4E-BP1/2 Deletion Enhances mRNA Cap-binding Complex Assembly and Protein Synthesis in Immobilized Skeletal Muscle But is Not Sufficient to Prevent Muscle Atrophy.

Skeletal muscle atrophy in long-term care patients is positively correlated with worse health outcomes and patient mortality. Immobilization-induced muscle atrophy is due to an imbalance between protein synthesis and protein degradation. The imbalance favors degradation due to anabolic resistance within the muscle. Targeting regulators of protein synthesis to increase mRNA translation under immobilized conditions may improve the protein synthesis/degradation imbalance and ameliorate immobilization-induced skeletal muscle atrophy. 4E-BP1 and 4E-BP2 (eukaryotic initiation factor 4E (eIF4E) binding proteins 1 and 2) are important regulators of mRNA translation initiation that bind to eIF4E and prevents its binding to the scaffold protein eIF4G, an essential and rate-limiting step of cap-dependent mRNA translation. Previous studies have implicated eIF4E and its binding proteins in disuse-induced repression of muscle protein synthesis. Consequently, we hypothesize that eliminating 4E-BP1/2 may enhance mRNA translation and protect against disuse-induced muscle atrophy. The hindlimb muscles of 8-9-week-old C57Bl/6 mixed-sex wild-type (WT) (20.25±2.39 g; n=19) and global 4E-BP1/4E-BP2 double knockout (DKO) (20.55±2.65 g; n=19) mice were compared after 3 days of unilateral hindlimb immobilization. Each mouse had one hindlimb immobilized in the plantarflexed position while the contralateral nonimmobilized hindlimb served as a control. Food and water were provided to the mice ad libitum. The hindlimb muscles were removed after a 4-6 h fast, and the weights were compared between both genotypes for both the immobilized and free limbs. The gastrocnemius muscles were homogenized, eIF4E was immunoprecipitated, and eIF4E, eIF4G, and 4E-BP1 in the immunoprecipitated sample was assessed by western blot analysis. In the immobilized gastrocnemius of the DKO mice the amount of eIF4G associated with eIF4E was higher than in control mice (p=0.001). In addition, mRNA translation, as assessed in a separate study by the incorporation of puromycin (0.04 μmol puromycin/g of body mass) into protein in the gastrocnemius, was also higher in muscle of the immobilized leg of DKO (n=24) compared to control mice (n=26) (17.1% increase, p=0.012). Surprisingly, eIF4E binding protein deficiency did not prevent muscle atrophy, as the mass of the gastrocnemius, plantaris, and soleus muscles was reduced to a similar extent in the immobilized compared to the control leg of both DKO and wildtype mice. To assess a possible role for upregulation of protein degradation in disuse-induced muscle atrophy, expression of the E3 ubiquitin ligase, MAFbx-1 (muscle atrophy f-box 1) was also assessed. MAFbx-1 expression was elevated in the gastrocnemius of the immobilized compared to the contralateral non-immobilized limb, though no significant difference was found between genotypes. More research into other components of the mRNA cap binding complex, e.g., eIF4A, and additional biomarkers of protein degradative pathways may reveal more details on the mechanism of immobilization-induced muscle atrophy.