminished muscle re-growth after immobilisation in elderly humans was associated with a lesser response in satellite cell proliferation in combination with an age-specific regulation of myostatin. In contrast, expression of local growth factors did not seem to explain the age-related difference in muscle mass recovery.

Skeletal muscle hypertrophy and regeneration: interplay between the myogenic regulatory factors (MRFs) and insulin-like growth factors (IGFs) pathways.

Nadège Zanou, Philippe Gailly

Adult skeletal muscle can regenerate in response to muscle damage. This ability is conferred by the presence of myogenic stem cells called satellite cells. In response to stimuli such as injury or exercise, these cells become activated and express myogenic regulatory factors (MRFs), i.e., transcription factors of the myogenic lineage including Myf5, MyoD, myogenin, and Mrf4 to proliferate and differentiate into myofibers. The MRF family of proteins controls the transcription of important muscle-specific proteins such as myosin heavy chain and muscle creatine kinase. Different growth factors are secreted during muscle repair among which insulin-like growth factors (IGFs) are the only ones that promote both muscle cell proliferation and differentiation and that play a key role in muscle regeneration and hypertrophy. Different isoforms of IGFs are expressed during muscle repair: IGF-IEa, IGF-IEb, or IGF-IEc (also known as mechano growth factor, MGF) and IGF-II. MGF is expressed first and is observed in satellite cells and in proliferating myoblasts whereas IGF-Ia and IGF-II expression occurs at the state of muscle fiber formation. Interestingly, several studies report the induction of MRFs in response to IGFs stimulation. Inversely, IGFs expression may also be regulated by MRFs. Various mechanisms are proposed to support these interactions. In this review, we describe the general process of muscle hypertrophy and regeneration and decipher the interactions between the two groups of factors involved in the process.

Mechano growth factor (MGF) promotes proliferation and inhibits differentiation of porcine satellite cells (PSCs) by down-regulation of key myogenic transcriptional factors.

Li-Li Qin, Xiao-Kui Li, Jian Xu +8 more

Porcine satellite cells represent an ideal model system for studying the cellular and molecular basis regulating myogenic stem cell proliferation and differentiation and for exploring the experimental conditions for myoblast transplantation. Here, we investigated the effects of mechano growth factor (MGF), a spliced variant of the IGF-1 gene, on porcine satellite cells. We show that MGF potently stimulated proliferation while inhibited differentiation of porcine satellite cells. MGF-treatment acutely down-regulates the expression of myogenic determination factor (MyoD) and the cyclin-dependent kinase inhibitor p21. MGF-treatment also markedly reduced the overall expression of cyclin B1 and key factors of the myogenic regulatory and myocyte enhancer families, including Myogenein and MEF2A. Taken together, the gene expression data from MGF-treated porcine satellite cells are in favor of a molecular model in which MGF inhibits porcine satellite cell differentiation by down-regulating either the activity or expression of MyoD, which, in turn, suppresses the expression of key genes required for cell cycle progression and differentiation, such as p21, Myogenin, and MEF2. Overall, our findings are in support of the previous suggestion that MGF may be used in vivo and in vitro to promote proliferation of myogenic stem cells to prevent and treat age-related muscle degenerative diseases.

[Expression of mechano-growth factor and its roles in tissue repairs and regeneration].

Bingyu Zhang, Guanbin Song, Qing Luo +1 more

To review the expression of mechano-growth factor (MGF) and its roles in tissue repairs and regeneration.

Age-related loss of muscle mass and strength.

Geoffrey Goldspink

Age-related muscle wasting and increased frailty are major socioeconomic as well as medical problems. In the quest to extend quality of life it is important to increase the strength of elderly people sufficiently so they can carry out everyday tasks and to prevent them falling and breaking bones that are brittle due to osteoporosis. Muscles generate the mechanical strain that contributes to the maintenance of other musculoskeletal tissues, and a vicious circle is established as muscle loss results in bone loss and weakening of tendons. Molecular and proteomic approaches now provide strategies for preventing age-related muscle wasting. Here, attention is paid to the role of the GH/IGF-1 axis and the special role of the IGFI-Ec (mechano growth factor/MGF) which is derived from the IGF-I gene by alternative splicing. During aging MGF levels decline but when administered MGF activates the muscle satellite (stem) cells that "kick start" local muscle repair and induces hypertrophy.

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.

Prashanth Kumar Kandalla, Geoffrey Goldspink, Gillian Butler-Browne +1 more

Loss of muscle mass and strength is a major problem during aging and the expression of Mechano Growth Factor (MGF), a member of the IGF-1 (insulin-like Growth Factor 1) super family, has been shown to be both exercise and age dependent. MGF, also called IGF-1Ec, has a unique E domain with a 49bp insert in humans (52bp in rodents; IGF-1Eb), which results in a reading frame shift during the IGF-1 gene splicing to produce a distinct mature isoform. We have studied the effects of the MGF-24aa-E peptide on proliferation and differentiation of primary human muscle cell cultures isolated from healthy subjects of different ages. We found that MGF-E peptide significantly increases the proliferative life span and delays senescence of satellite cells isolated from neonatal and young adult but not from old adult muscle, hypertrophy associated with a significant decrease in the percentage of reserve cells was observed in all cultures. It is concluded that the MGF-24aa-E peptide alone has a marked ability to enhance satellite cell activation, proliferation and fusion for muscle repair and maintenance and could provide a new strategy to combat age related sarcopenia without the oncogenic side effects observed for IGF1.

IGF-IEc expression, regulation and biological function in different tissues.

Zhongquan Dai, Feng Wu, Ella W Yeung +1 more

Insulin-like growth factor I (IGF-I) is an important growth factor for embryonic development, postnatal growth, tissue repair and maintenance of homeostasis. IGF-I functions and regulations are complex and tissue-specific. IGF-I mediates growth hormone signaling to target tissues during growth, but many IGF-I variants have been discovered, resulting in complex models to describe IGF-I function and regulation. Mechano-growth factor (MGF) is an alternative splicing variant of IGF-I and serves as a local tissue repair factor that responds to changes in physiological conditions or environmental stimuli. MGF expression is significantly increased in muscle, bone and tendon following damage resulting from mechanical stimuli and in the brain and heart following ischemia. MGF has been shown to activate satellite cells in muscle resulting in hypertrophy or regeneration, and functions as a neuroprotectant in brain ischemia. Both expression and processing of this IGF-I variant are tissue specific, but the functional mechanism is poorly understood. MGF and its short derivative have been examined as a potential therapy for muscular dystrophy and cerebral hypoxia-ischemia using experimental animals. Although the unique mode of action of MGF has been identified, the details remain elusive. Here we review the expression and regulation of MGF and the function of this IGF-I isoform in tissue protection.

Minireview: Mechano-growth factor: a putative product of IGF-I gene expression involved in tissue repair and regeneration.

Ronald W Matheny, Bradley C Nindl, Martin L Adamo

The discovery that IGF-I mRNAs encoding isoforms of the pro-IGF-I molecule are differentially regulated in response to mechanical stress in skeletal muscle has been the impetus for a number of studies designed to demonstrate that alternative splicing of IGF-I pre-mRNA involving exons 4, 5, and 6 gives rise to a unique peptide derived from pro-IGF-I that plays a novel role in myoblast proliferation. Research suggests that after injury to skeletal muscle, the IGF-IEb mRNA splice variant is up-regulated initially, followed by up-regulation of the IGF-IEa splice variant at later time points. Up-regulation of IGF-IEb mRNA correlates with markers of satellite cell and myoblast proliferation, whereas up-regulation of IGF-IEa mRNA is correlated with differentiation to mature myofibers. Due to the apparent role of IGF-IEb up-regulation in muscle remodeling, IGF-IEb mRNA was also named mechano-growth factor (MGF). A synthetically manufactured peptide (also termed MGF) corresponding to the 24 most C-terminal residues of IGF-IEb has been shown to promote cellular proliferation and survival. However, no analogous peptide product of the Igf1 gene has been identified in or isolated from cultured cells, their conditioned medium, or in vivo animal tissues or biological fluids. This review will discuss the relationship of the Igf1 gene to MGF and will differentiate actions of synthetic MGF from any known product of Igf1. Additionally, the role of MGF in satellite cell activation, aging, neuroprotection, and signaling will be discussed. A survey of outstanding questions relating to MGF will also be provided.

Effects of different intensities of resistance exercise on regulators of myogenesis.

Colin D Wilborn, Lemuel W Taylor, Michael Greenwood +2 more

A single bout of high-intensity resistance exercise is capable of activating the expression of various genes in skeletal muscle involved in hypertrophy such as myosin heavy chain (MHC) isoforms, myogenic regulatory factors (MRFs), and growth factors. However, the specific role exercise intensity plays on the expression of these genes is not well defined. The purpose of this study was to investigate the effects of exercise intensity on MHC (type I, IIA, IIX), MRF (Myo-D, myogenin, MRF-4, myf5), and growth factor (insulin-like growth factor [IGF]-1, IGF-1 receptor [IGF-R1], mechano-growth factor [MGF]) mRNA expression. Thirteen male participants (21.5 +/- 2.9 years, 86.1 +/- 19.5 kg, 69.7 +/- 2.7 in.) completed bouts of resistance exercise involving 4 sets of 18-20 repetitions with 60-65% 1 repetition maximum (1RM) and 4 sets of 8-10 repetitions with 80-85% 1RM. Vastus lateralis biopsies were obtained immediately before exercise, and at 30 minutes, 2 hours, and 6 hours after each bout. The levels of mRNA expression were determined using real-time polymerase chain reaction. Data were analyzed using 2 x 4 multivariate analysis of variance (p <or= 0.05). For both intensities, MHC type IIX, IGF-1, IGF-R1, MGF, Myo-D, myogenin, MRF-4, and myf5 mRNA were all significantly increased in response to resistance exercise by 2 hours after exercise, whereas myostatin and the cyclin-dependent kinase inhibitor p27(kip) were decreased at 2 hours after exercise (p < 0.05). Resistance exercise between 60-85% 1RM upregulates the mRNA expression of MHC and factors involved in myogenic activation of satellite cells while concomitantly decreasing expression of myogenic inhibitors.

Growth hormone stimulates mechano growth factor expression and activates myoblast transformation in C2C12 cells.

Mari Imanaka, Keiji Iida, Ayumi Murawaki +7 more

Mechano growth factor (MGF) is an alternatively spliced variant of insulin-like growth factor-I (IGF-I). Previous reports have revealed that the MGF in skeletal muscles is induced by mechanical overload or muscle injury. In the present study, we examined the effect of growth hormone (GH) on MGF expression in C2C12 mouse muscle cell lines since GH is the principal regulator of IGF-I. The MGF mRNA increased 1 h following GH stimulation whereas IGF-IEa mRNA, which encodes a systemic type of IGF-I, increased 4 h following GH stimulation. The diverse expression of MGF and IGF-IEa was also observed in the case of muscle injury by using bupivacaine in the same cell line. Furthermore, GH induced the increase of MyoD as well as M-cadherin expression, the peak of which was parallel to that of MGF. These results indicate that GH directly and preferentially increased MGF prior to the IGF-IEa expression in C2C12 cells, which may lead to the activation of muscle satellite cells.

Co-expression of IGF-1 family members with myogenic regulatory factors following acute damaging muscle-lengthening contractions in humans.

Bryon R McKay, Ciara E O'Reilly, Stuart M Phillips +2 more

Muscle regeneration following injury is dependent on the ability of muscle satellite cells to activate, proliferate and fuse with damaged fibres. This process is controlled by the myogenic regulatory factors (MRF). Little is known about the temporal relation of the MRF with the expression of known myogenic growth factors (i.e. IGF-1) in humans following muscle damage. Eight subjects (20.6 +/- 2.1 years; 81.4 +/- 9.8 kg) performed 300 lengthening contractions (180 deg s(-1)) of their knee extensors in one leg on a dynamometer. Blood and muscle samples were collected before and at 4 (T4), 24 (T24), 72 (T72) and 120 h (T120) post-exercise. Mechano growth factor (MGF), IGF-1Ea and IGF-1Eb mRNA were quantified. Serum IGF-1 did not change over the post-exercise time course. IGF-1Ea and IGF-1Eb mRNA increased approximately 4- to 6-fold by T72 (P < 0.01) and MGF mRNA expression peaked at T24 (P = 0.005). MyoD mRNA expression increased approximately 2-fold at T4 (P < 0.05). Myf5 expression peaked at T24 (P < 0.05), while MRF4 and myogenin mRNA expression peaked at T72 (P < 0.05). Myf5 expression strongly correlated with the increase in MGF mRNA (r(2) = 0.83; P = 0.03), while MRF4 was correlated with both IGF-1Ea and -Eb (r(2) = 0.90; r(2) = 0.81, respectively; P < 0.05). Immunofluorescence analysis showed IGF-1 protein expression localized to satellite cells at T24, and to satellite cells and the myofibre at T72 and T120; IGF-1 was not detected at T0 or T4. These results suggest that the temporal response of MGF is probably related to the activation/proliferation phase of the myogenic programme as marked by an increase in both Myf5 and MyoD, while IGF-1Ea and -Eb may be temporally related to differentiation as marked by an increase in MRF4 and myogenin expression following acute muscle damage.

Response of growth and myogenic factors in human skeletal muscle to strength training.

Y Liu, M Heinichen, K Wirth +2 more

To investigate the response to different strength training techniques of growth and myogenic factors in human skeletal muscle, with particular emphasis on satellite cell (SC) activation.

Loss of muscle strength during aging studied at the gene level.

Geoffrey Goldspink

Age-related muscle wasting and increased frailty is a major socioeconomic as well as a major medical problem. In our quest to extend the quality of life it is important to increase the strength of elderly people sufficiently so they can carry out everyday tasks and prevent them falling and breaking bones that are brittle because of osteoporosis. Muscles generate the mechanical strain that contributes to the maintenance of other musculoskeletal tissues and a vicious cycle is established when the muscles start to produce less force resulting in more bone loss and weakening of tendons. Another aspect that is less well appreciated is that muscle acts as a dynamic, metabolic store. In a traumatic situation, muscle provides amino acids to aid tissue repair processes and maintaining acid-base balance. At the present time there are strategies in addition to exercise for preventing age-related muscle wasting and these are briefly reviewed. Here, more attention is paid to the role of the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis and the discovery of mechano-growth factor (MGF). This is derived from the IGF-1 gene by alternative splicing and in the young is responsible for increasing contractile strength in response to exercise by activating the muscle satellite (stem) cells that kick-start local muscle repair and induce hypertrophy. Recent studies including gene transfer of this part of the IGF-1 gene and unique MGF peptides offer the prospect of treating muscle wasting during the aging process as well as muscle cachexia associated with many diseases.

Impairment of IGF-I gene splicing and MGF expression associated with muscle wasting.

Geoffrey Goldspink

The characterisation of a local tissue repair factor (mechano growth factor, MGF) that is produced by exercised and/or damaged muscle by differential splicing of the IGF-I gene provides understanding of how muscle is maintained in the young normal individual. Mechano growth factor, or MGF, is different to the systemic IGF-I as it has an insert of 49 base pairs in exon 5 that introduces a reading frame shift resulting in a C terminal peptide with unique properties. Muscle is a post-mitotic tissue and as cell replacement is not a means of tissue repair there has to be an efficient local repair mechanism otherwise the damaged cells undergo cell death. The extra nuclei for muscle repair and hypertrophy are provided by the muscle satellite (stem) cells. The pool of these stem cells is apparently replenished by the action of MGF, which is produced as a pulse following a mechanical challenge. Unfortunately, the production of MGF is deficient in certain diseases such as in the muscular dystrophies in which the mechanotransduction mechanism, which may involve the dystrophin complex, is defective. In elderly muscles, decreased levels of growth hormone apparently mean that there is less primary RNA transcript of the IGF-I gene to be spliced towards MGF. Consequently, there is an increasing inability to maintain muscle mass during ageing. Delivery of MGF and cDNA or peptide produces marked increases in the strength of normal as well as diseased muscle and, therefore, MGF has considerable potential as a generic means of treating muscle cachexia.

Growth factors and muscle ageing.

Geoffrey Goldspink, Stephen D R Harridge

Loss of muscle mass (sarcopenia) is one of the main problems associated with ageing as it has major health care as well as socioeconomic implications. The growth hormone (GH)/IGF-I axis is regarded as an important regulator of muscle mass. However, it is now appreciated that other tissues in addition to the liver express IGF-I and that there are local as well as systemic forms of IGF-I which have different functions. At least two different kinds of IGF-I that are expressed by skeletal muscle are derived from the IGF-I gene by alternative splicing, one of which is expressed in response to physical activity which has now been called 'mechano growth factor' (MGF). The other is similar to the systemic or liver type (IGF-IEa) and is important as the provider of mature IGF-I required for upregulating protein synthesis. MGF differs from systemic IGF-IEa in that it has a different peptide sequence which is responsible for replenishing the satellite (stem) cells in skeletal muscle. The ability to produce MGF declines with age, and this is commensurate with the decline in circulating GH levels. GH treatment up regulates the level of IGF-I gene expression in older people and when combined with resistance exercise more is spliced towards MGF and hence should improve the ability of muscle to respond to physical activity. The possibility of ameliorating sarcopenia using MGF is discussed.

Mechanical signals and IGF-I gene splicing in vitro in relation to development of skeletal muscle.

Umber Cheema, Robert Brown, Vivek Mudera +3 more

It has been shown that the insulin-like growth factor (IGF-I) gene is spliced in response to mechanical signals producing forms of IGF-I which have different actions. In order to study how mechanical signals influence this gene splicing in developing muscle, C2C12 cells were grown in three-dimensional (3D) culture and subjected to different regimens of mechanical strain. IGF-IEa which initiates the fusion of myoblasts to form myotubes was found to be constitutively expressed in myoblasts and myotubes (held under endogenous tension) and its expression upregulated by a single ramp stretch of 1-h duration but reduced by repeated cyclical stretch. In contrast, mechano growth factor (MGF), which is involved in the proliferation of mononucleated myoblasts that are required for secondary myotube formation and to establish the muscle satellite (stem) cell pool, showed no significant constitutive expression in static cultures, but was upregulated by a single ramp stretch and by cycling loading. The latter types of force simulate those generated in myoblasts by the first contractions of myotubes. These data indicate the importance of seeking to understand the physiological signals that determine the ratios of splice variants of some growth factor/tissue factor genes in the early stages of development of skeletal muscle.

Age-related loss of skeletal muscle function; impairment of gene expression.

G Goldspink

Mechano Growth Factor (MGF) is derived from the insulin-like growth factor (IGF-I) but its sequence differs from the systemic IGF-I produced by the liver. MGF is expressed by mechanically overloaded muscle and is involved in tissue repair and adaptation. It is expressed as a pulse following muscle damage and involved in the activation of muscle satellite (stem) cells. These donate nuclei to the muscle fibers that are required for repair and for the hypertrophy processes which may have similar regulatory mechanisms. Muscles in the elderly are unable to upregulate MGF in response to exercise. This is also true in certain diseases and this helps to explain muscle loss in those conditions. There is evidence that MGF is a local tissue repair factor as well as a growth factor and that it has an important role in damage limitation and inducing repair in other post-mitotic tissues. As there is no cell replacement in these tissues there has to be an effective local cellular repair mechanism. With advancing years this seems to become deficient and there is an increased chance that the damaged cells will undergo cell death leading to progressive loss of tissue function.

Expression and splicing of the insulin-like growth factor gene in rodent muscle is associated with muscle satellite (stem) cell activation following local tissue damage.

Maria Hill, Geoffrey Goldspink

Muscle satellite cells are mononuclear cells that remain in a quiescent state until activated when they proliferate and fuse with muscle fibres to donate nuclei, a process necessary for post-embryonic growth, hypertrophy and tissue repair in this post-mitotic tissue. These processes have been associated with expression of the insulin-like growth factor (IGF-I) gene that can undergo alternative splicing to generate different gene products with varying functions. To gain insight into the cellular mechanisms involved in local tissue repair, the time courses of expression of two IGF-I splice variants produced in muscle were determined together with marker genes for satellite cell activation following local muscle damage. Using real-time RT-PCR with specific primers, the mRNA transcripts in rat tibialis anterior muscles were measured at different time intervals following either mechanical damage imposed by electrical stimulation of the stretched muscle or damage caused by injection with bupivacaine. It was found that the autocrine splice variant mechano growth factor (MGF) was rapidly expressed and then declined within a few days following both types of damage. Systemic IGF-IEa was more slowly upregulated and its increase was commensurate with the rate of decline in MGF expression. Satellite cell activation as measured by M-cadherin and one of the muscle regulatory factors MyoD and the sequence of expression suggests that the initial pulse of MGF is responsible for satellite cell activation, as the systemic IGF-IEa mRNA expression peaks after the expression of these markers, including M-cadherin protein. Later splicing of the IGF-I gene away from MGF but towards IGF-IEa seems physiologically appropriate as IGF-IEa is the main source of mature IGF-I for upregulation of protein synthesis required to complete the repair.

Ageing and local growth factors in muscle.

Stephen D R Harridge

Muscle responds to mechanical overload by increasing its size. In contrast, as a muscle gets older it atrophies. The mechanisms regulating these differing responses are not fully understood. Animal studies have shown that older muscles are less well able to repair following contraction-induced injury than young muscles. It is becoming clear that local growth factors produced within the muscle may play important roles in both repair, adaptation and ageing. The growth hormone/insulin like growth factor 1 (GH/IGF-I) axis is important during growth and development, but circulating levels of these hormones decline in later life. However, many tissues including muscle, produce IGF-I for autocrine and paracrine actions. Genetic manipulation of IGF-I in muscle has shown that it has considerable anabolic affects on muscle both in young and old animals. Insulin like growth factor 1 exists in multiple isoforms and one isoform, which differs from the systemic or liver type (IGF-IEa), appears to be particularly sensitive to mechanical signals and to muscle damage. This isoform (IGF-IEc) has been termed mechano growth factor (MGF). The anabolic actions of IGF-I and MGF are through stimulating protein synthesis and by playing a role in the activation, proliferation and differentiation of satellite cells. These effects are discussed in relation to human studies of muscle adaptation to strength training in older people who seem to retain an ability to increase muscle mass and strength through this type of exercise.

Age-related loss of skeletal muscle function and the inability to express the autocrine form of insulin-like growth factor-1 (MGF) in response to mechanical overload.

V Owino, S Y Yang, G Goldspink

The response of insulin-like growth factor-1 (IGF-1) signalling and the capacity of skeletal muscle to adapt to mechanical overload was studied using synergistic muscle ablation. Overload of the plantaris and soleus resulted in marked hypertrophy and activation of satellite cells (as indicated by MyoD expression), particularly in young rats. Two muscle IGF-1 splice variants were measured and found to be differentially regulated at the RNA level. The significant changes associated with the inability of the older muscles to respond to mechanical overload included the considerably lower expression of the local splice variant mechano growth factor, and the failure to up-regulate IGF-1 receptor and MyoD mRNA.