microRNA – the further into the forest, the more firewood

Author: Peter Starokadomsky (together with Natalia Pavel).
Biomolecule

The traditional role of microRNAs (miRNAs) – small RNA molecules that do not encode proteins – is considered to be the repression of gene activity and, in particular, protein synthesis. However, a new study has shown (for the umpteenth time!) that the functions of these molecules are much broader: in certain cases, they can stimulate translation rather than block it.

In the last ten to fifteen years, the basic dogma of molecular biology ("DNA-RNA-protein") has been significantly loosened... that is, expanded due to the fact that a great many molecular mechanisms that break out of this harmonious concept have been discovered. Discoveries related to short, non-protein-coding RNA molecules have become revolutionary: this includes the famous phenomenon of RNA interference (awarded the Nobel Prize shortly after the discovery) and other mechanisms of RNA-dependent gene repression. One of the varieties of short RNA – microRNA (miRNA) is actively involved in the processes of individual development of the body, including temporary control, death, proliferation and differentiation of cells, embryonic laying of organs. They fine-tune gene expression at the post-transcriptional level, thereby adding another level of complexity to the sophisticated mechanism of intracellular regulation. Initially discovered in the "laboratory" nematode C.elegans, miRNAs were then found in many plants and animals, and relatively recently in unicellular organisms [1].

Previously, it was believed that short RNAs are used by cells in the process of RNA interference for specific degradation of unnecessary or harmful RNAs [2] - in particular, in this way a cell can destroy foreign genetic material of viruses, related retrotransposons and other mobile elements, as well as RNAs formed as a result of transcription of genomic repetitive sequences. Therefore, it was logical to assume that short RNAs (in particular, short interference RNA, kiRNA) serve as a kind of prototype of the "immune system" inside the cell. As our ideas about the participants and mechanisms of RNA-dependent gene repression developed, more and more interesting features were discovered, and a rich variety of ways of implementing this repression existing in nature was revealed.

In an actively dividing cell, miRNA, binding to a complementary sequence in a 3’-untranslated mRNA region, inhibits protein synthesis (translation). However, in a resting cell, the same event leads to the exact opposite effect. The picture is taken from [5].

The mechanism of action of most miRNAs is largely similar to RNA interference - short (21-25 nucleotides) single–stranded RNA as part of a protein complex (the key component of which is the Argonaute family protein) binds with high specificity to a complementary site in the 3’–untranslated region (3’-NTO) of the target mRNA. In plants whose miRNAs are completely complementary to the mRNA segment of the target, binding leads to the splitting of the mRNA by the Argonaute protein right in the middle of the miRNA-mRNA duplex – a situation closest to the "classical" RNA interference. In animals, miRNAs are not completely complementary to their target, and the binding result is different. For a long time it was believed that binding leads to the suppression of translation (the mechanism of which still remains a mystery) and does not cause any noticeable degradation of the target mRNA. However, it was later convincingly demonstrated that this is not the case for most miRNAs – proteins forming a complex with miRNAs stimulate the degradation of the target mRNA by attracting enzymes that remove the cap at the 5’ end and shorten the poly(A) tail at the 3’ end of the mRNA. (This is usually where the degradation of mRNAs that have served their time begins.) Surprisingly, it is still not entirely clear whether the suppression of translation is the cause or consequence of the onset of mRNA degradation.

Meanwhile, life once again demonstrates its unwillingness to fit into any unambiguous schemes: in the laboratory of Joan Steitz, it was discovered that miRNAs can effectively suppress translation by binding not only to the 3’-untranslated region of mRNA, but also to the 5’-NTO [3]. And recently another article of this successful laboratory appeared in the journal Science [4]. It says that under certain conditions (resembling the falling of cells into "hibernation", when cultured in the absence of serum in a nutrient medium), the interaction of miRNA and target mRNA leads to a strictly opposite effect - an increase in the synthesis of the target protein. This was shown for the mRNA of one of the cytokines, tumor necrosis factor α (TNF-α), and miRNA miR369-3, and then confirmed for miRNA let7-a and miRcxcr4 paired with artificially constructed mRNA targets.

Interestingly, the effect of the same miRNA depended on the state of the cells: in dividing cells, miRNA inhibited the translation of mRNA, and in resting cells (temporarily out of the cell cycle), on the contrary, it stimulated (see Figure). It is also curious that miRNAs functioned as part of a complex containing Argonaute 2 and FXR1 proteins (although the human genome encodes 4 related proteins of the Argonaute family, and all of them deal with miRNAs to one degree or another). It is these proteins that play the main role in the mechanism of the observed phenomenon, whereas miRNAs perform the function of a "replaceable adapter" through which proteins interact with various mRNA targets.

The question of the mechanism of action, as well as the avalanche of other, more specific questions caused by this publication, remain unanswered. But I remember the time when the phenomenon of RNA interference was just discovered - how then everything was clear to us and how logical it seemed!.. And now you can only spread your hands – the further into the forest, the more firewood.

Literaturebiomolecule: "microRNAs were first discovered in a unicellular organism";

S. Grigorovich (2003): "Small RNAs in big science. Part 1. The phenomenon of small RNAs" (Scientific.ru );
Lytle J.R., Yario T.A., Steitz J.A. (2007). Target mRNAs are repressed as efficiently by microRNA-binding sites in the 5’ UTR as in the 3′ UTR. Proc. Natl. Acad. Sci. U.S.A. 104, 9667-9672 (online);
Vasudevan S., Tong Y., Steitz J.A. (2007). Switching from Repression to Activation: MicroRNAs Can Up-Regulate Translation. Science 318, 1931-1934 (online);
Rusk N. (2008). When microRNAs activate translation. Nature Meth. 5, 122-123 (online).

Portal "Eternal youth" www.vechnayamolodost.ru08.02.2008