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MRF4 negatively regulates adult skeletal muscle growth by repressing MEF2 activity

ABSTRACT: The myogenic regulatory factor MRF4 is expressed at high levels in myofibers of adult skeletal muscle, but its function is unknown. Here we show that knockdown of MRF4 in adult muscle causes hypertrophy and prevents denervation-induced atrophy. This effect is accompanied by increased protein synthesis and the widespread activation of genes involved in muscle contraction, excitation-contraction coupling and energy metabolism, many of which are known targets of MEF2 transcription factors. Genes regulated by MEF2 represent the top-ranking gene set enriched after Mrf4 RNAi, and a MEF2 reporter is inhibited by co-transfected MRF4 and activated by Mrf4 RNAi. The role of MEF2 in mediating the effect of MRF4 knockdown is supported by the finding that Mrf4 RNAi-dependent increase in fiber size is prevented by dominant negative MEF2, while constitutively active MEF2 is able to induce myofiber hypertrophy. The nuclear localization of the MEF2 co-repressor HDAC4 is impaired by Mrf4 knockdown, suggesting that MRF4 acts by stabilizing a repressor complex that controls MEF2 activity. The demonstration that fiber size in adult skeletal muscle is controlled by the MRF4-MEF2 axis opens new perspectives in the search for therapeutic targets to prevent muscle wasting, in particular sarcopenia and cachexia.

ORGANISM(S): Rattus norvegicus

PROVIDER: GSE67069 | GEO | 2016/07/15

SECONDARY ACCESSION(S): PRJNA278851

REPOSITORIES: GEO

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MRF4 negatively regulates adult skeletal muscle growth by repressing MEF2 activity

Project description:The myogenic regulatory factor MRF4 is expressed at high levels in myofibers of adult skeletal muscle, but its function is unknown. Here we show that knockdown of MRF4 in adult muscle causes hypertrophy and prevents denervation-induced atrophy. This effect is accompanied by increased protein synthesis and the widespread activation of genes involved in muscle contraction, excitation-contraction coupling and energy metabolism, many of which are known targets of MEF2 transcription factors. Genes regulated by MEF2 represent the top-ranking gene set enriched after Mrf4 RNAi, and a MEF2 reporter is inhibited by co-transfected MRF4 and activated by Mrf4 RNAi. The role of MEF2 in mediating the effect of MRF4 knockdown is supported by the finding that Mrf4 RNAi-dependent increase in fiber size is prevented by dominant negative MEF2, while constitutively active MEF2 is able to induce myofiber hypertrophy. The nuclear localization of the MEF2 co-repressor HDAC4 is impaired by Mrf4 knockdown, suggesting that MRF4 acts by stabilizing a repressor complex that controls MEF2 activity. The demonstration that fiber size in adult skeletal muscle is controlled by the MRF4-MEF2 axis opens new perspectives in the search for therapeutic targets to prevent muscle wasting, in particular sarcopenia and cachexia. Adult innervated and denervated rat soleus muscles were transfected with shRNA to Mrf4 (M1) or to LacZ as a control. Muscles were dissected and examined after 7 days. For each group/condition we selected three different muscles (biological replicas).

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Transcriptional profiling of adult wild-type IFM and leg muslce in comparison to salm RNAi IFM.

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Myf6/MRF4 Mediated Myokine Signaling is Required for the Maintenance of the Adult Skeletal Muscle Stem Cell Pool

Project description:In metazoans, skeletal muscle evolved to contract and to produce force. Recent experimental evidence suggests, however, that skeletal muscle has also acquired endocrine functions and produces a vast array of myokines, but how myokine production is regulated and how they affect the resident stem cell population of the skeletal muscle is unknown. Here, we show that in adult skeletal muscle, Myf6/MRF4 is a major regulator of myokine expression. Genetic deletion of Myf6 in skeletal muscles leads to exhaustion of the muscle stem cell (MuSCs) pool in adult mice in a myokine-dependent manner but, surprisingly, does not disrupt muscle differentiation. Using ChIP-Seq and gene expression analyses of myogenic factors, we show that Myf6/MRF4 is essential for the transcriptional activation of many myokines and muscle-secreted proteins, including ligands for canonical signaling pathways such as EGFR, VEGFR and STAT3. Consequently, MuSCs from Myf6 knockout animals show impaired activation of those signaling pathways and exhibit impaired quiescence, defective self-renewal, but nevertheless undergo differentiation. Lastly, we show that induction of myokine expression during aerobic and anaerobic exercise closely correlates with Myf6 expression. Together, these findings indicate that control of myokine signaling by Myf6 is critical to maintain muscle stem cell pool in adult skeletal muscle.

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