Complexity in intracellular regulation of protein balance following unilateral diaphragm denervation

Abstract

The balance between protein synthesis and degradation determines if there is a net loss (atrophy) or net gain (hypertrophy) in muscle mass. Recent studies suggest that PI3K‐dependent activation of Akt is an important regulator of both protein synthesis (via Erk and mTOR) and degradation (via the transcription factor FoxO1). Unilateral denervation (DNV) of the diaphragm muscle (DIAm) increases protein synthesis at 3d and degradation by 5d post‐DNV such that protein breakdown becomes evident by 5d after DNV. We hypothesized that DIAm DNV results in time‐dependent changes in Akt activation, leading to initial mTOR and subsequent FoxO1 activation. Akt, Erk, and mTOR phosphorylation and FoxO1 nuclear translocation were measured by Western blot analyses. At 1h and 6h post‐DNV, pAkt/Akt and pErk/Erk decreased. pErk/Erk increased at 1d and 3d, and pAkt/Akt increased at 5d post‐DNV. Although mTOR phosphorylation and expression increased 5d post‐DNV, the ratio of phospho‐mTOR/mTOR did not change at any time point post‐DNV. Nuclear translocation of FoxO1 increased 1d post‐DNV. Taken together, these results indicate that concomitant changes in PI3K/Akt, Erk, mTOR, and FoxO1 activity all contribute to, but do not completely account for, the time‐dependent changes in protein balance observed after DNV. Regulation of other signaling pathways, e.g., AMPK, and/or trophic influences, e.g., neuregulin, must be considered.