263 GENE EXPRESSION PROFILING OF IMMATURE AND IN VITRO-MATURED BOVINE OOCYTES USING AFFYMETRIX GENECHIP TECHNOLOGY

Abstract

Previous studies by our group have demonstrated that oocyte maturation is a crucial event in the determination of subsequent developmental competence. The objective of the current study was to characterize changes in gene expression profiles of bovine oocytes during meiotic maturation. To this end, 5 replicate pools of 200 bovine cumulus–oocyte complexes (COCs)were collected from the ovaries of slaughtered heifers. Upon recovery, 100 COCs from each replicatewere immediately denuded, and the oocytes were snap frozen in liquid nitrogen. The remaining 100 COCs were matured in vitro in TCM-199 supplemented with 10% (v/v) fetal calf serum and 10 ngmL^-1 epidermal growth factor for 24 h at 39°C under an atmosphere of 5% CO₂ in air with maximum humidity. Following maturation, the remaining COCs were denuded and snap frozen. Total RNA was isolated (mean total RNA content 106.08±38.87 ng per 100 oocytes) and subjected to 2 rounds of amplification incorporating biotin-labeled nucleotides during the second in vitro transcription reaction (mean total RNA content 155.15±51.14 µg per 100 oocytes post-amplification). The resulting labeled antisense RNA was hybridized to a GeneChip Bovine Genome Arrays (Affymetrix, Inc., Santa Clara, CA, USA) (10 chips, 5 replicates each of immature and mature oocytes, n=100 oocytes/chip). Expression data were analysed using Genespring software (Agilent Technologies, Palo Alto, CA, USA), and data were normalized to the median. Overall, 54.9±1.3% and 53.3±3.3% of the 24 178 probe sets representing 23 000 transcripts spotted on the arrays were expressed in immature and in vitro-matured oocytes, respectively. Across the 5 array comparisons, 52 genes were consistently exclusively present in immature oocytes, whereas 16 genes were exclusively present in mature oocytes. A further 821 genes were found to be differentially expressed (≥2-fold) between the 2 groups (P <0.05), of which 209 were up-regulated and 612 were down-regulated in the in vitro-matured oocytes compared with their immature counterparts. The differentially expressed transcripts were classified according to their gene ontology (http://benzer.ubic.ca/ermineJ). The existing Affymetrix annotation was updated by blasting the sequences against bovine, human, and murine databases (≥90% homology; increasing molecular function annotation from 14% to 42%). In terms of olecular function, the majority of these genes were associated with protein or nucleic acid binding (>42%), catalytic activity (24%), signal transduction (7%), transporter activity (5%), and structural molecule activity (5%). In conclusion, we have stablished the molecular transcriptome blueprint of immature and in vitro-matured bovine oocytes. Through comparisons with in vivo-matured oocytes, this resource will be invaluable in determining genes that are involved in controlling the developmental competence of oocytes.

This research was funded by the Science Foundation Ireland (02/IN1/B78).