DRIMTIM

Deciphering RNAi machineries required for miRNAs Cell-to-Cell Transfer in Mammals

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 Obiettivo del progetto (Objective)

'Small RNAs are key post-transcriptional regulators of eukaryotic gene expression. Among the most fascinating aspects of small RNAs is their ability to cross cell boundaries owing to their non-cell-autonomy. Recently, the host laboratory demonstrated that 21-24bp siRNAs could act as mobile silencing signals in Arabidopsis. Interestingly, in C. Elegans, systemic silencing requires SID1, a transmembrane channel through which endogenous double-stranded RNA may be communicated to adjacent cells. Functional SID1 homologues and miRNAs found in secreted exosomes in mammals suggest that systemic RNA silencing might also operate in these organisms, raising the question of how this process might be regulated? Tight regulation is indeed anticipated given the exquisite expression patterns and developmental roles of many mammalian miRNAs. A first possibility for regulated miRNA movement entails that it might mostly occur between compatible “emitting” and “receiving” cells. This might be achieved via qualitatively differences in miRNAs effector complexes, localization or shear availability of silencing transporter systems. A second, non-mutually exclusive possibility is that release of miRNA through membranes might be polarized. The identification, in the host laboratory, of a requirement for multi-vesicular bodies for the assembly of miRNA effector complexes supports this idea. In fact, we propose that both mechanisms could be at work in specialized cells, such as secreting epithelia, to direct the selective release of miRNAs either along the epithelial cell layer or in body fluids. Using the mammary gland as a model system we will (i) decipher the molecular requirements for cell to “emit” or “receive” systemic miRNAs (ii) study how cell-polarization might affect miRNA cell-to-cell transfer (iii) investigate the in vivo relevance of our findings by characterizing miRNAs contained in milk, and by studying potential effects of systemic miRNA arising from grafted tumors in mice.'

Introduzione (Teaser)

Since their discovery, small RNA molecules have come to revolutionise the way biologists perceive regulation of eukaryotic gene expression. A European study reveals a novel function for microRNA (miRNA) molecules secreted in biological fluids such as milk.

Descrizione progetto (Article)

Among the most fascinating properties of small interfering (siRNA) and miRNA molecules is their ability to cross cell boundaries. Although different mechanisms may be in place for facilitating this transmission, the non-cell autonomous nature of the process raises the question of how it is regulated.

In answer to this, the EU-funded project 'Deciphering RNAi machineries required for miRNAs cell-to-cell transfer in mammals' (DRIMTIM) plans to test different hypotheses. DRIMTIM scientists will explore whether there are miRNA-producing and -receiving cells, and if the release of miRNA through the membrane is a polarised process. The latter is also supported by the fact that miRNA function is tightly linked to vesicles within cells.

To obtain information on the cell-to-cell movement of small RNA in mammals, the consortium has utilised cancer cells and the mammary gland system as models. It had to face the challenge of monitoring miRNA concentrations at the cellular level. For this purpose, they generated an innovative, sensitive and specific miRNA sensor strategy based on the fluorescent detection of miR21 in a transgenic cancer cell line. This cell line has no endogenous miR21 so it can serve as a sensor for the cell-to-cell movement of miR21.

In addition, researchers identified differences in RNA silencing complexes, which reinforce the differential capacity of cells to either emit or receive small RNAs. This could be a result of qualitative and quantitative differences in various factors implicated in the processing and secretion of miRNAs.

Finally, the DRIMTIM consortium investigated the physiological relevance of miRNA transfer. They found that mammalian milk contains abundant miRNAs and RNA silencing factors, and hypothesised that they serve as paracrine molecules, or as a long-distance communication system between mother and child. Experiments in mice demonstrated that miRNAs secreted in the milk are indeed transferred to the pups. Further investigation is, however, required to unequivocally demonstrate the function of these molecules.