Changes in global histone acetylation pattern in somatic cell nuclei after their transfer into oocytes at different stages of maturation

Accumulation of Chromatin Remodelling Enzyme and Histone Transcripts in Bovine Oocytes

During growth, the oocyte accumulates mRNAs that will be required in the later stages of oogenesis and early embryogenesis until the activation of the embryonic genome. Each of these developmental stages is controlled by multiple regulatory mechanisms that ensure proper protein production. Thus mRNAs are stabilized, stored, recruited, polyadenylated, translated and/or degraded over a period of several days. As a consequence, understanding the biological significance of changes in the abundance of transcripts during oocyte growth and differentiation is rather complex. Nevertheless the availability of transcriptomic platforms applicable to scarce samples such as oocytes has generated large amounts of data that depict the transcriptome of oocytes under different conditions. Despite several technical constrains related to protein determination in oocytes that still limit the possibility to verify certain hypothesis, it is now possible to use mRNA levels to start building plausible scenarios. To start deciphering the changes in the level of specific mRNAs involved in chromatin remodelling, we have performed a meta-analysis of existing microarray datasets from germinal vesicle (GV) stage bovine oocytes during the final stages of oocyte differentiation. We then analysed the expression profiles of histone and histone-remodelling enzyme mRNAs and correlated these with the major histone modifications known to occur at the same period, based on data available in the literature. We believe that this approach could reveal the function of specific enzymes in the oocyte. In turn, this information will be useful in future studies, which final ambitious goal is to decipher the 'oocyte-specific histone code'.

Changes in histone H4 acetylation during in vivo versus in vitro maturation of equine oocytes

Epigenetic modifications are established during gametogenesis and preimplantation embryonic development. Any disturbance of the normal natural environment during these critical phases could cause alterations of the epigenetic signature. Histone acetylation is an important epigenetic modification involved in the regulation of chromatin organization and gene expression. The present study was aimed to determine whether the proper establishment of post-translational histone H4 acetylation at lysine 8 (AcH4K8), 12 (AcH4K12) and 16 (AcH4K16) of equine oocytes is adversely affected during in vitro maturation (IVM) when compared with in vivo matured oocytes collected from naturally cycling mares not undergoing ovarian hyperstimulation. The acetylation patterns were investigated by means of indirect immunofluorescence staining with specific antibodies directed against the acetylated lysine residues. Our results indicate that the acetylation state of H4 is dependent on the chromatin configuration in immature germinal vesicle (GV) stage oocytes and it changes in a residue-specific manner along with the increase of chromatin condensation. In particular, the levels of AcH4K8 and AcH4K12 increased significantly, while AcH4K16 decreased significantly from the fibrillar to the condensed state of chromatin configuration within the GV. Moreover, during meiosis, K8 and K12 were substantially deacetylated without any differences between in vivo and in vitro conditions, while K16 displayed a strong acetylation in oocytes matured in vivo, and in contrast, it was markedly deacetylated following IVM. Although the functional meaning of residue-specific acetylation during oocyte differentiation and meiotic resumption needs further investigation, our results support the hypothesis that IVM conditions can adversely affect oocyte ability to regulate the epigenetic reprogramming, critical for successful meiosis and subsequent embryonic development.