Gene silencing and microRNA discovery

One of the body's primary strategies for regulating its genome is a kind of targeted gene silencing orchestrated by small molecules called microRNAs, or miRNAs.

First observed only a few years ago, these molecules appear to inactivate messenger RNA, itself responsible for translating genes into proteins. Scientists have been eager to know more about miRNAs, clearly important players on the genetic field despite having gone unnoticed for so long. How are they produced? And how do they work?

In a series of studies published over the past year, a research team at The Wistar Institute has provided considerable insight into the world of miRNAs. In their first study, which appeared last year in Nature, they identified a two-protein complex, called the microprocessor, which controls the earliest steps in the creation of miRNAs in the cell nucleus. In their next study, published in Nature earlier this year, the Wistar group described a three-protein complex that picks up the process in the cell cytoplasm and carries it through to the maturation of the finished miRNAs.

Now, in new findings published online November 3 in Cell, Wistar professor Ramin Shiekhattar, Ph.D., and his colleagues report that the three-protein complex has been identified as RISC, a previously glimpsed but ill-defined molecular complex known to be involved in gene silencing. RISC, the new study demonstrates, not only oversees production of miRNAs, as described in the earlier study this year, but is also responsible for miRNA specificity in silencing particular messenger RNAs.

In RISC, two of the three components, Dicer and Argonaute 2, are enzymes bound together by the third member, TRBP. Dicer cuts double-stranded precursor molecules shaped like hairpins into pairs of short single-stranded miRNAs – in essence, nipping off the bend in the pin. RISC then unzips the two single-stranded miRNAs from each other and identifies and holds one as a guide to help it find the specific messenger RNA to be inactivated. Using complementarity to match the guide miRNA to a particular length of its target messenger RNA, Dicer and TRBP then hand over the messenger RNA for cutting by Argonaute 2. Still holding the guide miRNA, RISC then scouts for additional copies of its target messenger RNA to cut. The cutting destroys the messenger RNA, effectively silencing the gene from which it was transcribed.

"The two enzymes in the complex are like two scissors working together in a concerted fashion, connected and coordinated by the third member of the complex," Shiekhattar explains.

Another scientific question surrounding RISC was also resolved by the current study. Some investigators had theorized that the activity of RISC required ATP for energy. ATP, or adenosine triphosphate, is a molecule used to store and release energy for tasks throughout the body.

"The work of RISC is being accomplished with no energy requirement whatsoever," Shiekhattar says. "All of the activity – the separation of the strands, the multiple cutting steps, everything – is being done in the absence of any energy use."

Instead, he says, the different molecules involved have stronger and weaker affinities for each other that govern their stepwise associations and disassociations as the process unfolds.