Mouse Nucleolus-Like Body Supports Transcription and Enhances Processing of rRna in Porcine Enucleolated Oocytes

Abstract Fully-grown mammalian oocytes contain morphologically evident but transcriptionally inactive nucleoli called “nucleolus-like bodies” (NLBs). These nuclear structures are essential for early embryonic development. Removing oocyte NLBs (enucleolation) before their activation leads to developmental failure, mainly at the time point of embryonic genome activation. The present study examined the developmental and expression dynamics of embryos derived from intra- (pig) and interspecies (mouse) nucleolus (NLB) transferred porcine oocytes after parthenogenetic activation. Activation of rDNA transcription and pre-rRNA processing in NLB re-injected embryos was observed by real-time qRT-PCR analysis, targeting the expression rates of 45S rRNA and levels of 18S rRNA. Reinjection of NLBs from mouse or porcine GV oocytes into the enucleolated MII stage porcine oocytes supported the embryonic development up to the blastocyst stage after their activation. Intra- and intergeneric NLB transferred embryos demonstrated rDNA transcription initiation at the 8-cell stage, corresponding with the embryonic genome activation in porcine parthenogenetic embryos (control). The measured levels of 18S rRNA in these experimental embryos showed delayed initiation of pre-rRNA processing at the blastocysts stage compared with the control (8-cell stage). However, the porcine embryos with re-injected mouse NLB displayed significantly higher levels of 18S rRNA than the control and experimental intraspecies group. In conclusion, NLBs from different mammal species (mouse) can enhance the quality of enucleolated porcine oocytes and thus support their early embryonic development after activation.