In contrast to oocytes matured in vitro, porcine embryos that result from in vivo maturation and fertilization have a high developmental competence and readily make the transition from oocyte to blastocyst. This observation led us to investigate the transcript profile differences between in vivo- and in vitro-matured porcine oocytes. For the in vivo-matured group, oviducts of 3 gilts of similar genetic background were flushed 2 days after detection of standing oestrus. MII oocytes were collected in pools of 10 and snap frozen in liquid nitrogen for RNA isolation. The in vitro-matured oocytes were obtained by euthanizing 3 gilts, again with a similar genetic background and recovering the ovaries. Follicles (2 to 8 mm in size) were aspirated and oocytes with multiple layers of cumulus cells and uniform cytoplasm were placed in M-199 supplemented with LH, FSH and epidermal growth factor for 42 h. Upon maturation, cumulus cells were stripped and the healthy MII oocytes were collected in pools of 10 and snap frozen. Total RNA was extracted from 3 pools of 10 oocytes for both treatments using an All prep DNA/RNA micro isolation kit (Qiagen, Valencia, CA, USA). Complementary DNA was synthesized using oligo (dT′) primed reverse transcriptase with superscript III (Invitrogen, Carlsbad, CA, USA). Second-strand cDNA was synthesized using DNA polymerase I and sequenced using Illumina Genome Analyzer II. All reads were aligned to a custom-built porcine transcriptome. There were over 18 million reads in the 2 maturation groups that tiled to the 34 433-member transcriptome: 1317 transcripts were detected with a P ≤ 0.1 (Students t-test), a minimum of 7 reads in at least 1 of the treatments and ≥2-fold difference. Real-time PCR was used on selected transcripts. Comparative CT Method was used on an IQ real-time PCR system with the Bio–Rad SYBR green mix. Statistical differences were determined using the Proc general linear model procedure of SAS (SAS Institute Inc., Cary, NC) and means separated with a l.s.d. (P ≤ 0.05). The misrepresented transcripts from the sequencing data were also characterized using the functional annotation tool DAVID. Twelve pathways were overrepresented in the in vitro-matured oocytes (the top 4 are pathways to cancer, spliceosome, cell cycle and ubiquitin-mediated proteolysis). Eight pathways were underrepresented in the in vitro-matured oocytes (the top 4 are cytoskeleton regulation, T-cell receptor signaling pathway, ubiquitin-mediated proteolysis and cell cycle). Eight transcripts were selected for real-time PCR. ZP2 was higher in the in vitro-matured oocytes as determined by both sequencing and real time. ATG4, HSP90, UBAP2 and SOX4 were not different, regardless of assay. SLC7A3, MRPS36 and PDHX2 were not different based on sequencing, but based on real-time MRPS36 and PDHX2, were higher in the in vivo group and SLC7A3 was higher in the in vitro group. In conclusion, there is an abundance of misregulated transcripts and altered pathways in in vitro-matured oocytes. This dataset is a tool that may provide clues to improve the in vitro maturation process so that in vitro-matured oocytes will be more like their in vivo-matured counterparts, thus improving developmental competence.
Funded by Food for the 21st Century.
Steroid hormones interact with natriuretic peptide C to delay nuclear maturation, to maintain oocyte–cumulus communication and to improve the quality of in vitro-produced embryos in cattle
