Time of Day Dependent Effects of Contractile Activity on the Phase of the Skeletal Muscle Clock

Exercise has been proposed to be a zeitgeber for the muscle circadian clock mechanism. However, this is not well defined and it is unknown if exercise timing induces directional shifts of the muscle clock. Our purpose herein was to assess the effect of one bout of treadmill exercise on skeletal muscle clock phase changes. We subjected PERIOD2::LUCIFERASE mice (n=30F) to one 60-minute treadmill exercise bout at three times of day. Exercise at ZT5, 5h after lights on, induced a phase advance (1.4{+/-}0.53h; p=0.038), whereas exercise at ZT11, 1h before lights off, induced a phase delay (-0.95{+/-}0.43h; p=0.0315). Exercise at ZT17, middle of the dark phase, did not alter muscle clock phase. Exercise induces diverse systemic changes so we developed an in-vitro model system to examine effects of contractile activity on muscle clock phase. Contractions applied at peak or trough Bmal1 expression induced significant phase delays (applied at peak: 1.3{+/-}0.02h; p=0.0425; applied at trough: 1.8{+/-}0.02h, p=0.0074). Contractions applied during the transition from peak to trough Bmal1 expression induced a phase advance (1.8{+/-}0.03h; p=0.0265). Lastly, contractions at different times of day resulted in differential changes of core-clock gene expression demonstrating an exercise and clock interaction, providing insight into potential mechanisms exercise-induced phase shifts. These data demonstrate that muscle contractions, as part of exercise, are sufficient to shift muscle circadian clock phase, likely through changes in core-clock gene expression. Additionally, our findings that exercise induces directional muscle clock phase changes confirms exercise is a bone fide environmental time cue for skeletal muscle.