Signaling Pathways Regulating Protein Synthesis in the Rat Soleus Muscle in the Early Period of Gravitational Unloading

Abstract

The effects of different durations of gravitational unloading (antiorthostatic suspension of rats for 1, 3, or 7 days) on the intensity protein synthesis, ribosomal RNA contents, and intracellular signal pathways involved in controlling protein biosynthesis were studied in rat soleus muscle. The contents of key markers of the anabolic signal pathways (p-AKT, p-GSK-3β, p-p70s6k, p-4E-BP1, p-90RSK1) were determined by gel electrophoresis followed by immunoblotting. The intensity of protein synthesis in soleus muscle was evaluated by puromycin labeling, i.e., the SUnSET method. A significant decrease in protein synthesis intensity in the soleus muscle was seen after 3 and 7 days of gravitational unloading. Gravitational unloading for 3 and 7 days led to a significant decrease in the content of 28S rRNA. Functional unloading for 24 h led to a significant increase in the content of p-p70s6k and a decrease in the content of p-4E-BP1 in the soleus muscle (p < 0.05). By day 3 of suspension, there were decreases in the contents of markers such as phospho-AKT and phospho-p90RSK1 as compared with controls (p < 0.05). After 7 days of gravitational unloading, there were significant decreases in the levels of phosphorylated AKT and GSK-3β compared with the control group (p < 0.05). These results lead to the conclusion that decreases in the intensity of protein synthesis in rat soleus muscle at the early stages of gravitational unloading may be associated with suppression of ribosome biogenesis and decreases in the activity of mTORC1-dependent signal pathways.