Molecular insights into human soleus muscle atrophy development: long-term dry immersion effects on the transcriptomic profile and post-translational signaling

Muscle disuse results in complex signaling alterations followed by structural and functional changes, such as atrophy, force decrease and slow-to-fast fiber-type shift. Little is known about human skeletal muscle signaling alterations under long-term muscle disuse. In this study, we describe the effects of 21-day dry immersion on human postural soleus muscle. We performed both transcriptomic analysis and Western blots to describe the states of the key signaling pathways regulating soleus muscle fiber size, fiber-type, and metabolism. 21-day dry immersion resulted in both slow-type and fast-type myofibers atrophy, downregulation of rRNA content, and mTOR signaling. 21-day dry immersion also leads to slow-to-fast fiber-type and gene expression shift, upregulation of p-eEF2, p-CaMKII, p-ACC content and downregulation of NFATc1 nuclear content. It also caused massive gene expression alterations associated with calcium signaling, cytoskeletal parameters, and downregulated mitochondrial signaling (including fusion, fission, and marker of mitochondrial density). Overall design: Eight young healthy male volunteers participated in this study. Their characteristics were next: the average age is 29.3 ± 3.56 years, the height is 175.8 ± 0.03 cm, the body weight is 73.24 ± 10.97 kg, the body mass index (BMI) is 23.2 ± 3.24 kg/m2 (mean ± standard deviation). A ground-based hypogravity model, dry immersion (DI), was used as an unloading model (Figure 1). The duration of DI was 21 days. Under an immersion environment, the volunteer's body is subjected to an impact of the buoyant force of water, and the support reactions are eliminated. The studies were carried out under standard conventional conditions, during which volunteers were placed one at a time in a pool measuring 200 x 100 x 100 cm, filled with water, in a horizontal position, the water temperature in the pool was maintained constant at +33 ± 0.5 °C. The daily routine in immersion was regulated by time intervals selected for examinations, preventive measures (if any), meals and sanitary and hygienic procedures, the participants of the study ate and slept inside the pool. The detailed description of the experiment, including diet and inclusion criteria, was published previously (17, 18). Briefly, the daily routine was as close as possible to the space station life, including 8 hours of sleep, 3 meals per day, a medical monitoring program and experimental studies. To carry out sanitary and hygienic measures, once a day the volunteers removed from the bath for 15–20 minutes. Periods of verticalization are excluded and all manipulations during the hygienic toilet of the volunteers are carried out in a horizontal position, if possible. All subjects gave written consent to participate in the experiment. The experimental protocol was approved by the Commission on Biomedical Ethics of the Institute of Biomedical Problems, Russian Academy of Sciences (Moscow, Russia) – Physiological Section of the Russian Committee on Bioethics under the Russian Federation Commission for UNESCO (protocol ? 483, August 3rd, 2018). All procedures complied with the Declaration of Helsinki, international and Russian legislation.