MicroRNAs (miRNAs) are short RNA molecules that function as negative regulators of protein translation and participate in a variety of cellular processes such as development and disease. Data from experiments analyzing the role of miRNA gene regulation in aging pathways have been variable, so the precise role of miRNAs in aging systems remains to be determined.
In this study, Drummond et al looked at muscle-specific primary (pri-miRNA: the longer, nuclear localized, unprocessed miRNA transcript) and mature miRNAs (miRNA-1, 133a, and 206) in aging muscle before and after exercise. The researchers obtained leg muscle biopsies from 6 young (29 ±2yrs) and 6 older men (70 ±2yrs) before and after anabolic exercise with essential amino acid consumption.
Aging differentially affects human skeletal muscle microRNA expression at rest and following resistance exercise and essential amino acid ingestion. At baseline, we found pri-miRNA-1-1, -1-2, -133a-1, and -133a-2 expression elevated in old compared to young (P<0.05). Pri-miRNA-1-2, -133a-1, and -133a-2 were reduced at 6h post-exercise only in the young compared to baseline whereas levels of pri-miRNA-206 were elevated at different post-exercise time points in old and young (P<0.05). As compared to baseline, miR-1 was reduced only in the young while Rheb protein increased in both age groups following the anabolic stimulus (P<0.05). We conclude that skeletal muscle primary and mature miRNAs expression in young men is readily altered by an anabolic stimulus of resistance exercise + essential amino acid ingestion. However, aging is associated with higher basal skeletal muscle pri-miRNA expression and a dysregulated miRNA response following the anabolic stimulus.
The authors were appropriately conservative with their conclusions, considering many of their data were challenging to interpret. For example, the older men had significantly higher pri-miRNA levels (miRNA-1 and 133a) prior to the exercise regimen that did not translate to higher levels of mature miRNA. To decipher these results the researchers looked at miRNA processing proteins to determine if reduced amounts of those proteins could be the cause for this discrepancy, but found that none of the proteins analyzed (Dicer, Drosha, or Exportin5) were dysregulated as a function of age. The authors speculate that additional exercise or longer experimental conditions may reveal changes in mature miRNA that reflect the primary miRNA results. As we learn more about the regulation and function of miRNAs the cause for these discrepant data should become clear.
The researchers did find a reduction in pri-miRNA-1-2 which translated to reduced miRNA-1 levels after exercise in young men, indicating an acute, direct relationship between the primary transcript and the mature miRNA-1. Interestingly, the older men did not show this reduction in either pri-miRNA-1 or mature miRNA-1 indicating a disrupted/delayed response to acute resistance exercise. This finding adds support to their previous conclusions from analyses on the same muscle biopsies published in a previous report where they observe that older men have a delayed response in protein synthesis after exercise.
To see whether the reduced level of miRNA-1 translated to higher protein levels of known and predicted targets of miRNA-1, the researchers looked at protein expression of IGF1, HDAC4, MEF2 and Rheb. They did find higher expression of Rheb in both young and old patients but these results do not appear to be attributable to reduced miRNA-1 expression considering only the young patients had reduced miRNA-1 levels. So while the researchers observed decreased expression of miRNA-1, they do not find any altered protein expression in the limited number of proteins analyzed. More extensive analysis will need to be completed to identify the role of miRNA-1 in this system.
One suggestion for a future experiment would be to repeat these experiments using an animal model. Utilizing an animal model will allow the researchers to obtain an initial understanding of the role of many miRNAs in the exercised or aging muscle; without preliminary evidence it seems premature to use human tissue for this experiment. While animal models may not always be a true reflection of what happens in humans, the use of other animal systems allow for optimization of experimental design. I am excited to learn about a role for miRNAs in aging muscle but I feel that this study’s scope was too limited to convince me just yet.