Abstract

The development of a functional nucleolus accompanying the major embryonic genome activation (EGA) is considered a marker for embryo quality and viability. However, the use of this marker is limited by the lack of accurate knowledge of the biology of embryonic nucleologenesis. The objective of this study was to elucidate the role of RNA polymerase I (RPI) and total transcriptional activity, reflecting EGA, for nucleologenesis in in vivo-developed porcine embryos. Late 4-cell-stage embryos were cultured in the absence (control) or presence of actinomycin D (AD; 0.2 µg mL^-1, 3 h for RPI inhibition; 2.0 µg mL^-1, 3 h for total transcriptional inhibition). Late 2-cell-stage embryos were cultured to the late 4-cell stage with 0.2 µg mL^-1 AD (long-term inhibition) to prevent EGA. Embryos were fixed at the late 4-cell stage and processed for RT-PCR (de novo synthesized rRNA), autoradiography (ARG, following culture with 3H-uridine for the last 20 min before fixation), TEM, FISH (probe-labeling rRNA and rDNA), silver staining of nucleolar proteins, and immunofluorescence for RPI. Control embryos displayed typical extranucleolar and nucleolar ARG labeling, fibrillo-granular nucleoli, and focal RPI localization signaling de novo rRNA synthesis in functional nucleoli, confirmed by RT-PCR. All nuclei showed large FISH clusters (rRNA and rDNA) that in 88% of the cases were co-localized with large foci of silver-stained nucleolar proteins. After RPI inhibition, only extranucleolar ARG labeling was detected and, instead of fibrillo-granular nucleoli, a segregated dense-fibrillar component and a granular component, but no fibrillar centers, were observed. RPI was dispersed in all nuclei, the number of nuclei presenting large FISH clusters decreased to 40%, and only 42% of nuclei showed nucleolar proteins localized to large foci. After total transcriptional inhibition and long-term inhibition, the nuclei did not display any ARG labeling and presented inactive nucleolus precursor bodies indicating lack of rRNA (RT-PCR) and total RNA synthesis. However, 40% of the nuclei in both groups presented large FISH clusters of rRNA. This rRNA is considered as partially processed residues of maternally inherited molecules, and their clustering is most likely independent of EGA. Inhibition of transcriptional activity at the time of EGA caused the dispersion of RPI (de novo synthesized) but did not influence the localization of silver-stained nucleolar proteins to large foci (41%). On the other hand, EGA inhibition caused the lack of RPI labeling and hampered the localization of nucleolar proteins to foci. Differences between these 2 groups could be due to the activation of RNA polymerase II before the 3-h AD treatment. In conclusion, RPI transcription and de novo protein synthesis are required for formation of functional nucleoli. However, the clustering of maternally inherited nucleolar transcripts is independent on transcriptional activity at the time of EGA. Failure in constituent RNA polymerase activation during EGA leads to pattern-specific changes in nucleologenesis, which may serve as a marker for early embryo quality.