Three-Dimensional Distribution of UBF and Nopp140 in Relationship to Ribosomal DNA Transcription During Mouse Preimplantation Development1

The nucleolus is a dynamic nuclear compartment that is mostly involved in ribosome subunit biogenesis; however, it may also play a role in many other biological processes, such as stress response and the cell cycle. Mainly using electron microscopy, several studies have tried to decipher how active nucleoli are set up during early development in mice. In this study, we analyzed nucleologenesis during mouse early embryonic development using 3D-immunofluorescent detection of UBF and Nopp140, two proteins associated with different nucleolar compartments. UBF is a transcription factor that helps maintain the euchromatic state of ribosomal genes; Nopp140 is a phosphoprotein that has been implicated in pre-rRNA processing. First, using detailed image analyses and the in situ proximity ligation assay technique, we demonstrate that UBF and Nopp140 dynamic redistribution between the two-cell and blastocyst stages (time of implantation) is correlated with morphological and structural modifications that occur in embryonic nucleolar compartments. Our results also support the hypothesis that nucleoli develop at the periphery of nucleolar precursor bodies. Finally, we show that the RNA polymerase I inhibitor CX-5461: 1) disrupts transcriptional activity, 2) alters preimplantation development, and 3) leads to a complete reorganization of UBF and Nopp140 distribution. Altogether, our results underscore that highly dynamic changes are occurring in the nucleoli of embryos and confirm a close link between ribosomal gene transcription and nucleologenesis during the early stages of development.