The general mechanism behind miRNA sounds simple enough: These single-stranded RNAs are complementary to some section of a mRNA, and binding between the two blocks the mRNA from making its protein. As a result, the miRNA reduces the output of that protein.
The details behind the miRNA pathway, though, get more complex. First, an miRNA’s gene and RNA polymerase II produce a string of RNA nucleotides—a primary transcript called pri-miRNA—that includes a double-stranded stem topped with a loop.
An enzyme complex, composed of Drosha and Pasha, cleaves the pri-miRNA to leave only the stem-and-loop section. This so-called pre-miRNA moves from the nucleus to the cytoplasm, where Dicer, another enzyme, clips off the loop. Further processing leaves only a short, single-stranded molecule, miRNA.
Despite unraveling these steps of the biochemical pathway from DNA to miRNA, many details remain unresolved. Beyond the protein complexes that participate in shaping miRNA, other molecules coordinate the cellular translocation from the nucleus to the cytoplasm.
Moreover, the RNA can get edited along the way. In fact, some miRNAs must be edited to be turned on, and others stop working after editing. In addition, mature miRNA might get stored in specific cellular compartments, even going back to the nucleus.
To really understand how miRNAs control gene expression under normal conditions, researchers must refine their knowledge of the mechanism behind this regulation.
For example, scientists want to know the details of how a particular miRNA regulates a specific target, because it might block some targets and merely modulate others. In addition, a better understanding of normal miRNA processes will help scientists understand what causes miRNA regulation to go wrong in a disease and how that problem might be treated or repaired.
In using miRNA as a research tool, though, researchers face further obstacles. For one thing, the short stretch of an miRNA allows it to bind with many mRNAs. In addition, to find out what miRNA does in a specific tissue, a scientist needs a way to get miRNA to the right location. The right spot is not enough, however; it must also avoid the wrong places.
This is particularly crucial in developing miRNA-based therapeutics. For instance, if a scientist developed an miRNA that battles cancer but is also toxic to the liver, it is a useless therapy.
In studying the normal mechanisms of miRNA, researchers must pay attention to tissue-specific functions as well as functions related to a specific developmental stage. Consequently, researchers must note place and time in miRNA-based studies.