Medical Definition of Myostatin

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Medical Definition of Myostatin

Medical Author: William C. Shiel Jr., MD, FACP, FACR

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Myostatin: A growth factor that regulates the size of muscles beginning in early embryonic development and continuing throughout life. Myostatin acts by inhibiting the growth of muscles, It prevents them from growing too large. Myostatin is also known as growth and differentiation factor 8 (GDF-8). It is a protein made up of two identical subunits. Each subunit contains 110 amino acids. The gene encoding myostatin is termed MSTN (or GDF8) and is on chromosome 2 in band 2q32.1.

Myostatin is a member of the transforming growth factor beta (TGF-beta) family. All of the members of this gene family regulate growth and differentiation from early embryogenesis to mature cell types and tissues.

Myostatin was first found to regulate muscle mass in mice from which the gene encoding myostatin had been knocked out (deleted). In these "mighty mice," there is muscle overgrowth due to an increase both in the number of myocytes (muscle cells) and the size (hypertrophy) of the myofibers (muscle fibers). Breeds of cattle with exceptional muscle development -- referred to as "double-muscled" cattle -- have a mutation in the bovine MSTN gene encoding myostatin.

A child born with very large muscles was discovered to have mutation of the MSTN gene encoding myostatin, providing very strong evidence that myostatin is a lead actor in regulating muscle mass in humans. Aside from the increase in the size of his muscles, the child appeared normal at age 4. The child has a loss-of-function mutation in the MSTN gene that inactivates myostatin It may be possible to increase muscle mass and strength by inactivating myostatin in people with muscle wasting due to disease (Schuelke M et al. New Engl J Med 350:2682,2004).

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Myostatin Is a Skeletal Muscle Target of Growth Hormone Anabolic Action
Wei Liu, Scott G. Thomas, Sylvia L. Asa, Nestor Gonzalez-Cadavid, Shalendar Bhasin, Shereen Ezzat
The Journal of Clinical Endocrinology & Metabolism, Volume 88, Issue 11, 1 November 2003, Pages 5490–5496, https://doi.org/10.1210/jc.2003-030497



Abstract

Myostatin is a cytokine that has recently been shown to selectively and potently inhibit myogenesis. To investigate the mechanisms of anabolic actions of GH on skeletal muscle growth, we examined the in vitro and in vivo effects of GH on myostatin regulation. Twelve GH-deficient hypopituitary adult subjects were treated with recombinant GH (5 μg/kg·d) in a double-blind, placebo-controlled fashion. Body composition and physical function were assessed and skeletal muscle biopsies from the vastus lateralis performed at 6-monthly intervals during 18 months of treatment. Myostatin mRNA expression was significantly inhibited to 31 ± 9% (P < 0.001) of control by GH but not by placebo administration (79 ± 11%) as determined by quantitative real-time PCR normalized for the housekeeping glyceraldehyde-3-phosphate dehydrogenase gene. The inhibitory effect of GH on myostatin was sustained after 12 and 18 months of GH treatment. These effects were associated with increases in lean body mass and translated into enhanced aerobic performance as determined by maximal oxygen uptake and ventilation threshold. Parallel in vitro studies of skeletal muscle cells demonstrated significant reduction of myostatin expression by myotubes in response to GH, compared with vehicle treatment. Conversely, GH receptor antagonism resulted in up-regulation of myostatin in myoblasts. Given the potent catabolic actions of myostatin, our data suggest that myostatin represents a potential key target for GH-induced anabolism.

GH DEFICIENCY (GHD) in adults is associated with excessive fatigue, impaired physical performance, and reduced skeletal muscle mass (1). That GHD is causally related to significant skeletal muscle impairment is supported by a wealth of studies demonstrating reversal of sarcopenia in response to systemic GH administration and increased circulating levels of its target IGF-I (2). We have previously used adult-onset GHD as a model to investigate the mechanisms of in vivo action of GH on skeletal muscle growth (1). We demonstrated that GH treatment increases submaximal measures of physical performance in concert with increases in skeletal muscle fiber size and local expression of IGF-I. The mechanism(s) by which GH mediates these anabolic actions on skeletal muscle growth, however, is not clear.

A number of candidate cytokines have been implicated in differentiated skeletal muscle growth. Of these, myostatin (also known as growth/differentiation factor-8) is a member of the TGFβ family that has gained attention due to its remarkable expression profile and dramatic actions. Myostatin mutations have been linked to the double-muscled phenotype in cattle (3), and mice with targeted disruption of the myostatin gene display marked increase in skeletal muscle mass (4). In humans, hypercatabolic states such as HIV-associated wasting have been characterized by marked up-regulation of myostatin (5), but little is known about its regulation. In this study, we tested the hypothesis that a potential mechanism for GH action on skeletal muscle may be suppression of locally synthesized myostatin. Using parallel examination of muscle biopsies from GH-deficient adults treated with GH or placebo and in vitro treatment of skeletal muscle cells, we demonstrate that GH effectively suppresses myostatin expression. Furthermore, we took advantage of a GH receptor (GHR) antagonist to demonstrate that attenuated GHR signaling results in significant up-regulation of myostatin. The data strongly implicate myostatin as an important target of GH action in skeletal muscle.