The emerging roles of placenta-specific microRNAs in regulating trophoblast proliferation during the first trimester.

In the human placenta, eight 'placenta-specific' microRNAs (miRNAs) are exclusively expressed and are associated with high cloning frequencies. The expression of placenta-specific miRNAs is known to be involved in preeclampsia, but the understanding of these miRNAs is still limited. The goal of this study was to investigate the levels and localisations of eight placenta-specific miRNAs in placental villi with different proliferative abilities during the first trimester. Immunohistochemical analyses indicated that placental trophoblast proliferation ability was associated with the weight of villi in the same gestational week during the first trimester. Of the eight placenta-specific miRNAs, quantitative real-time RT-PCR demonstrated that the expression of miR-517b and miR-1283 was increased in the lightest villi and decreased in the heaviest one, the expression of miR-519a was increased in the heaviest villi and decreased in the lightest one. In situ hybridisation analysis showed that miR-517b and miR-519a were located primarily in the trophoblast layer, while miR-1283 were expressed not only in the villous trophoblasts but also in some villous stroma cells. These findings suggest that miR-517b and miR-519a may play an important role in trophoblast proliferation during the first trimester.