Unr is a cytoplasmic RNA-binding protein with roles in the regulation of mRNA stability and translation. In this study, we have identified a novel function for Unr, which acts as a positive regulator of placental development. Unr expression studies in the developing placenta revealed the presence of Unr-rich foci apparently localized in the nuclei of trophoblast giant cells (TGCs). We determined that these foci are actually cross sections of a network of double-wall nuclear membranes invaginations containing a cytoplasmic core, related to the nucleoplasmic reticulum (NR), and accordingly named Unr-NRs. Unr-NRs constitute a novel type of NR, because they contain high levels of poly (A) RNA and translation factors and are sites of active translation. In murine tissues, Unr-NRs are only found in two polyploid cell types, TGCs and hepatocytes. In vitro, their formation is linked to stress and polyploidy, since in three cancer cell lines, cytotoxic drugs known to promote polyploidization induce their formation. Finally, we show that Unr is required, in vivo, for Unr-NRs formation, because these structures are lacking in Unr-null TGCs.
- Received October 18, 2016.
- Accepted March 31, 2017.
- © 2017. Published by The Company of Biologists Ltd