Identification and expression analysis of genes associated with bovine blastocyst formation

Normal preimplantation embryo development encompasses a series of events including first cleavage division, activation of the embryonic genome, compaction and blastocyst formation. First lineage differentiation starts at the blastocyst stage with the formation of the trophectoderm and the inner cell mass. The main objective of this study was the detection, identification and expression analysis of genes associated with blastocyst formation in order to help us better understand this process. This information could lead to improvements of in vitro embryo production procedures.

A subtractive cDNA library was constructed enriched for transcripts preferentially expressed at the blastocyst stage compared to the 2-cell and 8-cell stage. Sequence information was obtained for 65 randomly selected clones. The RNA expression levels of 12 candidate genes were determined throughout 3 stages of preimplantation embryo development (2-cell, 8-cell and blastocyst) and compared with the RNA expression levels of in vivo "golden standard" embryos using real-time PCR. The RNA expression profiles of 9 (75%) transcripts (KRT18, FN1, MYL6, ATP1B3, FTH1, HINT1, SLC25A5, ATP6V0B, RPL10) were in agreement with the subtractive cDNA cloning approach, whereas for the remaining 3 (25%) (ACTN1, COPE, EEF1A1) the RNA expression level was equal or even higher at the earlier developmental stages compared to the blastocyst stage. Moreover, significant differences in RNA expression levels were observed between in vitro and in vivo produced embryos. By immunofluorescent labelling, the protein expression of KRT18, FN1 and MYL6 was determined throughout bovine preimplantation embryo development and showed the same pattern as the RNA expression analyses.

By subtractive cDNA cloning, candidate genes involved in blastocyst formation were identified. For several candidate genes, important differences in gene expression were observed between in vivo and in vitro produced embryos, reflecting the influence of the in vitro culture system on the embryonic gene expression. Both RNA and protein expression analysis demonstrated that KRT18, FN1 and MYL6 are differentially expressed during preimplantation embryo development and those genes can be considered as markers for bovine blastocyst formation.