Akt (protein kinase B) isoform phosphorylation and signaling downstream of mTOR (mammalian target of rapamycin) in denervated atrophic and hypertrophic mouse skeletal muscle

Abstract

BackgroundThe present study examines the hypothesis that Akt (protein kinase B)/mTOR (mammalian target of rapamycin) signaling is increased in hypertrophic and decreased in atrophic denervated muscle. Protein expression and phosphorylation of Akt1, Akt2, glycogen synthase kinase-3beta (GSK-3beta), eukaryotic initiation factor 4E binding protein 1 (4EBP1), 70 kD ribosomal protein S6 kinase (p70S6K1) and ribosomal protein S6 (rpS6) were examined in six-days denervated mouse anterior tibial (atrophic) and hemidiaphragm (hypertrophic) muscles.ResultsIn denervated hypertrophic muscle expression of total Akt1, Akt2, GSK-3beta, p70S6K1 and rpS6 proteins increased 2–10 fold whereas total 4EBP1 protein remained unaltered. In denervated atrophic muscle Akt1 and Akt2 total protein increased 2–16 fold. A small increase in expression of total rpS6 protein was also observed with no apparent changes in levels of total GSK-3beta, 4EBP1 or p70S6K1 proteins. The level of phosphorylated proteins increased 3–13 fold for all the proteins in hypertrophic denervated muscle. No significant changes in phosphorylated Akt1 or GSK-3beta were detected in atrophic denervated muscle. The phosphorylation levels of Akt2, 4EBP1, p70S6K1 and rpS6 were increased 2–18 fold in atrophic denervated muscle.ConclusionsThe results are consistent with increased Akt/mTOR signaling in hypertrophic skeletal muscle. Decreased levels of phosphorylated Akt (S473/S474) were not observed in denervated atrophic muscle and results downstream of mTOR indicate increased protein synthesis in denervated atrophic anterior tibial muscle as well as in denervated hypertrophic hemidiaphragm muscle. Increased protein degradation, rather than decreased protein synthesis, is likely to be responsible for the loss of muscle mass in denervated atrophic muscles.

The effects of heat stress on morphological properties and intracellular signaling of denervated and intact soleus muscles in rats

“…7F, I). Similar results were also reported by Norrby et al (2012) and MacDonald et al (2014). Due to the high levels of total proteins, it is likely that the phosphorylation rates of Akt in denervated muscles were calculated low even immediately after heat stress.…”

Titin‐based mechanosensing modulates muscle hypertrophy

“…UDD is known to induce a transient hypertrophy,18, 19 yet, the trigger for this hypertrophy response was unknown. We show that this trigger is the passive, cyclic stretch of the denervated hemidiaphragm, imposed by the contracting contralateral diaphragm.…”

“…To this end, we turned to unilateral diaphragm denervation (UDD) which induces a transient but strong hypertrophy response in the denervated, inactive hemidiaphragm. 18,19 This makes UDD ideal for directly testing the role of titin-based mechanosensing in muscle hypertrophy. UDD was applied in two mouse models in which titin stiffness is genetically altered: the RBM20 ΔRRM model.…”

High mTORC1 signaling is maintained, while protein degradation pathways are perturbed in old murine skeletal muscles in the fasted state

“…Inactivation of AKT has been suggested as a permissive step for MuRF1 and Fbxo32 expression(Glass 2003), although in vivo MuRF1 induction may occur in conjunction with high levels of AKT phosphorylation(Machida et al 2012;Norrby et al 2012). Food deprivation induces MuRF1 and Fbxo32 expression concomitant with reduced AKT phosphorylation, and the sensitivity of skeletal muscle to length of food deprivation decreases with age: we have previously shown that 8 h of fasting during light hours is sufficient to induce MuRF1 and Fbxo32 expression mRNA in skeletal muscles of juvenile (3 weeks) mice, but not young adult (3 m) mice(Shavlakadze et al 2013b).…”