The differential Role of Insulin-like Growth Factor-1 Isoforms in Skeletal muscle

Abstract

Insulin-like growth factor-1 (IGF-1) has pleiotropic effects on various tissues during pre- and postnatal development by stimulating proliferation and differentiation, and plays a major role in hypertrophy and tissue remodeling in the fetal, perinatal, and adult organism. In skeletal muscle the role of IGF-1 is well established, but questions remain about the significance of the various isoforms that are produced from the single-copy IGF-1 gene. By the use of different promoters, differential splicing and post-translational modifications, at least six IGF-1 isoforms are generated from the IGF-1 gene, that differ in the N-terminal signal peptide (Class) and the C-terminal E-peptide. The aim of this work was to dissect the different roles of IGF-1 isoforms in skeletal muscle in vitro and in vivo. Cell culture experiments revealed that IGF-1Ea isoforms promoted myogenic differentiation and cell hypertrophy, resulting in enlarged myofibers, while IGF-1Eb isoforms instead did not show an effect on fiber size but on proliferation of myoblasts. Correlating with the results obtained in vitro, transgenic animals over-expressing IGF-1Ea isoforms showed pronounced muscle fiber hypertrophy, accompanied by an increase in force generation and strength, while IGF-1Eb isoforms showed very mild effects on muscle size and no changes in muscle strength, further implicating the Ea-peptide in the hypertrophic response. Analysis of the intracellular signals transduced by the different IGF-1 isoforms revealed a complex regulatory network, excluding certain pathways previously implicated in the induction of skeletal muscle hypertrophy in response to IGF-1. Preliminary analysis of regeneration in response to IGF-1 isoforms demonstrated that each isoform enhanced the regeneration process, suggesting that Eb-peptide-containing isoforms did so by stimulating the proliferation of satellite cells, while IGF-1Ea enhanced the growth of newly forming fibers. Class 2 IGF-1Eb was found to specifically induce a calcineurin isoform (CnAs1) that has been linked to enhanced regeneration. In addition, this thesis describes the cloning of a Class 2 IGF-1En isoform that was previously not described in rodent species, but known to exist in humans. The present work constitutes the first evidence for different functions of IGF-1 isoforms in vitro and in vivo, provides an overview of their variable effects in skeletal muscle and a strong basis for future research into their specific functions.