Mechanical loading stimulates hypertrophy in tissue‐engineered skeletal muscle: Molecular and phenotypic responses

Kathryn W Aguilar‐Agon,Mark P Lewis,Darren J Player,Neil R.W Martin,Andrew J Capel

doi:10.1002/jcp.28923

Kathryn W Aguilar‐Agon, Mark P Lewis + Show 3 more

Open Access

https://doi.org/10.1002/jcp.28923

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Abstract

Mechanical loading of skeletal muscle results in molecular and phenotypic adaptations typified by enhanced muscle size. Studies on humans are limited by the need for repeated sampling, and studies on animals have methodological and ethical limitations. In this investigation, three‐dimensional skeletal muscle was tissue‐engineered utilizing the murine cell line C2C12, which bears resemblance to native tissue and benefits from the advantages of conventional in vitro experiments. The work aimed to determine if mechanical loading induced an anabolic hypertrophic response, akin to that described in vivo after mechanical loading in the form of resistance exercise. Specifically, we temporally investigated candidate gene expression and Akt‐mechanistic target of rapamycin 1 signalling along with myotube growth and tissue function. Mechanical loading (construct length increase of 15%) significantly increased insulin‐like growth factor‐1 and MMP‐2 messenger RNA expression 21 hr after overload, and the levels of the atrophic gene MAFbx were significantly downregulated 45 hr after mechanical overload. In addition, p70S6 kinase and 4EBP‐1 phosphorylation were upregulated immediately after mechanical overload. Maximal contractile force was augmented 45 hr after load with a 265% increase in force, alongside significant hypertrophy of the myotubes within the engineered muscle. Overall, mechanical loading of tissue‐engineered skeletal muscle induced hypertrophy and improved force production.

Highlights

Skeletal muscle exhibits a high degree of plasticity and is responsive to both increased and decreased mechanical loading (Bodine, 2013; Sandri, 2008)
We hypothesised that progressive mechanical overload would induce an increase in specific mechano‐regulated gene transcripts and posttranslational responses similar to those observed in vivo, inducing myotube hypertrophy and augmenting maximal force production
Mechanical loading in both humans and animals result in skeletal muscle hypertrophy and increased maximal force production, with both components mediated through alterations in cellular signalling and gene expression

Summary

| INTRODUCTION

Skeletal muscle exhibits a high degree of plasticity and is responsive to both increased (exercise) and decreased (disuse) mechanical loading (Bodine, 2013; Sandri, 2008). Loading of skeletal muscle in vitro was traditionally accomplished through mechanical stretch and was first established in monolayer cultures of avian myotubes (Vandenburgh & Kaufman, 1979; Vandenburgh, Hatfaludy, Karlisch, & Shansky, 1989) In this seminal work, cyclic mechanical stretch of collagen embedded monolayer myotubes resulted in an increase in protein synthesis and myotube diameters and changes in the characteristic of skeletal muscle hypertrophy in vivo. Progressive mechanical overload of engineered muscle was used to determine its temporal effects on myotube hypertrophy, tissue function, mTOR signalling, and well‐known hypertrophic and atrophic transcripts (IGF‐1, MMP‐2, MMP‐9, MAFbx, and MuRF‐1). We hypothesised that progressive mechanical overload would induce an increase in specific mechano‐regulated gene transcripts and posttranslational responses similar to those observed in vivo, inducing myotube hypertrophy and augmenting maximal force production

| MATERIALS AND METHODS

| RESULTS

| DISCUSSION

Findings

CONFLICT OF INTERESTS