Overexpression of Latent TGFβ Binding Protein 4 in Muscle Ameliorates Muscular Dystrophy through Myostatin and TGFβ.

Kay-Marie Lamar,Andy H Vo,Judy U Earley,Jeffery D Molkentin,Lisa Wren,Michele Hadhazy,Elizabeth M Mcnally,Quan Q Gao,Sasha Bogdanovich,Brandon B Gardner,Tamari Miller,Gregory A Cox

doi:10.1371/journal.pgen.1006019

Abstract

Latent TGFβ binding proteins (LTBPs) regulate the extracellular availability of latent TGFβ. LTBP4 was identified as a genetic modifier of muscular dystrophy in mice and humans. An in-frame insertion polymorphism in the murine Ltbp4 gene associates with partial protection against muscular dystrophy. In humans, nonsynonymous single nucleotide polymorphisms in LTBP4 associate with prolonged ambulation in Duchenne muscular dystrophy. To better understand LTBP4 and its role in modifying muscular dystrophy, we created transgenic mice overexpressing the protective murine allele of LTBP4 specifically in mature myofibers using the human skeletal actin promoter. Overexpression of LTBP4 protein was associated with increased muscle mass and proportionally increased strength compared to age-matched controls. In order to assess the effects of LTBP4 in muscular dystrophy, LTBP4 overexpressing mice were bred to mdx mice, a model of Duchenne muscular dystrophy. In this model, increased LTBP4 led to greater muscle mass with proportionally increased strength, and decreased fibrosis. The increase in muscle mass and reduction in fibrosis were similar to what occurs when myostatin, a related TGFβ family member and negative regulator of muscle mass, was deleted in mdx mice. Supporting this, we found that myostatin forms a complex with LTBP4 and that overexpression of LTBP4 led to a decrease in myostatin levels. LTBP4 also interacted with TGFβ and GDF11, a protein highly related to myostatin. These data identify LTBP4 as a multi-TGFβ family ligand binding protein with the capacity to modify muscle disease through overexpression.

Highlights

Latent TGFβ binding proteins (LTBPs) are extracellular matrix (ECM) proteins that bind and sequester the small latent complex of TGFβ
Increased levels of latent TGFβ binding protein 4 (LTBP4) protein resulted in increased muscle mass
LTBP4 formed a complex with myostatin, a protein that when inhibited leads to muscle growth

Summary

Introduction

Latent TGFβ binding proteins (LTBPs) are extracellular matrix (ECM) proteins that bind and sequester the small latent complex of TGFβ. There are more than 30 TGFβ family members, and the primary TGFβ family members 1, 2 and 3 are known to coordinate cell growth during development and regulate response to injury in mature cell and tissue types (reviewed in [1,2,3,4]). Two GDFs are especially important for muscle health. Deletion of GDF8, known as myostatin, leads to doubling of muscle mass in mice, and naturally occurring mutations in the MSTN gene result in increased muscle mass in large animals and humans [5,6,7,8]. GDF11 is nearly identical to GDF8/myostatin in its active domain, and, controversial, GDF11 has been linked to muscle wasting in aging [9, 10]

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