Akt-mediated phosphorylation controls the activity of the Y-box protein MSY3 in skeletal muscle
Background: The Y-box protein MSY3/Csda represses myogenin transcription in skeletal muscle by binding a highly conserved cis-acting DNA element located just upstream of the myogenin minimal promoter (myogHCE). It is not known how this MSY3 activity is controlled in skeletal muscle. In this study, we provide multiple lines of evidence showing that the post-translational phosphorylation of MSY3 by Akt kinase modulates the MSY3 repression of myogenin. Methods: Skeletal muscle and myogenic C2C12 cells were used to study the effects of MSY3 phosphorylation in vivo and in vitro on its sub-cellular localization and activity, by blocking the IGF1/PI3K/Akt pathway, by Akt depletion and over-expression, and by mutating potential MSY3 phosphorylation sites. Results: We observed that, as skeletal muscle progressed from perinatal to postnatal and adult developmental stages, MSY3 protein became gradually dephosphorylated and accumulated in the nucleus. This correlated well with the reduction of phosphorylated active Akt. In C2C12 myogenic cells, blocking the IGF1/PI3K/Akt pathway using LY294002 inhibitor reduced MSY3 phosphorylation levels resulting in its accumulation in the nuclei. Knocking down Akt expression increased the amount of dephosphorylated MSY3 and reduced myogenin expression and muscle differentiation. MSY3 phosphorylation by Akt in vitro impaired its binding at the MyogHCE element, while blocking Akt increased MSY3 binding activity. While Akt over-expression rescued myogenin expression in MSY3 overexpressing myogenic cells, ablation of the Akt substrate, (Ser126 located in the MSY3 cold shock domain) promoted MSY3 accumulation in the nucleus and abolished this rescue. Furthermore, forced expression of Akt in adult skeletal muscle induced MSY3 phosphorylation and myogenin derepression. Conclusions: These results support the hypothesis that MSY3 phosphorylation by Akt interferes with MSY3 repression of myogenin circuit activity during muscle development. This study highlights a previously undescribed Akt-mediated signaling pathway involved in the repression of myogenin expression in myogenic cells and in mature muscle. Given the significance of myogenin regulation in adult muscle, the Akt/MSY3/myogenin regulatory circuit is a potential therapeutic target to counteract muscle degenerative disease.
© 2015 De Angelis et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Received: 20 February 2015; Accepted: 29 April 2015; Published: 29 May 2015. We thank Daniela Palacios and Pier Lorenzo Puri (both of Fondazione Santa Lucia, Roma, Italy) for critical reading; Brian Williams (California Institute of Technology, Pasadena, US) for critical reading and proofreading; Barbara Wold (California Institute of Technology, Pasadena US) for helpful discussions, advice and support; Anita Bansal (City of Hope National Medical Center, Duarte, US) for help in proofreading. This work was funded by the Brain Foundation Grant and Beckman Institute Grant to Barbara Wold. We also thank the French Muscular Dystrophy Association (AFM-Telethon) for funding Libera Berghella laboratory and research. Authors' contributions: LDA performed and analyzed experiments, and revised the manuscript. SB performed and analyzed experiments, and revised the manuscript. LB conceived of the study, designed, and analyzed experiments, wrote and finalized the manuscript. All authors read and approved the final manuscript. The authors declare that they have no competing interests.
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