Donahue, H. | Search bioRxiv

bioRxiv2020

Muscle Atrophy: Different Strains of Mice Respond Differently to Single Limb Immobilization

To examine whether genetic variability plays a role in muscle adaption to its mechanical environment, we examined hind limb immobilization in five different strains of mice: CAST/EiJ, NOD/ShiLtJ, NZO/HILtJ, 129S1/SvImJ and A/J. Mice had one limb immobilized by a cast for three weeks. The response to immobilization was dependent on the strain of mice examined. A/J mice lost the most body weight following immobilization and displayed a significant decrease in physical activity. None of the other strains displayed a significant decrease in activity. Food consumption was significantly increased in NOD/ShiLtJ mice. All other strains had non-significant changes in food consumption. Muscle mass/body weight was decreased by immobilization, to varying degrees, in all strains except 1291/SvImJ. Casting decreased absolute muscle mass in both quadriceps and gastrocnemius in NOD/ShiltJ and NZO/HILtJ mice, two strains that can develop diabetes, but not in the other strains. Three weeks of immobilization caused a significant increase in quadriceps levels of atrogenes in CAST/EiJ mice but not in other strains. Immobilization caused a significant increase in quadriceps and gastrocnemius levels of Myh4 in CAST/EiJ mice but not in any other strain. A similar trend was observed for Myh7 in gastrocnemius muscle. Immobilization resulted in a decrease the p-p70S6K1/total p706SK1 ratio in quadriceps of NOD/ShiLtJ mice and the gastrocnemius of A/J mice, but not in other strains. Immobilization did not affect the p-4EBP1/total 4EBP1 ratio in quadriceps of any of the strains examined. However, the p-4EBP1/total 4EBP1 ratio in gastrocnemius was greater in immobilized, relative to control, limbs in CAST/EiJ mice. Muscle mass normalized to body weight in both gastrocnemius and quadriceps displayed the greatest degree of heritability. These results reveal remarkable variability in responsiveness to immobilization across five different genetic strains of mice.

physiology↗

bioRxiv2020

Mechanical Loading Recovers Bone but not Muscle Lost During Unloading

Space travel and prolonged bed rest are examples of mechanical unloading that induce significant muscle and bone loss. To explore interactions between skeletal bone and muscle during reloading, we hypothesized that acute external mechanical loading of bone in combination with re-ambulation facilitates proportional recovery of bone and muscle lost during hind limb suspension (HLS) unloading. Adult male C57Bl/6J mice were assigned to a HLS or time-matched ground control (GC) group. After 2-weeks of HLS, separate groups of mice were studied at day 14 (no re-ambulation), day 28 (14 days re-ambulation) and day 56 (42 days re-ambulation); throughout the re-ambulation period, one limb received mechanical loading and the contralateral limb served as an internal control. HLS induced loss of trabecular bone volume (BV/TV; -51%{+/-}2%) and muscle weight (-15%{+/-}2%) compared to GC at day 14. At day 28, the left tibia (re-ambulation only) of HLS mice had recovered 20% of BV/TV lost during HLS, while the right tibia (re-ambulation and acute external mechanical loading) recovered to GC values of BV/TV ([~]100% recovery). At day 56, the right tibia continued to recover bone for some outcomes (trabecular BV/TV, trabecular thickness), while the left limb did not. Cortical bone displayed a delayed response to HLS, with a 10% greater decrease in BV/TV at day 28 compared to day 14. In contrast to bone, acute external mechanical loading during the re-ambulation period did not significantly increase muscle mass or protein synthesis in the gastrocnemius, compared to re-ambulation alone.

physiology↗