Regulation of Contractile Proteins and Protein Translational Signaling in Disused Muscle

Background/Aims: Muscle disuse can lead to muscle atrophy and impaired skeletal muscle function. How skeletal muscle modulates protein translational signaling in response to prolonged muscle disuse is not well understood. Using the hindlimb unloading (HU) model of muscle atrophy we examined how hindlimb unweighting affects protein translational signaling, including the activation of Akt/mTOR/p70S6K/S6 signaling and the inhibitory association of 4EBP1 with translation initiation factor eIF4E. Methods: Male F344BN rats were randomized into baseline control, or subjected to HU for 3, 7 or 14 days. Body weight, gastrocnemius muscle, and individual myofiber cross-sectional area were measured to evaluate the degree of muscle atrophy. The amounts of myosin and related muscle contractile proteins were assessed using SDS-PAGE and immunoblotting. Microarray analysis was used to evaluate changes in the mRNA expression of muscle contractile proteins. Total and phosphorylated proteins of Akt/mTOR/p70S6K/S6 pathway were determined via immunoblotting, while the association of 4EBP1 with eIF4E was detected via co-immunoprecipitation. Results: Unloading for 3 days significantly reduced cytosolic myosin content and was associated with increased binding of 4EBP1 to eIF4E, while prolonged unloading (14 days) was associated with the activation of Akt/mTOR/p70S6K/S6 signaling, decreased binding of 4EBP1 to eIF4E, increased cytosolic myosin and elevations in myofibrillar mRNA levels. Conclusion: Taken together, these data suggest that prolonged muscle disuse induces a biphasic translational signaling response that is associated with diminished and then increased muscle contractile protein expression