miRNAs in the regulation of embryonic stem cell differentiation

Embryonic Stem cells (ESCs) have the unique characteristics of self-renew and differentiate into all the cells derived from the three germ layers. These properties make them a limitless source of specialized cells for replacement therapies. However, the knowledge of the mechanisms controlling ESC differentiation into lineage-specific derivatives is necessary before using them for therapeutic purposes. Extracellular signalling, as that of bone morphogenetic protein 4 (BMP4), plays an important role in maintaining ESCs in undifferentiated state and in regulating the lineage commitment. Recently, it was identified a transmembrane protein, named Dies1, which suppression blocks ESC differentiation by interfering with the BMP4 signalling. Over the past few years, it has become evident the involvement of miRNAs in the ESC fate. Thus, we investigated whether a physiological modulation of Dies1 level by miRNAs could be a mechanism regulating ESC choice between pluripotency and differentiation. We demonstrated that miR-125a and miR-125b control Dies1 expression targeting its 3' UTR. Their overexpression impairs ESC differentiation, maintaining the cells in the epiblast state. This effect is due to a reduction of BMP4 signalling and a concomitant increase of Nodal/Activin pathway. This phenotype recapitulates that of Dies1 KD ESCs and is mediated by Dies1 suppression. Moreover we found that Dies1 is associated with BMP4 receptor complex and that BMP4 itself induces the expression of miR-125a at transcriptional level. This miRNA, in turn, controls BMP4 activity through Dies1 regulation. These results show that a feedback loop exists to set ESC sensitivity to BMP4, and it is mediated by miR-125a and Dies1. Interestingly, we found that miR-125b, opposite to miR-125a, is not directly regulated by Transforming Growth Factor-? (TGF-?) signals. These results demonstrate a new role of miR-125a and miR-125b in the regulation of the transition of ESCs to the epiblast stage, working on the control of TGF? signalling.