Combined Effects Of Unloading And Radiation On Skeletal Muscle In Mice

Abstract

In preparation for upcoming space missions to the Moon and Mars, there is a need to understand how space stressors (e.g. microgravity, radiation) affect different physiological systems. As skeletal muscle is a critical organ, not only for locomotion but also for overall body homeostasis, defining the molecular impact of microgravity and radiation on this tissue will help developing new, or fine-tuning current countermeasures to maintain health and physiological function of space travellers. PURPOSE: To investigate the effects of combined radiation and unloading on anabolic/catabolic and immune/inflammatory processes on skeletal muscle in mice. METHODS: Ten C57/BL6J mice were subjected to 14-d hind-limb unloading by tail suspension with an acute radiation session (dose=25 mGy, X-ray) on day 7 of unloading (HLUR). Ten mice were used as control (CTRL; similar cages, sham radiation). Mice were sacrified and soleus muscle was immediately dissected, weighed and frozen. Then, RNA was extracted and converted to cDNA. Gene expression of anabolic/catabolic (i.e. myostatin, MuRF-1, Atrogin-1, PGC-1α) and immune/inflammatory markers (i.e. CD4, CD8, IFNγ, CD11b, MHCII, TNF, IL-6) was assessed by RT-PCR. Independent t-tests were use to compare HLUR vs. CTRL. RESULTS: Soleus muscle weight was ~30% lower in HLUR vs. CTRL (P<0.001). Myostatin expression was greater in HLUR vs. CTRL (1.8-fold, P=0.014). MHCII expression was higher in HLUR vs. CTRL (2.4-fold, P<0.001). There was a trend for group differences (P<0.08) in CD11b and TNF mRNA content with HLUR showing greater values than CTRL. Gene expression of CD4, CD8 and IFNγ was barely detected in either group. CONCLUSIONS: The combination of unloading and radiation has a major impact on skeletal muscle. Apart from inducing muscle atrophy, as indicated by the decreased muscle weight and increased myostatin levels, these two space stressors altered the immune profile within the muscle. The increased gene expression of MHCII and CD11b indicates that the myeloid component of the immune system is activated upon unloading and radiation in skeletal muscle. In contrast, the almost undetected mRNA levels of CD4, CD8 and IFNγ may imply that unloading and acute radiation have little impact on the lymphoid component. These findings should be followed up with immunohistochemical analysis.