What exactly is a miRNA? miRNAs or microRNAs are tiny segments of RNA. Where most RNA segments are about 1,400 nucleotides long, miRNAs are typically only about 22 nucleotides long. While miRNAs are sometimes written out by themselves on DNA (as in the video above), it's more common to see miRNAs created as a byproduct of the creation of other kinds of RNA. Unlike many other types of RNA, miRNAs don't encode for proteins or assist in their transcription of RNAs to protein. Instead, miRNAs prevent proteins from being made from mRNAs by latching onto them before they can be transcribed. In the past few years, miRNAs have been implicated in certain types of cancer, and been used as biomarkers to help detect and diagnose diseases.
All of this is very exciting on its own, but more exciting still is a paper recently published in Nature Cell Research which suggests that miRNAs can actually travel from material in the digestive system of an organisms you eat into its bloodstream. Scientists at Nanjing University, China found a certain miRNA from rice, called MIR168a, in the bloodstreams of both humans and mice. By varying the diets of lab mice, they were subsequently able to show that the miRNAs found in mouse blood were being ingested.
This alone is an important discovery, but the researchers further "hypothesized that [plant miRNAs] may play a role in regulating the functions of mammalian cells and organs". Looking through the known sequences of mouse and human mRNAs, they identified about 50 proteins which MIR168a was likely to interfere with, including LDLRAP1, a gene that codes for a liver protein in both humans and mice. By exposing a certain type of liver cell (HepG2) to increased miRNAs, they determined that "Plant MIR168a...significantly decreased the LDLRAP1 protein level in the recipient HepG2 cells" while LDLRAP1 mRNA levels remained unaffected. In other words, the miRNA was preventing the transcription of the LDLRAP1 mRNA, as expected.
Why care about this result? It further clouds the waters regarding the role environment plays gene expression, an area of much of interest in biology right now. In addition, if miRNA uptake turns out to be common (or at least easy to induce) in humans then it could lead to whole new classes of oral drugs that target the expression of specific proteins.