Opposing effects of Gα12 and Gα13 loss on myotube size regulation via mTORC1 signaling.

To reduce the risk of diseases caused by a reduction in skeletal muscle mass and quality, it is important to understand the molecular mechanisms underlying the maintenance and improvement of skeletal muscle mass and quality. Gα12 and/or Gα13 have been implicated in the regulation of myotube size through the mechanistic target of rapamycin complex 1 (mTORC1) signaling; however, their specific and potentially distinct molecular mechanisms remain unknown. Knockdown and rescue experiments revealed that the loss of Gα12 decreased myotube size, whereas the loss of Gα13 increased it. Gα12 knockdown reduced the phosphorylation levels of mTORC1 signaling components (Akt, mTOR, and p70S6K) and the levels of puromycin-labeled proteins, whereas Gα13 knockdown increased these levels. Loss of Gα12 or Gα13 suppressed SRF-RE-dependent transcriptional activity. While expression of a constitutively active form of RhoA (RhoA-CA) activated SRF-RE activity, notably, RhoA-CA expression did not affect myotube size, nor did it alter myotube atrophy induced by Gα12 knockdown or hypertrophy induced by Gα13 knockdown. Depletion of Gα12 increased the mRNA expression of oxidative myosin heavy chain (MyHC) isoforms Myh7 and Myh2 and decreased the mRNA expression of Myh1 and Myh4, whereas depletion of Gα13 increased the mRNA expression of Myh7, Myh2, Myh1, and Myh4. These results indicate that loss of Gα12 induces myotube atrophy by suppressing mTORC1 signaling and protein synthesis, whereas loss of Gα13 induces myotube hypertrophy by enhancing these processes, likely independent of SRF-RE-mediated transcription. Notably, Gα12 and Gα13 oppositely regulated the mRNA expression of MyHC isoforms, particularly Myh1 and Myh4.