Exercise Induced Autocrine Myokines Mediate Myofiber Hypertrophy via PI(3)K/Akt/mTOR Signalling Pathway in Human Skeletal Muscle Tissues

Bakooshli, Mohsen Afshar 1 ; Bakooshli, Mohammad Ebrahim Afshar 1 ; Davoudi, Sadegh 1 ; Tung, Kayee 2 ; Ginsberg, Howard 1, 2, 3 ; Ahn, Henry 2 ; Gilbert, Penney M 1, 4, 5

1. Institute of Biomaterials and Biomedical Engineering, University of Toronto; 2. Department of Surgery, University of Toronto; 3. Li Ka Shing Knowledge Institute, Saint Michael’s Hospital; 4. Department of Biochemistry, University of Toronto; 5. Terrence Donnelly Centre for Cellular and Biomolecular Research, Toronto

Regular exercise has beneficial effects on physical well-being as well as to muscle health and is suggested as a non-pharmacological strategy to prevent muscle atrophy. Indeed, exercise induces a variety of changes in muscle properties such as muscle plasticity, metabolic regulation, contractile ability, fiber size, intracellular signaling, mitochondrial function, and transcriptional control. Moreover, symptoms of many muscle conditions, like Duchenne Muscular Dystrophy (DMD), do not manifest in the absence of muscle activity, and the therapeutic effects of many drugs often have synergistic effects when combined with exercise. However, the mechanisms that mediate the therapeutic effects of exercise remain obscure. 
In vitro models of human organs have improved our understanding of mechanisms that lead to disease as well as responses to clinical drugs. However, the majority of skeletal muscle culture models are two-dimensional platforms that are incapable of mimicking exercise and the few reports of three-dimensional (3D) models of skeletal muscle have not studied the effect of exercise in their systems. 
Here, we report the first 3D human skeletal muscle microtissue (hMMT) system in a high throughput 96-well plate platform capable of mimicking muscle exercise in response to light stimuli. We show that 5 days of light induced exercise leads to higher expression of myosin heavy chain and myofiber hypertrophy compared to sedentary/control hMMTs that lead to higher force generation by exercised tissues. Our pathway analysis indicated the activation of the PI(3)K/Akt/mTOR pathway as early as 3 hours post exercise leading to protein translation. In addition, our media supernatant studies indicated the modulation of the hMMT secretome post exercise. Moreover we show that the media from exercised hMMTs is sufficient to activate the PI(3)K/Akt/mTOR pathway in sedentary tissues confirming the autocrine role of exercise induced myokines from myofibers. Finally we confirmed the role of IL-4 as an exercised induced myokine from muscle fibers that is capable of activating PI(3)K/Akt/mTOR pathway in sedentary hMMTs.