Mapping of the Contraction‐Regulated Phosphoproteome Identifies TRIM28 as a Key Regulator of Skeletal Muscle Size

Abstract

Mechanical signals, such as those which are evoked during maximal intensity contractions (MIC), can induce an increase in skeletal muscle size. It has been widely concluded that this process is driven by the activation of rapamycin‐sensitive / mTORC1‐dependent signaling; however, recent studies have revealed that mTORC1‐independent signaling events might also be involved. Thus, in an effort to identify these events, we generated a comprehensive map of the MIC‐regulated, and rapamycin‐sensitive phosphoproteomes. In total, we identified over 2,400 unique MIC‐regulated phosphorylation events, of which, nearly 2,200 were unaffected by rapamycin. Interestingly, one of the most robust MIC‐regulated and rapamycin‐insensitive phosphorylation events was located on the S473 residue on a protein named TRIM28. This was intriguing because TRIM28 is a transcriptional intermediary factor that functions in a variety of biological processes and many of its regulatory effects are dependent on the phosphorylation of the S473 residue. Moreover, a recent study revealed that TRIM28 interacts with several skeletal muscle regulatory factors (e.g., MyoD and Mef2) and their transcriptional co‐repressors and co‐activators. Thus, to investigate the role of TRIM28 in the regulation of muscle size, we generated skeletal muscle specific and inducible TRIM28 knockout mice and then subjected these mice to various forms of mechanical stimuli. Collectively, our results indicated that: i) TRIM28 is not required for the mechanical activation of mTORC1 signaling, ii) TRIM28 plays a key role in the maintenance of normal muscle size, and iii) TRIM28 significantly contributes to the pathway via which mechanical stimuli induce hypertrophy. Taken together, these outcomes establish TRIM28 as a novel, mTORC1‐independent, component of the pathway via which mechanical signals regulate skeletal muscle size.Support or Funding InformationNIHR01AR057347