Investigating the Role of Exercise-Induced Circulating Extracellular Vesicles in Adult Myogenesis and Neurogenesis
Physical activity brings about a widespread physiological response and elicits the benecial adaptation of several tissues and organs. Furthermore, regular participation in physical activity reduces the risk of developing major non-communicable diseases such as cardiovascular disease, diabetes, cancer, osteoporosis, and dementia. Two important processes known to occur following physical activity are adult myogenesis and adult neurogenesis; both of which involve the activation and proliferation of specialised tissue-resident stem cells, which subsequently integrate into the existing tissue by dierentiating into mature functional cells. The molecular mechanisms regulating these processes following exercise are poorly understood to date. A possible contributing mechanism is the release of extracellular vesicles into the circulation during exercise: in many physiological and pathological conditions these vesicles carry functional cargo molecules which are protected from degradation in the circulating milieu by a robust lipid bilayer, enabling them to transfer signals over long distances and generate phenotypic changes in recipient cells. The origin and functional signicance of circulating extracellular vesicles remain virtually unexplored in the context of physical activity and adult myogenesis or neurogenesis. This project therefore aimed to enrich and count small circulating extracellular vesicles from healthy rested and exercised volunteers, and to assess their eects on recipient muscle and neural stem cell proliferation and dierentiation. Potential mechanistic actions were also explored by proteomic analyses. The number of circulating extracellular vesicles did not increase following moderate intensity cycling exercise, but the proliferation of myoblasts treated with these vesicles did. Moreover, proteomic analysis of myoblasts treated with post-exercise vesicles revealed an increase in several post-translational modications indicative of ERK1/2 and Akt pathway activation. Proliferating neural stem cells treated with plasma vesicles adhered to the culture surface in chain formations: potentially a sign of stem cell migration and the onset of dierentiation. Dierentiating neural stem cells treated with post-exercise plasma vesicles exhibited increased commitment to a neuronal fate over other (glial) neural cell types. Mass spectral analysis of the circulating extracellular vesicles conrmed enrichment of vesicle-associated proteins, but also hinted at contamination with other blood components such as lipoproteins and plasma proteins which impeded identication of any vesicle cargo increased by exercise. Methods used to enrich small extracellular vesicles from human plasma were consequently evaluated for their ability to separate these vesicles from other plasma components, to help shed light on which molecules might mediate changes in stem cell behaviour following physical activity. The work presented in this thesis suggests that exercise-induced circulating extracellular vesicles could play a role in mediating myoblast proliferation and neural stem cell migration processes, plus encourage commitment of neural stem cells to a neuronal fate. In the case of myoblasts, extracellular vesicles appear to be acting via ERK1/2 and Akt pathways to promote cell proliferation following exercise. Future technological advancements in separating plasma EVs from other blood components such as lipoproteins may allow subsequent work to further tease apart their origin and mechanistic action.
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