Protein Arginine Methyltransferases Exhibit Distinct Cellular Localization and Function During Skeletal Muscle Disuse

Protein arginine methyltransferase 1 (PRMT1), PRMT4 (also known as co‐activator‐associated arginine methyltransferase 1; CARM1), and PRMT5 catalyze the methylation of arginine residues on target proteins, thereby mediating intracellular processes such as signal transduction and transcriptional control. Although only a few studies have investigated PRMTs in skeletal muscle, evidence strongly suggests that these enzymes regulate skeletal muscle plasticity. However, the function of PRMTs in response to disuse‐induced muscle remodelling remains unknown. Thus, our study objective was to determine whether denervation‐induced muscle disuse alters the cellular localization and specific methyltransferase activities of PRMT1, PRMT4, and PRMT5 in skeletal muscle within the context of early signaling events that precede muscle atrophy. Mice were subjected to 6, 12, 24, 72, or 168 hours of unilateral hindlimb denervation (DEN). The contralateral limb served as an internal control. Western blot analyses were employed to determine nuclear and cytosolic protein expression levels in the DEN gastrocnemius (GAST) muscle, relative to the contralateral, non‐DEN, control GAST muscle across the experimental time course. Muscle mass significantly decreased by ~25% in the DEN hindlimb following 168 hours of disuse. The PRMTs exhibited remarkable enzyme‐specific spatial and temporal expression in skeletal muscle in response to DEN. Nuclear PRMT1 content significantly decreased by 40% after 6 hours of DEN prior to increasing by ~2.8‐fold (p < 0.05) after 72 and 168 hours of disuse. PRMT4 levels in the myonuclei were significantly augmented by 92% at 72 hours, before decreasing by 32% (p < 0.05) following 168 hours. Nuclear PRMT5 protein content increased 2.8‐fold (p < 0.05) after 12 hours, but was significantly reduced by 43–63% following 72 and 168 hours of denervation. These unique expression profiles suggest that PRMTs have distinct functions in response to muscle disuse. Furthermore, since ~85% of PRMT protein content was found within the cytosolic compartment, it was not surprising that cytosolic PRMT levels reflected whole muscle PRMT expression. Interestingly, the levels of myonuclear peroxisome proliferator‐activated receptor‐γ coactivator‐1α (PGC‐1α) protein content, a master regulator of skeletal muscle phenotype, decreased 33% (p < 0.05) following 6 hours of denervation, which suggests a coordinated expression of PRMT1 and PGC‐1α mediated by a common upstream regulator(s). To assess PRMT methyltransferase activity, we next examined the myonuclear content of histone 4 arginine 3 (H4R3), H3R17, and H3R8, which serve as specific targets for PRMT1, PRMT4, and PRMT5 methylation, respectively. H4R3 and H3R17 marks increased 3.2‐ and 8.8‐fold after 168 hours of DEN, whereas H3R8 methylation was elevated 1.7‐fold following 12 hours of disuse. Our results suggest that alterations in PRMT1, PRMT4, and PRMT5 localization and function in response to skeletal muscle disuse are rapid and dynamic. This study provides evidence that PRMTs participate in skeletal muscle remodelling that occurs prior to, as well as during, muscle atrophy. Support or Funding Information Natural Sciences and Engineering Research Council of Canada and Canada Research Chairs

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