The discovery that IGF-ImRNAs 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-IEamRNAis correlated with differentiation to mature myofibers. Due to the apparent role of IGF-IEb up-regulation in muscle remodeling, IGF-IEbmRNA 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.
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