Signalling role of skeletal muscle during exercise

Abstract Upon acute exercise skeletal muscle is immediately and heavily recruited, while other organs appear to play only a minor role during exercise. These other organs show significant changes and improvements in function, although they are not directly targeted by exercise. These improvements suggest that skeletal muscle can communicate with other organs. In the past fifteen years it became clear that skeletal muscle produces and secretes a variety of signalling proteins that are able to interact and communicate with other organs. These signalling proteins are called myokines and are likely the link between exercising muscle and the rest of the body. The aim of the research presented this thesis is to study the signalling role of skeletal muscle during exercise and to gain further insight in the local molecular changes in skeletal muscle induced by exercise. In the first part of this thesis the focus was on the local changes induced in skeletal muscle by acute exercise and exercise training. The aim was to gain more insight in the molecular basis of exercise-induced changes in skeletal muscle. First we performed a microarray analysis on human muscle biopsies taken from endurance, resistance and combined exercise training interventions. We showed that despite a substantial overlap between the three exercise training types, each of the exercise training types had an unique gene expression print. The gene expression print found in combined exercise training lacked some specific oxidative and PPAR related components compared to the gene expression print found in endurance exercise training. For acute exercise microarray analysis was performed on muscle biopsies taken before and after an one-legged cycling intervention from resting and exercising skeletal muscle. Results showed that acute exercise induced large gene expression changes in active skeletal muscle. Furthermore, results showed that acute exercise also induced gene expression changes in resting skeletal muscle and that these changes were likely systemically induced via free fatty acids. In the second part of this thesis the focus was on the signalling role of skeletal muscle during exercise. Secretome analysis was performed on the microarrays of the muscle biopsies taken before and after the one-legged cycling intervention. This secretome analysis resulted in a list of putative myokines of which a selection was measured in the plasma. These plasma measurements showed that CCL2 (MCP-1) and CX3CL1 (Fractalkine) increased plasma levels during acute exercise. The findings of the one-legged cycling study furthermore showed that Angptl4 mRNA levels were higher in the resting leg compared to the exercising leg. Follow-up studies using cell culture and mice models revealed that Angptl4 levels were increased in the resting leg via free fatty acids that activated PPARs. In the exercising leg the increased Angptl4 levels were inhibited via AMPK activation. This resulted in an influx of triglyceride derived fatty acids in the exercising, but not in the resting skeletal muscle. In conclusion, we showed that exercise not only elicits molecular changes in active or trained skeletal muscle, but also in non-active organs such as resting skeletal muscle. Furthermore, we were able to identify several myokines produced by skeletal muscle during exercise, of which CCL2, CX3CL1 and Angptl4 were the most promising. CCL2, CX3CL1 and Angptl4 all increased plasma levels during acute exercise. It remains unclear what the systemic role is of these myokines. For Angptl4 we were able to provide more insight in the mechanism and local functioning during exercise. We concluded that Angptl4 is important in the substrate distribution during exercise. From the results presented this thesis we conclude that skeletal muscle has an important signalling role during exercise, but that it remains unclear how important this signalling role is systemically.