Adeno-Associated Virus-8-Mediated Intravenous Transfer of Myostatin Propeptide Leads to Systemic Functional Improvements of Slow but Not Fast Muscle

Keith Foster,Ketan Patel,Dale Bickham,Frank S Walsh,George Dickson,Nancy A Curtin,Helen Foster,Capucine Trollet,Susannah L Kawar,Anthony Otto,Ian R Graham,Paul J Yaworsky

doi:10.1089/rej.2008.0815

Abstract

Myostatin is a member of the transformating growth factor-beta (TGF-beta) superfamily of proteins and is produced almost exclusively in skeletal muscle tissue, where it is secreted and circulates as a serum protein. Myostatin acts as a negative regulator of muscle mass through the canonical SMAD2/3/4 signaling pathway. Naturally occurring myostatin mutants exhibit a 'double muscling' phenotype in which muscle mass is dramatically increased as a result of both hypertrophy and hyperplasia. Myostatin is naturally inhibited by its own propeptide; therefore, we assessed the impact of adeno-associated virus-8 (AAV8) myostatin propeptide vectors when systemically introduced in MF-1 mice. We noted a significant systemic increase in muscle mass in both slow and fast muscle phenotypes, with no evidence of hyperplasia; however, the nuclei-to- cytoplasm ratio in all myofiber types was significantly reduced. An increase in muscle mass in slow (soleus) muscle led to an increase in force output; however, an increase in fast (extensor digitorum longus [EDL]) muscle mass did not increase force output. These results suggest that the use of gene therapeutic regimens of myostatin inhibition for age-related or disease-related muscle loss may have muscle-specific effects.

Highlights

THE LOSS OF SKELETAL MUSCLE TISSUE has a major impact on public health
Myostatin is naturally inhibited by its own propeptide; we assessed the impact of adeno-associated virus-8 (AAV8) myostatin propeptide vectors when systemically introduced in MF-1 mice
MF-1 male mice treated with AAV8ProMyo demonstrated a significant increase in gross mass by 4 weeks postinjection (p ϭ 0.036), with the difference being maintained until the end of the experiment, week 10 (p ϭ 0.014, Fig. 1B)

Summary

Introduction

A variety of conditions result in the loss of muscle, including disease-related loss (cachexia), age-associated loss (sarcopenia), enforced inactivity, such as bed rest and muscular dystrophies.[1] Skeletal muscle development is a complex process, with a number of regulatory factors for the different steps involved in muscle stem cell activation, proliferation, and postmitotic differentiation having been identified.[2] Skeletal muscle size is dynamic and responsive to extracellular signals such as mechanical load, neural activity, hormones, growth factors, and cytokines. Myostatin is a central determinant of muscle size and mass; its role has been established from the effects of gene-inactivating mutations in mouse.[3,4] Inactivation mutations of myostatin have been described in several mammals such as cattle, sheep, dogs, and humans,[5,6,7,8,9] with the central tenet being hypermuscularity and reduced adipogenesis. The overexpression of myostatin is a key feature in cachexia[10,11] and sarcopenia.[12]

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Results

Conclusion