Abstract
The epigenetic factors causing competence differences between SN (surrounded nucleolus) and NSN (non-surrounded nucleolus) oocytes, the significance for the increased histone acetylation and methylation in SN oocytes, and whether chromatin configuration or histone modification determines oocyte competence, are unclear. This study has addressed these issues by using the ovary-holding (OH) stress models where oocyte SN configuration was uncoupled from histone modifications and developmental potential. Prepubertal mouse ovaries containing high percentages of NSN oocytes were preserved at 37 or 39 °C for 1 or 2 h before examination for oocyte chromatin configuration, developmental competence, histone modification and apoptosis. Whereas 1-h OH at 37 °C caused a moderate apoptosis with increased oocyte competence, improved histone modification and a normal NSN-to-SN transition, harsher OH conditions induced a severe apoptosis with decreased oocyte competence, impaired histone modification and a pseudo (premature) NSN-to-SN transition. Observations on Fas/FasL expression and using the gld (generalized lymphoproliferative disorder) mice harboring FasL mutations indicated that OH triggered oocyte apoptosis with activation of the Fas signaling. It was concluded that OH stress caused oocyte apoptosis with activation of the Fas/FasL system and that oocyte competence was more closely correlated with histone modification than with chromatin configuration.
Highlights
The epigenetic factors causing competence differences between SN and non-surrounded nucleolus (NSN) oocytes, the significance for the increased histone acetylation and methylation in SN oocytes, and whether chromatin configuration or histone modification determines oocyte competence, are unclear
Similar changes in the chromatin configuration were observed in oocytes after Ovary holding (OH) of adult mouse ovaries, the proportion of NSN oocytes was very low in the adult ovaries
The c-IN and c-NSN configurations generated by the pseudo NSN-to-SN transition showed abnormally condensed chromatin masses, which were reminiscent of the pyknotic nuclei observed in apoptotic somatic cells
Summary
The epigenetic factors causing competence differences between SN (surrounded nucleolus) and NSN (non-surrounded nucleolus) oocytes, the significance for the increased histone acetylation and methylation in SN oocytes, and whether chromatin configuration or histone modification determines oocyte competence, are unclear. Whereas the SN percentage of stressed oocytes returned to normal, neither the level of histone acetylation and methylation in SN oocytes nor the developmental competence recovered following a post-restraint recovery These results suggested that the SN configuration was uncoupled from increased histone acetylation and methylation in the restraint-stressed oocytes, and that the developmental potential of SN oocytes is more closely correlated with epigenetic histone modification than with chromatin configuration, the conclusions need further verification using other models. Why restraint stress inhibited while OH stress facilitated the NSN-to-SN transition, and how the early atretic changes triggered by the low temperature OH and other stressors[18,19] improve oocyte competence, need further investigations
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