Short-, medium- and long-term effects on skeletal muscles in pre-flight tests of MDS mission

The Mouse Drawer System (MDS) is an automated rodent spaceflight payload designed to accommodate mice in individual cages on board the International Space Station. The MDS cage size is smaller than standard laboratory cages and might affect overall mouse activity and muscle properties. Therefore we examined the effects of 20-, 60- and 100-day housing in MDS cages on skeletal muscle properties, as a ground control experiment in view of the scheduled spaceflight mission. In particular, we evaluated muscle morphology and some physiological properties, as well as the expression of different genes, including critical atrophy genes. After 20 days, body weight of MDS mice slightly decreased compared to controls, whereas control and MDS mice showed similar weight in the 60- and 100-day tests. No major differences were observed in the mean fiber cross sectional area of fast (EDL) and slow (soleus) muscles from control and MDS mice. Electrophysiological properties were comparable in control and 60-day MDS mice. Expression of ubiquitin-ligases and autophagic genes was slightly up-regulated in the short-term MDS test only. SDS-PAGE analysis of myosin heavy chain composition demonstrated slight modifications only in the 20-day MDS EDL muscle and the 60-day MDS soleus muscle. In conclusion, our preliminary analyses point to minor adaptive changes in hind limb muscles of mice housed for 20 days in MDS cages, whereas no significant change was observed at later stages, suggesting that the MDS payload is suitable for studies on skeletal muscle in future spaceflight missions.