MGF is a split variant of IGF-1 but its sequence differs from the systemic IGF-1 produced by the liver. IGF-I is spliced towards MGF which initiates hypertrophy and repair of local muscle damage. 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 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 process, which may have similar regulatory mechanisms. MGF is essential for repair and therefore growth of new cells, similar to IGF-1. If MGF is not PEGylated, the half-life lasts only minutes therefore PEGylated MGF must be considered during the compounding process to ensure an appropriate half-life, thereby increasing duration of action.


Expression of IGF-1 Isoforms after Exercise-induced Muscle Damage in Humans: Characterization of the MGF E Peptide Actions In Vitro
Different insulin-like growth factor-1 (IGF-1) isoforms, namely IGF-1Eb and IGF-1Ec (MGF), have been proposed to have various functions in muscle repair and growth. To gain insight into the potentially differential actions of IGF- 1 isoforms in the regulation of muscle regeneration, we assessed the time course of their expressions at both mRNA and protein levels after exercise-induced muscle damage in humans. In addition, we characterized mature IGF-1 and synthetic MGF E peptide signaling in C2C12
myoblast-like cells in vitro. Ten healthy male volunteers were subjected to exercise-induced muscle damage and biopsy samples were taken from the exercised muscles before and 6 h, 2,5 and 16 days post-exercise. Muscle damage was documented by specific functional and biochemical responses post exercise. PCR-based analyses of muscle biopsy samples revealed a rapid and transient
up-regulation of MGF mRNA expression which was followed by a prolonged increase of IGF-1Ea and IGF-1Eb mRNA expression (p<0.05). Patterns similar to those for mRNA expression were detected for MGF and IGF-1Ea expression at the protein level. The action of synthetic MGF E peptide differed from that of mature IGF1 since its proliferative effect on C2C12 myoblast-like cells was not blocked by an anti-IGF-1 receptor neutralizing antibody and it did not phosphorylate Akt. Therefore, we conclude that the differential expression profile of IGF-1 isoforms in vivo and the possible IGF-1R- independent MGF E peptide signaling in skeletal muscle-like cells in vitro support the notion that tissue-specific mRNA expression of MGF isoform produces mature IGF-1 and MGF E peptides which possibly act as distinct mitogens in skeletal muscle regeneration.

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