Investigating the role of the circadian clock and timed exercise on mouse skeletal muscle function

An inactive, sedentary lifestyle directly impacts health and increases the risk of
premature death. Likewise, circadian rhythm disruption (e.g., shift-work) is linked to various
chronic diseases, including diabetes, cardiovascular disease, and cancer. Regular physical
exercise is a potent regulator of skeletal muscle and whole-body metabolism and can prevent
or even treat chronic medical disorders, including diabetes, cancer, and cardiovascular
disease. Moreover, proper sleeping schedules, meal-timing, and chrono-medication all can
ameliorate many of the aforementioned diseases. It is unclear, however, whether a chronotherapeutic
approach of exercise can amplify the beneficial health effects of regular training,
and whether these effects would be mediated by skeletal muscle.
The first part of my Ph.D. work evaluated the transcriptomic, proteomic, and
phosphoproteomic responses of mouse skeletal muscle to two widely used exercise
modalities at distinct phases of the light-dark cycle. We found that maximal treadmill exercise
capacity varies according to the time of day, and give evidence of subtle changes in systemic
and muscle energy levels in response to exercise around the clock. Moreover, we reveal a
timely activation of specific biological pathways within working muscles. For instance,
mechanisms directing vesicular trafficking, phosphorylation of key regulators of glucose
uptake and calcium metabolism, and putative secreted factors affecting hepatic glucose
production were induced explicitly by early daytime treadmill exercise and could potentially
enhance running exercise capacity. We furthermore established a completely new
methodology to examine the effects of spontaneous wheel running. Using this method, we
give evidence that exercise is a poor modifier of daily clock gene expression. On the other
hand, we show that daytime wheel-running activity is a potent modifier of oxidative
metabolism gene expression under constant environmental light conditions; however, foodintake
has a more profound effect on the skeletal muscle clock. With our large-scale
transcriptomic, proteomic, and phosphoproteomic data, we provide resources for future
research projects and validation studies. We also hope that the use of scheduled wheel
running at different times of the day could be useful in preclinical mouse models.
Mouse models of whole-body clock gene deletion highlighted the potential role for
circadian clock components in muscle cell development and repair, insulin sensitivity, glucose,
and lipid metabolism. Yet, global gene deletion leads to multiple effects in other organs, which
can be confounding factors for the evaluation of muscle-specific function. The second part of
my Ph.D. work was to characterize the specific contribution of the core clock transcription
factor RORα in skeletal muscle physiology and exercise behavior. We show that skeletal
muscle-specific deletion of RORα significantly decreased spontaneous wheel-running activity
in the absence of significant alteration in muscle structure, and systemic metabolism.
Interestingly, in response to exercise, a decrease in the expression of key regulators of
oxidative stress was observed. Furthermore, upon exposure to hypoxia, causing abnormal ROS
elevation, or directly to H2O2, we reveal that RORα overexpression protects from cell death.
Our data suggest that skeletal muscle RORα is important for modulating ROS metabolism and
possibly the skeletal muscle adaptations to exercise.
Taken together, our data provide new evidence that skeletal muscle molecular and
cellular responses are dependent on the time of the day and systemic energy level. Notably,
we propose that mechanisms promoting glucose delivery and uptake in muscle tissues,
together with those critical for the response to inflammation and the secretion of myokines,
are tied to a time period of the day. Our results, moreover, suggest that there is an optimal
time to exercise to improve muscle adaptation and probably performance, as well as glucose
homeostasis. Finally, the last part of my Ph.D. work gives evidence that the circadian
component RORα is required to influence ROS metabolism and the cellular adaptations of
skeletal muscle to a prolonged period of exercise training.