Søren Nielsen – University of Copenhagen

PhD Søren Nielsen

Centre of Inflammation and Metabolism

Affiliated with UNIK project no. 14

Title of thesis: Myomir Regulation and Function in Human Skeletal Muscle

Thesis summery: MicroRNAs (miRNAs) are a class of non-coding short RNAs that are 20-24 nucleotides in length and involved in post-transcriptional gene regulation. To date, the human genome encodes 2042 miRNAs, subsets of which create tissue specific expression signatures with coordinated control over numerous signalling pathways. When my thesis was initiated, it was established that a small number of miRNAs are highly enriched in skeletal muscle (myomiRs) (miR-1, miR-133a, miR- 133b and miR-206) and play an essential role in regulating myogenesis. Since then, it has been shown that these myomiRs are also important for the control of hypertrophy and skeletal muscle metabolism. In the current thesis, I investigate the regulation of myomiRs in humans in vivo. Physical activity is a well established factor for maintaining whole body metabolic fitness. As miRNAs are potent regulators of gene expression networks, it is important to research how they are regulated by altered physiological conditions in vivo in humans. This will increase our understanding on why physical exercise has such a fundamental importance for human health. Therefore, the first aim with my thesis was to investigate the myomiR expression pattern in human skeletal muscle in response to physical activity and aspects of aging. The second aim was to examine the global miRNA alterations in the circulation in response to endurance exercise and training and to investigate if skeletal muscle was a potential source of release.

I first investigated the effect of altered metabolic state, physical activity, aging and circulating testosterone levels on myomiR expression. We examined myomiR expression in skeletal muscle before and after training in two separate human studies and found a clear downregulation of myomiRs in both studies. We further established that myomiRs quickly adjusted their expression back to original levels when the training programme was halted for two weeks, despite the fact that subjects still maintained an increased fitness level. Moreover, three of the four myomiRs were decreased in young men compared to elderly men. The elderly men had a lower fitness level, which could be a possible reason for their higher myomiR expression. In support we found a correlation between miR-133a/b and VO2max. The elderly men also had decreased circulating testosterone levels. However, our follow up studies clearly demonstrated that testosterone induces mir-133a/b. Thus, the age-dependent decline in testosterone and increase in miR133a/b seemed to occur as two separate events. We then combined training with testosterone depletion which still resulted in a downregulation of myomiRs, indicating that physical fitness is a more dominant regulatory factor of myomiRs. We further observed a gender-specific myomiR signature, again coinciding with differences in physical fitness. Finally, while myomiR expression remained stable during a hyperinsulinaemic–euglycaemic clamp, an acute bout of endurance exercise transiently increased mir-1 and mir-133a. Predicted mRNA targets of the myomiRs include components of the MAPK and TGF- β pathways which we partly verified at protein level. Global examination of circulating miRNAs in plasma in response to acute exercise revealed 19 differentially expressed miRNAs where 10 were up-regulated and 8 down-regulated. Among the upregulated miRNAs where the muscle specific miR-1, miR-133a and miR-133b. In addition, endurance training demonstrated altered expression levels for 13 miRNAs where 2 miRNAs were upregulated and 11 downregulated, Including miR-133a.

In conclusion, I have demonstrated that myomiRs quickly adapt to altered physiological conditions in humans. While testosterone positively regulates miR-133a and miR-133b, its effects are overridden by skeletal muscle endurance fitness which regulates all four myomiRs. Moreover, the miRNA plasma signature, including the myomiRs is modified by exercise and skeletal muscle is likely to be one source of secreted miRNAs.


Søren Nielsen at PubMed