Nr4A1 mRNA regulation of FSH-induced meiotic resumption in bovine cumulus-oocyte complexes cultured in vitro.

OBJECTIVE: 1) further define the time course required for transcription initiation in bovine cumulus-oocyte-complexes (COC); 2) determine the pattern of expression for Nr4A1 and Egr1 mRNAs in bovine COC; and, 3) reduce Nr4A1 mRNA expression using small interfering RNAs (siRNA) to determine the effect on breakdown of the germinal vesicle (GVBD). METHODS: A series of experiments were performed to define the time required for transcription initiation during FSH-induced maturation in bovine COCs, determine the pattern of expression for candidate mRNAs during GVBD, and use RNAi to determine their potential role in GVBD by examining whether candidate-specific siRNA can reduce mRNA expression in bovine COCs and affect the occurrence of GVBD. RESULTS: Transcription required for GVBD in bovine COC occurred as early as 30 min after culture initiation. Expression of Nr4A1 mRNA increased (P <0.05) at 30 min after culture initiation, consistent with the time of transcription initiation required for GVBD. Expression of Egr1 mRNA did not differ during culture. Expression of Nr4A1 mRNA was decreased (P <0.05) in COC cultured with 50nM siNr4A1 or with 120 µM of the transcriptional inhibitor DRB compared to controls. The proportion of COC undergoing GVBD at 9 hr of culture in FSH and non-specific siRNA (siNS) treatment groups did not differ. However, fewer (P <0.05) COC underwent GVBD at 9 hr of culture when in the presence of DRB or 50nM siNr4A1 compared to controls. CONCLUSION: these data support a role for Nr4A1 in regulating FSH-mediated and transcription-dependent GVBD in bovine COC cultured in vitro.