The capability of reprogramming the male chromatin after fertilization is dependent on the quality of oocyte maturation

Abstract

The present experiments compared the ability of pig oocytes matured eitherin vivoorin vitroto structurally reorganize the penetrated sperm chromatin into male pronucleus (PN) and to carry out, in parallel, the epigenetic processes of global chromatin methylation and acetylation, 12–14 h afterin vitrofertilization (IVF). In addition, PN distribution of histone deacetylase (HDAC), a major enzyme interfacing DNA methylation and histone acetylation, was investigated. The ability of the oocyte to operate an efficient block to polyspermy was markedly affected by maturation. The monospermic fertilization rate was significantly higher forin vivothan forin vitromatured (IVM) oocytes(P< 0.01) which, furthermore, showed a reduced ability to transform the chromatin of penetrated sperm into male PN(P< 0.01). Indirect immunofluorescence analysis of global DNA methylation, histone acetylation and HDAC distribution (HDAC-1, -2 and -3), carried out in monospermic zygotes that reached the late PN stage, showed that IVM oocytes also had a reduced epigenetic competence. In fact, while in about 80% ofin vivomatured and IVF oocytes the male PN underwent a process of active demethylation and showed a condition of histone H4 hyperacetylation, only 40% of IVM/IVF zygotes displayed a similar PN remodelling asymmetry. Oocytes that carried out the first part of maturationin vivo(up to germinal vesicle breakdown; GVBD) and then completed the processin vitro, displayed the same PN asymmetry as oocytes matured entirelyin vivo. A crucial role of HDAC in the establishment of PN acetylation asymmetry seems to be confirmed by the use of HDAC inhibitors as well as by the abnormal distribution of the enzyme between the two PN in IVM zygotes. Collectively, these data demonstrated that some pig IVM oocytes fail to acquire full remodelling competence which is independent from their ooplasmic ability to morphologically reorganize the sperm nucleus into PN.