MGF (Mechano Growth Factor)

Introduction

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.

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.