MGF: a local growth factor or a local tissue repair factor?

Combining physiological and molecular biology methods have indicated how a factor expressed by stressed muscle induces local muscle fibre repair and adaptation

Geoff Goldspink
Royal Free and University College Medical School Royal Free Campus, London 

---

https://doi.org/10.36866/pn.53.22

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 is apparently involved in the activation of muscle satellite (stem) cells. These donate nuclei to the muscle fibres that are required for repair and for the hypertrophy processes which may have similar regulatory mechanisms.

Cloning of MGF

About 10 years ago our group set about cloning the growth factor(s) involved in the local regulation of muscle mass using a technique known as differential display. For this purpose we needed to have an animal model in which we could make a muscle grow rapidly. Previous work had shown that if the tibialis anterior in the mature rabbit was electrically stimulated whilst held in the stretched position by plaster cast immobilisation it increased in mass by 35% within seven days (Goldspink et al. 1992). The RNA in the stretch/stimulated muscles increased considerably but most of this was ribosomal RNA. Using various oligonucleotide primers and the RT-PCR technique, it was possible to detect RNA transcript that was only expressed in the stretched/stimulated muscle and not in resting control muscles. This was cloned and sequenced and it became evident that it was derived from the insulin-like growth factor gene by alternative splicing. The terminology of the IGF-I splice variants is a problem when attempting to apply it to non hepatic tissues (Hameed et al. 2003a). Therefore we named this newly discovered splice variant mechano growth factor (MGF) as it is expressed in response to mechanical stimulation (Yang et al. 1996, McKoy et al. 1999) and as it has a different downstream sequence to the liver type of IGF-I (Figure).

Different products depending on the splicing of the IGF-I gene

The systemic type of IGF-I (IGF-IEa) is expressed constitutively by many tissues, including skeletal muscle, but the mRNA of MGF is not detectable or barely detectable unless the muscle is exercised and/or damaged. The sequence of MGF has a 52 base insert in the rat and 49 base insert in the human in exon 5. As these inserts are not multiples of three, the 3fi downstream sequence is different in MGF to that of IGF-IEa. This reading frame shift has important functional consequences as the carboxy peptide is involved in the recognition of the binding proteins that stabilize and target these growth factors. Also, in the case of MGF, this part of the peptide acts as a separate growth factor involved in initiating muscle satellite (stem) cell activation in addition to its IGF-I receptor domain which increases protein synthesis.

As well as having a different carboxy peptide sequence, MGF expression kinetics are different to those of IGF-IEa as shown by Gregory Adam’s group in the USA who found that MGF is expressed earlier than IGF-IEa in response to exercise. Hill & Goldspink (2003) showed that following muscle damage MGF is produced as a pulse lasting a day or so followed by the splicing of the IGF-I gene towards IGF-IEa production that continues for a longer time.

Local damage repair and/or hypertrophy?

Intramuscular injection of its cDNA inserted into a plasmid vector demonstrated that MGF is a potent inducer of muscle hypertrophy. This resulted in a 25% increase in the fibre cross-sectional area of the injected muscle within two weeks (Goldspink, 2001). Similar experiments have also been carried out using liver IGF-IEa cDNA in viral constructs which resulted in a 25% increase in muscle mass, but this took over four months to develop (Musaro et al. 2001). In vivo experiments in which muscles of the rat were subjected to mechanical damage or injection of a myotoxic agent also demonstrated (Hill & Goldspink, 2003) that MGF precedes muscle satellite (stem) cell activation. This is in accord with the finding that when skeletal muscle cells in culture, were either transfected with the MGF cDNA or were treated with the MGF carboxy peptide they increased in number but stayed as monocleated myoblasts Yang & Goldspink, 2002). It appears that MGF plays a dual role in inducing satellite cell activation as well as protein synthesis and this is probably why it is much more potent than the liver type or IGF-IEa for inducing rapid muscle hypertrophy.

As muscle repair and hypertrophy appear to involve the same signalling, the question still needs to be answered as to whether local damage is a prerequisite for muscle hypertrophy? Therefore the saying ‘no gain without pain’ may have a physiological basis.

Failure to maintain muscle loss in ageing and disease

Muscle loss (sarcopenia) is one of the most obvious effects of ageing. The work of Owino et al. (2001) indicated that muscles in old rats when surgically overloaded were much less able to express MGF than those in younger animals. Hameed et al. (2003b) reported that this was the case for elderly male volunteers as compared to muscles of young men. None of the other parameters measured showed a marked age­related decline although it has previously been known that circulating growth hormone levels in the over 70s are much lower than those in teenagers (Rudman et al. 1981). Growth hormone is known to upregulate the IGF-I gene. With Michael Kjaer’s group in Copenhagen (Hameed et al. 2003c) we have recently found that administration of growth hormone combined with resistance exercise considerably improved MGF levels and increased cross-sectional area of the muscles in the elderly. Muscle loss is a major problem in certain hereditary diseases. In dystrophic muscles it seems that there is an inability to produce MGF in response to mechanical overload (Goldspink et al. 1996). The systemic type of IGF-IEa is also produced by muscle, including dystrophic muscle, but MGF is apparently required to ensure muscle fibre repair and survival. Therefore the role of the IGF-I splice variants including MGF seems to be very relevant to understanding the aetiology and the development of possible treatment in these muscle wasting conditions including sarcopenia.

References

Adams GR (2002) Exercise effects on muscle insulin-signalling and action. Invited Review. Autocrine/paracrine IGF-I and skeletal muscle adaptation. J Appl Physiol 93, 1159-1167.

Goldspink G, Scutt A, Loughna P, Wells D, Jaenicke T & Gerlach G-F (1992). Gene expression in skeletal muscle in response to mechanical signals. Am J Physiol 262, R326-R363.

Goldspink G, Yang SY, Skarli M & Vrbova G (1996). Local growth regulation is associated with an isoform of IGF-I that is expressed in normal muscles but not in dystrophic muscles. J Physiol 495, 162.

Goldspink G (2001). Method of treating muscular disorders. United States Patent No. US 6,221,842 B1.

Hameed M, Orrell RW, Cobbold M, Goldspink G & Harridge SDR (2003a) Clarification. J Physiol 549, 995.

Hameed M, Orrell RW, Cobbold M, Goldspink G & Harridge SDR (2003b). Expression of IGF-I splice variants in young and old human skeletal muscle after high resistance exercise. J Physiol 547, 247-254.

Hameed M, Lange KHW, Andersen JL, Schjerling P, Kjaer M, Harridge SDR & Goldspink G (2003c). The effect of recombinant human growth hormone and resistance training on IGF-I mRNA expression in the muscles of elderly men. J Physiol, in press.

Hill M & Goldspink G (2003). Expression and splicing of the insulin­like growth factor gene in rodent muscle is associated with muscle satellite (stem) cell activation following local tissue damage. J Physiol 549, 409-418.

McKoy G, Ashley W, Mander J, Yang SY, Williams N, Russell B & Goldspink G (1999). Expression of insulin-like growth factor-I splice variant and structural genes in rabbit skeletal muscle induced by stretch and stimulation. J Physiol 516, 583-592.

Musaro A, McCullagh K, Paul A, Houghton I, Dobrowolny G, Molinaro M, Barton ER, Sweeney HL & Rosenthal N (2001). Localized IGF-I transgene expression sustains hypertrophy and regeneration in senescent skeletal muscle. Nat Genet 27, 195-200.

Owino V, Yang SY & Golsdpink G (2001). 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. FEBS Lett 506, 259-263.

Rudman D, Kutner MH, Rogers CM, Lubin MF, Fleming GA & Bain RP (1981). Impaired growth hormone secretion in the adult population: relation to age and adiposity. J Clin Invest 67, 1361-1369.

Yang SY & Goldspink G (2002). Different roles of the IGF-IEc peptide (MGF) and mature IGF-I in myoblast proliferation and differentiation. FEBS Lett 522, 156-160.

Yang SY, Alnaqeeb M, Simpson H & Goldspink G (1996). Cloning and characterisation of an IGF-I isoform expressed in skeletal muscle subjected to stretch. J Muscle Res Cell Motil 17, 487-495.