Quick summary: MGF is a splice variant of the IGF produced by a frame shift if the IGF gene. It exhibits local effects in skeletal muscle and without modification is not systemic (can’t travel through the body). MGF increase the muscle stem cell count, so that more may fuse and become part of adult muscle cells. This is a process required for adult muscle cells to continue growing.
Complete Overview of MGF or IGF-IEc
From its sequence, MGF is derived from the IGF-I gene by alternative splicing and has different 3' exons to the liver or systemic type (IGF-IEa). It has a 49 base pair insert in the human, and a 52 base pair insert in rodents, within the E domain of exon 5. This insert results in a reading frame shift, with a different carboxy (C) terminal sequence to that of systemic IGF-IEa. MGF and the other IGF isoforms have the same 5' exons that encode the IGF-I ligand-binding domain. Processing of pro-peptide yields a mature peptide that is involved in upregulating protein synthesis. However, there is evidence that the carboxy-terminal of the MGF peptide also acts as a separate growth factor. This stimulates division of mononucleated myoblasts or satellite (stem) cells, thereby increasing the number available for local repair
During the early stage of skeletal muscle development, myoblasts (muscle stem cells) fuse to form syncytial myotubes, which become innervated and develop into muscle fibres. Thereafter, mitotic proliferation of nuclei within the muscle fibres ceases. However, during postnatal (after development) growth, additional nuclei are provided by satellite cells (myoblast) fusing with myotubules. Muscle damage-recovery seems to have a similar cellular mechanism, in that satellite cells become activated and fuse with the damaged muscle fibres (reviewed by Goldring et al. 2002). This is also pertinent to certain diseases such as muscular dystrophy in which muscle tissue is not maintained and which have been associated with a deficiency in active satellite (stem) cells (Megeney et al. 1996; Seale & Rudnicki, 2000) and in myogenic factors (Heslop et al. 2000). Skeletal muscle mass and regenerative capacity have also been shown to decline with age (Sadeh, 1988; Carlson et al. 2001). The reduced capacity to regenerate in older muscle seems to be due to the decreased ability to activate satellite cell proliferation (Chakravarthy et al. 2000). The markedly lower expression of MGF in older rat muscles (Owino et al. 2001) and human muscle (Hameed et al. 2003) in response to mechanical overload has been associated with the failure to activate satellite cells, leading to age-related muscle loss (Owino et al. 2001). Your muscle cels can not grow once they have reached a certain size unless they obtain more nuclei from the myoblast. MGF increases the myblast available to donate their nuclei to the adult muscle cell.
“MGF appears to have a dual action in that, like the other IGF-I isoforms, it upregulates protein synthesis as well as activating satellite cells. However, the latter role of MGF is probably more important as most of the mature IGF-I will be derived from IGF-IEa during the second phase of repair. Nevertheless, it has been shown that MGF is a potent inducer of muscle hypertrophy in experiments in which the cDNA of MGF was inserted into a plasmid vector and introduced by intramuscular injection. This resulted in a 20 % increase in the weight of the injected muscle within 2 weeks, and the analyses showed that this was due to an increase in the size of the muscle fibres (Goldspink, 2001). Similar experiments by other groups have also been carried out using a viral construct containing the liver type of IGF-I, which resulted in a 25 % increase in muscle mass, but this took over 4 months to develop (Musaro et al. 2001). Hence, the dual role MGF plays in inducing satellite cell activation as well as protein synthesis suggests it is much more potent than the liver type or IGF-IEa for inducing rapid hypertrophy.”
These results are based on actual transplantation of the DNA coding for the peptides. This is a permanent effect and much more potent than IM injections of the peptide itself. You will not see a 20% increase in muscle mass through IM injections as claimed above.