Study summary · research use only
Gene expression in muscle in response to exercise
Plain-language summary
Paraphrased from the published abstract below — not a verdict on whether anything works.
This review article discusses how muscle adapts to different work by changing fibre type and mass through quantitative and qualitative gene-expression changes, including myosin heavy chain isogenes. The authors describe the roles of oxidative metabolism, slow myosin expression, and calcium in regulating these switches. They report their group's cloning of three alternatively spliced forms of IGF-I in human muscle, including a mechanically activated autocrine/paracrine variant termed mechano growth factor (MGF) and a systemic liver-type form (IGF-IEa) with a different C-terminal sequence. They note MGF's role in activating satellite cells, its reduced expression in dystrophic and aged muscle, and its association with muscle repair and hypertrophy.
Abstract
Muscle has an intrinsic ability to adapt to different types of work by changing fibre type and muscle mass. This process involves quantitative and qualitative changes in gene expression including those of the myosin heavy chain (MyHC) isogenes that encode different types of molecular motors. Increased expression of slow MyHC and of metabolic genes result in increased fatigue resistance. Recently, there has been some insight into how oxidative metabolism, as well as slow myosin expression, is regulated and the role of calcium in initiating switches in gene expression. In relation to muscle mass and power output it has been appreciated that local as well as systemic factors are important. Our group have cloned three types of IGF-I in human muscle which are derived from the IGF-I gene by alternative splicing. The expression of one of these that appears to be an autocrine/paracrine splice variant is only detectable after mechanical stimulation (MGF) and a systemic type (IGF-IEa) that is produced by the liver and other tissue including muscle. As the result of a reading frame shift, the MGF peptide has a different C terminal sequence to IGF-IEa. Interestingly, the MGF C terminal peptide has been found to act as a separate growth factor and to initially activate mononluceated myoblasts (satellite cells). MGF also responds to different signals and has different expression kinetics to IGF-IEa. The mechanotransduction mechanism for this signalling may directly or indirectly involve the dystrophin complex as dystrophic muscle, unlike normal muscle, is unable to express MGF in response to overload. Also the ability to express MGF has been found to decline markedly during ageing. The deficiency in expressing MGF and activating satellite cells in dystrophic and aged muscles may explain why muscle mass is not maintained in these situations. However, in normal muscle MGF appears to initiate local muscle repair with its over expression resulting in hypertrophy.
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